Sputter Deposition Processes

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THIN FILM PROCESSES II 11-4 Sputter Deposition Processes ROBERT PARSONS Department of Physics The University of British Columbia Vancouver, British Columbia Canada I. Introduction A. General B. Features of a Sputter Coater C. Considerations of Film Properties A. Nonmagnetron Sources (Diode and Triode) B. Magnetron Sputter Sources C. Ion Beam Sputter Sources A. General B. Specific Applications A. Reactive Sputtering A. General B. Technical Considerations 11. Sputter Sources 111. Sputter Deposition of Conducting Films IV. Sputter Deposition of Dielectric Films V. Sputter Coating Systems VI. Emerging Technologies VII. Concluding Remarks References 177 177 179 180 183 183 184 188 188 188 189 191 191 200 200 200 203 203 204 1. INTRODUCTION A. General The intent of this chapter is to present a comprehensive treatment of sputter deposition of thin films, with the main emphasis on the practical, engineering aspects of the sputter technique. For a detailed discussion of 177 Copyright 0 1991 by Academic Press, Inc All rights of reproduction in any form reserved. ISBN 0-12.728251-3

Transcript of Sputter Deposition Processes

Page 1: Sputter Deposition Processes

THIN FILM PROCESSES II

11-4

Sputter Deposition Processes

ROBERT PARSONS Department of Physics The University of British Columbia Vancouver, British Columbia Canada

I. Introduction A. General B. Features of a Sputter Coater C. Considerations of Film Properties

A. Nonmagnetron Sources (Diode and Triode) B. Magnetron Sputter Sources C. Ion Beam Sputter Sources

A. General B. Specific Applications

A. Reactive Sputtering

A. General B. Technical Considerations

11. Sputter Sources

111. Sputter Deposition of Conducting Films

IV. Sputter Deposition of Dielectric Films

V. Sputter Coating Systems

VI. Emerging Technologies VII. Concluding Remarks

References

177 177 179 180 183 183 184 188 188 188 189 191 191 200 200 200 203 203 204

1. INTRODUCTION

A. General

The intent of this chapter is to present a comprehensive treatment of sputter deposition of thin films, with the main emphasis on the practical, engineering aspects of the sputter technique. For a detailed discussion of

177 Copyright 0 1991 by Academic Press, Inc

All rights of reproduction in any form reserved. ISBN 0-12.728251-3

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the basic underlying physics of sputtering, the reader is referred to Chapter 11.1 and the excellent review articles on cathode sputtering [l], discharge sputtering [2], magnetron sputtering [3, 41, and reactive sputtering [5 , 61.

Sputtering involves many interrelated physical and chemical processes. Although our basic understanding of this complex subject is incomplete, sputtering is a very mature technology, as evidenced by the many applica- tions of the process. Examples include magneto-optical storage media, compact disks, planarized coatings for multilayer circuits, optical multi- layer coatings for mirrors and filters, solar control and low emissivity window coatings, conductors and barrier layers for very large scale inte- grated circuits, solar cells, diamondlike coatings, transparent conducting electrodes, amorphous optical films for integrated optics devices, lumines- cent films, microcircuit photolithographic mask blanks, wear-resistant coatings for cutting tools, and decorative coatings. One of the main reasons for this development has been the apparent ease of extending results obtained empirically on a small, research-size sputter coater to a highly reliable, production process. Other reasons for using sputtering include:

Excellent film uniformity, particularly over large areas; * Surface smoothness and thickness control;

Deposition of films with nearly bulklike properties, which are pre- dictable and stable; Versatility; the sputter process is essentially a kinetic process involv- ing momentum exchange rather than a chemical and/or thermal process and, therefore, virtually any material can be introduced into a gas discharge or sputtered from the solid; Good adhesion; Either conformal or planarized coatings; and High rates, which are comparable to evaporation.

Sputter sources for film deposition can be categorized in two ways: glow discharge (diode, triode, and magnetron) and ion beam. In all cases the particles are ejected by the same basic mechanism of momentum exchange between energetic particles and surface atoms. However, as discussed in Chapter 11.1, the various source configurations cover quite different pro- cess parameters and all have their particular advantages. Nonmagnetron sources, especially ion beam sources and the rf planar diode, are widely used; however, the planar magnetron with its high efficiency and conve- nient geometry for scale-up is well established as the sputter source of choice. Ion beam sputtering can be subdivided into deposition and etching applications. Ion beams are used for film deposition in two basic configura- tions; primary ion beam deposition and ion beam sputtering (also called

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LEAK V A L V E

INERT REACTIVE PRESSURE GAUGE

SUBSTRATE SPUTTERED ATOM

SPUTTER SOURCE

SUBSTRATE

HIGH VACUUM PUMP

Fig. 1. General features of a sputter coater.

secondary ion beam deposition) [7 , 81. The latter technique is discussed in this chapter. Sputter etch applications are treated in Chapter V.2.

B. Features of a Sputter Coater

Figure 1 shows the standard parts of a sputter coater:

A stainless steel or mild steel chamber, which has been certified vacuum-tight with a helium leak detector;

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Pumping capacity that is capable of reducing the chamber pressure to about 1 x lop6 torr (or lower in the case of ultrahigh-vacuum applica- tions); Pressure gauges; In the case of the glow discharge sources, a means to raise the chamber pressure to about 5 X torr for sputter operation; for example, with a combination of mass flow controllers, and a variable orifice valve (“throttle”) to reduce the pumping speed; Sputter source and power supply; and Substrate holder.

In addition, the coater can have the following hardware:

Substrate heater;

Separate ion source for bombardment of the growing film; Multiple sputter sources for co-sputtering; Residual gas analyzer and/or optical emission monitor to measure partial pressures and sputtered flux; and

Power supply to apply a voltage to the substrate (for bias sputtering);

Automation control system.

C. Considerations of Film Properties

Before we consider specific sputter processes, it is useful to review the dependence of film microstructure on the growth parameters. By control of film microstructure, many film properties of practical importance, such as intrinsic stress, refractive index, surface roughness, and electrical resistiv- ity, can be promoted. Microstructure is determined primarily by the sur- face and near-surface environment during film growth-more specifically, the adatom mobility. The main parameters for the control of adatom mobility are substrate temperature and particle bombardment [9-131.

The microstructure of sputtered films is usually classified in terms of four zones [14, 151. Zone 1 structure, consisting of tapered columns and significant voids between columns, is prevalent which the ratio T/T, is less than about 0.3, where Tis the growth temperature and T, is the melting point of the deposited material. In this range of growth temperatures, adatom diffusion is negligible and, as a result of shadowing effects, most of the sputtered flux is deposited on high points on the film, with little material reaching the valleys. Factors that increase shadowing, such as increased angle of incidence of the coating flux, promote the growth of Zone 1. These factors should be considered in the case of substrate move- ment, and are particularly important for wide-angle geometries such as the

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cylindrical-post magnetron (Section 11,B) [16]. Zone 1 structure is also promoted by the presence of impurities such as oxygen [15] that, presum- ably, reduce the movement of adatoms. Zone 1 structures tend to be associated with rough surfaces, poor stability, and properties that are far from those of the bulk material [17].

Zone 2 is usually found when 0.3 < TIT,,, < 0.5, which is associated with significant adatom diffusion on grain surfaces. Zone 2 structures are characterized by columnar, platelet, or whisker grains separated by dense intercrystalline boundaries. In the case of many compound semiconductors of interest, an optimum growth situation occurs in the structure-sensitive properties, such as surface smoothness, when the growth temperature is within a few percent of one-third of the boiling point of the compound [18].

Zone 3 occurs at high relative temperatures, TIT,,, > 0.5, when diffu- sion within the grains is a significant mechanism of film growth. Zone 3 is associated with equiaxied grains and epitaxial growth on the substrate. By elevating the substrate temperature during film growth, semiconductor materials such as GaAs and Si have been epitaxially grown [19].

The fourth zone, Zone T (“transition”), is the result of bombardment- induced surface mobility. Films that would have been expected to be Zone 1 structures on the basis of TIT,,, can be grown with a very smooth surface and high density by bombarding the growing film with energetic particles during film growth 115, 20-251. The main bombarding species of importance in the case of magnetron sources are ions and energetic neutrals. The latter species originate from the target either as positive ions neutralized and reflected from the target surface [26], or as sputtered nega- tive ions that are accelerated in the dark space and then neutralized in the gas [27]. Film material is moved into the spaces between grains by forward sputtering [14, 281 and by energy deposited locally by the bombarding particles (i.e., thermal spikes). This movement of material leads to tightly packed fibrous grains.

Surface mobility and, consequently, Zone T structures, can be pro- moted by control of positive ion bombardment. Since the plasma is always the most positive part of the glow discharge, the self-bias voltage on a floating substrate is approximately equal to the energy of the bombarding, positive ions (usually 10-30 eV). The ion energies can be increased by applying a negative bias to the substrate. The upper limit to the ion energy is usually set by resputtering of the film-e.g., about 200 eV. Insulating substrates can be biased with the use of a rf matching network to the substrate carousel, or indirectly with the use of a positively biased elec- trode separate from the substrate. Alternatively, bombardment can be accomplished with use of an ion source [22, 23, 291 or an unbalanced magnetron (Section 11,B).

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Bombardment of the growing film is a well-known method [9,30-371 to make film properties more closely matching the bulk values. Zone T films, produced by bombardment, tend to have smooth surfaces, high densities, and other properties close to the bulk values. Thus, these films are usually the desired type of coatings for applications. Both the ion energy and the ion/atom arrival-rate ratio are important factors determining the effects of bombardment [38-401. For example, the energy range between 10 and 20 eV, with ion/atom arrival-rate ratio > 1, is necessary to produce di- amondlike thin films with a high percentage of sp3 to sp2 bonds [41, 421. Studies of optical thin films, such as SiOz, and T i02 , have shown that low-energy ions (e.g., 30 eV) are better than higher energies (e.g., 500 eV) for the improvement of optical quality, which is related to film stoichi- ometry and density [43]. Forward sputtering (or recoil implantation) of the film atoms into voids does not require as much energy as complete sputtering, since momentum reversal is not required. Because forward- sputtered atoms fill void regions much more easily than interstitial implan- tation into a perfect lattice, low-energy ion bombardment is an important factor in decreasing void content and, thereby, changing properties in both crystalline and amorphous films [22, 39, 44-49]. At low ion energies (e.g., 60 eV), the incorporation of gas is orders of magnitude lower and subsurface damage is less [.SO].

The term ion plating is being used when the substrate is “in contact” with the plasma, and the term ion-assisted deposition is used where the substrate is bombarded by an ion beam in a “vacuum” environment. In many commercial applications, such as optical and wear-protective coat- ings [51, 521, the use of either ion plating or ion-assisted deposition to densify films and to improve the adhesion [53-551 to the substrate is a crucial part of the deposition process. In addition, treatment of substrates prior to deposition with an O2 plasma helps to remove organic contami- nants and covers the substrate with a thin oxide layer that has a low sticking coefficient for organic materials. Vossen has discussed the preparation of substrates for film deposition using glow-discharge techniques [56 , and Chapter 1.21.

Bombardment effects can be also influenced by the pressure, angle of incidence, magnetic field configuration, discharge current, and working gas species [57,58]. For example, decreasing the pressure causes an increase in cathode voltage, which results in more energetic particle bombardment. As the angle of incidence increases, the effect of bombardment is gradually reduced. Thus, Zone T structure, seen at normal incidence, can change to Zone 1 at oblique angles [57].

Bombardment of the growing film usually causes high compressive stresses in the films. This is attributed to rearrangement of the condensing

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layers due to recoil implantation [9,30-331 and, to a lesser degree, entrap- ment of the incident working gas [59]. The passage from Zone 1 to Zone T is usually associated with a change in film stress from tensile to compres- sive. The underdense Zone 1 structure causes tensile stress, lower reflectance, higher resistivity, and more impurity contamination.

Optical thin films are usually refractory and, therefore, T / T , ratios are invariably very low. As a consequence, all optical films have a columnar structure, with the important differences being the closeness of the packing of the columns [60]. The volume of the film associated with voids between the columns is typically 5% for Zone T sputtered films, but it can be as high as 30% in Zone 1 films. Since the film is a composite of solid parts plus voids, its refractive index is expected to be less than that of bulk material.

I I . SPUlTER SOURCES

A. Nonmagnetron Sources (Diode and Triode)

The planar diode is the simplest sputter source. The cathode target is typically in the shape of a disk about 5 to 10 cm in diameter, consisting of the material of interest. The target is usually thermally bonded (e.g., solder or conducting epoxy) to a water-cooled backing plate, or directly water-cooled with the use of a vacuum/water O-ring seal. A ground shield is used to suppress undesirable sputtering of the sides and support structure of the source body. A detailed treatment of the glow discharge is given in Chapter 11.1.

An advantage of diode sputtering is the efficient use of target material. Since the diode electrodes can be large and the electric field between them quite uniform, as in a large parallel-plate capacitor, the ion flux is nearly constant over the target. The major weakness of the diode technique is inefficient use of secondary electrons. In using a diode sputter source, one must take into account bombardment of the growing film by energetic electrons, which significantly increases the substrate temperature, and accept low deposition rates compared to other sputter sources that are capable of low-pressure (e.g., < 5 mtorr) operation.

In a triode source, a heated filament is added to a diode source to provide electrons to sustain the glow discharge, independent of the target. In this manner, ionization efficiency is increased, and thus, the discharge is able to operate at lower pressures (0.5 to 1 mtorr) and lower target voltage. As a result, higher deposition rates (several thousand angstroms per minute) can be achieved with triodes than with planar diodes. The main disadvantage of the triode source is shortened lifetime of the fila- ment in the case of reactive gases.

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8. Magnetron Sputter Sources

The class of sputter sources called magnetrons has a magnetic field of about 50 to 500 gauss parallel to the target surface, which in combination with the electric field causes the secondary electrons to drift in a closed circuit, or “magnetron tunnel,” in front of the target surface [3, 4, 16, 61, and Chapter 11.11. This electron confinement significantly increases the efficiency and, as a result, a magnetron can operate at low pressures (e.g., 1-3 mtorr) and low voltage (e.g., 350 V). The current density at the cathode of a magnetron is peaked where the magnetic field lines are tangent to the surface of the cathode [62]. Therefore, the erosion of the target is nonuniform.

Figure 2 shows a cross-sectional view of a typical planar magnetron [4, 9, 63-65]. In the S-gun configuration, the target surface is conical [66]. Other magnetron configurations include cylindrical and hollow [ 161.

One of the disadvantages of the planar magnetron is poor utilization of the target material, which is typically 20-30% of the starting target material. This problem can be overcome by providing relative motion of the target with respect to the magnets [66]. For example, in the rotatable cylindrical magnetron, the target is in the shape of a tube that is rotated around a fixed magnet array. As a result, target utilization of up to 90% has been reported [66]. Inefficient target utilization can be a severe prob- lem in the case of magnetic targets. If the permeability of the target is high, magnetic flux leakage is concentrated at narrow regions on the target

Fig. 2. Cross sectional view of a planar magnetron sputter source.

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surface and, as a result, a narrow erosion track results. Alternatively, it may be practical to heat the target above the Curie temperature for the magnetic material.

Magnetrons work well with either dc or rf power. The most common type of rf magnetron configuration uses the chamber and other grounded fixtures for the second rf electrode. With this single-ended configuration, the area of the sputtered target surface is usually small compared with that of the ground electrode and, therefore, only the target electrode has a sufficiently large bias voltage for sputtering. If the length of the magnetron is less than about 30 cm, rf sputtering is as easy to implement as dc sputtering, provided the following points are observed:

Ground shielding, which is conformal and as close as possible (e.g., a

Good ground return, with flat bars rather than wire in order to

Matching box, with minimum length of cable from the matching box

few millimeters) to the target body;

reduce inductance; and

to the source.

While a small rf coater is straightforward to set up, scaling up rf sputtering to large sources is very difficult because of the following problems:

Nonuniform power delivery along the length of the source due to

“Cross-talk” between sources operated simultaneously; Problems with fabricating a high-power matching box; Arcing of the grounded fixtures located near the source, which can have a significant bias voltage if the ratio of the target area is compa- rable to that of the surrounding grounded surfaces;

standing wave patterns;

Concerns about the effects of rf leakage on personnel; and The cost of rf power supplies.

The magnetic field does not directly affect the ion motion; however, because of electrostatic attraction ‘the ions move with the electrons, keep- ing the plasma neutral. In a conventional magnetron, most of the discharge is confined close to the cathode surface and, therefore, bombardment of the growing film by electrons and ions is minimized. Substrate bombard- ment can be significantly increased by “unbalancing” the magnetics [67]. In an unbalanced magnetron, the flux from the north pole is unequal to that entering the south pole, as indicated in Fig. 3. There are two types of unbalanced magnetron configurations [68]. In type I, the flux from the central magnet is larger than that of the outer magnet; in type I1 (Fig. 3),

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;TARGET

Fig. 3. Schematic representation of an unbalanced magnetron (type 11).

the central flux is less than the outer flux. Type I has been shown to give low ion and electron currents at the substrate and low self-bias voltages [68]. Type I1 systems give large ion currents and large electron currents (about 100 times larger than the case of type I sources) to the substrate, and high self-bias voltages (about 20 to 30 volts). Electrons are channelled along field lines extending from the discharge region to the substrate. The ions are electrostatically dragged by the electrons and, thereby, bombard the substrate. The type 11, unbalanced magnetron is capable of giving ion fluxes at the substrate that are much larger than the flux of sputter atoms. Typical ion currents for such an arrangement are 3 to 10 mA/cm2 [67, 681. The ion flux is very dependent on the magnetic field configuration, dis- charge current, and substrate bias; however, it is roughly independent of the target composition, gas pressure, and gas composition.

In practice, it is difficult to construct a perfect, “balanced” magnetron because magnetic field lines are difficult to focus. Thus, magnetron sources are almost always accompanied by substrate bombardment by ions and electrons from the plasma discharge. For most dc magnetron systems, the ion fluxes are typically 5 1 0 % of the deposition flux [69]. For applications

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requiring the minimum possible bombardment, it may be necessary to increase the operating pressure and to increase the target-to-substrate distance .

It is common practice to use the ground shield, chamber walls, and other grounded hardware for the anode. The first field line, from the target, to intersect a grounded surface determines a "virtual anode sheet" [70]. As mentioned in Section II,C, the effect of introducing a positively biased anode is to impose a negative potential on the substrate, relative to the plasma potential.

The dc magnetron discharge is characterized by a superlinear depen- dence of the current on the cathode voltage. This behavior is understood in terms of gas heating and rarefaction resulting from collisions with the sputtered atoms [71,72, and Chapter 11.11. Typical magnetron characteris- tics are: cathode current density of 20 mA/cm2, discharge voltage 250 to 800 V, and minimum pressure of about 1 mtorr. The target-to-substrate distances reported in the literature vary from a few centimeters to 20 cm; however, about 6 cm is typical. Deposition rates of several thousand ang- stroms per minute are obtained in the case of most metals, and 100-2,000 A/min can be achieved for dielectrics.

As the target erodes, the plasma impedance changes because of the increased strength of the parallel component of the magnetic field at the surface of the target. For most applications, this does not pose a problem. If the magnets are accessible, the field can be kept constant at the target surface by adjusting the distance of the magnetic array with respect to the bottom of the erosion track. Electromagnetics are usually too bulky to be seriously considered for this purpose.

A design study of the magnetic field for cylindrical-post magnetron sources has been reported [73]. A cylindrical magnetron source has been designed for deposition on the inside wall of a long (20 cm), small-diameter (2.5 cm) closed-end tube [74]. In the hollow cathode device, electrons are reflected from the cathode wall [75,76]. If the anode is located outside the cathode cavity, the efficiency of plasma generation is high. There are two different types of hollow cathode devices. One of them is basically the inverted configuration of the cylindrical magnetron and does not rely on thermal effects for the plasma to be sustained. The second type of device, called a hot hollow cathode (HHC), relies on a very thermionic emission of electrons from the cathode surface, which is typically above 2,OOO"C. The HHC discharge is an arc in terms of its high current (1 to 1,000 A) and low voltage (V = 20-70 V). Reviews of both types of hollow cathode sources for various kinds of metallurgical processing that involve a powerful heat source (e.g., large evaporators) are found in the literature [77].

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C. Ion Beam Sputter Sources

The multiaperture Kaufman ion source is, by far, the most popular type of source for ion beam sputtering. Detailed descriptions of the Kaufman source, together with other types of sources (e.g., duplasmatron) and ion beam sputtering techniques, are found in the literature [7]. The unique features of ion beam sputtering are:

Complete isolation of the substrates from the ion generation process; Minimal interaction between the processes at the target and pro-

Control of the angle of ion impact and the spot size; Independent control of the ion energy and current density; and Low background pressure (typically 0.1 mtorr).

cesses at the substrate;

The first two points are important if the minimum possible heat load is to be delivered to the substrate; however, it may be a disadvantage if bom- bardment of the growing film is desired (see Section 1,C). Control of ion flux (the third and fourth points) is very useful for studies of sputter yield and deposition processes. Low background pressure (the last point) gives less gas incorporation and less scattering of sputtered particles.

Kaufman sources are commercially available that are capable of gener- ating an argon ion beam up to about 10 cm diameter and typically 0.5- 1.0 mA/cm2, with a variable beam energy in the range 500-2,000 eV with an energy spread of 1-10 eV. Applications of ion beam sputtering for device fabrication have been reviewed [79, 801. The main control on the ion density is the cathode heating current, which controls the rate of electron emission. A wide range of ion current densities can be achieved by leaving the discharge voltage and pressure fixed (typically 1 mtorr) and varying the cathode current. There are several types of gridless ion sources to overcome the current density limitations of gridded ion sources. The end-Hall type and the closed-drift ion source are two common configura- tions [81]. A detailed description of the end-Hall source, which is capable of generating low-energy , high-current beams of ions, has been published [82 , 831.

111. SPUTTER DEPOSITION OF CONDUCTING FILMS

A. General

Conducting films can be easily deposited by sputtering metallic targets in an inert gas, usually argon. When multicomponent targets are sputtered, an altered surface layer forms because of the difference in sputter yields of

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the components. If diffusion of the target components does not occur, the composition of the atoms sputtered from the altered layer is equal to the bulk composition of the target. Furthermore, if the sticking coefficients of the elements are approximately equal, the film composition is nearly equal to that of the starting alloy material. A dopant, such as nitrogen, can be introduced into the metal film during growth by using reactive sputtering techniques (Section IV,A).

Any type of sputter source, either dc or rf, can be used to fabricate metallic films. Modern metallizers usually have a dc planar magnetron source, with provision for substrate heating and biasing during film growth. Alloys can be fabricated using a single multicomponent target [84, 851 or by using multiple sources (“co-sputtering”), each with a elemental target, all focused on a common spot. Co-sputtering can be done with magnetron or ion-beam sources. The superior adhesion of sputtered metal films, compared to evaporated coatings, is attributed to bombardment of the film with energetic particles from the target and the plasma, which helps to clean the substrate of adsorbed atoms and promote local rearrangement of atoms [86].

B. Specific Applications

Examples of important developments in the area of sputtered metal films, which have occurred since the publication of “Thin Film Processes I,” are listed below. Sputter deposition of compound semiconductors and semiconducting oxides (e.g., indium tin oxide) is treated in the subsequent section.

1. Aluminum

Aluminum or one of its alloys has become increasingly important for metallization in very large scale integrated circuits. One of the several problems, hillock formation after a high-temperature process step, has re- ceived considerable attention as the degree of integration of ICs becomes higher and the widths of the interconnects become smaller. The simulta- neous suppression of whickers and hillock growth in aluminum alloy films is an important consideration for integrated circuit metallizations [87-911.

2 . Refractory Metals

Refractory metals, such as tungsten, are of interest in the elecronics industry because of their low resistivity, good thermal conductivity, hard- ness, and ability to withstand high-temperature processing. The structure and properties of sputtered tungsten films up to 28 microns in thickness,

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with a hardness several times that of bulk tungsten, have been reported ~921.

3. Planarization

Planarization of thin metal coatings and coverage of steps, such as vias a few microns in diameter, is required for precise pattern alignment and reliability in the realization of very large scale integration [93-981. As the sizes of vias and contacts become smaller, the aspect ratio of steps becomes high. The use of bias sputtering methods have been shown to improve step coverage significantly. This method is basically resputtering of the growing film, some of which is moved into the holes. Improvement in step coverage can be also achieved by enhancing the surface mobility (see Section 1,C) by low-energy (a few tens of electron volts) ion bombardment or by elevating the temperature (e.g., about 500°C) of the substrate. These techniques promote the movement of adatoms from the horizontal surface into vias and valleys, where they are trapped. Due to preferential sputtering of one element over another, the composition of an alloy film can be altered during bias sputtering [99]. For example, in the case of aluminum-copper, A1 is preferentially sputtered relative to Cu, which can significantly en- hance the Cu concentration in an ion-bombarded film.

The base pressure should be as low as possible during metallization because reactive gas contaminants form compounds that tend to reduce adatom mobility [ 1001.

4. Lift-off

Unlike (3), delineation of blanket-deposited films by a lift-off method is an important technique for device fabrication. For lift-off, it is necessary to leave the photoresist side wall uncoated to allow the solvent to penetrate under the sputtered layer. Vertical incidence of the deposit is readily achieved with evaporation; however, special care is required in the case of sputtering. A magnetron source capable of operating below 3 mtorr should be chosen in order to reduce gas scattering effects. The sputtered flux on the substrate should be kept as close to normal incidence as possible. The magnetics in the magnetron source should be balanced to minimize the escape of plasma from the source and, thereby, reduce heating of the photoresist. A magnetron sputter technique has been recently developed for lift-off patterning [ lol l . A hollow cathode electron source was im- plemented near the magnetron source to allow operation in the torr range, where gas scattering effects are negligible. An array of collimating tubes was placed just above the sample to restrict the depositing flux to normal incidence +/- 5".

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5. Mirrors

Light scatter, caused by surface microroughness in the case of metal films, can be significantly reduced by bombarding the growing film with ions [102]. It is thought that an adatom with high surface mobility is more likely to reach sites of abrupt change in surface structure, which are mainly responsible for the light scattering. The extreme stability of the sputter deposition process is well suited for the fabrication of coatings that re- quire very precise control of coating thickness, such as x-ray multilayer mirrors [ 1031.

6. Magnetic Materials

Magnetrons can efficiently deposit magnetic materials, provided that a sufficient magnetic field (minimum about 50 gauss) appears above the target surface to confine the electrons. For example, films of rare-earth transition metals have been deposited by sputter deposition [104]. A mag- netron with very strong permanent magnets, such as CoSm, is required to saturate the target material magnetically. With this type of arrangement, a $-inch-thick nickel target, and a high deposition rate of about 10 kA/min can be obtained [105]. Magnetic CoCr films suitable for high-density per- pendicular recording can be sputtered, with controlled coercivity and magnetic anisotropy [106-1091.

IV. SPUTTER DEPOSITION OF DIELECTRIC FILMS

A. Reactive Sputtering

Dielectric coatings, such as oxides and nitrides, can be deposited either by sputtering an insulating target of the desired material, using rf power [110-1121, or by reactively sputtering a metal target, with either rf or dc power, in a mixture of an inert and a suitable reactive gas [6]. Both techniques are widely used; however, reactive sputtering is usually the preferred method because of the following advantages:

Metal targets can be machined; Metal targets have high thermal conductivity and, therefore, can

Different types of dielectrics can be fabricated by choosing different

High-rate techniques give deposition rates comparable to those of

The traditional method of reactive sputtering involves bleeding suf- ficient reactive gas into the chamber to keep the target completely covered

handle high power densities (e.g., 50 W/cm2) without cracking;

reactive gas mixtures; and

pure metals.

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with dielectric. This “covered-mode’’ operation is limited by the low sput- ter yield of the dielectric layer. High-rate reactive sputtering, with deposi- tion rates comparable to those for metals in pure inert gas, can be achieved in the “metallic mode” [113, 1141. In this high-rate mode, the sputter- eroded area of the cathode remains bare, while sufficient reactive gas is present at the substrate to form the dielectric compound.

Figure 4 is useful for a qualitative description of the two modes of reactive sputtering. Here, 0, and 0, are, respectively, the degree of target coverage and degree of substrate coverage by dielectric compound (0 = 0 corresponds to pure metal; 0 = 1, to stoichiometric dielectric). The flow rate of the reactive gas is varied, while the flow rate of the inert gas, the pumping speed, and the target power are assumed to be held constant. The target coverage changes very little with initial increase in reactive gas flow because the bombarding ion flux keeps the eroded portion of the target clear of dielectric deposit. The reactive gas is removed by the external pump and is getter pumped by the sputtered metal flux. The latter pumping mechanism causes 0, to increase, and eventually the gas flow reaches a critical point, F,, , corresponding to complete reaction of all the metal deposit-i.e., essentially all of the deposit is stoichiometric or very nearly stoichiometric compound (e.g., TiN,95). At this operating point, the getter- ing pumping is saturated and, as a result, any further increase in reactive gas flow leads to a significant increase in reactive gas partial pressure. As a further consequence, a permanent dielectric layer on the target expands

1 W 17 4 Lf W > 0 0

0

A

t 1

6.2- 6, 6 , REACTIVE GAS FLOW 111)

Fig. 4. Idealized dependence of the degree of target coverage 0, and substrate coverage 0, as a function of reactive gas flow. 0 = 0 corresponds to pure metallic surface; and 0 = 1, to the stoichiometric dielectric compound.

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from the edges of the sputtered track [113], until a second critical point, F2+, corresponding to a completely covered cothode, is reached. For metallic-mode (or high rate) sputtering, the reactive gas flow is operated in the range from F1+ to F2+. Under these operating conditions, a high deposition rate corresponding to the sputter yield for the metal surface is achieved, while nearly stoichiometric dielectric is being deposited on the substrate (0, = 1). If the operating point is very close to F,, , (0, << l ) , the arrival rate of metal atoms at the substrate can be very nearly equal to that obtained with no reactive gas [115].

Because the sputter yield of a reactive compound on a target surface is usually much lower (e.g., by a factor of 10-20) than that of the metal, the target remains covered as the gas flow is decreased, until a new critical point, F1,2-, is reached. The hysteresis seen in Fig. 4 can, in principle, be eliminated by providing a large pump with a pumping speed that is high compared to that of getter pumping [116-1201. To operate a coater in the covered mode, the gas flow is usually set to a value just above F2+,

The fundamental parameters 0, and 0, are not easily measured, and therefore the reactive sputter process must be monitored by observable parameters, such as voltage, power, pressure, optical emission, and film properties. As shown by the example in Fig. 5 for the case of reactive sputtering of an A1 target in a mixture of Ar and N2 [121], measurement of the sputter parameters as a function of reactive gas flow can accurately locate the transition points F2+ and F1,2-; however, little information is obtained for Fl+. A major part of the problem associated with controlling

N2 FLOW RATE (SCCM)

Fig. 5. Variation of the partial pressure of nitrogen and the cathode voltage as a function of flow rate of the reactive gas, for sputtering an AI target in a mixture on NZ and Ar. The flow rate of the inert gas, pumping speed, and cathode current were held constant.

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high-rate reactive sputtering is the difficulty in knowing whether the reac- tive gas has reached the Fl + point, without exceeding the irreversible transition point F2+. The general behavior of the above characteristics is typical of all metal/reactive gas combinations; the main difference being whether the plasma impedance increases or decreases with the target transition [ 1221,

1. High-Rate Reactive Sputtering

Numerous different combinations of the process parameters and hard- ware have been studied in an effort to achieve stable sputter operation in the high-rate, metallic mode [123]. General points are discussed below.

a. Gas Flow Geometries It is common practice to introduce a mixture of inert and reactive gases

at an arbitrarily selected point in the vacuum chamber. This method is simple to implement; however, better process control can be obtained by consideration of the gas geometry. For example, in the case of large, planar magnetron sources (e.g., >2 ft. long), gradients in reactive-gas partial pressure along the length of the source usually cause arcing, un- stable operation, and nonuniform film properties. Thick dielectric layers formed on the inactive portion of the cathode surface (i,e., the center and the outside of the racetrack) tend to build up electrostatic charge and, by dielectric breakdown, initiate a vacuum arc. This problem is most notice- able near the ends of a long planar magnetron, where getter pumping of the sputtered flux drops off and, as a result, excess amounts of reactive gas accumulate.

Figure 6 shows two common types of geometries: (1) Inert and reactive gases are uniformly mixed and introduced uniformly over the target sur- face; and (2) inert gas is uniformly distributed over the cathode surface, and the reactive gas is uniformly distributed over the deposition region. A typical gas manifold consists of a tube with a line of small holes. The analysis of gas distribution is usually very complex, depending on the geometry and the operating conditions; therefore, it is usually easier to determine the spacing of the holes by trial and error. The space between the cathode and the ground shield is a convenient location for the introduc- tion of gases at the target surface.

Gas geometry (1) is useful for high-rate deposition of nitrides; geome- try (2), for oxides. Unlike oxygen, nitrogen molecules in their ground state are unable to react with a metal surface, and therefore activation is re- quired. Geometry (1) makes full use of the intense discharge region near the cathode to generate ions and other highly reactive species. The cou- pling of the substrate to the plasma-generated species can be maximized by

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REACTIVE

R \

i:: @

0’

GAS

W pa m

3 Ln

Fig. 6. Two common gas geometries. (1) Inert gas and reactive gas introduced near cathode surface. (2) Inert gas introduced near the cathode, and reactive gas uniformly introduced near the substrate.

operating at lo-w pressure (e.g., below about 3 mtorr), with a short target- to-substrate distance (e.g., 5 cm), and with the application of a negative substrate bias. Alternatively, an unbalanced magnetron can be used to provide bombardment of the growing film.

Gas geometry (1) is unsuitable for oxides because molecular oxygen strongly reacts with most metal surfaces. The key to high-rate reactive sputtering of oxides is to minimize the oxygen partial pressure in the near-region of the cathode surface. This is best done with gas geometry ( 2 ) . Because the sticking probability of oxygen is less than unity, additional measures are usually necessary to prevent the oxygen from reaching the target surface. Various approaches found in the literature are discussed below.

b. Geometrical Baffle A geometrical baffle, shown in Fig. 7 , can be used to restrict the oxygen

flow to the target region and to provide additional gettering surfaces for the reactive gas in proximity to the target [124-1301. The main disadvantage associated with this solution is the need for cleaning the baffle after a few hours of sputtering. Another disadvantage of the baffle is the termination

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196 ROBERT PARSONS

GETTER IN G REACTIVE SUR FACE \ BAFFL;

CENTRAL GROUND SHIELD

Fig. 7. Modified magnetron sputter configuration for control of high-rate reactive sputter- ing of oxides.

of the discharge region at the (grounded) baffle, which reduces bombard- ment-induced mobility effects (see Section 1,C). However, the latter prob- lem can be overcome by placing a positively biased electrode near the substrate to draw the plasma through the baffle apertures. Alternatively, the substrate can be negatively biased, e.g., with the use of an rf power

c. Gettering Surfaces With gas configuration ( 2 ) , the reactive gas at the target can be reduced

by increasing the separation between target and the substrate, and by using oxygen gettering surfaces, as shown in Fig. 7. To be effective, the gettering surfaces must intercept a significant portion (e.g., 10-30%) of the sput- tered flux to maintain an unsaturated getter-pumping surface. As in the case of the baffle, removal of film buildup on the gettering surfaces has to be part of the regular operational procedure. To be meaningful, gas separation between the target and the reactive gas inlet should be much more than the mean free path of the reactive gas. The mean free path is a few centimeters at typical working pressures for a magnetron source.

d. Scale-up With the exception of a few oxides, such as indium tin oxide, high-rate

reactive sputtering has been difficult to scale up to large-area applications (e.g., web coating) because of arcing (Section IV,A,l,a). Arcing becomes very noticeable when a sputter source is operated with currents greater than about 10 amps. Once triggered, an arc at these current levels can be

supply.

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self-sustained. The only practical method to terminate this type of arc is to momentarily interrupt the cathode power. Arc-quenching circuitry is a standard part of modern power supplies. In the case of tenacious oxides, such as A1203 and Si02 , the frequency of arcing sharply increases with increasing reactive gas flow. The duty cycle for actual sputtering decreases and, as a result, it is often impossible to reach the critical point, Fl+, for high-rate reactive sputtering. Very little information on. arc suppression has been published. One method, which the author has found particularly useful, is shown in Fig. 7. A ground shield/plasma shield combination is positioned several millimeters above the nonsputtered portions of the cathode, including the area inside the racetrack. With this attachment, stable, metallic-mode sputter operation has been demonstrated in the case of A1203 deposition with a 60-cm-long, planar magnetron [131]. An occa- sional arc (e.g., every few seconds) can be tolerated, since the magnetron is usually able to return to the metallic mode if the arc is quickly extin- guished.

2. Deposition Rates

The maximum deposition rate is given by the product of the normalized rate [65], the area of the eroded part of the cathode, and the maximum power that can be dissipated by the source configuration. Normalized rates in the range from 40 to 115 (A cm2)/(W min) have been reported for most materials of practical importance-e.g., A1203 [132, 1331, S O 2 [128], TiN [134], T i c [135]. Maximum power dissipations of about 50 W/cm2 can be achieved with directly water-cooled metallic targets.

3. ModiJed Techniques

Numerous studies of the reactive sputter process have been reported, each with a special emphasis on one aspect of the process and/or material properties. A brief summary is given below.

a. Gas Pulsing Periodic pulsing of the reactive gas during the reactive sputtering pro-

cess has been shown to form alternating layers of metal and dielectric [136-1381, This technique suppresses columnar growth, and thereby re- sults in very smooth surface finishes. The effect of the alternative layers inhibits the formation and movement of dislocations, and thus the coatings can have improved mechanical properties. Pulsing has also been used to maintain stable operation in the metallic mode, by periodically switching off the reactive gas for a short time in order to remove any traces of compound formation before it builds up [139,140].

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198 ROBERT PARSONS

b. Closed-Loop Control There is a choice of several process parameters for closed-loop control

of the target coverage. By maintaining a constant cathode voltage by variation of the discharge current [121], the reactive gas partial pressure [141], or the pumping speed, the degree of target coverage can be pre- cisely controlled in the case of nitrides, for which ion plating, rather than chemisorption, controls the reaction of the reactive gas species at the target surface [121]. The voltage-control technique has been used for the fabrica- tion of nonstoichiometric nitrides, such as AlN, granular metals or cer- mets; however, allowance had to be made for the drift of the operating characteristics and the efficiency of a planar magnetron due to erosion of the target [ 1421.

Stable operation in the high-rate mode has been achieved by closed- loop control of the reactive gas flow, with use of either mass spectroscopy [115,141] or optical emission spectroscopy [143] to monitor partial pres- sures and sputtered species. Optical emission is a very useful monitor of the reactive sputtering process because several peak emission intensities can be proportionally related to the sputtered metal flux and the partial pressure of the reactive gas [ 1441. These techniques have become standard practice in the case of hard nitride coatings such as TiN.

For roll-to-roll coating of transparent conducting films, both the trans- mittance and resistivity of the film have been used to monitor the F,, point and, thereby, to provide feedback control of the reactive gas flow [145].

c. Hard Coatings A critical review of hard coatings has been published [51]. The optical,

electrical, and mechanical properties of diamondlike carbon films have been studied by ion sputtering of graphite with argon ion co-bombardment and rf plasma decomposition of hydrocarbon compounds [146]. TIN films were deposited, with substrate temperatures between 200°C and 550°C, by reactive sputtering onto steel substrates [147], Ion-assisted, reactive sput- tering has been shown to be important for the fabrication of nitride, hard, and wear-resistant coatings [148-150], Improved wear resistance has been reported with the addition of A1 to TiN, ZrN, and Ti-Zr-N [151,152]. A very energetic deposition environment was provided when the substrates (e.g., drills) were positioned in the plane separating two opposite-facing magnetrons, and a negative bias was applied to the substrates to promote bombardment.

d. Multicomponent Semiconductors The use of magnetron sputtering for multicomponent semiconductors

such as GaAs and InSb has been limited in part by the availability of

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high-purity compound targets. Recent results involving metallorganic mag- netron sputtering (MOMS) have shown that InSb films can be produced with good compositional uniformity and surface morphology by reactively sputtering an antimony metal target in a reactive vapor of trimethylindium [153-1551. The reactive gas is prepared in a temperature-controlled sub- limer, which is then transferred to the sputtering chamber by flowing argon gas.

e. Amorphous Silicon a-Si : H films have been reactively sputtered with a planar magnetron

[156]. The hydrogen content of the films was controlled by adjusting the hydrogen partial pressure in the sputter discharge. Amorphous carbon films containing up to 35 at % of hydrogen were deposited by ion beam sputtering a carbon target in a hydrogen-argon gas mixture [157,158]. The magnetron discharge is insensitive to the presence of H2 in the discharge [ 1591,

f . Low-Frequency Sputtering Low-frequency power suppiess (e.g., 60-100 kHz) have been shown to

be useful for reactive magnetron sputtering [160]. For AlN, the deposition rate was 80% higher at 80 kHz than at 13.56 MHz for rf sputtering. To utilize this technique, two nearly identical targets are connected to the secondary winding of an isolation transformer. Unlike the rf case, in which the ions are accelerated by the self-bias voltage, at low frequencies the ions are accelerated by the full voltage modulation.

g. Unbalanced Magnetrons An unbalanced magnetron has been used to simultaneously reactively

sputter and bombard the growing films with an ion flux (up to 9 mA/cm2) equal to 10 times the atom deposition rate 142,681. Examples of coatings that were deposited with this technique are diamondlike a-C and TiN. The a-C films that were deposited at low sputtering power possessed the best diamondlike properties.

h. High-Temperature Superconductors High-temperature superconductors, such as Y-Ba-Cu-0 and Bi-Sr-

Ca-Cu-0, have been deposited using standard sputter techniques-e.g., rf-sputtering of a single, multicomponent target [ 161-1641. Resputtering effects, mainly associated with negative ion bombardment, are a very important factor in the determination of film composition 11651.

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200 ROBERT PARSONS

V. SPUlTER COATING SYSTEMS

A. General

Many different types of sputter coater systems are commercially avail- able, ranging from a small research-type coater to complex production coaters with multiple processing stages, including plasma cleaning, heat- ing, consecutive sputter deposition of several layers of either metal or dielectric, and etching. For example, a typical coater system for the manu- facture of compact disks consists of loading and unloading stations, a degassing chamber, magnetron cathodes, a disk conveyor system, and a transfer chamber for separating the loading and the sputtering process. Hundreds of in-line systems and roll-to-roll coaters are in active use worldwide [ 1661.

Before considering the detailed design of a coater system, one should prepare a list of coater specifications, which includes the coating materials, types of substrates, a range of acceptable film properties, minimum throughput requirement for present use, and a reasonable estimate of the maximum throughput requirement for future use. From these specifica- tions, equipment manufacturers or others experienced in the field are able to provide a reasonably accurate estimate of the costs and requirements in terms of time and space. Usually, many different options are available, depending on the choice of sputter process, types of sources, pumps, substrate handling apparatus, process control, etc. This section concludes by considering some of these technical points.

6. Technical Considerations

1. Targets

The procurement of sputter targets is an important consideration for a sputter system. Planar targets of an almost unlimited range of materials are commercially available-however, at what cost? Targets can be fabricated by mechanical, sintering, and metallurgical techniques. Sample films of alloys, which are suitable for research study and prototype products, can be mechanically fabricated. For example, A1 plugs can be inserted into a Ti plate for the fabrication of Ti-AI-N dielectrics. As another example, rare-earth chips have been placed on a transition-metal to fabricate magne- to-optical disks [167]. Two-source sputtering, with one source for one alloy component and the other source for the second component, can be used. However, this method requires substrate rotation to obtain a homo- geneous film. The best solution for the deposition of alloys is to use an alloy target with the same composition as the desired film. In the case of

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hot pressed targets, one should run warm water through the target cooling lines in order to prevent condensation and trapping of water in the porous target material.

Incomplete utilization of target material is often a major concern in the case of large magnetron sources. If direct water-cooling of the target is required for high-power operation, mechanical considerations require an oversize target compared to the size of the erosion track, and as result, target utilization is often limited to about 25%. Target utilization can be increased by flattening the magnetic field lines parallel to the target surface [168].

2. Film Uniformity

The deposition rate profile depends on the geometry and size of the source, the operating pressure, and the target-to-substrate distance. The deposition flux can be predicted with considerable accuracy by assuming a cosine emission profile. Deposition profiles have been calculated for annu- lar [169] and rectangular sources [170]. Substrate motion, together with aperture masking, is often used to improve film uniformity. The depostion probability and spatial distribution of atoms sputtered from a magnetron source have been measured for a variety of chamber pressures and target- to-substrate distances [171].

3. Growth Temperature

Magnetron sputtering is usually considered a “cold” deposition pro- cess, which is capable of coating heat-sensitive materials. The main sources of heating are secondary electrons generated by ion impact at the target, which are accelerated towards the substrate by the dark space voltage; the heat of condensation; sputtered atom kinetic energy; plasma radiation; and ion neutralization and reflection at the cathode [73, 1721.

4. Substrate Preparation

Glow discharge cleaning of the substrate prior to film deposition has been used for many years, especially for optical coatings. A shielded cathode system, using a glow discharge at high pressure (0.5 torr) and low voltage (500 V), provides only excited neutral bombardment, not high- energy particles [173]. The substrate should be shielded from any line-of- sight path between the glow discharge and the sample surface. A cleaning station using steam and hot water to clean the substrates prior to sputter coating has been incorporated into an in-line sputter-coater system [174]. The use of wet and dry air glow discharges to improve adhesion of Cr films to glass has been reported [175]. Titanium, chromium, niobium, tantalum,

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202 ROBERT PARSONS

and tungsten are often used for an interfacial layer to increase adhesion. A comprehensive discussion of the preparation of substrates for film deposi- tion has recently been published (56; see also Chapters 1.1 and 1.2).

Loading/unloading interlocks may be provided so that the target sur- face is not exposed to the atmosphere between depositions. Seizure of mechanical parts that occurs during sliding or rolling in a vacuum environ- ment can be prevented by use of appropriate lubricants [176].

5. Pumps

High-vacuum pumping [ 1771 of sputter coaters has traditionally been accomplished by diffusion pumps, oil-sealed mechanical pumps, and a liquid nitrogen cryotrap. Now there are two alternative methods of pump- ing available: turbomolecular and He-cryopumps. Contrary to some views, modern diffusion pumps with proper baffling can achieve ultrahigh-vacuum conditions. A turbomolecular pump and a diffusion pump have roughly the s m e pumping performance for the same size of inlet. The main advantages of a turbomolecular pump are its greater tolerance to sudden transient gas overload and its lack of a motive fluid to backstream. The major advan- tages of the diffusion pump are its lower cost and the availability of very large pumps (e.g., pumping speed 100,000 l/s) and high continuous gas loads (30 torr l/s). To reduce backstreaming from mechanical pumps, one should use traps, which must be periodically replaced, and/or introduce a controlled leak in the foreline to keep the pressure high enough (e.g., 10-100 mtorr) to prevent upstream migration of the mechanical pump oil. For the semiconductor industry, the He-cryopump is often used because of the promise of cleanliness.

6. Gas Pressure The operating pressure for sputtering can be achieved by throttling the

gas inlet and/or by downstream pressure control by means of a special throttling of the pumping speed, usually with a variable-orifice valve [178]. Although downstream pressure control has been shown to be superior to upstream control of gas flow, standard practice is to set the variable orifice roughly and to control the inlet gas flow with the use of a piezoelectric valve.

7. Automation Control

Automation control can vary widely in scope and type. At one extreme, a simple vacuum controller can be used, with interlocks for proper se- quencing of valves, together with local closed-loop controllers for gas flow and the power supply parameter (e.g. , current), with occasional operator supervision [179-1821. At the other extreme, a computer-based system can

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be used for turn-key operation, including robotic handling of substrates. It is important to remember that automation control can never substitute for poor process understanding. Automation becomes more important, if not crucial, as development progresses from the R&D stage to production.

There are many microcomputer-based systems, as well as program- mable logic controllers, that can be used for the development of a custom automation system. However, one should remember that it is very easy to underestimate greatly the amount of time and expenses associated with in-house software development by personnel who are not expert computer programmers.

Powerful software packages are commercially available for design of thin film structures, in particular optical multilayer coatings. In the near future we can expect the combination of design/control software packages that will automatically tailor the properties of sputtered coatings.

VI. EMERGING TECHNOLOGIES

A magnetron sputtering source has been integrated into an electron cyclotron resonance plasma deposition system [183]. Films of A1203 and TazOS were deposited by high-rate reactive sputtering. The main advan- tage expected from the ECR-magnetron configuration is the very high density plasma, which bombards the growing film.

A hollow cathode has been developed for etching and deposition ap- plications [184]. The device shows promise for planarizing layers.

Strongly adherent films of copper on sapphire have have deposited by pulsed laser treatment [185]. The laser energy had to be carefully con- trolled to prevent excessive evaporation or film damage.

Selective deposition of a metal film has been demonstrated by using rf-bias sputtering in an argon atmosphere [186, 1871. The substrate bias etching rate is set approximately equal to the deposition rate. Further studies have shown that it is possible to obtain interfaces between sub- strates and films deposited by the selective process that are as sharp as if the films were deposited without rf-biasing of the substrate [188.] How- ever, one has to choose a narrow operating window for the process param- eters, which provides a compromise between selectivity and interface sharpness.

VII. CONCLUDING REMARKS

It is hoped that this chapter provides a useful guide through the volumi- nous literature on sputter techniques and film properties. Sputtering is the method of choice for a wide range of coating applications. Both equipment

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and sputter processes are being continually improved to extend the appli- cations of the method. It is anticipated that future developments in the area of thin films will involve an integration of many different types of processes into more and more complex deposition systems. Sputtering will continue to play a very important role.

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