Thin Solid Films, 64 (1979) 433-438 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands 433

THE BORIDING OF CHROMIUM PHOTOMASKS FOR INCREASED ABRASION RESISTANCE*

STEVE LONG AND G. E. McGUIRE~"

Texas Instruments Incorporated, P.O. Box 225936, Dallas, Texas 7522 (U.S.A.)

(Received April 9, 1979; accepted April 26, 1979)

In the photolithographic processing of semiconductor material, 700-1000 A of chromium on soda-lime or borosilicate glass is the most commonly used masking material. During contact printing the chromium surface may become scratched or abraded so that the opaque regions lose the ability to block actinic radiation. The gas phase diffusion of boron into chromium is used to increase the hardness of the photomask surface and to improve the wear resistance. The boronizing process confers great resistance both to sliding wear and to abrasive wear. Boriding at elevated temperatures on a graphite substrate decreases the warpage of the photomask blank and improves the overall quality of the photomask without introducing defects or without affecting the plasma etching characteristics of the chromium. Thus the standard photomask fabrication techniques of resist application, patterning and etching may still be applied after boriding treatment to produce a longer life photomask.

1. INTRODUCTION

Diffusion of boron into the surfaces of various metals and alloys results in the formation of metallic borides which provide extremely hard wear-resistant and erosion-resistant surfaces 1-3. Boronization reduces both sliding wear and abrasive wear of mild steels by up to three orders of magnitude 4. A resistance to acids 4 and to flowing pressurized water 5 is imparted to mild steel and ferrous materials.

Boron diffusion has been achieved bymeans of a high temperature electrolytic process 6'7, by pack cementation 5 with powders and paste and by gas phase boriding a. Diffusion is typically carried out at temperatures in excess of 900 °C; however, low temperature liquid boriding has been carried out at temperatures in the range 550-700 °C 9

Various steels are the usual materials that are boronized; however, cobalt- based alloys10 and nickel, chromium and tungsten 11 have been successfully treated. In this study the low temperature gas phase diffusion of boron into 1000 A films of chromium on glass substrates was accomplished.

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 23-27, 1979. t Present address: Tektronix, Inc., Delivery Station 50-289, P.O. Box 500, Beaverton, Oreg. 97077, U.S.A.

434 s. LONG, G. E. McGUIRE

The photolithographic processing of semiconductor products requires the use of high quality masking materials to define certain features of the device. The basic requirements are that the film should be durable, scratch resistant and thermally stable, that it should adhere well to glass, that it should be resistant to attack by organic cleaners and that it should block the actinic radiation necessary for photoresist work. The most commonly used material is 700-1000 A chromium on soda-lime or borosilicate glass plates. Even though chromium fulfils the major requirements ofa photomask coating, the chromium surface does become scratched and damaged during usC 2. This adversely affects images produced from the photomask.

Several methods have been utilized to protect the chromium surface from damage: the application of a soft or lubricating material to the surface13; alternatively the use of an ultrahard protective overcoat such as SiaN, or SiC 12. In this study a hard wear-resistant surface is produced by the gas phase diffusion of boron into the chromium.

2. EXPERIMENTAL

Chromium films 700-1000/~ thick were sputter deposited from a Materials Research Corporation model 8800 sputtering system onto (60-90) x 10-3 in soda- lime glass plates. The chromium-coated plates were cleaned by the standard photomask clean-up prior to being placed horizontally on a graphite boat; they were then loaded individually into an Applied Materials Silox Reactor model 2000. The chromium blanks were loaded in the same configuration each time to control the variable gas flow across the surface. Diffusion of boron was preceded and followed by a 3 min N z purge. One complete cycle was carried out before introducing the chromium blanks in order to purge the system thoroughly. Semiconductor-grade 1~o diborane in argon, flowing at a constant 0.868 1 min-1, provided the source of the boron. The 1~o diborane in argon was diluted by N z at a flow rate of 18.7 1 min -1 or 30.3 1 min -1. The diffusion temperature was varied between 375 and 600 °C while the diffusion time was varied from 1 min to 90 min.

After treatment, the plates were tested for abrasion resistance using a 600 grit SiC powder obtained from Buhler Ltd. which was ground into the chromium surface by the tip of a steel rod 0.318 cm in diameter rotating at 1 rev min- 1 for 3 min under a 200 gfload. The test produced circular scratches in the chromium film, the severity of which is indicative of the abrasion resistance of the chromium. Each plate was washed free of the SiC powder and was blown dry. A transmission densitometer was used to measure the minimum optical density N of the plate in the region of the abrasion pattern and also to measure the average optical density N O in regions adjacent to the abrasion pattern. The average optical density was used because the optical density varied over the plate.

A parameter a, which will serve as a measure of abrasion resistance, is expressed a s

a = X / A (1)

where A is the area exposed to the incident light and X is the portion of the area A which has been abraded away. Assuming 100% transmission through the abraded

BORIDING OF Cr PHOTOMASKS 435

portion of A, then

transmission = transmission in the unabraded area of A times the fraction of area A unabraded

+ transmission in the abraded area of A times the fraction of area A abraded

1 0 0 x l 0 - N _ AAX (IOOx lO-No)+X Io0

This equation satisfies the following boundary conditions: (i) w h e n X = 0 , N = N O

(ii) when X = A, N = 0 Solving for a yields

10-N_ 10-No - 1 - 1 0 -N°

~, as determined by measuring N and No, was found to correspond to the appearance of photomicrographs that were taken of abrasion patterns.

The flatness was measured before and after diffusion on an air gauge flatness tester accurate to +0.6 ~tm. The test involved placing the beveled corner of the plate in the lower left-hand comer of the holder and taking five measurements, one at each corner and one measurement at the center of the plate. If the spread in the five readings is less than 3 ~tm, then the plate is classified as master grade; if the spread is between 3 and 10 ~tm, then the plate is classified as print grade.

3. RESULTS AND DISCUSSION

The dependence of the abrasion resistance on the diffusion time is demonstrated in the series of photomicrographs in Fig. 1 which show the results of abrasion tests on an untreated plate and on plates diffused for 3, 9 and 15 min at 375 °C. Both the number of scratches and the area over which gouging occurs decreased markedly with increasing diffusion time. If the negative natural logarithm of the abrasion parameter ~ is plotted against the diffusion time as in Fig. 2, a sharp initial increase in abrasion resistance is observed which levels off after a period tma x of approximately 15 min. By varying the flow of N z, the diluent gas, from 18.71 min - 1 to 30.31 m i n - ~, tma x was shifted. These limited data suggest a possible dependence of tma x o n the diborane partial pressure over the range of pressures investigated.

The relationship between the abrasion resistance and the diffusion temperature for a fixed time of 6 min is shown in Fig. 3. At higher temperatures the abrasion resistance shows a slight decrease. Using Auger spectroscopy and in-depth profiling it was possible to show an increase in the amount of silicon in the chromium layer due to diffusion from the substrate. The decrease in abrasion resistance at higher temperatures has been associated with the increased silicon content. The lack of any major dependence of the abrasion resistance on the temperature suggests that the process is limited by the transport of boron through the gas at the surface and not by solid state diffusion.

Although it was anticipated that significant warpage would occur during heat

436 s. LONG, G, E. McGUIRE

(a) (b)

(c) (d)

Fig. 1. The dependence of the abrasion resistance on the diffusion time is demonstrated in the photomicrographs of chromium blanks abraded using SiC grit after boriding at 375 °C: (a) t = 0 min; (b) t = 3 min; (c) t = 9 min; (d) t = 15 min. The test produced circular scratches in the chromium film, the severity of which decreased with increasing diffusion time.

- I n ~

6.0--

S.0--

4 . 0 - -

I I 10 20

I I ~ I I I I 30 40 SO 60 70 80 90

Dif fusion Time (min.)

Fig. 2. When the negative natural logarithm of the abrasion parameter X/A is plotted against the diffusion time, a sharp initial increase in abrasion resistance is observed which levels offafter a period of approximately 15 min (the boriding temperature was 375 °C).

t r ea tmen t , this d id n o t t u r n o u t to be the case. In genera l , m a s t e r - g r a d e p la tes were still c lassif ied as m a s t e r g r ade af ter t r ea tmen t . T h e d a t a in T a b l e I s h o w the f la tness

tests for n ine p la tes m e a s u r e d before a n d af ter bor id ing . W i t h few excep t ions the

B O R I D I N G O F Cr P H O T O M A S K S 437

lowest of the five measurements on each treated plate was the center measurement; however, no such relationship was found on the plates before treatment. The changes in flatness that occur possibly arise from stress relief at the diffusion temperature where the plate molds itself to the fiat graphite boat. Any thermal stresses that occur when the plate is withdrawn from the furnace would be small and fairly uniform from plate to plate.

5 . 5 - -

5 . 0 - -

I I I I I 350 400 450 500 550

Temperature (°C)

Fig. 3. When the negative natural logarithm of the abrasion resistance parameter X/A is plotted against the diffusion temperature, a max imum is observed at approximately 415 °C (t = 6 min). The dependence of the abrasion resistance parameter on temperature is believed to be due to the transport of boron through the gas phase at the chromium surface and to the solid state diffusion of boron and silicon into the chromium.

TABLE I FLATNESS MEASUREMENT OF PHOTOMASK

Plate no. Temperature Time Spread before diffusion Spread after diffusion (°C) (min) (Itm) (~tm)

1 375 1 1.3 0.5 2 375 3 2.0 1.0 3 375 5 2.8 1.8 4 375 7 0.8 2.0 5 375 11 0.8 1.0 6 400 5 2.0 2.8 7 425 5 2.5 1.3 8 450 5 2.5 1.0 9 475 5 1.0 2.3

In order to check the etching characteristics of the borided chromium blanks, plates were coated with photoresist, were patterned by electron beam lithography, were plasma etched and were then cleaned in an 02 plasma. There was no detectable difference in the plasma etch rate of the pure chromium and the borided chromium. Wet etching was not investigated.

The defect density was examined both before and after boron diffusion without any observable difference. Visibly there is no difference except when oxygen is present in the gas phase which results in a thin oxide film on the chromium surface. Boriding was also carried out on a patterned chromium photomask in anticipation of differences that might occur in the plasma etch rate. Chemical attack along the edges of the chromium features during boriding resulted in rough irregular borders.

438 s. LONG, G. E. McGUIRE

The borders appear to offer sites for preferential interaction. On a cont inuous sheet of ch romium no preferential at tack was observed.

Boron diffuses interstitially into the ch romium lattice, forming a boron-r ich surface layer composed mainly of diborides 11. The initial effect increases the hardness of the chromium; however, the strain produced in the ch romium lattice by the presence of bo ron is reversible and may be relieved by the out-diffusion of bo ron at room temperature over an extended period of time. An abrasion test of borided chromium after a period of 3 months showed a decrease in the abrasion resistance over the initial value shortly after treatment.

4. CONCLUSION

The gas phase diffusion of bo ron into a chromium pho tomask was shown to increase the hardness of the pho tomask surface and to improve the wear resistance, Boron diffusion may be accomplished at temperatures as low as 375 °C whereas the diffusion time is dependent on the partial pressure of diborane. Boriding does not alter the plasma etching characteristics of the chromium nor does it introduce addit ional defects. The thermal cycle relieves stresses in the chromium pho tomask so that warpage is decreased, resulting in a master-grade mask with an extended useful life against abrasive wear.

ACKNOWLEDGMENTS

The authors would.l ike to express their gratitute to George DeJean, Rama Shaw and James Yuan for valuable technical assistance provided during this investigation.

REFERENCES

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