Silicon nitride films were deposited by RF sputtering on 304 stainless steel substrates in a planar RF sputtering apparatus. The sputtering was performed from a Si3N4 target in a sputtering atmosphere of argon and nitrogen. The rate of deposition, the composition of the coatings, the surface microhardness and the adhesion of the coatings to the substrates were investigated as a function of the process parameters, such as: substrate target distance, fraction nitrogen in the sputtering atmosphere and sputtering pressure. Silicon rich coating was obtained for fraction nitrogen below 0.2. The rate of deposition decreases continuously with increasing fraction nitrogen and decreasing sputtering pressure. It was found that the adherence of the coatings improves with decreasing sputtering pressure, almost independently of their composition

Aron, P. R.

Grill, A.

English

NASA Technical Reports Server

NASA Technical Memorandum 82819 
Sputtered Silicon i4itride Coatings 
for Wear Protectio:~ 
A S - - 1 )  S P U T I E B E E  S l l l C G l i  b l l R 1 D E  162-203 14 
C O A T i P G S  POb YEAb EhC'IECTlCh (hASA) 10 p 
HC AOLJBF A 0 3  CSCL 070 Unclas  
G3/23 353b9 
Alfred Grill and Paul R. Aron 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
lntemational Conference on Metallurgical Coatings and Process Technology 
sponsored by the American Vacuum Society 
San Diego, California, April 4-9, 1982 
https://ntrs.nasa.gov/search.jsp?R=19820012440 2020-03-21T08:37:42+00:00Z
SPUTTERED SILICOR WITRIE COATIW6S FOR E A R  WK)lECTIO# 
by Alfred 6 r i l l *  and Paul R. Aron 
Alational Aeronautics and Space Administraton 
Lewis Research Center 
Cleveland, Ohio 
ABSTRACT 
Sil icon n i t r i de  f i lms were deposited by rf sputtering on 304 stainless 
steel substrates i n  a planar rf sputtering apparatus. The sputtering was 
perfomed fm a Sij l lq target i n  a sputtering atmosphere o f  arpon and 
nitrogen. The rate o f  deposition, the composition of the coatings, the 
surface mic. ohardness and the adhesion o f  the coat ings t o  the substrates 
have been investigated as a function o f  the process parameters, such as: 
substrate-target distance, fract ion nitrogen i n  the sputtering atmosphere 
PB and sputterinq pressure. Sil icon r i c h  coating have been obtained f o r  frac- s 
- 
I 
w t ion  nitroqen below 0.2. The rate of deposition decreases continuously with 
increasing fract ion nitrogen and decreasing sputtering pressure. It was 
found that the adherence of the coatinqs improves with decreasing sputtering 
pressure, a lmst  independent 1 y of the i r  composition. 
Sil icon n i t r i de  i s  characterized by high chemical s t ab i l i t y  and good 
dielectr ic and mechanical properties. As a coating it i s  mainly used i n  the 
microelectronic but i t i s  also used fo r  formaton of glass t o  
metal sealrS5 fo r  electrode insul t i o n  i n  high temperature chemistry and 
*Ben Gurion University o f  the Negev, Beer Sheva, Israel and National 
Research Counci 1 - NASA Research Associate. 
has beerr proposed as an ant iref lect ion coating f o r  solar thermal collec- 
torso6 I t s  ~ h a n i c a l  properties, especially i t s  high hardness7 
(ABO0 ~~r-*)  and i t s  chemical stabi l i ty,  mahe the s i l i con n i t r i de  a 
candidate for use as a wear resist ing coating f o r  metals. 
Sil icon n i t r i de  coatings have been produced by glow discharge 
1nethods.8*~ CVD," by sputterinq from s i l icon n i t r i de  t a q t s  6-11 or 
by reactive sputtering from s i l i con target.'* Due t o  the d i f f i cu l t i e s  o f  
controll ing i q u r i t y  levels and stoichiaaetry,6 sputtering methods are not 
generally used f o r  microelectronic applications o f  t h i s  material since the 
relevant electr ical  properties are very sensitive t o  these parameters. RF 
sputtering does, however, o f fe r  the advantaqe of low substrate temperature. 
This feature can be of consequence i n  the application of wear resistant 
coatings since the mechanical properties o f  the substrates may be affected 
by high temperatures. 
The aim of the present restkrch i s  t o  investigate the wear properties 
o f  s i l icon n i t r i de  coatings deposited by rf sputtering from a s i l i con 
n i t r i de  target and t o  relate these properties t o  the parameters of the 
deposition process. 
2. EXPERIMENTAL 
The s i l icon n i t r i de  coatings were deposited i n  a diode type rf sputter- 
ing system from a 15 cm diameter cofflrnercial high pur i t y  s i l icon n i t r i de  
target prepared by powder technology. The substrates were of 304 stainless 
steel polished with 1 urn diamond. The sputter chamber was i n i t i a l l y  pumped 
t o  less than 5*10-~ Pa with a LN2 trapped o i l  diffusion pump. The 
deposition was performed at a rf power density of 2.7 ~ * c r n - ~  i n  flowing 
gas mixtures of A r  and N2. Since the pumping speed was di f ferent for A r  
*Id N2 i n  the working pressure rage, the fract ion nitrogen has been 
calibrated with a mass spectrometer for di f ferent gas mixtures and to ta l  
pssures.  The to ta l  pressure was kept constant during deposition by a 
Veeco APC 1000 pressure controller. 
The rate of deposition was determined by measuring t t z  height of  a 
step, formed by masking a part of the substrate during dtqwsition, with a 
profi lomter. The microhardness o f  the coated surface was measured with a 
Knoop d i m d  indenter at  a load o f  1.02xl0-~ 1 (10 9). 
The f i l m  composition was determined with Auger Electron Spectroscopy 
(AES) while simultaneously sputterinq with 3 kV ~ r *  ions, t o  remove con- 
taminated surface layers u n t i l  a constant spectrum was obtained. Although 
preferential sputtering i s  e ~ ~ e c t e d ~ ' ~ ~  and absolute caposit ion cannot 
be obtained i n  th is  way, the relat ive dependence of the composition on 
deposition conditions should be exposed. Due t o  the high res is t i v i t y  of the 
f i lms it was necessary t o  work with f i lms less than 0.1 pm thick t o  avoid 
charging problems. AES measurements were made i n  a baked u l t r a  high vacuum 
system which was ion pumped to  <2.7~10-~ Pa then backfi l led with u l t r a  
high pur i ty  argon t o  5 - 1 4 ~  Pa. A comercia1 single pass cylindrical 
mirror analyser with an integral electron gun was used. The gun was 
operated at a voltage of 2 kV and a currezt of 2 vA. The detection system 
was operated i n  the dN/dE mode with a modulator amplituoe of  *l volt. 
The adhesion of  the di f ferent f i lrns to  the substrates was cornpared by a 
scratch test. The scratch was produced on the coated surface by sl iding a 
spherical diamond indenter of radius 86 urn over the surf ace under a load of  
0.0102 N. The formed tracks were examined by scanninq electron microscopy. 
The inf lumce of the following parameters on the deposition process and 
properties of the coating were investigated: 
(1) Target-substrate distance (3.3 and 4.5 cm) 
(2) Fraction nitrogen i n  the A r  + N2 sputtering gas (0 t o  0.1) 
(3) Total pressure i n  the sputtering chamber (1.1 and 2.9 Pa) 
3. RESULTS A#0 DISCUSSION 
The thickness measurea#nts indicated that f o r  a1 1 considered condi- 
tions, the deposition rate was about 30 percent lower a t  e target-substrate 
distance of 4.5 cm than at 3.3 cm. The AES measurements showed that there 
i s  no composition difference between the coatings deposited a t  the two 
target-substrate distances. Since 1.2 cm thick disks have t o  be coated for 
the avai 1 ~ b l e  wear test  apparatus, the f 01 lowing characterization was per- 
formed only f o r  coatings deposited at a target substrate distance of 3.3 cm. 
Rate o f  Deposition 
Figure 1 presents the rate of deposition as a function of fract ion 
nitrogen, FN , i n  the sputtering atmosphere, at two sputtering pressures. 
2 
These rates were determined fo r  coatings deposited f o r  1 hr. A gradually 
decreasing rate of deposition, with increasing f i s  observed at both 
N2 
pressures. An increase of fN over 0.25 does not result  i n  a severely 
2 
reduced rate of deposition as reported by ~ o m i n i a k . ~  The rate of deposi- 
t i on  i s  about 70 percent higher at 2.9 Pa than at 1.1 Pa a t  a l l  gas 
compositions. 
Cornposi t ion 
The AES data with respect t o  composition i s  sunmarized i n  Fig. 2. I n  
Fig. 2(A) the ra t i o  of the nitrogen t o  s i l i con peak height (dNldE) i s  
plotted as a function o f  fraction nitrogen. These rat ios are uncorrected 
f o r  sens i t i v i t y  factors o r  preferent ial  sputtering effects. The data does 
demonstrate that  f o r  fi , 0.2 the s i l i con  t o  nitrapen r a t i o  i s  indcpendcnt 
2 
o f  f~ i n  agrcclnent wi th  the work o f  ~ominlak.6 &sides nitrogen and 
2 
s i l i c o n  a sametimes pwnninent feature o f  the BES spectrum was the KLL ox- 
peak a t  508 kV. Figure 2(B) exhibi ts the range of the ra t ios  b e t e n  the 
oxygea and s i l i con  peak imp1 itudes observed i n  t h i s  work. Wo consistent 
dependence on t o t a l  sputtering pressure was observed. It does appear how- 
ever that the oxygen impurity level  tends t o  increase with fi( i n  roughly 2 
the same manner as the deposition ra te  decreases (Fig. 1). The only other 
signif icant AES peak seen was due t o  carbon a t  272 eV, t yp i ca l l y  o f  an 
amplitude that  would suggest a contamination level  of a t  most a few per- 
cent. It has been shown,15 however, that the carbon seen under these 
conditions may be an ar t i fac t  o f  the methane background i n  the AES vacuum 
system. 
Figure 3 presents the microhardness o f  the coated surfaces as a func- 
t ion o f  f ract ion nitrogen i n  the sputtering atmosphere. This microhardness 
was measured on coatings about 1 urn thick, thus the presented resul ts do not 
represent the hardness o f  the coating but a combination o f  hardness of 
coating and substrate. It can be seen that the hardness of the coated 
surface i s  about 650 Kg mn-' as compared t o  the substrate hardness of 
300 Kg R o u t  the same hardness was obtained f o r  coatings deposited 
a t  both 1.1 Pa and 2.9 Pa f o r  f below 0.15. It was not possible t o  mea- 
N2 
sure the microhardness o f  samples coated a t  2.9 Pa and f above 0.20 since 
N2 
the coating developed cracks around the indentation. These cracks suggest 
low adhesion between the corresponding coatings and substrates. 
Adhesion 
The scratch tes t  was performed on f i lms about 1 um thick. The scratch 
tests on samples deposited a t  2.9 Pa resulted i n  breaking up the coating 
over a wide area around the s l id ing  track and a scratch track i n  the s u b  
strate. This behavior i s  independent o f  the f and composition. 
N2 
A typ ica l  scratch track i s  shown i n  Fig. 4(A). Scratches produced on coat- 
ings deposited a t  1.1 Pa resulted i n  only a p a r t i a l  cracking o f  the coating 
as seen i n  Fig. 4(B). The diamond indenter d id  not scratch the substrate. 
Again, the behavior was the s r  f o r  a l l  fN investigated. 
2 
The difference between the behavior of  the coatings deposited a t  the 
two different pressures indicates that  the coatings deposited a t  the lower 
pressure have better adherence than these deposited at  the higher pressure. 
This i s  consistent with the microhardness measurements. However the d i f fe r -  
ent behavior o f  the coatings deposited a t  the two d i f ferent  pressures may 
also be a resu l t  o f  d i f ferent  internal  stresses i n  these coatings. I n  order 
t o  check the ef fect  of  the thickness o f  the coating on the adhesion o f  the 
coating, a 0.5 m thick coating was deposited a t  2.9 Pa and fN = 0.25. 
2 
This coating, too, developed cracks around the indentation produced by the 
microhardness tester and showed the same scratch track as the thicker coat- 
ing. This resul t  indicates that  o f  a l l  investiqated parameters the t o t a l  
pressure of the sputtering atmosphere i s  the most important parameter 
affecting adhesion of the coatings t o  the substrates. The coatings de- 
posited a t  the lower pressure may be able t o  protect the metal 1 i c  substrate 
against wear better than the coatings deposited at  the higher pressure. 
4. CO#CLUSIOIQ 
Si l icon n i t r i d e  coatings of different compositions have been deposited 
by rf sputtering i n  gas mixtures o f  A r  + N2. It was found that: 
1. The canposition o f  the coatinq i s  not dependent on the substrate- 
target distance. 
2. The microhardness o f  the coated surf ace and the adhesion o f  the 
coating t o  the substrate i s  almost independent o f  the concentration o f  
nitrogen i n  the sputtering atmosphere and on the composition of the coating. 
3. The adhesion o f  the coating t o  the substrate increases with de- 
creasing sputtering pressure. 
4. The adhesion o f  the coating t o  the substrate i s  not dependent on 
sputtering ra te  and on the composition o f  the sputtering atmosphere. 
REFERENCES 
1. H. Seki, S. Ohosaka, M. Kanda, Y. Kawasaki, H. Yamazaki and 
M. Fujimoto, Rev. Elec. Comun. Lab, 20 (1972) 810. 
2. 0. Swaroop, Proc. Annual Conf. on Electr ical  Insulation and Die lect r ic  
Phenomena, Buck H i l l  Falls, Pa., IEEE, Washington, DC (1972) 75. 
3. P. K. Chaudari and 0. C. Beyar, 3. Electrochein. Soc., 121 (1976) 89C. 
4. M. Sekimoto, H. Yoshihara, T. Ohkubo and Y. Saitoh, Jap. 3. Appl. 
Phys., 20 (1981). L669. 
5. A. I. Stoller, W. Schilp and 3. Benbenek, RCA Rev., 31 (1970) 443. 
6. 6. 3. Kominiak, 3. Electrochem. Soc., 122 (1975) 1271. 
7. Engineering Property Data on Selected Ceramics, Volume I: Nitrides, 
Bat te l le  Colufflbus Labs., March 1976, NTIS AD-A023773, p. 47. 
8. W. A. Pliskin, 3. Wac. Sci. Technol., 14 (1977) 1064. 
9. A. Rheinberq, 3. Electrochem. Soc., 121 (1976) 85C. 
10. P. C. Pareckh, Solid-State Electron., 16 (1973) 954. 
11. 6. Reisse, W. Gunther and F. Henny, Wiss. Zed. Techn. Hochsch., 
Kar l  Marx Stadt, 20 (1978) 835. 
12. C. J. nogab and E. Lugujjo, 3. hppl. Phys., 47 (1976) 1302. 
13. S. Thomas and R. 3. Mattox, J. Electrochem. Soc., 124 (1977) 1942. 
1 J. F. Delord, A. 6. Schrott and S. C. Fain, Jr., 3. Vac. Sci. Technol., 
17 (1980) 517. 
15. S. Ingrey, M. Johnson, and R. W. Streater, American Vacuum Society 28th 
National Synposium, Anaheim, California, Now. 2-6, 1981. 
FIGURE CAPTIONS 
Figure 1. - Rate of deposition vs. fraction nitrogen in Ar + N2 sputtering 
atmosphere. 
Figure 2. - The ratios of the intensities of the AES peaks of nitrogen to 
silicon and oxygen to silicon vs. fraction nitrogen in the Ar + N2 
sputtering atmosphere. 
Figure 3. - Surface microhardness vs. fraction nitrogen in sputtering 
atmosphere. 
Figure 4. - SEM micrographs of scratch tracks on si 1 icon nitride coated 
304 SS samples. 
A. coating deposited at 2.9 Pa 
8. coating deposited at 1.1 Pa. 
Flgure L - Rated @osHbn vr, fradion nitmgan in Ar + N2 
sputterins atmosphera 
L2- 
LO 
.8 
- 
P .6 
-- 
- 
3 - 1% SPUTILRNG $ 
PRESSURE 
0 LlPa 
0 29Pa 
0 15-Z5Pa 
0 -  I 1 I . I  I I I 1 I 
0 .1  .2 3 .4 5 .6 .7 .8 
(bl 
' ~ 2  



Sputtered silicon nitride coatings for wear protection

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