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Laser damage of CdS and ZnS thin films
Thin Solid Films, 34 (1976) 119-123 © Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
1 19
LASER DAMAGE OF CdS AND ZnS THIN FILMS* K. M. LEUNG, C. C. TANG AND L. G. DESHAZER**
Center for Laser Studies, University o f Southern California, Los Angeles, Calif. 90007 (U.S.A.) (Received August 25, 1975)
1. INTRODUCTION In past years, much research 1 has been directed towards understanding laser damage to thin dielectric films. Ordinary linear absorption of laser light by the dielectric film is believed to be the chief cause of damage, although this has not yet been adequately confirmed by thermal calculations. The difficulty in such a verification has been that the absorption coefficients of the dielectric films studied to date are not accurately known since their absorption is too low to be measured by standard techniques, and varies with the deposition procedure. When the absorption coefficient can be measured accurately, e.g. for highly absorptive metal films, thermal calculations based on linear absorption compare well with the laser damage results 2. However, metal films differ considerably from dielectrics in their thermal properties. In order to understand the applicability of these thermal calculations of laser damage to dielectric f'rims, we studied ruby laser damage to CdS films, which have a weak but measurable absorptivity, and compared it with that of ZnS films, which have an absorption too low to be measured. 2. PREPARATION OF SPECIMENS
2.1. Film deposition and coating defects Films of ZnS and CdS were deposited on BSC-2 glass and pyrex glass substrates by evaporation of materials of 99% purity in a quartz crucible from a resistance-heated boat under a vacuum of 10 -s torr. All fdm deposition was optically monitored and the thickness was determined from transmission spectra at 6943 ,~. CdS films of thicknesses ;k/4, ;k/2 and X and ZnS films X/4 thick were made. In both cases, the substrates were cleaned for 2 rain in a glow discharge at 400 torr before evaporation of the film material. The deposition rate was 75 A s -1 for ZnS and 70 A s -1 for CdS. Different types of coating defects were observed in the thin films. These defects were gross structural defects with sizes greater than the physical thickness of the film and formed hills and cracks and other defects not readily associated with foreign inclusions. The distribution and nature of these defects were examined with a scanning electron microscope (SEM) at a magnification of 1000. On the ZnS films used in this * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-10. ** Temporary address: U.S. Office of Naval Research, 223 Old Marylebone Road, London NW1, Gt. Britain.
120
K. M. LEUNG, C. C. TANG, L. G. DESHAZER
tOO-
CdS-
9C
X/4
RACKS
IOSHADOWS 70
28
SHADOW w/STRIPS
5 IRREGULAR
I
w/DOT 54 HILLS
aJ 36
OVERALL do
HILLS
OVERALL d o
Ff'rrED d* Ji
f lo
r'o 2'0 '~o 5'0 uEAN DISTANCEBETWEENDEFECTS, ~m
o.,
FITTED do I
' ,~b ' 8'o ',~o' MEAN
I
' 'z6o 160 DISTANCE BETWEEN DEFECTS, p.m
Fig. 1. A schematic plot of defect size vs. defect separation for a single quarter-wave film of ZnS. Fig. 2. A schematic plot of defect size vs. defect separation for a single quarter-wave film of CdS.
experiment, five general types of defects were observed and our results are summarized in Fig. 1. Three defect types were found for the CdS films, as shown in Fig. 2. The mean distance between defects is labelled do.
2.2. Optical absorption The optical absorption of the thin films was measured at 6943 A with a Caw 14 spectrophotometer and by using low intensity ruby laser transmission. The absorption for the ZnS film was too low to be measured by the standard techniques available to us. Table I lists the absorption coefficients for CdS films measured by the two methods. Tile larger value of a obtained with the Cary 14 instrument is due to a scattering loss; by using a small laser beam to probe the film, this scattering was considerably reduced and a smaller value was measured which is believed to be closer to the correct a. This smaller value of a was used in the subsequent calculations. 3. EXPERIMENT
The procedure used in measuring the damage thresholds of CdS and ZnS thin films has been described by Newnam and DeShazer 3. The Q-switched ruby laser oscillator was operated in a single longitudinal and lowest order transverse mode having a pulse width of 17 ns with peak powers up to 0.5 MW. The spatial intensity profile of the laser beam had a near Gaussian distribution as determined by a pinhole scan and a TABLE I LINEAR ABSORPTION COEFFICIENT OF CdS Material
~ using Cary 14 spectrometer (1 cm 2 aperture)
a using low intensity ruby laser (725 ~rn spot size)
CdS
19 470 cm - l
5950 cm -1
LASER DAMAGE OF CdS AND ZnS THIN FILMS
121
television monitor system. The spatial extent of the laser beam is described by the radius of the 1/e 2 value of the peak intensity. This radius is called the "spot size" Wo. The chief method of monitoring the onset of laser-induced damage to the thin films was by the observation of laser-induced scatter (LIS). For this LIS technique, a low intensity He-Ne laser beam was used to indicate the scatter from the coating before and after the ruby laser irradiation. 4. RESULTS 4.1. ZnS films By varying the spot size of the laser beam, the damage thresholds of X/4 ZnS films were determined by the onset of LIS. Figure 3 displays these data along with a
)./4, ZnS F~LM Id~ 10.3 d/canl d. • 3 o ~
((n,,4)
| --
X I
o
5~
~o
J~o
i
2O0
6POT SJZE,W,(pm)
Fig. 3. Spot-size dependence for a single quarter-wave thick f'tim of ZnS on a glass substrate. The solid curve is a plot of the equation J = 1 + (r/- 1) exp{---~Trln2(Wo/do) 2 } where J = I/ld and ~=3. theoretical curve obtained using the model suggested by DeShazer et al. 4 For spot sizes greater than 125/am, the damage threshold I d was 10.3 J cm -2 and the fitted do was 30/am, compared with the SEM measured d o of 20/am. The result indicates that coating defects play an important role in the damage process, and defects such as "shadow-with-strips" and "irregular hills" are enhancing the electric field of the laser light. 4.2. CdS j~lms Table II lists the average damage thresholds with various laser spot sizes for three CdS films of different thicknesses. Figure 4 compares the data with the fitted curve for d o = 175/am. As shown in Fig. 2, the measured overall do was 10/am, indicating that these defects are not responsible for the laser damage. However, it is possible that a large defect or a particular type of defect separated by 175/am could be present. When the trim was viewed between crossed polarizers, two types of structural defects were found. One was a ridge 800/am in length and the other a circular spot 150 tam in diameter. These additional defects may be due to inhomogeneous adhesion between film and substrate.
122
K. M. LEUNG, C. C. TANG, L. G. DESHAZER
T A B L E II DAMAGE THRESHOLD O F CdS FILMS AT 6943 A
Spot size (/am)
Average damage threshold (J cm -2) ~/4 film ~12 film
725 592 500 321 233 162 126 108 81
0.39 0.39 0.51 0.83 1.50 1.53 1.86 1.87 -
(0.28)
0.34 0.39 0.45 0.89 1.31 1.43 1.68
h film (0.29)
0.34 0.38 0.59 0.88 1.56 1.87 -
(0.32)
Theoretical values are enclosed in parentheses. 5
X/4. Cd5 FILM 1=, 0.4J/©m= do= 175 ~ m (¢ftted)
4
~2 Q
__¢..-_
,~o
2~o ~8o 40'0 58o 6do SPOT 51ZE,W.(#~)
r~o
Fig. 4. Spot-size dependence for a single quarter-wave thick f'dm o f CdS on a glass substrate. r/=5. Fig. 5. Scanning electron micrograph o f a damage site near threshold on a single quarter-wave film o f ZnS using LIS diagnosis.
4.3. Linear absorption in CdS f i l m s
If linear absorption of a laser pulse causes damage by heating the film to its melting point, then the energy e absorbed per unit volume to melt the film is e = p ( C p ( T m p - 20 °C) + AH}
(I)
where p, Cp, Trap and AH are the film density, specific heat, melting temperature and latent heat of fusion. If the film thickness is h and the percentage absorption in the sample is/3, then the damage threshold intensity will be I 0 = eh/~
J cm-2
(2)
In this derivation, we have assumed that the film density is equal to the bulk density* with no loss of heat from the irradiated area. For a single quarter-wave CdS film, using its measured/~ of 9.8% (or t~ = 6000 cm-l), the predicted damage threshold is 0.28 J cm -2. This value is near the measured damage threshold of 0.39 J c m - 2 * The density o f thin films is equal to t h e packing fraction f times the bulk density Pb, where f = (n 2 - 1) (rib 2 + 1)/(rib 2 - 1) (n 7 + 2) (ref. 5).
LASER DAMAGE OF CdS AND ZnS THIN FILMS
123
TABLE III COMPARISON OF QUARTER-WAVE THICK DIELECTRIC COATINGS Properties
C.cIS
ZnS
Refractive index Melting point at 100 atm Region of transparency Thermal conductivity Thermal diffusivity Defect damage threshold Intrinsic damage threshold
2.506 1750 *C 0.6-16/~m 0.038 cal cm -1 s -1 K-1 0.0896 cm2 s -1 0.39 J cm -2 2.0 J cm -2
2.32 1745 °C 0.4-24/~m 0.0635 cal cm -l s-1 K -1 0.13 cm2 s -1 10 J cm -2 31 J cm-2
4.4. Damage morphology o f Z n S and CdS films The damage morphology of ZnS films as observed with the SEM was distinctly different from that o f the CdS films. For ZnS fdms a damage site caused b y a single laser shot contained many small craters, supporting the idea that the laser damage nucleates on the defects (Fig. 5). However, for CdS the breakdown region on the film was observed to be a uniform circular area.
5. CONCLUSION Table III summarizes the results and gives a comparison o f the parameters o f the two thin film materials. Intrinsic absorption o f the laser light was the cause o f damage for CdS; this was confirmed b y thermodynamic calculations assuming thermal melting o f the thin Films due to linear absorption o f laser light. For ZnS fdms, the damage threshold decreased as the spot size o f the laser beam increased, showing that the coating defects o f ZnS films have a dominant role in the damage process. ACKNOWLEDGMENTS The authors wish to thank H. R. Owen o f the University o f Southern California for the preparation o f the thin fdm specimens for this study, and J. Worrall for the SEM work. REFERENCES 1 A.J. Glass and A. H. Guenther (eds.), Laser Induced Damage in Optical Materials, Nat. Bur. Stand. (U.S.), Spec. Publ. No. 341 (1970), No. 356 (1971), No. 372 (1972), No. 38 7 (1973), No. 414 (1974), U.S. GPO, Washington, D.C. 2 U. C. Pack and A. Kestenbaum, J. Appl. Phys., 44 (1973) 2260. 3 B. E. Newnam and L. G. DeShazer, in A. J. Glass and A. H. Guenther (eds.), Laser Induced Damage in OpticaI Materials - 1972, Nat. Bur. Stand. (U.S.), Spec. Publ. No. 372, U.S. GPO, Washington, D.C., 1972, pp. 123-134. 4 L. G. DeShazer, B. E. Newnam and K. M. Leung, Appl. Phys. Lett., 23 (1973) 607. 5 G. Bauer, Ann. Phys., 19 (1934) 434.
1 19
LASER DAMAGE OF CdS AND ZnS THIN FILMS* K. M. LEUNG, C. C. TANG AND L. G. DESHAZER**
Center for Laser Studies, University o f Southern California, Los Angeles, Calif. 90007 (U.S.A.) (Received August 25, 1975)
1. INTRODUCTION In past years, much research 1 has been directed towards understanding laser damage to thin dielectric films. Ordinary linear absorption of laser light by the dielectric film is believed to be the chief cause of damage, although this has not yet been adequately confirmed by thermal calculations. The difficulty in such a verification has been that the absorption coefficients of the dielectric films studied to date are not accurately known since their absorption is too low to be measured by standard techniques, and varies with the deposition procedure. When the absorption coefficient can be measured accurately, e.g. for highly absorptive metal films, thermal calculations based on linear absorption compare well with the laser damage results 2. However, metal films differ considerably from dielectrics in their thermal properties. In order to understand the applicability of these thermal calculations of laser damage to dielectric f'rims, we studied ruby laser damage to CdS films, which have a weak but measurable absorptivity, and compared it with that of ZnS films, which have an absorption too low to be measured. 2. PREPARATION OF SPECIMENS
2.1. Film deposition and coating defects Films of ZnS and CdS were deposited on BSC-2 glass and pyrex glass substrates by evaporation of materials of 99% purity in a quartz crucible from a resistance-heated boat under a vacuum of 10 -s torr. All fdm deposition was optically monitored and the thickness was determined from transmission spectra at 6943 ,~. CdS films of thicknesses ;k/4, ;k/2 and X and ZnS films X/4 thick were made. In both cases, the substrates were cleaned for 2 rain in a glow discharge at 400 torr before evaporation of the film material. The deposition rate was 75 A s -1 for ZnS and 70 A s -1 for CdS. Different types of coating defects were observed in the thin films. These defects were gross structural defects with sizes greater than the physical thickness of the film and formed hills and cracks and other defects not readily associated with foreign inclusions. The distribution and nature of these defects were examined with a scanning electron microscope (SEM) at a magnification of 1000. On the ZnS films used in this * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-10. ** Temporary address: U.S. Office of Naval Research, 223 Old Marylebone Road, London NW1, Gt. Britain.
120
K. M. LEUNG, C. C. TANG, L. G. DESHAZER
tOO-
CdS-
9C
X/4
RACKS
IOSHADOWS 70
28
SHADOW w/STRIPS
5 IRREGULAR
I
w/DOT 54 HILLS
aJ 36
OVERALL do
HILLS
OVERALL d o
Ff'rrED d* Ji
f lo
r'o 2'0 '~o 5'0 uEAN DISTANCEBETWEENDEFECTS, ~m
o.,
FITTED do I
' ,~b ' 8'o ',~o' MEAN
I
' 'z6o 160 DISTANCE BETWEEN DEFECTS, p.m
Fig. 1. A schematic plot of defect size vs. defect separation for a single quarter-wave film of ZnS. Fig. 2. A schematic plot of defect size vs. defect separation for a single quarter-wave film of CdS.
experiment, five general types of defects were observed and our results are summarized in Fig. 1. Three defect types were found for the CdS films, as shown in Fig. 2. The mean distance between defects is labelled do.
2.2. Optical absorption The optical absorption of the thin films was measured at 6943 A with a Caw 14 spectrophotometer and by using low intensity ruby laser transmission. The absorption for the ZnS film was too low to be measured by the standard techniques available to us. Table I lists the absorption coefficients for CdS films measured by the two methods. Tile larger value of a obtained with the Cary 14 instrument is due to a scattering loss; by using a small laser beam to probe the film, this scattering was considerably reduced and a smaller value was measured which is believed to be closer to the correct a. This smaller value of a was used in the subsequent calculations. 3. EXPERIMENT
The procedure used in measuring the damage thresholds of CdS and ZnS thin films has been described by Newnam and DeShazer 3. The Q-switched ruby laser oscillator was operated in a single longitudinal and lowest order transverse mode having a pulse width of 17 ns with peak powers up to 0.5 MW. The spatial intensity profile of the laser beam had a near Gaussian distribution as determined by a pinhole scan and a TABLE I LINEAR ABSORPTION COEFFICIENT OF CdS Material
~ using Cary 14 spectrometer (1 cm 2 aperture)
a using low intensity ruby laser (725 ~rn spot size)
CdS
19 470 cm - l
5950 cm -1
LASER DAMAGE OF CdS AND ZnS THIN FILMS
121
television monitor system. The spatial extent of the laser beam is described by the radius of the 1/e 2 value of the peak intensity. This radius is called the "spot size" Wo. The chief method of monitoring the onset of laser-induced damage to the thin films was by the observation of laser-induced scatter (LIS). For this LIS technique, a low intensity He-Ne laser beam was used to indicate the scatter from the coating before and after the ruby laser irradiation. 4. RESULTS 4.1. ZnS films By varying the spot size of the laser beam, the damage thresholds of X/4 ZnS films were determined by the onset of LIS. Figure 3 displays these data along with a
)./4, ZnS F~LM Id~ 10.3 d/canl d. • 3 o ~
((n,,4)
| --
X I
o
5~
~o
J~o
i
2O0
6POT SJZE,W,(pm)
Fig. 3. Spot-size dependence for a single quarter-wave thick f'tim of ZnS on a glass substrate. The solid curve is a plot of the equation J = 1 + (r/- 1) exp{---~Trln2(Wo/do) 2 } where J = I/ld and ~=3. theoretical curve obtained using the model suggested by DeShazer et al. 4 For spot sizes greater than 125/am, the damage threshold I d was 10.3 J cm -2 and the fitted do was 30/am, compared with the SEM measured d o of 20/am. The result indicates that coating defects play an important role in the damage process, and defects such as "shadow-with-strips" and "irregular hills" are enhancing the electric field of the laser light. 4.2. CdS j~lms Table II lists the average damage thresholds with various laser spot sizes for three CdS films of different thicknesses. Figure 4 compares the data with the fitted curve for d o = 175/am. As shown in Fig. 2, the measured overall do was 10/am, indicating that these defects are not responsible for the laser damage. However, it is possible that a large defect or a particular type of defect separated by 175/am could be present. When the trim was viewed between crossed polarizers, two types of structural defects were found. One was a ridge 800/am in length and the other a circular spot 150 tam in diameter. These additional defects may be due to inhomogeneous adhesion between film and substrate.
122
K. M. LEUNG, C. C. TANG, L. G. DESHAZER
T A B L E II DAMAGE THRESHOLD O F CdS FILMS AT 6943 A
Spot size (/am)
Average damage threshold (J cm -2) ~/4 film ~12 film
725 592 500 321 233 162 126 108 81
0.39 0.39 0.51 0.83 1.50 1.53 1.86 1.87 -
(0.28)
0.34 0.39 0.45 0.89 1.31 1.43 1.68
h film (0.29)
0.34 0.38 0.59 0.88 1.56 1.87 -
(0.32)
Theoretical values are enclosed in parentheses. 5
X/4. Cd5 FILM 1=, 0.4J/©m= do= 175 ~ m (¢ftted)
4
~2 Q
__¢..-_
,~o
2~o ~8o 40'0 58o 6do SPOT 51ZE,W.(#~)
r~o
Fig. 4. Spot-size dependence for a single quarter-wave thick f'dm o f CdS on a glass substrate. r/=5. Fig. 5. Scanning electron micrograph o f a damage site near threshold on a single quarter-wave film o f ZnS using LIS diagnosis.
4.3. Linear absorption in CdS f i l m s
If linear absorption of a laser pulse causes damage by heating the film to its melting point, then the energy e absorbed per unit volume to melt the film is e = p ( C p ( T m p - 20 °C) + AH}
(I)
where p, Cp, Trap and AH are the film density, specific heat, melting temperature and latent heat of fusion. If the film thickness is h and the percentage absorption in the sample is/3, then the damage threshold intensity will be I 0 = eh/~
J cm-2
(2)
In this derivation, we have assumed that the film density is equal to the bulk density* with no loss of heat from the irradiated area. For a single quarter-wave CdS film, using its measured/~ of 9.8% (or t~ = 6000 cm-l), the predicted damage threshold is 0.28 J cm -2. This value is near the measured damage threshold of 0.39 J c m - 2 * The density o f thin films is equal to t h e packing fraction f times the bulk density Pb, where f = (n 2 - 1) (rib 2 + 1)/(rib 2 - 1) (n 7 + 2) (ref. 5).
LASER DAMAGE OF CdS AND ZnS THIN FILMS
123
TABLE III COMPARISON OF QUARTER-WAVE THICK DIELECTRIC COATINGS Properties
C.cIS
ZnS
Refractive index Melting point at 100 atm Region of transparency Thermal conductivity Thermal diffusivity Defect damage threshold Intrinsic damage threshold
2.506 1750 *C 0.6-16/~m 0.038 cal cm -1 s -1 K-1 0.0896 cm2 s -1 0.39 J cm -2 2.0 J cm -2
2.32 1745 °C 0.4-24/~m 0.0635 cal cm -l s-1 K -1 0.13 cm2 s -1 10 J cm -2 31 J cm-2
4.4. Damage morphology o f Z n S and CdS films The damage morphology of ZnS films as observed with the SEM was distinctly different from that o f the CdS films. For ZnS fdms a damage site caused b y a single laser shot contained many small craters, supporting the idea that the laser damage nucleates on the defects (Fig. 5). However, for CdS the breakdown region on the film was observed to be a uniform circular area.
5. CONCLUSION Table III summarizes the results and gives a comparison o f the parameters o f the two thin film materials. Intrinsic absorption o f the laser light was the cause o f damage for CdS; this was confirmed b y thermodynamic calculations assuming thermal melting o f the thin Films due to linear absorption o f laser light. For ZnS fdms, the damage threshold decreased as the spot size o f the laser beam increased, showing that the coating defects o f ZnS films have a dominant role in the damage process. ACKNOWLEDGMENTS The authors wish to thank H. R. Owen o f the University o f Southern California for the preparation o f the thin fdm specimens for this study, and J. Worrall for the SEM work. REFERENCES 1 A.J. Glass and A. H. Guenther (eds.), Laser Induced Damage in Optical Materials, Nat. Bur. Stand. (U.S.), Spec. Publ. No. 341 (1970), No. 356 (1971), No. 372 (1972), No. 38 7 (1973), No. 414 (1974), U.S. GPO, Washington, D.C. 2 U. C. Pack and A. Kestenbaum, J. Appl. Phys., 44 (1973) 2260. 3 B. E. Newnam and L. G. DeShazer, in A. J. Glass and A. H. Guenther (eds.), Laser Induced Damage in OpticaI Materials - 1972, Nat. Bur. Stand. (U.S.), Spec. Publ. No. 372, U.S. GPO, Washington, D.C., 1972, pp. 123-134. 4 L. G. DeShazer, B. E. Newnam and K. M. Leung, Appl. Phys. Lett., 23 (1973) 607. 5 G. Bauer, Ann. Phys., 19 (1934) 434.