Pergamon
`
`PII: S0042-207X(98)00265-6
`
`Vacuum/volume 51/number 4/pages 641to 646/1998
`ª 1998 Published by Elsevier Science Ltd
`All rights reserved. Printed in Great Britain
`0042-207X/98 Sl - see front matter
`
`High-rate reactive DC magnetron sputtering of
`oxide and nitride superlattice coatings
`
`W D Sproul,*, Sputtered Films, Inc., 320 Nopal Street, Santa Barbara, California 93103, U.S.A.
`
`Over the past 10 years, there have been three major advancements in reactive sputtering technology that now make it
`possible to deposit both conductive and non-conductive fully-dense films at high rates. These three advances are unba-
`lanced magnetron sputtering, partial pressure control of the reactive gas, and pulsed dc power. Multicathode unba-
`lanced magnetron sputtering systems provide a dense secondary plasma that is used for producing a well-adhered,
`fully dense film that is difficult to achieve with conventional magnetron sputtering. Online automatic partial pressure
`control of the reactive gas prevents the poisoning of the target surface during deposition, which leads to compound
`film deposition rates that approach or are equal to those for the pure metal rate. Pulsed dc power, where the polarity
`of the voltage on the sputtering target is alternately switched briefly between negative and positive, prevents arcing on
`the target surface during the deposition of nonconducting films. With both pulsed dc power and partial pressure control
`of the reactive gas, films such as aluminum oxide can now be deposited reactively at rates up to 78% of the pure metal
`rate. The reactive unbalanced magnetron sputtering process is used to deposit polycrystalline nitride superlattice films
`such as TiN/NbN or TiN/VN with hardnesses exceeding 50 GPa, which is more than double the hardness of either com-
`ponent in the multilayered film. The nitride superlattice work is being extended to oxide films, and initial results are
`encouraging. Nanometer scale, multilayer Al2O3/ZrO2 and Y2O3/ZrO2 films have been deposited at high rates. The
`Al2O3/ZrO2 films are amorphous and optically clear, whereas the Y2O3/ZrO2 films are crystalline as well as being opti-
`cally clear. ª1998 Published by Elsevier Science Ltd. All rights reserved
`
`Introduction
`
`Unbalanced magnetron sputtering
`
`Since the middle 1980s, there have been three major advance-
`ments in sputtering technology that have greatly a€ected the
`ability to reactively sputter fully dense, well-adhered films at
`high deposition rates. In 1986, Window and Savvides1–3 intro-
`duced the concept of unbalanced magnetron sputtering, and in
`the years since, it has been widely embraced by the sputtering
`community. Combined with partial pressure control of the
`reactive gas during the reactive sputter deposition of coatings,
`unbalanced magnetron sputtering today is one of the primary
`techniques for the deposition of hard coatings.
`Most recently, the introduction of pulsed direct current (dc)
`power is already having an important e€ect on the reactive
`sputtering of non-conducting films such as aluminum oxide
`(Al2O3). The combination of partial pressure control of the
`reactive gas, unbalanced magnetron sputtering, and pulsed dc
`power is a powerful tool for the high-rate reactive deposition
`of compound films. Each of these three techniques will be
`reviewed, and then they will be looked at together to show the
`full potential for the synergistic e€ects for the deposition of
`non-conducting films.
`
`*To whom all correspondence should be addressed
`
`A conventional magnetron sputtering cathode has magnets
`located along the outer edge and the centerline or at the center
`if the cathode is round. If the strength of the inner and outer
`magnets
`is
`roughly equal,
`the magnetron is
`said to be
`balanced, and most of
`the magnetic field lines will
`loop
`between the inner and outer magnets as is shown in Figure 1.
`If one of the sets of magnets is made stronger than the other,
`then the magnetron becomes unbalanced. Typically the outer
`set of magnets in the magnetron cathode is made stronger
`than the inner ones. Although there is still linkage between
`magnetic fields of the inner and outer magnets, not all of the
`field line will make the link. The excess field lines from the
`stronger magnets will radiate away from the magnet surfaces
`as is shown in Figure 2.
`During magnetron sputtering, energetic electrons escape
`from the primary magnetic trap between the inner and outer
`magnets, and in a balanced magnetron, these electrons go to
`the anode. It is the primary electron trap that is responsible
`for the formation of the dense plasma directly in front of the
`sputtering target and for the high deposition rate of the mag-
`netron cathode compared to a diode cathode.
`In an unbalanced magnetron, the escaping energetic elec-
`trons are trapped by the excess magnetic field lines, and the
`
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`
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`APPLIED MATERIALS EXHIBIT 1036
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`

`

`W D Sproul: High-rate reactive DC magnetron sputtering
`
`Figure 1. Schematic drawing of a balanced magnetron sputtering cath-
`ode.
`
`electrons spiral along the field lines and undergo ionizing col-
`lisions with gas atoms. A secondary plasma is formed away
`from the target surface from these ionizing collisions, and this
`secondary plasma can be used for ion-assisted deposition of
`the growing film. The current density collected on the sub-
`strate during unbalanced magnetron sputtering is usually an
`order of magnitude higher than it is in conventional balanced
`magnetron sputtering, and substrate current densities are typi-
`cally 5–10 mA cm(cid:255)2 with unbalanced magnetron sputtering.
`These current densities match or exceed the substrate current
`densities found in other ion-assisted deposition techniques.
`When multiple unbalanced magnetron cathodes are used in
`the same chamber, it is important to link their magnetic fields
`in order to maximize the trapping of electrons. In an opposed,
`two cathode system, the polarity of the outer and inner mag-
`nets on one cathode should be opposite to that polarity of the
`magnets in the cathode that it is facing; i.e., north pole should
`face south pole and vice-versa. The magnetic trap cannot be
`complete if an odd number of cathodes are used. It is necess-
`
`Figure 2. Schematic drawing of an unbalanced magnetron sputtering
`cathode.
`
`642
`
`ary to have an even number of cathodes to prevent a hole in
`the magnetic trap.
`Four cathode rectangular unbalanced magnetron systems
`link magnetic fields with the cathode next to it and not to the
`one opposite it in the chamber. This linking provides good
`magnetic trapping of the electrons in one plane, but not in
`another. At the top and bottom of the cathodes, the magnetic
`field lines are in opposite directions, and there are holes in the
`magnetic trap. To overcome this problem, steel plates are
`placed at the top and bottom of the cathodes, and an electro-
`static charge on these plates prevents the electrons from escap-
`ing from the trap. The electrostatic charge can come simply by
`letting the plates electrically float in the plasma.
`Ion-assisted deposition is very important for forming fully
`dense, well-adhered hard coatings. Both the ion-current den-
`sity and the ion energy (bias voltage) play significant roles in
`ion-assisted deposition. In balanced magnetron sputtering, the
`ion current density is limited, and what is lacking in ion cur-
`rent density has to be made up with the energy of the arriving
`ions. Typical ion current densities in balanced magnetron sput-
`tering are less than 1 mA cm(cid:255)2, which produces low ion to
`arriving neutral species ratios. High bias voltages can be used
`to overcome partially the low ion to neutral ratio, but high
`bias voltages produce more damage than can be annealed out
`by the ion energy input.
`Unbalanced magnetron sputtering, by producing a dense
`secondary plasma around the substrate, provides a high ion
`current density, on the order of 1–5 mA cm(cid:255)2, and the ion
`energy does not have to be as high as it is in balanced magne-
`tron sputtering. Ion to neutral ratios greater than one are
`often reported for the unbalanced magnetron sputtering of
`hard coatings such as titanium nitride. Fully dense coatings
`are usually produced when the negative substrate bias voltage
`is in the 100–150 V range.
`
`Reactive sputtering
`
`Reactive sputtering is the sputtering of a metallic target in the
`presence of a gas that will react with the metal atom ejected
`from the target surface. Historically mass flow control has
`been used to control the amount of reactive gas flowing into
`the chamber, but flow control of the reactive gas can lead to
`problems. If the target is set at a fixed power and the flow of
`the reactive gas is increased, initially all of the reactive gas will
`be consumed by the reaction with the metal.
`However, a point is reached as is shown in Figure 3 (point
`A) for the reactive sputtering of titanium in an argon/oxygen
`atmosphere where the amount of reactive gas in the chamber
`is su(cid:129)cient to react with the surface of the target. When this
`happens and the oxide compound covers the surface of the
`target (the target is said to be poisoned), the sputtering rate
`drops rapidly because the sputtering rate of the compound is
`much less than that for the metal. Since the rate has decreased,
`not as much reactive gas is consumed, and its partial pressure
`jumps rapidly from point A to point B as is shown in Figure 3.
`With flow control, it is very di(cid:129)cult to operate between points
`A and B, and there is a range of compositions that is forbid-
`den between these two points.
`Partial pressure control of the reactive gas overcomes the
`problems of the flow control.4–6 Using a sensor such as a
`quadrupole mass spectrometer that can provide a quick feed-
`
`Page 2 of 6
`
`

`

`W D Sproul: High-rate reactive DC magnetron sputtering
`
`Figure 3. Hysteresis plot for the reactive sputtering of titanium in an argon/oxygen atmosphere with flow control of the reactive gas. The target
`power was 8 kW, and the total pressure during deposition was 1.1 Pa.
`
`back signal for the partial pressure of the reactive gas, it is
`possible to control the partial pressure of the reactive gas at
`any desired set point as is shown in Figure 4 for the reactive
`sputtering of titanium in an argon/oxygen atmosphere. There
`are no forbidden compositions with partial pressure control,
`and it is possible to operate at any point between A and B in
`Figure 4. At point B, the target is fully poisoned, and the sput-
`tering rate is very low. As the partial pressure is lowered
`toward point A, the deposition rate increases, and the chal-
`lenge is to operate at as low a partial pressure that will pro-
`duce the desired composition.
`There are two main benefits of partial pressure control of
`the reactive gas. The first is that it is possible to reactively
`sputter hard compounds such as TiN at the same deposition
`rate as is found for the pure metal.4 No higher rate can be
`achieved. Secondly, partial pressure control provides precise
`
`control of the composition of the compound, and it is possible
`to produce the same compound material in every run.
`Reactive sputtering of oxides until just recently had been a
`di(cid:129)cult task. Oxygen reacts much more quickly with the target
`surface than does nitrogen, and it often forms an insulating
`compound on the target surface, which leads to di(cid:129)culty in
`sputtering the desired material. When an insulating material
`forms on the surface of the sputtering target during depo-
`sition, those insulating surfaces build up a charge and then dis-
`charge during dc reactive sputtering, which results in arcing.
`This arcing is particularly violent for reactive dc sputtering of
`Al2O3, and it can result in damage to the power supply and
`liquid droplet ejection from the target surface.
`Radio frequency (rf) power can be used for the reactive
`sputtering of oxides, but it has its own set of problems.
`Essentially half of the power is not used for sputtering, and
`
`Figure 4. Hysteresis plot for the reactive sputtering of titanium in an argon/oxygen atmosphere with partial pressure control of the reactive gas.
`The target power was 5 kW, and the total pressure during deposition was 1.1 Pa.
`
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`
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`

`

`W D Sproul: High-rate reactive DC magnetron sputtering
`
`the deposition rates for reactive rf power are much lower than
`that for the pure metal. For example, aluminum oxide reactive
`sputters with rf power at only 2–3% of the metal deposition
`rate.
`One method used to overcome the problems of using dc
`power for reactive sputtering of nonconducting coatings is to
`shield the sputtering target from the reactive gas. Typically,
`the target is enclosed in a box, with a mesh screen over the
`target to let the sputtered atoms out. The argon sputtering gas
`is injected into the system next to the target, and the reactive
`gas is injected next to the substrate. Although this method
`does allow dc power to be used for the sputtering of noncon-
`ducting coatings, the screen does reduce the sputtering rate
`since it intercepts part of the sputtered flux. Keeping the
`screen open is a problem with this method, and constant main-
`tenance of the screen is required.
`
`Pulsed dc power
`
`Within the past few years, it has been shown7–11 that bipolar
`pulsed dc power can be used for the reactive sputter deposition
`of oxides. With bipolar pulsed power, the polarity of the target
`power is switched from negative to positive, and during the
`positive pulse any charging of the oxide layer is discharged
`when electrons are attracted to the positive surface. During
`the negative pulse, ions are attracted to the target surface, and
`sputtering takes place initially from all surfaces on the target
`even those that have formed a compound since the charge on
`that surface has been neutralized during the positive pulse.
`Bipolar pulsed power is classified as either symmetric or
`asymmetric, which refers to the pulse height in the positive
`and negative directions.12 Symmetric bipolar pulsed dc power
`has equal pulse heights in both the positive and negative direc-
`tions, as is shown in Figure 5, and the width of both the posi-
`tive and negative pulses can be varied independently as can
`the time o€ between pulses. Symmetric bipolar pulsed dc
`power is often used for the reactive deposition of an oxide
`coating from two magnetron cathodes. These two cathodes,
`which are located side by side, are both connected to the same
`symmetric bipolar pulsed dc power supplied. One power lead
`goes to one cathode, and the other power lead is connected to
`the second cathode. With this electrical hookup, one sputtering
`target is the anode for the system, while the other is the cath-
`
`Figure 5. Schematic representation of symmetric bipolar pulsed dc
`power.
`
`644
`
`Figure 6. Schematic representation of asymmetric bipolar pulsed dc
`power.
`
`ode. When the polarity of the voltage on the targets changes,
`the anode and cathode switch as well. Sputtering from the
`cathode surface during the negative pulse keeps the target sur-
`face clean, and when it switches to act as an anode, it is not
`covered by an oxide. This procedure avoids the disappearing
`anode problem, which can occur in pulsed dc sputtering of ox-
`ides when all surfaces in the chamber become covered with an
`insulating oxide.
`Asymmetric bipolar pulsed dc power, on the other hand,
`has unequal pulse heights. The negative pulse height is greater
`than the positive one, and there is no o€ time between pulses
`as is shown in Figure 6. The width of the positive pulse is a
`fraction of the negative pulse width, and its width is usually
`10–20% of the width of the negative one. A significant portion
`of the power cycle is spent in the sputtering mode, and the de-
`position rate from asymmetric power can be close to that of
`pure dc power. The frequency of pulsed dc power covers a
`wide range from 0 (normal dc) up to 250 kHz, and typical
`operating frequencies for the pulsed dc power during reactive
`sputtering of oxides are in the 20–100 kHz range.
`The frequency selected is a function of the material being
`reactively sputtered. Whereas no arcing can be achieved for
`the reactive sputtering of titanium dioxide at a pulsing fre-
`quency of 30 kHz, it takes a frequency between 50 and 70 kHz
`for all arcing to disappear for aluminum oxide.11 With both
`partial pressure control of the reactive gas and asymmetric
`pulsed dc power, we have been able to reactively sputter
`aluminum oxide with no arcing at a frequency of 70 kHz.
`Although in theory pulsed dc power has a rectangular wave
`form, in fact it does not. There can be overshoot in the nega-
`tive pulse and ringing on the positive pulse as is shown in
`Figure 7. This overshoot can be significant, and the target vol-
`tage shown on the power supply can be quite misleading.13, 14
`For the example shown in Figure 7, the average target voltage
`is about (cid:255)450 volts, which was displayed on the front panel of
`the power supply, but in fact the peak-to-peak voltage was
`about 1500 volts. Such high voltages will produce much more
`energetic particles during part of the pulse cycle, and the e€ect
`of these energetic particles on the structure and properties of
`the coating is still being evaluated. Initially there has been no
`noticeable e€ect of these high energetic neutrals on the proper-
`ties of the coatings, but there may be applications where this
`high energy could be detrimental. In many ways, the voltages
`
`Page 4 of 6
`
`

`

`W D Sproul: High-rate reactive DC magnetron sputtering
`
`Figure 7. Trace of pulsed dc power during the reactive sputter of aluminum in an argon/oxygen atmosphere.
`
`produced during asymmetric pulsed dc sputtering are similar
`to the voltages found in dc diode systems.
`
`Reactive unbalanced magnetron sputtering of multilayered
`coatings
`
`Reactive unbalanced magnetron sputtering in an opposed-
`cathode system has been used very successfully to deposit nan-
`ometer-scale multilayer nitride and oxide films that have
`enhanced physical properties.15–19 The first work in this area
`was with titanium nitride/niobium nitride (TiN/NbN) and tita-
`nium nitride/vanadium nitride (TiN/VN) coatings. Partial
`pressure control of the reactive gas was crucial to achieve the
`cubic form of NbN, and the high degree of ion bombardment
`from the unbalanced magnetron sources led to fully dense
`
`well-adhered films. The hardness of the TiN/NbN and TiN/
`VN films was greater than 50 GPa, which is more than twice
`the hardness of either component in these two multilayered
`films, when the superlattice period, which is the bilayer thick-
`ness of the TiN and NbN or TiN and VN, was in the range of
`50 to 80 A˚ . The individual
`layer thicknesses were approxi-
`mately equal.
`The importance of the combination of pulsed dc power and
`partial pressure control of the reactive gas really came into the
`spotlight with the reactive unbalanced magnetron sputtering of
`non-conducting oxides
`such as aluminum oxide (Al2O3).
`Without this combination of pulsed dc power and partial
`pressure control, it really was not possible to reactively sputter
`Al2O3 at high deposition rates in a practical way. A portion of
`the hysteresis loop for the reactive sputtering of aluminum in
`
`Figure 8. Nose region of the hysteresis curve for the reactive sputtering of aluminum in an argon/oxygen atmosphere. Full curve is shown in the
`inset in the lower left corner of the diagram.
`
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`
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`
`

`

`W D Sproul: High-rate reactive DC magnetron sputtering
`
`an oxygen/argon atmosphere20, 21 is shown in Figure 8. As is
`shown in this figure, small changes in the partial pressure of
`oxygen lead to large changes in the deposition rate, and rates
`as high as 78% of the metal deposition rate can be achieved
`for an optically clear amorphous film of Al2O3. To achieve
`this sensitive control of the reactive sputtering of Al2O3, both
`pulsed dc power and partial pressure control of the reactive
`gas must be used.
`Other oxide films have also been deposited using the com-
`bined pulsed dc power and partial pressure control of the reac-
`tive gas in the unbalanced magnetron sputtering system.
`Oxides of
`titanium, zirconium, hafnium, chromium, mag-
`nesium, silicon, yttrium, tantalum, and an alloy of zirconium
`and yttrium have all been successfully deposited using this
`technology.
`Multilayered oxide films have been deposited very success-
`fully via reactive unbalanced magnetron sputtering using
`pulsed dc power and partial pressure control of the reactive
`gas. Aluminum oxide/zirconium oxide (Al2O3/Zr02) films were
`deposited simultaneously in an opposed-cathode unbalanced
`magnetron sputtering system,19 and the substrate was rotated
`between the two cathodes. The deposition rate was approxi-
`mately 75% of each metal rate, and both individual
`layer
`thicknesses of the Al2O3 and ZrO2 were about 45 A˚
`each,
`which gave a bilayer thickness of 90 A˚ . The films were opti-
`cally clear, and the structure of the multilayer film was amor-
`phous as determined by X-ray di€raction.
`Most recently, multilayer oxide films of yttrium oxide and
`zirconium oxide (Y2O3/ZrO2) have been deposited in the
`opposed-cathode unbalanced magnetron sputtering system
`again using pulsed dc power and partial pressure control of
`the reactive gas.14 The Y2O3/ZrO2 films were similar to the
`Al2O3/ZrO2 in that they were optically clear, but they were
`di€erent because the Y2O3/ZrO2 were crystalline with a cubic
`structure. The cubic structure of the Y2O3 forced the ZrO2,
`which normally has a monoclinic structure, into a cubic struc-
`ture. The overall film had a dense columnar structure, and
`within a column there was a true superlattice structure for the
`Y2O3/ZrO2 film. Satellite peaks were clearly visible in the X-
`ray di€raction patterns from these films. Work is currently
`underway to determine other properties of these multilayer
`oxide films.
`
`Summary
`
`Advances in reactive sputtering have made it much easier to
`deposit
`fully dense well adhered coatings. Non-conducting
`coatings such as aluminum oxide, which could not be done
`with conventional dc power, can now be reactively sputter
`deposited in a very controlled, stable way at very high depo-
`sition rates if pulsed dc power is used in conjunction with par-
`tial pressure control of the reactive gas. The high ion flux that
`is available with unbalanced magnetron systems assures a high
`
`ion to neutral ratio, which in turn leads to dense films. Other
`oxide coatings including oxides of silicon, titanium, zirconium,
`hafnium, chromium, and tantalum have also been deposited at
`very high rates compared to convention deposition techniques
`when pulsed dc power and partial pressure control are used.
`This advancement in reactive sputtering technology is opening
`up many new opportunities for the oxide films.
`
`References
`
`1. Window, B. and Savvides, N., J. Vac. Sci. Technol. A, 1986, 4(2),
`196.
`2. Window, B. and Savvides, N., J. Vac. Sci. Technol. A, 1986, 4(3),
`453.
`3. Window, B. and Savvides, N., J. Vac. Sci. Technol. A, 1986, 4(3),
`504.
`4. Sproul, W. D., Thin Solid Films, 1983, 107, 141.
`5. Sproul, W. D., Surf. Coat. Technol., 1987, 33, 73.
`6. Sproul, W. D., Rudnik, P. J., Graham, M. E., Gogol, C. A. and
`Mueller, R. M., Surf. Coat. Technol., 1989, 39/40, 499.
`7. Schiller, S., Goedicke, K., Reschke, J., Kirchho€, V., Schneider,
`S. and Milde, F., Surf. Coat. Technol., 1993, 61, 331.
`8. Frach, P., Heisig, U., Gottfried, Chr. and Walde, H., Surf. Coat.
`Technol., 1993, 59, 177.
`9. Graham, M.E. and Sproul, W.D., 37th Annual Technical
`Conference
`Proceedings,
`Society
`of
`Vacuum Coaters,
`Albuquerque, New Mexico, 1994, p. 275.
`10. Sproul, W. D., Graham, M. E., Wong, M. S., Lopez, S., Li,
`D. and Scholl, R. A., J. Vac. Sci. Technol. A., 1995, 13(3), 1188.
`11. Schiller, S., Goedicke, K., Kirhho€, V. and Kopte, T., 38th
`Annual Technical Proceedings, Society of Vacuum Coaters,
`Albuquerque, NM, 1995, p. 293.
`12. Sellers, J., Asymmetric Bipolar Pulsed DC, ENI Tech Note, ENI,
`Division of Astec America,
`Inc.,
`100 Highpower Road,
`Rochester, NY 14623.
`13. Schneider,
`J.M., Sproul, W.D. and Matthews, A., Phase
`Formation and Mechanical Properties of Alumina Coatings
`Prepared at Substrate Temperatures <5008C by Ionized and
`Conventional Sputtering, paper presented at
`the International
`Conference on Metallurgical Coatings and Thin Films, Town and
`Country Hotel, San Diego, California, April 23, 1997 and
`accepted for publication in Surface and Coatings Technology.
`14. Sproul, W.D., Wong, M.S., Yashar, P., Wang, Y.Y., Barnett,
`S.A. and Chung, Y.W., Stabilization of Metastable Phases in
`Polycrystalline Nanometer-Scale Multilayered Coatings, paper
`presented at
`the
`International Conference on Metallurgical
`Coatings and Thin Films, Town and Country Hotel, San Diego,
`California, April 23, 1997.
`15. Chu, X., Wong, M. S., Sproul, W. D., Rohde, S. L. and Barnett,
`S. A., J. Vac. Sci. Technol. A., 1992, 10(4), 1604.
`16. Chu, X., Wong, M. S., Sproul, W. D. and Barnett, S. A., Surf.
`Coat. Technol., 1993, 57, 13.
`17. Chu, X., Wong, M. S., Sproul, W. D. and Barnett, S. A., Mat.
`Res. Soc. Symp. Proc., 1993, 286, 379.
`18. Sproul, W. D., Science, 1996, 273, 889.
`19. Sproul, W. D., Surf. Coat. Technol., 1996, 86-87, 170.
`20. Schneider, J.M., Sproul, W.D., Wong, M.-S. and Matthews, A.,
`Very High Rate Pulsed DC Sputter Deposition of AlOx Coatings,
`accepted for publication in Surface and Coatings Technology.
`21. Schneider, J.M., Sproul, W.D., Lefkow, A.R.T., Rechner, J. and
`Matthews, A., 39th Annual Technical Proceedings, Society of
`Vacuum Coaters, Albuquerque, NM, 1996, p. 168.
`
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`Page 6 of 6
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