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8. S. B. Desu, J. Am. Ceram. Soc., 72, 1615 (1989).
9. K. F. J e n s e n and D. B. Graves, This Journal, 130, 1950
(1983).
10. P. Van Der Puttle, L. J. Giling, and J. Bloem, J. Cryst.
Growth, 31, 299 (1975).
11. A. Yekel and S. Middelman, This Journal, 134, 1275
(1987).
12. J. Juza and J. Cermak, ibid., 129, 1627 (1982).
13. T. R. Hughes, "The Finite Element Method," Prentice
Hall, Inc., Englewood Cliffs, N J (1987).
14. O. C. Zeinkiewicz, "The Finite Element Method in Engineering Science," McGraw-Hill, Inc., New York
(1973).
15. P. V. Danckwerts, Chem. Eng. Sci., 2, 1 (1953).
Yields of the Plasma Oxidation of Silicon by Neutral Oxygen
Atoms and Negative Oxygen Atom Ions
C. Vinckier and S. De Jaegere
Laboratory for Analytical and Inorganic Chemistry, K. U. Leuven, Celestijnenlaan 20OF, B-3030 Leuven, Belgium
ABSTRACT
The yield for the plasma oxidation of silicon is defined for both ground state oxygen atoms (O3P) and oxygen atom
ions (O-). On the basis of these yields it is shown that the role of oxygen atoms is negligible compared to the negative oxygen atom ions in plasma oxidation. The negative ion oxidation yield as a function of temperature follows an Arrhenius behavior with an apparent temperature coefficient of 0.20 -+ 0.033 eV. Other factors such as the oxygen pressure, the silicon
bias voltage, or the reactor geometry have only a minor effect on the oxidation yields. Calculated negative ion fluxes from
the plasma toward the silicon substrate seem to be too low to explain the observed initial silicon dioxide growth rates,
which supports a surface-catalyzed formation path for the negative oxygen atom ions.
While it is well known that the oxidation of silicon proceeds much faster in oxygen-containing plasmas than in
conventional thermal oxidation systems (1-6), the overall
oxidation mechanism is not completely characterized. The
application of a positive bias voltage enhances the oxidation rate (3-7), and thus negative charge carriers play a
major role under plasma anodization conditions. A number of studies indicate that the negative oxygen ion O- is a
crucial oxidation precursor (5-8). It is indeed shown (5) that
a direct correlation exists between the O- concentration
and the observed oxidation rate of silicon. A convincing
argument is the titration of O- by the addition of N20, leading to the fast conversion of O- into 02-. In this experiment
a strong reduction of the oxidation rate was observed even
though it is claimed the total negative ion concentration is
larger than in pure oxygen. Although the negative ion O- is
certainly present as such in the gas phase, some authors
(6, 9) believe that it is formed to a large extent in a catalyzed surface reaction
e- + Oad
surface
~
Oad-
[1]
The deposition of a thin ZrO2 layer on the silicon substrate strongly enhances the oxidation rate, which is explained by an increased formation rate of O- ions at the oxide/plasma interface (9b). However, in plasma afterglows
in the absence of an electric field or when the silicon substrate is unbiased and its potential is floating, the role of Oions is uncertain. In our previous work, where the silicon
substrates were placed in the afterglow far outside of the
microwave plasma, it was shown that oxygen atoms in
their electronic ground state (O3P) were the only oxidizing
species present in the gas phase (10, 11). In recent work on
the oxidation of silicon substrates by microwave discharged oxygen plasmas, it was also suggested that oxygen atoms dominate the oxide growth kinetics (12).
In this paper silicon oxidation yields will be defined for
oxygen atoms 6o,sio2 and for negative oxygen ions ~O-,SiO2"
It will be shown that in plasma oxidation systems the role
of oxygen atoms as such is negligible due to their very low
oxidation yield <~o.sio2-The yield of negative oxygen atom
ions ~o-.sio2 which can be derived from plasma anodization
studies is orders of magnitude higher. It only slightly depends on the reactor vessel design and experimental conditions such as oxygen pressure or bias voltage, but fol-
lows an Arrhenius temperature dependence. On the basis
of a calculated negative ion flux from the gas phase towards the silicon substrate, the additional heterogeneous
formation path for O- ions by reaction [1] seems to be required to explain the observed initial silicium dioxide
growth rates.
Yields
Atomic oxygen yields for Si02 production in microwaveinduced plasma afterglows.--The oxidation of phosphorous-doped <100> silicon wafers in the afterglow of
microwave induced plasmas containing O j A r mixtures
has been described in our earlier work (10, 11). It was
shown that O(~P) atoms are the ultimate gas phase species
responsible for the silicon oxidation. Additionally, it was
found that the oxide growth rate was not influenced when
the flow velocity varied between 1 and 5 m s -1. This means
that the convective transport of the active species (i.e., 0
atoms) is not a rate determining factor in the oxidation
process. Recently it was also noticed that the geometrical
orientation of the wafer had no effect on the oxidation rate
with the wafer mounted either perpendicular or parallel to
the gas flow (13). Under these circumstances the O atom
flux, Fo, is directly given by
[0] x c
Fo - - 4
[2]
where [O] is the oxygen atom concentration in the gas
phase, c = (8 RT/wM) ~2 the thermal velocity, R is the universal gas constant, T the temperature in Kelvin, and M the
atomic mass of oxygen.
The yield of silicon dioxide ~o,sio2 by reaction of atomic
oxygen with the surface can be defined as the n u m b e r of
SiO2 entities formed per unit of surface and time, Nsio2, divided by half of the oxygen atom flux Fo, since two O
atoms are needed to form one SiO2 entity
~o,sio2 -
2 x Ns~o2
Fo
[3]
The formation rate Ns~o2can then be expressed in terms of
the silicon dioxide growth rate Rsio2 as follows
Rsio2(A h-') - Nsi~ x Msio2 x K
NA • dsioz
[4]
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, Inc.
w i t h Msio2b e i n g t h e m o l a r w e i g h t of SiO2 (60 g mol-~), NA
t h e A v o g a d r o n u m b e r (6 x 10~3), dsioz t h e d e n s i t y of SiO2
(2.27 g cm-3), a n d K a n u m e r i c a l f a c t o r t a k i n g i n t o a c c o u n t
t h e c o n v e r s i o n of t h e silicon d i o x i d e g r o w t h rate f r o m c m
s -1 t o / ~ h - L T h i s f a c t o r is e q u a l to 3.6 x 10 ~ a n d h a s t h e dim e n s i o n s of/~ s c m -~ h - L B y c o m b i n i n g Eq. [3] a n d [4], a n d
a f t e r i n s e r t i n g all t h e n u m e r i c a l v a l u e s of t h e v a r i o u s par a m e t e r s , o n e finds
1.26 • 10 n • Rsio2
r
-
[5]
Fo
E q u a t i o n [5] allows u s to c a l c u l a t e r
in o u r p r e v i o u s
afterglow oxidation experiments. The extrapolated
g r o w t h rate Rsto2 to a r e a c t i o n t i m e t = 0 w a s e q u a l to
140.4 A h -1 a t 847 K [Table III, Ref. (10)] w i t h a n O a t o m
c o n c e n t r a t i o n o f 1.5 x 10 ~ a t c m 3. T h i s c o r r e s p o n d s to a n
O a t o m flux Fo e q u a l to 3.98 x 10 ~9 at c m ~2 s -1. I n s e r t i n g
t h e v a l u e s of RsLo~ a n d Fo in Eq. [5] l e a d s to a yield r
e q u a l to 4.4 x 10- at 847 K.
T h e a c c u r a c y of t h e o x i d a t i o n yield $o,s~oJ2 d e p e n d s prim a r i l y o n b o t h Rs~o~ a n d Fo. T h e v a l u e of Rs~o2 is o b t a i n e d
b y a n u m e r i c a l t r e a t m e n t of t h e e x p e r i m e n t a l l y g e n e r a t e d
g r o w t h c u r v e s (10) w h e r e t h e e r r o r is less t h a n 10%. T h e
m a g n i t u d e of t h e O a t o m flux is m a i n l y d e t e r m i n e d b y t h e
accuracy of the O atom concentration. The latter has been
m e a s u r e d b y t h e c h e m i c a l t i t r a t i o n t e c h n i q u e w i t h NO2,
which yields the absolute O atom concentration within
20%. T h e overall a c c u r a c y of r
must thus be of the
o r d e r of -+30%.
I n v i e w o f t h i s e x t r e m e l y low y i e l d it is c l e a r t h a t t h e silic o n d i o x i d e g r o w t h r a t e is n o t at all t r a n s p o r t c o n t r o l l e d ,
s i n c e t h e O a t o m c o n s u m p t i o n at t h e s u r f a c e is negligible.
T h i s j u s t i f i e s t h e u s e o f t h e t h e r m a l v e l o c i t y c in Eq. [2] for
t h e c a l c u l a t i o n of Fo. I n o u r p r e v i o u s w o r k (10) w e h a v e
t a k e n t h e m a c r o s c o p i c t r a n s p o r t v e l o c i t y v to c a l c u l a t e Fo
a n d t h i s led to a h i g h e r b u t i n c o r r e c t v a l u e for r
of
1.82 x 10 -5. I t s h o u l d also b e s t r e s s e d h e r e t h a t c a l c u l a t e d
yield is a n u p p e r l i m i t for t h e e x p e r i m e n t a l c o n d i t i o n s at
847 K [Table III, Ref. (10)], s i n c e t h e g r o w t h r a t e follows a
p a r a b o l i c d e p e n d e n c e o n t h e o x i d a t i o n time. I n t h i s c a s e
Rs~o2 d e c r e a s e s as a f u n c t i o n o f t i m e b y m o r e t h a n a f a c t o r
o f 10 b e t w e e n t = 0 a n d 5h a n d t h u s r
must drop with
t h e s a m e factor.
T h i s e x t r e m e l y l o w o x i d a t i o n yield r
c a n i n principle b e e x p l a i n e d b y e i t h e r a v e r y l o w r e a c t i o n r a t e o f oxyg e n a t o m s at t h e silicon/silicon d i o x i d e interface, or b y a
loss p r o c e s s o f t h e o x y g e n a t o m s d u r i n g t h e i r t r a n s p o r t
t h r o u g h t h e silicon d i o x i d e layer. S i n c e u n d e r t h e foregoi n g e x p e r i m e n t a l c o n d i t i o n s a p a r a b o l i c g r o w t h r a t e dep e n d e n c e o n t h e o x i d a t i o n t i m e is o b s e r v e d , t h e o x i d a t i o n
process must be transport controlled. The low oxidation
yield is t h u s m o s t p r o b a b l y d u e to a fast r e c o m b i n a t i o n of
t h e O a t o m s i n t h e f r e s h l y f o r m e d SiO2 layer. A k i n e t i c
m o d e l i n g c a i c u l a t i o n is n o w b e i n g u n d e r t a k e n to c h e c k
t h i s possibility.
In their work on the microwave discharge oxidation of
floating silicon s a m p l e s , K i m u r a a n d c o - w o r k e r s (12)
629
c a m e to t h e c o n c l u s i o n t h a t n e u t r a l o x y g e n a t o m s m a y
p l a y a n i m p o r t a n t role i n t h e o x i d a t i o n m e c h a n i s m . S i n c e
t h e y also c a r r i e d o u t a n e x p e r i m e n t at 853 K w e c a n u s e
t h e d e r i v e d v a l u e of r
to e s t i m a t e t h e i r g r o w t h rate.
A l t h o u g h t h e O a t o m c o n c e n t r a t i o n is n o t k n o w n i n t h e i r
e x p e r i m e n t , o n e c a n set a 100% d i s s o c i a t i o n yield of t h e
m o l e c u l a r o x y g e n p r e s e n t in t h e p l a s m a i n o r d e r to estimate the maximum possible O atom concentration. Since
t h e t o t a l O2 p r e s s u r e w a s 2.7 • 10 -2 Pa, t h e m a x i m u m
a c h i e v a b l e O a t o m c o n c e n t r a t i o n w a s 4.58 • 10 ~2 at c m ~3,
r e s u l t i n g i n a m a x i m u m of 1.27 • 1017 at c m -2 s - ' for t h e
flux Fo i n t h e i r e x p e r i m e n t a l c o n d i t i o n s . W h e n v a l u e s of
Fo a n d $o,s~o~ are i n s e r t e d in Eq. [5], o n e o b t a i n s t h e m a x i m u m a t t a i n a b l e g r o w t h rate, Rs~o2, of 0.43 A h -~. T h i s is at
l e a s t a f a c t o r of a h u n d r e d t o o l o w to e x p l a i n t h e o b s e r v e d
g r o w t h rateS. H e n c e w e m a y c o n c l u d e t h a t o x y g e n a t o m s
in t h e gas p h a s e c a n o n l y p l a y a v e r y m i n o r role i n t h i s t y p e
of p l a s m a o x i d a t i o n s y s t e m , w i t h t h e silicon s u b s t r a t e u n b i a s e d a n d at a floating potential.
Negative ion yields for Si02 production under plasma
anodization.--Assuming t h a t t h e n e g a t i v e o x y g e n a t o m
i o n O- is t h e m a j o r species u n d e r p l a s m a a n o d i z a t i o n conditions, a n a n a l o g o u s e q u a t i o n to Eq. [5] c a n n o w b e der i v e d for t h e o x i d a t i o n yield of O- ions ~o-.s,o2- T h e O a t o m
flux in t h e d e n o m i n a t o r m u s t t h e n b e r e p l a c e d b y t h e nega t i v e i o n O- flux. W h e n t h e a n o d i z a t i o n c u r r e n t Ia is exp r e s s e d in m A c m -z, t h e O- flux is e q u a l to
6.25 x 1015 x f x [a or
2.01 x 10 5 Rsio2
4o-,sio2 -
[6]
fxla
w h e r e t h e f a c t o r f i s t h e ratio of t h e O - c o n c e n t r a t i o n to t h e
t o t a l c o n c e n t r a t i o n of n e g a t i v e c h a r g e c a r r i e r s i n t h e substrate. F o r m o s t of t h e r e s u l t s r e p o r t e d in t h e l i t e r a t u r e o n
p l a s m a o x i d a t i o n t h i s f a c t o r f i s n o t k n o w n , so w e will set it
e q u a l to 1 i n o r d e r to d e r i v e m i n i m u m v a l u e s for r
T h e c a l c u l a t e d yields d)o- s102 for a w i d e v a r i e t y of e x p e r i m e n t a l s e t u p s a n d c o n d i t i o n s are g i v e n i n T a b l e I. W h e n a
parabolic growth rate was observed we have calculated the
e x t r a p o l a t e d v a l u e of Rsi02 at t = 0. F r o m T a b l e I o n e imm e d i a t e l y sees t h a t r ,si02 is at l e a s t a f a c t o r of a t h o u s a n d
larger than r
at c o m p a r a b l e t e m p e r a t u r e s . I n reality
t h e yields ~ O - SiO2 m u s t b e m u c h larger, s i n c e i n s o m e cases
(9, 15) t h e ratio o f t h e n e g a t i v e i o n flux Fo- to t h e t o t a l flux
of n e g a t i v e c h a r g e carriers is of t h e o r d e r of 10 -2. T h i s
w o u l d b r i n g t h e real i o n i c yield o f n e g a t i v e o x y g e n a t o m
i o n s O- i n t h e r a n g e f r o m 0.01 to 1.
I n o r d e r to d e r i v e t h e t e m p e r a t u r e d e p e n d e n c e of t h e ion
yield, a p l o t is m a d e of In r ,s,o~ vs. 1/T. F i g u r e 1, w h e r e
o n e sees t h a t t h e t e m p e r a t u r e d e p e n d e n c e for m o s t of t h e
c a l c u l a t e d yields follows a r e a s o n a b l y g o o d A r r h e n i u s behavior. I n t h i s w a y Fig. 1 i l l u s t r a t e s t h a t 4)o-.s~o2 is m a i n l y
d e t e r m i n e d b y t h e t e m p e r a t u r e , a n d o n l y to a m i n o r e x t e n t
b y t h e o t h e r e x p e r i m e n t a l c o n d i t i o n s s u c h as t h e o x y g e n
p r e s s u r e or t h e bias voltage. F r o m t h e s l o p e of t h e line a
t e m p e r a t u r e coefficient of 0.20 • 0.033 e V c a n b e d e r i v e d .
Table I. Experimental conditions and calculated yields ~o-,s~o2 obtained under plasma anodization conditions
Plasma
Rsioo
(A h :1)
T
(K)
P
(10-3 tort)
Ia
(10-2A cm -~)
Bias
(V)
MW
MW
MW
RF
RF
IVIW
MW
1300
500
840
14.0005
18.0005
24.0005
18.0005
40
40
50-60
200
200
150
150
60
20
10
100
30
277.7
277.7
50
18
10-20
0-~ 100
0 -o 300
300
50
MW
MW
3000
18.000
498a
498a
673
878
873
723c
723c
673
823
543-703e
.100
600
12.2
20-30e
50
60-80e
4)0 .si02
Ref.
4.35 • 10 4
5.02 • 10_4
1.68 • 10-3
2.8 x 10-3
1.2 • 10-2
1.77 x 10_2
1.3 x 10_3
1.0 • 10 -3 d
4.94 x 10-2
1.5 x 10-2
(7)
(7)
(8)
(14)
(3)
(2)
(2)
(9)
(15)
(1)
a Sample temperature is taken equal to 225~
b Rs~o2is calculated at t = 0 (extrapolation).
c Average temperature.
d Quoted yield in ReL (9).
e Range in values as given in Ref. (1).
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The Electrochemical Society, tnc.
630
stant for the dissociative a t t a c h m e n t reaction k~ b e c o m e s
e q u a l to 2.2 • 10 -l~ c m 3 m o l e c -1 s '. F o r the reverse reaction the rate constant k8 for the reaction b e t w e e n t h e r m a l
O- ions and o x y g e n atoms is f o u n d to be 1.7 x 10 -1~ c m 3
m o l e c 1 s-1 (17), and so the e q u i l i b r i u m c o n s t a n t K~ is
e q u a l to
,~,, la)
In ~bo-,Si 02
"'5
Ke .
[O-][O]
. . .
[e ][02]
k7
k8
1.29
[9]
or the ratio [O-]/[e-] in the plasma gas is g o v e r n e d by the
m o l e c u l a r o x y g e n dissociation yield
[O ]
[02]
-
[e-]
1300v}C~
---7
Isov}O 9
X ~ ( 18V1
--8
FIG]
X
1.5
2
}
I
V'r *m'3
2.5
*
I
Fig. 1. Natural logarithm of the ionic oxidation yield ~o-,s~o2 as a
function of 1/T. The symbols refer to the following authors and references: * Ligenza (1); Aa Sugano (3); Ab Sugano (14); 9 Eccleston
(15); G Kraitchman (2); [~ Fu (8); 9 Roppel (7); 9 quoted by Perri~re
(9). The numbers given between brackets are the applied bias voltages.
We h a v e not i n c l u d e d the Ligenza (1) or s o m e of the
S u g a n o (3) data since the yields are a p p a r e n t l y h i g h e r t h a n
in the o t h e r e x p e r i m e n t s at similar t e m p e r a t u r e s . The
m o s t p r o b a b l e e x p l a n a t i o n for their high yield m u s t be
s o u g h t in a m o r e favorable f factor and thus a higher ratio
of t h e O- flux to the total n e g a t i v e charge carrier flux.
Concentration of negative charge carriers in plasmas.In order to establish the possible role of n e g a t i v e o x y g e n
a t o m ions generated in the plasma itself, one should estim a t e first the c o n c e n t r a t i o n ratio [O-]/[e-]. Most p r o b a b l y
their c o n c e n t r a t i o n in the plasma at low p r e s s u r e is gove r n e d by the partial e q u i l i b r i u m
7
e- + O2 ~---O- + O
8
[7, 8]
In v i e w of this partial e q u i l i b r i u m i n t e r c o n n e c t i n g the concentrations of electrons, n e g a t i v e ions, and o x y g e n atoms,
any e x t e r n a l factor affecting one of t h e s e c o n c e n t r a t i o n s
will h a v e an influence on the o b s e r v e d oxidation rates. The
t h r e s h o l d for reaction [7] to occur lies at an electron e n e r g y
E~ of 4.3 eV (16). The cross section q7 for dissociative electron a t t a c h m e n t reaches a peak v a l u e of 1.6 • 10 -18 c m 2 at
an e l e c t r o n energy of 6.5 eV (16), and h e n c e the rate con-
1 . 2 9 -
[10]
[O]
H e n c e high o x i d a t i o n rates can be realized w h e n the
[02]/[0] ratio is relatively high. This could be one of the reasons w h y the calculated yields for the Ligenza (1) and Su~
gano (3) e x p e r i m e n t s are high in v i e w of the high 02 pres~
sures in their system.
A n o t h e r i m p o r t a n t factor of course is the v a l u e of the
e q u i l i b r i u m c o n s t a n t he, w h i c h itself strongly d e p e n d s on
t h e electron energy Ee. For the electron energies of 9 and
10 eV, the cross section q7 decreases to t h e r e s p e c t i v e
values of 0.1 • 10 -18 and.0.04 x 10 -'8 c m 2, w h i c h leads to
the rate c o n s t a n t k7 of r e s p e c t i v e l y 1.6 • 10 -11 and
6.87 • 10 -12 each in units of cm 3 m o l e c -1 s-'. A lower for1-nation rate of O- ions m u s t of course result in a lower
[O-]/[e-] ratio in the plasma gas.
W h e t h e r the n e g a t i v e ion c o n c e n t r a t i o n in the p l a s m a itself is sufficient to attain the calculated yields ~o-.s~o2, or an
additional h e t e r o g e n e o u s f o r m a t i o n path (reaction [1]) is
required, cannot be definitely established for t h e p l a s m a
anodization condition. The m a j o r u n k n o w n factor is the
n e g a t i v e ion flux in the gas phase towards the silicon substrate, w h i c h d e p e n d s on the local field strength in the
plasma. When t h e silicon substrate is floating though, the
n e g a t i v e ion flux can be e s t i m a t e d on the basis of Eq. [2].
T h e n e g a t i v e ion c o n c e n t r a t i o n can be calculated on the
basis of the partial equilibrium, and for the ion t h e therm a l v e l o c i t y will be assumed. We h a v e n o w selected the
e x p e r i m e n t a l conditions of K i m u r a etal. (12, 18) in
w h i c h w e e s t i m a t e an average 30% dissociation yield of 02
in t h e m i c r o w a v e discharge. F o r the c u s p p l a s m a w i t h an
e l e c t r o n e n e r g y of a b o u t 9 eV the e q u i l i b r i u m c o n s t a n t K~
is e q u a l to 9.0 • 10 -2, while for the m i r r o r p l a s m a w h e r e E~
is of the order of 10 eV, the v a l u e of K~ b e c o m e s e q u a l to
3.8 x 10 -2. The o x i d a t i o n rate Rsio~ is calculated at t = 0
t h r o u g h an extrapolation f r o m the first e x p e r i m e n t a l
points. The other parameters and the calculated yields
q~o-,sio2 are g i v e n in Table II. One sees that for the average
e x p e r i m e n t a l conditions, yields ~o-,s~o2 larger t h a n one are
f o u n d for the p l a s m a - g e n e r a t e d O- ions. This is of course
i m p o s s i b l e and m a y l e n d s u p p o r t for the h e t e r o g e n e o u s
f o r m a t i o n path for negative a t o m ions in reaction [1]. This
confirms the findings of other authors in a n u m b e r of
p l a s m a o x i d a t i o n systems of various substrates (19, 20). It
s h o u l d be p o i n t e d out, however, that the a b o v e is only true
at t h e initial stage of the o x i d e growth. O n c e the o x i d e
t h i c k n e s s b e c o m e s larger t h a n 200 to 350A, the g r o w t h rate
sharply decreases. In that case the role of reaction [1] as
m a j o r O- source b e c o m e s less certain and the n e g a t i v e ion
flux out of the plasma gas m i g h t be sufficient to e x p l a i n
t h e o b s e r v e d growth rates.
Conclusion
The yields for the o x i d a t i o n of silicon by neutral o x y g e n
a t o m s and atomic o x y g e n n e g a t i v e ions, r e s p e c t i v e l y
r
and $o-,s~o~, h a v e b e e n defined in this paper. T h e
Table II. Plasma composition and ~o-,s~o2for the average conditions in the Kimura et al. experiment (12, 18)
[e]av
• 101~ cn1-3
Cusp
Mirror
1.3 -+ 0.5
5.9 -+ 3
X
[10;)c~_3
1.31
2.5
X
101FcO~_2 s i
3.47
6.6
R~%
~)o- SiO2
400
950
1.45
1.81
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J. Electrochem. Soc., Vol. 137, No. 2, February 1990 9 The ElectrochemicalSociety.,Inc.
(bo,sio2 yield could be exactly derived from the plasma
afterglow oxidation of silicon and it was found to be very
low and only of the order of 4.4 _+ 1.2 x 10 v at 847 K. This
low yield excludes the role of oxygen atoms as such as oxidizing species in any plasma oxidation system, either with
the silicon sample floating or under anodization conditions. The yield ~o-.s~o2 derived in various plasma anodization systems is orders of magnitude larger and does not
seem to be influenced m u c h by experimental conditions
such as the oxygen pressure, bias voltage, or any geometrical features of the reactor system. Only the temperature
has a determining effect on ~o-.sior
Concerning the role of the negative oxygen atom ions
formed either in the plasma gas itself or in a heterogeneous
reaction at the surface, our calculated yield ~o .s~o2 for a
floating substrate indicates that a heterogeneous mechanism is required at the initial stage of the oxidation process. Once the silicon dioxide layers become of the order
of 200 to 350A, the observed growth rates can be explained
on the basis of negative oxygen atom ions formed in the
plasma itself.
Acknowledgments
We thank the Inter-University Institute for Scientific Research (Belgium) for their financial support. We also are
grateful to IMEC vzw for their scientific cooperation. C.V.
is a Research Director of the National Fund for Scientific
Research, Belgium. The technical assistance of M. Blondeel is also acknowledged.
Manuscript submitted March 28, 1989; revised manuscript received Sept. 5, 1989.
The Catholic University of Leuven assisted in meeting
the publication costs of this article.
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Design and Characterization of a Thermally Stable Ohmic
Contact Metallization on n-GaAs
Ram P. Gupta and W. S. Khokle
Central Electronics Engineering Research Institute, Pilani (Rajasthan) 333 031, India
J. Wuerfl and H. L. Hartnagel
Institute Fuer Hochfrequenztechnik, Technische Hochschule Darmstadt, Germany
ABSTRACT
A new metallization system comprising of Ge-WSiz-Au is developed for thermally stable ohmic contacts to n-GaAs.
Physical properties of materials in the layers and their chemical reaction stability are considered in designing the multilayered structure. Metallized samples are annealed at various temperatures and systematical]y characterized using electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM) to assess the stability of the metallization. Contact resistance is measured to determine its suitability for device application.
It is observed that 1000A thick layer o f W 9 St2 acts as an
effective diffusion barrier in the system and interdiffusive
effects are absent for temperatures up to 460~ Contact resistance better than 5 • 10 ~~ 9cm ~is measured on n-GaAs
(n = 1016 cm-3). The metallization demonstrated resistance
stability up to 700~ The reliability of contact is examined
by thermal aging at 350~ for 200h.
For high-temperature applications of microelectronics,
the solid-state devices are required to function reliably in
hot ambients (>300~ (1-3). The excellent material properties combined with the recent technological advancements have made GaAs a leading contender among
the large gap semiconductors (4-10) for high-temperation
applications. However, one of the most difficult problems
associated with GaAs devices intended for operation at elevated temperatures is the failure of interface integrity between the metal and the semiconductor (11, 12-15). Even in
room temperature preparation of metal-semiconductor
junctions, interdiffusion and interracial reactions occur
and these get accentuated at high temperatures (11, 15, 16).
This thermally induced interaction alters the interface
metallurgy and morphology which profoundly influences
the electrical behavior of the junction. Therefore, the ultimate limit in performance, parametric stability, and longterm reliability of GaAs devices are determined by the
properties of metallization system used for contacts.
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