Metallization Techniques

VLSI ELECTRONICS: MICROSTRUCTURE SCIENCE, VOL. 15

Chapter O Metallization Techniques D. W. SKELLY, T.-M. LU,* and D. W. WOODRUFF General Electric Corporate Research and Development Schenectady, New York 12301

General Considerations A. Introduction B. Vacuum Considerations C. Wafer Preparation D. Sources of Material Deposition Techniques A. Sputtering B. Chemical Vapor Deposition of Metals C. Evaporation D. Molecular-Beam Epitaxy (MBE) E. Ionized Cluster Beam Deposition Techniques References

101 101 102 104 105 107 107 119 141 145 149 153

I. GENERAL CONSIDERATIONS A. Introduction

The major driving forces for selecting new metals for VLSI, other than aluminum, which now holds a prime position in the industry, are electro­ migration and step coverage. Electromigration now accounts for a large fraction of device field failures. Although there are many alloys of alumi­ num that extend the electromigration life of devices, as the devices decrease in size and the number per chip increases, the current density required in the metal leads increases and statistical electromigration failures occur sooner. Ultimately, this limits the frequency at which high density chips may be operated [1]. Step coverage, already borderline in larger devices Present address: Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York, 12181. 101 Copyright © 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

102

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

(> 2 μπή, becomes unacceptable with aluminum and its alloys in smaller devices using multilayer metalization at these higher current densities. VLSI metallization has many criteria that must be met by the new materials and processes, depending on specific design rules and objectives. In general, metal layers are judged on resistivity, etchability, contact resist­ ance to underlying and overlying structures, step coverage, electromigration, adhesion to oxides, high-temperature processability, compatibility with subsequent chemical processing, and ease of processing. All of these factors must be considered when choosing a metal or process. It is clear that the VLSI industry is working within these criteria toward greater use of refractory metals, such as tungsten and molybdenum, to eliminate electromigration, and the chemical vapor deposition process, in order to improve step coverage. This chapter will treat in detail five techniques of metallization that may be used in VLSI. Sputtering and evaporation are the most commercially advanced and widely used methods to deposit various metals on many different types of devices. The more important features and aspects of these methods are described here in an overview fashion, but vast literature is available for specific materials deposited by these methods. Chemical vapor deposition (CVD) has received much recent attention and is becom­ ing commercially available specifically for VLSI applications. It is expected to have complimentary and competitive features when compared to sput­ tering and evaporation and is discussed in the greatest detail. Molecularbeam epitaxy (MBE) and ion cluster beam deposition, although they are presently applied to a very small group of devices on a developmental basis, are presented because they produce materials having unique proper­ ties not available by other means. Much of the VLSI market is expected to be in custom devices, and the preparation of such chips will require a broad spectrum of materials and compatible processing techniques for optimum versatility, especially as the number of levels of metallization increases. B. Vacuum Considerations

Most vapor deposition systems require the best vacuum feasible with considerations made for film quality, throughput, and cost. It is well known that impurity atoms arriving at the surface of a sensitive, freshly deposited film will react rapidly with the film atoms. Even if the surfaces do not react, the impurities may cause alterations in nucleation and growth of the film, which can seriously change properties such as contact resist­ ance, adhesion, resistivity, and grain size. For many years diffusion pumps have served as high vacuum pumps,

3. Metallization Techniques

103

but the need for higher quality metal films is displacing this type of system. Well-trapped oil-based systems do not contribute to contamination but, in regular manufacturing use, operator errors or equipment failures often lead to diffusion of oil vapor into the deposition chamber. Even low-vaporpressure oils, adsorbed on surfaces, can be knocked into the vapor state by stray ions, electrons, or atoms from the deposition. Diffusion pumps are being rapidly replaced by closed-loop helium refrig­ eration pumps. These pumps have extremely high pumping rates for con­ densable gases and often eliminate the need for liquid nitrogen trapping. Operator error may result in 4 to 6 hr of lost time to regenerate the pump system but does not result in system contamination. Adsorbed and trapped gases, on all of the system walls and fixtures, will continue to outgas for prolonged periods of time. These gases, released into the system during deposition, continually bombard all the surfaces. Impu­ rity gases (e.g., H 2 0, C0 2 , CO, 0 2 , N2) may make up a large fraction of the film content if the deposition rate of metal is comparable to the arrival rate of impurities. In most cases high-rate deposition systems with the lowest impurity gas pressure will produce the highest quality films. Some deposition systems do not have load locks; that is, the main deposition chamber must be opened every time a set of wafers is loaded. Except with prolonged pumping, possibly with heating, these systems tend to produce films of higher resistivity than films produced by load-locked systems, especially with reactive materials such as aluminum, molybde­ num, titanium, and tungsten. Load locked systems or deposition techniques using a high flow rate of pure gases will be advantageous in maintaining film properties. Recent systems using load locks with vacuum systems comparable to the main deposition chamber further prevent exposure of sputtering or evaporation sources to atmospheric gases. Load locks with heaters allow outgassing of wafers and carriers prior to exposure to the deposition chamber. The advent of inexpensive quadrupole mass spectrometers makes it possible, as well as advisable, to continuously monitor the deposition gases to unambiguously identify contamination gases. Most CVD processes developed in recent years are low-pressure pro­ cesses (LPCVD) and thus also use vacuum equipment, although the re­ quirements are somewhat different. In LPCVD, one must maintain pres­ sures in the 0.1-10-Torr range with continuous flows of hundreds to thousands of standard cubic centimeters per minute (seem), in many cases using reactive gases. Systems of this kind typically use a rough pump with fore traps to reduce pump oil contamination and pump oil filtration to remove particulates and acids. One common addition to increase pumping speed is a Roots blower package. Future systems may require high vacuum

104

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

initial pumpout in addition to load locks to maintain lower background levels of air and moisture. Where hydrogen fluoride is present, many of the typical pump oils will react [2]. The pumps used to maintain the high flow of these systems also require special consideration to maximize the reproducibility of film properties [3]. C. Wafer Preparation

The preparation of the wafer surface prior to deposition is a critical step. Surface contamination can reduce adhesion of metals to silicon, silicon oxide, nitrides, or other metals and can increase contact resistance to other conductors. Surface contamination can result from cleaning steps, natural oxides, adsorbed gases, etching and photoresist residues, as well as inten­ tional doping of oxides and silicon for special properties. Impurities can act as nucleation sites on which the entire metal film can grow, reducing direct contact of the metal to the surface of the wafer. This means that a large fraction of the surface may not contribute to adhesion. The arrival energy of sputtered atoms is considerably greater than that of evaporated atoms [4] (Fig. 1); thus sputtering has a greater probability of altering the wafer surface and desorbing impurity atoms. Even when the arriving atom does not have sufficient energy to react with the impurities it may in fact disrupt the surface sufficiently to enhance both contact resistance and adhesion. Palladium, for example, will react with the silicon to form a suicide even in the presence of grown oxides. Chemical vapor deposited tungsten reacts with silicon to a depth that thoroughly mixes the interfacial silicon with tungsten. Sputter etching, has historically been considered an excellent method of pretreating a surface prior to deposition. The wafer is made into a sputter cathode by applying an rf potential to the surface. The bombarding ions strip the surface of a few or a few thousand atomic layers to expose fresh

KINETIC ENERGY (eV)

Fig. 1. Range of arrival energies for atoms and ions at the wafer surface from various sources. (From Takagi [4] with permission Elsevier Sequoia S. A.)

3. Metallization Techniques

105

material. Sputter etching can be applied as a cleaning method even though sputtering is not used to deposit the metal. Unfortunately this technique may bury neutral or charged sputtering gas atoms in the surface of the silicon or oxide. In addition, the wafer carrier is also sputtered, and atoms of the etched carrier surface can be redeposited on the freshly exposed wafers. In some systems, exposed rf power leads can also support a dis­ charge that can generate impurities that will be deposited during cleaning. Reactive ion etching has been recommended as a cleaning process with­ out reexposing the wafers to atmosphere before deposition; however, welltested commercial equipment have not yet appeared, which are directly attached to deposition equipment. In those cases where the reaction prod­ ucts are very volatile, the surface can be left exceptionally clean. Care must be taken in the selection of these processes because nonvolatile residues can form and further complicate the cleaning process. Although poor deposition technique may result in contamination of the metals with oxygen, nitrogen, or other reactive species that may cause poor adhesion or high contact resistance; most often, ineffective premetallization cleaning is at fault. D. Sources of Material

All of the methods of deposition discussed in this chapter can increase the amount of impurity in the starting materials. It is best to start with the highest grade of metal available. The most serious impurities are iron, copper, gold, sodium, lithium, or potassium, which are rapid diffusers in silicon, as well as arsenic and boron, which can modify doping levels. There are limits of cost, availability, purity, and form. Not all elements that are available in limited quantities for engineering are also available for production in the shape required or the purity level desired. Most sputter­ ing sources for production, for example, are rather large in order to provide uniformity of deposition thickness over the wafers. These large complex shapes must be formed or machined (see Fig. 2). Most often they are hot or cold pressed from powders of various purities or machined from large pieces formed by vacuum melting. Pressed powder sources can be porous, and the inner surface of the pores can be filled with various gases absorbed during the preparation processes. These gases will be released during the deposition process and can be incorporated in the final film. In addition, contamination of the starting powders may originate in the ore from which the element was obtained or may result from the extraction process itself. Tungsten is a good example, because its development as a

106

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

Fig. 2. Partially spent magnetron sputtering targets of various shapes. Location of mag­ nets produces a variety of erosion patterns in the originally flat-metal surfaces.

metallic layer is being accelerated in VLSI applications. Both the ores and the methods of concentration of the ores [5] contribute to many forms of contamination. VLSI materials make up a very small fraction of the total use of these starting powders (tungsten carbide abrasive making up the major fraction) so that most suppliers begin with powders having the acceptable impurity levels for other applications. In recent years, at least one manufacturer has prepared special material for use in VLSI, with special attention being paid to sodium levels as well as alpha ray emitting impurities, which contribute to random errors in memory chips [6]. It is interesting to note that in 1986 tungsten was selling as ultra-highpurity commercial sputtering targets, at up to $2000/kg; as 99.97% com­ mercial sputtering targets, at up to $500/kg; as tungsten hexafluoride for CVD deposition gas at up to $4050/kg of tungsten; and as tungsten com­ mercial powder (99.95%) for $21/kg [7]. The major tasks are purification and target shape formation in the case of sputtering. Film costs are usually not an issue in custom circuits, but are a major issue for large volumes of VLSI devices.

107

3. Metallization Techniques II. DEPOSITION TECHNIQUES A. Sputtering 1. Basic Principles

Sputtering is a well-established deposition technique with sophisticated and automated equipment available for metallization. It is presently the method of choice for most VLSI applications. Details of the sputtering processes and mechanisms are well covered in Refs. 8-10. A basic diagram of the sputtering process is shown in Fig. 3. It is based on the bombardment of a solid surface or "target" with energetic ions, which knock off surface or near-surface atoms by energy transfer. These released neutral atoms land on the wafers to become part of the film coating. In all types of sputtering, the source or target material is negatively charged by a power supply. Electrons leaving the surface of the target travel toward an anode or positive electrode until they strike and ionize one or more atoms in their path. These atoms are usually an inert gas such as argon. The positive ions formed in the collision will be accelerated back to the negatively charged target surface. The energy of this collision is deposited in the target crystal lattice as phonon excitation. Some of these excitations lead to the release of neutral atoms from the surface, which travel to the substrate in a random way and grow into a film. The deposition rate (À/sec) of metal will depend on the number of ions arriving at the target surface, the efficiency with which they sputter mate-

CATHODE (TARGET OR SOURCE OF METAL) GLOW DISCHARGE REGION

/

WAFER (SUBSTRATE) Q

= METAL ATOMS

Fig. 3. Basic sputtering diagram.

/

CATHODE DARK SPACE

108

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

rial from the surface, and the distance from the target at which the rate is measured: D ex J · S Id,

(1)

where D is the deposition rate, / the inert gas ion current density, S the sputtering yield for the particular metal, and / the target-substrate dis­ tance. For a discharge used for sputtering, the current density is proportional to the voltage F applied (Fig. 4) as well as the pressure P of gas present [11] Joe V». p9

(2)

when /7 is 1-2 depending on the discharge configuration. The efficiency S with which atoms are sputtered off the target surface (atoms sputtered per ion arriving) is dependent on the relative masses of the arriving ions and the sputtered species, the energy of the ions E, and the energy required to remove an atom from the surface U (heat of sublima­ tion) S*E-

λ'/U,

(3)

where λ' is a function of the relative masses of the ions and atoms. Table I shows the efficiency for the release of atoms by argon ions of various energies. Each element has a different efficiency at the same ion ALUMINUM TARGETS 6000

h

PLANAR DIODE Ar PRESSURE = 6.5 Pa

3000 CYLINDRICAL-POST MAGNETRON Ar PRESSURE = 0.13 Pa

DISCHARGE VOLTAGE 1000 (V) 600 300

RECTANGULAR TYPE PLANAR MAGNETRON Ar PRESSURE = 0.13 Pa

CYLINDRICAL-HOLLOW MAGNETRON Ar PRESSURE = 0.13 Pa

100

I

I I I I Mill

RING TYPE PLANAR MAGNETRON Ar PRESSURE = 0.13 Pa

J

» I I I I III I

0.3 0.6 1 3 6 10 DISCHARGE CURRENT (A) Fig. 4. Typical current - voltage characteristics for a planar diode sputtering source and for various types of planar and cylindrical magnetrons. All sources operating with Al targets at the Ar working-gas pressures indicated. (From Bunshah [10]. Courtesy Telic Company.) 0.1

109

3. Metallization Techniques TABLE I Sputtering Yields (Atoms Sputtered per Ion) of Some VLSI Metals0 Bombarding Argon Ion Energy (eV) Metal

100

200

300

600

AL Cr Mo Pt Ta Ti W

0.11 0.30 0.13 0.20 0.10 0.08 0.07

0.35 0.67 0.40 0.63 0.28 0.22 0.29

0.65 0.87 0.58 0.95 0.41 0.33 0.40

1.24 1.30 0.93 1.56 0.62 0.58 0.62

a

From B. Chapman, "Glow Discharge Processes," p. 369. Wiley, New York, 1980.

energy, depending on the mass and binding energy of that element in its lattice. This means that the rate of removal of atoms from the target will be different for each element, and some elements will sputter more efficiently than others. There is a maximum ion energy that leads to a maximum efficiency for sputtering. At energies less than the maximum, the fraction of energy which is coupled into removal of atoms is less; at energies greater than the maximum, the depth of penetration of the ion into the target is too great for excited atoms to reach the surface. The maximum deposition rate will be governed not only by the basic efficiency of sputtering for a particular element but by practical limitations of power available to the target, voltages applied without breakdown of insulators, maximum wafer and target temperature, stable discharge pres­ sures, and film properties such as stress and resistivity, which may be a function of the deposition rate. 2. The Presence of Foreign Vapors

The inert gas must be present to sustain the discharge that provides ions for bombardment of the target. Because sputtering is an energy transfer process, heavier inert gas atoms will give a greater sputtering yield or efficiency per ion bombarding the target. The heavier gases, kryton and xenon, however, are much more expensive. Argon is comparatively plenti­ ful and represents a very practical compromise between the increased efficiency and cost. Although the gas is inert, it may be incorporated into sputtered metal

110

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

films, sometimes more than 5 at. % [12] and, depending on its mass, may effect properties such as stress. Impurity atoms, such as water, carbon dioxide, or oxygen, contained in the gas can be even more important to film properties. The highest grade of gas compatible with process cost and film properties should be used. The presence of both inert gas and discharge is significantly different from evaporation, in which high vacuum and long mean free paths can give very pure films. Inert gas atoms and ions colliding with walls can dislodge reactive gases from the deposition system hardware. At the pres­ sures required for efficient discharges the collision rates of reactive gas molecules with fresh film can be high, thus to minimize impurity effects the highest practical deposition rates are usually sought. The instantaneous deposition rate for high-quality aluminum, for example, is usually in excess of 10,000 À/min. The method of bonding the sputtering target to the cathode can contrib­ ute significantly to film properties. Many targets are soldered, epoxied, or clamped to a water-cooled backing plate. The thermal conduction may be inefficient because the contact is poor or the water cooling is insufficient for the amount of power being applied to the discharge. As the target temperature increases, the substrate will receive more heat from radiation, and the solder or epoxy (if any) will increase in temperature. If the target is made of pressed powder there may be sufficient voids for diffusion of the bonding material to the target surface resulting in contamination of the wafers with indium, tin, hydrocarbon, or residual flux materials that may contain sodium or other fast diffusors. In the worst case, the target may fall off of the backing plate. Target purities are often quoted on the basis of as-received powders by the vendor, rather than finished, bonded targets, since the bonding process can contribute to contamination. 3. Film Uniformity

The rate of erosion for sputtering targets of all types is nonuniform, and wafer motion of some kind must be used to achieve film uniformity. Figure 2 shows the erosion patterns of several types of sputtering cathodes. The film uniformity of a particular system will be the result of a combina­ tion of cathode size and erosion pattern, wafer motion, and temperature. 4. Step Coverage

The degree to which deposited metals cover steps over topography is important to the yield and reliability of devices. In evaporated and sput­ tered films the most difficult steps to cover are those with straight walls. This is especially true with the aluminum and its alloys. But in many cases

111

3. Metallization Techniques

it is desirable to have straight-walled steps, as in small vias, so that the density of the design can be maximized. Often it is easier to produce the straight walls by dry etch techniques. The normal step coverage of aluminum on straight-walled vias is shown in Fig. 5a. When the aspect ratio (diameter/height) is 2/1, the step coverage is typically < 20% for aluminum alloy films and decreases to less than 5% as the aspect ratio approaches 1/1. This conforms remarkably well with theoretical predictions [13,14]. While tapering of the via markedly im­ proves the step coverage the increased area needed to accomodate the larger top of the via adversely affects the circuit density. One of the most important conclusions of the theoretical works is that simple changes in sputtering or evaporation parameters that affect the metal atoms before they arrive at the wafer surface will not significantly change the step coverage on small deep vias. This includes, in particular, those parameters that broaden the angle of arrival of the atoms at the surface, such as introduction of planetary motion or use of a source of larger area. Therefore the only parameters that can affect the coverage are those that can provide rearrangement of atoms that have already arrived on the surface (e.g., temperature, ion bombardment, and elimination of species that retard surface migration). Smith [15] and Skelly and Gruenke [16] have reported that dc-bias voltage applied to the wafers at relatively low-power levels can lead to improved step coverage (Fig. 5b). Similarly, at low rf power, Mclntyre and Wright [17] saw this effect. These authors suggest a mechanism of in­ creased surface mobility due to ion bombardment, but the temperatures at which this occurs are very near the melting point of aluminum where mobility is already high and is only aided by ion bombardment.

(a)

(b)

Fig. 5. Step coverage of aluminum in a small, straight-walled via, 1-μπι, deep by (a) dc magnetron sputtering and (b) dc magnetron sputtering with a dc bias applied to the wafer. Aluminum has been etched away from under a brittle topcoat which more accurately defines the aluminum profile.

112

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

Homma and Tsunekawa [18] reported that rf-bias sputtering of alumi­ num, at high bias power, can greatly improve step coverage in a similar manner to the improvements and mechanism for rf-bias sputtered quartz [19]. Mogami et al. [20] have reported identical results for molybdenum depositions. In these rf-bias cases a great deal of energy is applied to the bias. Under these conditions, the wafers are being simultaneously coated and etched. The bias power, applied to the wafers turns them into sputtering targets, from which material is being eroded at the same time that the normal sputtering target is depositing material onto the wafers. Figure 6 shows the mechanism for bias-sputtered quartz. Sputtering is most efficient at corners of steps where the angle of incidence is nearer to 45°. The bias power is most effective at eroding material from the corners of steps where buildup can shadow those portions least effectively coated by line-of-sight deposition. This process does not enhance the coating of steps but slows the buildup of films on areas that can shadow the coating of the step walls. Although bias sputtering techniques have been known for a long time they have not been applied to metals in a routine way for improved step coverage. Commercial equipment is only beginning to respond to this need for manufacturing. Most of the equipment presently installed cannot be converted to rf-bias capability conveniently. Even low levels of ion bombardment can improve step coverage. The number of ions that arrive at the wafer surface in a particular system will be an accidental characteristic of the design. Step coverage may therefore vary depending on the type of sputtering system used. Step coverage in aluminum films is closely associated with surface tem­ peratures [14]. Those systems that elevate the temperature, either as a control parameter or because of the type of sputtering system used, may enhance step coverage. This is especially true of the low-melting materials, such as aluminum and its alloys. In higher melting materials, such as tungsten or molybdenum, very little increased mobility is expected. Atoms that arrive are rapidly accommodated and demobilized. The temperature of wafers during deposition is not well known. The heat flux from the sputtering system on wafers caused by ions, electrons, and photons for some types of cathode configuration has been described by Class and Hieronymi [21]. The actual temperature achieved by a particular wafer will be a complex function of energy absorption and emission char­ acteristics of both the front and back of the wafer as well as those of the wafer carrier and system walls, and may be a function of the VLSI circuit design density and previous layers. It will also be a function of the sput­ tered material because a great deal of energy is carried by secondary electrons and reflected inert gas neutrals, which may be more efficiently

3. Metallization Techniques

113

BIAS ETCH RATE PER UNIT POWER (A/Min/kW)

0

25

50

75

100

125

150

NEGATIVE SUBSTRATE BIAS VOLTAGE (V)

(a) PLANARIZED Si0 2 COATING

Fig. 6. Schematic of planarization of sputtered Si0 2 over steps by an rf bias applied to the wafer: (a) bias etch rates on flat (O) and 45° sloped surfaces (Δ) as a function of wafer bias and (b) effect of differences in etch rate on Si0 2 film growth. (Courtesy Temescal, a division of the BOC Group Inc.)

produced by particular alloys or elements. Although temperature can be measured in particular cases, there is no general method for determining the actual wafer temperature, and most system readout is remote from the wafer and not accurate. 5. Adhesion

It has been shown [22,23] that significant changes in nucleation, adhe­ sion, crystalography, and morphology can be produced in films with even a

114

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

small flux of ions on the surface, in addition to the neutral species. The degree of bombardment will depend on the accidental and incidental equipment design. In the future, sputtering and other deposition processes will include ion bombardment sources to enhance adhesion and other film properties in a controlled manner. It is unclear whether the improvement in adhesion arises from the ion bombardment effect on nucleation or on dislodging of poorly adherent impurity atoms that may block the interface. Film stress can strongly affect adhesion. All of the stress in a film eventually is applied to the single layer of interfacial atoms at the substrate surface. The strength of this bond is limited and is never as great as the interatomic bonds between pure metal atoms unless there has been signifi­ cant chemical reaction. The stresses in films arise from several sources in addition to thermal stress associated with the difference between the expansion coefficient of the film and that of the substrate. Thermal changes can occur during film growth. Often the wafer is at room temperature at the onset of deposition and can be more than 450 °C higher at the end of deposition. It is well known that grain size and shape are strong functions of the deposition temperature [24,25]. The change in growth pattern caused by changes in deposition temperature can cause severe stress. In some cases film stress can be controlled by adjustment of sputtering parameters such as deposition power and inert gas pressure [26a]. In the case of low-melting materials, these superimposed process-related stresses will be relieved by thermal annealing, and the value of the residual stress returns to that related to thermal coefficient of expansion differences. Films of high-melting metals will be highly stressed and less affected by annealing. Clearly, the interfacial bond depends also on the nature of the metal and the bonding surface. In the case of oxide surfaces strong bonds can be made if the metal can react to reduce the oxide to form a mixed interface. But under the best of deposition conditions, with clean interfaces and low film stress, some films do not react and show poor or borderline adhesion on silicon oxide surfaces. Such is the case with molybdenum and tungsten on silicon oxide, making it difficult to use these materials with confidence, directly on oxides. 6. Variations of Sputtering

There are several important variations on the sputtering process and each has unique deposition characteristics that affect the film properties. Each of the popular types will be discussed with respect to those known

115

3. Metallization Techniques

characteristics. The detailed descriptions of these processes have been care­ fully provided in Refs. 10 and 11. a. Diode Sputtering. Diode sputtering is shown in Fig. 7. It is character­ ized by elevated substrate temperatures, severe electron bombardment of wafers, and limited deposition rates. The elevated wafer temperatures are related to the electron bombard­ ment of the wafers. In this discharge, the wafers are part of the electrical circuit, and a significant number of energetic electrons arrive at the wafer surface. In the diode discharge, electrons leave the surface of the target (source of material), which is negatively charged. These electrons are accel­ erated across the dark space where they collide with argon atoms and ionize them. The primary electrons along with the electrons resulting from the ionization may still be within the accelerating field of the dark space and may be accelerated across it without another collision. Once in the field free region of the glow, these electrons are free to travel to the wafers, carrying whatever energy remains after their collisions. The amount of energy remaining will depend on the number of collisions during the transit across the glow region, which in turn depends on the inert gas pressure. The deposition rate in diode systems is ultimately dependent on the limit of power that can be applied to the discharge. As the target voltage is increased, the deposition rate will increase, as shown in Fig. 4. For voltages above 1000 V, special precautions have to be taken to prevent arc discharges both inside and outside the system. These arcs can breakdown insulators. Diode sputtering, as well as all the other variations, are relatively energy

•

Cathode

·

Target material

πΦ

•

σ;: b

•

Argon I

ô. il

#

^ Ö/ö.\. ò.

·\

<■>

ò Substrate

ò· ·

-

#

|

1· ι — • Argon molecules ΘArgon ions • Electrons OTarget material atoms

· |

1 ·' \^ZT* »

Fig. 7. Diode sputtering. (Courtesy Circuits Processing Apparatus, Inc.)

116

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

inefficient processes. Only about 2% of the energy applied to the discharge results in the deposition of films. On the average, a 600-eV ion will only produce one or two sputtered atoms with less than 10 eV of energy. The remainder appears as heat either on the cathode, anode, or system struc­ ture including the wafers. In a rough approximation, 90% of the energy appears on the cathode surface as heat during the bombardment with ions. This elevates the temperature of the target surface and contributes to the heating of the substrate by radiation. b. Magnetron Sputtering. Magnetron sputtering is presently the method of choice for VLSI because the wafers are significantly cooler and the power to the target can be increased to achieve ten times the deposition rate of a diode target. Figure 8 shows schematically a magnetron sputtering system. It differs from the diode system primarily in having a strong magnet set located (usually) behind the target, outside the vacuum chamber. The poles are arranged to confine electrons that leave the target to a region very near the surface by constraining them in a helical path around the magnetic field lines. Most of the electrons are reduced to a very low energy because they collide with more argon atoms in their longer helical path. They cannot escape the field to carry heat to the substrate. In addition, the cathodes are often configured with a separate anode that is positively charged and can collect the thermalized electrons, almost completely removing the wafers from the electrical circuit. The impedance of the discharge using magnetron cathodes is lower than

• 1

er: Argon

· Magnet · Ί

L

· Magnet

Target material

·#

#

'

· '

<#
• Argon molecules Θ Argon ions • Electrons O Target material atoms

Vacuum· pump

Fig. 8. Magnetron sputtering. (Courtesy Circuits Processing Apparatus, Inc.)

3. Metallization Techniques

117

that for simple diode cathodes. This means that much greater power can be applied to the magnetron cathode without exceeding the limits of the electrical isolation. The voltage applied to the cathode can either be dc or rf. Most metal depositions are made by dc. The rf system is usually operated at 13.6 MHz. The usual application for rf is the deposition of insulators in cases where the voltage applied is at such a high frequency that the charging and discharging of the capacitor creates a plasma, even though one of the electrodes is an insulator. By alternating of the voltage, the target during one cycle is the cathode and during the next becomes the anode. When this occurs the wafers can become the cathode and therefore can be bombarded by high-energy positive ions, which can cause such damage as buried charge in oxide layers and often trapped argon. There are many variations of the magnetron configuration [11]. The magnetic field can be induced by either electomagnets or permanent mag­ nets. The cathode can even be rod shaped. The rod is placed as the axis of a right cylinder and the wafers located on the inside of the cylinder facing the rod. This configuration accommodates a large number of wafers coated in a batch fashion. The magnetic field allows the shaping of the deposition plasma to accommodate a variety of wafer handling processes, from largebatches to single-wafer, cassette-to-cassette systems. c. Alloys. Sputtering is superior to all other forms of thin-film deposi­ tion for metal alloys. The sputtered vapor, as it leaves the target surface, will have nearly the same composition as the bulk after the target surface has been sputtered long enough to reach equilibrium. However, during the transport of atoms to the wafer surface and before the newly arrived atoms are fully accommodated by deposition film, some atoms can be lost, resulting in significant shifts in stoichiometry. The major losses are due to scattering of atoms by the inert atmosphere (argon) and by preferential reevaporation of atoms from the wafer surface. For example, films produced by sputtering targets containing tungsten/ 10% titanium can be reduced as low as tungsten/5% titanium in the film [26b and c]. This is probably caused primarily by the large difference in the atomic masses of tungsten ( 184) and titanium (48), which results in a larger scattering angle for titanium. More titanium is therefore lost to the system walls. The sputtered vapor, widely scattered from the target, will be highly enriched in the lighter elements. This fact may impact wafers produced in those sputtering systems where wafers are transported past a sputtering target for the sake of thickness uniformity. Wafers, in this case, will initially see vapor enriched in the lighter elements at the periphery of the target. This may have significant effects upon contact resistance, which is depen­ dent primarily on the initial metal layer interface with silicon.

118

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

In an equilibrium situation, with a well-cooled target, the target surface concentration, scattering losses and film composition will remain constant. Once the target composition has been corrected to account for losses, film composition will remain quite accurate. In general, the theories of multicomponent sputtering have not been worked out well enough to permit an accurate prediction of the composi­ tion of the film whether the target is a compound, alloy, or mixture. The film must be analyzed and the composition corrected empirically. Targets consisting of very coarse agglomorates of crystals of the various components are suitable sources, if the sputtering efficiency of the compo­ nents does not differ so greatly that grains of certain components are sputtered away before fresh material is exposed. In the limit of very large pieces, the target can consist of an array of overlapping plates of metals. Plate area can be adjusted to correct the composition, and depletion of components can be monitored visually. Coarse targets of this type are more suitable for surveying of a series of compositions rather than manufactur­ ing. Fine-grained, dense, homogeneous targets of alloys or mixtures are better suited for consistent manufacturing. d. System Considerations. One of the most serious problems confront­ ing VLSI is the control of particulates. Like all mechanical systems, sput­ tering and evaporation equipment do generate particulates, depending on the type of transport mechanism used. In addition, because the deposition process is isotropie, most of the mechanical components of the system become coated with significant quantities of metal. Many of components of the system are much closer to the target than the wafers and receive much heavier deposition, often several millimeters thick over the life of a target. Batch systems, which expose these heavy coatings to atmospheric gases on a routine basis while loading wafers, will generate much higher stress levels in the films due to oxidation and adsorption of gases, and therefore will generate much more particulate due to flaking off of the layers. Those layers which do not flake off will continue to reemit significant quantities of the adsorbed gases during deposition, reducing the purity of deposited metals. In general, systems not equipped with load locks will not be acceptable for VLSI. Systems opened for maintenance must be thoroughly cleaned of films exposed to atmosphere before continuing to process, in order to maintain consistent film properties and a low number of particles. Whether the system has a load lock to prevent exposure of the deposition chamber to atmospheric gases or the entire chamber is vented to atmo­ sphere, the venting process can generate a significant number of particles. Vent gases, even from liquid nitrogen boil off, can generate massive quan-

119

3. Metallization Techniques

tities of near and submicron particles. Line filters, often not supplied by equipment vendors, are necessary. Sputter-up and sputter-sideways systems (Fig. 9) significantly reduce particles because they are drawn away from the wafers by gravity. But particles from fracturing films are often ejected at high velocity and may be electrically charged, so that gravitational forces are easily overcome. These systems reduce but do not eliminate particles. B. Chemical Vapor Deposition of Metals 1. Introduction

Chemical vapor deposition (CVD) has been used as a deposition tech­ nique for many years [27] and for a wide variety of end uses. Its use in microelectronics is widespread as well: for epitaxial silicon, polycrystalline silicon, silicon dioxide, and silicon nitride. Although metals have been deposited by CVD for microelectronics since as early as the 1970s [28], this area has only recently found substantial commercial acceptance. Included in this discussion are the refractory metal suicides in that they are good conductors, and the development of CVD metal processes is strongly tied to that of the suicides. The major difference between CVD and sputtering as a deposition rWAFER

r

r

TARGET

PARTICLES

τχπ

PARTICLES

WAFER

TARGET (SOURCE OF METAL) "SPUTTER-DOWN"

"SPUTTER-UP"

h

WAFER

K1 PARTICLES

Fig. 9.

"SPUTTER-SIDEWAYS" Some target orientations and probable resulting particle trajectories.

120

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

technique is chemistry. Whereas sputtering is a technique that creates a supersaturated metal vapor from which the metal condenses on a "cold" surface, CVD uses volatile compounds containing the deposition species as transport agents, with these agents being chemically reacted on a "hot" surface, creating the desired deposit and other chemical species as the reaction product. Although this difference between the techniques may be obvious, it has major implications concerning the deposit properties and its interactions with the rest of the circuit components. This difference can result in both advantages and disadvantages, making it a process which both compliments and competes with sputtering. Chemical vapor deposition has two major advantages that make it unique and very attractive as a deposition technique. First, it produces deposits that generally have excellent step coverage. Figure 10 compares electron micrographs of sputtered aluminum and CVD tungsten over comparable structures. The improvement is obvious and quite important in terms of metallization schemes. The second advantage for CVD is selectivity. Under appropriate conditions for certain CVD reactions, depo­ sition can be limited to metallic or "reactive" surfaces (such as silicon) and prevented from occurring on insulator surfaces such as oxides and nitrides. This technique has been widely explored for CVD tungsten and is poten­ tially available for other CVD materials as well (e.g., selective silicon and gallium arsenide have been reported [29]). Selective deposition is selfaligned and requires no photolithographic steps, improving process reli­ ability while it simplifies the process. Chemical vapor deposition of metals is a technique in the throws of rapid development and discovery. Much is yet to be discovered about reaction mechanisms, appropriate reactor designs, new CVD materials,

(a) (b) Fig. 10. Comparison of step coverage of (a) CVD tungsten and (b) sputtered aluminum over similar topography. Note that aluminum is etched away from between two Si0 2 layers in this SEM.

3. Metallization Techniques

121

and interactions with device surroundings. This section is an attempt to review what is known to date on this subject, and to introduce the appro­ priate chemistry and reactor design concepts for a more complete under­ standing of the process. 2. Fundamentals of Chemical Vapor Deposition

Chemical vapor deposition (CVD) covers a wide range of deposition techniques which have, as a common thread, a chemical reaction as the principle step that forms the deposit. This is in contrast to sputtering, which forms deposits by condensation of a supersaturated vapor. Trans­ port of the depositing species, even those which have intrinsically low vapor pressures, such as carbon, silicon dioxide, or tungsten, is brought about by the use of volatile compounds ofthat species. A commonly used source for silicon deposition is the chlorosilane family of silicon com­ pounds. In general, these source compounds can be halides, hydrides, or organometallics that have high enough vapor pressures to allow convenient introduction into the CVD system. The chemical reactions that produce metals or semiconducting layers are either reductions of the halides with hydrogen or thermal decompositions of the hydrides or organometallics. Oxide films are produced either by oxidation of the hydrides, halides, or organometallics, or by thermal decomposition of alkoxy-organometallics. Kern and Ban [30] give an excellent general review of CVD. a. Rate Limiting Steps. Chemical vapor deposition is quite similar to catalysis reactions on solid catalyst surfaces in terms of the analysis of the problem [31]. In both systems one considers the transport of reactants into the system, diffusion of the reactants to the reacting surface, adsorption, dissociation, surface migration, chemical reaction, product formation, desorption, and product diffusion and transport away from the surface. This process is illustrated in Fig. 11. Since these processes all occur in series, the slowest of them determines the rate of reaction and is considered to be the rate-limiting step. Operating conditions can vary widely in CVD processing, and as a result a particular reaction can exhibit a number of different rate-limiting steps. These differ­ ences affect the composition and temperature dependence of the rate as well as the deposit morphology, the deposit uniformity, and overall reactor performance. Processes that are controlled by chemical reactions on the surface are typically ones that have high activation energies and thus are quite sensi­ tive to temperature. These processes tend to produce films that are very conformai to the surface on which they are deposited and are relatively

122

D. W. Skelly, T.-M. Lu, and D. W. Woodruff (A) REACTANT TRANSPORT

(B) REACTANT DIFFUSION

(C) REACTANT ADSORPTION

(G) PRODUCT TRANSPORT

(F) PRODUCT Λ DIFFUSION / \

(D) SURFACE REACTION

(E) PRODUCT DESORPTION

SOLID SURFACE Fig.

Schematic of the series of steps occurring in a CVD reaction.

smooth. Being temperature sensitive, they require very good temperature uniformity in order to achieve film thickness uniformity. Low-pressure LPCVD processes [32] are run at low pressure in order to specifically take advantage of this surface-reaction rate-limiting step. Because of the high diffusivity of gases at low pressures, wafers can be stacked quite close together and still not feel the effects of diffusion resistance to the reaction, thus allowing reactors with high loadings and high productivity. On the other hand, those processes that are controlled by the diffusion of reactants to the surface (or products away from the surface) are relatively temperature insensitive. In these cases, the local rate of deposition is dependent on the flow field, and thus the fluid properties and flow geome­ tries. In these processes, uniformity becomes more difficult to achieve, and wafer loadings are considerably lower. Atmospheric pressure reactors can exhibit either surface reaction or diffusion-limited kinetics, depending on the temperature of reaction and in some cases on the gas composition. b. Reaction Thermodynamics. Whereas detailed kinetic information can predict just how fast a reaction will go under certain conditions, this information is sometimes difficult to find or determine. Another source of useful information is in the chemical thermodynamics of a reacting sys­ tem. Thermodynamics gives information about the system at equilibrium;

3. Metallization Techniques

123

although this is not a typical condition of most CVD reactions, it at least indicates the state that the system is reacting toward. Thermodynamic calculations are based only on the free energy of formation of a compound as a function of temperature (for available information for a larger number of the compounds used or formed by CVD reactions see Refs. 33 and 34). On a small scale one can calculate the change in free energy of particular reaction to see if it is energetically favored, that is, has a negative free energy of reaction. For example, the reaction of WF6 with silicon to produce SiF4 and tungsten metal has a — 179.4 kcal/mole (WF6) free-en­ ergy change at 600 K, and thus is a possible reaction, and in fact does occur. Although reactions may be shown to be possible in this sense, kinetics (in particular high activation energies) can make these reactions so slow as to be insignificant. On a larger scale, computer codes are available that compute the gas composition and solid products of CVD systems such as silicon deposition from the chlorosilanes [35,36]. Again, these are equilibrium calculations and only indicate the products that would be present at equilibrium. This information can be very useful in kinetic expressions, though, in that those reactions that are fast in comparison with the rate-limiting step are often at chemical equilibrium. It is also important to recognize the difference in the information ob­ tained when the solid phase is included (two-phase calculations) and when it is not (single-phase). Two-phase calculations indicate how much of the reactant might be converted to deposit and what the chemical composition is at the reacting surface during deposition. Single-phase calculations indi­ cate gas-phase reactions that can occur away from the surface and could possibly affect processability, such as gas-phase nucleation. c. Homogeneous Nucleation in CVD Processes. Homogeneous nuclea­ tion in CVD processes refers to the formation of the desired product in an undesirable form, that of a powder. It occurs when the driving force for the reaction is too great, due to either high reactant partial pressures or high temperatures. Herrick and Woodruff [37] have applied classical nucleation theory to this problem for silicon CVD from the chlorosilanes using singlephase thermodynamics to calculate the gas composition of silicon vapor. It was shown that the nucleation rate goes through a maximum with respect to temperature for each of the chlorosilane family, giving a low-tempera­ ture limit above which noticeable powder formation occurs, and an upper limit above which appreciable nucleation does not occur. They have also shown [38] that moving to lower pressures reduces the lower temperature limit by favoring the reaction to silicon vapor.

124

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

3. Reactor Designs and Concepts

Chemical vapor deposition reactors come in a variety of forms, the nature of which depends on the trade-offs made between performance critieria for a particular application. These criteria can include total wafer loading (i.e., throughput), uniformity, sensitivity to particulates, sensitivity to contaminants, ultimate vacuum requirements, and so on. The form of the reactor also depends on the sensitivity of the process to temperature and flow conditions. In the following section, the principles of heat, mass, and momentum transfer are discussed with respect to CVD reactors and their designs. a. Atmospheric versus Low Pressure. Chemical vapor deposition was originally developed as an atmospheric pressure process. While some CVD processes are still run at atmospheric pressure, most new CVD processes are developed at low pressures. The advantages are such that there are few processes that do not benefit from them. There are two major benefits to low-pressure CVD (LPCVD). First, these processes are generally run at 0.5- 1 Torr. Since the diffusivity of gases is inversely proportional to the pressure, it is increased by nearly three orders of magnitude for these conditions. Because of this, the rate-limiting step for LPCVD reactions is almost always a surface-reaction step rather than diffusion step, which makes the flow geometry much less dominant in the design of reactors. High diffusivity also allows a stacked arrangement of wafers in hot-wall tube reactors, because the reactants can diffuse between wafers much faster than the reaction occurs on the surface. This increases the throughput of wafers considerably. Despite lower pressures, growth rates are generally not reduced to unacceptable levels. b. Transport Phenomena in CVD Reactors. The transport equations are those that govern the flow fields, the heat transfer and temperature profiles, and the mass transfer and concentration profiles in any chemical reactor. For simplified geometries they may be solved for exact solutions [39,40], or may be subjected to finite element analysis computer solutions for more complex geometries. The wide variation in reactor geometry, design, and conditions eliminates the possibility of one general solution to these equations. Rather than developing the set of equations and boundary conditions, only a few guiding principles will be discussed here. The flow in any system is characterized by the dimensionless Reynolds number, given by, Re = DpV/p,

(4)

where D is a characteristic length of the system, such as the diameter, p the

125

3. Metallization Techniques

fluid density, V the average fluid velocity, and μ the fluid viscosity. The viscosity of these gases is generally of the order of 10-30 mP, increasing with the square root of temperature and invariant with pressure down to the point where the mean free path is of the order of reactor geometry. The Reynolds number is the ratio of the inertial forces in a fluid to the viscous forces. For systems with Re < 1200, the flow is considered to be laminar (or viscous), while for Re > 2200 the flow is turbulent. Reynolds numbers between 1200 and 2200 are in an unstable transition region and are difficult to predict. Flow in LPCVD reactors generally has quite low Reynolds numbers, on the order of 1 to 10. Atmospheric pressure reactors generally have Reynolds numbers in the hundreds, and are thus still in laminar flow, although geometry can be such that there are turbulent regions. In the analysis of turbulent systems, the fluid layer near the surface is at lower velocity and can be in laminar flow. When phenomena in this near-surface region are important, as for CVD, boundary layer theory [41 ] is used to describe the heat and mass transport and flow fields in this boundary layer. Boundary layer theory is not literally applicable to laminar flow systems although the boundary layer is invoked inappropriately in many cases. Mass transfer through the fluid occurs both by convection and diffusion. In addition to the Reynolds number, the Schmidt number, and the mass transfer Peclet number are important for characterization here. They are given, respectively, by, Sc =μ/ρΟΑΒ

(5)

Pe = Re X Sc,

(6)

and where DAB is the diffusivity of component A in fluid B. Gas diffusivities are in the range of 0.1 - 1 cm2/sec at standard conditions, increasing with the inverse of the pressure and increased temperature. The Peclet number gives an indication of the relative importance of convective transport to diffusive transport of mass. A high value indicates that the composition profile in a reactor is highly dependent on the flow field, where a low value indicates a high degree of backmixing due to high diffusivity. Designing reactors with a low Peclet number is an effective way to maintain a relatively constant composition profile over the volume of the reactor for improved uniformity, although the effective reactant concentration is low and the product concentration high. The heat transfer is perhaps the most difficult of these subjects to deal with because three modes of heating can occur simultaneously: conductive, convective, and radiative. Conduction and convection occur through the

126

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

fluid, heating it in the process. This process is governed by the Prandtl number and the Peclet number for heat transfer: Pr = Cpß/k

(7)

Pe = Re X Pr,

(8)

and where Cp is the fluid heat capacity and k the fluid thermal conductivity. The Peclet number for heat transfer is analagous to that for mass transfer, giving the relative importance of the convective mode to the conductive mode of transferring heat. The Prandtl number for gases is generally in the range of 0.65-0.95. In parallel with the convective-conductive mode is the radiative heat transfer. This occurs between two solid surfaces with no fluid heating involved. For LPCVD, this tends to be the dominant mode of heat transfer from source to wafer. The radiation heat flux is given by q/A = aFl2(elT4l-e2T42),

(9)

where σ is the Stefan - Boltzmann constant, Fl2 the view factor, and et the emissivity of the respective surfaces. The view factor in typical reactors is generally equal to unity, but the emissivities can vary widely, even as a function of time, due to the deposit. It can also vary with the nature of the wafer and result in inconsistencies from run to run. c. Utilization. An inlet flow of reactant gas has a finite amount of material that it can give up as deposit. Typically one reactant will be limiting in this way while the other will be in excess. The utilization is defined in terms of the reactant supplied in lesser amounts, and is simply the fraction or percent of that reactant used in one pass of the reactor. A 10% utilization indicates that 90% of the reactant passes through the reactor unreacted. While this may appear to be an inefficient way to operate, it provides a much more uniform atmosphere from which to deposit. In the case of experiments designed to obtain kinetic parameters, the utilization should be as low as possible and certainly less than 5%. Many times CVD reactors are run at conditions that are at or near 100% utilization. These are characterized by having a first-order dependence on the reactant concentration and a total volumetric deposition rate equal to the inlet flow of reactant. In such cases, the amount of material deposited is limited by the rate of delivery of material to the surface, rather than by diffusion or surface reactions. d. Reactor Configurations. There are two basic reactor types to be considered, hot-wall and cold-wall. In hot-wall reactors, one creates an

3. Metallization Techniques

127

isothermal cavity inside a quartz vessel (although not always a tube) by surrounding it with heater elements. The reaction gases contact and usually deposit on the wall, wafer carriers, as well as on the wafers, result­ ing in a much higher utilization than for a cold-wall reactor with the same inlet flow. The primary advantage in these reactors is the high degree of temperature uniformity and control attained because the wafers are sur­ rounded by the heat source. There are also the advantages of simplicity of design and fabrication, which generally lead to lower cost, and high wafer loadings, which lead to high reactor throughput. The key issues for these reactors is loss of uniformity due to high utilization of the reactants and particulate formation due to gas heating and reactions on the wall. For cold-wall reactors, the wafer and the heater plate on which it sits are the only hot surfaces in the reactor. The other surfaces are generally maintained at roughly room temperature, cooled by either air or water. This arrangement allows for control over the flow of gases past the wafer and lower utilization, at the expense of temperature control and uniform­ ity. The wafer loading is also generally quite low. These reactors are most advantageous in cases where a reaction has a low activation energy and needs fine control of the flow field or a low utilization. e. CVD Reactor Enhancements. As a result of the drive to reduce the temperature history of circuit wafers, a number of techniques have been explored and developed in order to increase reaction rate at lower tempera­ tures. Plasmas, lasers, and photo techniques have all been considered. Plasma-enhanced CVD (PECVD) has been the most successful and widely used, with fully developed commercial processes for Si0 2 , Si3N4, and amorphous Si. Photo-enhanced CVD has also been developed to commer­ cialization, but has found less wide acceptance, and laser techniques are still in the early stages of development. 4. CVD of Metals

Chemical vapor deposition of metals for integrated circuits has been investigated for many years [42]. Its recent acceptance [43] has been driven by a need for better metallization step coverage, diffusion barriers, and a characteristic unique in thin films—selective deposition. Only tungsten deposition has been commercialized to date, but preliminary work on aluminum CVD has shown some promise, as has earlier work on molybde­ num. a. Tungsten Chemical Vapor Deposition. Tungsten has been deposited from chloride sources [44-46] and organometallics [47-49], but is princi­ pally deposited from tungsten hexafluoride, WF6 [50]. Considerable work

128

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

has been done on metallurgical tungsten coatings using atmospheric pres­ sure CVD [51], but nearly all semiconductor tungsten metallization is done at reduced pressures with a few exceptions [52]. Plasma CVD of tungsten has been attempted [53-55], but results in a high resistivity tungsten structure that must be annealed at high temperatures to yield a good film. Tungsten CVD encompasses a number of related processes run in slightly different ways, each of which are appropriate for a set of different applications [43]. Thin selective tungsten is the most fully developed pro­ cess and is used for contact pads, diffusion barriers, and gate shunts [56]. Thick selective tungsten processes are being developed [57,58] that have application in contact hole and via filling to reduce the step heights that must be covered by sputtering processes. Nonselective tungsten deposition could also be used for contact hole and via filling in conjunction with a suitable etchback process [59,60] and is also appropriate for replacing sputtered aluminum as full metal layer [61,62]. One other possibility, which has not been explored in much detail, is that of using nonselective tungsten as a gate metal directly on the gate oxide, although adhesion and WF6 attack of the thin gate oxide are formidable problems for this tech­ nology. Chemical vapor deposition of tungsten on a silicon substrate proceeds by a well-established two-step process [63]. The first reaction is that of WF6 with the silicon substrate: 2WF6 + 3Si => 2W + 3SiF4.

(10)

This reaction is self-limiting on a freshly cleaned silicon surface, stopping at about 200 A. The newly created tungsten film is a good diffusion barrier, preventing any further reaction. Due to stoichiometry and densities, this reaction consumes 1.89 À of Si for every angstrom of tungsten deposited, or roughly 380 À for the self-limited film just described. This reaction accounts in part for the excellent contact resistance of tungsten to silicon and also for the encroachment and possibly the wormhole forming pro­ cesses. The other primary reaction that occurs is the hydrogen reduction reac­ tion, WF6 + 3H2 => W + 6HF.

(11)

The kinetics of this reaction have been studied by many investigators [64-66] including works specifically looking at selective deposition [63,67,68]. It is generally believed that the rate-limiting step is the dissocia­ tion of hydrogen on the growing tungsten surface. The activation energy is reported by many as 0.7 eV (16.4 kcal/mole), although there is conflicting

129

3. Metallization Techniques

data [69]. Evidence for hydrogen dissociation as a rate-limiting step comes from the observed zeroth-order dependence of the rate on WF6 partial pressure, and half-order dependence on the total and the hydrogen partial pressures. Thus rate equation is r = £ ( P H 2 W W F 6 ) ° exp[- Ì6A/RT].

(12)

Pauleau et al have argued that there is a reduction in rate when the HF partial pressure becomes significant [67], but have shown only substantiat­ ing trends. The resistivity of the deposited tungsten film is a function of thickness up to nearly a micron [70,71]. Typical results are shown in Fig. 12. The trend indicates a higher oxygen concentration in the initial deposit than is seen later in the cycle. Even the thickest deposits reach only 7 μΩ cm as compared to a bulk value of 5.4 μΩ cm. i. SELECTIVE CVD TUNGSTEN. The thin selective tungsten CVD pro­ cess is typically done in a hot-tube LPCVD reactor [63,72], as shown in Fig. 13. One notable variation is a hot-wall domed reactor [73], which allows flow between the wafer stack instead of along the stack axis as in a standard tube reactor. Both reactor types make use of the hot-wall configu­ ration to enhance temperature (and thus deposit) uniformity. The thin selective process is run at 0.2 to 1.5 Torr, generally around 0.5 Torr. Flow rates tend to be chosen, which give this pressure, and, thus, are determined

I

30

I

I

1

I

*

'

I

25 E

ü

Οί

20

È 15

> »-

[2 io oc 5 J

0

0.2

0.4 0.6 0.8 THICKNESS (Jim)

»

1.0

Fig. 12. CVD tungsten resistivity as a function of deposit thickness. (From Learn and Foster [70]. Courtesy American Institute of Physics.)

130

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

PRESSURE TRANSDUCER

N2

n2

WF6

Fig. 13. Schematic of a hot wall, horizontal tube LPCVD reactor for tungsten deposition. This type of system is most widely used for thin selective tungsten deposition.

by the pump size and the vacuum line conductance. The ratio of hydrogen to WF6 in the feed gas ranges anywhere from 3 (stoichiometric) to 1000 or more, and can have an effect on the process performance. Cold-wall reactors are generally not used for thin selective processes, though in principle there is nothing to prevent such an arrangement. Under these conditions the utilization of the total WF6 flow is usually less than 20%. The selectivity of this reaction is due to the ability of the tungsten surface to dissociate the hydrogen molecule with relative ease compared to an oxide or nitride surface. Tungsten nucleates on oxides only when there is some external assistance to the dissociation of hydrogen. This can be in the form of impurities left on the oxide surface, high temperature, or hydrogen spillover from adjacent metallic areas. Impurities on the surface can in­ clude incomplete photoresist cleanup, residue from heavily used cleaning baths, and improper handling techniques. Temperature and time also play an important role. Most investigators report loss of selectivity after 3040 min of deposition at 300°C, while Broadbent and Stacy [50] report selectivity at 425 °C for a 2-min deposit. Both conditions result in a deposit that is approximately 1200 À thick. Another important observation is that the oxide areas near depositing areas (in particular, large ones), lose selectivity faster than areas removed

3. Metallization Techniques

131

from deposition, and has been attributed to release of SiF4 during the initial reaction [72]. Similar effects have also been observed on oxides near non-silicon metallic areas. Another possible contribution to the loss of selectivity is believed to be a phenomenon called hydrogen spillover, where hydrogen is dissociated on a metallic surface, and then surface diffuses from the metal onto the oxide surface. This behavior has been reported and studied for catalyst systems [74] and is reasonably extended to the loss of selectivity. Thick Selective Process. Reactor design also plays an important role as demonstrated by Wilson and co-workers [57,58]. Their quartz lamp heated, cold-wall system (see Fig. 14) has demonstrated 2.5//m-thick se­ lective deposits at temperatures as high as 700 °C. Figure 15 shows an electron micrograph of a 2 //m-deep via in Si0 2 , etched down to molybde­ num, filled by the Wilson process. Experiments have shown that the quartz lamps are only important as heating sources and are not responsible for any photochemistry. Other Selective Tungsten Issues. Substrate preparation is quite impor­ tant to the reproducible operation of this process. It is important that any oxide be clear of any processing residue in order to maintain selectivity, and also that any silicon areas have the native oxide removed just prior to deposition. If excessive native oxide remains, the first reaction between Si and WF6 is interrupted on the silicon surface, and the deposit becomes very rough and irregular [75,76]. Encroachment and "wormholes" are phenomena that are closely related to the WF6/Si reaction [71,76]. Enchroachment, shown in Fig. 16, is the lateral embodiment of the silicon reduction reaction that extends under the Si0 2 . It can reach from hundreds of angstroms to over 1 //m, many times further than the vertical reaction. It is the result of an enhanced reaction with the silicon directly under the oxide, and is a function of dopant, doping level, process temperature, and H 2 /WF 6 ratio. It can also be a function of the oxide type in that the local stress contributes to this phenomenon. Although it can be problematic, it is controllable. Wormholes, shown in Fig. 17, are tunnels of roughly constant diameter that are formed at the W-Si-Si0 2 corners. They extend up to a micron into the silicon and appear to have random paths. These paths very often have associated with them a small particle at the path end. Paine et al. [77] have shown the presence of a tungsten suicide phase at the tungstensilicon interface and have proposed that the tungsten-rich particle at the end of the path is a suicide particle that catalyses the consumption of silicon by either HF or F atoms. As a contact material, CVD tungsten has very good properties [72] in

QUARTZ VACUUM TUBE

INLET ( i o ) H 2 / N 2 INLET

WF C INLET

PORT ASSEMBLY

Θ

PRESSURE TRANSDUCER

Fig. 14. Top view schematic of a reactor for thick selective tungsten CVD. Selective tungsten of up to 2.5-μτη thickness has been demonstrated in such a reactor. (From Wilson et al [57,58]. Courtesy IEEE, © 1985.)

Θ

0 N 2 , BACKFILL

ABSORPTION TRAP

Θ Θ

QUARTZ WATER FILTERS (2)

Θ

foi

Γ 7 ) PROJECTOR LAMPS (8)

VACUUM PUMP

( T ) RIGHT-ANGLE VALVES (2)

M j

^y^j>

^

3. Metallization Techniques

133

Fig. 15. Thick selective tungsten deposited in a via cut in Si0 2 down to molybdenum. The selective tungsten plug is 2-μτη deep. No clean-up of the upper oxide surface has been performed after deposition. (From Wilson et al. [57]. Courtesy Materials Research Society.)

comparisons with aluminum-silicon contacts. These W-Si contacts are generally stable to processing temperatures of 600 °C. Blewer and Tracy [78] have demonstrated suppression of the silicidation reaction at the interface by introducing controlled amounts of oxygen at the interface. With 50% interfacial oxygen, temperatures of up to 1100°C can be used without causing silicidation. ii. NONSELECTIVE CVD TUNGSTEN. Nonselective or blanket CVD tungsten is a process very similar to selective tungsten, with process condi­ tions such that the tungsten deposits on all hot surfaces, including the oxides. As mentioned earlier, this process is useful either when patterned into a metallization layer or when etched back to provide tungsten plugs in contact holes or vias in order to reduce the aspect ratio seen by a subse­ quent sputtered metal. There are two basic approaches to processing nonselective tungsten: (1) in a hot-wall reactor using the standard WF 6 /H 2 chemistry and (2) in a cold-wall system using WF6/SiH4 chemistry. De­ posits have also been made from the carbonyls [45,46], but this chemistry as yet has not been applied commercially for VLSI.

134

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

Fig. 16. SEM of encroachment of selective CVD tungsten under the oxide contact hole. The thin selective tungsten is covered with sputtered molybdenum in this view.

Adhesion of blanket tungsten to Si0 2 is a major obstacle for this process. By itself, CVD tungsten will not adhere to the oxide due to low nucleation density, low oxygen affinity, and high film stress [79]. Most solutions to the problem involve some sort of adhesion-promoting layer deposited between the tungsten and the oxide. In the hotwall approach the user must provide such a layer prior to deposition of the tungsten. Sputtered molybdenum has been demonstrated as an adequate adhesion layer for an etchback plug process [79] using up to 2 μτη of tungsten, but has not been proven for tungsten lines. The choice of material for the adhesion layer requires both a strong oxide-forming material, and one which is not reduced to a fluoride (in particular, nonvolatile fluorides) by WF6 [71,79,80]. Another solution is in situ deposition of the adhesion layer as practiced in the WF6/SiF4 process. Here, a thin layer of tungsten suicide is deposited as part of the tungsten processing thus avoiding breaking vacuum between adhesion layer and tungsten deposition. The hot-wall process can be run in either the tube reactor mode or in the domed hot-wall reactor mentioned in the previous section. The basic

3. Metallization Techniques

135

Fig. 17. TEM of wormholes in the Si-Si0 2 -W corner of a contact hole.

kinetics of the reaction are as mentioned for selective tungsten, although the process is generally run at higher temperatures (400-500°C). Because the reaction proceeds on oxides, the consumption of WF6 becomes much higher. Both the entire wafer surface and the walls of the reactor now support the reaction. Depletion of the WF6 and resulting nonuniformity becomes a concern. In the tubular reactor, this has a greater effect than in the domed reactor since depletion occurs along the length of a wafer stack rather that across its diameter. The cold-wall process is one that has evolved from an existing tungsten suicide process. It uses the silane reduction of WF6 to form both the suicide adhesion layer and the tungsten deposit. As a result, the tungsten from this process has 1-2% Si incorporated in it and has a resistivity of 1012 μΩ cm compared with 8-10 μΩ cm from a conventional WF 6 /H 2 pro­ cess [81]. The activation energy of this reaction is much smaller than that for hydrogen reduction, around 0.1 eV, and the kinetics are not well understood. The main advantage of CVD processing is the excellent step coverage of the deposit over wafer topography. Figure 18 shows the step coverage on a 2-//m diameter via, l-//m deep, as a function of deposit thickness using the WF 6 /H 2 process in a cold-wall reactor. The films can be seen to have a

136

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

Fig. 18. SEMs illustrating the progressive coverage in a 2-//m diameter, ί-μτη deep via, through oxide, down to sputtered molybdenum. The step coverage is 100% for the entire deposition.

columnar structure, and increasing roughness with deposit thickness. The vias become filled at a deposit thickness equal to the via radius as expected, without formation of voids. At a thickness equal to approximately one diameter the dimple on top is nearly gone. As for many other processing steps, the blanket tungsten process is closely tied to the subsequent etching step. Microloading of an etch process puts strict limits on acceptable nonuniformities in deposit thickness. Reac­ tive ion etching of film areas containing higher levels of oxygen tends to be faster, forming a sidewall void [82]. This can be prevented by using an oxygen gettering step prior to tungsten deposition. iii. PRACTICAL CONSIDERATIONS. The major cause of irreproducibility in tungsten CVD is equipment leakage. The tungsten film properties and growth rate are strongly tied to the integrity of the vacuum system. Prob­ lems can arise from inboard leaks (across valve seats), outboard leaks (across compression fittings), and from virtual leaks (air or moisture ad­ sorbed on reactor walls). Although operated at medium vacuum condi-

3. Metallization Techniques

137

tions, these systems should be thought of in terms of O'Hanlon's [83] "clean high-flow system." It is expected that ultraclean high-flow reactors will become increasingly important. For similar reasons it has been found that a foreline trap of alumina as well as a circulating oil filtration system are advantageous in keeping product acids and particulates out of the pump oil. Also, the proper removal of photoresist and other processing contaminants is essential for consistent processing. b. Aluminum CVD. In contrast to the CVD tungsten processes, alumi­ num CVD is not well developed. The most successful work in this area has been done using organometallic aluminum compounds, and particularly tri-isobutyl aluminum, or TIBAL [84-86], though no commercially avail­ able process yet exists. Aluminum has also been deposited from an alumi­ num chloride source [87]. The TIBAL process chemistry involves a revers­ ible reaction of TIBAL to diisobutyl aluminum hydride (DIBAH) at about 50 °C, followed by an irreversible reaction of DIBAH to aluminum at 220-300 °C. Both reactions release isobutylene gas, the latter reaction also releasing hydrogen. Depositions are typically run at 260°C and 100200 Torr with deposition rates of 100-200 Â/min. The films deposited showed excellent step coverage as expected from a CVD process, but were quite rough. The resistivity was comparable to sputtered aluminum, in the range of 2.8 to 3.5 μΩ cm, and were deter­ mined to be at least 99.7% pure. Aluminum from the chloride source also was of high quality and was less rough than from the TIBAL process. There are three major problems with CVD of aluminum that need to be addressed. The first is that the organometallic aluminum compounds are quite difficult to handle. TIBAL is pyrophoric, explosive in contact with water, and is toxic. Because of this, it must be cold trapped during process­ ing rather than exhausting through the vacuum pump, and then must be disposed of after processing. Other aluminum organometallics have similar properties. This makes organometallic chemical vapor deposition (OMCVD) of aluminum unattractive. Aluminum chlorides were studied in response to these concerns. The second problem for CVD of aluminum is that the initial nucleation density of the deposit is quite low and lacks sufficient control. The TIBAL process uses a TiCl4 nucleation agent, whereas a number of nucleation schemes were attempted for the chloride process. A consistent production process will require improvements in this area. Electromigration is the third problem for CVD of aluminum. Levy et al [85] have shown CVD Al to be as good as pure sputtered aluminum. For electromigration resistance, other metals such as copper must be alloyed with the aluminum. This appears to be necessary for CVD aluminum as well, and has not to date been described.

138

p. W. Skelly, T.-M. Lu, and D. W. Woodruff

c. Molybdenum CVD. The similarities in the chemical nature of tung­ sten and molybdenum [88] suggest that molybdenum might be an interest­ ing alternative metal with potential advantages. Despite these similarities, molybdenum CVD is much less advanced than tungsten. There have been reports of molybdenum CVD from carbony [48,89-92], chloride [93-95], and fluoride [95,96], including plasma deposition [53,54]. Deposition from the carbonyl carries the problem of carbon codeposition, although deposits with resistivities near bulk (5.7 μΩ cm) have been demonstrated [48]. Good films have also been demonstrated from chloride sources, but this remains a difficult technology due to the low vapor pressure, solid source MoCl5 [97]. Selective deposition of Mo has been described [96], but with little success. The MoF6/Si reaction is not self-limiting for some condi­ tions, degrading patterns. Plasma deposition of molybdenum [53] has been equally disappointing, showing very high incorporation of fluorine, and thus very high resistivity. d. Laser-Assisted CVD of Metals. Laser-assisted CVD (LCVD) of metals has been reviewed by Solanki et al. [98] and Rytz-Froidevaux et al. [99]. This field comprises two general approaches. The first is the use of lasers to heat the substrate wafer, either locally or over the entire area. This allows direct writing of metal lines locally [100]. In practice this technique would be limited to special applications due to low wafer throughput. The second LCVD technique uses laser photons to dissociate source gases in the vapor near the wafer surface. This again may be done locally or over larger areas. Depositions of W, Mo, AI, and Cr have been reported [98]. The use of lasers effectively lowers the deposition temperature. Gas sources for this application generally must be chosen based on photon absorption ability,which invariably leads to use of organometallics. More work is required to reduce both resistivity and impurities. 5. CVD of Refractory Metal Suicides

The electronics industry has been developing refractory metal suicide technology for a number of years, primarily focusing on four candidates [101]: TiSi2, TaSi2, WSi2, and MoSi2. Their use has been primarily seen as an enhancement to existing LSI processes to stretch the technology one step further. The suicides would replace or enhance the doped-polysilicon gate material, and also make contact pads over active area [102- 105]. The suicides can be formed either by solid-phase reaction of silicon with the metal, or by deposition processes such as sputtering or CVD. Chemical vapor deposition has the step coverage advantage normally put forth, and also gives independent control of the stoichiometry from run to run and very good resistivity. Despite considerable use in the integrated

3. Metallization Techniques

139

circuits industry, there has been little indepth reporting of suicide CVD processes in comparison with tungsten. Issues that have been examined are rate of deposition, film purity, deposit uniformity, and film stress. These will be discussed later with respect to systems that have been studied or commercialized. The common thread that runs through all suicide CVD process is the reaction of silane with the metal halide to form the metal suicide [106]. This can involve special reactor designs to prevent gas-phase nucleation, where necessary. Reactor designs can run the gamut from thermal to plasma, hot wall to cold wall. There is no preferred single approach. a. Tungsten Disilicide. Tungsten disilicide (WSi2) has been deposited by plasma CVD [53,107] and by thermal reduction using silane [108112]. Plasma enhancement of the deposition produces films that have high resistivity for Si/W ratios greater than 2, even after annealing, and so has not been pursued actively. The thermal process is done in a cold-wall reactor, shown in Fig. 19. Typical conditions are 400 °C, 100-300 mTorr total pressure, 100 seem SiH4, and 10 seem WF 6 . The process uses excess silane and controls the deposition rate by the feed rate of WF6 in depletion mode. The process has a very low activation energy, though the temperature does affect the de­ posit stoichiometry, as does the WF6 flow rate. As deposited films had resistivities of 600-900 μΩ cm and could be annealed to 35-60 μΩ cm. b. Titanium Suicide. Titanium suicide has been deposited by laser CVD [113], by thermal reduction [106], and by plasma-enhanced CVD [114]. The latter has been commercialized in a process using TiCl4 and SiH4 gases in a hot-wall tube reactor similar to those used for deposition of oxides and nitrides. Deposition typically occurs at 2 Torr and 450 °C with a 3:1 ratio of silane to TiCl4. The films are somewhat silicon rich, annealing to 15-20 μΩ cm at moderate annealing conditions. c. Tantalum Suicide and Molybdenum Suicide. The suicides of both tantalum [106,115] and molybdenum [106] have been deposited by the reactions of silane with TaCl4 and MoCl5, respectively, but no commercial process has been developed to date for either process. Due to the low vapor pressure of these chlorides, the mass flow control of these compounds is difficult, thus making other processes more attractive. 6. Areas for New or Expanded Work

As stated earlier in this section, CVD of metals for VLSI is still at an early stage of development as a field. There are a number of directions for this field to take, with numerous challenges ahead. For example, in the area of reactor design and analysis much can be done yet to improve the

140

D. W. Skelly, T.-M. Lu, and D. W. Woodruff 8 Wafer Turret Assembly Water Cooled Dome and Skirt 8 Distributed Exhaust Headers

2500 W 13.56 MHz RF Generator

Fig. 19. Schematic diagram of a cold-wall reactor designed for deposition of tungsten suicide using the reaction of silane with tungsten hexaflouride. (Courtesy Genus Corpora­ tion.)

understanding of these systems, leading to better designs. Among the improvements expected are a move towards high or ultrahigh cleanliness systems in order to improve film quality and process reliability. Enhanced processes such as laser, photo, and plasma-assisted processes will continue to be investigated in order to find lower temperature processes. Better kinetic understanding of the surface chemistry of these metal CVD pro­ cesses is also a needed step. New materials will be of interest, particularly new source materials for aluminum CVD that might give the right combi­ nation of properties for this important material. Finally, it is expected that the move towards use of metals for the gate material will in part be addressed by CVD.

3. Metallization Techniques

141

C. Evaporation

In the evaporation-deposition techniques, the deposition materials is vaporized in a vacuum from its liquid (or solid) phase, and the vapor is then transported and deposited onto the substrate. The vacuum is < 10"5 Torr. At this pressure, the residual gas molecules have a mean free path of the order of 1 m. Therefore the evaporated vapor suffers no colli­ sion from the residual gas and is able to achieve a straight-line travel from the source to the surface of the substrate. The condensation of the vapor on the substrate is achieved through a nucleation and growth process. De­ pending on the vapor and substrate material, substrate cleanness, substrate temperature, and deposition rate, the initial shape of the nuclei can be either two or three dimensional [116]. These parameters also affect surface morphology and the grain size of the thin film. 1. Evaporation Sources

The commonly used evaporation sources are resistance-heated, induc­ tively heated, and electron-beam heated sources. Because of the reaction between the source material and the évaporant at an elevated temperature, one has to select an appropriate type of source for the particular évaporant one uses. One of the simplest kinds of source is the direct-resistance-heated type using refractory metals such as W, Mo, and Ta as the support. These refractory metals have relatively high melting temperature and low vapor pressure. The metal support can have various shapes. Examples are wire baskets and dimpled foil, with or without an alumina coating. The resist­ ance of these sources is small and they require only a relatively low voltage power supply. The greatest advantage of the resistance-heated source lies in its simplicity and freedom from ionizing radiation. The disadvantages are the possibility of source contamination, especially when operated at a high temperature, and the requirement for recharge if a long deposition time is desired. In an induction-heated source, a rf-induction coil is wound around the crucible where the évaporant is placed. The évaporant is heated by eddy currents generated in the évaporant. For Al evaporation, the crucible is normally made out of boron nitride. A boron nitride crucible is not desirable if the évaporant is Ti or Be since they react with nitride. In that case, a water-cooled copper crucible can be used [117]. Like the resistanceheated evaporation, the induction-heated evaporation is free from ionizating radiation and can achieve a high deposition rate. Induction heating can be more efficient than resistance heating since the radiative conduction heat losses are smaller. However, excessive rf heating can generate molten

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

142

droplets and alloying and reaction of the évaporant with crucible material — undesirable events for VLSI quality films. A high deposition rate and extended deposition times can be achieved using electron bombardment heating. In this technique, a 5 to 10-kV electron beam is supplied from a hot-cathode electron gun and upon impingement on the deposition material, temperatures exceeding 3000 °C can be obtained. In addition to Al and its alloys, other elements, including high-temperature metals, such as Ti, Mo, and W, can be evaporated using the electron-beam (e-beam) technique. Multiple sources can be used for coevaporation. During the evaporation, interactions between the évapor­ ant and the supporting materials can be greatly reduced since excess heat­ ing of the supporting material can be avoided. The heating and energy transfer are localized at the evaporating surface. For Al deposition, very often the source consists of a water-cooled copper hearth where the charge is placed. In order to prevent the geometrical restriction in the arragement of the electron source and substrate, very often a bent-beam electron gun is employed [118]. Figure 20 shows a schematic of a typical setup for e-beam evaporation. An additional advantage of using the bent-beam configura­ tion is that it can prevent the impurities generated by the hot filament from reaching the substrate. The e-beam evaporated Al films were shown to have superior electromigration resistance as compared to films deposited by other techniques such as In-source or S-gun magnetron sputtering. This is probably due to the more uniform Al grain size and a preferred (111) orientation produced by the e-beam evaporation technique [119]. The greatest disadvantage of the e-beam heating is the generation of ionization radiation during the high-energy electron bombardment of the évaporant. e" BEAM

"2^ DEPOSITION

^

y

/

®

®

WATER COOLED Cu HEARTH

Fig. 20. Schematic of a bent-beam electron evaporation setup. The electron beam is bent 270° by a magnet to hit the deposition material.

143

3. Metallization Techniques

This radiation can cause damage to the devices and subsequent heat treatment to heal the damage may be necessary. Excess heating can also create metal droplets during the evaporation. 2. Principles of Evaporation

The interpretation of the evaporation phenomena has been largely based on the kinetics theory of gas. The concept of the equilibrium vapor pres­ sure leads to the Hertz-Knudson evaporation rate equation: ^ ^

= {2nmkT)-^P,

(13)

where dNe/dt is the rate of molecules evaporating from a surface of area A, m the molecular mass, k the Boltzman's constant, T the surface tempera­ ture, and P the equilibrium pressure at temperature T. The evaporation rate from the molten surface is thus assumed to be the same as the impingement rate of the gas molecules on the surface. Integration of Eq. (13) gives the total number of molecules leaving the surface. The directionality of the evaporating molecules can be derived from the effusion of gases from an ideal Knudsen cell. In the cell the molecules are assumed to obey a Maxwellian velocity distribution. The spatial distribution of the emitted gas can be shown to obey a cosine law, cos φ, where φ is the angle measured from the normal of the evaporating surface. The cosine law of emission from a Knudsen cell is assumed to be also valid for a liquid or solid surface. The directionality of the evaporation can be an important issue in the metallization of integrated circuits be­ cause it can generate a nonuniform coating on the substrate. 3. Uniformity and Step Coverage

VLSI metallization requires uniformly thick metallization coating on large area substrates and shadowless step coverage for small microcircuit structures. Both the angular dependence of the emission law (~cos φ) and the orientation of the film-gathering surface contribute to uniformity and step-coverage problems. From Fig. 21, it is seen that the flux of material per unit area to the receiving surface, F, is given by Foe— cos φ cos Θ,

(14)

where r is the distance between the source and the receiving surface and Θ the angle of incidence measured from the normal to the receiving surface. The fundamental assumption is that evaporation is a line-of-sight depo-

144

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

sition. To ensure the uniformity and good step coverage over VLSI topog­ raphy, a rotating planetary over the evaporation source must be used [ 120]. The wafers are mounted on a spherical surface, such that Θ = 0, and the source is on the same spherical surface. The deposition rate is the same everywhere on the spherical surface. By allowing the receiving surface to rotate both around the r' and r0 axes, as shown in Fig. 21, good step coverage (50- 100% sidewall coating) and uniformity may be achieved on tapered-wall structures. For VLSI liftoff process applications, one requires minimal step cover­ age. In this case, a dome-shape planetary system (Fig. 22) is more suitable. In this configuration, the wafers are placed on a hemispherical surface with the source being the center of the sphere. The geometry will give the least sidewall coating that is required in the liftoff process, but the coating will not be uniform. The uniformity problem can be overcome by inserting a mask with varying radial area in front of the dome to reduce the coating near the center of the dome. In addition to step coverage, the principle disadvantage of evaporation lies in a high probability of source contamination with mobile carriers. At the elevated temperatures used, diffusion of these impurities throughout the source is probable once an initial contact with impurities occurs. X-ray flux from e-beam sources can cause damage in semiconductor and insula­ tor materials.

i

^v

. WAFERS

0 r

o

Fig. 21. Geometry of a rotating planetary system for conformai step coverage: cos Θ = cos(/> = r/2r0. In this configuration, the deposition rate is the same everywhere on the spherical surface.

3. Metallization Techniques

145 ^WAFERS

MASK

Fig. 22. Geometry of a dome-shape planetary system for the liftoff process applications. This geometry gives the least sidewall coating. The mask in front of the dome is to ensure the uniformity of the film.

D. Molecular-Beam Epitaxy (MBE)

Metal deposition at ultra-high vacuum (UHV) conditions with a pres­ sure of <10~ 9 Torr can provide atomically abrupt and well-controlled interfaces. In particular, the ability to grow epitaxial metal layers on semi­ conductor not only gives uniform and stable contacts for conventional VLSI applications, but also leads to the possibility of fabricating various advanced VLSI structures and novel devices [121]. Examples of such devices are metal-base transistors, permeable-base transistors, and multilayered-quantum-well and superlattice devices. Recently [122], a singlecrystal Si/CoSi2/Si structure has been demonstrated that has the same transistor action, voltage gain, and power gain as a vertical metal-base transistor. The growth of metallic superlattices [123] also requires UHV conditions. Metal epitaxial films can be grown using the conventional Si MBE chambers which will be described next. 1. MBE Chambers and Sources

Unlike the III-V MBE systems, a "standard" Si MBE system has not been realized. One of the more advanced Si MBE equipment is the multichamber system that includes a fast entry lock, a preparation-analysis chamber, and a growth chamber. Each chamber is isolated by independent vacuum valves and is separately pumped. An example of such machine is

146

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

shown in Fig. 23 [124]. Details of an MBE source is shown in Fig. 24. With a multichamber system, sample exchange can be done without breaking the vacuum. With this arrangement, one can improve the film quality as well as increase the throughput. The system is designed to allow several 3or 4-in. wafers to be introduced simultaneously into the chamber for deposition. The substrates are rotated during the growth to improve the film uniformity. In the preparation-analysis chamber, in situ cleaning can be performed by fast ion bombardment of the substrate (e.g., Ar + ). The substrate can be heated to 1200°C by using multiple radiation baffles. Cooling is necessary to minimize the heating of the surroundings. In order to prevent possible reaction between Si and metals at high temperature, Si wafers should not be tightly held by any metal components. The preparation-analysis chamber is normally equipped with several surface analysis tools such as Auger electron spectroscopy (AES) for compositional analysis, and lowenergy electron diffraction (LEED), or reflection high-energy electron dif­ fraction (RHEED) for structural analysis. Auger electron spectroscopy can be used to detect impurities down to 1% concentration. Electron diffrac­ tion allows one to observe the surface diffraction pattern that contains the information on the geometrical perfection of the clean surface. The growth chamber normally contains molecular-beam sources, beam

DEPOSITION CHAMBER

PREPARATION & ANALYSIS CHAMBER

TO COLD TRAP & TURBO PUMP

E-GUN FOR RHEED

Hl-T HEAT CLEANING STATION

SPUTTER CLEANING STATION

FASTENTRY LOAD LOCK

AES & XPS STATION

VIEW PORT & SUBSTRATE MANIPULATOR

Fig. 23. A schematic of a three-chamber MBE system: top view. (Provided by L. J. Schowalter. With permission of VG Semicon, East Grinstead, England.)

147

3. Metallization Techniques UHV CHAMBER

MOLECULAR BEAMS

LV/////I 4

t

SUBSTRATE

\ SHUTTERS

FURNACES Fig. 24.

Schematic of a MBE sources. (Provided by L. J. Schowalter.)

shutters, thickness monitors, a RHEED diffractometer, and a mass spec­ trometer. To grow epitaxial Si or metal suicides on the Si substrate, elec­ tron evaporation is employed. Knudsen cells can be used for dopant evaporation. (Other doping techniques such as ionization doping [125] have also been used recently.) Each beam has an individual shutter for multiple-layer growth. The deposition rate is measured by a quartz thick­ ness monitor. RHEED is used to monitor the surface morphology and structural properties during film growth. All components in the chambers are UHV compatible and bakable. 2. In Situ Surface Cleaning

A key to the success of growing epitaxial films by molecular-beam epitaxy method is the preparation of an atomically clean surface. Defects and contaminants in the surface act as nucleation centers for lattice defects such as stacking faults and dislocations in the growing film. If no foreign atom (typical elements are oxygen and carbon) is detected by AES, and if the surface reconstructs to give very sharp superlattice diffraction beams as measured by LEED or RHEED, the surface is said to be clean and free of high-density geometrical defects. Clean and low defect Si (100) and Si ( 111 ) surfaces give (2X1) and (7 X 7) superstructures, respectively. Although heating of the Si substrate to 1200°C can remove most of the surface contaminants, it causes a change in the dopant profile that is not desirable from the standpoint of device applications. Several low-tempera­ ture cleaning techniques have been employed. One commonly used tech­ nique is argon ion sputtering (1 - 10 kV) of the surface, which can remove

148

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

surface impurities very effectively. The surface damage caused by the sputtering can be healed by annealing the substrate at 850°C for a few minutes. However, it is not clear whether the entrapment of Ar atoms and the possible existence of point defects caused by the sputtering can affect the device performance. Research has showed [126] that surface contaminants can also be re­ moved by a short-time, high-energy laser-beam irradiation. Although the surface temperature is very high during the irradiation, the heating area is localized and the time very short. From the practical point of view, it is equivalent to a low-temperature cleaning process. Except for the cost of such high-power laser system, this technique seems very promising. Perhaps the simplest and most economic low-temperature cleaning tech­ nique is the newly developed chemical-thermal cleaning technique. It has been shown [127] that carbon and heavy metal contaminants can be eliminated by repetitive oxidation of the surface in boiling HN0 3 and HCl. The sample is then loaded into the UHV chamber. Prior to growth, the thin oxide layer can be decomposed and desorbed by in situ heating at about 8 50 °C. If a low-energy Si beam is incident on the surface at the time during the flash desorption, the decomposition temperature of the oxide can be reduced to below 750°C. 3. Metal MBE

After the surface is cleaned, the sample is transferred to the deposition chamber for metal MBE growth. For metal suicide formation, one fre­ quently employs the e-beam evaporation technique. Epitaxial suicides can be formed either by room temperature deposition of metals on clean Si substrates and followed by high-temperature annealing under UHV [128] or by codeposition of metal and silicon on heated silicon substrates [129]. The later growth method has been shown to give higher quality suicide films and lower growth temperatures. Examples of epitaxial suicides are NiSi2 and CoSi2. Their growth temperature ranges from 500 to 800°C. For metallic superlattice formation, thermal evaporation, e-beam evapo­ ration, and sputter deposition have all been used in the past [123]. The energy distribution of the beam, which affects the structural proper­ ties of the metallic multilayer, depends on the deposition technique as well as the relative position of the source and substrate. Optimum growth condition can be obtained by proper energy selection, control of the sub­ strate temperature and the deposition rate [123]. Examples of metallic epitaxy are Nb/Cu, Cu/Ni, Ag/Ti, Au/Ni, and Cu/Pd. Despite its low throughput (one to several angstroms per second deposi­ tion rate) and the fairly expensive UHV equipment involved, molecular-

3. Metallization Techniques

149

beam epitaxy has attracted increasing attention in recent years due to its ability to grow films of superior quality that no other method can match. E. Ionized Cluster Beam Deposition Techniques

Ionized cluster beam (ICB) deposition is a newly developed technique [ 130] in which ultrasmall clusters, up to several thousand atoms, are used in the deposition process. The ideas behind this deposition technique can be summarized as follows. The energy of the beam, supplied by accelera­ tion of ionized clusters, is spread over the atoms within the individual clusters. For example, when a 10-kV potential is applied to a singly charged cluster of 1000 atoms, each atom only carries an energy of 10 eV. Upon impact on the substrate these clusters are decomposed to create enhancedsurface mobility and reactivity. High-quality thin films and interfaces can therefore be formed at a relatively low substrate temperature. This tech­ nique is very different from the deposition of thin films using 0.1 - 1 μτη size molten clusters [131]. In the later case, instead of breaking into indi­ vidual atoms, the molten clusters splatter upon impact with the substrate resulting in a rougher surface. Several methods have been suggested to form such a cluster beam. In the pure nozzle expansion type of cluster formation [130], the deposition material is vaporized in a crucible at an elevated temperature such that the crucible pressure rises to several torr. Small clusters are believed to be generated during the rapid expansion of the hot vapors at a relatively high pressure through a small crucible opening (nozzle) into a vacuum region with a pressure of 10~6 Torr. The structure of these clusters is not well understood. These clusters are subsequently ionized by electron bombard­ ment prior to the deposition. Figure 25 shows a schematic of the ICB apparatus. Cluster formation of this kind has been a controversial subject for more than a decade. Conflicting results [130,132] have been reported on the measurement of metal or semiconductor clusters with this jet expansion technique. The existence of an abundance of clusters with a size of— 1000 atoms per cluster has not been demonstrated unambiguously in any experiment. Nevertheless, thin films with remarkable properties [ 130] have been produced with this deposition technique. Unlike the pure nozzle expansion technique, the carrier gas-assisted evaporation technique can unambiguously produce metal or semiconduc­ tor clusters with the desired size. The clusters produced in this method are cold and possess crystalline structures. It has been shown that the breakage of the cluster can still occur upon impact with the surface if a sufficiently high acceleration potential is applied to the ionized cluster beam. Lower

150

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

ACCELERATION ELECTRODE IONIZATION ANODE IONIZATION FILAMENT COOLING WATER PIPE ELECTRON BOMBARDMENT FILAMENT

TURN TABLE

Fig. 25. A schematic of the pure nozzle expansion type of ICB system. (With permission of Eaton Corporation, Boston, Mass.)

temperature epitaxy has been achieved with this deposition technique [132]. So far, most research activity on ICB deposition has been with the pure nozzle expansion type. As previously mentioned, there is no unambiguous or strong evidence for the existence of an abundance of clusters in this type of expansion. The high-quality thin films obtained so far might well be a result of the bombardment of the substrate by a beam consisting predomi­ nantly of monomers, but partially ionized and at supersonic velocity. This process may also generate high surface mobility, and therefore produce desirable thin films. Despite this controversy, we shall still refer to this as an ICB technique. 1. ICB Sources

The main components of the pure expansion type of ICB source are the crucible, its heating accessories, and the beam ionization and acceleration electrodes. The crucible is made out of high-purity graphite or other hightemperature materials, such as tantalum, and has a small nozzle at the top. The nozzle diameter D has to be larger than the mean free path of the vapor. Typically D = 1-2 mm. This crucible is quite different from the Knudsen cell for molecular-beam deposition in which D is smaller than the mean free path of the vapor. The length of the nozzle throat L is an important parameter that controls the degree of supersaturation one wishes to obtain. Qualitatively, the longer the nozzle throat, the more collisions one obtains during the expansion that leads to possibly larger clusters. The

3. Metallization Techniques

151

simplest types of nozzle geometry that has been used is of cylindrical shape with L/D= 1 [130]. To heat the crucible, either electron bombardment or radiation heating can be used. In order to obtain a vapor pressure of several torr, at the interior of the crucible, one requires a substantially higher crucible temper­ ature than is normally used in the conventional evaporation methods. For Al, the temperature has to be greater than 1600°C in order to achieve a pressure of several torr. Appropriate shielding or cooling of the source is therefore necessary to protect the surrounding, to maintain a good vacuum in the chamber, and to prevent overheating of the substrate. Electrons of several hundred volts of potential are used to bombard and ionize the species ejected from the crucible. The current can be varied from 0-300 mÀ. A fraction of the ejected material is ionized (singly charged) and is accelerated towards the substrate together with neutrals by applying a substrate bias potential of 0 - 10 kV. The bias potential allows one to control the energy of the depositing material. Multiple-ICB crucibles can be used simultaneously for alloy or compound film formation. A single crucible with multiple nozzles has also been considered for gaining higher deposition rate [130,133]. In the case of a carrier-gas-assisted cluster source, an open cup crucible is used to evaporate the deposition material. For the same deposition rate, the open cup crucible is operated at a much lower temperature than the nozzle-type crucible. Less contamination is expected. A cold carrier gas, such as helium, is sent to the chamber during the evaporation to quench the ejected vapor. After the clusters are created, they pass through a small hole into the deposition chamber that is differentially pumped. The beam is ionized by electron bombardment before reaching the substrate. 2. Principle of Metal Clusters Formation

While the basic principle of carrier-gas-assisted cluster formation is well established [134], serious effort aimed at understanding the formation of metal (or semiconductor) clusters during the pure nozzle expansion only began recently. In the carrier-gas-assisted cluster formation, the evaporated metal vapor is cooled down rapidly by a cold inert gas. The metal vapor then becomes suersaturated and the condensation of vapor into aggregates of atoms occurs. The size of the clusters can be controlled by the metal vapor pressure and the inert gas pressure. On the other hand, no carrier gas is used in the pure nozzle expansion type of cluster formation [130]. The ejection of hot metal vapors from the crucible through the small nozzle can be treated approximately as an adiabatic expansion process. During the expansion, the vapor cools down

152

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

rapidly through multiple collisions and become supersaturated. Part of the thermal energy has transformed into the translational kinetic energy of the vapor stream. Both the temperature and pressure drop as the vapor ex­ pands. Despite their high surface tension, it has been shown [135,136] that most metals and semiconductors have low free-energy barriers for cluster nucleation. Therefore, the condensation of the supersaturated vapor into aggregates of atoms can occur homogeneously. The velocity of the beam increases and the spread of the velocity be­ comes narrower as the expansion continues. One frequently uses the Mach number M, which is defined as the ratio of the vapor velocity to the sound velocity at the appropriate vapor temperature and pressure, to describe the expansion history. Typically, the Mach number can reach 6 after the expansion [136], which is an indication of very deep supersaturation. Based on a non-steady-state, step-wise cluster growth theory, an attempt has been made recently [137] to evaluate quantitatively the cluster size distribution for a hot Ag vapor (crucible pressure is 9 Torr) through a nozzle exit of 1-mm diameter. The result showed that it is possible to form a metal cluster size greater than 100 atoms/cluster in the nozzle expansion through homogeneous nucleation. (However, it is doubtful that the cruci­ ble pressure can reach several torr in a typical ICB deposition experiment.) The growth of the clusters is essentially completed within 1 cm from the nozzle throat. After that, the collision frequency is drastically reduced and the beam enters the molecular flow region. A completely different scheme base on the heterogeneous nucleation theory has also been proposed [138] to treat the metal cluster formation in the pure nozzle jet expansion. More accurate measurements of the cluster size is certainly needed in order to gain a clearer picture of the clustering process and of the ultimate size that the clusters can grow with such an expansion geometry. 3. Comments on the ICB Deposition Technique

The pure nozzle expansion type of ICB technique has been used to deposit a variety of metal, semiconductor, and polymer films [130]. The films showed high packing density, strong adhesion, and good metal con­ tact. Because of the unidirectional nature of the beam, potential applica­ tions in viafillingand liftoff processes were also discussed [139]. Normally, the ICB deposition technique is operated with a vacuum of ~ 10~7 Tonbase pressure. If a higher vacuum is used (< 10~9 Torr), one can grow [140] single-crystal Al on the Si substrate using the ICB technique inspite of the very large lattice mismatch (—25%) between Al and Si. Single-crystal Al-Si interface has been shown to possess unusual structural and electrical stability against heat treatment [141] and extremely high electromigration resistance [142].

3. Metallization Techniques

153

There are two major disadvantages in using ICB as a metallization technique. First of all, a crucible must be used in the technique. In order to generate a high vapor pressure inside the crucible, one has to heat the crucible to a high temperature. It is, therefore, not suitable for the metalli­ zation of high-temperature materials such as W and Mo. Secondly, in order to maintain a high pressure in the crucible, the nozzle exit must not be too large. This arrangement restricts the deposition rate. Typically, a rate of 5-10 A/sec. can be achieved for Al deposition. Construction of a multiple-nozzle source can increase the deposition rate. However, this means excess heating is necessary to maintain the high crucible pressure. Reaction between the deposition material and the crucible may then occur. Charged monomers always exist in the ionized jet beam unless they are mass separated. When a potential of several kilovolts is applied to the beam, these ionized monomers will accelerate towards the substrate to­ gether with the ionized clusters. The effect of these energetic monomers on the properties of the thin film has not been studied thus far. The potential of employing clusters of sizes less than 1000 atoms/cluster using the carrier-gas-assisted evaporation technique for metallization has not been explored so far, especially with respect to VLSI applications. REFERENCES 1. B. Hoeneisen and C. A. Mead, Solid-State Electron. 15, 819 ( 1972). 2. K. A. Aitchison and R. A. Hogle, in "Tungsten and Other Refractory Metals for VLSI Applications" (R. S. Blewer, ed.). Mater. Res. Soc, Pittsburgh, Pennsylvania, 1986. 3. J. F. O'Hanlon, /. Vac. Sci. Technoi, A 1, 228 (1983). 4. T. Takagi, Thin Solid Films 92, 1 ( 1982). 5. S. W. H. Yih and C. T. Wang, "Tungsten: Sources, Metallurgy, Properties and Appli­ cations." Plenum, New York, 1981. 6. H. Oikawa and T. Amazawa, Proc. Int. Symp. VLSI Technoi, 3rdS5-5, 131 (1985). 7. Private communication, D. W. Skelly with editors of "Chemical Economics Hand­ book," SRI International, 1986 data. 8. "Thin Film Processes" (J. Vossen and W. Kern, eds.). Academic Press, New York, 1978. 9. B. Chapman, "Glow Discharge Processes." Wiley, New York, 1980. 10. J. A. Thornton, In "Deposition Technologies for Films and Coatings," (R. Bunshah, ed.), p. 201. Noyes Pubi., New York, 1980. 11. "Handbook of Thin Film Technology" (L. I. Maissel and R. Glang, eds.), p. 4 - 18. McGraw-Hill, New York, 1970. 12. D. W. Hoffman and J. A. Thornton, J. Vac. Sci. Technoi. 17(1), 380 (1980). 13. I. A. Blech and H. A. Vander Pias, J. Appi. Phys. 54, 3489 (1983). 14. I. A. Blech, D. B. Fraser, and S. E. Haszko, J. Vac. Sci. Technoi. 15, 13 (1978). 15. J. F. Smith, Solid State Technoi. 27, 135(1984). 16. D. W. Skelly and L. A. Gruenke, J. Vac. Sci. Technoi, A 4, 457 (1986). 17. N. Mclntyre and S. J. Wright, Vacuum 34, 963 (1984).

154

D. W. Skelly. T.-M. Lu, and D. W. Woodruff

18. Y. Homma and S. Tsunekawa, J. Electrochem. Soc. 132, 1466 (1985). 19. C. Y. Ting, V. J. Vivalda, and H. G. Schaefer, J. Vac. Sci. Technol. 15, 1105 (1978). 20. T. Mogami, M. Morimoto, and H. Okabayashi, Conf. Solid State Devices Mater. 16th, Kobe, Jpn., Ext. Abstr. p. 43 (1984). 21. W. Class and R. Hieronymi, Solid State Technol. 25( 12), 55 ( 1982). 22. J. M. E. Harper, J. J. Cuomo, R. J. Gambino, and H. R. Kaufman, "Nuclear Instru­ ments and Methods in Physics Research," B 7/8, p. 886. North-Holland Pubi., Am­ sterdam, 1985. 23. J. E. Greene and S. A. Barnett, J. Vac. Sci. Technol. 21, 285 (1982). 24. B. A. Movchan and A. U. Demchishn, Fiz. Metall. 28, 653 (1969). 25. J. A. Thorton, J. Vac. Sci. Technol. 11, 666 (1974). 26a. J. A. Thornton and D. W. Hoffman, J. Vac. Sci. Technol. 14, 164 (1977). 26b. L. Hartsough, A. Koch, J. Moulder, and T. Sigmon, J. Vac. Sci. Technol. 17, 392 (1980). 26c. M. Hill, Solid State Technol. 23(1), 53 (1980). 27. C. F. Powell, in "Vapor Deposition" (C. F. Powell, J. H. Oxley, and J. M. Blocher, Jr., ed.), pp. 249-420. Wiley, New York, 1966. 28. D. M. Brown, W. R. Cady, J. W. Sprague, and P. J. Salvagni, IEEE Trans. Electron Devices ΕΌ-1Η, 931-940 (1971). 29. P. Rai-Choudhury and D. K. Schroder, J Electro. Chem. Soc. 118, 107-110 (1971). 30. W. Kern and V. S. Ban, in "Thin Film Processes" (J. L. Vossen and W. Kern, eds.), Chap. III-2, Academic Press, New York, 1978. 31. Smith, J. M., "Chemical Engineering Kinetics," 2nd Ed. McGraw-Hill, New York, 1970. 32. W. Kern and R. S. Rosler, J. Vac. Sci. Technol. 14, 1082 (1977). 33. D. R. Stull and H. Prophet, "JANAF Thermochemical Tables," 2nd Ed. Nati. Bur. Stand., Washington, D.C., 1971. 34. A Glassner, Argonne Nati. Lab. [Rep.] ANL ANL-5750. 35. L. P. Hunt and E. Sirtl, J. Electrochem. Soc. 119, No. 12 (1972). 36. K. E. Spear, Proc. Int. Conf. Chem. Vap. Deposition, 9th pp. 81-97 (1984). 37. C. S. Herrick and D. W. Woodruff, J. Electrochem. Soc. 131, 2417-2422 (1984). 38. D. W. Woodruff and C. S. Herrick, Electrochem. Soc. Meet., 167th, Ext. Abstr. 85-1, Abstr. No. 278(1985). 39. R. B. Bird, W. E. Stewart, and E. Lightfoot, "Transport Phenomena." Wiley, New York, 1960. 40. K. F. Jensen and D. B. Graves, / Electrochem. Soc. 130, 1950-1957 (1983). 41. H. Schlicting, "Boundary-Layer Theory," McGraw-Hill, New York, 1968. 42. C. F. Powell, J. H. Oxley, and J. M. Blocher, Jr., eds., "Vapor Deposition." Wiley, New York, 1966. 43. R. S. Blewer, ed., "Tungsten and Other Refractory Metals for VLSI Applications." Mater. Res. Soc, Pittsburgh, Pennsylvania, 1986. 44. C. M. Melliar-Smith, A. C. Adams, R. H. Kaiser, and R. A. Kushner, J. Electrochem. Soc. 121,298-303(1974). 45. C. F. Powell, in Ref. 42, pp. 322-326. 46. G. Wahl and P. Batzies, Proc. Int. CVD Conf., 4th, Electrochem. Soc, pp. 363-374, (1973). 47. L. H. Kaplan, and F. M. d'Heurle, J. Electrochem. Soc. 117, 693-700 (1970). 48. H. Peter, W. Hey, and J. N. Fordemwalt, in Ref. 43, pp. 393-398. 49. M. Diem, M. Fisk, and J. Goldman, Thin Solid Films 107, 39-43 (1983). 50. E. K. Broadbent and W. T. Stacy, Solid State Technol. 28( 12), 51 - 59 ( 1985).

3. Metallization Techniques

155

51. K. H. Meyer, Int. Conf. Chem. Vap. Deposition, 3rd, Am. Nucl. Soc. Proc. pp. 625642(1972). 52. R. J. Mianowski, K. Y. Tsao, and H. A. Waggener, in Ref. 43, pp. 145-160. 53. C. C. Tang, J. K. Chu, and D. W. Hess, Solid State Technol 26(3), 125 (1983). 54. J. K. Chu, C. C. Tang, and D. W. Hess, Appi. Phys. Lett. 41, 75 (1982). 55. D. W. Hess, in "VLSI Electronics: Microstructure Science," N. G. Einspruch and D. M. Brown, eds.), Vol. 8, pp. 55-68. Academic Press, Orlando, Florida, 1984. 56. N. E. Miller and I. Beinglass, Solid State Technol. 25(12), 85-90 (1982). 57. R. H. Wilson, R. W. Stoll, and M. A. Calacone, in Ref. 43, pp. 35-42. 58. R. H. Wilson, R. W. Stoll, and M. A. Calacone, V-MIC Conf. pp. 343-349 (1985). 59. G. C. Smith, V-MIC Conf, pp. 350-356 (1985). 60. G. C. Smith, in Ref. 43, pp. 323 - 330. 61. W. A. Metz and E. A. Beam, in Ref. 43, pp. 249-256. 62. J. P. Roland, N. E. Hendrickson, D. D. Kessler, D. E. Novy, Jr., and D. W. Quint, Hewlett-Packard J. Aug., 30-32 (1983). 63. E. K. Broadbent and C. L. Ramiller, J. Electrochem. Soc. 131, 1427-1433 (1984). 64. W. A. Bryant, J. Electrochem. Soc. 125, 1534-1543 (1978). 65. H. Cheung, Int. Conf. Chem. Vap. Deposition, 3rd, Am. Nucl. Soc. Proc. pp. 136-144 (1972). 66. C. McConica and K. Krishnamani, in Ref. 43, pp. 433-442. 67. Y. Pauleau and Ph. Lami, J. Electrochem. Soc. 132, 2779-2784 (1985). 68. Y. Pauleau, Ph. Lami, B. Minghetti, and A. Tissier, in Ref. 43, pp. 135-144. 69. Wilson, R. H., in Ref. 43, p. 221. 70. A. J. Learn and D. W. Foster, J. Appi. Phys. 58, 2001 -2007 (1985). 71. M. L. Green and R. A. Levy, J. Electrochem. Soc. 132, 1243-1250 (1985). 72. K. C. Saraswat, S. Swirhun, and J. P. McVittie, Proc—Electrochem. Soc. 84-7, 409-419(1984). 73. A. J. Learn, J. Electrochem. Soc. 132, 390-393 (1985). 74. W. C. Conners, Jr., in "Hydrogen in Metals" (Z. Paal, ed.). Springer-Verlag, New York, 1986. 75. K. Y. Tsao and H. H. Busta, J. Electrochem. Soc. 131, 2703-2708 (1984). 76. W. T. Stacy, E. K. Broadbent, and M. H. Norcott, J. Electrochem. Soc. 132, 444-448 (1985). 77. D. C. Paine, J. C. Bravman, and K. C. Saraswat, in Ref. 43, pp. 117-124. 78. R. S. Blewer and M. E. Tracy, in Ref. 43, pp. 53-62. 79. D. W. Woodruff, R. H. Wilson, and R. A. Sanchez-Martinez, in Ref. 43, pp. 173-186. 80. M. Bowman and J. Carlsson, Proc. Int. Conf. Chem. Vap. Deposition, 9th p. 150. Electrochem. Soc, Princeton, New Jersey, 1984. 81. C. Fuhs, E. J. Mclnerney, L. Watson, and N. Zetterquist, in Ref. 43, pp. 257-266. 82. R. J. Saia, B. Gorowitz, D. W. Woodruff, and D. M. Brown, Ext. Abstr. Electrochem. Soc. Meet., 170th, Vol. 86-2. San Diego, Calif. 1986. 83. J. F. O'Hanlon, /. Vac. Sci. Technol., A 1, 228-232 (1983). 84. M. J. Cooke, R. A. Heinecke, R. C. Stern, and J. W. C. Maes, Solid State Technol. 25(12), 62-65(1982). 85. R. A. Levy, M. L. Green, and P. K. Gallagher, J. Electrochem. Soc. 131, 2175-2182 (1984). 86. M. L. Green, R. A. Levy, R. G. Nuzzo, and E. Coleman, Thin Solid Films 114, 367-377(1984). 87. R. A. Levy, P. K. Gallagher, R. Contolini, and F. Schrey, J. Electrochem. Soc. 132, 457-463(1985).

156

D. W. Skelly, T.-M. Lu, and D. W. Woodruff

88. C. L. Rollinson, in "Comprehensive Inorganic Chemistry" (J. C. Bailar, Jr., H. J. Emeleus, R. Nyholm, and A. F. Trotman-Dickenson, eds.), Chap. 36. Pergamon, Oxford, 1973. 89. G. E. Carver and B. O. Séraphin, Appi. Phys. Lett. 34, 279-281 (1979). 90. G. E. Carver, Thin Solid Films 63, 169 -174 ( 1979). 91. W. J. McCreary, Proc. Conf Chem. Vap. Deposition, Int. Conf, 5th, pp. 714-725 (1975). 92. J. A. Papke and R. D. Stevenson, Proc. Conf Chem. Vap. Deposition Refract. Met., Alloys, Compd, pp. 193-204(1967). 93. D. K. Seto, V. Y. Doo, and S. Dash, Proc. Int. Conf. Chem. Vap. Deposition, 2nd, pp. 659-691 (1970). 94. K. Hieber and M. Stoltz, Proc. Conf. Chem. Vap. Deposition, Int. Conf, 5th. pp. 436-449(1975). 95. R. R. Jaeger and S. T. Cohen, Proc. Int. Conf. Chem. Vap. Deposition, 3rd, pp. 500-512(1972). 96. M. Ayukawa, K. Yamagishi, K. Matsuda, and H. Shimizu, 1984 Fall Meet. Jpn. Soc. Appi. Phys. Rep. 14p-C-14; summarized and translated by Science Forum, Inc., JST News 3(6), 37-38(1984). 97. D. K. Seto, V. Y. Doo, and S. Dash, Proc. Int. Conf Chem. Vap. Deposition, 2nd, pp. 659-691 (1970). 98. R. Solanki, C. A. Moore, and G. J. Collins, Solid State Technol. 28(6), 220-227 (1985). 99. Y. Rytz-Froidevaux, R. P. Salathe, and H. H. Gilgen, Appi. Phys. A 37, 121-138 (1985). 100. Y.S.Liu, in Ref. 43, pp. 43-52. 101. S. P. Murarka, "Silicides for VLSI Applications," Academic Press, New York, 1983. 102. S. P. Murarka, J. Vac. Sci. Technol. 17, 775-792 (1980). 103. F. Mohammadi, Solid State Technol. 24( 1 ), 65 - 72 ( 1981 ). 104. A. K. Sinha, J. Vac. Sci. Technol. 19, 778-785 (1981). 105. S. P. Murarka, Proc.—Electrochem. Soc. 81-5, 551-561 (1981). 106. D.S. Williams and E. Coleman, Workshop Refract. Met. Silicides VLSI, U. C Berkeley Ext., San Juan Bautista, Calif, 1985. 107. K. Akitmoto and K. Watanabe, Appi. Phys. Lett. 39, 445-447 (1981). 108. D. L. Brors, J. A. Fair, K. A. Monnig, and K. C. Saraswat, Solid State Technol. 26(4), 183-186(1983). 109. K. Monnig, Workshop Refract. Met. Silicides VLSI II, U. C Berkeley Ext., San Juan Bautista, Calif, 1984. 110. S. Sachdev and T. T. Doan, Workshop Refract. Met. Silicides VLSI II, U. C Berkeley Ext., San Juan Bautista, Calif, 1984. 111. J. A. Fair, Workshop Refract. Met. Silicides VLSI, U. C Berkeley Ext., 1983. 112. W. I. Lehrer and J. M. Pierce, Proc.—Electrochem. Soc. 81-5, 588-595 (1981). 113. K. W. Beeson, G. A. West, and A. Gupta, Workshop Refract. Met. Silicides VLSI, U. C Berkeley Ext. 1985. 114. R. S. Rosier and G. M. Engle, J. Vac. Sci. Technol., B 2, 733 (1985). 115. W. I Lehrer, J. M. Pierce, E. Goo, and S. Justi, Int. Symp. Very Large Scale Integr. Sci. Technol. pp. 258-264 (1982). 116. C. A. Neugebauer, in "Handbook of Thin Film Technology" (L. I. Maissel and R. Gland, eds.), p. 8-3. McGraw-Hill, New York, 1970. 117. R. F. Bunshah and R. S. Juntz, "Beryllium Technology," Vol. 1, p. 1. Gordon & Breach, New York, 1966.

3. Metallization Techniques

157

118. L. Holland, Br. Pat. 754, 102 (1951); R. A. Denton and A. D. Greene, Proc. Electron Beam Symp., 5th p. 180. Alloyd Electron. Corp., Cambridge, Massachusetts, 1963. 119. S. Vaidya, D. B. Fraser, and A. K. Sinha, Annu. Proc. Reliab. Phys. Symp. 18, 165 (1980). 120. I. A. Blech, D. B. Fraser, and S. E. Haszko, J. Vac. Sci. Technol. 15, 13 (1978). W. Fichtner, in "VLSI Technology" (S. M. Sze, ed.), p. 385. McGraw-Hill, New York, 1983. 121. For review see K. L. Wang, Solid State Technol. 28( 10), p. 137 ( 1985). 122. J. C. Hensel, A. F. J. Levi, R. T. Tung, and J. M. Gibson, Appi. Phys. Lett. 47, 157 (1985). 123. For review see I. K. Schuller and C. M. Falco, in "VLSI Electronics" (N. G. Einspruch, ed.), Vol. 4, p. 183. Academic Press, New York, 1982. 124. L. J. Schowalter, private communication. 125. H. Sugiura, J. Appi Phys. 51, 2630 (1980); Y. Ota, J. Vac. Sci. Technol. A 2, 393 (1984). 126. D. M. Zehner, C. W. White, and G. W. Ownby, Appi. Phys. Lett. 36, 56 (1980). 127. A. Ishizaka, K. Nakagawa, and Y. Shiraki, Proc. Int. Symp. Mol. Beam Epitaxy Relat. Clean Surf. Tech., 2nd, Tokyo p. 183 (1982). 128. H. Ishiwara, S. Nagatoma, and S. Furukawa, Nucl. Instrum. Methods 149, 417 (1978); S. Saitoh, H. Ishiwara, and S. Furukawa, Appi. Phys. Lett. 37, 203 (1980). 129. J. C. Bean and J. M. Poate, Appi Phys. Lett. 37, 743 (1980); R. T. Tung, J. M. Gibson, and J. M. Poate, Appi. Phys. Lett. 42, 888 (1983). Y. C. Kao, M. Tejwani, Y. H. Xie, T. L. Lin, and K. L. Wang, J. Vac. Sci. Technol., B 3,596(1985). 130. T. Takagi, / Vac. Sci. Technol., A 2, 382 (1984); I. Yamada and T. Takagi, Thin Solid Films 80, 105 (1981); H. Inokawa, H. Hsui, S. C. Cheng, and T. Takagi, Thin Solid Films 22, 137(1982). 131. C. D'Cruz, K. Pourrezaei, and A. Wagner, J. Appi. Phys. 58, 2724 ( 1985). 132. A. E. T. Kuiper, G. E. Thomas, and W. J. Schouten, /. Cryst. Growth 51, 17 (1981). 133. T. Negishi, M. Itoh, and F. Kimijima, Proc. Int. Workshop Ioniz. Cluster Beam Tech. Kyoto p. 21 (1986). 134. C. G. Granqvist and R. A. Buhrman, J. Appi. Phys. 47, 2200 (1976). 135. S.-N. Yang and T.-M Lu, J. Appi. Phys. 58, 541 (1985); Chem. Phys. Lett. 127, 512 (1986). 136. I. Yamada, Proc. Int. Ion Eng. Congr. — ISIAT (Ion Source Ion Assisted Technol.), IPAT (Ion Plat. All. Technol.), Kyoto, 1983; I. Yamada, T. Takagi, P. R. Younger, and J. Blake, SPIE, Los Angeles Tech. Symp. Opt. Electro-Opt. Eng. p. 530 (1985). 137. S.-N Yang and T.-M Lu, Appi. Phys. Lett. 48 H22 (1986). 138. W. Knauer, Proc. Int. Workshop Ioniz. Cluster Beam Tech., Kyoto p. 41 (1986). 139. R. Ramanarayanan, J. Wong, T.-M. Lu, and D. Skelly, J. Vac. Sci. Technol. B4(5), 1180(1986). 140. I. Yamada, H. Inokawa, and T. Takagi, J. Appi. Phys. 56, 2746 (1984). 141. I. Yamada, C. J. Palmstrom, E. Kennedy, J. W. Mayer, H. Inokawa, and T. Takagi, in "Layered Structure, Epitaxy and Interfaces" (J. M. Gibson and L. R. Dawson, eds.), Vol. 37, p. 402. Mater. Res. Soc. Pittsburgh, Pennsylvania, 1985. 142. C. G. Chen, private communication.