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Hard transparent dielectric coatings
Thin Solid Films, 39(1976) 155-163 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
155
H A R D TRANSPA R E N T D I E L E C T R I C COATINGS* W. SLUSARK, JR., B. LALEVIC AND G. TAYLOR Department of Electrical Engineering, Rutgers University, New Brunswick, N.J. 08903 (U.S.A.) (Received June 22, 1976; accepted June 22, 1976)
Hard transparent corrosion-resistant dielectric coatings were investigated (a) for application to the direct protection of glass surfaces, (b) for application to the protection of thin film metallic window coatings used for reflection or transmission of solar radiation and (c) for use in codeposited metal-ceramic coatings. The dielectric coatings w e r e A1203, S i O 2 and SiC in the thickness range 100-100000/~. The coatings were deposited under varying conditions of substrate temperature, gas composition, r.f. power and substrate preparation. The results of the following measurements on the dielectric coatings are presented: diamond pyramid hardness tests, optical reflectance and transmittance in the region 0.6-4.0 eV and corrosion resistance tests under cyclic conditions. The results showed a considerable improvement in the scratch resistance and hardness of glass by layering of hard dielectric coatings 2-5 Ixm thick.
1. INTRODUCTION Many ceramic materials, both conductive and non-conductive, have large values for their hardness and compressive strengths. They exhibit a wide range of resistivities from insulators such as A120 3 to low resistivity metallic compounds such as WC (50 ~tf~cm) and TiB 2 (15 laf2 cm). Both A120 3 and SiO2 are known 1 to have low absorbance in the U V - V I S - N I R portions of the spectrum. Both A120 3 and SiO 2 have large microhardness values, 3000 and 820 kg m m - 2 respectively2. In thin film form, these materials should provide a hard transparent corrosionresistant coating which would offer direct protection of a glass surface or any thin film metal window coating by improving the scratch and corrosion resistance of the surface 3. In addition, SiC was chosen to be investigated because of its large microhardness value of 2700 kg m m - 2. The application for these films would be as protective surfaces for architectural, automotive and aircraft glass windows as well as for transparent heating panels and antireflection coatings for solar arrays. Hardness, optical and abrasion properties and the stoichiometry of ceramic thin films depend on their method of deposition 4. Furthermore, flexibility in the choice of deposition parameters is needed since these materials, when deposited on *Paper presentedat the InternationalConferenceon MetallurgicalCoatings,San Francisco,California, U.S.A., April 5-8, 1976.
156
w . SLUSARK, JR., B. LALEVIC, G. TAYLOR
glass, are subject to brittle fracture during deposition through stress concentration at a Griffith's crack-type fault 5. Ideally, it is desirable to have the surface of the coating and, if possible, the interfacial surface of the coating and substrate under a net compressive stress 6. In this paper, we present the effects of deposition procedures on three materials: A1203, SiO 2 and SiC. These films were deposited in diode (MRC) and triode (NRC) r.f. sputtering systems and also in an electron beam evaporator. We present results for qualitative scratch hardness tests, using a tungsten carbide tipped pen. A quantitative diamond pyramid hardness (DPH) test was performed to evaluate the SiO 2 and A1203 films, using Wilson's 7 model M O - T u k o n microhardness tester equipped with a 136 ° diamond pyramid indenter. Hardness numbers were obtained from the measured length of indentation and the associated conversion tables. Optical data were obtained for S i O 2 and SiC in the range 0.6-4.0 eV. Corrosion tests consisting of cyclic weather and humidity variations were also performed. 2.
EXPERIMENTAL PROCEDURE
Films of A1203, SiO2 and SiC were deposited on plate-glass substrates approximately ~ in in thickness and 33 in square. The glass was polished with a slurry of cerium oxide powder in isopropyl alcohol and water. It was rinsed with the alcohol-water mixture and was wiped dry with a lint-free tissue. In the M R C system, the substrate was heated to 200 °C by means of a substrate heater during evacuation of the chamber. The substrate was cooled to r o o m temperature before deposition. In the N R C and evaporator systems, the substrates were heated to 100°C using a hot air gun before being placed in the vacuum chambers. Immediately before deposition, in all the systems, the substrates were cleaned in an oxygen glow discharge for 10 min at 10 x 10 - 3 Torr ofO2 at 10 kV. The cyclic humidity and weather testing of the films and the D P H tests were performed at the P P G Inc. Laboratories. In the weather test, the samples were cyclically heated to the temperature of boiling water and were then cooled. Water condensate was repeatedly formed and evaporated. In the salt-spray test, the samples were exposed to a 5 ~ salt spray at r o o m temperature for 15 d. Transmission and reflection measurements were made, in the range 0.6-4.0 eV, using a Cary 14 double-beam double-monochromator U V - V I S - N I R recording spectrophotometer. A model 1413 specular reflectance attachment was used to measure the reflectance of the samples. The accessory uses the W - V configuration of Strong 8. The thickness of the films was determined by optical interference techniques. 3.
RESULTS AND DISCUSSION
3.1. Diode-sputtered films Both A l 2 0 3 and S i O 2 w e r e deposited under varying conditions of substrate temperature, gas composition and r.f. power in the diode unit. The A1203 films were sputtered from a high purity A1203 target, ~ in thick and 5 in in diameter, which was mounted with conductive epoxy on an aluminum electrode. The SiO 2 films were
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DIELECTRIC
157
COATINGS
sputtered from a high purity target of S i O 2 , the same thickness and diameter as the A120 3 target, which was also mounted on an aluminum electrode with conductive epoxy. The deposition rates for A 1 2 O 3 and g i G 2 a s a function of r.f. power are shown in Fig. 1 for the M R C diode system. The target-to-substrate distance was 3½ in, and the background pressure of argon during sputtering was 2 × 10-3 Torr for these films. The deposition rates for A1203, plotted for r.f. power levels of 200, 400 and 600 W, show a linear relationship. The deposition rates for S i O 2 , plotted for r.f. power levels of 200, 450 and 600 W, show a nearly linear relationship with rates approximately twice those for A120 3. 1,4
1.2
1.0 0 m 0
0.8
OJ {E
0.6
_g 0.4 w 0.2
0 0
i 0.1
i 02
0.3
L 04
i 0,5
0,6
RF POWER (KILOWATTS}
Fig. 1. The deposition rate as a function of r.f. power for AI2O ~ and SiO 2 in an MRC diode sputtering system: A , silica; x , alumina.
The initial work on diode-sputtered A 1 2 O 3 and S i O 2 o n glass indicated that there was a thermal contact problem between the glass substrate and the watercooled substrate support. The poor contact between the substrate and the substrate holder resulted in frosted, weakly bonded films which were easily scratched and which had very poor adhesion. The poor thermal contact between the substrate and the substrate holder also led, in m a n y cases, to fracture of the glass substrate during deposition of the films. This condition was never observed in the much shorter depositions of metallic films. Silicone vacuum grease was used to improve the thermal contact between the glass substrate and the substrate holder. This resulted in successful deposition of both A 1 2 0 3 and SiO 2, with much improved adhesion and no visible film contamination from the grease. All the films deposited by this method were optically transparent, but initially m a n y displayed varying degrees of discoloration. The r.f. power was varied in several runs but had no effect on the appearance of the films. Annealing of the films at 600 °C in air for 2 h removed the discoloration, resulting in clear, optically
158
W . SLUSARK, JR., B. LALEVIC, G. T A Y L O R
transparent films. It was concluded that the discoloration was due to an oxygen deficiency in the films. Several A1203 films were then made with oxygen introduced into the system during sputtering. The oxygen varied from 0 to 80 ~ of the total gas in the system (the other gas being argon), with a total background pressure of 2 x 10 -3 Torr. Partial pressures of oxygen (PPOs) of greater than 20 ~ resulted in clear transparent films. Similarly, for SiO2 films, PPOs of greater than 2 0 ~ resulted in clear transparent films. Coleman 9 showed that a PPO of at least 5 x 10- 5 Torr is needed to produce SiO 2 films from either an SiO or an SiO2 target. The refractive index was calculated for an A120 3 film 4.0 ~tm thick deposited in an 8 0 ~ PPO at 600 nm. The value of the refractive index was 1.79, in good agreement with values reported 2 for crystalline A120 3 of 1.70-1.77. An A120 3 film 10.6 Ixm thick (80 ~ PPO) was examined in a Norelco X-ray diffractometer. An intensity scan was made in the as-deposited condition and again after annealing at 600°C for 14 h in air. Both results were identical, showing completely amorphous films in the as-deposited and annealed conditions. The results of the scratch and adhesion tests are shown in Table I as a function of deposition conditions for both A120 3 and SIO2. TABLE I RESULTS OF QUALITATIVESCRATCH AND ADHESION TESTS FOR DIODE-SPUTTERED FILMS
Film
Thickness (p,m)
PPO during deposition (%)
Scratch test
Adhesion test
AI2Oa a AI203 AI203 SiO2 a SiO 2 SiO 2
3.0 3.5 3.6 3.0 3.0 3.1
0 0 80 0 0 50
1 2 3 1 3 3
P G G P G E
a P o o r t h e r m a l s u b s t r a t e - t o - h o l d e r contact. Scratch test: 1, p o o r e r t h a n glass; 2, as g o o d as glass; 3, better t h a n glass. A d h e s i o n test :'P, p o o r ; G, g o o d ; E, excellent.
The scratch resistance of the coated film was qualitatively compared with the scratch resistance of an uncoated portion of the glass substrate by scribing both in succession with a tungsten carbide tipped pen. A Scotch-tape test was used to measure the adhesion of the films. A rating of excellent indicates that no film was removed after repeated testing; a rating of good indicates that small portions of the film were removed after repeated testing; p o o r indicates that some portion of the film was removed during the first test. It is seen that, for both materials, the adhesion and scratch resistance of the films were improved by improving the thermal contact between the glass substrate and the substrate holder. Sputtering in a PPO significantly improved the scratch resistance of the materials, as shown in Table I. The results of the D P H tests are shown in Table II for A1203 and SiO 2 films of two different thicknesses. In both cases, the thicker films of both materials had a greater than 40 ~ improvement over the uncoated glass surface. As the thickness of the
HARD TRANSPARENT DIELECTRIC COATINGS
159
T A B L E II A l 2 0 a AND
D P H MEASUREMENTS OF
Film
S i O 2 COATINGS ON GLASS
Thickness (I.tm)
Glass SiO 2 (diode) SiO 2 (diode) AI20 3 (diode) AI20 3 (diode) A120 3 (triode)
Improvement in DPH over uncoated surface (%)
DPH (kg m m - 2)
6.0 4.1 1.6 3.5 1.0
lOOg
200g
average
470 690 580 580 670 610
500 730 560 560 660 560
480 710 570 570 660 580
-48 20 20 41 23
dielectric coating decreased, the improvement in the hardness of the sample dropped to 20 ~ for both the S i O 2 film 4.1 lam thick and the A l 2 0 3 film 1.6 lam thick.
3.2. Triode-sputteredfilms Films of A1203, SiO 2 and SiC were deposited in the N R C triode sputtering system. The A1203 films were sputtered from a plasma-sprayed Al203 target onto an aluminum electrode. The S i O 2 and SiC films were sputtered from high purity targets which were clamped to copper electrodes. The background pressure during sputtering was 1 x 10 - 3 Torr, consisting of a mixture of argon and oxygen. The experimental arrangement has been reported elsewhere 1o. There was no difficulty with substrate fracture in the triode system. Because the plasma is confined between two electrodes and does not come into contact with either the target or the substrate, the substrate temperature is much lower than that in the diode system. However, it was necessary to sputter both A1203 and SiO2 in a PPO to obtain clear transparent films. The transmittance and reflectance for a 1180 •~ SiO2 film on glass is shown in Fig. 2. The film has a flat reflectance and transmittance characteristic through the VIS and N I R regions with very low absorbance. ioo T z
f
80
II-
~
6o
I z Io
20 f 0
i
i
0.4
0.6
I
0.8
i
i
a
a
i
1.0
1,2
1.4.
1.6
1.8
WAVELENGTH
2.0
(MICROMETERS)
Fig. 2. The optical characteristics o f an N R C triode-sputtered SiO2 film 1180 A thick on glass.
160
w.
SLUSARK,
JR., B. LALEVIC,
G. TAYLOR
The color of the SiC films in transmission begins with a light sand color for films i,1 the range 80-350 A. The films show a light-gray tint at 650 A and a yellowish tint at 1280 A. Between 2000 and 2400 A, the films take a greenish-yellow transmission color. The transmittance and reflectance of three SiC films are shown in Fig. 3. They are characterized by a relatively high transmittance and low reflectance in the I R and a high reflectance in the VIS region. A quarter-wave interference effect is apparent in the samples 1280 and 2000 A thick. The important optical parameters for T and R in the application to window coatings are given in Table III. The P factors for the 670 A coating are 8.8 and 4 . 9 ~ for T a n d R1 (film side reflectance) respectively, but they are considerably higher (44, 38 ~ ) for the 1280 A sample. Ideally, the transmitted light should have a saturation index P of less than 3 ~ with a dominant wavelength (DW) of about 550 nm to achieve a neutral gray tone in reflection. The I R solar reflectance is about 30 ~ for all three films with the highest value for the 1280 A sample. The scratch resistance and adhesion of the A120 3 and SiO 2 were at least as good as those for diode-sputtered films. The scratch resistance for the SiC films was significantly better than that of glass, with excellent adhesion. The results for the D P H tests for a triode-sputtered A120 3 film are shown in Table II. The improvement in the hardness is equivalent to that of a diode-sputtered A120 3 film of the same thickness. i00
z F-
60
~.
4o
~
2o
~
o IOO 1280A
o
z
~
~.
4o
o I00
IJJ n.'
80
~
60
_j LI. I
40
2000A
T
20 0,4. 0.5 0.6
O.'f
0 . 8 - 1,2
1.6 2.0
WAVELENGTH (MICROMETERS)
Fig. 3. The optical characteristics of NRC triode-sputtered SiC films on glass as a function of SiC thickness. 3.3. Electron beam evaporation A number of A1203 and SiO 2 films were deposited in a Denton electron beam
III
580.6
574.8
1280
2000
36.9
44.0
8.8
31.2
46.8
40.3
Lum.
DW(nm) P(%)
3.2
8.0
24.8
UV
% solar transmission
Solar transmission factors
580.0
(A)
670
Thickness
SELECTED OPTICAL PARAMETERS FOR TRIODE-SPUTTERED S i C
TABLE
482.4 574.8
45.8
63.2
568.1
29.1
38.0
4.9
DW(nm) P(%)
46.8
49.8
Total solar energy
48.2
61.4
IR
Solar reflectance factors
40.3
24.5
55.6
Lum.
32.8
32.9
35.3
UV
% solar reflectance
30.9
42.2
36.3
IR
31.8
33.3
45.6
Total solar energy
a
0 >
m t""
Z
z
,.q >
>
162
w . SLUSARK, JR., B. LALEVIC, G. TAYLOR
evaporator. Films with thicknesses from 0.03 to 1.74 Ixm were deposited at rates from 0.03 to 0.58 lam min- 1. The film adhesion for both materials ranged from poor to fair. In both cases, the scratch resistance of the films was not as good as that for glass.
3.4. Weather testing Weather testing of the A 1 2 0 3 and S i O 2 coatings deposited in the M R C diode system was carried out using a glass-metal-dielectric structure. Any deficiency in the dielectric coatings would appear as an attack on the center metal film. Samples of chromium, stainless steel and titanium coated with A 1 2 0 3 and S i O 2 w e r e tested in the P P G Inc. cyclic humidity tester and salt-spray tester for 2 weeks. In both tests, outstanding protection of the metallic film was provided by the S i O 2 overlayer film, no signs of attack being observed. The A120 a films were beginning to show minor signs of attack after 1 week at scratch marks made with a diamond p o i n t - - a considerable improvement over the unprotected films (which are attacked after 1 d) but inferior to the SiO2-protected film. The results of the tests are shown in Table IV.
TABLE IV C Y C L I C H U M I D I T Y A N D S A L T - S P R A Y TEST F O R D I O D E - S P U T T E R E D FILMS
Composition
Conductor thickness ( A )
Insulator thickness (~tm)
Rating
Cr-SiO2 Ti-SiO 2 SS-SiO z Cr-A1203 Ti-AI203 SS-A1203
100 150 118 50 225 118
0.8 0.8 0.8 0.32 1.0 0.40
1 1 1 2 2 2-
SS, stainless steel. Rating: 1, no observable attack; 2, slight attack at scratch marks and edges on cyclic humidity test.
4. CONCLUSIONS We have shown that A 1 2 0 3 and S i O 2 films can be deposited in either an r.f. diode or triode sputtering system with little difference in their mechanical properties. The films are seen to be at least as good as glass, if not superior, in their resistance to scratching using a tungsten carbide pen. Electron beam evaporated films, while offering the possibility of much larger deposition rates, did not match the mechanical properties of sputtered films. D P H tests showed that the film coatings were superior to unprotected glass. The effect of substrate preparation and the method of deposition is reflected in the adhesive properties of the coatings. The N R C triode system offers the advantage of lower substrate temperatures during deposition with no possibility of surface damage due to contact with the plasma during sputtering. Layering of metallic films with a protective coating provided an effective corrosion resistance and is a feasible method of protecting both the metal film and the underlying glass surface.
HARD TRANSPARENT DIELECTRIC COATINGS
163
ACKNOWLEDGMENTS T h e a u t h o r s w i t h to a c k n o w l e d g e the c o n t r i b u t i o n o f the late D r . N i c h o l a s F u s c h i l l o , the p r i n c i p a l r e s e a r c h i n v e s t i g a t o r a n d d i r e c t o r o f the solid state l a b o r a t o r y d u r i n g this work. T h e w o r k was s u p p o r t e d b y P P G I n d u s t r i e s . REFERENCES 1 G. Hass and R. Thun, Phys. Thin Films, 2 (1964) 239. American Institute o f Physics Handbook, 2nd edn., McGraw-Hill, New York, 1963. 3 G. Hass, J. Opt. Soc. Am., 39 (1949) 532. 4 L. Holland, Vacuum Deposition of Thin Films, Chapman and Hall, London, 1966, Chap. 16. 5 A.A. Griffith, Philos. Trans. R. Soc. London, Ser. A, 221 (1920) 163. 6 F. Ernsberger, Research Into Glass, Vol. 2, PPG Industries, Pittsburgh, Pa., 1970, p. 21. 7 A. F. Wilson, Instruction Manual: Tukon Hardness Tester, Mechanical Instrument Division, American Chain & Cable Company, New York, 1957. 8 J. Strong, Procedures in Experimental Physics, Prentice-Hall, Englewood Cliffs, N.J., 1938, p. 288. 9 W. Coleman, Appl. Opt., 13 (4) (1974) 946. 10 G. Taylor, B. Lalevic and W. Slusark, Jr., Properties of metal~lielectric codeposited films, Thin Solid Films, 39 (1976) 165. 2
155
H A R D TRANSPA R E N T D I E L E C T R I C COATINGS* W. SLUSARK, JR., B. LALEVIC AND G. TAYLOR Department of Electrical Engineering, Rutgers University, New Brunswick, N.J. 08903 (U.S.A.) (Received June 22, 1976; accepted June 22, 1976)
Hard transparent corrosion-resistant dielectric coatings were investigated (a) for application to the direct protection of glass surfaces, (b) for application to the protection of thin film metallic window coatings used for reflection or transmission of solar radiation and (c) for use in codeposited metal-ceramic coatings. The dielectric coatings w e r e A1203, S i O 2 and SiC in the thickness range 100-100000/~. The coatings were deposited under varying conditions of substrate temperature, gas composition, r.f. power and substrate preparation. The results of the following measurements on the dielectric coatings are presented: diamond pyramid hardness tests, optical reflectance and transmittance in the region 0.6-4.0 eV and corrosion resistance tests under cyclic conditions. The results showed a considerable improvement in the scratch resistance and hardness of glass by layering of hard dielectric coatings 2-5 Ixm thick.
1. INTRODUCTION Many ceramic materials, both conductive and non-conductive, have large values for their hardness and compressive strengths. They exhibit a wide range of resistivities from insulators such as A120 3 to low resistivity metallic compounds such as WC (50 ~tf~cm) and TiB 2 (15 laf2 cm). Both A120 3 and SiO2 are known 1 to have low absorbance in the U V - V I S - N I R portions of the spectrum. Both A120 3 and SiO 2 have large microhardness values, 3000 and 820 kg m m - 2 respectively2. In thin film form, these materials should provide a hard transparent corrosionresistant coating which would offer direct protection of a glass surface or any thin film metal window coating by improving the scratch and corrosion resistance of the surface 3. In addition, SiC was chosen to be investigated because of its large microhardness value of 2700 kg m m - 2. The application for these films would be as protective surfaces for architectural, automotive and aircraft glass windows as well as for transparent heating panels and antireflection coatings for solar arrays. Hardness, optical and abrasion properties and the stoichiometry of ceramic thin films depend on their method of deposition 4. Furthermore, flexibility in the choice of deposition parameters is needed since these materials, when deposited on *Paper presentedat the InternationalConferenceon MetallurgicalCoatings,San Francisco,California, U.S.A., April 5-8, 1976.
156
w . SLUSARK, JR., B. LALEVIC, G. TAYLOR
glass, are subject to brittle fracture during deposition through stress concentration at a Griffith's crack-type fault 5. Ideally, it is desirable to have the surface of the coating and, if possible, the interfacial surface of the coating and substrate under a net compressive stress 6. In this paper, we present the effects of deposition procedures on three materials: A1203, SiO 2 and SiC. These films were deposited in diode (MRC) and triode (NRC) r.f. sputtering systems and also in an electron beam evaporator. We present results for qualitative scratch hardness tests, using a tungsten carbide tipped pen. A quantitative diamond pyramid hardness (DPH) test was performed to evaluate the SiO 2 and A1203 films, using Wilson's 7 model M O - T u k o n microhardness tester equipped with a 136 ° diamond pyramid indenter. Hardness numbers were obtained from the measured length of indentation and the associated conversion tables. Optical data were obtained for S i O 2 and SiC in the range 0.6-4.0 eV. Corrosion tests consisting of cyclic weather and humidity variations were also performed. 2.
EXPERIMENTAL PROCEDURE
Films of A1203, SiO2 and SiC were deposited on plate-glass substrates approximately ~ in in thickness and 33 in square. The glass was polished with a slurry of cerium oxide powder in isopropyl alcohol and water. It was rinsed with the alcohol-water mixture and was wiped dry with a lint-free tissue. In the M R C system, the substrate was heated to 200 °C by means of a substrate heater during evacuation of the chamber. The substrate was cooled to r o o m temperature before deposition. In the N R C and evaporator systems, the substrates were heated to 100°C using a hot air gun before being placed in the vacuum chambers. Immediately before deposition, in all the systems, the substrates were cleaned in an oxygen glow discharge for 10 min at 10 x 10 - 3 Torr ofO2 at 10 kV. The cyclic humidity and weather testing of the films and the D P H tests were performed at the P P G Inc. Laboratories. In the weather test, the samples were cyclically heated to the temperature of boiling water and were then cooled. Water condensate was repeatedly formed and evaporated. In the salt-spray test, the samples were exposed to a 5 ~ salt spray at r o o m temperature for 15 d. Transmission and reflection measurements were made, in the range 0.6-4.0 eV, using a Cary 14 double-beam double-monochromator U V - V I S - N I R recording spectrophotometer. A model 1413 specular reflectance attachment was used to measure the reflectance of the samples. The accessory uses the W - V configuration of Strong 8. The thickness of the films was determined by optical interference techniques. 3.
RESULTS AND DISCUSSION
3.1. Diode-sputtered films Both A l 2 0 3 and S i O 2 w e r e deposited under varying conditions of substrate temperature, gas composition and r.f. power in the diode unit. The A1203 films were sputtered from a high purity A1203 target, ~ in thick and 5 in in diameter, which was mounted with conductive epoxy on an aluminum electrode. The SiO 2 films were
HARD
TRANSPARENT
DIELECTRIC
157
COATINGS
sputtered from a high purity target of S i O 2 , the same thickness and diameter as the A120 3 target, which was also mounted on an aluminum electrode with conductive epoxy. The deposition rates for A 1 2 O 3 and g i G 2 a s a function of r.f. power are shown in Fig. 1 for the M R C diode system. The target-to-substrate distance was 3½ in, and the background pressure of argon during sputtering was 2 × 10-3 Torr for these films. The deposition rates for A1203, plotted for r.f. power levels of 200, 400 and 600 W, show a linear relationship. The deposition rates for S i O 2 , plotted for r.f. power levels of 200, 450 and 600 W, show a nearly linear relationship with rates approximately twice those for A120 3. 1,4
1.2
1.0 0 m 0
0.8
OJ {E
0.6
_g 0.4 w 0.2
0 0
i 0.1
i 02
0.3
L 04
i 0,5
0,6
RF POWER (KILOWATTS}
Fig. 1. The deposition rate as a function of r.f. power for AI2O ~ and SiO 2 in an MRC diode sputtering system: A , silica; x , alumina.
The initial work on diode-sputtered A 1 2 O 3 and S i O 2 o n glass indicated that there was a thermal contact problem between the glass substrate and the watercooled substrate support. The poor contact between the substrate and the substrate holder resulted in frosted, weakly bonded films which were easily scratched and which had very poor adhesion. The poor thermal contact between the substrate and the substrate holder also led, in m a n y cases, to fracture of the glass substrate during deposition of the films. This condition was never observed in the much shorter depositions of metallic films. Silicone vacuum grease was used to improve the thermal contact between the glass substrate and the substrate holder. This resulted in successful deposition of both A 1 2 0 3 and SiO 2, with much improved adhesion and no visible film contamination from the grease. All the films deposited by this method were optically transparent, but initially m a n y displayed varying degrees of discoloration. The r.f. power was varied in several runs but had no effect on the appearance of the films. Annealing of the films at 600 °C in air for 2 h removed the discoloration, resulting in clear, optically
158
W . SLUSARK, JR., B. LALEVIC, G. T A Y L O R
transparent films. It was concluded that the discoloration was due to an oxygen deficiency in the films. Several A1203 films were then made with oxygen introduced into the system during sputtering. The oxygen varied from 0 to 80 ~ of the total gas in the system (the other gas being argon), with a total background pressure of 2 x 10 -3 Torr. Partial pressures of oxygen (PPOs) of greater than 20 ~ resulted in clear transparent films. Similarly, for SiO2 films, PPOs of greater than 2 0 ~ resulted in clear transparent films. Coleman 9 showed that a PPO of at least 5 x 10- 5 Torr is needed to produce SiO 2 films from either an SiO or an SiO2 target. The refractive index was calculated for an A120 3 film 4.0 ~tm thick deposited in an 8 0 ~ PPO at 600 nm. The value of the refractive index was 1.79, in good agreement with values reported 2 for crystalline A120 3 of 1.70-1.77. An A120 3 film 10.6 Ixm thick (80 ~ PPO) was examined in a Norelco X-ray diffractometer. An intensity scan was made in the as-deposited condition and again after annealing at 600°C for 14 h in air. Both results were identical, showing completely amorphous films in the as-deposited and annealed conditions. The results of the scratch and adhesion tests are shown in Table I as a function of deposition conditions for both A120 3 and SIO2. TABLE I RESULTS OF QUALITATIVESCRATCH AND ADHESION TESTS FOR DIODE-SPUTTERED FILMS
Film
Thickness (p,m)
PPO during deposition (%)
Scratch test
Adhesion test
AI2Oa a AI203 AI203 SiO2 a SiO 2 SiO 2
3.0 3.5 3.6 3.0 3.0 3.1
0 0 80 0 0 50
1 2 3 1 3 3
P G G P G E
a P o o r t h e r m a l s u b s t r a t e - t o - h o l d e r contact. Scratch test: 1, p o o r e r t h a n glass; 2, as g o o d as glass; 3, better t h a n glass. A d h e s i o n test :'P, p o o r ; G, g o o d ; E, excellent.
The scratch resistance of the coated film was qualitatively compared with the scratch resistance of an uncoated portion of the glass substrate by scribing both in succession with a tungsten carbide tipped pen. A Scotch-tape test was used to measure the adhesion of the films. A rating of excellent indicates that no film was removed after repeated testing; a rating of good indicates that small portions of the film were removed after repeated testing; p o o r indicates that some portion of the film was removed during the first test. It is seen that, for both materials, the adhesion and scratch resistance of the films were improved by improving the thermal contact between the glass substrate and the substrate holder. Sputtering in a PPO significantly improved the scratch resistance of the materials, as shown in Table I. The results of the D P H tests are shown in Table II for A1203 and SiO 2 films of two different thicknesses. In both cases, the thicker films of both materials had a greater than 40 ~ improvement over the uncoated glass surface. As the thickness of the
HARD TRANSPARENT DIELECTRIC COATINGS
159
T A B L E II A l 2 0 a AND
D P H MEASUREMENTS OF
Film
S i O 2 COATINGS ON GLASS
Thickness (I.tm)
Glass SiO 2 (diode) SiO 2 (diode) AI20 3 (diode) AI20 3 (diode) A120 3 (triode)
Improvement in DPH over uncoated surface (%)
DPH (kg m m - 2)
6.0 4.1 1.6 3.5 1.0
lOOg
200g
average
470 690 580 580 670 610
500 730 560 560 660 560
480 710 570 570 660 580
-48 20 20 41 23
dielectric coating decreased, the improvement in the hardness of the sample dropped to 20 ~ for both the S i O 2 film 4.1 lam thick and the A l 2 0 3 film 1.6 lam thick.
3.2. Triode-sputteredfilms Films of A1203, SiO 2 and SiC were deposited in the N R C triode sputtering system. The A1203 films were sputtered from a plasma-sprayed Al203 target onto an aluminum electrode. The S i O 2 and SiC films were sputtered from high purity targets which were clamped to copper electrodes. The background pressure during sputtering was 1 x 10 - 3 Torr, consisting of a mixture of argon and oxygen. The experimental arrangement has been reported elsewhere 1o. There was no difficulty with substrate fracture in the triode system. Because the plasma is confined between two electrodes and does not come into contact with either the target or the substrate, the substrate temperature is much lower than that in the diode system. However, it was necessary to sputter both A1203 and SiO2 in a PPO to obtain clear transparent films. The transmittance and reflectance for a 1180 •~ SiO2 film on glass is shown in Fig. 2. The film has a flat reflectance and transmittance characteristic through the VIS and N I R regions with very low absorbance. ioo T z
f
80
II-
~
6o
I z Io
20 f 0
i
i
0.4
0.6
I
0.8
i
i
a
a
i
1.0
1,2
1.4.
1.6
1.8
WAVELENGTH
2.0
(MICROMETERS)
Fig. 2. The optical characteristics o f an N R C triode-sputtered SiO2 film 1180 A thick on glass.
160
w.
SLUSARK,
JR., B. LALEVIC,
G. TAYLOR
The color of the SiC films in transmission begins with a light sand color for films i,1 the range 80-350 A. The films show a light-gray tint at 650 A and a yellowish tint at 1280 A. Between 2000 and 2400 A, the films take a greenish-yellow transmission color. The transmittance and reflectance of three SiC films are shown in Fig. 3. They are characterized by a relatively high transmittance and low reflectance in the I R and a high reflectance in the VIS region. A quarter-wave interference effect is apparent in the samples 1280 and 2000 A thick. The important optical parameters for T and R in the application to window coatings are given in Table III. The P factors for the 670 A coating are 8.8 and 4 . 9 ~ for T a n d R1 (film side reflectance) respectively, but they are considerably higher (44, 38 ~ ) for the 1280 A sample. Ideally, the transmitted light should have a saturation index P of less than 3 ~ with a dominant wavelength (DW) of about 550 nm to achieve a neutral gray tone in reflection. The I R solar reflectance is about 30 ~ for all three films with the highest value for the 1280 A sample. The scratch resistance and adhesion of the A120 3 and SiO 2 were at least as good as those for diode-sputtered films. The scratch resistance for the SiC films was significantly better than that of glass, with excellent adhesion. The results for the D P H tests for a triode-sputtered A120 3 film are shown in Table II. The improvement in the hardness is equivalent to that of a diode-sputtered A120 3 film of the same thickness. i00
z F-
60
~.
4o
~
2o
~
o IOO 1280A
o
z
~
~.
4o
o I00
IJJ n.'
80
~
60
_j LI. I
40
2000A
T
20 0,4. 0.5 0.6
O.'f
0 . 8 - 1,2
1.6 2.0
WAVELENGTH (MICROMETERS)
Fig. 3. The optical characteristics of NRC triode-sputtered SiC films on glass as a function of SiC thickness. 3.3. Electron beam evaporation A number of A1203 and SiO 2 films were deposited in a Denton electron beam
III
580.6
574.8
1280
2000
36.9
44.0
8.8
31.2
46.8
40.3
Lum.
DW(nm) P(%)
3.2
8.0
24.8
UV
% solar transmission
Solar transmission factors
580.0
(A)
670
Thickness
SELECTED OPTICAL PARAMETERS FOR TRIODE-SPUTTERED S i C
TABLE
482.4 574.8
45.8
63.2
568.1
29.1
38.0
4.9
DW(nm) P(%)
46.8
49.8
Total solar energy
48.2
61.4
IR
Solar reflectance factors
40.3
24.5
55.6
Lum.
32.8
32.9
35.3
UV
% solar reflectance
30.9
42.2
36.3
IR
31.8
33.3
45.6
Total solar energy
a
0 >
m t""
Z
z
,.q >
>
162
w . SLUSARK, JR., B. LALEVIC, G. TAYLOR
evaporator. Films with thicknesses from 0.03 to 1.74 Ixm were deposited at rates from 0.03 to 0.58 lam min- 1. The film adhesion for both materials ranged from poor to fair. In both cases, the scratch resistance of the films was not as good as that for glass.
3.4. Weather testing Weather testing of the A 1 2 0 3 and S i O 2 coatings deposited in the M R C diode system was carried out using a glass-metal-dielectric structure. Any deficiency in the dielectric coatings would appear as an attack on the center metal film. Samples of chromium, stainless steel and titanium coated with A 1 2 0 3 and S i O 2 w e r e tested in the P P G Inc. cyclic humidity tester and salt-spray tester for 2 weeks. In both tests, outstanding protection of the metallic film was provided by the S i O 2 overlayer film, no signs of attack being observed. The A120 a films were beginning to show minor signs of attack after 1 week at scratch marks made with a diamond p o i n t - - a considerable improvement over the unprotected films (which are attacked after 1 d) but inferior to the SiO2-protected film. The results of the tests are shown in Table IV.
TABLE IV C Y C L I C H U M I D I T Y A N D S A L T - S P R A Y TEST F O R D I O D E - S P U T T E R E D FILMS
Composition
Conductor thickness ( A )
Insulator thickness (~tm)
Rating
Cr-SiO2 Ti-SiO 2 SS-SiO z Cr-A1203 Ti-AI203 SS-A1203
100 150 118 50 225 118
0.8 0.8 0.8 0.32 1.0 0.40
1 1 1 2 2 2-
SS, stainless steel. Rating: 1, no observable attack; 2, slight attack at scratch marks and edges on cyclic humidity test.
4. CONCLUSIONS We have shown that A 1 2 0 3 and S i O 2 films can be deposited in either an r.f. diode or triode sputtering system with little difference in their mechanical properties. The films are seen to be at least as good as glass, if not superior, in their resistance to scratching using a tungsten carbide pen. Electron beam evaporated films, while offering the possibility of much larger deposition rates, did not match the mechanical properties of sputtered films. D P H tests showed that the film coatings were superior to unprotected glass. The effect of substrate preparation and the method of deposition is reflected in the adhesive properties of the coatings. The N R C triode system offers the advantage of lower substrate temperatures during deposition with no possibility of surface damage due to contact with the plasma during sputtering. Layering of metallic films with a protective coating provided an effective corrosion resistance and is a feasible method of protecting both the metal film and the underlying glass surface.
HARD TRANSPARENT DIELECTRIC COATINGS
163
ACKNOWLEDGMENTS T h e a u t h o r s w i t h to a c k n o w l e d g e the c o n t r i b u t i o n o f the late D r . N i c h o l a s F u s c h i l l o , the p r i n c i p a l r e s e a r c h i n v e s t i g a t o r a n d d i r e c t o r o f the solid state l a b o r a t o r y d u r i n g this work. T h e w o r k was s u p p o r t e d b y P P G I n d u s t r i e s . REFERENCES 1 G. Hass and R. Thun, Phys. Thin Films, 2 (1964) 239. American Institute o f Physics Handbook, 2nd edn., McGraw-Hill, New York, 1963. 3 G. Hass, J. Opt. Soc. Am., 39 (1949) 532. 4 L. Holland, Vacuum Deposition of Thin Films, Chapman and Hall, London, 1966, Chap. 16. 5 A.A. Griffith, Philos. Trans. R. Soc. London, Ser. A, 221 (1920) 163. 6 F. Ernsberger, Research Into Glass, Vol. 2, PPG Industries, Pittsburgh, Pa., 1970, p. 21. 7 A. F. Wilson, Instruction Manual: Tukon Hardness Tester, Mechanical Instrument Division, American Chain & Cable Company, New York, 1957. 8 J. Strong, Procedures in Experimental Physics, Prentice-Hall, Englewood Cliffs, N.J., 1938, p. 288. 9 W. Coleman, Appl. Opt., 13 (4) (1974) 946. 10 G. Taylor, B. Lalevic and W. Slusark, Jr., Properties of metal~lielectric codeposited films, Thin Solid Films, 39 (1976) 165. 2