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Characterization of stress in the absorber of x-ray masks using a holographic technique
Microelectronic Engineering 3 (1985) 573-579 North-Holland
573
Characterization of Stress in the Absorber of X-ray Masks Using a Holographic Technique R. E. Acosta, A. D. Wilson, and J. V. Powers IBM T. J. Watson Research Center P. 0. Box 218 Yorktown Heights, NY 10598 USA
The distortion of X-ray masks is directly related to the stress of the absorber used. Because of this, it is very important be able to determine the value of the absorber stress in order to be able to control, or reduce, the distortion of the masks. A simple technique, double exposure holographic interferometry, is described. Its application in measuring the stress of electrodeposited gold films, and the effect that several deposition parameters have on the gold stress are described.
Introduction Distortion of X-ray masks due to stress of the absorber could render the masks unusable if the stress is not minimized. The ability to accurately measure the stress is clearly a prerequisite to controlling it. Most currently available methods for determining the stress in metal films involve measuring the distortion of the underlying substrate, and from this distortion calculating the stress. Because the distortion of the substrate is directly proportional to the thickness of the film, and inversely proportional to the thickness of the substrate squared, the distortion is apt to be a very small quantity. For the case of films deposited on single crystal silicon substrates the distortion can be obtained with the aid of an X-ray diffraction technique 1. In this case it is necessary to determine the crystalline integrity of the substrate before the deposition of the film to be studied, and to insure that no damage to the crystal occurs during the process of deposition of the film. For non-crystalline substrates the curvature of the substrate can be determined either by mechanical or by optical means. In the former case use is made of Tallysurf-like instruments, with the consequent lack of precision for the very small distortion involved. Distortion of the substrate can be determined optically by simply determining the change in focussing distance at different points in the substrate using an optical microscope; by interferometry using a reference optical flat (i.e., Newton rings) 2~3; or variations of this method 4; or using laser interferometry with a two-frequency laser 5. All of these methods require the use of substrates that were flat originally, and in some cases optically flat (a condition not frequently found), or that some complicated method be used to subtract the original bending of the substrate from that caused by the film. 0167-93 17/85/$3.30 0 1985, Elsevier Science Publishers B.V. (North-Holland)
574
Experimental
R.E. Acosta et al. /Absorber stress of X-ray mash-s
Technique
We have developed a procedure for determining the stress of electrodeposited films of gold using the technique of double exposure holographic interferometry 6. This technique has the required resolution and does not demand an optically flat substrate. In fact, the holographic technique works well on either a flat or a distorted substrate. This application of holography is basically an interferometric process, but instead of using an optical flat as reference, the two wavefronts that are superimposed come from the unstressed sample and from the sample under stress. Because the sample is displaced as a result of the applied stress, interference fringes are observed which are a direct measure of the strain in the substrate. In the approach that we have used, the specimen, after electrodeposition of the gold film, is mounted on a suitable holder and the first holographic exposure is made. Then, part or all of the gold film is etched away, in situ, and a second holographic exposure is made on the same photographic plate. (Before the second hologram is made the substrate must be rinsed and dried, also in situ). After processing of the photographic plate the holographic image is reconstructed with the coherent light source used originally to expose it. A photograph of the hologram is made, or the fringes are counted directly, to determine the net displacement of the substrate, and thus the stress induced by the (etched) gold film. A photograph of a typical hologram is shown in Figure 1.
Fig. 1. Photograph of a typical double exposure hologram showing the clarity of the interference fringes. The in-plane displacement of the substrate is given by the number of fringes multiplied by l/4 X of the light used to make the hologram.
R. E. Acosta et al. / Absorber stress of X-ray masks
575
The stress of the film is calculated using Stoney’s equation 7: 1 d: GRXd,XI_vX6’
E,
1
where: R = radius of the curvature caused by the stress of the film d,,= thickness of the substrate d, = thickness of the stressed film, and E, and Y = Young’s modulus and Poisson’s ratio of the substrate material, respectively. For large R the relationship the substrate,is given by: 1 A -=R r2
between R and A, the displacement
perpendicular
to the plane of
where r = distance from center of the substrate. A is given by the number of fringes counted over the surface of the substrate multiplied by the quarter wavelength of the light used to make the hologram. The set-up that we used is schematically represented in Figure 2. The source of coherent light used was a He-Ne laser (output power 15 mWatt). The gold films were etched using a RI-I, aqueous solution, and the substrates were rinsed and dried using water and iso-propyl alcohol, DOUBLE EXPOSURE HOLOGRAPHIC INTERFEROMETRY
-0
r
LASER
Figure 2. Set-up for double exposure holographic interferometry. The optical path length is adjusted to be equal for the object and the reference beams. Spatial filters are used in each of the beams to reduce noice.
516
R. E. Acosta
et al. / Absorber
stress of X-ray masks
respectively. Since the interference fringes arise from the displacement of the substrate due to the removal of the (stressed) gold, it is absolutely necessary that the substrate be rigidly held and that the etching, rinsing, and drying steps be performed without altering the position of the substrate. With proper care these steps are easily performed without affecting the substrate. We verified this using gold-free substrates in our set-up, and subjecting them to all the steps regularly used to etch the substrates used for the measurements of stress. The fact that double-exposed holograms were consistently obtained where there was no evidence of interference fringes reassured us of the soundness of the technique and verified the stability of our experimental set-up. One other requirement of the technique is that, for ease of observation of the fringes, the substrates must be diffusively reflecting. In our experiments we used one-side polished silicon substrates (with the film to be measured deposited on the polished side). In mounting the specimen on the holographic set-up, the specimen was positioned so that the laser light was incident on the unpolished (diffusively reflecting) side of the wafer. One way to get around this requirement is to use a light diffuser between the specimen and the laser beam. One very important advantage of the double exposure holographic interferometry technique is that the stress of metal films may be measured directly, without interference by the stress in the metal used as adhesion layer: since the etchant chosen specifically dissolves the gold layer, the displacement measured is that due solely to removal of the gold layer (i.e., the “reference” plane includes any and all underlayers between the gold and the substrate).
Results Using this technique we have measured the stress of the electrodeposited fabrication of X-ray masks.
gold that we use for
Single crystal silicon wafers were used as substrates. Silicon wafers were chosen because of easy availability in a polished, clean state, and not by any special requirements dictated by the technique, as explained above. dn order to make the substrates suitable for electrodeposition a thin conducting layer of 300 A gold was evaporated on them. To insure proper adhesion of the gold to the silicon an adhesion layer of 50 A evaporated chrome was pre-deposited on the substrates. Electrodeposition of the thick gold layer was carried out from a non-cyanide plating solution. Because it is not possible to discriminate between the electrodeposited gold and the evaporated one during the etching, the method actually determines the average stress of the two layers of gold. It is known 3 that evaporated gold has a very large stress, of the order of 109dynes/cm2. However, the effect of this large stress is comparatively small on the measured average, given the thinness of the evaporated conducting layer. Where the evaporated gold seems to exert a very large effect is on the actual stress of the initial layers of electrodeposited gold. Figure 3 shows a plot of measured stress vs thickness of the deposit. It is observed that the stress decreases as the thickness of the deposit increases, reaching a value of 2.8 x lo7 dynes/cm 2 for a thickness of the order of 30 pm. We believe that this effect arises from the evaporated gold affecting the nucleation habit of the electrodeposited gold: as the thickness of the deposit grows the influence of the substrate becomes smaller and smaller. The value of the stress of thick electrodeposited gold may only be of academic interest: the stress affecting the distortion of the X-ray lithography mask substrates is the total, electrodeposited plus evaporated gold. For the gold thickness needed for synchrotron radiation X-ray lithography 8, 0.5 pm, the average stress is 4 to 5 x 10Xdynes/cm2
577
R. E. Acosta et al. 1 Absorber stress of X-ray masks
T
I
5
IO
I5 THICKNESS
2C
25
29
(pm)
Figure 3. Effect of film thickness on the average stress.
In the practice of electrodeposition of metals it is common to vary several parameters in order to obtain a deposit with the desired characteristics of brightness, uniformity, etc. Of these parameters we investigated the effect of plating with solutions at different temperatures, at different deposition rates, and using different amounts of the brightener additive recommended by the manufacturer. *
578
R.E. Acosta et al. /Absorber stress of X-ray masks
The deposition temperature may affect the stress of electrodeposited films in a dual fashion. The nucleation and crystal growth of the deposit may be affected and thus the (intrinsic) stress of the growing film. Also, because the substrate and the deposited film may have different expansion coefficients, as the case is for silicon and gold, when the gold is deposited at a high temperature the deposited film will be in a state of stress (a “thermal” stress) upon cooling. Table I shows the stress of films deposited at three different temperatures. Knowing the expansion coefficients of gold and silicon it is possible to calculate the value of the thermal stress induced on the deposited gold. Table I also gives the stress of the films deposited at high temperature after correction for the thermal stress. These results indicate that the intrinsic stress of the films is not adversely affected. One should not be misled by this previous statement: the films deposited at 75 “C &J have a higher stress, and as such will affect the distortion of the masks. Table I Dep. Temp. Stress at RT Stress at RT, corrected “C 10 s dyne/cm* 10 * dyne/cm* 25 2.2 ___ 50 75
3.9 4.0
2.0 1.0
Improper control of the plating solution operating parameters, improper agitation, presence of contaminants, etc. may lead to deposits that are not satisfactory. However, even under these conditions satisfactory deposits may be obtained by increasing the amount of additive used in the solution. Table II shows the effect of increasing the amount of additive used on the stress of the fihn obtained: excess additive results in deposits with a larger stress.
Amt. Additive “normal” 2 x “normal” 4 x “normal”
Table II Stress, 10 8 dyne/cm 2.2 7.0 7.0
2
Table III shows the effect of changing the rate of electrodeposition of the gold (i.e., changing the current density): deposition at fast rates may result in films with stress two-to-three times the stress of films deposited at low rate.
Table III Current Density, mA/cm * Stress, 10 s dyne/cm 1.5 3.8 3.0 4.9 6.0 5.8 9.0 8.9
2
In the fabrication of X-ray masks the plating base is removed using Ar ion milling 8. It has been found 9 that the distortion of the mask is considerably reduced after ion milling. This has been attributed to a reduction in the stress of the absorber. On the other hand, when a “plating base” (ie 50 A Cr plus 300 A Au) was evaporated on top of the electrodeposited films and this “plating base” was subsequently removed by ion milling, no significant change in stress was detectable. One explanation for this is that when masks are ion milled the heat generated by
R. E. Acosta et al. 1 Absorber stress of X-ray masks
579
the process cannot be conducted away by the very thin substrate membrane, while in the present experiments the thicker substrates can absorb much more heat, and also conduct it away in a more effective manner. Accordingly, the effect of heating on the stress of the electrodeposited gold was investigated. Gold films electrodeposited at room temperature were heated in a nitrogen atmosphere to 100, 150 and 200 ‘C, for a period of 1 hour. The stress measured after cooling to room temperature was found to be 65%, 45%, and 45% of the original stress, respectively. This tentatively supports the conclusion that the reduced distortion is due to heating, but more experiments will have to be conducted to verify it.
CONCLUSIONS Double exposure holographic interferometry of the absorber used in X-ray masks.
is a simple, reliable technique
to measure stress
So far we have used the double exposure holographic interferometry technique to determine the stress of uniform (i.e., non-patterned) films electrodeposited on silicon wafers. However, the technique can also be applied to study absorber films on membrane substrates, where the holographic image will contain information about the distribution of the net substrate displacement/distortion/deformation. In some instances this can be be very useful data, e.g., to study the effects of film stress on X-ray masks, where the pattern is not uniform but rather that of a typical integrated circuit. On the other hand, deconvolution of this information may not be simple to carry out.
REFERENCES 1. Segmuller, A. and Murakami, M. “Analytical Techniques for Thin Films”, IBM Research Report RC 10077, to be published in Treatise on Materials Science and Technology, T. N. Tu and R. Rosenberg, Eds. (Academic Press, New York) 2) Gate,P. B. and Hall, L. H., J. ECS, 119 (1972) 491-5. 3) Bruns, A., Harms, M., Luthje, H., and Matthiessen, B., in Proceedings of ME83. 4) Rossnagel,S. M., Gilstrap, P., and Rujkorakarn, R., J. Vat. Sci. Technol, 21 (1982) 1045-6. 5) van Nie, A. G., Solid State Technology, (Jan. 1980), 81-4. 6) Magill, P. S. and Young, T., J. Vat. Sci. Technol., 4 (1967) 47-8. 7) Klokholm, E., Rev. Sci. Instr., 40 (1969) 1054-g. 8) Acosta, R. E., Maldonado, J. R., Towart, L. K., and Warlaumont, J. M., in Proceedings of SPIE 448 (1984) 114-8. Reprinted in Solid State Technology, October 1984,205-g 9) Acosta, R. E., Maldonado, J. R., Fair, R., Viswanathan, R., and Wilson, A. D., This proceedings.
573
Characterization of Stress in the Absorber of X-ray Masks Using a Holographic Technique R. E. Acosta, A. D. Wilson, and J. V. Powers IBM T. J. Watson Research Center P. 0. Box 218 Yorktown Heights, NY 10598 USA
The distortion of X-ray masks is directly related to the stress of the absorber used. Because of this, it is very important be able to determine the value of the absorber stress in order to be able to control, or reduce, the distortion of the masks. A simple technique, double exposure holographic interferometry, is described. Its application in measuring the stress of electrodeposited gold films, and the effect that several deposition parameters have on the gold stress are described.
Introduction Distortion of X-ray masks due to stress of the absorber could render the masks unusable if the stress is not minimized. The ability to accurately measure the stress is clearly a prerequisite to controlling it. Most currently available methods for determining the stress in metal films involve measuring the distortion of the underlying substrate, and from this distortion calculating the stress. Because the distortion of the substrate is directly proportional to the thickness of the film, and inversely proportional to the thickness of the substrate squared, the distortion is apt to be a very small quantity. For the case of films deposited on single crystal silicon substrates the distortion can be obtained with the aid of an X-ray diffraction technique 1. In this case it is necessary to determine the crystalline integrity of the substrate before the deposition of the film to be studied, and to insure that no damage to the crystal occurs during the process of deposition of the film. For non-crystalline substrates the curvature of the substrate can be determined either by mechanical or by optical means. In the former case use is made of Tallysurf-like instruments, with the consequent lack of precision for the very small distortion involved. Distortion of the substrate can be determined optically by simply determining the change in focussing distance at different points in the substrate using an optical microscope; by interferometry using a reference optical flat (i.e., Newton rings) 2~3; or variations of this method 4; or using laser interferometry with a two-frequency laser 5. All of these methods require the use of substrates that were flat originally, and in some cases optically flat (a condition not frequently found), or that some complicated method be used to subtract the original bending of the substrate from that caused by the film. 0167-93 17/85/$3.30 0 1985, Elsevier Science Publishers B.V. (North-Holland)
574
Experimental
R.E. Acosta et al. /Absorber stress of X-ray mash-s
Technique
We have developed a procedure for determining the stress of electrodeposited films of gold using the technique of double exposure holographic interferometry 6. This technique has the required resolution and does not demand an optically flat substrate. In fact, the holographic technique works well on either a flat or a distorted substrate. This application of holography is basically an interferometric process, but instead of using an optical flat as reference, the two wavefronts that are superimposed come from the unstressed sample and from the sample under stress. Because the sample is displaced as a result of the applied stress, interference fringes are observed which are a direct measure of the strain in the substrate. In the approach that we have used, the specimen, after electrodeposition of the gold film, is mounted on a suitable holder and the first holographic exposure is made. Then, part or all of the gold film is etched away, in situ, and a second holographic exposure is made on the same photographic plate. (Before the second hologram is made the substrate must be rinsed and dried, also in situ). After processing of the photographic plate the holographic image is reconstructed with the coherent light source used originally to expose it. A photograph of the hologram is made, or the fringes are counted directly, to determine the net displacement of the substrate, and thus the stress induced by the (etched) gold film. A photograph of a typical hologram is shown in Figure 1.
Fig. 1. Photograph of a typical double exposure hologram showing the clarity of the interference fringes. The in-plane displacement of the substrate is given by the number of fringes multiplied by l/4 X of the light used to make the hologram.
R. E. Acosta et al. / Absorber stress of X-ray masks
575
The stress of the film is calculated using Stoney’s equation 7: 1 d: GRXd,XI_vX6’
E,
1
where: R = radius of the curvature caused by the stress of the film d,,= thickness of the substrate d, = thickness of the stressed film, and E, and Y = Young’s modulus and Poisson’s ratio of the substrate material, respectively. For large R the relationship the substrate,is given by: 1 A -=R r2
between R and A, the displacement
perpendicular
to the plane of
where r = distance from center of the substrate. A is given by the number of fringes counted over the surface of the substrate multiplied by the quarter wavelength of the light used to make the hologram. The set-up that we used is schematically represented in Figure 2. The source of coherent light used was a He-Ne laser (output power 15 mWatt). The gold films were etched using a RI-I, aqueous solution, and the substrates were rinsed and dried using water and iso-propyl alcohol, DOUBLE EXPOSURE HOLOGRAPHIC INTERFEROMETRY
-0
r
LASER
Figure 2. Set-up for double exposure holographic interferometry. The optical path length is adjusted to be equal for the object and the reference beams. Spatial filters are used in each of the beams to reduce noice.
516
R. E. Acosta
et al. / Absorber
stress of X-ray masks
respectively. Since the interference fringes arise from the displacement of the substrate due to the removal of the (stressed) gold, it is absolutely necessary that the substrate be rigidly held and that the etching, rinsing, and drying steps be performed without altering the position of the substrate. With proper care these steps are easily performed without affecting the substrate. We verified this using gold-free substrates in our set-up, and subjecting them to all the steps regularly used to etch the substrates used for the measurements of stress. The fact that double-exposed holograms were consistently obtained where there was no evidence of interference fringes reassured us of the soundness of the technique and verified the stability of our experimental set-up. One other requirement of the technique is that, for ease of observation of the fringes, the substrates must be diffusively reflecting. In our experiments we used one-side polished silicon substrates (with the film to be measured deposited on the polished side). In mounting the specimen on the holographic set-up, the specimen was positioned so that the laser light was incident on the unpolished (diffusively reflecting) side of the wafer. One way to get around this requirement is to use a light diffuser between the specimen and the laser beam. One very important advantage of the double exposure holographic interferometry technique is that the stress of metal films may be measured directly, without interference by the stress in the metal used as adhesion layer: since the etchant chosen specifically dissolves the gold layer, the displacement measured is that due solely to removal of the gold layer (i.e., the “reference” plane includes any and all underlayers between the gold and the substrate).
Results Using this technique we have measured the stress of the electrodeposited fabrication of X-ray masks.
gold that we use for
Single crystal silicon wafers were used as substrates. Silicon wafers were chosen because of easy availability in a polished, clean state, and not by any special requirements dictated by the technique, as explained above. dn order to make the substrates suitable for electrodeposition a thin conducting layer of 300 A gold was evaporated on them. To insure proper adhesion of the gold to the silicon an adhesion layer of 50 A evaporated chrome was pre-deposited on the substrates. Electrodeposition of the thick gold layer was carried out from a non-cyanide plating solution. Because it is not possible to discriminate between the electrodeposited gold and the evaporated one during the etching, the method actually determines the average stress of the two layers of gold. It is known 3 that evaporated gold has a very large stress, of the order of 109dynes/cm2. However, the effect of this large stress is comparatively small on the measured average, given the thinness of the evaporated conducting layer. Where the evaporated gold seems to exert a very large effect is on the actual stress of the initial layers of electrodeposited gold. Figure 3 shows a plot of measured stress vs thickness of the deposit. It is observed that the stress decreases as the thickness of the deposit increases, reaching a value of 2.8 x lo7 dynes/cm 2 for a thickness of the order of 30 pm. We believe that this effect arises from the evaporated gold affecting the nucleation habit of the electrodeposited gold: as the thickness of the deposit grows the influence of the substrate becomes smaller and smaller. The value of the stress of thick electrodeposited gold may only be of academic interest: the stress affecting the distortion of the X-ray lithography mask substrates is the total, electrodeposited plus evaporated gold. For the gold thickness needed for synchrotron radiation X-ray lithography 8, 0.5 pm, the average stress is 4 to 5 x 10Xdynes/cm2
577
R. E. Acosta et al. 1 Absorber stress of X-ray masks
T
I
5
IO
I5 THICKNESS
2C
25
29
(pm)
Figure 3. Effect of film thickness on the average stress.
In the practice of electrodeposition of metals it is common to vary several parameters in order to obtain a deposit with the desired characteristics of brightness, uniformity, etc. Of these parameters we investigated the effect of plating with solutions at different temperatures, at different deposition rates, and using different amounts of the brightener additive recommended by the manufacturer. *
578
R.E. Acosta et al. /Absorber stress of X-ray masks
The deposition temperature may affect the stress of electrodeposited films in a dual fashion. The nucleation and crystal growth of the deposit may be affected and thus the (intrinsic) stress of the growing film. Also, because the substrate and the deposited film may have different expansion coefficients, as the case is for silicon and gold, when the gold is deposited at a high temperature the deposited film will be in a state of stress (a “thermal” stress) upon cooling. Table I shows the stress of films deposited at three different temperatures. Knowing the expansion coefficients of gold and silicon it is possible to calculate the value of the thermal stress induced on the deposited gold. Table I also gives the stress of the films deposited at high temperature after correction for the thermal stress. These results indicate that the intrinsic stress of the films is not adversely affected. One should not be misled by this previous statement: the films deposited at 75 “C &J have a higher stress, and as such will affect the distortion of the masks. Table I Dep. Temp. Stress at RT Stress at RT, corrected “C 10 s dyne/cm* 10 * dyne/cm* 25 2.2 ___ 50 75
3.9 4.0
2.0 1.0
Improper control of the plating solution operating parameters, improper agitation, presence of contaminants, etc. may lead to deposits that are not satisfactory. However, even under these conditions satisfactory deposits may be obtained by increasing the amount of additive used in the solution. Table II shows the effect of increasing the amount of additive used on the stress of the fihn obtained: excess additive results in deposits with a larger stress.
Amt. Additive “normal” 2 x “normal” 4 x “normal”
Table II Stress, 10 8 dyne/cm 2.2 7.0 7.0
2
Table III shows the effect of changing the rate of electrodeposition of the gold (i.e., changing the current density): deposition at fast rates may result in films with stress two-to-three times the stress of films deposited at low rate.
Table III Current Density, mA/cm * Stress, 10 s dyne/cm 1.5 3.8 3.0 4.9 6.0 5.8 9.0 8.9
2
In the fabrication of X-ray masks the plating base is removed using Ar ion milling 8. It has been found 9 that the distortion of the mask is considerably reduced after ion milling. This has been attributed to a reduction in the stress of the absorber. On the other hand, when a “plating base” (ie 50 A Cr plus 300 A Au) was evaporated on top of the electrodeposited films and this “plating base” was subsequently removed by ion milling, no significant change in stress was detectable. One explanation for this is that when masks are ion milled the heat generated by
R. E. Acosta et al. 1 Absorber stress of X-ray masks
579
the process cannot be conducted away by the very thin substrate membrane, while in the present experiments the thicker substrates can absorb much more heat, and also conduct it away in a more effective manner. Accordingly, the effect of heating on the stress of the electrodeposited gold was investigated. Gold films electrodeposited at room temperature were heated in a nitrogen atmosphere to 100, 150 and 200 ‘C, for a period of 1 hour. The stress measured after cooling to room temperature was found to be 65%, 45%, and 45% of the original stress, respectively. This tentatively supports the conclusion that the reduced distortion is due to heating, but more experiments will have to be conducted to verify it.
CONCLUSIONS Double exposure holographic interferometry of the absorber used in X-ray masks.
is a simple, reliable technique
to measure stress
So far we have used the double exposure holographic interferometry technique to determine the stress of uniform (i.e., non-patterned) films electrodeposited on silicon wafers. However, the technique can also be applied to study absorber films on membrane substrates, where the holographic image will contain information about the distribution of the net substrate displacement/distortion/deformation. In some instances this can be be very useful data, e.g., to study the effects of film stress on X-ray masks, where the pattern is not uniform but rather that of a typical integrated circuit. On the other hand, deconvolution of this information may not be simple to carry out.
REFERENCES 1. Segmuller, A. and Murakami, M. “Analytical Techniques for Thin Films”, IBM Research Report RC 10077, to be published in Treatise on Materials Science and Technology, T. N. Tu and R. Rosenberg, Eds. (Academic Press, New York) 2) Gate,P. B. and Hall, L. H., J. ECS, 119 (1972) 491-5. 3) Bruns, A., Harms, M., Luthje, H., and Matthiessen, B., in Proceedings of ME83. 4) Rossnagel,S. M., Gilstrap, P., and Rujkorakarn, R., J. Vat. Sci. Technol, 21 (1982) 1045-6. 5) van Nie, A. G., Solid State Technology, (Jan. 1980), 81-4. 6) Magill, P. S. and Young, T., J. Vat. Sci. Technol., 4 (1967) 47-8. 7) Klokholm, E., Rev. Sci. Instr., 40 (1969) 1054-g. 8) Acosta, R. E., Maldonado, J. R., Towart, L. K., and Warlaumont, J. M., in Proceedings of SPIE 448 (1984) 114-8. Reprinted in Solid State Technology, October 1984,205-g 9) Acosta, R. E., Maldonado, J. R., Fair, R., Viswanathan, R., and Wilson, A. D., This proceedings.