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Electric-field-assisted deposition of optical coatings
Thin Solid Films, 156 (1988) 153 160
153
PREPARATION AND CHARACTERIZATION
E L E C T R I C - F I E L D - A S S I S T E D D E P O S I T I O N OF OPTICAL C O A T I N G S P. F. GU*
Department ~f Optical Engineering, Zhe/iang University, Hangzhou (China) (Received May 5, 1987; accepted August 24, 1987)
It is possible to improve the stability of optical coatings if an electric field is applied during deposition. A.c. and d.c. electric fields have been used in the preparation of narrow-band filters consisting of ZnS and cryolite layers and edge filters of ZnS and MgFz. As a result, the shift in the wavelength of prominent features in the spectral transmittance is reduced considerably even for coatings subjected to 100% relative humidity.
1. INTRODUCTION
Stabilization of narrow-band and edge filters is a serious problem. It is related to the microstructure of thin films, specifically to voids giving rise to capillary action. The optical consequence is that the wavelength of prominent features of the spectral transmittance of a coating is invariably shifted towards the red end of the spectrum when the coating is exposed to the atmosphere 1. The effects of stabilization can be reduced in a number of ways. The first is to select materials with higher packing density as well as the most appropriate multilayer structure. For example, ZnS and ZnSe are used for the spacer layer and the layers next to the spacer in so-called phase dispersion filters. Secondly, ionassisted deposition may be used. Martin e t al. 2 achieved considerable improvements in hard oxide layers and F a b r y - P e r o t filters by the use of argon and oxygen ions with energies considerably in excess of the thermal energies of the vapour stream. Not only was the packing density of films increased so that the shift in wavelength of peak transmittance of a narrow-band spectral filter of Z r O 2 and S i O 2 w a s reduced to 0.6 nm from 8 nm for the conventional process, but the residual absorption in films was also improved because an oxygen atmosphere helps to compensate for the dissociation which occurs when oxides are evaporated. The third method to be described here is the use of electric fields during deposition 3. A.c. or d.c. electric fields were applied in the vacuum chamber during the preparation of narrow-band * Present address: Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast N. Ireland. 0040-6090/88/$3.50
~" Elsevier Sequoia/Printed in The Netherlands
154
P.F. GU
spectral filters of ZnS and cryolite and edge filters of ZnS and MgF 2. The experiment established that the procedure is useful for improving the stabilization of optical coatings, especially of narrow-band spectral filters. The shift in wavelength of the transmittance peak of spectral filters prepared in the presence of suitable electric fields is negligible even in 100% relative humidity (RH). 2. EXPERIMENTAL PROCEDURES
Cryolite is widely used as a low-index thin film material in combination with high-index ZnS films for the preparation of multilayer optical coatings. However, dissociation and fractionation occur during the evaporation of cryolite and ZnS, and a fraction of the atoms will be ionized. An electric field is therefore introduced into the vacuum chamber to accelerate these ions. The arrangement of the electrodes in the vacuum chamber is illustrated in Fig. 1. An a.c. voltage of about 3400 V is applied to an electrode which consists of two semicircular rings of aluminium, whereas in the d.c. case about - 2400 V is applied to a ring 180 m m in diameter. The distance of the electrodes from the substrate is about 40 ram.
40m~
/*0mm
mm
3 -2II~ kV
50 Hz
" i ,f 4
",,i,,"4
-fiT"
{a)
(b)
Fig. l. Arrangement for preparing narrow-band spectral filters of ZnS and cryolite: (1) substrate; (2) holder; (3) electrode;(4) source. Because the wavelength shift of peak transmittance in spectral filters of ZnS and cryolite is much greater with cryolite spacers than with ZnS spacers, the following multilayer designs were chosen for experimental coatings: air/(HL)ZH 4LH(LH)2/glass
2o = 560 nm
a i r / H L H 4L(HL) aH 4L HLH/glass
2o = 510 nm
air/(HL)2H 1.75L Ag 1.75L(HL)2H/glass
20 = 510nm
and
where H and L refer to quarter-wave layers of ZnS and cryolite respectively and 20 is the design wavelength corresponding to the wavelength of peak transmittance. The filters were prepared by an electric-field-assisted deposition process in a commercial coating unit with a base pressure of about 6.6 × 10- a Pa. The films were deposited on plate glass substrates which were at ambient temperature or just above. Both ZnS
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
155
and cryolite were evaporated by an electron beam at evaporation rates of about 1.3 nm s- 1 and 2.2 nm s- 1 respectively. For edge filters, the low-index layers are usually made of MgF 2 because of their durability and the high-index layers of ZnS. Because the change in wavelength of short-wavelength-pass filters due to water vapour sorption is always greater than that of long wavelength-pass filters and the effect of the number of layers on the shift is negligible, the multilayer structure for these investigations is air/(L/2)H(LH)3/ glass, where H and L are quarter-wave layers (at)~o = 600 nm) of ZnS and MgF2 respectively. The arrangement of electrodes for preparing ZnS/MgF2 edge filters is shown in Fig. 2.
(a)
(b)
ic)
Fig. 2. Arrangement for preparing edge filters of ZnS and MgF 2.
3. EXPERIMENTAL RESULTS The changes in the wavelength of peak transmittance as well as the peak transmittances of filters of ZnS and cryolite prepared with and without electric fields are listed in TableI. The measurements were taken by means of a spectrophotometer. (Specord UV-VIS) immediately after removal from the vacuum chamber. The samples were kept in a container under 100~o RH. It appears that the overall change in peak wavelength of conventional F a b r y - P e r o t filters was more than 50 nm before the coatings crazed. However, electric-field-assisted deposition appreciably improved the optical stabilization of narrow-band filters of ZnS and cryolite, which did not show any changes in the peak wavelength after a few days in 100~o RH. It should be noted that even at the beginning of the damage phase, after a few days, no shift in the peak wavelength could be detected, although the transmittance began to decrease. The effect of electric-field-assisted deposition on the properties of double halfwave filters and induced transmission filters is also satisfactory. Because the silver layer in the induced transmission filter was easily damaged in 100~o RH, the transmittance of filters with and without an electric field decreased after two days so that the shift in peak wavelength in induced transmission filters is much less than in F a b r y - P e r o t and double half-wave filters. It appeared that the filters were damaged seriously before water penetration was complete. For short-wavelength-pass edge filters of ZnS and MgF 2 three arrangements were tried as shown in Fig. 2, and the results are given in Table II. Evidently the effect of electric fields on the wavelength shift of the band edge is not as good as for narrow-band filters. It must be mentioned that, after water penetration, the ratio of refractive indices and hence the width of the rejection zone are reduced, and the
k/I
"FABLE 1 STABILIZATIONOF ZnS/CRYOLITE INTERFERENCEFILTERS
Filler o'pe
Preparation process
Monitor wavelen~,th
Filter properties
2 o (nm)
Initial
hi 100°o RH 20 (nm)
Tm,x
A). (nm)
Time (h)
8.5 8.0 8.0 8.0 8.5 8.0
620 612 561 562 561 561
0.45 0.48 0.80 0.76 0.77 0.78
26.5 24.0 8.0 8.5 8.5 9.5
50 56 120 96 72 78
0.92 0.89 0.93
14.5 15.0 15.5
542 517 514
0.27 0.52 0.48
36.0 16.5 18.0
96 73 66
0.63 0.53
13.5 12.0
525 519
0.18 0.25
14.0 13.5
48 48
2o (nm)
Ymax
D.c. tield( -- 240(/V)
560 560 560 560 560 560
561 561 561 562 560 561
0.88 0.90 0.89 0.91 0.87 0.88
Air FLF/glass (F" - HLH4 LHL H)
Conventional A.c. field ( ~ 3400 V, 50 Hz)
510 5t0 510
514 515 513
Air M1.75 LAgl.75 LM/ glass(M = (HL) 2 H)
Conventional A.C. field ( ~ 3400 V, 50 Hz)
510 510
515 518
Air/M4LM/glass (M = (HL)ZH)
Conventional A.c. field ( ~ 3400 V, 50 Hz)
A/;. (nm)
O,
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
157
T A B L E II STABILIZATIONOF Z n S / M g F 2 EDGE FILTERS(AIR/(L/2)H(LH)3/GLASS)
Preparation process
Voltage (kV)
Conventional
D.c. field (Fig. 2(a)) A.c. field (Fig. 2(b))
A.c. field (Fig. 2(c))
- 2.4
1.5 2.2 2.6 3.0 2.2 2.2 3.3 2.2 3.3
(°C)
Red shift of short wavelength edge in 100% RH (rim)
Mean pass-band transmittance
50 150 200 50
20 18 9 13
0.88 0.88 0.88 0.88
150
12
0.88
200 50 50 50 50 150 5O 50 150 150
3 13 7 4 1 1 11 8 6 5
0.88 0.80 0.75 0.70 0.50 0.50 0.88 0.88 0.85 0.85
Substrate temperature
optical thicknesses of the layers and consequently the wavelength of the shortwavelength edge of the stop band are increased. Because of the two effects the wavelength shift in short-wavelength-pass filters is much greater than in longwavelength-pass filters. It should be noted that the arrangement shown in Fig. 2(b) may result in serious absorption so that transmittance in the pass band is reduced as the voltage or the temperature of the substrate is increased. 4.
CONCLUDING REMARKS
It appears that the extraordinary changes in peak wavelength of narrow-band filters of ZnS and cryolite and short-pass edge filters of ZnS and MgF 2 are caused by water penetration, accompanied by changes in chemical composition. The packing densities of ZnS, Na3A1F 6 and MgF 2 were measured by the frequency changes of a quartz crystal oscillator before and after water penetration. The results are shown in Table III. The red shift in the wavelength of peak transmittance for F a b r y - P e r o t filters calculated using values of the packing density in Table II! for ZnS and Na3A1F6 films with electric-field-assisted deposition was 10.5 nm. This is not consistent with the measured values. It is possible that the chemical composition of the films is also an important factor. !t has been well known for many years that dissociation can occur during evaporation. Thus, the chemical composition and hence the optical performance of evaporated films depend strongly on preparation conditions. For example, the refractive index of bulk cryolite is listed as 1.365, but for thin films measured values between 1.28 and 1.36 have been reported. Using surface electromagnetic wave and ellipsometric techniques, it was found that the index of cryolite films prepared by conventional deposition was as
158
P.F. GU
TABLE III PACKING DENSITY OF ZnS, NaaA1F 0 AND M g F 2 WITH DEPOSITION
Material
AND WITHOUT
ELECTRIC-FIELD-ASSISTED
Packing density q/films ~
ZnS Na3AIF 6 MgF 2
Conventional
Electric field assisted
0.93 0.82 0.74
0.98 0.90 0.90
a Substrate at ambient temperature.
high as 1.45 after water penetration in 100~o RH. The changes in the surface composition accompanying this effect have been investigated using Auger electron spectroscopy. Figure 3 shows the Auger spectrum of cryolite in bulk and thin film form. It can be seen that for the vapour-deposited cryolite film (Fig. 3(b)) there is a noticeable decrease in the fluorine and sodium Auger signals compared with the bulk, a shift in the position of the aluminium peak and a considerable increase in the aluminium Auger signals. This may be explained as follows. Since a larger number of A1F3 molecules is present in the vapour phase during the formation of the films4, when the film is exposed to a moist atmosphere the refractive index rises to a value exceeding that of the bulk material owing to the formation of a hydrate: fluellite (A1F3.H20 ) has a refractive index of 1.49 (ref. 5). In addition, it is well known that the packing density of NaF is much higher than that of AIF3. After the electric field is introduced the amount of the fluorine and the sodium in the cryolite is relatively larger (Fig. 3(c)). The composition ratio of the cryolite prepared with and without electric field is shown in Table IV. dN(E) dE
(a)
A~!4
/[ 6528
986
1387.2 aN(E) dE
[b)
(c/
f
I l e A l
~/
I] 6528
41'
oVA ,
'~f 572~'
68
II IJ
986
I/ Y
No g86
13821
Z,
3go6
t/ F 0~ 6528 5066
tb)
----ev
-• eV Fig. 3. Auger spectra of bulk and thin film cryolite: (a) bulk; (b) thin film deposited without electric field; (c) thin film deposited with electric field. Fig. 4. Auger spectra of M g F 2 films: (a) film deposited without electric field; (b) film deposited with electric field.
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
1 59
TABLE IV COMPOSITIONRATIOSIN CRYOLITEFROM AUGERELECTRON SPECTROSCOPY
Na.'Al:Fratio
Preparation process
Sample 1
Sample 2
Sample 3
Without electric field Electric field assisted
1:2.3:11 1:0.57:6
1: 2.3:12 1:0.77:4.6
1: 2.4:12 1:0.61:4
Perfect cryolite
1:0.33:2
Figure 4 depicts the Auger spectra of MgF2 with and without an electric field applied as shown in Fig. 2(a). Clearly, the Auger signal of fluorine is increased and that of Mg ~+ decreased in MgF 2 prepared with an electric field, and hence the fluorine loss is restricted. It is worth pointing out that the filters prepared with and without an electric field show different damage behaviours in humid atmospheres. For filters prepared with an electric field no water penetration patterns were observed. After the filters were subjected to 100% RH for a few days there were still no water penetration patterns, but a number of pits were found on the surface of the coatings. Many bright spots surrounded by dark rings could be seen. This means they are not penetration patterns caused by water penetration but damage spots of a different form from that in filters prepared conventionally. Figure 5 shows the measured transmittance of the filter air/(HL)2H 4L H(LH)// AlzO 3 (2 = 560 nm) in the 3 ~tm water absorption band. The dip in transmittance for the conventional filter is due to water absorption at a wavelength of 3 lam. S
8O
~ 70
70
oo[ 50"
2.5
z 6o J 3
(a)
' 4
'
50 •
5
WAVELENGTH(~m]
25
(b)
~ 3
~
4
h
5
WAVELENGTH(~m]
Fig. 5. Measured transmittance of the filter air/(HL)2H 4L H(LH)2/AI203 (20 = 560nm at 3 rtm; in water absorption band) (a) deposited with an electric field and (b) deposited without an electric field: spectra 1, after deposition; spectra 2, 17 h in 100% RH; spectra 3, 42 h in 100% RH.
It is concluded that electric-field-assisted deposition is a useful technique for preparing multilayer coatings, especially for narrow-band filters with ZnS and cryolite. ACKNOWLEDGMENTS
The author would like to thank Professor P. H. Lissberger for suggesting useful revisions of the manuscript and Professor J. F. Tang for constructive suggestions
160
e . F . OU
about the experiment. The author is also grateful to the Surface Physics Group led by Professor Y. B. Qu for the Auger electron spectroscopy and to Mr. Bao Zhaolong and Mr. Bao Yugang for financial support for research at The Queen's University of Belfast. REFERENCES 1 2 3 4 5
D . R . Gibson and P. H. Lissberger, Appl. Opt., 22 (1983) 269. P.J. Martin, H. A. Macleod, R. P. Netterfield, C. G. Pacey and W. G. Sainty, Appl. Opt., 22 (1983) 178. Sh. A. F u r m a n , M. D. Levina and V. D. Vvedenskii, Soy. J. Opt. Technol., 46 (1979) 405. H . K . Pulker and C. Zaminer, Thin Solid Films, 5 (1970) 42 I. W. Heitmann, Thin Solid Films, 5 (1970) 61.
153
PREPARATION AND CHARACTERIZATION
E L E C T R I C - F I E L D - A S S I S T E D D E P O S I T I O N OF OPTICAL C O A T I N G S P. F. GU*
Department ~f Optical Engineering, Zhe/iang University, Hangzhou (China) (Received May 5, 1987; accepted August 24, 1987)
It is possible to improve the stability of optical coatings if an electric field is applied during deposition. A.c. and d.c. electric fields have been used in the preparation of narrow-band filters consisting of ZnS and cryolite layers and edge filters of ZnS and MgFz. As a result, the shift in the wavelength of prominent features in the spectral transmittance is reduced considerably even for coatings subjected to 100% relative humidity.
1. INTRODUCTION
Stabilization of narrow-band and edge filters is a serious problem. It is related to the microstructure of thin films, specifically to voids giving rise to capillary action. The optical consequence is that the wavelength of prominent features of the spectral transmittance of a coating is invariably shifted towards the red end of the spectrum when the coating is exposed to the atmosphere 1. The effects of stabilization can be reduced in a number of ways. The first is to select materials with higher packing density as well as the most appropriate multilayer structure. For example, ZnS and ZnSe are used for the spacer layer and the layers next to the spacer in so-called phase dispersion filters. Secondly, ionassisted deposition may be used. Martin e t al. 2 achieved considerable improvements in hard oxide layers and F a b r y - P e r o t filters by the use of argon and oxygen ions with energies considerably in excess of the thermal energies of the vapour stream. Not only was the packing density of films increased so that the shift in wavelength of peak transmittance of a narrow-band spectral filter of Z r O 2 and S i O 2 w a s reduced to 0.6 nm from 8 nm for the conventional process, but the residual absorption in films was also improved because an oxygen atmosphere helps to compensate for the dissociation which occurs when oxides are evaporated. The third method to be described here is the use of electric fields during deposition 3. A.c. or d.c. electric fields were applied in the vacuum chamber during the preparation of narrow-band * Present address: Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast N. Ireland. 0040-6090/88/$3.50
~" Elsevier Sequoia/Printed in The Netherlands
154
P.F. GU
spectral filters of ZnS and cryolite and edge filters of ZnS and MgF 2. The experiment established that the procedure is useful for improving the stabilization of optical coatings, especially of narrow-band spectral filters. The shift in wavelength of the transmittance peak of spectral filters prepared in the presence of suitable electric fields is negligible even in 100% relative humidity (RH). 2. EXPERIMENTAL PROCEDURES
Cryolite is widely used as a low-index thin film material in combination with high-index ZnS films for the preparation of multilayer optical coatings. However, dissociation and fractionation occur during the evaporation of cryolite and ZnS, and a fraction of the atoms will be ionized. An electric field is therefore introduced into the vacuum chamber to accelerate these ions. The arrangement of the electrodes in the vacuum chamber is illustrated in Fig. 1. An a.c. voltage of about 3400 V is applied to an electrode which consists of two semicircular rings of aluminium, whereas in the d.c. case about - 2400 V is applied to a ring 180 m m in diameter. The distance of the electrodes from the substrate is about 40 ram.
40m~
/*0mm
mm
3 -2II~ kV
50 Hz
" i ,f 4
",,i,,"4
-fiT"
{a)
(b)
Fig. l. Arrangement for preparing narrow-band spectral filters of ZnS and cryolite: (1) substrate; (2) holder; (3) electrode;(4) source. Because the wavelength shift of peak transmittance in spectral filters of ZnS and cryolite is much greater with cryolite spacers than with ZnS spacers, the following multilayer designs were chosen for experimental coatings: air/(HL)ZH 4LH(LH)2/glass
2o = 560 nm
a i r / H L H 4L(HL) aH 4L HLH/glass
2o = 510 nm
air/(HL)2H 1.75L Ag 1.75L(HL)2H/glass
20 = 510nm
and
where H and L refer to quarter-wave layers of ZnS and cryolite respectively and 20 is the design wavelength corresponding to the wavelength of peak transmittance. The filters were prepared by an electric-field-assisted deposition process in a commercial coating unit with a base pressure of about 6.6 × 10- a Pa. The films were deposited on plate glass substrates which were at ambient temperature or just above. Both ZnS
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
155
and cryolite were evaporated by an electron beam at evaporation rates of about 1.3 nm s- 1 and 2.2 nm s- 1 respectively. For edge filters, the low-index layers are usually made of MgF 2 because of their durability and the high-index layers of ZnS. Because the change in wavelength of short-wavelength-pass filters due to water vapour sorption is always greater than that of long wavelength-pass filters and the effect of the number of layers on the shift is negligible, the multilayer structure for these investigations is air/(L/2)H(LH)3/ glass, where H and L are quarter-wave layers (at)~o = 600 nm) of ZnS and MgF2 respectively. The arrangement of electrodes for preparing ZnS/MgF2 edge filters is shown in Fig. 2.
(a)
(b)
ic)
Fig. 2. Arrangement for preparing edge filters of ZnS and MgF 2.
3. EXPERIMENTAL RESULTS The changes in the wavelength of peak transmittance as well as the peak transmittances of filters of ZnS and cryolite prepared with and without electric fields are listed in TableI. The measurements were taken by means of a spectrophotometer. (Specord UV-VIS) immediately after removal from the vacuum chamber. The samples were kept in a container under 100~o RH. It appears that the overall change in peak wavelength of conventional F a b r y - P e r o t filters was more than 50 nm before the coatings crazed. However, electric-field-assisted deposition appreciably improved the optical stabilization of narrow-band filters of ZnS and cryolite, which did not show any changes in the peak wavelength after a few days in 100~o RH. It should be noted that even at the beginning of the damage phase, after a few days, no shift in the peak wavelength could be detected, although the transmittance began to decrease. The effect of electric-field-assisted deposition on the properties of double halfwave filters and induced transmission filters is also satisfactory. Because the silver layer in the induced transmission filter was easily damaged in 100~o RH, the transmittance of filters with and without an electric field decreased after two days so that the shift in peak wavelength in induced transmission filters is much less than in F a b r y - P e r o t and double half-wave filters. It appeared that the filters were damaged seriously before water penetration was complete. For short-wavelength-pass edge filters of ZnS and MgF 2 three arrangements were tried as shown in Fig. 2, and the results are given in Table II. Evidently the effect of electric fields on the wavelength shift of the band edge is not as good as for narrow-band filters. It must be mentioned that, after water penetration, the ratio of refractive indices and hence the width of the rejection zone are reduced, and the
k/I
"FABLE 1 STABILIZATIONOF ZnS/CRYOLITE INTERFERENCEFILTERS
Filler o'pe
Preparation process
Monitor wavelen~,th
Filter properties
2 o (nm)
Initial
hi 100°o RH 20 (nm)
Tm,x
A). (nm)
Time (h)
8.5 8.0 8.0 8.0 8.5 8.0
620 612 561 562 561 561
0.45 0.48 0.80 0.76 0.77 0.78
26.5 24.0 8.0 8.5 8.5 9.5
50 56 120 96 72 78
0.92 0.89 0.93
14.5 15.0 15.5
542 517 514
0.27 0.52 0.48
36.0 16.5 18.0
96 73 66
0.63 0.53
13.5 12.0
525 519
0.18 0.25
14.0 13.5
48 48
2o (nm)
Ymax
D.c. tield( -- 240(/V)
560 560 560 560 560 560
561 561 561 562 560 561
0.88 0.90 0.89 0.91 0.87 0.88
Air FLF/glass (F" - HLH4 LHL H)
Conventional A.c. field ( ~ 3400 V, 50 Hz)
510 5t0 510
514 515 513
Air M1.75 LAgl.75 LM/ glass(M = (HL) 2 H)
Conventional A.C. field ( ~ 3400 V, 50 Hz)
510 510
515 518
Air/M4LM/glass (M = (HL)ZH)
Conventional A.c. field ( ~ 3400 V, 50 Hz)
A/;. (nm)
O,
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
157
T A B L E II STABILIZATIONOF Z n S / M g F 2 EDGE FILTERS(AIR/(L/2)H(LH)3/GLASS)
Preparation process
Voltage (kV)
Conventional
D.c. field (Fig. 2(a)) A.c. field (Fig. 2(b))
A.c. field (Fig. 2(c))
- 2.4
1.5 2.2 2.6 3.0 2.2 2.2 3.3 2.2 3.3
(°C)
Red shift of short wavelength edge in 100% RH (rim)
Mean pass-band transmittance
50 150 200 50
20 18 9 13
0.88 0.88 0.88 0.88
150
12
0.88
200 50 50 50 50 150 5O 50 150 150
3 13 7 4 1 1 11 8 6 5
0.88 0.80 0.75 0.70 0.50 0.50 0.88 0.88 0.85 0.85
Substrate temperature
optical thicknesses of the layers and consequently the wavelength of the shortwavelength edge of the stop band are increased. Because of the two effects the wavelength shift in short-wavelength-pass filters is much greater than in longwavelength-pass filters. It should be noted that the arrangement shown in Fig. 2(b) may result in serious absorption so that transmittance in the pass band is reduced as the voltage or the temperature of the substrate is increased. 4.
CONCLUDING REMARKS
It appears that the extraordinary changes in peak wavelength of narrow-band filters of ZnS and cryolite and short-pass edge filters of ZnS and MgF 2 are caused by water penetration, accompanied by changes in chemical composition. The packing densities of ZnS, Na3A1F 6 and MgF 2 were measured by the frequency changes of a quartz crystal oscillator before and after water penetration. The results are shown in Table III. The red shift in the wavelength of peak transmittance for F a b r y - P e r o t filters calculated using values of the packing density in Table II! for ZnS and Na3A1F6 films with electric-field-assisted deposition was 10.5 nm. This is not consistent with the measured values. It is possible that the chemical composition of the films is also an important factor. !t has been well known for many years that dissociation can occur during evaporation. Thus, the chemical composition and hence the optical performance of evaporated films depend strongly on preparation conditions. For example, the refractive index of bulk cryolite is listed as 1.365, but for thin films measured values between 1.28 and 1.36 have been reported. Using surface electromagnetic wave and ellipsometric techniques, it was found that the index of cryolite films prepared by conventional deposition was as
158
P.F. GU
TABLE III PACKING DENSITY OF ZnS, NaaA1F 0 AND M g F 2 WITH DEPOSITION
Material
AND WITHOUT
ELECTRIC-FIELD-ASSISTED
Packing density q/films ~
ZnS Na3AIF 6 MgF 2
Conventional
Electric field assisted
0.93 0.82 0.74
0.98 0.90 0.90
a Substrate at ambient temperature.
high as 1.45 after water penetration in 100~o RH. The changes in the surface composition accompanying this effect have been investigated using Auger electron spectroscopy. Figure 3 shows the Auger spectrum of cryolite in bulk and thin film form. It can be seen that for the vapour-deposited cryolite film (Fig. 3(b)) there is a noticeable decrease in the fluorine and sodium Auger signals compared with the bulk, a shift in the position of the aluminium peak and a considerable increase in the aluminium Auger signals. This may be explained as follows. Since a larger number of A1F3 molecules is present in the vapour phase during the formation of the films4, when the film is exposed to a moist atmosphere the refractive index rises to a value exceeding that of the bulk material owing to the formation of a hydrate: fluellite (A1F3.H20 ) has a refractive index of 1.49 (ref. 5). In addition, it is well known that the packing density of NaF is much higher than that of AIF3. After the electric field is introduced the amount of the fluorine and the sodium in the cryolite is relatively larger (Fig. 3(c)). The composition ratio of the cryolite prepared with and without electric field is shown in Table IV. dN(E) dE
(a)
A~!4
/[ 6528
986
1387.2 aN(E) dE
[b)
(c/
f
I l e A l
~/
I] 6528
41'
oVA ,
'~f 572~'
68
II IJ
986
I/ Y
No g86
13821
Z,
3go6
t/ F 0~ 6528 5066
tb)
----ev
-• eV Fig. 3. Auger spectra of bulk and thin film cryolite: (a) bulk; (b) thin film deposited without electric field; (c) thin film deposited with electric field. Fig. 4. Auger spectra of M g F 2 films: (a) film deposited without electric field; (b) film deposited with electric field.
ELECTRICAL-FIELD-ASSISTED DEPOSITION OF OPTICAL COATINGS
1 59
TABLE IV COMPOSITIONRATIOSIN CRYOLITEFROM AUGERELECTRON SPECTROSCOPY
Na.'Al:Fratio
Preparation process
Sample 1
Sample 2
Sample 3
Without electric field Electric field assisted
1:2.3:11 1:0.57:6
1: 2.3:12 1:0.77:4.6
1: 2.4:12 1:0.61:4
Perfect cryolite
1:0.33:2
Figure 4 depicts the Auger spectra of MgF2 with and without an electric field applied as shown in Fig. 2(a). Clearly, the Auger signal of fluorine is increased and that of Mg ~+ decreased in MgF 2 prepared with an electric field, and hence the fluorine loss is restricted. It is worth pointing out that the filters prepared with and without an electric field show different damage behaviours in humid atmospheres. For filters prepared with an electric field no water penetration patterns were observed. After the filters were subjected to 100% RH for a few days there were still no water penetration patterns, but a number of pits were found on the surface of the coatings. Many bright spots surrounded by dark rings could be seen. This means they are not penetration patterns caused by water penetration but damage spots of a different form from that in filters prepared conventionally. Figure 5 shows the measured transmittance of the filter air/(HL)2H 4L H(LH)// AlzO 3 (2 = 560 nm) in the 3 ~tm water absorption band. The dip in transmittance for the conventional filter is due to water absorption at a wavelength of 3 lam. S
8O
~ 70
70
oo[ 50"
2.5
z 6o J 3
(a)
' 4
'
50 •
5
WAVELENGTH(~m]
25
(b)
~ 3
~
4
h
5
WAVELENGTH(~m]
Fig. 5. Measured transmittance of the filter air/(HL)2H 4L H(LH)2/AI203 (20 = 560nm at 3 rtm; in water absorption band) (a) deposited with an electric field and (b) deposited without an electric field: spectra 1, after deposition; spectra 2, 17 h in 100% RH; spectra 3, 42 h in 100% RH.
It is concluded that electric-field-assisted deposition is a useful technique for preparing multilayer coatings, especially for narrow-band filters with ZnS and cryolite. ACKNOWLEDGMENTS
The author would like to thank Professor P. H. Lissberger for suggesting useful revisions of the manuscript and Professor J. F. Tang for constructive suggestions
160
e . F . OU
about the experiment. The author is also grateful to the Surface Physics Group led by Professor Y. B. Qu for the Auger electron spectroscopy and to Mr. Bao Zhaolong and Mr. Bao Yugang for financial support for research at The Queen's University of Belfast. REFERENCES 1 2 3 4 5
D . R . Gibson and P. H. Lissberger, Appl. Opt., 22 (1983) 269. P.J. Martin, H. A. Macleod, R. P. Netterfield, C. G. Pacey and W. G. Sainty, Appl. Opt., 22 (1983) 178. Sh. A. F u r m a n , M. D. Levina and V. D. Vvedenskii, Soy. J. Opt. Technol., 46 (1979) 405. H . K . Pulker and C. Zaminer, Thin Solid Films, 5 (1970) 42 I. W. Heitmann, Thin Solid Films, 5 (1970) 61.