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PROPERTIES OF PURE SILICON AMORPHOUS FILMS PREPARED BY rf-BIAS SPUTTERING
M. Suzuki, T. Maekawa, Y. Kakimoto, T. Bandow
To cite this version:
M. Suzuki, T. Maekawa, Y. Kakimoto, T. Bandow. PROPERTIES OF PURE SILICON AMOR-
PHOUS FILMS PREPARED BY rf-BIAS SPUTTERING. Journal de Physique Colloques, 1981, 42
(C4), pp.C4-623-C4-626. �10.1051/jphyscol:19814136�. �jpa-00220756�
JOURXAL DE PHYSIQUE
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C4-623P R O P E R T I E S O F PURE S I L I C O N AMORPHOUS F I L M S PREPARED BY r f - B I A S S P U T T E R I N G
?I. S u z u k i , T. Maekawa, Y. Kakimoto and T.
ando ow.'.
f ' a c u i t y
o /'L'ecbrno Logy, Kanazawa Uni.)erui ty, K~~nazclwcc
325,Japan
* I s h i k a ~ m :"d,'cknica.Z
CoZ l e g e ,Tsubata
929-03,Japan
A b s t r a c t . - P u r e a - S i f i l m s w e r e p r e p a r e d by r f - b i a s s p u t t e r i n g w i t h --
m a g n e t r o n t a r g e t . The s u b s t r a t e was ion-bombarded d u r i n g d e p o s i t i o n by r f - i n d u c e d b i a s a t t e m p t i n g t o remove l o o s e l y bound m a t e r i a l s f r o m t h e s u r f a c e . The e l e c t r i c a l a n d o p t i c a l p r o p e r t i e s o f s p u t t e r e d a - S i f i l m s w e r e f o u n d t o Ile improved by s u b s t r a t e b i a s w i t h o u t u s i n g d a n g l i n g bond
t e r m i n a t o r s , when s p u t t e r i n g v o l . t a g e was l.ower t h a n 1 kV and A r p r e s s u r e was h i g h e r t h a n -200 mTorr. ESR m e a s u r e m e n t s r e v e a l e d t h a t t h e d e n s i t y o f d e f e c t s t a t e s was r e d u c e d by ion-bombardment d u r i n g d e p o s i t i o n .
I n t r o d u c t i o n . - The d c n s i t y of d e f e c t s t a t e s i n a - S i f i l m s c a n h e c o n t r o l l e d by so-cal..Led h y d r o g e n a t i o n ( 1 . 2 ) . T h e amount o f H i n t h o s e f i l m s i s u s u a l l y t o o l a r g e t o t e r m i n a t e i n h c r e n t d a n g l i n g b o n d s . A l t h o u g h t h e e f f u s i o n o f H , w h i c h r e s u l t s i n t h e d e t e r i o r a t i o n o f f i l m q u a l i t y , t a k e s p l a c e a b o v e 3 5 0 ° C ( 3 , 4 ) , i t would g r a d u a l l y g o on e v e n a t room t e m p e r a t u r e . F l . u o r i n a t i o n would h e o n e o f t h e ways o f o v e r c o m i n g t h i s d i f f i c u l t y ( 5 ) . Ilowever, a n a t t e m p t t o r e d u c e t h e d a n g l i n g - b o n d d e n s i t y i n a - S i f i l m s w i t h o u t u s i n g m i x t u r e s a r c a l s o i n t e r e s t i n g a n d i m p o r t a n t n o t o n l y i n p h y s i c s o f t e t r a h e d r a l l y bonded random n e t w o r k s b u t a l s o i n d e v i c e a p p l i c a t i o n s .
P u r e a - S i f i l m s a r e e a s i l y o b t a i n e d by c o n v e n t i o n a l vacuum e v a p o r a t i o n o r s p u t t e r i n g . Tn t h e c a s e o f s p u t t e r i n g t h e p r o p e r t i e s o f f i l m s c a n b e c o n t r o l l e d i n w i d e r a n g e by c h a n g i n g s p u t t e r i n g c o n d i t i o n s s u c h a s g a s p r e s s u r e and s p u t t e r i n g v o l t a g e . I n f a c t , a r c 1 . a t i v e l y s m a l l s p i n d e n s i t y h a s b e e n a c h i e v e d i n non-hydrogen- a t e d a - S i f i l m s p r e p a r e d by h i g h p r e s s u r e s p u t t e r i n g and c o n d u c t i v i t y c o n t r o l h a s a l s o b e e n a t t a i n e d by i m p u r i t y d o p i n g u s i n g c o - s p u t t e r i n g t e c h n i q u e ( 6 , 7 ) . A l t h o u g h t h e s e r e s u l t s a r e n o t s a t i s f a c t o r y compared w i t h t h o s e i n GU a - S i f i l m s , i t i s e x p e c t -
ed t h a t t h e p r o p e r t i e s of s p u t t e r e d a - S i f i l m s a r e f u r t h e r improved by i n t r o d u c i n g a new p a r a m e t e r i n t o s p u t t e r i n g c o n d i t i o n s . Then we a p p l i e d r f - b i a s s p u t t e r i n g w i t h m a g n e t r o n t a r g e t t o t h e f a b r i c a t i o n o f p u r e a - S i f i l m s . I t was found t h a t r f - i n d t l c e d
s u b s t r a t e b i a s s t r o n g l y a f f e c t s f i l m q u a l i t y .
Sample P r e p a r a t i o n . - A s c h e m a t i c d i a g r a m o f t h e a p p a r a t u s u s e d f o r d e p o s i t i o n o f b i a s s p u t t e r e d a - S i f i l m s i s shown i n " i g . 1 . The s u b s t r a t e w a s ion-bombarded d u r i n g d e p o s i t i o n by r f - i n d u c e d b i a s attempting t o remove l o o s e l y bound m a t e r i a l s from t h e s u r f a c e o f t h e f i l m . The m a g n e t r o n t a r g e t was u s e d t o l o w e r s p u t t e r i n g v o l t a g e w i t h o u t . t h e s e r i o u s r e d u c t i o n o f d e p o s i t i o n r a t e , s o t h a t damage g e n e r a t e d by h i g h e n e r g y n e u t r a l s r e f l e c t e d from t h e t a r g e t m i g h t h e d i m i n i s h e d .
An n - t y p e w a f e r ( r e s i s t i v i t y - 8 0 0 ncm ) w i t h d i a m e t e r o f 1 0 cm was u s e d a s t h e t a r g e t , o n w h i c h magnetic: f l u x d c n s i t y o f - 4 0 0 G a u s s was a p p l i e d . The s u b s t r a t e was ? l a c e d o n t h e b i a s e l e c t r o d e w i t h d i a n e t e r o f 5 cm t o w h i c h r f power was s u p p l i e d . The p e a k - t o - p e a k v a l u e and t h e power o f r f - b i a s was m e a s u r e d t o know t h e e n e r g y a n d t h e r a t e o f bombardment. The t a r g e t - s u b s t r a t e s p a c i n g was s e t t l e d a t 5 . 5 cm and 5 n i n e - p u r e A r g a s was u s e d a s t h e s p u t t e r i n g g a s .
R e s u l t s . - The e f f e c t s o f s p u t t e r i n g v o l t a g e , s p u t t e r i n g g a s p r e s s u r e and b i a s power o n t h e ESR s p i n d e n s i t y a r e shown i n F i g . 2 . The d c s p u t t e r i n g v o l t a g e was 1 . 5 kV f o r t h e s p u t t e r i n g i n -90 mTorr a n d - 1 2 5 mTorr, 1 kV f o r -225 mTorr a n d 0 . 8 kV
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814136
JOURNAL DE PHYSIQUE
OSC. scope
Power Met
Fig.1. Schematic diagram of rf-bias sputtering with magnetron target.
for-280 mTorr. The results of zero bias and 1 0 W bias are shown for each case. But in-280 mTorr the reult of 5 W is added, which was obtained in the differ- ent sputtering conditions where magnetic field was increased to -700 Gauss and the sputtering
voltage was reduced to 0.6 kV.
The deposition rate kept almost unchanged (-1 i/sec) for all cases.
It is noticeable that the application of substrate bias largely reduced the spin density when Ar pressure was high. The g- value and A H of the signal was -2.0053 and -7 Gauss, which would
be ascribed to dangling-bond electrons.
Photoconductivity AQp was measured at room temperature under illumination of Xe-lamp ( .40 m ~ / c d )
.
dry of the samples prepared under conditions of 0.8 kV and of -280 mTorr increased with increasing bias power and saturated as shown in Fig.3.Thus it is certain that film quality was improved by substrate bias when Ar pressure was high and sputtering voltage was low.
Dark conductivity U also
:
low
x : OW
Ar. Press. (mtorr )
increased with increasing biaspower and reached the maximum Fig.2. Effects of sputtering voltage, Ar around 5 W bias. It was found pressure and bias power on the ESR spin from the plot of InU-1/T that the density.
activation type conduction became *Magnetron target was reinforced and dominant as Ar pressure and bias sputtering voltage was reduced to 0.6 kV.
power was increased. These behaviors are consistent with the decrease of the spin density and the increase of photoconductivi- tY.
The decrease of the activation energy -3 for conduction would be due to the incorpo-
-
ration o f unknown donar impurities. Thermo- 0 0
electric power of the biased samples was
-
negative and the carrier activation energy
-
0obtained from the slope of thermoelectric power vs.l/T plot coincides fairly well with the activation energy for conduction. ",5
-
Substrate bias caused preferable
-
changes in the optical absorption edge. The
g
optical energy gap of biased samples prepared 4 -6- under conditions of 0.8 kV and -280 mTorr was
g
01.7-1.85 eV and the slope of ( d ~ w ) ~ ~ vs k@
plot became steeper than that of zero bias samples as shown in Fig.4, whereelis optical absorption coefficient and 4co is photon ener- gy.
X-ray diffraction pattern did not show -80 any crystalline peak. Although the postdepo- sition oxygen contamination was observed in -9
infra-red spectra of zero bias samples as 0
5
10 reported in high pressure sputtering ( 8 ) ,Si-0 vibrational peak was hardly observed for Bias Power (w)
biased samples. Fig.3. Variation of dUp with
bias power. Preparation condi- tions: Ar pressure -280 mTorr, sputtering voltage 0.8 kv.
Fbwer (w)
0 0
A 1 Discrlssion and Summary.- It was found
300 -
2 that the electrical. and optical proper-1 0 ties of a-Si films prepared under condi-
tions of low sputtering voltage and high
A Ar pressure were markedly improved by
$ > rf-induced substrate bias. The ion-bom-
bardment is expected to remove loosely bound materials from the surface if the
u r n -
-
energy and the rate of bombardment issuitable. High Ar pressure and low sput-
s
tering voltage are known to reduce film3
damage generated by high energy neutrals4c
u
reflected from the target (8,9). In thev present work, low voltage sputtering
100 -
was attained because of high sputteringgas pressure and of the magnetron target.
In the case of high pressure sput- tering, in which remarkable effects o f
0
131 . 8 20 22
-220 V even for 10 IJ bias, and the rateh o
( e V ) of the bombardment was estimated to be over 1016 ionslcn? sec. So rather low energy and high rate bombardment is Fig.4. Variation of optical absorption considered to be suitable for the im- edge with bias power. Preparation condi- provement of film quality. Anyway, the tions are the same as in Fig.3. reduction of a number of defect states,
,: ..
1
,',
;, , , '
r . r ,'
,' ,'J :
substrate bias were observed, the peak- to-peak value of rf-bias, a measure of the energy of bombardment atoms, was
C4-626 JOURNAL DE PHYSiQUE
is consi.dered to be due to synthetical effects of ion-bombardment and low voltage sputtering, and it is noticeable that the density of defect states was fairly reduced without using dangling bond terminators.
There are so many factors determining film quality in bias sputtering with mag- netron target. At the present stage, the minimum spin density is --1~10'~cm-3, which was obtained for the samples prepared in the highest Ar pressure (-280 mTorr) and in
the lowest voltage sputtering ( 0.6 kV) which was attained using the reinforced mag- netron target. l'hcse results suggest that the properties of bias sputtered a-Si films will be further improved by controlling preparation conditions.
Acknowledgem-.- The authors would like to express their thanks to Yessrs. Shuichi Okano and Xakoto Suzuki for stimulating discussion, and to Messrs. Yatahiko Yanada, Masakatsu Morita, Xasahiro Kawada and Hiroshi Koarashi for their assistance in sample preparations and measurements.
This work is supported by the Grant in Aid for Scientific Research from the Ministry of Education of Japan and in part by the Iwatani Naoji Foundation's Grant.
References.
--
(1) Paul k ' . , Lewis A.J., ConnellG.A.X., and Moustakas T.!)., Solid State Commun.
2
(1979) 969.
(2) Spear W.E., LeCom1,er P.G., Solid State Commun.
17
(1975) 1193.(3) Fritzsche H., I'roc. 7th Tnt. Conf. on Amorphous and Liquid Semiconductors, ed.
W.E.Spear (CICL,Edinburgh,l978) p. 3.
(4) McMillan J.A. and Peterson E.M., J.Appl.Phys.
50
(1979) 5238.(5) Ovshinsky S.R. and ?ladan A . , slature
276
(1978) 482.(6) Suzuki N., Nakao A., Maekawa T., Kumeda M. and Shimizu T., Jpn.3.Appl.Phys. Suppl.
1.9-2 (1980) 85.
(7) Suzuki M., Maekawa T., Sakao A., Kumeda M. and Shimizu T., Solid State Commun.
36 (1980) 393.
(8) G;crson D.A., :{oddel i., Paesler M.A. and Paul W., J.Vac.Sci.Techno1. 15(1978) 906.
(9) Paulel~icz Ir'.T., J.App1 .Phys.
