Micro-machining: A survey of the most commonly used processes

Sensors and Actuators, 17 (1989) 1 2 3 - 138

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MICRO-MACHINING : A SURVEY OF THE MOST COMMONLY USED PROCESSES

GILLES DELAPIERRE CEA/IRDI Division LETI-DOPT, CEN/G 85 X 38041 Grenoble Cedex (France)

Abstract Thanks to micro-machining processes such as chemical anisotropic etching, plasma etching and `sacrificial layer' methods, it is now possible to manufacture small, cheap and reliable micro-mechanical components in the same way as micro-electronic ones . This paper briefly reviews relevant processes . Various materials such as silicon, gallium arsenide and quartz are considered .

1 . Introduction It has become common practice to group under the title of 'micromachining' all the processes of the type used in micro-electronics but more specifically used to manufacture three-dimensional micro-mechanical components. The strength of this technique lies in the systematic use of `batch processing' and physical or chemical processes, with well-known advantages : Very low production costs ; no theoretical limit to the possible miniaturization ; very precise control of the material structure and composition ; use of small quantities of material, which can be rare or very pure ; the only possible method that can provide access to some physical phenomena (elastic limit of single-crystals, field effect etc .) . The main tools that can be used to manufacture three-dimensional components (dimensions of the same order of magnitude in all directions) despite the fundamentally two-dimensional character of micro-electronic technologies (thin film, microlithography etc .) are anisotropy and selectivity of etching processes, with three main families of processes that are able to produce such structures : chemical anisotropic etching of single crystals; plasma etching ; and sacrificial layer methods . This paper attempts to give a general idea of what is feasible with these processes . A more complete treatment of each technique is given in the references.

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2 . Chemical anisotropic etching of single crystals Mineralogists have known for a long time that the etching speed of a single crystal strongly depends upon the crystalline orientation of the surface being etched [1, 2,j the origin of this anisotropy being not clearly explained . The atomic density of the crystalline surface being etched seems to be a determining factor, the slowest speed generally corresponding to the densest planes . The method most frequently used to determine the etching speed diagram is to study the contour evolution of spheres, cylinders or profiles along mask edges . Convex forms (spheres, cylinders) tend to be limited to surfaces etched rapidly . Concave forms (e.g. a hole in a substrate) will give the contrary effect . Once the dependence of the dissolution rate with respect to the orientation is known, it is possible to foresee the profile that will be obtained by using, for example, the Wulff-Jaccodine graphical method [2 - 5] schematized in Fig . 1 . The points 0, and 0 2 correspond to the edges of the etching mask . The origin of the polar etch-rate diagram is located at 0 1 and 0 2 , and the left and right sides of the diagram are considered respectively . The profile is then obtained by tracing perpendicular lines at the end of the R rate vectors, and only keeping the points of these lines that can be reached from 0 102 without intersecting any others . The shape determined in this manner corresponds to the minimum integral of the etching action . The strength of the method resides mainly in the fact that, sometimes, polar diagrams that can be used for all wafer orientations are obtainable by observing only a few etching profiles on a few substrates . A good example of this method is given by Shaw in the case of GaAs etching in acidic hydrogen peroxide [6, 71 . The polar plot of the etch rate of GaAs in H 2 SO4 :30% H 20 2 :H 20 solutions of different composition by volume is given in Fig . 2 . These (011) polar diagrams have been obtained by observing etching profiles on grooves parallel to the two orthogonal (011) directions on a (100) GaAs wafer . Planar facets appear on the groove walls, which correspond

polar plot of etch rates

etching mask MPPW

etching profile

j

Fig . 1 . Wulff-Jaccodine's construction [2 - 5 ] .

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1 :8 :1 1 :8 :40 1 :8 :80 1 :8 :160-

[011]

1 :8 :1 1 :8 :40 1 :8:80 1 :8 :16

[1x01

Fig. 2 . Polar diagrams of etching rate of GaAs in H2SO4 :H202 :H20 solutions (by volume of concentrated reagents) [6] .

to minimum etch-rate directions . Five minimum etch-rate vectors have been measured in this example . The complete etch-rate diagram is then estimated by joining these five points by arcs of a circle . The radii of these arcs are determined using a minimum rate criterion [4, 61, which states that to avoid appearance of planar facets other than those two observed at coordinates (r 1 , 0 1 ) and (r2 , 6 2), intermediate (r,, B 1 ) vectors must satisfy the condition r 1 sin(e1 - 0 2) + r2 sin (0 1 -0 2 ) r; sin ( 0 1 -0 2) This theoretical extrapolation is completed by some trials of a few minor modifications of the diagram, until one obtains excellent fitting of the predicted profiles to those observed on the (100) wafer . In Fig. 3, excellent agreement has been obtained between actual and predicted wall profiles on a (511) wafer, starting from the polar diagram previously determined on a (100) wafer . Another material that has been extensively studied is quartz . Although it is less widely used in micro-electronic applications, its piezoelectric properties make it a material of choice for all sensors using resonant transducers . Several million quartz watch movements are manufactured in the world every year thanks to this technology . The most complete etching diagram that we have found is given in Fig . 4 and was established by Ueda et al., who systematically studied the profiles of 21 substrates with different orientations [8, 9] . The etching rate in the z direction is about 1 .3 pm/min . A very useful property of quartz is that it has a practically zero etching speed for all the planes parallel to its z axis (optical axis) . This property



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( a)

L

(b)

A

k

PP

(C)

-•I 10

F µm

N [iO0]

Fig . 3 . Predicted (a, b) and actual (c) profiles for opposing sides of a mesa etched in a (511)-oriented GaAs slide using the 1 :8 :40 solution [ 7 ] .

Fig. 4 . Normalized three-dimensional polar plot of the etching rate of quartz in NH4 : HF 2 at 82 °C (different angle of view) [9] .

permits the realization of various shapes with steep flanks perpendicular to the z axis in any direction on `z cut' substrates . An accelerometer (Fig . 5) we have manufactured using this process [10] illustrates this possibility well . Since it is not directly relevant to micro-machining, the working principle of the sensor is summarized in the Figure caption . The magnetic field B,. is generated by permanent magnets, not represented in the Figure . The thickness of the wafer is about 150 pm . From the micro-machining point of view, the most critical dimension is the 5 pm beam width in the x direction, which requires a good control of underetching . Figure 4 is not sufficiently accurate to determine the etching speed of a surface parallel to the z axis . To control this very slow speed, a high quality Cr-Au mask deposited on a quartz surface exempt of lattice defects is necessary . Figure 6 shows the results we have obtained on a substrate etched for one hour in a 3 :2 by volume mixture of 48% HF :40% NH4F at a



127 500µm

feedback coil

seismic mass

feedback coil

suspension beam

differential capacitive detector

Fig . 5 . Micro-machined quartz accelerometer (a seismic mass is linked to the substrate by two beams flexible in the x direction ; a capacitive detector measures the mass displacement due to acceleration A x ; a Laplace feedback force IyBz counteracts the acceleration force as a result of the coil) .

3

L v0

C 2 7 V 0 m CC

7

direction of i; Fig . 6 . Etching rate of quartz for planes parallel to the z axis .

temperature of 85 °C . The etching rate in the z direction was about 2 .5 pm/min . Undercutting for directions other than the ones given in the plot is obtained by considering that the z optical axis and x electrical axis are

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respectively three- and two-fold rotation axes (y + 30 ° direction coincides with x direction) . Profiles other than flanks perpendicular to the substrate are possible to machine in quartz . Figure 7 shows an example of a 30 pm thick membrane we have etched in a 125 Am thick substrate . The membrane area is about 1 mm 2 . One of the problems with quartz is that the thickness of a membrane can only be controlled by the etch time . This requires the bath temperature to be precisely monitored . (A 1 °C change of bath temperature induces a change of about 0 .1 pm/min in the etching rate) . This problem is more easily resolved in silicon, which is the most commonly used material for obvious reasons : the best understood technology ; large surface/low priced substrates ; possibility of detector and circuit integration on the same substrate . Numerous articles have been written about this subject [11 - 18] with a complete review by Petersen in 1982 [11] . Some of the principal elements are recalled here . Nearly all the profiles are etched following the (111) planes ; these are the surfaces that are attacked the slowest . Principal anisotropic etching agents [11, 18] are : ethylenediamine-pyrocatecholwater (typically 750 ml-120 g-240 ml at 115 ° C) with an etch rate of 1 .25 µm/min and differential (100)/(111) etch rate of 35 ; hydrazine-water (typically 50%-50% at 118 °C in nitrogen environment) with an etch rate of 3 gm/min and differential (100)/(111) etch rate of 16 ; KOH-water (typically 44 g-100 ml at 85 °C) with an etch rate of 1 .4 µm/min and differential (100)/(111) etch rate of 400 . The most commonly used bath is KOH . This bath presents the best compromise between anisotropy, etching speed, dopant dependence and safety . The KOH bath attacks Si0 2 etching masks, but this problem can be resolved by using Si 3 N4, which is abundantly available from micro-electronic manufacturers . The most widely used substrates are (100) and the profiles are generally of membrane or cantilever beam type . Their dimensions perpendicular to the substrate are controlled using the `etch stop' technique [19 - 23] : a layer of p + , which acts as a membrane, is deposited epitaxially

Fig. 7 . 30 pm thick quartz membranes in a 125 pm thick substrate .

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on n silicon, which is attacked . This system ensures that the etching stops automatically on reaching the p + layer because the etch rate is slower in heavily boron-doped substrates (50 times slower for EDP on 5 X 10 19 cm-3 ; 20 times slower for KOH on 1020 CM-3 ; small dependence for hydrazine) . A more recent method consists of etching a reversely polarized p-n diode . When the attack reaches the junction, the diode disappears and the current passes, causing an anodic oxidation that stops the etching . The example of a supple membrane shown in Fig . 8 illustrates the possibilities of these `etch stop' techniques [25] . When flanks perpendicular to the substrate are needed (110) wafers can be used, but the sides of the mask must be perfectly aligned with the (111) planes, which considerably limits the permitted forms . In addition, if the

(a)

p

vv (b)

~

Fig . 8 . 1 pm thick supple membrane made by the dopant-dependent `etch stop' technique . (a) Boron diffusion (5 X 10 19 cm-3) on anisotropically pre-etched V-grooves . (b) Rear etching in EDP [25] .

Fig. 9 . Deep shafts (three at bottom, one at top left) which have been laser-drilled, then etched . Blocking (111) planes are visible in the other non-laser-drilled shallow pits [ 26 ] .

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holes are too small, the other (111) planes non-perpendicular to the substrate will stop the attack if the substrate has not yet been etched through, when only (111) planes are still in contact with the etching solution . An interesting solution to this problem is proposed by Barth et al. [26] . It consists of roughly piercing the substrate with a Nd-YAG laser, and then finishing the process using chemical etching, which will stop at the (111) planes (Fig . 9) . The combined use of different technologies, as shown in this example, probably has a promising future . Another technology that is now common in VLSI and will probably become more widespread in micro-machining is plasma etching.

3 . Plasma etching This technique uses a plasma instead of a liquid as the source of chemical reagents . It presents two main advantages when used for micromachining : the profiles obtainable, are not dictated by the crystalline orientation of the substrate and the plasma does not exert large forces on the microstructures . However, access to these advantages must be paid for by some increase in the complexity of the set-up and by having to manage a large number of parameters . Typical ones are : nature and flow of gases, nature and area of substrates and walls of the chamber, electrode structures, electromagnetic parameters of excitation and geometry of the chamber . Various combinations are possible, leading to numerous families of etching processes [27 - 30] . The ideal plasma micro-machining process incorporating the highest speed, anisotropy and selectivity, has not yet been found . Nevertheless, with the help of the research being carried out, it is reasonable to assume that such processes will become available within a few years . Only reactive ion etching (RIE), which presents the best compromise, will be reviewed here . The principle of RIE is shown in Fig . 10, with an example of silicon etching using CF 4. The plasma is obtained by glow discharge in a low-pressure gas (between 10-2 and 1 Torr) . The substrates are placed on the r .f. electrode so that a large self-bias potential difference exists between the substrate and the

disc

low

F, CF n

CF4

CF3 gas Inlet

d ischarge

SI F4 , CF n

.1 1 Si F4

T

r ng

dark

exhaust to pump

blocking capacitor

self bias potential

oRF power

Fig. 10 . Basic principle of RIE with CF 4 etching of silicon as an example .



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plasma (a few hundred volts) . This potential difference causes the positive ions to accelerate from the plasma to the substrate . The ion bombardment, which is perpendicular to the substrate if the pressure is not too high, can cause some directivity of etching to take place . The mechanisms most often proposed for the enhancement of chemical reactions on ion-bombarded surfaces are : (a) chemically-enhanced physical sputtering (radicals such as SiF X have a higher sputtering yield than elemental silicon) ; (b) damage-induced chemical reaction (creation of active sites by breaking atomic bonding) ; (c) chemical sputtering (chemical reactions activated by the ion beam energy giving rise to volatile products) . Polymer film deposition reactions also play a prominent part in the occurrence of anisotropy . Polymer films can sometimes deposit in a plasma and inhibit chemical reactions . Ion bombardment, by retarding the onset of polymerization, can help to maintain etching at the bottom of grooves when it is inhibited on the non-bombarded side walls . For example, when using fluorocarbon gases to etch silicon, the proportion of F to C is one of the determining factors [31, 32] (Fig . 11) . If the F/C ratio is between two and three, whether we have etching or deposition depends on the bias voltage . On the side walls, ions are seen as having zero bias voltage (kinetic energy `parallel' to the surface) and polymerization tends to dominate . On the bottom of grooves, the available energy is 200 eV and etching reactions dominate . One way to adjust the C/F ratio is to add H 2 or 0 2 to the gas . For example, if 20% H 2 is added to CF 4, the F concentration will decrease (creation of HF) and translate the working point from a C/F ratio of four (isotropic etching) to a value of between two and three . 02 has the opposite effect due to the creation of CO 2 . In fact, high anisotropy is difficult to achieve when using fluorinated plasmas because the fluorine remains very reactive, even in the absence of H2 Addition gases

C2F4

C4F10

C2F6

02 Addition CF4

4 fluorine to carbon ratio (F/C) of gas phase etching species

Fig . 11 . Influence of the fluorine to carbon ratio on the etching vs. polymerization processes in fluorocarbon plasmas [ 31 ].

132 ion bombardment . Anisotropic etching can only be successful when using these gases if the etching speed is reasonably slow . The more recently introduced chlorinated plasmas (CF 3C1, C 2 F 5 C1, CC1 4 etc .) are better adapted for use in the anisotropic etching of silicon (they are also used for the etching of Al, Cr, GaAs and InP) . The etching is achieved by Cl, and the polymerization by the CF 2 and CF 3 radicals. On the surfaces not bombarded by the ions, the chlorine reacts with absorbed species to form, for example, CF 3 C1 and therefore is prevented from attacking the silicon . As an example of how it is possible to play with the gas composition, Fig . 12 shows profiles we have obtained by using three different gas mixtures [33, 34] . When using SF 6 , the etching is rapid but isotropic . When using C 2 F 5 Cl, the etching is anisotropic but much slower . When a mixture of SF 6 and C 2 F 5 Cl is used, a compromise between speed and anisotropy is obtained . The mask used is Si0 2 ; it is the etching selectivity of Si/Si0 2 that limits the depth of etch . It is possible to achieve a selectivity ratio of about five and still obtain anisotropic etching . In terms of selectivity, RIE falls somewhere between physical sputter etching and chemical etching processes . In fluorine-based gases, the Si/SiO 2 selectivity arises from the fact that SiO 2 etches very slowly in the absence of ion bombardment, but this is only appropriate if isotropic etching is acceptable . With Cl-based gases, SiO 2 is etched very slowly even with energetic ion bombardment ; this is not the case for Si, so that some directional etching is possible along with high Si/Si0 2 etch rate ratios . Figure 13 shows 5 pm deep grooves in silicon obtained using CC1 4 with a 1 pm mask of Si0 2 . Selective etching of Si0 2 /Si is also feasible in hydrofluorocarbon gases such as CHF 3 . Anisotropy is easier to obtain because Si0 2 does not react with fluorine in the absence of ion bombardment . In this case we have an ionenhanced chemical reaction mechanism (the breaking of Si-O bonding by CHF 2 + ions, which facilitate Si reactions with absorbed F) . The slow etching of Si is due to absorbed carbon, which inhibits the formation of volatile SiF 4 . A selectivity ratio of 15 :1 has been obtained for Si0 2 etching speeds of the order of 300 A/min [35] . Absorbed carbon does not inhibit etching

SI C

Fig . 12 . RIE of silicon with three different gases : (a) SF 6, 30 cm 3 /min 10 mTorr, 80 W ; (b) C 2 F5 Cl, 30 cm 3/min, 10 mTorr, 60 W ; (c) SF 6 15 cm 3/min + C 2 F5 Cl 45 cm 3/min, 30 mTorr, 1 W/cm2. Thicknesses of profiles obtained are (a) 1 .7 pm, (b) 0 .35 pm ; (c) 0 .8 pm .

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Fig . 13 . 5 pm deep grooves etched in silicon with CC14, 10 cm 3/min ; N2 , 40 cm 3/min, 60 mTorr, 1 W/cm 2 .

of Si0 2 because it reacts with the oxygen of Si0 2 to give rise to volatile CO 2 or CO . From the micro-machining point of view, an etching depth of a few tens of micrometers is a bit limited and further developments have to be carried out if we want to etch anisotropically grooves a few hundreds of micrometers in depth. One of the promising methods consists of using microwave excitation [36], which enables increased reactive particle generation to occur independently of substrate polarization . Without polarization this process is naturally isotropic . If r .f. polarization is maintained, some amount of directivity can be kept . Using this method, silicon etching rates of 15 pm/min have been reached with a Si/SiO 2 selectivity of 100 . Figure 14 shows an example of 50 µm deep etching of silicon, which has been obtained in a SF 6/0 2 plasma at an etching rate of 5 pm/min and with a Si/SiO 2 selectivity of 60 . The pressure was 20 mTorr, HF input power 5 W/cm 2 at 2 .45 GHz and input r .f. polarization power 0 .3 W/cm2 at 13 .56 MHz . Some anisotropy is visible on the photograph, nevertheless it has to be improved and research

Fig. 14 . 50 pm deep etching of silicon using a microwave SF 6-02 plasma .

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is in progress to solve this problem . By changing the chemistry of the plasma and the masking material, very deep anisotropic dry etching seems feasible .

4 . Sacrificial layer methods This Section groups together the processes in which the mechanical object to be manufactured is not `machined' in the substrate, but in the thin layers that have been deposited on it . In this case, components are of the three-dimensional type more by their degrees of mechanical freedom than by their dimensions . Nearly all these processes use the `sacrificial layer' technique, which consists of liberating mechanical structures that have been deposited or doped by under-etching another underlying thin layer . Although probably the oldest technique [37], the `sacrificial layer' method has not achieved the hoped for industrial development . This is perhaps due to the poor mechanical quality of polycrystalline materials [381 . A revived interest in these procedures has nevertheless appeared in the last few years [38 - 411 because of the achievements in controlling the internal stresses of polycrystalline silicon [42] or other materials such as Si0 2 and Si3 N 4 . Another reason is probably that these `additive' procedures could be used to manufacture mechanical structures above integrated circuits, after they have been processed in a `classical' silicon foundry . This could solve the problem of compatibility of processes and then favour development of `smart sensor' concepts at a reasonable cost . Figure 15 shows a recent spectacular example where, for the first time, mechanical components fully free in rotation and translation have been manufactured using micro-machining techniques [43] .

1st polysilicon

2nd PSG

2nd polyellicon 1st polysilicon /

J .

re

iSSS

were

1st PSG

Fig . 15 . Polysilicon mechanical components fully free in rotation and translation obtained by the sacrificial layer method (2nd PSG is the sacrificial layer that will be underetched) [43] .



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A lot of processes are available to deposit layers and it is not possible, even briefly, to describe them here . Those giving good mechanical properties are generally of the CVD type . We would also like to mention evaporation, because it is the only process with some directivity . For three-dimensional structures, where it is necessary to define shapes of layers on walls other than the substrate surface, microlithography cannot be used . Pending the development of other processes such as laser-assisted deposition, this can only be done well by using mechanical shadow masks during evaporation . For example, this method has been used to deposit the electrodes on the flanks of the quartz accelerometer we have presented in Fig . 5 . A good illustration of the advantages obtainable using the directivity of the evaporation technique is given in Fig . 16 . This is the only example we have found of a component whose shape is defined during a deposition process . The pointed profile is obtained thanks to the progressive closing of the hole in the substrate that occurs during the evaporation . The `closure layer' is removed afterwards by means of etching the Ni layer . These micro-tips are used as an electron cold source [44], which shows that even good old vacuum tubes could be micro-machined .

closure layer

axis of evaporation and rotation

Fig . 16 . Sharp microtips obtained thanks to the high directivity of evaporation [44] .

5 . Conclusion We have looked at the technologies known as 'micro-machining', considering them above all as a new way of working material . It seems to us that the enormous potential that exists for cost reduction, miniaturization and improvement of the reliability of these techniques alone justifies that they should be widely applied to sensors even independently of the `smart sensor' concept, which is closely linked to silicon [45] . Evidently the examples we have given here show that silicon technologies will remain dominant for a long time to come, the economic reasons weighing as heavily as the technical ones . However, other materials ideally suited to the needs of sensors can be worked in the same way with the same advantages, GaAs and quartz being the most obvious examples . In

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this field, there is probably a great reservoir of innovation that only our imagination prevents us from exploring .

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