Amorphous hydrogenated carbon (a-C:H) films were deposited on silicon and quartz substrates by a 30 kHz plasma discharge technique using methane. Rapid thermal processing of the films was accomplished in nitrogen gas using tungsten halogen light. The rapid thermal processing was done at several fixed temperatures (up to 600 C), as a function of time (up to 1800 sec). The films were characterized by optical absorption and by ellipsometry in the near UV and the visible. The bandgap, estimated from extrapolation of the linear part of a Tauc plot, decreases both with the annealing temperature and the annealing time, with the temperature dependence being the dominating factor. The density of states parameter increases up to 25 percent and the refractive index changes up to 20 percent with temperature increase. Possible explanations of the mechanisms involved in these processes are discussed

Alterovitz, Samuel A.

Pouch, John J.

Warner, Joseph D.

English

NASA Technical Reports Server

~ 
~ P ~- ~~ 
I, u 
L 
NASA Technical Memorandum 89859 
Rapid Thermal Annealing of Amorphous 
Hydrogenated Carbon (a-C:H) Films 
Samuel A. Alterovitz, John J. Pouch, and Joseph D. Warner 
Lewis Research Center 
Cleveland, Ohio 
(ldasli-TM-89859) EidPIC TBE6EAL APYEALIIJG CP N87-20821 
AMIBPHOUS RPDEOCEEAIEE CAEECA {a-C:H) EILHS 
I ( U A S A )  e p C S C L  20L _- uncias  
63/76 45385 
Prepared for the 
Anaheim, California, April 2 1-25, 1987 
t 1987 Spring Meeting of the Materials Research Society 
L 
https://ntrs.nasa.gov/search.jsp?R=19870011388 2020-03-20T11:53:45+00:00Z
R A P I D  THERMAL ANNEALING OF AMORPHOUS HYDROGENATED CARBON (a-C:H) FILMS 
SAMUEL A. ALTEROVITZ, JOHN J. POUCH, AND JOSEPH D. WARNER 
N a t i o n a l  Aeronaut ics  and Space A d m i n i s t r a t i o n ,  Lewis Research Center,  
Cleveland, Ohio 44135 
ABSTRACT 
Amorphous hydrogenated carbon (a-C:H) f i l m s  were d e p o s i t e d  on s i l i c o n  
and q u a r t z  s u b s t r a t e s  by a 30 kHz plasma d i scha rge  techn ique  u s i n g  methane. 
Rapld thermal  p rocess ing  o f  t h e  f i l m s  was accompl ished i n  n i t r o g e n  gas u s l n g  
t u n g s t e n  halogen l i g h t .  The r a p i d  the rma l  p rocess ing  was done a t  s e v e r a l  
f i x e d  temperatures (up t o  600 "C), as a f u n c t i o n  o f  t i m e  (up t o  1800 sec) .  
The f i l m s  were c h a r a c t e r i z e d  by o p t i c a l  a b s o r p t i o n  and by e l l i p s o m e t r y  i n  
m t h e  near UV and t h e  v i s i b l e .  The bandgap, es t ima ted  f r o m  e x t r a p o l a t i o n  o f  
LD t h e  l i n e a r  p a r t  o f  a Tauc p l o t ,  decreases b o t h  w i t h  t h e  a n n e a l i n g  tempera- 
I t u r e  and t h e  annea l i ng  t ime,  w i t h  the temperature dependence b e i n g  t h e  domi- 
n a t i n g  f a c t o r .  The d e n s i t y  o f  s ta tes  parameter i nc reases  up t o  25 pe rcen t  
and t h e  r e f r a c t i v e  i ndex  changes up t o  20 pe rcen t  w i t h  temperature i nc rease .  
P o s s i b l e  exp lana t ions  o f  t h e  mechanisms i n v o l v e d  i n  these  processes a r e  
d iscussed.  
l-l 
m 
w 
INTRODUCTION 
Rapid thermal  p rocess ing  i s  an emerging technology.  I t  has seve ra l  
a p p l i c a t i o n s  i n  e l e c t r o n i c  m a t e r i a l s  [1,2]  m o s t l y  a t  r e l a t i v e l y  h i g h  temper- 
a t u r e s  (>lo0 "C). However, i n  many cases, p rocess ing  a t  temperatures below 
600 " C  f o r  t imes o f  20 sec o r  lower a r e  adequate. Some examples o f  t h e  
l a t t e r  are:  ohmic c o n t a c t s  on 111-V compound semiconductors [3 ] ,  improv ing  
m e t a l i z a t i o n  p r o p e r t i e s  on S i  [ 4 ]  o r  r e d u c t i o n  o f  t r a p p i n g  i n  a S i02  
i n s u l a t i n g  f i l m  [5]. 
a t  moderate temperatures (<600 "C) on amorphous hydrogenated carbon (a-C:H) 
f i l m s .  T h i s  i s ,  t o  our  knowledge, t h e  f i r s t  r e p o r t  o f  a s tudy  o f  t h e  
e f f e c t s  o f  a c'ombination o f  t i m e  and temperature on t h e  p r o p e r t i e s  o f  a-C:H 
f i l m s .  Most o f  t h e  c h a r a c t e r i z a t i o n  has been done u s i n g  o p t i c a l  a b s o r p t i o n  
i n  t h e  v i s i b l e .  I n  a d d i t i o n ,  thermal annea l i ng  e f f e c t s  ob ta ined  f o r  s h o r t  
p rocess ing  t i m e  (20  sec) have been s t u d i e d  by e l l i p s o m e t r y .  T h l s  work w i l l  
p r o v i d e  much needed !nformat!on f o r  a p p l i c a t i o n s  o f  a-C:H f!!ms where rap3d 
the rma l  p rocess ing  may be r e q u i r e d .  One such a p p l i c a t i o n  i s  i n  111-V semi- 
conduc t ing  e l e c t r o n i c  m a t e r i a l s ,  where a-C:H f i l m s  show p r o m i s i n g  r e s u l t s  
I n  t h i s  s tudy we w i l l  p resen t  t h e  e f f e c t s  o f  r a p i d  the rma l  p rocess ing  
L6.71. 
EXPERIMENTAL 
Samples were prepared on quar t z  and S i  s u b s t r a t e s  u s i n g  a 30 kHz plasma 
d e p o s i t i o n  u n i t .  Measurements o f  the f i l m  p r o p e r t i e s  were done u s i n g  a 
U V - V i s  spectrometer and a r o t a t i n g  ana lyze r  e l l i p s o m e t e r .  D e t a i l s  o f  t h e  
p r e p a r a t i o n  and c h a r a c t e r i z a t i o n  techniques were r e p o r t e d  elsewhere [ 8 ]  and 
w i l l  n o t  be repeated here.  The only change i n  t h e  c h a r a c t e r i z a t i o n  tech-  
n i q u e  was t h e  use o f  a Xe lamp as l i g h t  source f o r  t h e  e l l i p s o m e t e r ,  i n s t e a d  
o f  t h e  Hg lamp. We used a 4200 A cu t  o f f  f i l t e r  t o  e l i m i n a t e  h i g h  energy 
s t r a y  l i g h t  f o r  measurements above t h i s  wavelength.  
The r a p i d  h e a t i n g  module [9] has a r e c t a n g u l a r  q u a r t z  chamber, 5 i n .  
wide, 8.5 i n .  long,  and 1 i n .  h igh .  I t  i s  heated by a microprocessor  con- 
t r o l l e d  bank o f  1 .5  kW tungs ten  halogen lamps, w i t h  seven lamps on t o p  and 
1 
six on the bottom. Two mass flow controllers were used to control the proc- 
essing gas. We used dry nitrogen gas with flow rates of order two liter/ 
minute. The sample holder i s  a 3 in. quartz rlng on which we placed a S i  
wafer. To enhance light absorption, the S i  wafer was coated with 200 A Ta 
followed by 3000 A Au. 
ing the metallic coating. The temperature was measured by a type K ther- 
mocouple in contact with the bottom of the S i  wafer. 
one-half inch of contact between the two. A typical 600 "C heating cycle 
i s  shown in Fig. 1. We have carefully avoided temperature overshoot. The 
heating rate was of order 40 "C/sec for 600 "C processing, with lower values 
. for lower temperatures. For long processing times (1 min. or more), temper- 
ature oscillations of order +-5 "C were observed. Samples were processed 
with both the film pointing upward and downward. The heating current used 
for processing quartz or S i  substrates were different, as the quartz plate 
has much more heat capacity than the S i  substrate. 
The sample was placed on top of the S i  wafer, touch- 
There i s  at least 
I I I I I I I I I 
T I E E .  SEC 
FIGURE I .  - TWERATURE VERSUS TI* FMI 80 SEC. 600 O c  RAPID THERM PROCESSING OF a -  C:H 
F ILM ON OUARTZ. 
RESULTS 
The samples used in this study were prepared in two deposition runs. 
Both runs were nominally equal, using 150 W, 70 sccm flow rate methane 
plasma (corresponding to 315 mrn). Absorption in the UV-visible range was 
measured on quartz substrates. Absorbance versus wavelength for samples 
processed 20 and 160 sec and the reference, are shown in Fig. 2. The opti- 
cal bandgap Eo was found [lo] using a Tauc plot, i.e. (AE)1/2 versus 
E, where E I s  the energy. For amorphous materials, this plot i s  expected 
to be linear. A ty ical plot i s  shown in Fig. 3. We can see that the line- 
processing. However, the slight difference in absorption observed in Fig. 2 
between the samples heated for 20 and 160 sec, i s  now quantitatively 
described by a change in the optical bandgap 
range of (AE)1/2 versus E the same for all isothermal runs. This proce- 
dure gave us both consistency in comparing results for different samples and 
the best least square fits to linear dependency. 
the quartz substrate reference samples made in the two deposition runs were 
2.4 and 2.6 eV. Results of the optical bandgap on quartz substrates versus 
the processing time are shown in Figs. 4(a) and 4(b). There i s  a slight 
decrease of Eo 
runs. 
arity range of (AE) ! I2 versus E i s  markedly reduced with the thermal 
Eo. We kept the fitting 
The optical bandgaps of 
at longer processing times, especially for the 400 "C 
2 
4 
I 4 
8000 10 ooo 12 ooo 
YAMLENGTH, i 
FIGURE 2. - A B S M I W E  A VERSUS WAVELENGTH FOR a - C:H FILBS ON WART.?, ANNEALED AT 
500 OC FOR 20 AND 160 SEC. AND A REFERENCE. 
N : 2t  
N 
0 
/ r m  SEC 
E. eV 
FIGURE 3. - TAUC PLOT OF ( ~ ~ 1 ' 2  VERSUS ENERGY E FOR a - C:H FILE ON QUARTZ. 
ANKALED AT 500 OC FOR 20 AND 160 SEC. AND A REFERENCE. T H  STRAIGHT L I E S  SHOW 
THE FlnlNG RANGE. 
3 
TEIBERATURE. 
OC 
0 400 
3.0 r 0 600 
2 . 5 F  c 
T I E .  SEC 
(A) SHCRT T I E  ANNEALS. (6) L W  T I E  A N W S .  
FIGURE 4. - OPTICAL ENERGY GAP Eo MRSUS ANKALING T I E  FOR a - C:H F I L S  ON WARTZ. 
ANNEALED AT TWO EWERATURES. 
Ellipsometric measurements were done on a-C:H films on Si substrates. 
q and A results were inverted using a three-phase model [ l l ]  to obtain 
the refractlve indices n, extinction coefficients K (at each wavelength), 
and the thickness. 
processed samples, gives excellent fits. The results for K were analyzed 
in the form of (~E)1/2 versus E, as shown in Fig. 5. Here u is the 
absorption coefficient, u = 4nK/X. 
smaller than for the films on quartz substrates. 
versus E graph, also denoted 13, is called the density of states parameter 
[8,10]. 
Table I. 
We found that the three phase model, especially for 
The optical bandgaps found were markedly 
The slope of the (aE)1/2 
The values obtained for 6 ,  Eo, and the thicknesses are given in 
Results of the refractive index are shown in Fig. 6. 
E. ev 
FIGURE 5. - TAUC PLOT OF ((1~11'2 VERSUS ENERGY E FOR a - C:H FILNS 01( SI. 
ANfi€ALEO FOR '20 SEC AT 400 OC AND 600 OC. AND A REFERENCE. 
4 
- 
TEWERATURE. T I E .  
OC SEC 
2.m r . 25 _. 
1.90 c 
J 1.80 
1.70 I O 1  
- 
3500 (too0 4m so00 5500 6ooo 6500 
WAVELENGTH. i\ 
FIGURE 6. - REFRMTlVE INDEX fl VERSUS WAVELENGTH FOR a - C:H FICIIS ON 
Si. ANNEALED FOR 20 SEC AT 1100 OC AND 600 OC. AND A REFEREM.  
A comparison was performed f o r  samples processed w i t h  t h e  f i l m  p o i n t i n g  
upward o r  downward. 
peak a t  -2000 A was markedly reduced versus t h e  f i l m s  p o i n t i n g  down, 
e s p e c i a l l y  f o r  l o n g  p rocess ing  t imes and h i g h  temperatures.  
40 pe rcen t  r e d u c t i o n  f o r  300 sec a t  500 "C.) However, t h e  o p t i c a l  bandgap 
Eo was unchanged t o  w i t h i n  b e t t e r  than 1 pe rcen t .  We a t t r i b u t e  t h i s  
l a r g e  r e d u c t i o n  i n  absorbance t o  a r e d u c t i o n  i n  f i l m  th i ckness .  T h i s  f a c t  
i s  p robab ly  due t o  smal l  amounts of oxygen p resen t  i n  t h e  p rocess ing  gas. 
We found t h a t  f o r  f i l m s  p o i n t i n g  up, t h e  absorbance 
(Up t o  
DISCUSSION 
The main r e s u l t  ob ta ined  i n  t h i s  work i s  shown i n  F i g .  4.  The main 
p a r t  o f  t h e  r e d u c t i o n  i n  t h e  o p t i c a l  bandgap i s  ob ta ined  a t  s h o r t  t imes .  
T h i s  f a c t  can a l s o  be deduced by the r e s u l t  ob ta ined  by l a s e r  annea l i ng  
[12] ,  when p rocess ing  t i m e  i s  much s h o r t e r  t han  those  r e p o r t e d  here.  How- 
ever,  t h e r e  i s  a d e f i n i t e  i nc rease  i n  a b s o r p t i o n  ( F i g .  2) and decrease i n  
bandgap ( F i g .  4) f o r  i n c r e a s i n g  process ing t imes o f  o r d e r  102-103 sec. 
The mechanism i n v o l v e d  should be a two-step process. There i s  known t o  be 
a two-stage p y r o l y s i s  o f  o rgan ic  m a t e r i a l  i n t o  g r a p h i t e  [13]  f o r  tempera- 
t u r e s  i n  t h i s  range, namely c a r b o n i z a t i o n  and p o l y m e r i z a t i o n .  Carbon iza t i on  
s tage  i n c l u d e s  l o s s  o f  v o l a t i l e  mat ter ,  which we i d e n t i f y  w i t h  hydrogen l o s s  
i n  t h i s  case [14] .  Th is  s tage occurs i n  t h e  temperature range 400 t o  600 O C  
i n  a-C:H. P o l y m e r i z a t i o n  s tage inc ludes  t h e  f o r m a t i o n  o f  g r a p h i t i c  c r y s t a l -  
!!tes o r  sheets.  If we assume t h a t  t h e  pn!yrner!zat!on !r a d i f f u s i n n  
dependent process w i t h  r e l a t i v e l y  long t i m e  cons tan t  ( o f  o r d e r  l o 3  sec),  
t h e n  we can deduce t h a t  t h e  two processes o f  c a r b o n i z a t i o n  and po lymer i za -  
t i o n  occur  s imu l taneous ly  i n  our  f i l m s .  
versus t i m e  a t  ve ry  s h o r t  process ing t i m e  i s  due t o  t h e  hydrogen l o s s ,  w h i l e  
t h e  subsequent decrease i n  
More work i s  r e q u i r e d  t o  check t h e  v a l i d i t y  o f  t h i s  assumption. 
a decrease i n  peak h e i g h t  and a s h i f t  i n  t h e  peak p o s i t i o n .  The s h i f t  has 
been observed e a r l i e r  [ 14 ]  and shows changes i n  t h e  carbon bonding. The 
decrease i n  peak h e i g h t  i s  a t t r i b u t e d  m o s t l y  t o  l o s s  o f  m a t e r i a l  i n  t h i s  
case, as i t  i s  o p p o s i t e  t o  t h e  r e s u l t s  ob ta ined  a t  400 O C  by us and t o  pub- 
l i s h e d  r e s u l t s  [14] .  
have been rep laced  by S i  ( F i g s .  4 and 5 ) ,  a l t hough  t h e  samples were prepared 
i n  t h e  same d e p o s i t i o n  run .  The main reason i s  t h e  f a c t  t h a t  S i  i s  a con- 
d u c t o r  w'tth r e s i s t i v i t y  o f  o rde r  0.01 Rcm, w h i l e  q u a r t z  i s  an i n s u l a t o r .  
A s  a r e s u l t ,  t h e  plasma above t h e  subs t ra tes ,  and subsequent ly  t h e  f i l m s ,  
a r e  d i f f e r e n t  i n  each case. 
The a b r u p t  decrease o f  t h e  bandgap 
i s  due t o  i n c r e a s e  i n  c l u s t e r  s i z e  [13].  Eo 
The absorbance A versus wavelength A p l o t  a t  600 " C  ( F i g .  2)  shows 
A l a r g e  decrease i n  bandgap has been observed when q u a r t z  s u b s t r a t e s  
5 
The e l l i p s o m e t r i c  r e s u l t s  on f i l m s  processed f o r  20 sec p a r t i a l l y  
reproduce the r e s u l t s  ob ta ined  e a r l i e r  [ 1 5 ]  f o r  1 h r  annealed f i l m s .  For  
t h e  600 O C  f i l m ,  we found inc reased  r e f r a c t i v e  i n d i c e s  a t  low energ ies  and 
inc reased  dens i t y  o f  s t a t e s  parameter B versus t h e  r e f e r e n c e  f i l m  
(Tab le  I). However, t h e  400 O C  f i l m  shows a decrease i n  t h e  r e f r a c t i v e  
i ndex  and s l i g h t  i n c r e a s e  i n  th i ckness ,  w h i l e  t h e  parameter 
same. Th is  f a c t  may be exp la ined  by mechanical  changes i n  t h e  f i l m  as t h e  
v o l a t i l e  gas I s  expe l l ed ,  and vo ids  a r e  l e f t  behind. 
B s tays  t h e  
T, o c  B, crn-1/2ev-1/2 E,, eV 
Reference 
400 
600 
250 
250 
31 8 
2.02 
1 - 9 0  
0.75 
a-C:H 
1690 
CONCLUSIONS 
An abrupt  decrease o f  t h e  o p t i c a l  bandgap Eo has been observed f o r  
s h o r t  t i m e  ( o f  o rde r  20 sec) r a p i d  thermal  p rocess ing .  A smal l ,  a d d i t i o n a l  
decrease o f  Eo i s  observed f o r  t imes o f  o r d e r  103 sec. A two-stage 
mechanism i s  proposed t o  e x p l a i n  t h e  r e s u l t .  
AC K NOW L E D GMEN’I 
The authors would l i k e  t o  thank M r .  N.S. Shoemaker f o r  t e c h n i c a l  
a s s i s t a n c e  w i t h  t h e  measurements. 
REFERENCES 
1. J.S. W i l l i ams  and S.J. Pearton, i n  Energy Beam-Solid I n t e r a c t i o n s  and 
T rans ien t  Thermal Process ing/ l984,  D.K. B iegelson,  G.A. Rozgonyi and 
C . V .  Shank, eds., M a t e r i a l s  Research Soc ie ty ,  P i t t s b u r g h ,  PA, p .  427 
(1985).  
2. N.M. Johnson, i n  Energy Beam-Solid I n t e r a c t i o n s  and T r a n s i e n t  Thermal 
Processing/ l984, D.K. B iegelson,  G.A. Rozgonyi and C . V .  Shank, eds., 
M a t e r i a l s  Research Soc ie ty ,  P i t t s b u r g h ,  PA, p. 265 (1985).  
3. S .  Yasuami, Y. S a i t o ,  and A. Hojo, Jpn. J .  Appl .  Phys. 23. 379 (1984).  
4. T.J.  F a i t h  and C.P. Wu, Appl .  Phys. L e t t .  45, 470 (1984).  
5.  Z.A. Weinberg, D.R.  Young, J.A. C a l i s e ,  S.A. Cohen, J.C. DeLuca, and 
6. J.D. Lamb and J.A. Woollam, J. Appl. Phys. 2, 5420 (1985).  
7. J.E. Oh, J.D. Lamb, P.G. Snyder, J.A. Woollam, and D . C .  L i u ,  S o l i d  
8. S.A. A l t e r o v i t z ,  J.D. Warner, D .C .  L i u ,  and 3.5.  Pouch, J r l .  
9. Model 2106 made by Process Products  Corp. 
V.R.  Del ine,  Appl. Phys. L e t t .  45, 1204 (1984) .  
S t a t e  E l e c t r o n i c s  29, 933 (1986).  
Electrochem. SOC. 133. 2339 (1986).  
M a t e r i a l s ,  2nd ed., Clarendon Press, Oxford,  p. 289 (1979).  
Phys., 54, 1559 (1983). 
10. N.F .  Mot t  and E.A .  Davis,  E l e c t r o n i c  Processes i n  N o n - C r y s t a l l i n e  
11. S.A. A l t e r o v i t z ,  G.H. Bu-Abbud, J.A. Woollam, and D . C .  L i u ,  J. Appl .  
12. S .  Prawer, R .  K a l i s h ,  and M. Adel, Appl .  Phys. L e t t .  48, 1585 (1986) .  
13. J. Robertson, Adv. Phys. 35, 317 (1986) .  
14. J.C. Angus, P. K o i d l ,  and S .  Domitz, Plasma Deposi ted T h i n  F i l m s .  J. 
1 5 .  F.W. Smith, J. Appl .  Phys. 55, 764 (1984) .  
Mor t  and F .  Jansen, eds., C R C  Press, Boca Raton, p. 89 (1986).  
6 
- -  ___. _ _  _. - _ _ _ _  
1. Report No 1 ?.Government Accession No. 
7. Key Words (Suggested by Author(s)) 
NASA TM-89859 I 
18. Distribution Statement 
4. Title and Subtitle 
9. Security Classif. (of this report) 
Unclassi f i ed 
Rapid Thermal Anneal ing of Amorphous Hydrogenated 
Carbon (a-C:H) Films 
20. Security Classif. (of this page) 21. No. of pages 22. Price' 
Unclass i f i ed 7 A02 
7. Author(s) 
Samuel A. Alterovitz, John 3. Pouch, and 
Joseph D. Warner 
9. Parforming Organization Name and Address 
National Aeronautics and Space Adminlstration 
Lewis Research Center 
Cleveland, Ohio 44135 
2. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, D.C. 20546 
____ 
3. Recipient's Catalog No. 
5. Report Date 
6. Performing Organization Code 
506-44-2C 
8. Performing Organization Repoi No. 
E-351 9 
10. Work Unit No. 
11. Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
5. Supplementary Notes 
Prepared for the 1987 Spring Meeting of the Materials Research Society, Anaheim, 
California, April 21-25, 1987. 
6. Abstract 
Amorphous hydrogenated carbon (a-C:H) films were deposited on silicon and quartz 
substrates by a 30 kHz plasma discharge technique using methane. Rapid thermal 
processing o f  the films was accomplished in nitrogen gas using tungsten halogen 
light. The rapid thermal processing was done at several fixed temperatures (up 
to 600 " C ) ,  as a function o f  time (up to 1800 sec). 
by optical absorptlon and by ellipsometry in the near UV and the visible. The 
bandgap, estimated from extrapolation of the linear part of a Tauc plot, 
decreases both wlth the annealing temperature and the anneallng time, with the 
temperature dependence being the dominating factor. The density of states param- 
eter increases up to 25 percent and the refractive index changes up to 20 percent 
with temperature increase. Possible explanations of the mechanisms involved in 
these processes are discussed. 
The films were characterized 
Rapid thermal processing 
Amorphous carbon films 
Unclassified - unlimited 
STAR Category 76 
*For sale by the National Technical Information Service, Springfield. Virginia 22161 


Rapid thermal annealing of Amorphous Hydrogenated Carbon (a-C:H) films

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