Sputtered titania borosilicate glass films

Thin Solid Films, 202 (1991) 321 331

321

PREPARATION AND CHARACTERIZATION

S P U T T E R E D T I T A N I A B O R O S I L I C A T E GLASS FILMS J. A. HAWTHORNE*

Di,splay Engineering Inc., 480 Pesconi Court, Santa Rosa, CA 95401 (U.S.A.) S. N. HOUDE-WALTER~

The Institute gfl Optics, University qfl Rochester, Rochester, N Y 14627 (U.S.A.) E. M. VOGEL

Bell Communications Research, 331 Newman Springs Road, NVC3Z-283, Red Bank, N J07701 (U.S.A.) (Received October 10, 1990; accepted January 21, 1991)

Moderately high refractive index titania borosilicate glasses are sputtered into glassy films. Deposition parameters are used to control the stoichiometry and optical properties of the films. Compositions unobtainable by batch melting are achieved with TiO2 levels up to 47 mol.~o. The films are extremely durable with average optical intensity damage thresholds of 13.5 G W cm -2. Multilayer films have similar damage thresholds. Total optical losses at 633 nm are 8 dB c m - 1 when waveguided in a 0.3 gm film on a fused quartz substrate.

1. INTRODUCTION

Glass has a unique ability to accommodate high dopant levels in a variety of stoichiometries in its disordered structure. While there are limits to the range of compositions that will form a vitreous material in most multicomponent systems, the composition range over which a vitreous material is obtained may be extended by rapid quenching. Since sputtered materials quench rapidly at the substrate surface, one may expect a larger range of glass compositions in sputtered films than is obtainable by batch melting. Multicomponent oxide glasses have been sputtered into thin films for optical waveguide applications for many years, but the literature is primarily devoted to Corning 7059 aluminum borosilicate glass 1-6. Sputtered 7059 films generally have different compositions from their bulk counterparts 3, consistent with the preferential sputtering of the more weakly bound components ~' 8. Pitt 2 and later Jerominek e t al. 6 showed that the resultant optical properties of sputtered 7059 glass could be varied by changing the cathode power and the percentage of oxygen present in the sputtering gas and by using different cathode configurations. These sputtered films initially showed large scattering losses and extensive work has been done to reduce the optical losses in these films. Laser annealing and surface coatings have been * J. A . Hawthorne was at the University of Rochester at the time this work was done. t A u t h o r to whom correspondence should be addressed.

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J. A. H A W T H O R N E , S. N. H O U D E - W A L T E R , E. M. V O G E L

successfully employed to reduce index variation and surface roughness scattering in 7059 films to losses of 0.4 dB cm i (ref. 9). The purpose of this study is to determine how well sputtering can be used to extend the composition range of certain multicomponent glasses. The glass used in this study was based on a titania borosilicate glass designed for all-optical switching 1°. This glass, with an equimolar composition of SiO 2, TiO2, N b z O 5, B 2 0 3 and K 2 0 , devitrified in the large volume melts required to produce 4 in and 8in sputtering targets. The glass was stabilized by replacing the 20')~i Nb205 modifier with an additional 200Ji, of the SiO 2 glass former. This allowed fabrication ofa 4 in target without devitrification of the glass. The modified glass composition is designated E98C glass. The measured optical properties of bulk E98C glass are nd = 1.7184 (refractive index at He d line), vd = 27.65 (Abbe dispersion number) and ~d = 0.5 c m - 1 (optical absorption coefficient at the He d line). 2. E X P E R I M E N T A L DETAILS

2.1. Optical property measurements Optical properties of the sputtered films were measured using a combination of techniques, most of which used the sputtered film layer as a planar optical waveguide. Polished fused quartz microscope slides were used as substrates with an r.m.s, surface roughness of 5 ~ , as measured with an AlphaStep profilometer. Coupling of light into the waveguides was accomplished by grating coupling 11 m line measurements ~1 were performed to measure the refractive indices of the films. The refractive index of the sputtered films was solved by using the exact solution of the grating boundary value problem ~2. Chromatic dispersion of the films was measured by successively coupling three different wavelengths (632.8 nm from a helium neon laser and 514.5 nm and 457.9nm from an argon ion laser) into the waveguides. The values of ha and vd were obtained by interpolation of the measured points using Buchdal's method ~~. Transmission envelope measurements 14 were also taken over 300-1300nm using a Perkin Elmer spectrophotometer as a cross-check on the values ofn d, vd and the film thickness. In addition, the sputtered film thicknesses were independently measured using an AlphaStep surface profilometer. Agreement between the m line, transmission envelope and profilometer measurements was excellent. 2.2. Magnetron sputtering Magnetron sputtering was used to deposit the E98C glass initially. Films were sputtered using cathode power densities between 1.48 and 3.08 W cm - 2 and O2:Ar gas mixture ratios of 100:1 and 50:50. The deposition rate using a cathode power density of 1.85 W cm 2 was 16.7 A min 1 Overall, the magnetron-sputtered films exhibited a lower refractive index than the target glass over the entire range of sputtering parameters (Table 1). The target was severely damaged by the sputtering, including significant cracking and melting of its surface. The low refractive index and the heat-related damage to the target suggest that there may have been chemical dissociation of the target oxides during sputtering. The low refractive indices of the magnetron-sputtered films also suggest

323

SPUTTERED TITANIA BOROSILICATE GLASS

TABLE I SUMMARY OF SPUTTERED

FILM DATA

Film identification

Sputtering parameters (cathode configuration," target power density; sputtering gas composition)

Refractive index nd

Abbe number

56-1 56-2 5-13 5-29

Magnetron; Magnetron; Magnetron; Magnetron;

1.577 1.631 1.638 1.622

32.01 41.77 34.84 61.6

7 9 5 8 12

Diode, Diode, Diode, Diode, Diode,

target; 3.7 W cm 2; 100% Ar target; 4.3 W cm - 2; 100% Ar target;4.3Wcm 2;90%Ar, 100/oO2 target; 4.3 W cm 2; 80% At, 20% O2 target; 4.9 W cm 2; 100% Ar

1.709 1.726 1.756 1.828 1.68

31.9 28.38 30.37 18.9 40.0

18, 22

Diode, 8 i n d i a m e t e r t a r g e t ; l . 0 8 W c m

2;80%Ar, 20%02

1.662

34.18

1.48 W 1.48 W 1.85 W 3.08 W

cm-2; cm -2; cm -2; cm-2;

4 in diameter 4 in diameter 4 in diameter 4 in diameter 4 in diameter

50% Ar, 50% 02 100% 0 2 100% 0 2 100% 0 2

that they may be TiO 2 poor, and so magnetron sputtering of the titania borosilicate glass was abandoned in favor of r.f. diode sputtering.

2.3. Diode-sputtered.films In diode sputtering, the plasma is not confined to the proximity of the target surface. As a result, diode sputtering should result in less heat-related target damage and may produce less chemical dissociation of the target oxides. The initial diode sputtering system configuration used for this study had a 4 in diameter target, stainless steel cathode shielding and aluminum substrate fixture. As with the magnetron films, the cathode power and gas ratio parameters were varied. The sputtering rate, using a cathode power density of 4.3 W cm 2, was 6.4 A m i n - ~. The diode-sputtered films had a higher refractive index and lower dispersion than the bulk E98C glass. The refractive index of the diode-sputtered film increased with the addition of oxygen (Fig. 1), which would indicate there was still chemical 1.85

i

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i

~

,

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4" diode: 4.3 W/cm2 • 8" diode: 1.08 W/em2 .... Bulk value --="" magnetmn: 1.5 W/cm2

1.80

1.75 ........................................

•~ 1.7o e~

1.65

1.60 I

I

r

I

I

I

0

20

40

60

80

I00

Percent Oxygen Partial Pressure Fig. 1. Refractive index at a wavelength of 633 nm vs. oxygen partial pressure for the magnetron- and diode-sputtered films.

324

J. A. H A W T H O R N E , S. N. H O U D E - W A L T E R , E. M. VOGEl,

dissociation of the target oxides. The amount of oxygen that could be used was limited, however. Films sputtered with oxygen ratios higher than 20To had a frosted appearance. This has been observed with oxygen reactive diode sputtering of other glasses and may be attributable to excessive substrate heating due to b o m b a r d m e n t by energetic oxygen ions 2. The magnetron-sputtered films did not exhibit frosting, presumably because the magnetron plasma confinement reduces substrate bombardment. The film refractive index was observed to increase with cathode power densities up to 4.3 W cm 2 (Fig. 2), thereafter decreasing. Chromatic dispersion of the films increased with the O2:Ar gas mixture ratio, as shown in Table I.

1.72 1.70 1.68

.... Bulk material --m--- Magneu'on sputtered; 100% 0 2

•

1.66 1.64

taL-~

1.62 I

1

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i

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2 3 4 Cathode power density [W/era2]

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i

5

Fig. 2. R e f r a c t i v e index at a w a v e l e n g t h of 633 n m e s . c a t h o d e p o w e r d e n s i t y for the m a g n e t r o n - a n d d i o d e - s p u t t e r e d films.

3.

A N A L Y S I S OF E X P E R I M E N T A L R E S U L T S

3.1. Film composition andstoichiometD' Neutralized secondary ion mass spectroscopy (SIMS) was performed on two samples (films 5 and 9) to determine their compositions. Film 5 was sputtered at 4.3W cm 2 in 10~J0 0 2 and film 9 was sputtered at 4.3W cm 2 in 10~"/,,/oAt. The compositions of the two films are given in Table II. The SIMS analysis shows an increase in the percentage of TiOx and a decrease in the S i O 2 relative to the bulk composition. This may be due to resputtering of the deposited film and subsequent loss of material as a result of the small area ratio between the substrate and the target. It is worth noting that both films contain greater than 40mol.'~ TiO2. The film compositions are well outside what is normally considered the glass forming region for titania borosilicate glasses as melted from batch 15. Uniformity of composition was examined as a function of depth in the film using SIMS profiling. Figure 3 shows the SIMS data for film 9. The oxide components vary by at most -+-5 mol '~ within the film, and consequently the films can be considered fairly uniform in composition. Early films showed discoloration when viewed in transmission. The optical

325

SPUTTERED TITANIA BOROSILICATE GLASS

TABLE 1I COMPOSITIONS

AND OPTICAL

PROPERTIES OF FILMS 9 AND 5

Target (nominal value) a

Film 9 (SIMS

Film 11 (SIMS)

Composition (mol%) SiO 2 TiO2 B20 3 KzO

40 20 20 20

21.2 42.0 8.6 28.2

26.2 46.8 11.2 15.7

Optical properties nd %

1.718 27.65

1.726 28.38

1.756 30.37

See Table I for sputtering parameters. a Target composition is nominal batch composition.

i

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80

ti~miLml •~

I t

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40 2o

. . . . . . . . . . . . ~[ih~6fi"-"" - "--" .........

I

boron

,I '1 ,I

u

L

I

I

0.00

0.05

I

I

0.10 0.15 film depth [lam)

I

/

0.20

0.25

Fig. 3. SIMS profile for film 9 demonstrating uniformity of composition to within _+5}o as a function of depth in the film.

absorption of the film could be due to a different bonding state of the titanium ~5; for example, titanium could be present as Ti 3 ÷. This would cause the absorption band edge to shift toward visible wavelengths. Electron spectroscopy for chemical analysis (ESCA) was performed on film 7 (100~o Ar, 3 . 7 W c m 2 as in Table I) to determine the bonding state of the titanium. The ESCA showed that the titanium was present only as TiOz, so film absorption resulting from the reduction of TiO2 was unlikely. The source of contamination was determined by replacing the E98C target with a quartz target and sputtering a quartz film onto a quartz substrate. This allowed SIMS analysis of films without masking by a strong titanium signal. The analysis revealed copper, silver, chromium and aluminum metals in the discolored films. The source of this metal contamination was thought to be sputtering of the chamber fixturing and the cathode housing. The sputtering system was modified to reduce metal contamination. The 4 in diameter target was replaced by an 8 in diameter target so that the stainless steel cathode shield could be replaced by a quartz shield. The aluminum substrate fixture

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J. A. HAWTHORNE, S. N. HOUDE-WALTER, E. M. VOGEL

was replaced by a q u a r t z fixture a n d a q u a r t z shield was placed over the stainless steel s u b s t r a t e fixture s u p p o r t plate. A schematic d i a g r a m of the modified system is s h o w n in Fig. 4.

Substrate f / SSTcathodeshield fixtUreplateSuppOrtwi Nth. replacedwith~ quartz shield [ ~ quartzshield SST substrate fixture relpaced with quartz 4" dia. targetand quartz spacer plate. Replaced~ with 8" alia.target

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Fig. 4. Schematic diagram of the diode sputtering chamber after modifications were made to reduce metal contamination. Quartz plates cover or replace stainless steel (SST) surfaces: 8 in diameter target allows replacement of SST cathode shield with quartz shield. T h e c o n t a m i n a t i o n test was r e p e a t e d using the new s p u t t e r i n g c o n f i g u r a t i o n a n d an 8 in d i a m e t e r q u a r t z target. T h e s u b s e q u e n t S I M S profiles in the q u a r t z films c o n f i r m e d that copper, silver a n d c h r o m i u m c o n c e n t r a t i o n s were reduced by a p p r o x i m a t e l y half by s u i t a b l y m a s k i n g the c h a m b e r furniture. A l u m i n u m cont a m i n a t i o n r e m a i n e d high, however, a n d m a y be a t t r i b u t a b l e to the a l u m i n u m c a t h o d e s u p p o r t plate at the b o t t o m of the c h a m b e r . The resulting films were clear when viewed in transmission, but the optical a b s o r p t i o n coefficients of the films are u n k n o w n . T a b l e III s u m m a r i z e s the c o n t a m i n a t i o n test results. After reconfiguration, a single c a t h o d e p o w e r density a n d oxygen ratio (20'y,,) were used for the films s p u t t e r e d using the 8 in d i a m e t e r E98C target. The c a t h o d e p o w e r density was four times lower than for the 4 i n d i a m e t e r target as a result of i m p e d a n c e m i s m a t c h i n g a s s o c i a t e d with the r.f. p o w e r s u p p l y a n d c h a m b e r configuration. The s p u t t e r i n g rates using c a t h o d e p o w e r densities of 1.08 W cm 2 were 4.9/~ min - 1. The refractive index a n d dispersion of these films were lower t h a n those of the bulk glass, but the refractive index was still higher t h a n in the

327

S P U T T E R E D T I T A N I A BOROSILICATE GLASS

TABLE III SECONDARY I()N MASSSPECTROSCOPY ANALYSISOF METAL CONTAMINATESIN SPUTTERED SiO 2 FILMS BEFORE (4 in DIAMETERTARGET) AND AFTER (8 in DIAMETERTARGET) CHAMBER MODIFICATIONS

Component

Si A1 Cu Ag Cr

Concentrations(atoms c m "~) 4 in diameter

8 in diameter

1 × 1020 6× 10~v 7×101~ 3×10 ~5 3 × 10~

1 × 1020 1 x 1()1~ 1.5×10 Iv 6×10 ~4 2 × 10~

magnetron-sputtered films. S I M S analysis of films sputtered from the 8 in target revealed film compositions that were close to those of the target batch values (Table IV). The higher refractive index of the 4in diameter target films can mainly be attributed to the higher titanium concentration; however, the metal c o n t a m i n a t i o n m a y have also contributed slightly to the high refractive index. T A B L E 1V COMPARISON OF FILM COMPOSITIONAS MEASUREDBY SECONDARY ION MASSSPECTROSCOPY ANALYISFROM THE 8 in TARGET (FILM 22) AND 4 in TARGET (FILM 5)

Component

SiO 2 TiO 2 B20 3 K2O

Compositions (mol.%) Fihn 22

Film 5

Targel

34.3 25.7 14.2 25.8

26.2 46.8 11.2 15.7

40 20 20 20

See T a b l e I for s p u t t e r i n g p a r a m e t e r s . T h e n o m i n a l b a t c h c o m p o s i t i o n of the target is given for reference.

The difference in properties of films sputtered from the 4 and 8 in diameter targets is not surprising. Different c o m p o n e n t s of m u l t i c o m p o n e n t targets are k n o w n to sputter at different rates depending on the magnitude of the potential applied to the target 2. It is also possible that the targets m a y have slightly different compositions because of volatilization during melting, which would in turn contribute to differences in film composition. A s u m m a r y of the m a g n e t r o n and diode sputtering parameters, refractive indices and dispersion is given in Table I and Figs. I and 2.

3.2. Scattering losses Most sputtered films suffer from optical scattering losses. The scattering can usually be attributed to surface roughness of the film, to refractive index variations caused by the porosity of the film microstructure, and to film defects, such as pinholes and dust. W h e n a film is used as a waveguide, scattering losses are also dependent on the film thickness, the choice of substrate glass and whether a cover

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J. A. HAWTHORNE, S. N. HOUDE-WALTER, E. M. VOGEL

layer is used. The waveguide configurations reported here were not optimized to reduce scattering, however. Annealing is commonly used to reduce scattering by reflowing the film and promoting lateral diffusion. Initial measurements of the as-deposited films using an AlphaStep profilometer showed a typical r.m.s, film surface roughness of 75/~. Consequently, annealing was required before sufficiently good waveguiding could be achieved to make scattering loss measurements. Several anneal cycles were investigated. The first anneal cycle was a 50 K h ' r a m p up to 673K, a n S h s o a k , and cool to room temperature at15 K h l i n a 5 i n 2 (gauge) 10030 0 2 atmosphere. Out-of-plane scattering losses were measured by scanning a fiber along the surface of the waveguide I ~' at a wavelength of 633 nm. The scattering loss of the annealed fihn was reduced from more than 30dB cm ~ to 13.8 + 0.8 dB cm 1. Increasing the anneal temperature to 723 K, using the same soak time, ramp up and down rates and atmosphere, further reduced the loss to 8.0 + 0.7 dB cm 1. The annealing also reduced the r.m.s, film surface roughness to 38/~ Subsequent anneals at 773 K for 8 h and 793 K for 36 h, again with the same ramp rates and atmosphere, showed no further improvement on the 8 dB cm 1 loss value. The 723 K oxygen anneal, while decreasing scattering losses, also increased the refractive index slightly and increased the dispersion significantly. This is shown in Table V for film 18, which was sputtered under identical conditions and has identical optical properties to film 22. Consequently, the two films are expected to have the same compositions and Tables IV and V may be directly compared. The film compositions indicate that the diode-sputtered films using a 20'},{; oxygen ratio still had an oxygen-deficient stoichiometry. TABLE V COMPARISON OE REFRACTIVE INDEX A N D DISPERSION OE FILM ] 8 AS DEPOSITED AN[) AFTER AN 8 h A N N E A L AT 450

C U N D E R AN OXYGEN F L O W

Processing

tl d

Vd

Unannealed Annealed

1.662 1.672

34.2 21.4

Target

1.718

27.65

The normalized scattered power was modeled as out-of-plane scattering by surface roughness at the film-cover and film substrate interfaces 17. The out-ofplane scattering loss due to surface roughness was calculated for the annealed samples using an r.m.s, film roughness of 38/~, with a resultant predicted scatter loss of 3.4dB cm '. The experimental loss measurements are 2.3 times larger than the calculated value, suggesting that refractive index inhomogeneities exist within the film layer. In-plane scattering at a wavelength of 633 nm was characterized as well by measuring the intensity distribution along the "m line" coupled out of the waveguide ~6. This can readily be modeled as in-plane scattering by both surface roughness and film volume index variations 18. In-plane scatter due to the 38/i, r.m.s.

SPUTTERED TITANIA BOROSILICATE GLASS

329

surface roughness alone was calculated. (In-plane scatter due to index fluctuations were not included since index variations in the film could not be directly measured.) Again, the calculations showed that the measured scattered power is significantly greater (by a factor of 3) than what would be expected for a perfectly homogeneous film with a surface roughness of 38 ,~. By assuming that the total measured loss is due only to surface roughness and index variation scattering (i.e. no residual absorption), a value for the r.m.s, index variation was established by fitting the measured data to the in-plane scattering model. The r.m.s, index variation An was 0.0023, for an assumed correlation length of 1 ~tm 16 Further corroboration of scattering from within the film was sought by examining a fractured sample in a scanning electron microscope. The film layers appeared non-porous and topographically uniform (i.e. no voids) down to the resolution limit of the microscope (65',~). One might speculate that the films contain composition or density fluctuations that lead to the optical scatter, but more work needs to be done to identify the scattering sites.

3.3. Mechanical properties Internal mechanical stresses were tested qualitatively by thermally shocking the films. The films were taken from 293 K to 77 K by submersion in liquid nitrogen. Afterwards, the films were inspected with the aid of an optical microscope and also by propagating a guided wave and looking for an increase in scattering. No damage or increase in scattering was observed. Adhesion was tested by trying to peel the film offwith masking tape and trying to rub it off using an eraser, both before and after thermally shocking the films. The films did not delaminate.

3.4. Optical damage thresholds" The films were also tested for their laser damage threshold. Testing was performed using a neodymium-doped glass laser, operating at 1.05/am, with a m a x i m u m average energy output of 2 J and 1 ns pulse width. The film was repeatedly pulsed using various energy outputs and beam areas and inspected under an optical microscope for damage after each pulse. The film was able to withstand repeated pulses of average intensity 10 G W c m - 2 without signs of visible damage. The onset of damage was observed at an average intensity of 13.5 G W c m - 2 or a fluence of 13.5J cm -2. The average damage threshold for a single-layer film o f T i O 2 for 1 ns pulses at 1.06~tm is 4.73J cm 2 and the m a x i m u m damage threshold is 16J cm 2 (ref. 19). Thus the damage threshold of the E98C film is comparable with the damage threshold of a single-component film. A two layer antireflection (AR) coating was fabricated using a quarter-wave layer of E98C glass adjacent to the substrate and a quarter-wave layer of quartz place on top of the E98C layer. The film was designed to have a reflection coefficient of 0.3~o for a normal incident wave at 1.05 ~m, assuming the same refractive index as in the bulk for the sputtered SiO2 (the refractive index of the sputtered SiO2 layer was unknown). The reflection of the film was measured in a Perkin-Elmer spectrometer, calibrated to a quartz reference. The reflection was minimum at 1.16 lam and the magnitude was 1.9~o, or 60~o lower than for an uncoated quartz surface. The damage threshold of the two-layer AR coating was 7.2GW cm -2

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J. A. H A W T H O R N E , S. N. H O U D E - W A L T E R , E. M. VOGEL

average intensity or a fluence of 7.2 J cm 2. The average damage threshold of 1.06 gm AR coatings for 1 ns pulses and for a variety of coating materials is 9.2 J cm 2. Again the E98C glass AR coating damage threshold is comparable with those of the more traditional AR coating materials, such as AlzO 3, M g F > TiO2 and Z r 0 2 19 4.

SUMMARY AND CONCLUSIONS

This study shows that a glass containing a modifier of 20mol. ')4, TiO 2 can be sputtered and form a glassy film with TiO 2 concentrations as high as 47 mol.'}~,, well above the concentration obtainable in a batch melt glass. The extended glass forming region may be attributable to the rapid quench during the transition from the vapor phase to the solid phase at the substrate. Films sputtered with a magnetron cathode formed as the glass but these films were oxygen deficient, even when sputtered in 100% oxygen. The films sputtered using a diode cathode also formed as glasses and required only a 203~, oxygen partial pressure during sputtering. The increase in the film refractive index after an oxygen anneal indicates that the as-sputtered diode films were also oxygen deficient. The diode configuration did not cause target damage, but masking the chamber fixtures was required to avoid metal contamination of the films. Annealing in an resistive coil oven reduced the total film losses to 8 dB cm 1 when employed as an optical waveguide at a wavelength of 633 nm. Reconfiguration of the waveguide dimensions, use of a cover layer, and possibly laser annealing are expected to reduce these losses further. The films are extremely durable. The quantitative adhesion and stress testing showed that these films were able to handle thermal shock without damage. The laser damage thresholds of both single-layer and double-layer films were comparable with the damage thresholds of simple oxide films, i.e. on the order of 10GW cm 2. ACKNOWLEDGMENTS The authors are indebted to Jose Gallego of Pilkington Research for several helpful discussions on sputtering oxides, to Dennis Hall of the University of Rochester for the loan of the sputtering equipment, and to Ansgar Schmid and Semyon Papernov of the Laboratory of Laser Energetics, University of Rochester, for the high optical fluence damage testing. The authors are pleased to acknowledge the A R O - A F O S R Joint Services Optics Program for support of this project. REFERENCES

l J.E. Goell and R. D. Standley, Sputtered glass waveguide for integrated optical circuits, Bell Svst. Tech. J., 48 (1969) 3445. 2 C.W. Pitt, Sputtered-glass optical waveguides, Eh, ctron. Left., 9 (1973)401. 3 Y. Shimomoto, H. Matsumaru and T. Nishimura, Optical characteristics of Coming 7059 glass films deposited by rf sputtering, Jpn. J. Appl. Phys., Suppl. 2, (Part 1) (1974) 701. 4 D.J. Walter and J. Houghton,The roughness parameters of glass tilms, Vacuum, 27 (1976) 7.

SPUTTERED TITANIA BOROSILICATE GLASS

5 6 7 8 9 10

11 12 13 14 15 16 17 18 19

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C.W. Pitt, Thin fihn optical waveguides fabricated by ion-assisted techniques, Thin Solid Films, 86 (1981) 137. H. Jeromenik, S. Patela, J. Y. D. Pomerleau, C. Delisle and R. Tremblay, Some properties of r.f. planar magnetron-sputtered Corning 7059 glass films, Thin Solid Films, 146 (1987) 191. J.L. Vossen, Control of film properties by if-sputtering techniques, J. Vac. Sci Technol., 8 (1971) SI2. J.L. Vossen and W. Kern, Thin Film Processes, Academic Press, New York, 1978. S. Dutta, H. E. Jackson and J. T. Boyd, Extremely low-loss glass thin-film optical waveguides utilizing surface coating and laser annealing, J. AppL Phys., 52 ( 1981 ) 3873. E.M. Vogel, S. G. Kosinski, D. M. Kroll, J. L. Jackel, S. R. Friberg, M. K. Oliver and J. D. Powers, Structural and optical study of silicate glasses for nonlinear optical devices, J. Non-Cryst. Solids, 107 (1989) 244. T. Tamir, Beam and waveguide couplers. In T. Tamir (ed.), Integrated Optics, Topics in Applied Physics, Vol. 7, Springer, New York, 1979. K . C . Chang, V. Shah and T. Tamir, Scattering and guiding of waves by dielectric gratings with arbitrary profiles, J. Opt. Soc. Ant., 70 (1980) 804. P . N . Robb and R. 1. Mercado, Calculation of refractive indices using Buchdahl's chromatic coordinate, Appl. Opt., 22 (1983) 1198. J.C. Manifacier, J. Gasiot and J. P. Fillard, A simple method for the determination of the optical constants n, k and the thickness of a weakly absorbing thin film, J. Phys. E, 9 (1976) 1002. M.B. Volf, Chemical Approach to Glass, Elsevier, New York, 1984, Chap. 17. J . A . Hawthorne, Investigation of r.f. sputtered high refractive index glasses for planar optical waveguides, M.Sc. Thesis, University of Rochester, 1990. G . H . Ames, Attenuation in planar optical waveguides, M.Sc. Thesis, University of Rochester, 1982. D . G . Hall, In-plane scattering in planar optical waveguides: refractive-index fluctuations and surface roughness, J. Opt. Soc. Am. A, 2 (1985) 747. F. Rainer, R. P. Gonzales and A. J. Morgan, Laser damage database at 1064 nm, Bouhler Damage Syrup., Bouhter, Co, 1989.