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IR analysis of water absorption in optical multilayer structures
Thin Solid Films, 67 (1980) 239-244 © Elsevier Sequoia S.A., Lausanne Printed in the Netherlands
239
IR ANALYSIS OF WATER ABSORPTION IN OPTICAL MULTILAYER STRUCTURES NIELS ERIK HOLM AND ORLA CHRISTENSEN Industriselskabet FERRO P E R M A/ S, 2950 Vedbaek (Denmark) (Received September 7, 1979; accepted October 9, 1979)
The destruction by water absorption of a Fabry-Perot-type bandpass filter, deposited by conventional evaporation onto silicon and fused quartz, was studied. IR spectra were used to determine the total amount of absorbed water as a function of the time of exposure to humidity. Saturation occurred and a correlation between the water content, the centre wavelength shift and the increase of the metric filter thickness was established. It is concluded that filter protection, including cemented cover glass and epoxy resin seals, cannot prevent destruction. Higher film densities might reduce or eliminate the moisture sensitivity problem.
1. INTRODUCTION It has been established that evaporated optical thin film structures are moisture sensitive 1. It has thus been observed that the exposure of bandpass filters for example to high levels of humidity results in (1) a shift of the pass band towards longer wavelengths, (2) a broadening of the pass band and (3) a decrease in the transmittance. Various techniques are commonly used to protect these structures against humidity. These include cemented cover glasses which have an uncoated rim at the substrate edge and epoxy resin sealing of the edge of the cemented filter in a guard ring. Such measures are taken to ensure the stability required in optical and electronic equipment using interference filters. Ritter 2, Christmas and Richmond ~ and McLeod and Richmond3'have reviewed the humidity penetration process and have discussed various experimental approaches for demonstrating the effect of water vapour in both single films and multilayer structures. In most of these methods optical performance is checked as water absorption proceeds. There seems to be very little direct evidence for the presence of water vapour in damaged multilayer filters. In this work quantitative results on the water content of damaged filters are presented and a correlation with changes in the spectral response is established. 2. EXPERIMENTAL
Single films of both high refractive index (ZnS) and low refractive index (NaaAIF6) materials were examined as well as a Fabry-Perot bandpass filter centred
240
y . E . HOLM, O. CHRISTENSEN
at 546 nm and having the configuration HLHLH(8L)HLHLH, using the conventional notation. Standard procedures were used for substrate cleaning and film deposition by filament heating. The films were evaporated onto fused quartz and silicon substrates and process control was carried out by transmission-mode monitoring. Both the single films and the interference filters were exposed to 100'~, relative humidity at room temperature. The following data were collected on the bandpass filters at intervals: IR spectra at about 3 and 6 tam, known to be bands for water absorption, visible spectra and metric thickness measurements using Tolansky interferometry 4. Only the metric thickness as a function of the exposure time was recorded for the single films. No difficulties were found in counting the Tolansky fringes without the use of conventional metal overcoatings. It was thus possible to measure on the same substrate the thickness evolution as a function of the exposure time and additionally to eliminate possible vacuum-induced water vapour release due to the metallization process. The difference in phase shift between reflection from the bandpass filter and from the uncoated part of the substrate was calculated, thereby giving errors of less than 50 nm in the thickness measurements.
10
08 1
530 , c
06
I
/
5 z
i
hU 6 0 0 > ,<
O4 3
uJ i-z Ld
qP
!
//
o/°
~
/
' i
/
570
/
/
/
/
/ '
i
i ol °
OQ: 500
. _
. 700 WAVELE NGTH(nm) 600
I 540 .
.
.
.
. 20
.
. 30
40
~ 50
• 60
70 ~ 7 ' ~ 8 20 4 6 HOURS
Fig. 1. The spectral drift of a Fabry-Perot filter caused by exposure to 100'~o relative humidity at room temperature: curve 1, the "dry filter"; curve 2, after 24 h of humidity exposure; curve 3, after 78 h of humidity exposure. Fig. 2. The position of the centre wavelength as a function of the total time of humidity exposure. Film fracturing took place after 246 h, probably because of stress relief.
WATER ABSORPTION IN OPTICAL MULTILAYER STRUCTURES
241
3. RESULTS Figure 1 shows the filter response in the visible spectrum for the "dry filter" and for the same filter after 24 and 78 h of humidity exposure. These data were obtained from the quartz substrate filter and show the characteristic shift towards higher wavelengths, the decrease in transmission and the broadening of the pass band. Figure 2 gives more detailed information concerning the location of the centre wavelength as a function of the total time of humidity exposure (the centre wavelength is defined as the mean wavelength between the half maximum points). Figure 3 shows typical IR absorption spectra obtained with a "dry filter" from the same pump down inserted in the reference beam.of the IR spectrometer. The peaks at both 3 and 6 Ixm are present for the silicon substrate sample, whereas only the 3 ~tm peak is found for the quartz substrate sample because the transparency range for quartz is limited to approximately 4 lam. 4
3
FR EQU E NCY (x 103c.~1) ~ 1
1.0
2
z
05
0
R
25
i
i
i
33
5
10
WAVELENGT H ( p rn )
Fig. 3. Typical IR absorption spectra: - - - , silicon substrate; - - - , quartz substrate.
The peak obtained from the silicon substrate sample at 3 ~tm is apparently well resolved, whereas the broadening of the 3 lam peak of the quartz substrate sample and of the 6 ~tm peak of the silicon substrate sample is ascribed to background contributions to the signal. Figure 4 shows log=(To/T ) plotted against the exposure time, where T is the transmittance at the IR absorption centre and TOis the initial transmittance value. Silicon substrate data for both the 3 and 6 lam peaks are shown, whereas only the 3 lam results are plotted for the quartz substrate filter. The results of the metric thickness measurements are shown in Fig. 5. Similar measurements indicated no thickness changes (less than 29/0)for single films of both the high and low refractive index materials upon moisture exposure. 4. DISCUSSION It is well established that water absorption does not take place uniformly over
242
N. E. HOLM, O. CHRISTENSEN
an exposed surface. The water penetration follows a more complicated pattern which is related to the columnar structure of the films (see Fig. 6). However, on a macroscopic level Figs. 2, 5 and 4 clearly demonstrate that there is a correlation between the centre wavelength shift, the thickness increase and the water content; all seem to be simple saturation processes which have the same time constant, in this particular case approximately 40 h. <
09r
/
I
°tl osF
× -
-
- -
7
IgL I
04
03i
c~4 1 8
/
×
.........
~17!
I /"
[o
OlO2r[//+~'+j'
+
/
/~-
<9
3-
I0
20
30
40
50
60
70
80 246
HOURS
0
10
20
30
40
50
164
HOURS
Fig. 4. The absorbance as a function of the total time of humidity exposure: x, silicon substrate, 3 lam peak; O, silicon substrate, 6 ~tm peak; + , quartz substrate, 3 I~m peak. Fig. 5. The metric filter thickness as a function of the humidity exposure.
(a)
(b)
Fig. 6. O p t i c a l m i c r o g r a p h s s h o w i n g ( a ) p a r t l y a n d ( b ) f u l l y s a t u r a t e d f i l t e r s o b t a i n e d a f t e r 4 4 a n d 1 6 4 h o f humidity exposure respectively. The samples were illuminated by 589 n m sodium light.
Evaporated ZnS films deposited onto thermally floating substrates usually have the bulk refractive index and a density very close to unity. Na3AIF 6 films, however, have lower refractive index and density values than those of the bulk materiaP. If it is assumed in this work that the ZnS films have bulk properties 6, then the effective refractive index and the packing density of Na3AIF 6 can be calculated.
WATER
ABSORPTION
IN OPTICAL
MULTILAYER
STRUCTURES
243
4.1. Dryfilter The filter was initially centred at a wavelength of 546 nm, determined by the eight quarter-wave spacer. The metric thickness of the entire structure was measured to be 1570 nm, resulting from the presence of a total of six high refractive index quarter-wave layers and 12 low refractive index quarter-wave layers. This yields a film refractive index nmm value for Na3AIF 6 of 1.33, which is in good agreement with previously reported data s. The packing density can then be calculated 7 from p = ( n f i l m - 1 ) / ( n b u l k - - 1) and a value of 0.87 is obtained. 4.2. Saturated filter Upon saturation with water the filter became centred at 625 nm. If it is assumed that the penetration of water does change the density of Na3A1F 6, then the filling of the porous spacer with water having a refractive index of 1.33 will result in an effective refractive index of 1.36. Combination with the metric thickness of the spacer found earlier would then predict a saturated centre wavelength of 560 nm which disagrees with the measured value. Consequently, an additional source of increasing the optical thickness of the spacer must explain the observed wavelength shift. The total amount of water available for filling the pores of the 12 quarter-wave layers of Na3AIF 6 can be estimated from the data of Fig. 4 and the findings of Plyler and Griff 8. These authors have studied IR absorption in water films of various thicknesses. From these findings and the saturation value attained by the 3 lxm peak of the silicon substrate filter, th.e recorded total water content would correspond to an equivalent water film having a thickness of 510 nm. The water of course does not exist as a continuous single film but is distributed within the filter structure. The amount of water needed to fill 13%, i.e. 1 - p , of the thickness of 12 NaaAIF 6 layers amounts to a total of 160 nm. This leaves 350 nm of water to be accounted for. However, if this value is added to the measured thickness of the dry filter, then the sum agrees within error with the measured thickness of the saturated filter (Fig. 5). The data obtained in this work do not give any indication of either the exact water absorption mechanism or the precise location of the water. However, if the assumption is maintained that the water is mainly destroying the Na3AIF 6 layers and their interfaces with ZnS, then the excess water can be accounted for. The reflectors on both sides of the spacer represent four quarter-waves and eight interfaces, whereas the spacer has eight quarter-waves and two interfaces. If, in the saturated filter, it is assumed that equal amounts of water can be bound in an interface and forced into a quarter-wave then one-half of the water should belong to the spacer and the rest to the reflectors. In this case the measured thickness of the saturated filter agrees with the IR water content data and with the measured original thickness of the dry filter; forcing more water into the spacer also increases its optical thickness and drives the centre wavelength towards still higher values, which is consistent with the observations. 5.
CONCLUSIONS
Fabry-Perot bandpass filters consisting of five-layer quarter-wave stacks surrounding an eight quarter-wave low refractive index spacer centred at 546 nm were evaporated onto silicon and fused quartz substrates. ZnS and NaaA1F6 were
244
Y. E. HOLM, O. CHRISTENSEN
used as the high and low refractive index materials respectively. The filters were not protected by optical cement and covers, but were allowed to absorb water by exposure to 100% relative humidity at room temperature. The IR absorption at 3 and 6 Ixm, the metric thickness and the filter centre wavelength shift were recorded as functions of the total time of humidity exposure. The water penetration pattern is known to be rather complicated on the microscopic level and is related to the film structure. However, the three macroscopic parameters studied in this work all indicate a simple saturation process characterized by a time constant of approximately 40 h. These findings establish a direct correlation between the water content and the centre wavelength shift. The actual water content of the saturated filter greatly exceeds the amount of water needed to fill the porous Na3A1F 6 layers. The extra water was accounted for using simple calculations, the validity of which, however, cannot be verified by the present data. The matter could perhaps be clarified by nuclear reactions. Even so the present results agree with the observed increase of the metric thickness of the filters. Practical filters are protected with cemented cover glasses and epoxy resin diffusion barriers on the edges. Although these measures are taken to prevent moisture absorption, water does diffuse through epoxy resin 9. The penetrating water will eventually result in total destruction since the forces created by film expansion will, upon extended humidity exposure, exceed the bond strength of the optical cement thus releasing the protective cover. The results of this work suggest that, in order to eliminate moisture sensitivity, the density of the individual films in optical multilayer structures should be increased, perhaps by using other materials or other deposition techniques or both. ACKNOWLEDGMENTS
The authors would like to express their gratitude to the Institute of Chemistry, University of Aarhus, for allowing the IR spectroscopy measurements to be carried out there. Special thanks are due to J. Bentsen and G. Pedersen for technical assistance and many fruitful discussions. One of us (N.E.H.) would like to acknowledge a grant from the Industrial Research Committee of the Academy of Applied Sciences (ATV). REFERENCES l 2 3 4
T.M. Christmas and D. Richmond, Opt. Laser Technol., 9 (1977) 109. E. Ritter, Phys. Thin Film, 8 (1975) 1. H.A. MacLeod and D. Richmond, Thin Solid Films, 37 (1976) 163. L.I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, p. 11-8. 5 S. Ogura and H. A. MacLeod, Thin Solid Films, 34 (1976) 371. 6 N.E. Holm, unpublished results, 1976. 7 S. Ogura, N. Sugawava and R. Hiraga, Thin Solid Films, 30 (1975) 3. 8 E.K. Plyler and N. Griff, Appl. Opt., 4 (1965) 1663. 9 M.E. Sweet, Proc. 28th Electronic Components Conf., Anaheim, Cal(l:, IEEE, New York, 1978, p. 33.
239
IR ANALYSIS OF WATER ABSORPTION IN OPTICAL MULTILAYER STRUCTURES NIELS ERIK HOLM AND ORLA CHRISTENSEN Industriselskabet FERRO P E R M A/ S, 2950 Vedbaek (Denmark) (Received September 7, 1979; accepted October 9, 1979)
The destruction by water absorption of a Fabry-Perot-type bandpass filter, deposited by conventional evaporation onto silicon and fused quartz, was studied. IR spectra were used to determine the total amount of absorbed water as a function of the time of exposure to humidity. Saturation occurred and a correlation between the water content, the centre wavelength shift and the increase of the metric filter thickness was established. It is concluded that filter protection, including cemented cover glass and epoxy resin seals, cannot prevent destruction. Higher film densities might reduce or eliminate the moisture sensitivity problem.
1. INTRODUCTION It has been established that evaporated optical thin film structures are moisture sensitive 1. It has thus been observed that the exposure of bandpass filters for example to high levels of humidity results in (1) a shift of the pass band towards longer wavelengths, (2) a broadening of the pass band and (3) a decrease in the transmittance. Various techniques are commonly used to protect these structures against humidity. These include cemented cover glasses which have an uncoated rim at the substrate edge and epoxy resin sealing of the edge of the cemented filter in a guard ring. Such measures are taken to ensure the stability required in optical and electronic equipment using interference filters. Ritter 2, Christmas and Richmond ~ and McLeod and Richmond3'have reviewed the humidity penetration process and have discussed various experimental approaches for demonstrating the effect of water vapour in both single films and multilayer structures. In most of these methods optical performance is checked as water absorption proceeds. There seems to be very little direct evidence for the presence of water vapour in damaged multilayer filters. In this work quantitative results on the water content of damaged filters are presented and a correlation with changes in the spectral response is established. 2. EXPERIMENTAL
Single films of both high refractive index (ZnS) and low refractive index (NaaAIF6) materials were examined as well as a Fabry-Perot bandpass filter centred
240
y . E . HOLM, O. CHRISTENSEN
at 546 nm and having the configuration HLHLH(8L)HLHLH, using the conventional notation. Standard procedures were used for substrate cleaning and film deposition by filament heating. The films were evaporated onto fused quartz and silicon substrates and process control was carried out by transmission-mode monitoring. Both the single films and the interference filters were exposed to 100'~, relative humidity at room temperature. The following data were collected on the bandpass filters at intervals: IR spectra at about 3 and 6 tam, known to be bands for water absorption, visible spectra and metric thickness measurements using Tolansky interferometry 4. Only the metric thickness as a function of the exposure time was recorded for the single films. No difficulties were found in counting the Tolansky fringes without the use of conventional metal overcoatings. It was thus possible to measure on the same substrate the thickness evolution as a function of the exposure time and additionally to eliminate possible vacuum-induced water vapour release due to the metallization process. The difference in phase shift between reflection from the bandpass filter and from the uncoated part of the substrate was calculated, thereby giving errors of less than 50 nm in the thickness measurements.
10
08 1
530 , c
06
I
/
5 z
i
hU 6 0 0 > ,<
O4 3
uJ i-z Ld
qP
!
//
o/°
~
/
' i
/
570
/
/
/
/
/ '
i
i ol °
OQ: 500
. _
. 700 WAVELE NGTH(nm) 600
I 540 .
.
.
.
. 20
.
. 30
40
~ 50
• 60
70 ~ 7 ' ~ 8 20 4 6 HOURS
Fig. 1. The spectral drift of a Fabry-Perot filter caused by exposure to 100'~o relative humidity at room temperature: curve 1, the "dry filter"; curve 2, after 24 h of humidity exposure; curve 3, after 78 h of humidity exposure. Fig. 2. The position of the centre wavelength as a function of the total time of humidity exposure. Film fracturing took place after 246 h, probably because of stress relief.
WATER ABSORPTION IN OPTICAL MULTILAYER STRUCTURES
241
3. RESULTS Figure 1 shows the filter response in the visible spectrum for the "dry filter" and for the same filter after 24 and 78 h of humidity exposure. These data were obtained from the quartz substrate filter and show the characteristic shift towards higher wavelengths, the decrease in transmission and the broadening of the pass band. Figure 2 gives more detailed information concerning the location of the centre wavelength as a function of the total time of humidity exposure (the centre wavelength is defined as the mean wavelength between the half maximum points). Figure 3 shows typical IR absorption spectra obtained with a "dry filter" from the same pump down inserted in the reference beam.of the IR spectrometer. The peaks at both 3 and 6 Ixm are present for the silicon substrate sample, whereas only the 3 ~tm peak is found for the quartz substrate sample because the transparency range for quartz is limited to approximately 4 lam. 4
3
FR EQU E NCY (x 103c.~1) ~ 1
1.0
2
z
05
0
R
25
i
i
i
33
5
10
WAVELENGT H ( p rn )
Fig. 3. Typical IR absorption spectra: - - - , silicon substrate; - - - , quartz substrate.
The peak obtained from the silicon substrate sample at 3 ~tm is apparently well resolved, whereas the broadening of the 3 lam peak of the quartz substrate sample and of the 6 ~tm peak of the silicon substrate sample is ascribed to background contributions to the signal. Figure 4 shows log=(To/T ) plotted against the exposure time, where T is the transmittance at the IR absorption centre and TOis the initial transmittance value. Silicon substrate data for both the 3 and 6 lam peaks are shown, whereas only the 3 lam results are plotted for the quartz substrate filter. The results of the metric thickness measurements are shown in Fig. 5. Similar measurements indicated no thickness changes (less than 29/0)for single films of both the high and low refractive index materials upon moisture exposure. 4. DISCUSSION It is well established that water absorption does not take place uniformly over
242
N. E. HOLM, O. CHRISTENSEN
an exposed surface. The water penetration follows a more complicated pattern which is related to the columnar structure of the films (see Fig. 6). However, on a macroscopic level Figs. 2, 5 and 4 clearly demonstrate that there is a correlation between the centre wavelength shift, the thickness increase and the water content; all seem to be simple saturation processes which have the same time constant, in this particular case approximately 40 h. <
09r
/
I
°tl osF
× -
-
- -
7
IgL I
04
03i
c~4 1 8
/
×
.........
~17!
I /"
[o
OlO2r[//+~'+j'
+
/
/~-
<9
3-
I0
20
30
40
50
60
70
80 246
HOURS
0
10
20
30
40
50
164
HOURS
Fig. 4. The absorbance as a function of the total time of humidity exposure: x, silicon substrate, 3 lam peak; O, silicon substrate, 6 ~tm peak; + , quartz substrate, 3 I~m peak. Fig. 5. The metric filter thickness as a function of the humidity exposure.
(a)
(b)
Fig. 6. O p t i c a l m i c r o g r a p h s s h o w i n g ( a ) p a r t l y a n d ( b ) f u l l y s a t u r a t e d f i l t e r s o b t a i n e d a f t e r 4 4 a n d 1 6 4 h o f humidity exposure respectively. The samples were illuminated by 589 n m sodium light.
Evaporated ZnS films deposited onto thermally floating substrates usually have the bulk refractive index and a density very close to unity. Na3AIF 6 films, however, have lower refractive index and density values than those of the bulk materiaP. If it is assumed in this work that the ZnS films have bulk properties 6, then the effective refractive index and the packing density of Na3AIF 6 can be calculated.
WATER
ABSORPTION
IN OPTICAL
MULTILAYER
STRUCTURES
243
4.1. Dryfilter The filter was initially centred at a wavelength of 546 nm, determined by the eight quarter-wave spacer. The metric thickness of the entire structure was measured to be 1570 nm, resulting from the presence of a total of six high refractive index quarter-wave layers and 12 low refractive index quarter-wave layers. This yields a film refractive index nmm value for Na3AIF 6 of 1.33, which is in good agreement with previously reported data s. The packing density can then be calculated 7 from p = ( n f i l m - 1 ) / ( n b u l k - - 1) and a value of 0.87 is obtained. 4.2. Saturated filter Upon saturation with water the filter became centred at 625 nm. If it is assumed that the penetration of water does change the density of Na3A1F 6, then the filling of the porous spacer with water having a refractive index of 1.33 will result in an effective refractive index of 1.36. Combination with the metric thickness of the spacer found earlier would then predict a saturated centre wavelength of 560 nm which disagrees with the measured value. Consequently, an additional source of increasing the optical thickness of the spacer must explain the observed wavelength shift. The total amount of water available for filling the pores of the 12 quarter-wave layers of Na3AIF 6 can be estimated from the data of Fig. 4 and the findings of Plyler and Griff 8. These authors have studied IR absorption in water films of various thicknesses. From these findings and the saturation value attained by the 3 lxm peak of the silicon substrate filter, th.e recorded total water content would correspond to an equivalent water film having a thickness of 510 nm. The water of course does not exist as a continuous single film but is distributed within the filter structure. The amount of water needed to fill 13%, i.e. 1 - p , of the thickness of 12 NaaAIF 6 layers amounts to a total of 160 nm. This leaves 350 nm of water to be accounted for. However, if this value is added to the measured thickness of the dry filter, then the sum agrees within error with the measured thickness of the saturated filter (Fig. 5). The data obtained in this work do not give any indication of either the exact water absorption mechanism or the precise location of the water. However, if the assumption is maintained that the water is mainly destroying the Na3AIF 6 layers and their interfaces with ZnS, then the excess water can be accounted for. The reflectors on both sides of the spacer represent four quarter-waves and eight interfaces, whereas the spacer has eight quarter-waves and two interfaces. If, in the saturated filter, it is assumed that equal amounts of water can be bound in an interface and forced into a quarter-wave then one-half of the water should belong to the spacer and the rest to the reflectors. In this case the measured thickness of the saturated filter agrees with the IR water content data and with the measured original thickness of the dry filter; forcing more water into the spacer also increases its optical thickness and drives the centre wavelength towards still higher values, which is consistent with the observations. 5.
CONCLUSIONS
Fabry-Perot bandpass filters consisting of five-layer quarter-wave stacks surrounding an eight quarter-wave low refractive index spacer centred at 546 nm were evaporated onto silicon and fused quartz substrates. ZnS and NaaA1F6 were
244
Y. E. HOLM, O. CHRISTENSEN
used as the high and low refractive index materials respectively. The filters were not protected by optical cement and covers, but were allowed to absorb water by exposure to 100% relative humidity at room temperature. The IR absorption at 3 and 6 Ixm, the metric thickness and the filter centre wavelength shift were recorded as functions of the total time of humidity exposure. The water penetration pattern is known to be rather complicated on the microscopic level and is related to the film structure. However, the three macroscopic parameters studied in this work all indicate a simple saturation process characterized by a time constant of approximately 40 h. These findings establish a direct correlation between the water content and the centre wavelength shift. The actual water content of the saturated filter greatly exceeds the amount of water needed to fill the porous Na3A1F 6 layers. The extra water was accounted for using simple calculations, the validity of which, however, cannot be verified by the present data. The matter could perhaps be clarified by nuclear reactions. Even so the present results agree with the observed increase of the metric thickness of the filters. Practical filters are protected with cemented cover glasses and epoxy resin diffusion barriers on the edges. Although these measures are taken to prevent moisture absorption, water does diffuse through epoxy resin 9. The penetrating water will eventually result in total destruction since the forces created by film expansion will, upon extended humidity exposure, exceed the bond strength of the optical cement thus releasing the protective cover. The results of this work suggest that, in order to eliminate moisture sensitivity, the density of the individual films in optical multilayer structures should be increased, perhaps by using other materials or other deposition techniques or both. ACKNOWLEDGMENTS
The authors would like to express their gratitude to the Institute of Chemistry, University of Aarhus, for allowing the IR spectroscopy measurements to be carried out there. Special thanks are due to J. Bentsen and G. Pedersen for technical assistance and many fruitful discussions. One of us (N.E.H.) would like to acknowledge a grant from the Industrial Research Committee of the Academy of Applied Sciences (ATV). REFERENCES l 2 3 4
T.M. Christmas and D. Richmond, Opt. Laser Technol., 9 (1977) 109. E. Ritter, Phys. Thin Film, 8 (1975) 1. H.A. MacLeod and D. Richmond, Thin Solid Films, 37 (1976) 163. L.I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGraw-Hill, New York, 1970, p. 11-8. 5 S. Ogura and H. A. MacLeod, Thin Solid Films, 34 (1976) 371. 6 N.E. Holm, unpublished results, 1976. 7 S. Ogura, N. Sugawava and R. Hiraga, Thin Solid Films, 30 (1975) 3. 8 E.K. Plyler and N. Griff, Appl. Opt., 4 (1965) 1663. 9 M.E. Sweet, Proc. 28th Electronic Components Conf., Anaheim, Cal(l:, IEEE, New York, 1978, p. 33.