Dry Etching

Chapter 13

Dry Etching

13.1. Introduction The success of micro-fabrication technology is crucially dependent on the manufacture of a growing number of integrated circuits and products as exemplified by the pervasiveness of computers in human life. In order to produce these products it is essential to introduce fine features and patterns on films that on deposition are continuous. Dry etching technology is indispensable for fabricating three-dimensional building blocks for micro-electromechanical systems (MEMS) applications. Hence, most of the plasma etching science and technology has evolved from needs in the semiconductor industry. A typical micro-fabrication process consists of many repetitions of the following sequence of steps: pattern generation (known as mask making), pattern replication (accomplished by lithography techniques) and pattern transfer (done primarily by etching). The fabrication of microelectronic devices requires the transfer of patterns defined by a mask onto a layer of photoresist deposited on a substrate in which the device has to be fabricated. The various steps in the transfer of pattern from the mask to the wafer in the subtractive method are illustrated schematically in Fig. 13.1. The wafer surface is coated with a thin film of photosensitive polymer called resist. Appropriate geometrical areas are delineated on a positive photoresist by exposing them to radiation through a patterned mask. The radiation produces chemical changes in the resist, which make the exposed region of a positive photoresist more soluble in a developer solution than the unexposed regions. The exposed positive-photoresist is dissolved in a developer solution whereas the unexposed photoresist stays on the substrate as a mask after a polymerizing bake out. Once the pattern is established in the photosensitive material, it provides the mask in the next step in manufacturing. The resist therefore, completely covers the wafer over the selected regions, leaving the other parts fully exposed. Etching is done only in the exposed regions. A vast variety of

1074 Dry Etching

Wafer

Deposition of resist

Energetic radiation on through a patterned mask Exposure of positive resist

Positive resist in which the exposed resist is dissolved in a developer and the unexposed resist is polymerized by a bake out

Unexposed photoresist stays on substrate as a mask. Etch wafer in the opened region

Remove photoresist

Figure 13.1: Subtractive pattern transfer by etching.

materials, structures and geometries are encountered during the etching process that imposes a wide range of requirements for the etching process. For example, when etching is carried out strictly by a chemical solution, the removal of material, normal to the exposed surface may proceed at the same rate as in the lateral dimension as shown in Fig. 13.2(a). The situation is referred to as isotropic etching. Special chemical solutions are available to etch the materials so that lateral etching rate is much slower than the etching rate normal to the exposed surface. This is especially true when the substrate is a single crystal so that it is possible to make use of the different etching rates in different crystallographic directions. A situation, shown in Fig. 13.2(c) is termed directional or anisotropical etching. Aspect ratios (i.e. the ratio of depth to height) in excess of 650:1 can be obtained by these techniques.

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x

Mask

Substrate z

(a) Isotropic etch

(b) Vertical etch

(c) Directional etch

Figure 13.2: Directionality of the etching processes.

Etching done with the help of liquid solutions is called wet etching as opposed to dry etching that does not involve any liquids. When the thickness of the etched film is small relative to the minimum pattern dimensions, undercutting of the film to sizes larger than that permitted by pattern size is not acceptable. One needs to have etching techniques that produce a vertical etching profile as shown in Fig. 13.2(b) in order to attain small feature sizes. However, a slight taper in the etching profile may be advantageous from the point of view of deposition in and over crevices and steps in a pattern. The importance of plasma or dry etching stems from the ability to achieve vertical etching so that the dimension defined by lithography is preserved in the etching process. To etch straight trench walls, the ions must impinge on the wafer at normal incidence. This requires that the plasma sheath edge must be planar all the way across the wafer. Currently, all pattern transfer steps from the mask to the underlying substrate are performed by dry etching techniques because of the small line widths that are current in today’s manufacture of semiconductor devices. Plasma etching is central to the success of manufacturing microelectronic devices because it is the only technology that can produce reproducible patterns of the dimensions required. There are several other techniques of

1076 Dry Etching

pattern transfer. For example, if the material to be patterned is deposited over the mask, a process called additive pattern transfer can reproduce the pattern. In the final step, the unwanted material is lifted off from their substrate by dissolving the masking pattern. There are also a limited number of situations, where the pattern transfer, can be accomplished without the need for lithographic step and exploits the existence of a patterned structure of some kind. This is achieved in the so-called self-aligned pattern transfer, where the selective chemical reaction occurs only on certain regions of the substrate, which are to be patterned, and not on the mask material. In special situations, one can directly write the pattern on the substrate. These however, are not of concern in this chapter. Etching has also been utilized to obtain planarization of the surface. Optical lithography can be successfully carried out if the surface of the film is planar because the depth of focus is typically 1 mm, whereas large height variations may occur as a result of some processing steps. Planarization involves the deposition of an organic layer, such as polyimide, which fills the depressions coupled with some etching process to remove material from the high points. In plasma etching, a glow discharge is utilized to generate radicals, metastables and ions that are obtained from a relatively nonreactive gas. The production of radicals, metastables and ions occurs through homogeneous gas phase collisions in the plasma. The species that are produced are such that some of them react with the material to be etched by forming a volatile product through heterogeneous reactions. Thus, the normally nonreactive gas is turned into several highly reactive species. The etch product spontaneously desorbs from the etched material into the gas phase, which is subsequently removed by the vacuum pumping system, leaving the material in an etched condition. The use of the plasma has the additional advantage that etching can be carried out at or around room temperature. It is important to emphasize at the outset that plasma etching can only be considered in relation to the total process involved in the fabrication of any device rather than individual interaction between a particular chemical radical and the material it is supposed to etch. In fact, a successful etch process will depend upon previous process steps, and may affect subsequent process steps and may change if either of them is modified. The constraints on the total system must always be considered in dry etching. Ultimately, it involves making trade-offs among selectivity, uniformity and anisotropy. Consider for example, a structure (Fig. 13.3), where a photoresist of thickness, tph , is covering a thin film of polysilicon of thickness, tpoly , which has been deposited over an oxide of thickness, tox . A lithographic process has opened up a window of width wm . In order to etch the polysilicon, we can subject the structure to plasma etching when both the photoresist and polysilicon will be subject to the plasma

Dry Etching 1077

t ph

wm

Resist

t poly

Polysilicon

t ox

Oxide

Figure 13.3: The various films that have to be considered in determining etch rates.

conditions. If the etching has to be completed in a time t, then the rate required to etch the polysilicon is Rpoly ¼ tpoly =t. During this time the photoresist may also be etched at a rate, Rphoto ¼ tphoto =t. If we want to keep the photoresist without significant erosion while the polysilicon is etched, it is clear that the ratio S ¼ Rpoly =Rphoto should be very much greater than tpoly =tphoto . Therefore, an immediate criteria to accomplish etching successfully is to make sure that one selects the appropriate selectivity, S, acceptable to the manufacture of the device. Selectivity needed for etching can therefore, simply be estimated by the ratio of thickness of the film to be etched to the thickness of the film acting as the etch stop. Note that the etching rate of the thin film has two components: horizontal and vertical etching rates. If finer features are to be patterned the selectivity has to be high. The thickness of the layers, control of profile and the critical dimension uniformity are determined by the selectivity. Etching is rarely uniform, so that it is necessary to over-etch the polysilicon at some locations in order to clear it from all other unmasked regions. The uniformity is usually defined as the maximum difference in etching rates over a wafer and/or within an etcher or from batch to batch. Variations in the etching rate are due to the surface topography, nonuniform thickness and variation in the etch rate in the reactor due to localized depletion of gas phase radicals. The etching of a wafer from center to edge evenly requires that the plasma is uniform in density, temperature and potential. Uniformity also refers to the ability of maintaining etch rates from wafer to wafer in the same reactor. This means there will be regions when the plasma will be in contact with the oxide as shown in Fig. 13.3. In other words, there should also be selectivity in etching

1078 Dry Etching

between the polysilicon and the gate oxide, which is to be far in excess of tpoly =tox . Since tox is usually very small, the selectivity criteria imposed by the underlying thin film can be a very important concern for process to be successful. The main criteria for etch selectivity is that it should be high enough to preserve a thin oxide film that is on the substrate of the film to be patterned. The most important reason for utilizing plasma etching is the directionality in etching it provides in the fabrication of high-aspect ratios, such as deep trenches necessary in producing many semiconductor devices. A measure of the directionality of etching is provided by the degree of anisotropy in etching (AE), which is defined by l AE ¼ 1
, d

(13:1)

where l is the amount of undercutting at the end of etching, and d is the thickness of the etched layer as shown in Fig. 13.4. If l ¼ d, the etching is completely isotropic, and if l ¼ 0 the etching is completely vertical. Note that the dimensional loss that is incurred by an etching process depends upon the etch aspect ratio (depth/width ratio). Vertical etching profiles permit maximum density packing on the chip. In addition to all the above considerations about selectivity, uniformity and anisotropy, one frequently finds additional problems from erosion of the sidewalls of the mask due to lateral etching, development of facets in photoresist due to sputtering so that careful evaluation of all these considerations as they affect pattern reproducibility is needed in relationship to the etching process. The most effective trade off between line size control, overetch and under layer preservation may be different for each process, and an understanding of which of these (if any) is needed to be maximized is the key to successful process development.

l Mask

d

Film

Substrate

Figure 13.4: Anisotropy in etching.

Dry Etching 1079

Plasma sources have to be adaptable to the size requirements of the industrial needs. For example, a silicon wafer can be 12 in. in diameter whereas a flat panel display can be very large in area. When thin oxides are involved, plasma processing can damage the oxide. Electrons reaching a trench to be etched cause a charge buildup that can drive the current through the insulating layer. The ability to etch requires different combinations of gases to be used. This puts a premium on operating the plasma reactor under a variety of conditions. The reliability of a plasma tool is of course a major concern for the industry as well as the space required for each reactor, since a large number of reactors of necessity is required in a fabrication facility. The materials used in the construction of the reactor have to be benign to the plasma and its various neutral and reactive components. The dry etching in plasma can proceed in one of three ways; ion sputtering, chemical reaction and ion-assisted chemical reaction. Energetic ions crossing the plasma sheath can transfer large amounts of energy to the material on which they fall, which can result in the ejection of atoms by sputtering. This is purely a physical phenomenon and is least selective of the material that it etches and the material is removed as atoms. In addition, the rates of etching are low, surfaces have a tendency to show faceting and one can damage the substrate (Fig. 13.5a). Faceting occurs because the sputter yield of a material is usually a strong function of the angle at which ions are directed at the surface. If the atoms sputtered are not to return to the surface, low gas pressures are required giving mean free path equivalent to reactor dimensions. Otherwise, collisions in the gas phase will reflect and redeposit the sputtered species. The radicals created in plasma can bring about chemical reaction that results in etching when the product of the chemical reaction is volatile. This type of etching tends to be isotropic and highly selective and any sputtered atom has a tendency to return on the substrate because of the high-vapor pressures that one encounters during etching (Fig. 13.5b). The ion-assisted etching is of primary interest in dry etching because it not only gives vertical etching possibility but also gives rise to high rates of etching (Fig. 13.5c). A special feature of dry etching occurs when the chemical reaction produces a very thin film, which inhibits etching while at the same time isotropic etching occurs between the radicals and the material. If the inhibitor film forms on the vertical surface, where it is immune from sputtering by the ions, one can achieve vertical etching (Fig. 13.5d). The chemical volatilization of the material during dry etching can result in a situation where the area exposed to etching will determine the etch rate. This manifests itself as loading effect, which has been one of the many reasons for dry etching reactors to prefer single-wafer etching. One can use ion-assisted gas chemistry to advantage not only to get anisotropy in etching but also to improve selectivity in etching. Frequently, a number of different gaseous mixtures are

1080 Dry Etching + Ion

(a) Sputtering

Neutral +

Volatile product

(b) Chemical

Neutral +

+

Volatile product

(c) Ion enhanced energetic Ion

Neutral +

+

Volatile product (d) Ion enhanced inhibitor

Ion

Inhibitor

Figure 13.5: The variety of processes that may be involved in plasma etching.

used at different times in etching to take advantage of sidewall passivation to insure vertical etching as well as obtain selectivity in etching. Control of ion energy and ion flux independently requires direct ion beam etching at low pressures. There are clearly many reactor configurations, which have evolved depending on the large varieties of needs and structures and materials that have to be etched. The monitoring of the etching process during etching is desired in every case even though it is not always possible to attain this. Finally, one has to make sure that the damage introduced from ion etching is removable or is at a level acceptable to the particular process and device fabrication. The largely empirical route to optimize the etching operation is very expensive and can be beneficial if we have realistic models of the etching process, and this appears still far from being realized. Plasma etching processes are judged by their ability to give the acceptable etching rate, uniformity in etching over the material surface as well as among the substrates, directionality of the etching and the selectivity of the materials to be etched.

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13.2. Chemical Volatilization The principal requirement in plasma etching is the volatility of the etch product. The etching gas has to be chosen such that in a plasma environment, the gas produces radicals, which can react with material to be etched to form a volatile species. A radical is an atom or an aggregate of atoms, which is electrically neutral, but which exists in a state of incomplete chemical bonding, so that it is very reactive. The hot electrons in the plasma are responsible for the unusual chemistry that occurs in the plasma. The energy to ionize neutrals is much higher than the energy to dissociate molecules. There are usually more electrons with enough energy to dissociate molecules than those that have enough energy to ionize molecules. It is usually the case that the gas introduced into the plasma does not react with surface to be etched according to thermodynamics, so that all reactive fragments are produced when the gas dissociates in the plasma due to collisions with electrons. The creation of the radicals in the plasma and transporting them to the surface to be etched is clearly the basic step that is necessary in the etching process. Radicals are thought to exist in plasma in much higher numbers than ions because they are generated at a faster rate and they survive longer in plasma than ions. The primary function of the plasma reduces to maintaining a supply of gaseous etchant species. Plasma etching was first utilized to remove carbonaceous materials from products. In this case, oxygen was utilized as the gas in the plasma. The oxygen atoms produced in the plasma react with carbonaceous material and hydrogen to produce volatile products, such as CO, CO2 and H2 O. This process was initially termed plasma ashing or plasma stripping (Irving, 1971). Plasma etching is typically used for the removal of organic fragments from inorganic samples. The plasma ashing could be carried out in a simple barrel reactor at temperatures varying between room temperature and 300 8C. The technique was used for stripping photoresists since 1960s, where one typically uses CF4 or SF4 with oxygen. The purpose of fluorine atoms in the plasma is to enhance dissociation of oxygen, which combines with photoresist to form water vapor and carbon dioxide. If the concentration of CF4 is low, one produces excited fluorine atoms, which also assist in etching. At high concentration of CF4 , the polymerization reaction of tetrafluoromethane competes with etching reaction with fluorine and passivation of the surface. Any inorganic contaminants on the surface that do not form volatile oxides are not removed in plasma etching. This reactor consisted of a cylindrical dielectric vessel with an oxygen admission system at one end and a vacuum pump at the other (Fig. 13.6). The RF power is supplied by placing the metal electrodes on opposite sides of the vessel or by encircling the cylindrical vessel with an RF coil. The material to be etched is placed on racks in the center of the vessel. Inserting a cylindrical mesh of metal

1082 Dry Etching

RF

Wafers To pump Etch gas in Etch tunnel

RF

Figure 13.6: Barrel reactor for etching with an etch tunnel.

(known as an etch tunnel) to isolate the material from the electrons, protects those devices that are prone to radiation damage by electron bombardment but may prove to be the source of contamination and sputtered material with reactor usage. The rapid removal of the carbonaceous material by stripping depends on the supply of active species. The process is therefore, performed at high pressures, where ion energy is low and the reactant supply is high. The reactors are inexpensive, but uniformity in etching is rarely achieved and the rate of reaction changes radically from point to point because of reactor heating from the RF power input and the heats associated with etching reactions. The application of plasma etching to silicon occurred with use of CF4 gas in a plasma. CF4 gas is inert and does not react with silicon. However, in plasma, it produces many radicals (mainly F and CF3 ) that react with silicon to form a volatile gaseous product SiF4 , thereby affecting the etching of silicon. Many other materials, such as W, Ta, Ge, Ti, Mo, etc., can also react with CF4 plasma to achieve volatile fluorides and thus, can be etched in CF4 plasma. The selection of gaseous material to etch a particular material is important. For example, chlorine used in plasma can form a volatile chloride of aluminum when aluminum is to be etched. However, fluorine-containing gas in plasma forms an involatile fluoride of aluminum and is not a suitable gas for etching aluminum. Plasma etching with a material involves a reaction between the active gas species and the material to be etched. One can distinguish three separate steps involved in the mechanism of etching. The gas must first adsorb on the surface of the material forming a chemical bond with surface atoms. The atoms on the surface must subsequently rearrange themselves to form the molecules of the product. The product molecule must then desorb from the surface. If any of these steps do not occur spontaneously, etching will not occur. It is therefore,

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quite important to know the gas–surface interaction in detail both in the presence and in the absence of energetic radiation. Many of the gases commonly used in a plasma etching do not readily chemisorb on the material that one wishes to etch (Winters, 1978). Hence, they do not etch the material without the assistance of the plasma environment. On the other hand, molecular chlorine chemisorbs on silicon, but it dissociates into chlorine atoms rather than forming SiCl4 (Pandey et al., 1977). Hence, etching does not occur. There are many situations, where the radical in the plasma form a nonvolatile compound on reaction with the material to be etched. Hence, the reaction terminates after a certain limiting thickness of the involatile product is formed. The spontaneous oxidations of silicon, aluminum are examples of this kind. Example 13.1 List the stages involved in the etching of silicon by CF4 plasma. CF4 gas is dissociated and ionized in the plasma to produce F and various other ions and radicals in the plasma. Consider the species formation in the discharge by electron impact as: CF4 þ e ¼ CF3 þ F þ e CF3 þ e ¼

CFþ 3

þ 2e

(dissociation reaction): (ionization reaction):

The etchant must be absorbed on the wafer surface Si(surface) þ F(g) ¼ Si-F(ads): The product of etching must form on the surface, possibly by successive fluorination Si-F(ads) þ F(g) ¼ SiF2 (ads): SiF2 (ads) þ F(g) ¼ SiF3 (ads): SiF3 (ads) þ F(g) ¼ SiF4 (ads): The etch product must be sufficiently volatile to desorb giving the final etch effluent SiF4 (g) as: SiF4 (ads) ¼ SiF4 (g): The interaction of gaseous species in plasma with material to be etched is much more involved and follows many reaction pathways as discussed by Flamm (1981) in terms of an ‘‘etchant-unsaturated species’’ model. Consider the following different reaction paths that are possible in the plasma. Many of these paths are initiated by the electron impact of gaseous species.

1084 Dry Etching

An electron interacting with a halocarbon can create saturated species, unsaturated species and atoms. For example, we have 2e þ 2CF4 ¼ CF3 þ CF2 þ 3F þ 2e:

(13:2)

The above type of reaction produces the necessary radicals that can interact with the material to be etched. The concentration of the various radicals is clearly important and is often not known in detail. The reactive species can combine with unsaturated species to form saturated species. For example, F þ CF2 ¼ CF3 ,

(13:3)

and the above reaction is one of the recombinations. This can dilute the concentration of the active species available for etching. One notes gas phase oxidant additives, such as O2 , F2 , etc., can dissociate and react with unsaturated species changing the relative concentration of species as well as reaction pathways. The most desired reaction path occurs when the atoms interact with the surface atoms producing a volatile product. This is illustrated by 4F þ Si ¼ SiF4 :

(13:4)

Etching will occur as a result of the above reaction. The unsaturated species can react with itself aided by the surface in a polymerization type reaction. Hence, we may have reactions like nCF2 þ surface ¼ (CF2 )n :

(13:5)

Clearly, the polymerization reaction produces a film, which is generally undesirable if it forms in unwanted regions. Reactive radicals with low volatility can deposit in the system so that the knowledge of the sticking coefficients of the free radicals is of critical importance. Methods of influencing the gas phase reactions have to be undertaken so that deposition can be utilized advantageously to provide vertical etching and to avoid excessive deposition or nonconformal deposition. The interaction between the etch gas and the substrate can thus result in etching, recombination or film formation depending on the predominant reaction that occurs between the various radicals and gas species. The etching of a material by forming a volatile product with the radical produced in the plasma is strictly speaking a chemical etching process, where gases are involved as an etchant and as a product. As such, the etching occurs isotropically as shown in Fig. 13.2a. The selectivity in etching can be extremely high owing to the large difference in an etchant’s chemical affinity for the various materials. The initial applications of plasma etching in resist stripping, patterning silicon nitride and for polysilicon were essential to achieve isotropic etching and did not really focus on controlling pattern dimensions. The advantage of using gaseous radicals is that the control of gaseous product is relatively

Dry Etching 1085

easy and no residues from etching process are left on the material as in the case of wet etching. The etching rate is very rapid as the pressures used are in the several hundred Pascal range so that radicals impact the surface from all different directions. This is adequate in many situations, and the presence of plasma enables chemical reactions to occur at room temperature or thereabout. It is also easy to handle gases than liquid etchant that is contaminated with the etchant material from environmental concerns. The disposal of large quantities of etching liquids becomes a costly affair reaching and exceeding the cost of the etchant itself. The gases involved in dry etching are generally toxic and can be handled relatively easily because of their relatively small quantities. In addition, dry processes are inherently clean from the point of view of residues and particulates. On the other hand, the equipment costs of plasma etching units are substantially larger than wet chemical methods. Furthermore, dry etching techniques are capable of being integrated into automatic processes in manufacturing. One of the important requirements in etching is to achieve uniformity in etching across the entire area of the surface that is exposed for etching. One of the most important variables governing the uniformity is the gas flow and the manner in which gas is distributed into the reactor over the substrate. It is desired that the design of the inlet and outlet of gases are such as to expose a fresh and evenly distributed flow of gases. Gas flow rates are determined by the maximum pumping speed and the desired working pressure. The flow rate should be kept at a minimum level with respect to the etch rate. One can define a utilization factor U where U¼

rate of formation of etch product , flow rate of etch gas

(13:6)

and this should be greater than about 0.1 for assuring uniform etching. A parameter that is of importance that is related to the utilization factor is the residence time t of the feed gas, which depends on the flow rate and pressure in the gas. The residence time is given by t¼

Vp , Q

(13:7)

where V is the reactor volume, p is the steady pressure and Q is the flow rate. (1 sccm ¼ 4:48  1017 molecules=s). A number of additives are added to the gas in the plasma and perform a variety of functions. Oxidants are often used to increase etchant concentrations or suppress polymer formation. This is utilized when O2 , Cl2 are added to CF4 , CCl4 . The density of fluorine atoms increases as oxygen is added to fluorocarbon discharges. Fluorine atoms have a tendency to combine with carbon radicals in the carbon-tetrafluoride plasma and become unavailable for

1086 Dry Etching

surface etching reactions. The presence of oxygen in the plasma has the effect of providing a sink for the carbon radicals by promoting their recombination with oxygen atoms produced in the plasma. The mechanism by which this happens is still not very clear but may have to do with forming CO or CO2 by some mechanism thereby releasing four F atoms from CF4 . A number of gases function as scavengers for radicals. Molecular hydrogen is known to react with fluorine to form HF so that the addition of hydrogen in a CF4 plasma decreases the concentration of F dramatically and thereby the etch rate of silicon also decreases. However, the etch rate of SiO2 is not affected that much so that this gives an opportunity to make etching thin layer of SiO2 without etching silicon underneath possible. This is because the oxygen released from silica goes to attack the carbon atoms again releasing more fluorine. Hydrogen added to fluorocarbon feeds promotes CFx film growth. Inhibitors and formers are typically unsaturated radicals, like CF2 and CCl2 that form sidewall films and assist in inducing anisotropy. These species are easily desorbed by ion bombardment so that the films only form on sides not subject to ion bombardment. When thin oxide films form on some materials, one requires small amount of native oxide etchants, like C2 F6 for SiO2 . Some of these additives can also scavenge water and oxygen. Inert gases are also employed to stabilize the plasma or to reduce the etching rate by dilution or improve heat transfer. In some situations, one can add additives, which combine with the primary etchant to form ternary compounds that are more volatile than binary compounds. This is employed in etching MoSi2 in Cl2 when oxygen is added to the gas stream. It is evident that there is considerable gas chemistry to be accomplished in formulating etch gases for materials. The etching rate therefore, depends on the plasma density and the density of reactive radicals. Therefore, input power, operating gas pressure, additions of gas to the reactor and the gas flow rate are to be controlled in any plasma-etching reactor. Increase in plasma power increases the plasma density often as the square root of the power. The rate of production of radicals is controlled by electron temperature, their energy distribution and collision cross section of the dissociation reaction. Example 13.2 Calculate the gas flow rate for a large batch reactor working under a pressure of 0.01 torr and has a volume of 100 L if a residence time of 4 s is desired. t¼

PV , Q

where P is the pressure in the chamber, V is the volume of the chamber that usually includes the connecting pipes, Q is the gas throughput and t is the residence time of the gas. Hence, Q ¼ 0:25 torr L=s ¼ 20 sccm:

Dry Etching 1087

Example 13.3 A single-wafer reactor of volume 1 L processes wafers at a pressure of 0.001 torr for a residence time of 0.4 s. What is the pump rate required? PV 0:01 torr  1:0 L ¼ ¼ 200 sccm: t 0:4 s The pumping speed S required is

The flow rate required is Q ¼

S¼

Q 200 sccm 200 sccm  79 torr  L=s=sccm ¼ ¼ ¼ 1580000 L=s: P 0:01 torr 0:01 torr

Example 13.4 Calculate the flux of CH4 hitting silicon substrate and compare it with number of silicon atoms removed per unit area in unit time, when the etch rate is 1000 0 /min. The density of molecules is given by N=V ¼ P=(kB T). Hence, we have at 300 K, Density, r ¼ 2:4137  1020 P molecules=m3 . The average speed of the molecules is given by rffiffiffiffiffiffiffiffiffiffiffiffiffiffirffiffiffiffiffi rffiffiffiffiffi 8kB N0 T T v¼ ¼ 4:6023 : M M p For CF4 , M ¼ 88:0046=1000 kg and v ¼ 269 m=s. The flux of molecules f ¼ (1=4)rv ¼ 3:16  1021 P=m2 =s. For a lineargrowth rate,G, of 1000 0 /min, for silicon,therate of removal of silicon atoms, R, is given by Gr0 N0 =M, where M ¼ 28:09 g=mol, r0 ¼ 2:33 g=cm3 , so that R ¼ 8:35  1019 molecules=m2 =s. We find even at a pressure of 1 mtorr ¼ 133  10
3 Pa, the flux of molecules is 4:2  1020 molecules=m2 =s. Even if 10% CH4 gives rise to radicals there are sufficient radicals, in the plasma to produce the given etch rate.

13.3. Loading Effects In plasma etching, a fraction of the gas used for etching is consumed in the process. In some situations, the amount of gas consumed can be quite large for example, as with the evolution of SiF4 in the etching of silicon. The etch rate thus, becomes an important process variable in plasma etching. One of the serious consequences of the consumption of the active species is the phenomenon of ‘‘loading effect’’, which relates to the etch rate that decreases with increase in the area of etchable material exposed to the plasma. When the etch

1088 Dry Etching

rate depends upon the amount of etchable surface exposed to the plasma, the system is said to show a loading effect. Most isotropic etchants show this effect. The example of the loading effect is given in Fig. 13.7, where the etch rate of aluminum is plotted as a function of the area being etched in a CCl4 plasma. One also infers from the strong loading effect that the etching species are primarily removed by the material that is being etched and not by the walls of the system or by vacuum pumping of the system. If the etchant is depleted mainly by the surface reaction, then small increases in flow rate will produce large increases in the etch rates. When the loading effect occurs there is particularly a difficult situation that is encountered during the ‘‘endpoint’’ of etching. When the endpoint of etching nears, the area of the material to be etched decreases, the concentration of the active species increases and the etching rate increases. This is exactly the time at which one wishes to have slow etch rate in order to terminate the etching process properly.

5000

400 W, 225
C 10 m torr CCI4

Etch rate (Å/min)

4000

3000

2000

1000

0

0

5

10

15

Loading: number of 5.7 cm diameter AI disks

Figure 13.7: The loading effect (etch rate versus batch size) observed during the etching of aluminum in CCl4 in a reactive ion configuration.

Dry Etching 1089

One may try various approaches to diminish the amount of reactive species when the endpoint of etching is reached. The use of increase in the pumping speed of the vacuum system does not appear to be a practical solution, as very large pumps are required to compete with pumping of active species by a large area of the material. One way to increase area of wafer to be etched is to artificially increase the area by introducing large number of dummy wafers in the system. One may then try to introduce an etch gas or gas mixture in which the active species are consumed by processes other than etching. If the etching is not stopped in time severe undercutting may occur. Let the etchant be labeled F, and let us suppose there are m wafers to be etched each with an area Aw in the reactor. The mass balance of F may be written as rF V ¼ Ak*sF nF þ kvF nF V þ mAw kwF nF ,

(13:8)

where rF is the rate at which F is generated per unit volume and V is the volume, A is the area of the empty reactor, nF is the concentration of F in the gas, ksF * is the net rate constant for loss of F on reactor walls, kvF is the loss rate per unit volume of the gas and kwF is the loss from etching the material. The etch rate Rm is given by Rm ¼ akwF nF ,

(13:9)

where a converts the units from linear etch rate to material consumption in molecules=cm3 . Solving for nF and substituting in Eq. (13.9), we obtain R0 Aw kwF ¼ 1 þ mFF ¼ 1 þ m , Rm AKsF

(13:10)

where R0 is the etch rate in an empty reactor (m ¼ 0) and Rm is the etch rate when m wafers are present, FF is the ratio of the F consumed by etching a wafer (rate kwF ) to that lost by recombination (AksF ). The loading effect is shown in Fig. 13.8 as R0 =R as a function of m. When there is etchant loss with little or no loading effect, the volume and heterogeneous radical recombination reactions dominate rather than etching. One way to avoid loading effect is to have kloss  ketch or use of a plasma in which the principal etchant loss process is insensitive to the etching reaction. One can also avoid loading effect if the ion bombardment fluxes rather than the etchant supply controls the etching reaction rate. The theory of loading effect has been formulated for n simultaneous etchants (Flamm et al., 1982). In microelectronic fabrication, one encounters a situation when densely populated lines are present along with isolated lines in the same wafer. One observes micro-loading effects (Fig. 13.9), in which the difference in local pattern density will make one area of wafer etch at a different rate than others. In the design of device layouts, these effects have to be taken into consideration.

1090 Dry Etching

6 5.5 5 4.5

R/R0

4 3.5 3 2.5 2 1.5 1

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Number of wafers

Figure 13.8: R=R0 versus m demonstrating loading effect.

Figure 13.9: Micro-loading effect.

13.4. Ion-Assisted Gas–Surface Chemistry The success of plasma etching is due to the role played by positive ions in altering the manner in which the active gas reacts with a solid surface. The glow discharge provides a source of energetic particle bombardment. The crucial step in the dry etching process has to be the creation of reactants in the plasma and

Dry Etching 1091

transport them to the surface to be etched. The sheath field, which can be quite strong, provides the positive ions into the boundary surface at normal incidence thereby achieving vertical etching conditions. Coburn and Winters (1979) have clearly demonstrated the role of ions in the gas–surface chemistry. They utilized XeF2 gas and 450 eV Arþ ions in their experiment. They measured the etch rate directly by using a quartz microbalance. The XeF2 gas etches silicon at a very low rate. This is because the fluorine atoms in XeF2 are held apart from each other by the xenon atom in the center, so that the fluorine atom in the molecule behaves very much like free fluorine atoms as opposed to the more stable fluorine molecules. A small ( 5 0 =min) purely chemical etch reaction is observed. When the Arþ ions beam is turned on, the etch rate increases dramatically (about 10 times more) as shown in Fig. 13.10. It is clear that the reaction rate increases tremendously when both the etch gas and the argon ion bombardment are acting together. When the XeF2 gas is turned off, the etching rate drops precipitously so that under purely physical bombardment of Arþ ions, the etch rate is once again very small. We thus, see that ion bombardment for the etched surfaces will increase the reaction rate of spontaneously occurring processes. The key result is that the silicon erosion rate obtained for a silicon surface simultaneously exposed to XeF2 and to Ar ion beam is much greater than the sum rates for exposure to the ion beam and etchant separately. Similar effects have been found in many other systems. XeF2 Gas only

Ar+ ion beam only

Ar+ ion beam + XeF2 gas

70

Silicon etch rate (Å /min)

60 50 40 30 20 10 0

0

100

200

300

400

500

600

700

800

900

Time (s)

Figure 13.10: Ion-assisted gas–surface chemistry using Arþ ions and XeF2 on silicon (volatile reaction product).

1092 Dry Etching

Even when the gas does not etch the material to any significant extent, the presence of ion bombardment enhances the etch rate. An example of such a situation is the role of chlorine molecules with Arþ ions on silicon (Fig. 13.11). The flux of chlorine molecules alone does not etch undoped silicon but in the presence of ion bombardment silicon chlorides form and are easily evaporated from the system. Chlorine base etching of silicon exposed to ions have been described as ion initiated etching in contrast to fluorine based etching, which is said to be ion-assisted etching. One should note that highly doped silicon etches spontaneously in a chlorine discharge. In chlorine etching of silicon, atomic chlorine does not appreciably etch p-type and undoped silicon at room temperature, but etches n-type polysilicon spontaneously with one or two orders of magnitude increase in the etching rate. The large electron density in the valence band causes the Fermi level to bend upwards. This is termed field enhanced etching and is related to band bending at the surface, which facilitates charge transfer from silicon lattice to the electronegative and chemisorbed silicon atoms, making the Si–Cl bond more ionic. This allows more flexibility in the bonding geometry and creates more chemisorptions sites. The incorporation of chlorine atoms is enhanced and the etching rate increases. Differences between other reactive halogens and silicon etch rate are attributed to steric hindrance effects.

Silicon etch rate (Å/min)

+ Ar ion beam only

+ Ar ion beam + Cl2 gas

10

5

0 100

200

300

400

500

600

Time (s)

Figure 13.11: Ion-assisted gas–surface chemistry using Arþ and Cl2 on silicon.

Dry Etching 1093

Example 13.5 Write a sequence of plausible reactions for etching silicon in chlorine plasma in the presence of ion flux. We postulate the dissociation of Cl2 in the plasma as: Cl2 þ e ¼ 2Cl þ e

(dissociation reaction):

This reaction may be followed by ionization reaction as: Cl þ e ¼ Clþ þ 2e (ionization reaction): Then the chlorine has to be adsorbed on the silicon surface with help of ion flux Iþ as: Si(surface) þ Cl(g) þ Iþ ¼ Si-Cl(ads) (surface adsorption): There is very little evidence for SiCl4 desorption from etched silicon, so that one has to abandon the idea of successive chlorination as unlikely. Evidence exists for singly and doubly chlorinated silicon, with SiCl being the dominant species suggesting that ion stimulated associative adsorption may be occurring as: SiCl(ads) þ SiCl(ads) þ Iþ ¼ SiCl2 (g) þ Si(surface): Chlorination or association to form the final product SiCl4 may follow the above reaction. There is hardly any etching of silicon by chlorine at 300 K. This means that ion irradiation is essential for chlorine etching. This also implies that etching can be highly anisotropic. If the reaction product is involatile, the etch rate of aluminum with Arþ ions alone due to sputtering is reduced when the XeF2 gas is present. The product AlF3 has very low vapor pressure. The etch rate is reduced when ion beam and the fluorine gas are both present (Fig. 13.12). Gases that do not adsorb on silicon, do not participate in etching even in the presence of ion bombardment as when Arþ ions bombard silicon exposed to CF4 gas (Fig. 13.13). Reactive ions yield higher etching rates than that of inert ions due to the formation of volatile species with the reactive etchant. For example, the sputtering yield of poly silicon by Clþ ions is a linear function of the square root of the ion energy and is much higher than that by Arþ ions. Even the threshold energy for sputtering is reduced due to the formation of a heavily chlorinated layer that reduces the surface binding energy. The mechanisms that lead to the observed ion enhanced etching are imperfectly understood. One mechanism attributes the effect of ions to the chemical modification of the surface layer, which has a larger sputtering yield than the

1094 Dry Etching

Ar+ ion beam only

Ar+ ion beam + F2 gas

Silicon etch rate (Å /min)

15

10

5

0 100

200

300

400

500

600

700

Time (s)

Figure 13.12: Ion-assisted gas chemistry using Arþ ions and F2 on aluminum.

+ Ar + Cf4 gas

Ar+ only

Silicon etch rate (Å /min)

15

10

5

0 200

400

600

Time (s)

Figure 13.13: An example of nonreactive gas CF4 in that the Arþ sputter etch rate of silicon is unaffected by a flux of CF4 molecules.

Dry Etching 1095

unmodified surface (Mauer et al., 1978). A second mechanism postulates that the lattice damage induced by ion bombardment enhances the reaction rate of etchant species as compared to the reaction rate with the undamaged material (Flamm, 1981). A third mechanism assumes that the energy supplied to the reaction layer by ion bombardment is utilized to increase the mobility of the molecules, which forms volatile product, and desorbs from the surface (Tu et al., 1981). Other mechanisms that are proposed involve ion enhanced sputtering and ion-assisted/thermal activation. The individual mechanism that is predominant appears to be dependent on the material/active gas combination. In other words, depending upon the nature of the active species and the material it has to etch, ion bombardment can enhance the etch rate by influencing one of three essential steps in etching-adsorption, product formation, desorption of volatile product. It is of course possible for several different mechanisms to be operating concurrently. In addition to ions, electrons also are known to influence gas–surface chemistry. One of the carefully studied examples is the etching of SiO2 by XeF2 . Bombardment of SiO2 with electrons will etch the oxide when XeF2 is present, whereas without the electron bombardment XeF2 shows hardly any reaction with SiO2 (Fig. 13.14). The effect is not due to the heating effect of the electron beam as increasing the current density does not affect etch rate. The field of radiation-assisted gas–surface chemistry is vast. We have observed that the rate at which active gas reacts with a solid surface can be considerably enhanced by simultaneous irradiation with energetic particles, primarily positive ions. Radiation-assisted gas–surface chemistry also includes effects of photons, electrons on the gas solid surface reactions. As far as dry etching is concerned, the role of ion-assisted gas–surface chemistry is twofold. First, it increases the reaction rate of spontaneously occurring processes when ion bombardment occurs simultaneously. Second, it can promote reactions that do not occur without energetic radiation. We note that if the etching is limited by surface reaction kinetics, the rate of etching is a function of temperature. Furthermore etching by surface limited reaction tends to be isotropic since the reactant gas has no strong preferential directionality. Etching limited by electron impact reaction in the plasma or ion bombardment induced surface kinetics is relatively insensitive to temperature.

13.5. Anisotropy in Etching The most important reason for utilizing plasma etching is directionality in etching that it provides. Accurate pattern transfer that maintains the features of the dimensions of the mask implies that the etching has to be anisotropic.

1096 Dry Etching

9000

8000 SiO2 P = 6 × 10−4 torr. 45 µA, 50 mA /cm2, 1500 eV Rate ~ 200 Å /min

7000

Amount of SiO2 removed (Å)

6000

5000

4000

3000

2000 XeF2 in system 1000

Electron beam on

0

0

500

1000

1500

2000

2500

3000

Time (s)

Figure 13.14: Electron enhanced etching of silicon dioxide by XeF2 .

The etching should occur preferentially in a direction normal to the substrate than in the lateral direction. The directionality in etching is attributed to the role played by the bombarding ions. When the mean free path of ions in plasma is long compared to the feature depth, the plasma sheath field makes ions strike the horizontal surface almost exclusively at normal incidence. This is the feature that enables the chemical reaction to accelerate in the vertical direction because vertical feature walls largely escape the effects of perpendicular-going ion bombardment (Fig. 13.15). This arises from the fact that sputtering is strongly sensitive to the angle of incidence of ion and is zero for grazing angle of

Dry Etching 1097

Positive ions and electrons

Mask Material to be etched

Figure 13.15: Illustrative figure to emphasize the fact that sidewalls on etched features are not subject to extensive energetic particle bombardment.

incidence. Photons, electrons can also influence the etching rate and may have a role to play in determining the etching profile, but the principal concern is the role of ions. In the majority of situations, the ions fall normal to the feature to be etched. We assume therefore, the dimensions of the surface topographical structure is not of the same order as the thickness of the sheath between the bulk plasma and the etched surface. One has to remember that the requirements of etching are not simply a matter of obtaining a vertical profile because a sloped profile may be necessary to guarantee adequate step coverage during a subsequent deposition. In addition, the etched surface should have good morphology and be free of physical or electrical damage. It is important to note that bombarding with ions alone can cause the removal of atoms by sputtering. The principal species ejected from the material that is being bombarded by ions is the neutral atoms of the element of which the material is composed. In the majority of cases, these elements have very low vapor pressure and they condense any where the sputtered atom hits the surface. The re-deposition due to etching by ion beam bombardment alone limits the high aspect ratio pattern transfer capability (Ho and Poulin, 1987). The products of dry etching have high-vapor pressures and will not usually condense or react with surface but will be simply be reflected back into the gas phase. The importance of the volatility of the etch product in dry etching is therefore, the crucial factor in its acceptance in semiconductor fabrication. A second serious issue with just using ion beams is that the sputtered atoms may undergo many collisions in the relatively short mean free path gas phase and eventually land up and return to the surface where it came from or other places. This is often called ‘‘cross talk’’ and can be unpredictable as to its location and its influence on the device being fabricated (Melngailis, 1987). Besides, whenever the species that does not form volatile products arrives at a surface, microroughness of the surface can be expected. The species that do not evaporate

1098 Dry Etching

may collect and form small clusters resulting in miniature masks on the surface inhibiting the etching of the underlying material further adding to the problem of defining the etch geometry. Since the etch rate of ion-assisted etching gas is much faster than the etch rate of surface that are not subject to ion bombardment, we can at once see the formation of anisotropy etching behavior, since the sidewalls of the region to be etched receive no or little ion bombardment. The manner in which ions hit the etch surface now causes directional etching. The relationship between the shape of the etched profile and the dependence on etch rate on ion bombardment is shown in Fig. 13.16, for a hypothetical Si and SiO2 system. With a bias of
150 V on the sample, the etch rate of the bottom surface of the etched feature (Vz ), whereas the lateral etch rate can be approximated by the zero bias etch rate (Vx ). Therefore, we see the etched result in SiO2 has a vertical sidewall whereas for Si (Vx 6¼ 0) the sidewalls are sloping even though anisotropic. Adjusting the composition of the gases introduced into making the plasma can control the profile of etching. A good illustration of this feature is the

Etch rate

Si

SiO2 vx Vz

−100

0

–200

Bias voltage on wafer (V)

x

Mask

z

Si

SiO2

Figure 13.16: The shape of the etched profile and dependence on the etch rate on the wafer potential.

Dry Etching 1099

control of etching rate of silicon by adding H2 to CF4 . When there is no bias, implying that the silicon is not subjected to ion bombardment there is a dependence on etch rate versus gas composition. As shown in Fig. 13.17, the etch rate of unbiased silicon drops to zero when 10% hydrogen is added to CF4 . However, if bias is applied, say
150 V, the amount of hydrogen required to stop the etching of silicon is 30%. In the region between 10 and 30% hydrogen, etching is taking place in the biased surface as fluorocarbon polymer is forming on the unbiased surface. The anisotropy in plasma etching is brought about by directed ion flux in the plasma environment and is the single most important aspect of assuring the fidelity in the faithful replication of the mask pattern in the film, where devices are fabricated. Typically, the final etched feature must be within 10% of the dimensions of the mask. The lack of anisotropy results in undercutting beneath the resist mask. Adjusting the composition of the etch gas to produce a film on the sidewalls is one method of controlling the undercutting. For example,

Silicon etch rate

vz

Bias = −150 vx A 0

10

20 No bias

30 Percerntage of H2 in CF4

x = vx z vz

x

Mask z

Si

Pure CF4 etch gas

Mask

Si

10% of H2 in Cf4 etch gas

Figure 13.17: Illustrative plot to demonstrate the way in which the shape of the etched wall profile can be influenced by decreasing the fluorine to carbon ratio.

1100 Dry Etching

addition of 12% CCl3 F to SF6 in etching polysilicon prevents side attack by fluorine atoms even when substantial over-etching occurs. Example 13.6 The rate of reaction for silicon and fluorine atoms in the absence of positive ion bombardment, is known to follow the etch rate law of the form
12

R(F, Si) ¼ 2:9  10

nF T

1=2

 
0:108 , exp kB T

and the etch rate of SiO2 is given by
13

R(F, SiO2 ) ¼ 6:14  10

nF T

1=2

 
0:163 , exp kB T

where R is the rate in 0 =min, nF is the number of fluorine atoms in cm3 and kB in eV/K. In order to etch silicon selectively with respect to silicon dioxide, what ratio of CF4 to O2 would you recommend? The ratio of etching rate of silicon to silica at 300 K is given by R(F, Si) ¼ 39:7: R(F, SiO2 ) To achieve selectivity of oxide over silicon, one has to adjust the chemistry of gases in the plasma. Creation of fluorine deficient environment can be beneficial. Scavengers for fluorine are H2 , C2 H4 and CH4 .

13.6. Passivation of Sidewalls Anisotropic etching is one of the hallmarks of dry etching. In order to achieve this there are two basic ways. One can design the etch chemistry so that the etch rate of the film is actively enhanced in the unmasked parts that are subject to ion bombardment. On the other hand, if the plasma is such that it can passivate the sidewalls of the features that are being etched, which are not subject to ion bombardment, then ion bombardment can keep the bottom of the feature etchable and clean. Sidewall passivation makes it possible to achieve directionality in etching for etchant/substrate systems, which normally exhibit isotropic etching characteristics. The dependence of etch rate on dopant concentration of silicon has been well established experimentally (Mogab and Levinstein, 1980). Highly doped n-type

Dry Etching 1101

silicon etches easily in chlorine plasma, even in the absence of ion bombardment. The etch rate is lower with intrinsic silicon. The etch rate of p-type silicon is a factor of six lower than n-type silicon. The dopant concentration has to be higher than 1019 to observe any doping effect on the etching rate of silicon. The experimental observations suggest that the etch rate has a good correlation with the density of conduction band electrons. When etching has to be done through several layers of silicon with different doping concentrations, the control of etch profile with ion bombardment exclusively is difficult or impossible. Here, sidewall passivation or sidewall blocking is employed to have control on etching profiles. The sidewall passivation exploits the formation of a feature of the film on the sidewalls, which slows down or completely stops lateral etching. The glow discharge chemistry is chosen so that etching inhibiting films can form as long as they are not exposed to ion bombardment. Consider the etching process of silicon in chlorine plasma to which oxygen has been added. The sidewalls are not exposed to ion bombardment but a film can grow on the sidewalls. Etching can continue at the bottom of the feature as it is subjected to ion bombardment. The passivation film that forms is the result of oxygen in the plasma. A nearly stoichiometric SiO2 forms on the sidewalls of the silicon that is being etched. The radicals in the plasma, such as SiCl4 , SiCl3 , SiCl2 , SiCl and Si reach the sidewalls and they are adsorbed there with large sticking coefficients. When these radicals interact with oxygen, they form SiO2 , and the products of reaction are Cl2 , H2 O and Cl2 O, which are volatile gases, and are easily desorbed from the sidewalls. The presence of SiO2 in the sidewalls and the absence of ion bombardment on the sidewalls assure that the sidewalls are protected from etching. One possibility of preventing lateral etching is when one takes advantage of forming a protective polymerized film on the sidewalls. Consider the halocarbon discharge in terms of the relative stoichiometry of the discharge. It is useful to represent the stoichiometry of the glow discharge in terms of the halogen to carbon ratio (e.g. F/C ratio). In assessing this ratio, we can consider those species that are active and enter into etching and polymerization reactions. In other words, gases that do not significantly interact with the surface, such as CF4 , CO, CO2 , COF2 , SiF4 , HF, etc., are ignored. Species, þ þ such as F, CF3 , CF2 , CF, C, CFþ 3 , CF2 , CF , etc., are taken into consideration. When Si consumes F forming SiF4 without consuming carbon, F/C ratio decreases with etching of silicon. If hydrogen is introduced into the discharge, it consumes fluorine, forming HF so that F/C ratio is once again reduced. Consequently, the F/C ratio can be reduced by either using a larger saturated fluorocarbon, such as C2 F6 , or C3 F8 or by adding these gases to CF4 . Thus, gases containing less fluorine per carbon can also lower the F/C ratio. One can also

1102 Dry Etching

influence C/F ratio by the addition of hydrogen to the plasma. In a similar fashion, the addition of oxygen increases the F/C ratio, when C is consumed by the formation of CO and CO2 . Clearly, the nature of the gases added to the plasma are able to control F/C ratio, as can be gauged by the addition of gases, like CO2 , F2 , NO2 , etc. It appears that the molecular nature of the gases in the plasma is less important than the atomic composition of the discharge gases. Ultimately, when one tries to go to higher etch rates, there is a boundary between the regions where polymerization occurs as opposed to the region where etching occurs as shown in Fig. 13.18 (Coburn and Kay, 1979). Ion bombardment appears to have a major effect on this boundary between etching and polymerization pushing the balance toward etching. One can achieve a situation, where the etching is occurring on the cathode and polymerization on the sidewall. The substrate bias can also affect the boundary between the polymerization and etching regions. The ion-etching step has to be followed by a surface-cleaning step. In this step the goal is the removal of etch passivation layers, such as the fluorocarbon film in silicon dioxide etching, chlorine containing residues for aluminum etching to

Loading H2 addition O2 addition C2F4

C4F10

C2F6

CF6

Bias applied to surface (V)

−200

Etching

−100

Polymerization 0 1

2

3

4

Fluorine-to-carbon ratios (F/C) of gas phase etching species

Figure 13.18: Illustrative plot of the boundary between polymerizing and etching conditions as influenced by the fluorine to carbon ratio of the chemically reactive species and the bias applied to a surface in the discharge.

Dry Etching 1103

avoid long term corrosion effects, sidewall passivation film in silicon trench etching, the annealing of lattice damage, etc. Example 13.7 One concept that has proved useful in the analysis of the behavior of fluorocarbon discharges is the F/C ratio. Explain the effect of this ratio as influenced by (a) oxygen, (b) hydrogen in the etching of silicon. In determining the F/C ratio, one ignores those gases, which do not react significantly with surfaces, such as CF4 , CO, CO2 , COF2 , SiF4 , HF, etc. The F/C ratio is determined only by the active species involved in the plasma, such as F, CF3 , CF2 , CF and other ions of these radicals. F/C ratio generated by the dissociation of CF4 is 4. Etching of Si forms SiF4 , which consumes F but not C. Hence, F/C ratio decreases when silicon is etched. Adding hydrogen combines with fluorine to form HF, which is relatively nonreactive. Hence, it reduces F without affecting C. Hence, adding hydrogen reduces F/C ratio and reduces the number of active species in the plasma. Adding C2 F6 or C3 F8 gases to CF4 introduces gases that have a F/C ratio less than 4, so that the overall F/C ratio also decreases. Introducing oxygen into the system consumes carbon in the formation of CO and CO2 so that the F/C ratio increases. Etching of Si consumes F decreasing F/C ratio, whereas the etching of SiO2 consumes both F and C thereby keeping the F/C ratio approximately constant so that minimal changes occur in etching. Carbon with a moderate amount of fluorine forms a film, whereas the abundance of fluorine with little carbon etches silicon. The ratio F/C has also been used to demark the regions that are predominantly etching from those regions where polymeric films form when bias is applied to the substrate. It is found that when there is F between two and three times as much as C, the ions can clean the trench bottom but not the sidewalls and very anisotropic etching can occur.

13.7. Selectivity One of the requirements of dry etching is the selectivity that the etching process reveals. Selectivity means that the etch rate of the material to be removed should be much larger than the etch rate of all the other materials exposed to the plasma, such as the material of the mask and the immediate material underlying the film that is being etched. Selectivity is imposed on the process

1104 Dry Etching

because one should avoid eroding the mask, which will ultimately lead to dimensional inaccuracies, and furthermore the underlying material cannot be attacked when the endpoint of the etch process is reached. Ideally speaking, selectivity that is desired is to etch one material without etching another. It is necessary that the etching stops as soon as the material to be etched is completely etched. The degree to which differential etching of one material relative to another is possible in plasma is one of the several factors that govern the fidelity of pattern transfer. Selectivity may be understood as applying to materials in the same patterned layer, or to the material in the layer immediately below or to the relative etching between the mask and the substrate below. This is one of the difficulties with dry etching when etching can be indifferent to the material it is etching. Therefore, many techniques have evolved to control the etching selectivity by exploiting the different etch rates on different materials. There are two basic ways in which selectivity can be achieved. One can design the etch chemistry in such a way that the ion bombarded region etches faster than the sidewalls, where there is no ion bombardment. It may be by suitable choice of reactants and conditions in the reactor, one can adjust the parameter space of etching to achieve passivation of the sidewalls, whereas the bottom of the features to be etched is kept clean because of ion bombardment. Etch selectivity implies knowledge of the different materials and their etching characteristics. Etching of silicon in chlorine-based plasma may be taken as an example to illustrate the selectivity. In these plasmas, there is absence of thermal etching at room temperature and there is considerable selectivity with respect to the etching of SiO2 . The former effect is thought to be due to the fact that Cl atoms can only chemisorb on the surface and cannot penetrate the silicon lattice without the assistance of ion bombardment. Therefore, higher anisotropies are easily achieved with Cl based plasma than with F based plasmas. If the chlorine-based plasma is combined with some form of sidewall passivation, one can achieve most stringent anisotropy requirements (Westerheim et al., 1942). If controlled sidewall angles are desired, one can utilize combinations of mixtures of gases containing Cl and fluorine. Example 13.8 How to achieve selectivity between Si and SiO2 etching in CF4 plasma? Hydrogen gas added to CF4 discharge causes the etching rate of silicon to decrease whilst the etching rate of SiO2 remains largely unaffected. There is evidently a composition of CF4 and hydrogen gas for which one can achieve selectivity between etching of SiO2 and Si.

Dry Etching 1105

This is useful when we may wish to etch through SiO2 but stop etching when we reach the Si material. The hydrogen combines with F to form HF so that F is no longer available to etch Si.

13.8. Ion Beam Based Etching Dry etching tasks can be carried out utilizing ion beams. In ion beam techniques, the discharge is physically separated from the substrate. The most important difference between dry etching using glow discharge plasma with ion beam based methods of etching is in the operating pressure used. The ion based etching operations typically operate at pressures of 10
4 torr or lower, whereas the glow discharge etching operations are typically conducted under pressures, which are 10–1000 times higher. The low pressures allow the ion beam to traverse from the ion source to the material to be etched without any collisions on their way so that backscattering problems are avoided. Particle flux at the substrate consists mainly of ions expected from the source and to a lesser extent neutrals effusing from the source. In addition to having control on ion energy, one can also adjust the orientation of the substrate with respect to the ion beam. This permits generation of tapered profiles (Hawkins, 1979; Lee, 1979). The sputter yield difference between materials is within a factor of three of each other, so that selectivity with ion beam methods is poor. Since the gas flow rates are very low at low pressures, if etching process evolves significant amount of gases, then large vacuum pumps are needed to maintain the appropriate concentration of the etchant in the system so that the etch product does not become the major constituent in the etch system. High etch rate and large areas to be etched are therefore more difficult to be handled with purely ion beam based methods. The major advantages of ion beam milling are high resolution, good uniformity, high anisotropy, residue free etching, good control over these parameters and relative freedom from radiation damage effects resulting from the physical isolation of the plasma from the source. The disadvantages are lack of sensitivity, possible trenching and re-deposition, faceting of the photoresist, damage from energetic bombardment and low rates of etching. Sputtering is the only process, which can remove involatile products from the surface. Ion beam based etching employs reactive gases in one of two ways. One of the simplest ways is to admit the reactive gas through the ion source to give at the substrate a flux of ions and a small amount of neutral radicals. This setup is sometimes called reactive ion beam etching (RIE). Reactive ion beam etching is plasma based etching technique characterized by a combination of physical

1106 Dry Etching

sputtering with chemical activity of reactive species. The ratio of the chemical to the physical components of the etch process are gas flow source-to-substrate distance, gas mixtures of the ion source and the residual vacuum environment. A second way is to introduce the neutral reactants into the etch chamber directly. The ions then are typically produced by discharge of inert gas. This process has been termed ion beam assisted chemical etching and is potentially more flexible because ion bombardment of the surface and exposure to the neutral beam can be controlled independently. For example, by exploiting the control over the physical component (ion beam current density and energy and chemical component flow rate into the etching chamber), one can obtain controlled sidewall profiles. The development of large area stable sources for producing ion beams has been the major concern in utilizing ion beam methods in manufacturing. Furthermore, most ion sources operate satisfactorily only above several hundred electron volts, where damage from ions becomes intolerable. The low pressure, line of sight nature of ion beam techniques provides a flexibility of directional bombardment that is unavailable in other techniques used in etching. The ability to have independent control of ion energy and flux provides us with valuable data to understand ion-surface reactions. The etch rate R in m/s from a known value of sputtering yield, S, for a given ion flux, J (A=m2 ) is given by R ¼ 1:04  10
8

SJW , r

(13:11)

where r is the density (kg=m3 ) and W is the atomic weight (kg / mol). The etch rate of each system requires calibration since published values are not reliable due to the role of small amounts of residual gas, and the charge exchange reaction that alters the flux of ion beams. Features that have topographic structure must take into account the angular dependence of etch rates. Since sputtering yield is maximum at some angle to the direction of incidence (typically 60–708). If the sputtering yield rises faster than 1/cos u, the resulting etch rate shows a maximum as a function of angle, whereas for some other cases, the maximum yield occurs at normal incidence only. The topographical features are amplified in etching if the sputtering is dependent on angle. Inert ion beam etching is capable of pattering with aspect ratios that are high, but for low aspect ratios the mask material limits the resolution. For aspect ratios exceeding unity it is necessary for the wall shapes to be more sloped, and reactive gas ions are necessary to generate volatile etch products. The mask shape requires control to produce faithful reproduction of pattern, and tilting and rotating of sample have also been used to improve ion beam etching configurations (Lee, 1979).

Dry Etching 1107

Chemically reactive gases added to the etch system can produce higher etch rates and provide etch rate selectivity. Typically, one wants to advance the etch rate of the material to be etched and retard the rate of masking material. Addition of oxygen has been shown to change the etch rates of many reactive materials. Example 13.9 Show by a simple model the combined effect of ion beam flux and neutral beam flux as enhancing etch rate. Consider the etching of silicon in a plasma environment of chlorine. The surface density of the reactant, Cl, is governed by the balance between adsorption and desorption, both of which can be influenced by ion bombardment. When we have steady state conditions, @u ¼ 0 ¼ Jn S0 (1
u)
Jþ hu, @t where Jn is the neutral flux and S0 is the reactant sticking coefficient when the surface coverage is zero, Jþ is the ion flux and h is the probability that a reactant will be desorbed per incident ion. The surface coverage, u, can therefore be written u¼

1 , 1 þ rj

where rj , the effective neutral to ion flux ratio is given by rj ¼

Jþ h : Jn S0

If we ignore the contribution to etch rate from sputtering as well as spontaneous etching in the absence of ion bombardment, we obtain the etch rate, R, as R ¼ Jþ Yu ¼

Jþ Y , 1 þ rj

where Y is the probability that a substrate atom will be removed per incident ion. We thus, see when rj is large, the etching is limited by the neutral flux so that the etch rate is not dependent on the ion flux. If rj is small, the etch rate is basically limited by ion flux. The etch rate is large when both ion flux and neutral flux are present as observed in many cases.

1108 Dry Etching

13.9. Etching Reactor Configurations As the dry etching has proven its popularity, more and more demands are made on the reactor configurations that are suitable for a given dry etching process. We can consider some of the concerns that have been addressed in different etch system configurations. It is clear that the presence of ions is needed for dry etching. Even if one does not wish to have directionality in etching, one can simply increase the rate of etching when the ion bombardment is present. This is the situation when CF4 is used to etch SiO2 . The trend in plasma etching configurations is to etch one wafer at a time rather than process simultaneously many wafers. Single-wafer processing offers numerous advantages. One of the critical problems in multiwafer processing is lack of uniformity in etching, which is avoided to a large extent in single-wafer processing. The endpoint detection in etching is lot easier to deal with when only a single wafer is processed. There are very few problems in scaling up the process and etch rate that can currently be achieved are rapid enough to not severely handicap the throughput. Automation, yield and capital costs are also favorable in single-wafer processing. However, in order to achieve the high rates of etching one should be careful to make certain that problems associated with higher temperatures, less selectivity, ion bombardment and sputtering damage are under acceptable limits. Many of the reactors provide the opportunity to control input power, reactor pressure, excitation frequency, temperature, flow rate and feed composition. The reactor geometry and the materials of construction are also very important in reactor performance, particularly in view of the corrosive nature of the many gases used in etching. The specific tasks for which a reactor is used limit the type of materials that one can use in the construction of the process chamber, which is generally restricted to specially prepared aluminum alloys, 316 stainless steel or quartz. Similarly, none of the materials used for seals, such as Viton-O-rings, perfluoroelastomers act as universal solutions in all cases. When electrodes are present inside the chamber, they require coating by some material that can evaporate in the plasma without disturbing the etching process itself. By making sure that all powered electrodes are covered with a dark space shield one can void the sputtering from unprotected surfaces. Venting to atmosphere, or leaks from cooling systems can introduce water vapor, which can have harmful effects in etching, so that the standard procedure is to employ load lock chamber to assure reproducibility in results. Due to the unique situation of each reactor system with the etching gases involved, it is essential in order to assure safety of operation and prevention of particulates, and to insure continuous monitoring of pressure and flow properties of gas in the system, close adherence to a maintenance schedule provided by the manufacturer and constant calibration checks are a requirement. In the absence of

Dry Etching 1109

careful considerations discussed so far, the reactor is likely to exhibit ‘‘memory effects’’ that are difficult to control and permanently damage the reproducibility of results. Etching characteristics of materials are also a function of the film properties. For example, the etching characteristics of refractory metals and silicide films are dependent on stoichiometry, crystallinity, level of contaminant and concentration of dopants. Some films that are annealed also influence etching characteristics. Demands made on etching can be varied depending on the materials and its applications. For example, in III–V compounds there are at least three different needs in etching: to produce macroscopic features with high selectivity, anisotropy and fast etch rates; surfaces of optical quality with controlled shapes; and channels or mesa with optimized electronic surface properties. We observe that the relative volatility and reactivity for the group-III and group-V elements can lead to non-stoichiometric surface features, which can in turn have disastrous consequences for the device that one is fabricating. One of the most widely used reactors generates plasma between two parallel plate electrodes (Fig. 13.19). The substrate is placed on the electrode, which is driven by a capacitively coupled RF voltage. The negative bias that develops at the cathode allows the ions to bombard the surface of the substrate in a direction normal to the surface provided the pressures are low so that collisions of ions on their way to the cathode are avoided. The ion flux to the wafer that is to be etched depends upon the character of the sheath between the plasma and the surface. Therefore, care has to be taken to ensure that the plasma is spatially RF-driven electrode (13.56 MHz) Gases in

Plasma

High V power supply

Pump

Figure 13.19: Plasma etching in a parallel plate reactor.

1110 Dry Etching

homogeneous and avoid macroscopic topography or fixtures near the waferbearing electrode. Uniform currents can then be delivered to large areas thereby achieving large area etching uniformity. In glow discharge methods, the feed gas molecules will be dissociated into radicals and are extremely chemical reactive and therefore, dominate the etching reactions. There is also a difficulty in specifying the ion flux and energy of ions precisely in glow discharge methods so that the parameter space to control the etching is much more difficult. Even though the relationship between electron density, temperature, ion flux and ion energy can be related to RF voltage, frequency, gas pressure and electrode spacing, the internal plasma parameters cannot be independently varied. For example, at a fixed gas pressure and RF frequency, increasing RF voltage can increase plasma density but with increased ion energies, which can cause damage to exposed surface. This coupling between the plasma density and ion energy is a serious limitation of these reactors. Sputtering of electrode material can cause severe problems due to backscattering material onto the substrate. Wafers can be placed on a grounded surface or a powered electrode. The latter situation is referred to as reactive ion etching. The reactive ion etching arrangement generally causes the wafers to be subjected to ion bombardment at higher energies than in the plasma etching mode because of the larger negative potentials established on the powered electrode compared to the grounded electrode. The lower operating pressure also increases the energy of the ions reaching the substrate that is etched. At frequencies of the order of 30–300 MHz, higher plasma densities can be generated with low applied voltage, which implies high process rates with low damage. Sheath thickness decreases with increased frequency reducing the number of collisions an ion experiences in the sheaths and thereby improves the anisotropy at fixed pressure. Besides radicals, there are electrons that overcome the sheat potential, ultraviolet light emanating from excited and metastable radicals that hit the etched surface. The compromises that are necessary for all these reactions to occur properly at the substrate are difficult to attain in processing. The motivation to search alternating etch methods arises from the dilemma one faces when etching narrow and deep structures, such as deep trenches. It is found that there is considerable drop in etch rate inside the trenches and this is particularly severe when the pressures are high. However, reducing the pressure to lower values to minimize the effects reduces the etch rate to unacceptable levels. So, it is necessary to increase the concentration of reactive species in the plasma at very low pressures. Higher etch rates imply higher plasma densities. This can be achieved by the introduction of magnetic fields to confine electrons in plasma (Fig. 13.20). The magnetic field is parallel to the two electrodes and can be established either by a permanent magnet or by an externally azimuthally rotating electromagnetic field. The magnetic field serves

Dry Etching 1111

−5

⫻10 2.8

2.6

Viscosity (Pa)

2.4

2.2

Argon

2

Nitrogen

1.8

Water vapor

1.6

1.4 200

220

240

260

280

300

320

340

360

380

400

Temperature (K)

Figure 13.20: Magnetic confinement reactive ion etcher.

to confine electron trajectories leading to a higher ionization and dissociation efficiency. The plasma density in a magnetic ion etcher (MIE) is typically three to five times higher than in conventional parallel plate reactor. The self-bias voltage is also reduced. The substrate cannot be biased independently because of the self-induced DC bias and cannot be controlled independent of the plasma density. The use of magnetic field provided benefits, such as higher ion density, an increased etch rate, as well as a reduced DC bias and a narrower sheath at the powered electrode. In addition, the discharge could be sustained at lower pressure. The etching uniformity is not very good because of inhomogeneities in the magnetic field and frequently there is a loss of anisotropy when the reactor operates in excessive neutral to ion flux ratio or low ion energy. The field direction is rotated electrically with a short period to achieve the plasma uniformity. The overall effect of applied magnetic field is to provide low energy high ion flux discharge. A more efficient excitation of the plasma and therefore, an enhanced discharge can be obtained by inductive coupling (Fig. 13.21). In inductive coupling

1112 Dry Etching

Figure 13.21: Inductively coupled plasma etcher.

a changing magnetic field induces an alternating voltage that sustains the discharge. The inductively coupled sources achieve high-density plasma without the magnetic field and allow the substrate to be biased independently (Hopwood, 1992). The inductive coupled plasma is created by the application of RF power to a nonresonant inductive coil. The most widely used inductively coupled plasma (ICP) configuration is the planar inductor developed by Keller et al. (1993). A flat pancake coil is driven by an RF source through a matching network to excite an RF magnetic field B(t). The alternating magnetic field generates an electric field E(t), driving plasma current, which is dissipated by Ohmic loss. If the coil current is I0 exp (ivt) and there are N turns then the oscillation current resembles an azimuthal current K(t) ¼ NI0 exp (ivt),

(13:12)

which generates an oscillating sheath of magnetic flux in the radial direction, Br (t) ¼
o NI0 exp (ivt):

(13:13)

The time varying magnetic flux density Br (t) generates an azimuthal electric field Eu (t) according to Faraday’s law @Eu (t) ¼
ivBr (t): @z

(13:14)

Assuming the main spatial variation of Eu is along z, we write Eu  ez=d ,

(13:15)

Dry Etching 1113

where d is the skin depth for ohmic absorption. We have thus, @Eu (t) Eu ¼
, @z d

(13:16)

Eu (t) ¼ ivdBr (t):

(13:17)

and

In this fashion, the plasma electrons can gain sufficient energy between collisions to ionize most process gases. The electric filed Eu drives a plasma current Ju , which can be found from a simple momentum balance equation as: me

@Ve ¼
qEu
me Ve ve : @t

(13:18)

The electron velocity is thus, Ve ¼


qEu , me =(iv þ v)

(13:19)

giving rise to a plasma current Ju ¼
ne qVe ¼

ne q2 Eu v
iv : me v2 þ v2

(13:20)

The average power distribute to the plasma is therefore, 1 ne q2 E2u v Pav ¼ Re(Ju )Re(Eu ) ¼ : 2 2 2me v þ v2

(13:21)

The inductively coupled plasma is apparently elctrodeless so that it produces less contamination of plasma generated from electrode erosion as in capacitive coupled plasma. The chief goal is to control the ion bombardment energy and plasma density independent of one another. Electron cyclotron resonance etching reactor employs microwave power corresponding to the electron cyclotron resonance condition of the electron. The most efficient microwave power supplies commercially available are at 2.45 GHz and 910 MHz and these limit the ECR sources. The physical limits of magnets required to create a uniform plasma are also a limiting factor. The lack of electrodes and the ability to create high density of charged and excited species at low pressures makes this technique an attractive processing for etching of thin films (Suzuki et al., 1977). Large diameter wafers preclude the processing of multiple wafers at a time and single wafers have to be processed. The throughput can be maintained only if the etch rate can be increased. The high-density plasma creation (1011 ---1012 cm
3 ) therefore, takes on an increased importance for future reactors. The high density of plasma generated stably

1114 Dry Etching

under low pressure (0.2–10 mtorr), generates a large number of chemically active species. Therefore, a high etching rate is achieved even at low pressures. The plasma potential is between 15 and 30 eV without substrate heating thereby reducing the damage to the substrate. The RF or DC biasing of the substrate controls the ion energy. Control of the ion neutral flux is achieved by varying the microwave power and neutral gas pressure. Electron cyclotron resonance sources typically use microwave radiation to excite a right hand circularly polarized mode propagating along a static nonuniform magnetic field (one kilogauss), supplied either by a current driven coil or by an arrangement of permanent magnets (Fig. 13.22). Microwave energy is coupled to the natural resonant frequency of the electron in the presence of a static magnetic field. The plasma electrons gyrate around the magnetic field with an angular frequency, the cyclotron frequency qB/m. The direction of electron rotation is the right hand circular direction for the magnetic field line, that is the direction to cancel the magnetic field by the diamagnetism of the electron. The microwave in the waveguide is usually a linearly polarized wave, which can be thought of as a sum of left hand circularly polarized wave and right hand circularly polarized wave. When the electric field of the microwave is perpendicular to the magnetic field

Figure 13.22: Electron cyclotron resonance etch reactor.

Dry Etching 1115

and the right hand circular wave transmitting in the direction of the field satisfies v ¼ vc , electrons are continuously accelerated by the electric field of the microwave. When resonance conditions are set up, that is when the excitation frequency is equal to the resonant frequency of the electron, the electrons are continuously accelerated. We can discuss the transmission of the microwaves by the dispersion relationship that shows the relationship between the angular frequency v and wave number k. In the absence of the magnetic field, there are two significant problems. The input power is reflected above a critical plasma density and the pressure is low for satisfactory operation. The input power signal propagates according to c2 k2 ¼ v2 :

(13:22)

c2 k2 ¼ v2
v2p ,

(13:23)

With B ¼ 0, we find

where vp is the plasma frequency and c is the velocity of light. Thus, above the critical density for which v2p > v2 , k2 < 0 and k is imaginary. Therefore, the wave is cut off and the input power will be reflected. When the microwave is introduced parallel to the magnetic field the dispersion relation takes the form for the right hand circularly polarized wave c2 k2 ¼ v2


v2p 1
vc =v

:

(13:24)

As long as vc > v, k2 > 0. The input microwave signal propagates along B through the plasma electron density, ne , if the wave is launched from a high field side vc > v. The electron gas is directly heated by the electric fields and in turn the heated electron gas transfers its energy to neutral and ions gases by elastic and inelastic collisions. Due to the heavy mass of ions, the energy transferred to the ions can be safely ignored. The expression for the power transfer to plasma with applied field is given by (Asmussen, 1989) " # ne q2 E 2 v v þ : (13:25) Pav ¼ 4me (v
vc )2 þ v2 (v þ vc )2 þ v2 The power transfer can be significant v ¼ vc . The ECR discharge uses 2.45 GHz magnetron microwave sources that can deliver 0.3–6 kW. The resonance magnetic field at 2.45 GHz is 0.0875 T. The attraction of ECR reactors stem from the possibility of creating large densities of excited and charged species without the use of electrodes (Mantei and Ryle, 1991). The discharge occupies a finite volume and is bounded by the discharge container walls that

1116 Dry Etching

are either conductive waveguide walls or microwave transparent material like quartz. The thickness d of the ECR region is given by ( )1=5 1:4c (eE=me cv)2 : (13:26) d¼ v [(c=v)(1=B)(dB=dz)]3 When dB/dz gets smaller, the ECR region becomes thicker, the probability of an electron existing in the ECR region increases and the heating efficiency of the electron increases. When the gradient of the magnetic field is small, the wavelength l of the microwave decreases gradually until it is absorbed in the ECR region, by the phenomenon, which has been termed as beach effect. When one takes into account the Doppler shift effect, the thickness of the ECR region will be larger than that given by d above. The electron energy in the ECR region Wk parallel to the magnetic field at the position of the magnetic field B is given by   B Wk ¼ W0 1
, (13:27) B0 where B0 is the magnetic flux density in the ECR region, and W0 is the electron energy perpendicular to the magnetic field and B is the magnetic field density at the position of the electron. The electrons are transported along the magnetic field lines. Similarly, ions are transported along the magnetic field lines of force by ambipolar diffusion due to the necessity to keep quasi neutrality. The excited species and particles diffuse out of the discharge enclosure through an opening into a processing zone. Changing the current in the coils can easily alter the position of the ECR region, where the plasma is mainly generated. Since the life times of the ions and radical are different, the influence of the magnetic field on the diffusion of ions and neutrals also differs. There is therefore, the possibility that the transport of gases and ions from the ECR region to the wafer and the ion/radical flux ratio can be controlled. Different methods of coupling microwave energy into the plasma have been explored. In electron cyclotron resonance, the coupling is accomplished by microwave energy exciting the natural resonant frequency of electron gas in the presence of a magnetic field (Asmussen, 1989). In the majority of cases, the substrate is positioned downstream from the discharge. Charge species recombination is much faster than most general neutral gas phase reactions. One can therefore, avoid ion bombardment damage if we place the wafers away from the discharge region. Therefore, in a down stream afterglow reactor, neutral etchant radical flows from the plasma to the area where one can place wafers and also control their temperature. However, because of the absence of ions applications for this type of reactor are minimal. The energy of ions bombarding the surface is more or less controlled

Dry Etching 1117

independently of the discharge conditions, by applying a capacitive coupled RF voltage to the substrate electrode. This allows one to control the etching profile with high accuracy and reduces the ion bombardment damage of the wafer. Since the degree of dissociation is rather high, one has to deal with fact that relative amounts of the various species may be different than conventional discharges so that special recipes may have to be developed for etching. In addition, achieving of uniform etch rate along the entire wafer can pose serious concerns. A microwave processing system consists of the following components: (a) power source, (b) transmission lines, (c) microwave applicator and (d) microwave plasma load. The design of the system should be such as to transfer the power between the microwave applicator and the plasma load as efficiently as possible. This requires balancing the admittance of microwave oscillator, plasma load and the transmission line. There are also a variety of configurations that can be employed depending on: (i) the substrate is inside the active microwave discharge, (ii) outside the discharge in the processing chamber and (iii) though the substrate is outside in the processing chamber, it could be subject to a new discharge set-up in the processing chamber. Among the problems of these reactors, one has the following concerns. The resonance zone is subjected to nonuniformities due to microwave modes. The coupling of the microwave power to the plasma is governed by the design of the magnetic field to produce resonance effect, where small variations in the field produces large variations in the plasma uniformity. The magnetic field lines terminate on the substrate and charged particles follow magnetic field lines. The manner in which the magnetic field lines terminate the substrate is of major concern in the design of these reactors. The helicon source employs regular RF power at frequencies between 2 and 70 MHz in combination with an axial magnetic field to excite the plasma via helicon wave mode. Helicon resonator sources consist of coaxial RF coil surrounded by a grounded coaxial can (Fig. 13.23). The coil length is tuned to be approximately a quarter wavelength of the operating frequency and the RF power input tap position is adjusted to match the RF generator impedance. The coil coupling to the plasma is primarily inductive in the high-density regime. However, the voltage at the open circuit end of the coil can be very large, and any capacitive coupling can raise the plasma potential to high levels. Helicon resonators therefore, usually have a slotted cylindrical shield between the coil and the plasma to eliminate unwanted capacitive coupling. Helicon resonators operate over the frequency range with fairly high input power levels (< 5 kW) due to losses in the coil. Plasma densities exceed 1012 cm
3 with good radial uniformity has been achieved. The ability to control the ion energy and bias the substrate independently has been taken to a next higher level in ion beam techniques (Fig. 13.24). In ion beam techniques, the plasma is generated in a separate chamber and ions are

1118 Dry Etching

kL Gas inlet −162 mm

M#1

Antenna

kL C1

x y

C2

0.0 mm Detector

z

To reactor M#2

+162 mm +z y 0

30

60

mm

Figure 13.23: Helicon wave plasma reactor.

Discharge chamber Beam grid Acc-grid −100 to 200 V 0V 200 −1000 V

High-energy ion Ar+ Substrate

55 V PR

e−

Figure 13.24: Ion beam etcher.

Dry Etching 1119

accelerated toward the substrate surface by means of several grids. The flux and the energy of ions can be independently controlled in these methods. Example 13.10 Estimate the velocities of the ions parallel and perpendicular to the surface that is etched when the sheath potential is Vsh . If the ions fall through the sheath potential Vsh with no scattering, their velocity normal to the surface is given by sffiffiffiffiffiffiffiffiffiffiffiffi 2qVsh , V? ¼ mi where mi is the ion mass. In the velocity parallel to the wafer surface, the velocities of the ions are expected to have a thermal distribution, since that was their condition before they fell into the influence of the sheath potential. We ignore any small component of the sheath electric field that may be present parallel to the surface. Hence, sffiffiffiffiffiffiffiffiffiffiffiffiffi 2kB Ti , Vk ¼ mi where Ti is the temperature of the ions. Therefore, sffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffi Vk k B Ti Ti  ¼ : V? qVsh Te Since at room temperature, Ti is about 1/40 eV and Te is typically 2 eV, we have the result Vk 1 : ¼ V? 8:94 Particles with this ratio of velocitie move at an angle tan u ¼

Vk , V?

which gives u ¼ 6:4 . One has to significantly increase the sheath voltage to avoid the thermal contribution to the velocity being important, so that the horizontal etch rate can be reduced. We can now examine the main characterizing parameters for plasma etching. In RF discharges, one of the most influential parameters for discharge

1120 Dry Etching

phenomenon, which controls plasma etching, is the pressure. Pressure controls the RF voltage amplitude, which is closely related to the sheath voltage drop and the energy of the ions bombarding the surface. The relationship between the pressure and the ion energy is schematically shown in Fig. 13.25. With decreasing pressure, the characteristic potential drops across the sheaths and the voltage applied to the two electrodes increases sharply. The self-bias voltage is proportional to the peak RF applied voltage and therefore, that also rises. The mean free path of species is inversely proportional to pressure so that the rise in potential induces higher energy ion fluxes to the substrate surface. Higher ion energies induce etching, even though from the point of view of selectivity and damage to the device and loss of selectivity in etching, this may be undesirable. The average electron energy decreases with increase in pressure. Electron energy and electron molecule ionization rate constants tend to decrease with increasing pressure. The change in pressure can affect the surface of flux of neutrals to ions to the extent that one can achieve essentially chemical etching with little contribution from ions or vice versa. The relationship of electrical measurements to pressure has been carried out by several people and interpreted by them to yield several characteristics of the plasma (Godyak et al., 1991; Lieberman, 1988). The excitation frequency in an RF discharge has an important influence on plasma chemistry and etching. The excitation frequency can change the spatial Chamber Pressure

Beam Energy

High >100 mtorr

Low

Low 10 −100

Anisotropy Selectivity

Plasma etching Plasma assisted chemical reaction

Low

Very good

Medium

Reactive ion etching Physical bombardment + chemical reaction

Medium

Good

High

Physical sputtering (ion mill) Physical bombardment

High

Poor

mtorr

very low <10 mtorr

Dry etching

Figure 13.25: Pressure versus time of etching.

Dry Etching 1121

distribution of species and electric fields across the discharge. Frequency determines whether the energy and density of species are constant in time, or whether they fluctuate during a period of the applied field. Frequency can also change the electron energy distribution function, which has an effect on controlling the electron molecule reaction channels that can predominate. Frequency also determines the minimum voltage necessary to start and maintain the plasma and thereby determines the energy with which ions bombard the surface. Each process that occurs in the plasma has a relaxation time t i . Varieties of these processes are ambipolar diffusion, ion–ion recombination, ion–electron recombination, atom–atom recombination and free diffusion; and these have an effect on density of the plasma. Charge transfer reactions, momentum transfer collisions and attachment have an effect on velocity fluctuations. Electron energy collisional relaxation influences the energy. If vt i 1, the dynamic process will be representative of the instantaneous condition induced by the time varying field. If vt i  1, the process is too slow to respond and it reaches a state of equilibrium with time averaged conditions. The complex phenomena associated with frequency effects on etching has precluded a large number of investigations in this field, and requires a lot more attention to fundamental studies for data gathering. Temperature has an important influence on plasma etching because of the way the temperature at the surface of the substrate affects the kinetics of reactions. The temperature has a dominant effect of etch rates, selectivity and degradation of resist masks. Temperature also affects the morphology of the surface that is etched making the surface rough at higher temperatures. The role of diffusion and physisorption are also tremendously influenced by temperature.

13.10. Plasma Diagnostics and Process Control The demands made on uniformity of etching or deposition of films utilizing plasma sources imply that one must be able to achieve uniform plasma. There is therefore, a need to measure plasma parameters and confirm the uniformity of plasma in any reactor. It is also necessary to measure the density and energies of species in the plasma as they determine the reaction rates. The development of diagnostic measures is an extensive field of research in itself (Hutchinson, 1987; Auciello and Flamm, 1989). Sensor methods, which are simple and unobtrusive, are useful as process monitors, endpoint detectors and process controllers to improve process flexibility and reliability. One of the simplest diagnostic techniques to measure with great spatial resolution is the use of a probe. A probe is an electrically conductive electrode

1122 Dry Etching

of convenient shape that is inserted into the plasma with a view to measure its properties. The voltage on this electrode is varied with respect to a reference electrode by an external power supply, and the current I, to its surface is measured in a variety of conditions that include different frequencies, pressures, reactive gases and magnetic fields. The plasma properties are deduced from the current voltage characteristic of the probe. The quantities elucidated from these studies are electron density, electron energy and electron energy distribution and plasma potential. Even though the basic operation of the probe is simple, the derivation of the appropriate information from the characteristics of the probe is quite an effort in itself and is available in many books (Cherrington, 1982). The design of the Langmuir probe is done with due attention to the hostile environment in which the probe has to operate. The current versus voltage measured in a Langmuir probe in an ideal case shows five distinct features. The point at which the curve crosses the V axis gives the floating potential Vf . To the left of this potential one obtains the ion saturation current, Isat . To the right of Vf , electron current is drawn and grows exponentially. Eventually, this current bends down at the knee portion and saturates at the electron saturation value Ies (Fig. 13.26). The exponential region of the current voltage curve can be plotted in a semi-logarithmic plot to obtain the electron temperature. Theories of probes are available to infer the other plasma parameters (Huddlestone and Leonard, 1965).

I

Ion saturation

Electron retardation

Vplasma

Electron saturation

V

V flosting Planar

Cylindrical

Spherical

Figure 13.26: Current versus voltage measured in a Langmuir probe.

Dry Etching 1123

Particle energy analyzers can be used to obtain information of the electron and ion energy distribution functions albeit with more perturbation of the plasma (Stenzel et al., 1983). The most commonly used energy analyzer is the retarding field analyzer or a multigrid analyzer. This is a series of grids used to select particles of a given charge sign and analyze the energy distribution of these particles. With the help of three grids and a collector, one can measure either ions or electrons. Electrostatic sector analyzers have also been used even though the signal obtainable is much reduced in this case. When etching features in a substrate one cannot assume that the etch rate is uniform over the entire surface. This means there will be situation when one has to over-etch the substrate. However, this gives problems in retaining the geometry of the etched region. This is especially important when etching thin planar films, such as extremely thin gate oxides. In order to maintain the selectivity and avoid overetching, the endpoint detection of etching becomes an extremely important aspect of process control. A schematic sequence of events in plasma etching process may be controlled by observing the etching process as a function of time is shown in Fig. 13.27. There is an initial period during which the plasma produces the necessary radicals to etch the species. This is followed by a stage in which the radicals react with material to be etched to produce a volatile product. As the film is etched and when the substrate is reached new secondary products of etching can form. There are several principles that are exploited to infer the endpoint of etching. One of them is to measure the surface either in situ or by detecting the endpoint in etching. It is also possible to measure the key species in the reactor, which are either the etching species or the product species. It is also possible to measure the properties of the discharge itself to infer the endpoint of etching. A source of nonuniformity in etching is the aspect ratio dependent etching (ARDE). This is etching nonuniformity among features with different aspect ratios (ratio of depth to width opening of the feature). The reason for this phenomenon is that the bottoms of the features are shadowed by the feature sidewalls from the plasma. This can cause less number of particles (ions and neutrals) to arrive at the bottom of the features. Therefore, the features etch differently depending on the aspect ratio. If higher aspect ratios are etched slower it is sometimes referred to as forward lag in the industry, otherwise reverse lag. It is frequently found that both forward lag reverse lag are observed when the same etching mechanism is operating. All metrology measurements to ascertain the fidelity of pattern transfer are made by scanning electron microscopy. Film formation on the vertical surfaces and their absence in the horizontal surfaces can also be established. Deposits and roughness features of the etched region can easily be delineated. These approaches are essential in the development stages of dry etching. Initially, the pressure in the system rises as the gas is heated and dissociated when RF power is applied. The etching involves the gas-solid reaction and

1124 Dry Etching

Etching non-ideally: micro-trenching through the underlying layer Etching non-ideally: faceting

Etching non-ideally: bowing

Etching non-ideally: trenching

Etching non-ideally: notching

Figure 13.27: Defects in transferring pattern due to etching.

usually causes a decrease in the number of gas molecules so that the system pressure will drop when there is a constant resistance to flow. However, by fixing the feed gas rate constant and adjusting through an automatic throttle vale the pressure in the system as in many modern reactors, the monitoring pressure is not available for endpoint detection of etching. Laser interferometry is a popular technique to monitor the progress of etching in situ. This is especially possible for films that are transparent. A helium neon laser with a wavelength of 632.8 nm is directed at the material surface. At normal incidence, interference maxima and minima occur when the thickness of the film, d, satisfies the relationship   l , (13:28) dd ¼ 2n

Dry Etching 1125

where n is the refractive index of the transparent layer and dd is the spacing between adjacent maxima. The intensity of light varies sinusoidally with film thickness and the etch rate can be monitored by noting the time interval between corresponding points in the signal trace. The technique permits real time monitoring of etch rate. Opaque films, like metal films cannot be monitored by this technique. A change in reflectivity can however be observed upon complete removal of the metallic film. The signal strength from small area can often be a problem in the technique. Optical emission from glow discharges has been widely used by emission spectroscopy for discharge characterization and process control. Electron impact ionization of molecules raises them to an excited state, followed by a relaxation to a lower energy state releasing a photon containing energy equal to the difference between the two energy states. The diagnostic technique of optical spectroscopy relies on the light collected by a lens and focused onto the slit of a spectrometer. Quartz or sapphire windows are used and may have to be cleaned from deposits to insure their transparency. A monochromator is used to spectrally disperse the collected optical emission and a photomultiplier will then convert the spatial radiation into electrical signals. Optical multichannel analyzers are also popular to measure rapidly and accelerate the spatial scans of emission spectrum. The identification of the species in the discharge and their state of excitation are made through observation of some characteristic lines. Therefore, the appearance or disappearance of certain characteristic lines can be used for endpoint detection in etching. Generally, the reactive radical emissions are used for this purpose. By combining the intensities of the different spectral lines, it is possible to determine the molecular species present. The relative intensities of two spectral lines with different excitation thresholds can give electron temperature. The relative intensity of an ion line and a neutral line can be used to estimate the ion fraction. Doppler broadening can be interpreted to give the velocity of the emitting ion, whereas Stark broadening may be used to analyze the information on density. A more popular technique is one of actinometry. A known concentration of impurity is introduced, and the intensity of two neighboring spectral lines, one from known gas and the other from the sample are compared. Assuming both species are bombarded by the same electron energy distribution, and from known concentration of the actinometer, the density of the sample is obtained. With further refinements, one could study chemical kinetics of surface reactions as well. Simple emission spectroscopy cannot be used to measure ground-state concentrations directly, since the emission intensity is a complicated function of ground-state concentration, electron energy distribution, electron impact excitation cross section and the solid angle and spectral optical transmission of the optical system. However, by adding a small percentage of some inert trace gas

1126 Dry Etching

(called an actinometer) to the discharge, the emission intensity ratio of reactive radical to a suitably selected tracer gas can provide estimate of the ground-state concentration of the radical (Coburn and Chen, 1980). Laser induced fluorescence is another popular technique to monitor etching process. In order to obtain quantitative information on the ground-state species, one needs to photoexcite it with a field of known intensity and spectral width, instead of impact excitation by electrons whose concentration and energy distribution is normally unknown. This is widely used in laser-induced fluorescence technique. Laser radiation needs to be transmitted to the discharge chamber. Tunable dye lasers are used and directed at 908 to the emission collection axis. The laser radiation is concentrated near a single wavelength, the fluorescent intensity relates directly to the ground-state concentrations. Laser induced fluorescence posseses high spatial and temporal resolutions. Uncertainties in quenching and excited states result in uncertainty in measurement. Most laser induced fluorescence measurements have concentrated on molecular species because they are easily photoexcitable. Many radicals and products of etching have been determined experimentally. Atomic species measurements are somewhat more difficult because of their high excitation energy and they require two photon excitations. Nevertheless, O and Cl have also been probed by this technique. Optogalvanic spectroscopy is also utilized sometimes in monitoring etching process. Optogalvanic spectroscopy is based on measuring a change in the discharge current resulting form absorption of optical radiation, usually from a laser. Such a change of current can result from direct ionization of neutral molecules, or detachment of negative ions. In either case, increase in electron concentration causes an increase in plasma conductivity. Optogalvanic effects involving atomic, molecular, charged particles, inert and reactive species have all been reported. Photodetachment of negative ions by laser radiation can be used to distinguish the identity of negative ions and measure their concentration. Different chemical species are associated with the start and end of the etching process. Hence, they can be monitored by mass spectrometry. Mass spectrometric sampling of glow discharges has provided information for many aspects of gas discharge behavior. Either one analyzes the ion species after extraction from the glow discharge or a fraction of the neutral species is extracted and ionized by exposing it to electron beam impact. Typically, one constituent can be tracked to observe the progress of etching. Mass spectrometry is compelled to sample the species at the plasma boundary. Some attempt has been made to measure accessible discharge parameters to follow the etching process. In particular, the impedance of the discharge as given by the DC self-bias voltage at constant RF power can be measured. One

Dry Etching 1127

can also measure the pressure at a constant gas flow rate. These rely on a macroscopic change in the discharge, so that small areas that are etched are hard to follow. One would expect that the discharge characteristics change because of the volatile etch product in the discharge itself. One finds that when the wafers are mounted on the powered electrodes, changes in ion induced secondary electron emission coefficient of the cathode can bring about changes in the nature of the discharge.

13.11. Etching-Induced Damage The challenges to a successful plasma etching include maintaining etch uniformity, etch selectivity, high etch rate and reducing the substrate damage. The undesired side effects in dry etching are related to the damage caused by ion bombardment of the surface with ions and neutral plasma particles. The energy of ions and neutrals is sufficiently high to remove the material from the surface and redeposit elsewhere in the system including the substrate. In addition, the surface to be etched is exposed to electrons and high-energy photons. Sputtering occurs from any surface that is in contact with the plasma, provided the sheath potential is larger than the sputtering threshold (typically 30 eV or less). Therefore, the walls and fixtures of the chamber are potential candidates from which sputtering can occur. Elements that are not prone to form volatile halides in halocarbon plasma are particularly troublesome as they are redeposited on the substrate. The major mechanisms for particulate contamination in plasma processes are mechanical abrasion of moving robotic parts, the flaking of deposited films onto the wafer, arcing, formation of particulates in the plasma and degradation of materials. Deposited metals are very problematic to device integrity as they have large diffusion coefficients in semiconductors and can harm the electrical integrity of the devices. Even if the devices do not degrade (due to large interface–surface density and degradation of life time), redeposited sputtered material contributes to the formation of rough surfaces because of micro masking. These, in turn can lead to the formation of micro-cracks during deposition of subsequent layers of films. Thermal annealing usually may not remove the damage. Low pressure and a long mean free path are essential for the material to leave the vicinity of the surface without being backscattered and redeposited. Besides giving slow etching rates, sputtering can also form facets and trenches near etched features. Ion implantation, especially of carbon can drive carbon deep into the device. One can find surface reconstruction after etching is over and this may result in some device degradation in some situations.

1128 Dry Etching

During etching to produce a large aspect ratio hole, over-etching can occur at the corner of the bottom, and this is sometimes known as ‘‘notching’’. This is attributed to charge up at these areas. A pulse discharge method is utilized to solve this problem. This method reduces the reactive ion bombardment energy, by cutting off the electric input power before the formation of the ion sheath, resulting in the reduction of reactant acceleration (Samukawa, 1994; Ahn et al., 1996). Other etching anomalies include faceting, trenching, micro trenching and bowing as shown in Fig. 13.27. Damage also results in polymers when they are processed in the plasma. Electron, ultraviolet radiation as well as ion beams also damage polymers. The unintended sources of polymers are from Teflon cover plates, and outgassing of solvents from photoresists. The damage consists of breaking the polymer bonds and forming polymer fragments, which are extremely reactive. These polymer fragments can cross-link and form polymeric film on vertical walls, where there is no ion bombardment. The cross-linked polymers are extremely brittle and can crack rather easily. When the polymers adhere firmly to the sidewalls they can be used to promote vertical etching as we have already observed. The accumulation rate of polymers in the horizontal plane is minimal because of the bombardment of ions. Polymers can also deposit on the walls of the chamber from where they are to be removed by cleaning. The presence of polymeric films on the walls of the reactor is potential source for contaminants and requires frequent cleaning with oxygen plasma. Surface quality is an important feature of dry etching process. This can be influenced by many variables, such as temperature, ion bombardment, etchant and crystallographic orientation. One desires an etched surface as smooth as the unetched surface, which is possible in some cases. Under other situations, the etched surface can be pitted, rough or covered with cones and spikes or substrate material. Contamination from sputtering, etch residues, electrical damage as a result of contamination, radiation damage from ion bombardment and charging of surfaces may all contribute to degradation of surface quality. The damage can also manifest in the form of non-stoichiometric region, defect aggregates and point defects. The electrical effects of these etchinginduced defects include compensation of dopants, a decrease in carrier mobility and an increase in flicker noise. Local regions can alter their stoichiometry substantially and can have large effects on Schottky barrier heights. Damage can occur to devices fabricated while they are subject to plasma etching. The damage is essentially due to radiation from ions, electrons and ultraviolet light, soft X-rays. The damage appears in several forms. Ion impact can cause atomic displacement. X-ray and ultraviolet light can cause electron– hole pairs and secondary ionization, where electrons formed by primary processes create defect centers. These influence the operating characteristics of the devices.

Dry Etching 1129

Some of the defects are reversible and can be removed by annealing. Many other defects are irreversible. Device damage often requires thermal annealing treatments for their removal. The temperatures of annealing can be as high as 900 8C so that shallow junction devices pose special problems in this regard. The charged particles of the plasma accumulate on the insulator film of the wafer. The accumulated space charge will build up a high electric field and induce breakdown in the film. Sputtered particles and electrostatic breakdown of thin films caused by charge that is deposited during the plasma process cycle are irreversible. The need for particulate control in a plasma reactor is becoming more and more crucial. The particulates are generated from hardware movement that is associated with wafer transport, valves opening and closing and other mechanical parts. This source of damage has to be controlled by reducing the plasma potential with respect to the chamber surface to below the sputtering threshold (typically 15 eV). Normally, all dielectrics and metals exposed in the reaction chamber should not have sharp edges since the discharge tends to concentrate on the edges and becomes a source of particles. Corrosion and material degradation are sources of particulates and are specific to a given process. The products of etching may coat and flake later on giving rise to particulates requiring extensive cleaning and the consequent down time of the reactors. Particulates can also result from improper pumping procedures as well as contamination in pipelines leading to the reactor chamber. Careful attention to materials of construction and proper maintenance and cleaning procedures can reduce the role of particulates from destroying the etching process step. To minimize particulate contamination, clustered plasma tools use a wafer handler to pass wafers from one process chamber to another in vacuum environment. In addition, one can increase the throughput, and provide high yield advantages, since wafers are exposed to less contamination. Example 13.11 Estimate the etch rate dependence of reactive ion etching damage as given by Oehrlein et al. (1985). In RIE etching the reactive ions will damage the substrate and accumulate. At the same time the reactive ion etching of the substrate will occur, which will consume the damaged layer. The time necessary to reach a steady state depends on the etch rate ER. It has been shown that for an allocation xi fixed with respect to the substrate surface, the impurity concentration will have reached its maximum possible to within 1% after a time tmax , where tmax

2:69DRP þ RP
xi , ER

1130 Dry Etching

where DRp is the straggle, Rp is the range of the impinging ions. For low ion energies, it is safe to assume that Rp is very nearly the same as DRp . If xi ¼ Rp , then for xi ¼ 2:3 nm tmax

6:2 : ER

For ER of 600 nm/min, 60 nm/min and 0.6 nm/min the time to reach steady state is respectively, 0.6, 6 and 600 s. If the dose rate is 1015 ions=cm2=s, then the maximum retained dose is 6  1014 , 6  1015 and 6  107 ions=cm2 . Higher etch rates will reveal lower impurity damage.

13.12. Modeling of Dry Etching Developing accurate models and simulation tools assist in the development of new processes and aid in understanding the problems and issues surrounding each particular process. The complexity of dry etching has eluded development of such models even though many partial tools of simulation are available. The ever-shrinking device dimensions with accompanying higher aspect ratios have made profile control in plasma etching processes a formidable task. This area has demanded increasing attention of researchers and many approaches are constantly being tried. Predictive profile simulation during etching is sought as a means to reduce time and cost associated with trial and error process development and/or equipment design. For a process engineer, the inputs of the system are gas pressure, gas flows, power and other controllable chamber conditions that are specific to a given reactor. The output that the process engineer expects is the same as feature scale simulation results, like etch rates and etch profiles. Relying on estimates of fluxes and other input parameters the feature scale simulator is enough and gives trends of feature evolution for different reactor conditions. However, neutral fluxes are not easily established and one hopes to have more accurate numbers. The etching features have great dependence on the aspect ratio of the feature to be etched. The smaller the feature to be etched, slower is the etch rate. Higher aspect ratio (which reduces the view factor) and charging within the feature cause a reduction of ions and neutrals to the bottom of the feature. Lowering the pressure (reducing the collisions) increases the directionality of the ions. Similar effects are seen in high density, low-pressure higher energy ions. The greater the ratio of energy of the ions to electrons less will be the surface charging. There are several important parameters to consider while studying the evolution of etch

Dry Etching 1131

features. The collisionality in the sheath influences the ion bombardment directionality, which is governed by the thickness of the sheath in terms of mean free path lengths. The view factor to the plasma and the ion angular/energy distribution determine the flux on each element of the feature surface. The ions can scatter from the surface features and produce significant fluxes at other parts of the surface profile. One of the features produced is the so-called trenching, which is due to more rapid etching of the surface at the base of the sidewall. If the resist does not have a vertical profile or becomes faceted, ion scattering into the opposite sidewall and undercutting can also be observed due to ion scattering and this is dependent on the slope of the sidewall and the angular dispersion of the ion bombardment. Micro trenches can form at the base of the sidewall, where the flux of ions is the largest due to the fact that the flux is a sum of the direct flux from the plasma and the flux reflected of the sidewalls. Passivation is an essential feature in achieving highly directional etching and depends upon the production of depositing species in the plasma. The presence of excess amount of passivation leads to the narrowing of features. The presence of tapered features affects ion scattering, which can alter the etched profile. The nonvolatile etch products can redeposit and modify feature profile. Charging of surfaces can arise because in the presence of a magnetic field plasma can support a potential gradient across the wafer when conducting surfaces and nonconducting surfaces can charge to different potentials. Charging can also arise by the differing angular dispersion of the ions and the electrons, where the electrons are isotropic whereas the ions would be highly directional. This phenomenon is dependent on aspect ratio, the ion directionality and electron energy. The etching of features is dominated by the feature aspect ratio. The etching of smaller features is slower than that of larger features. Some of the causes contributing to the low etch rate of small features are: reduction of ions and neutrals at the bottom of the feature, reduced view factor, charging within the feature. Some of the ways to overcome these difficulties is by lowering the process pressure, increasing the ion energy, ion to electron ratio to control charging effects. The continued decrease in the line width has prompted the use of low pressure, high-density plasma sources. The post-etching corrosion can be a major problem. For example, residues of chlorine left after etching aluminum can react with water and when in contact with another metal, such as TiW can start the corrosion process by galvanic corrosion. The search for high dielectric materials, such as ZrO2 , HfO2 and ZrSix Oy to replace the limitations of SiO2 when the thickness of the dielectric has to be reduced to below 10 0 because of device considerations presents unique problems in etching because of the general difficulty of these materials to reactive gases. Metals and their nitrides and oxides are explored for alternative gate materials, which require proper identification of etch procedures to be

1132 Dry Etching

incorporated in device structures. Copper metallization sought as an alternative to aluminum metallization presents problems in dry etching. The complete simulation of plasma etching chamber proceeds at least at three levels. At the global level, the whole chamber is simulated. Inputs are process conditions, such as total gas pressure, flow rate and power. The outputs are partial pressure of gases, fluxes toward surface and electron temperature. At the intermediate level, we deal with region of the reactor close to the wafer surface. Most important of intermediate simulation is the nature of ions. The ion temperature and the pre-sheath and sheath conditions are important. The sheath region determines the energy distribution of ions. Pre-sheath region has a decisive effect on the final angular distribution of ions. The third level is the feature scale level, where the ideal would be to reproduce the etching feature completely. The gas phase species (ions and neutrals) undergo several complex processes once they arrive on the surface. There are several approaches to modeling these processes. One method is the feature scale simulations using Monte Carlo techniques by following each species on the surface. The long times required to run these types of computer simulations make these techniques very impractical. Consequently, many ad hoc models that rely on sticking probability assumptions, surface kinetics, chemical reactions, etc., are invoked to put simulation to manageable size. However, the integration of all the various levels of simulations from global to feature scale simulations is still far away from realization. Example 13.12 Assuming the radius of CF4 to be 2  10
10 m, and temperature is 300 K, at a pressure of 1 mtorr, estimate the elastic mean free path. Consider the radius of the feed gas to be r. Area the molecule of the gas presents is A ¼ pr2 . At temperature of T, and pressure, P, the density of neutral molecules is given by P=(kB T). The mean free path is s¼

1 1 : P=(kB T) pr2

Hence, the mean free path is given by 300K  1:38  10
23 J=K mtorr torr 1 ¼ 0:25 m:
3 1 mtorr 10 torr 133 Pa p(2  10
10 )2 These are bigger than the dimensions of most reactors. Inelastic mean free paths are typically an order of magnitude smaller.

Dry Etching 1133

Problems 13.1. Sketch the etch profiles in silicon by electron cyclotron resonance etching with an SF6 plasma and a photoresist mask as a function of temperature of the substrate (Tachi et al., 1988). 13.2. Outline the plausible steps in the dry etching of GaAs with BCl3 =Ar gas mixture. How is the selectivity with respect to etching AlGaAs achieved? 13.3. Consider a cylindrical reactor of radius 5 cm. The particles travel by diffusion along the reactor. If the particle velocity is 300 m/s, estimate the diffusion coefficient. How long does it take for the particle to go the distance 2a. What is the total distance traveled by the particles? 13.4. What would you expect in a plasma chamber when there is moisture in the system? 13.5. Calculate the thickness of the ECR region for an electric field 2 kV/m, magnetic field of 875 G and for frequency 2.45 GHz when dB/dz is 5 kG/m. 13.6. If the frequency of microwave is 2.45 GHz and argon gas is used, what is the best pressure for plasma generation? 13.7. Silicon is being deposited at an average growth rate of 10 nm/min. What is the growth rate in number of atoms per meter2 per second? 13.8. The electron affinity for a fluorine atom in free space is 3.45 eV. If the electrostatic image force increases the affinity calculate the affinity for a fluorine atom at a distance of 10 from the SiFx surface.

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1134 Dry Etching Hopwood, J. (1992), Review of inductively coupled plasma for plasma processing, Plasma Sources Sci. Technol., 1, 109. Huddlestone, R. H., and Leonard, S. L. (Eds.) (1965), Plasma Diagnostic Techniques, Academic press, New York. Hutchinson, I. H. (1987), Principles of Plasma Diagnostics, Cambridge University Press, Cambridge. Irving, S. M. (1971), A plasma oxidation process for removing photoresist films, Solid State Technol., 14(6), 47. Keller, J. H., Forster, J. C., and Barnes, M. S. (1993), J. Vac. Sci. Technol. A11, 2487. Lee, R. E. (1979), J. Vac. Sci. Technol., 16, 164. Lieberman, M. A. (1988), Analytical solution for capacitive RF sheath, IEEE Trans. Plasma Sci., 16, 638. Manos, D. M., and Flamm, D. L. (Eds.) (1989), Plasma Etching, Academic Press, New York. Mauer, J. L., Logan, J. S., Zielinski, L. B., and Schwartz, G. C. (1978), J. Vac Sci. Technol., 15, 1734. Melngailis, J. (1987), J. Vac. Sci. Technol., B5, 469. Mogab, C. J., and Levenstein, H. J. (1980), J. Vac. Sci. Technol. 17, 1721. Oehrlein, G. S., Tromp, R. M., Tsang, J. C., Lee, Y. H., and Petrilllo, E. J. (1985), J. Electrochem. Soc., 132, 1441. Pandey, K. C., Sakurai, T., and Hagstrum, H. D. (1977), Phy. Rev., 16, 3649. Samukawa, S. (1994), Pulse-time-modulated electron cyclotron resonance plasma etching in highly selective, highly anisotropic, and notch-free polycrystalline silicon patterning, Appl. Phys. Lett., 64, 3398. Stenzel, R. L., Gekelman, W., Wild, N., Urrutia, J. M., and Whelan, D. (1983), Rev. Sci. Instrum., 54, 1302. Suzuki, K., Okudaira, S., Sakudo, N., and Kanomata, I. (1977), Microwave plasma etching, Jpn. J. Appl. Phys., 16, 1979. Tachi, S., Tsujimoto, K., and Okuduaiara, S. (1988), Appl. Phys. Lett., 52, 1170. Tu, Y. Y., Chuang, T. J., and Winters, H. F. (1981), Phys. Rev., B23, 823. Westerheim, A. C., Labun, A. H., Dubash, J. H., Arnold, J. C., Sawin, J. H. and Yu-Wang, V., (1995), Journal of Vacuum Science & Technology A: Vacuum Surfaces, and Films, 13(3), 853. Winters, H. F. (1978), J. Appl. Phys., 49, 5165. Yu, Y. C. S., Hacherl, C. A., Patton, E. E., Lane, E. L., Yamaguchi, T., and Dottarara, S. S. (1990), J. Electrochem. Soc., 137, 1942.