Comparison of Aluminum Etch Rates in Carbon
Tetrachloride and Boron Trichloride Plasmas
K. Tokunaga,* F. C. Redeker,* D. A. Danner, and D. W. Hess*
Department of Chemical Engineering, University oJ California, Berkeley, California 94720
ABSTRACT
Etch rates of a l u m i n u m a n d "native a l u m i n u m oxide" films were studied
as a f u n c t i o n of substrate t e m p e r a t u r e in a parallel plate plasma etcher using
CC14 a n d BC13 plasmas. Differences in the i n h i b i t i o n period for CC14 vs. BCI~
etching were a t t r i b u t e d to the relative abilities of these etchants to scavenge
oxygen and w a t e r vapor and to etch n a t i v e a l u m i n u m oxide. The t e m p e r a t u r e
dependence of the etch rates suggested basic differences in the r a t e - c o n t r o l l i n g
steps for CC14 vs. BC18 plasma etching. The surface chemistry of the electrode
materials played an i m p o r t a n t role in the etch rates observed with CC14 a n d
BCIs.
Plasma or reactive ion etching of a l u m i n u m and a l u m i n u m alloys is vitally i m p o r t a n t to the VLSI effort.
A n u m b e r of papers have appeared recently describing
the etching of a l u m i n u m films in carbon tetrachloride
or boron trichloride plasmas using parallel plate plasma
etch systems (1-10). However, these publications have
not directly compared etch rate r e s u l t s for carbon
tetrachloride and boron trichloride in the same etch
system. I n this paper we investigate the etch rates of
a l u m i n u m films i n carbon tetrachloride and boron trichloride as functions of substrate t e m p e r a t u r e i n a
parallel plate plasma etcher.
Experimental Procedure
The detailed operation of the parallel plate plasma
etcher used in this study has b e e n described p r e v i ously (10), so only a brief description will be given
here. F i l a m e n t - e v a p o r a t e d or S - g u n sputtered alum i n u m films on either P y r e x cover slips or on oxidized
2 in. diam silicon wafers were placed on the grounded
lower electrode, which was t e m p e r a t u r e controlled.
P o w e r from a 13.56 MHz rf generator was supplied to
the upper electrode, which was separated from the
lower electrode b y a distance of 2.5 cm. Etch gases
e n t e r e d the system through holes i n the hollow u p p e r
electrode, and were exhausted through the lower e l e c t r o d e support column.
The etch r u n s described below were carried out at
100W rf power (0.3 W / c m 2) and 13 Pa (0.1 Torr)
pressure. Since etching proceeded from the substrate
edge to the center, the etch time was taken as the time
required to completely remove the a l u m i n u m film.
CC14 etching.--Using anodized a l u m i n u m e l e c t r o d e s
(10), controllable a l u m i n u m etching was achieved in
pure CC14 plasmas as shown i n Fig. 1. However, Fig.
1 indicates that a large discrepancy (approximately a
factor of 100) is observed b e t w e e n the etch rate of
the "oxide" (which is composed of oxygen and w a t e r
vapor scavenging, and native a l u m i n u m oxide etching)
and the etch rate of the a l u m i n u m .
I n an attempt to d e t e r m i n e a n o r d e r - o f - m a g n i t u d e
estimate of the etch rate of native a l u m i n u m oxide,
a l u m i n u m oxide was sputtered onto silicon wafers
using a Model 304 R a n d e x rf sputtering system. T h e
sputtering atmosphere was 20% 02/80% Ar, at a pressure of 0.01 Torr, which resulted in an a l u m i n u m o x i d e
film with a refractive i n d e x of 1.60. At 100~ s u b strate temperature, the sputtered a l u m i n u m oxide etch
rate was N0.1 nm/sec, which is a p p r o x i m a t e l y a factor
of two higher than that indicated in Fig. 1 for "oxide."
This difference is no doubt related to the fact t h a t
native a l u m i n u m oxide has a different composition t h a n
T (~
10.0
150 125
r
,
I00
75
50
,
,
,
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(D
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E
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v
Many investigators have noted that the native a l u m i n u m oxide film present on a l u m i n u m surfaces etches
slowly (compared to a l u m i n u m ) in c h l o r i n e - c o n t a i n i n g
etch gases (2-6, 10, 11). By utilizing both a t h i n (30-40
n m ) and a thick (>500 n m ) a l u m i n u m film d u r i n g
each etch run, an estimate of the etch rates of a l u m i n u m and of native a l u m i n u m oxide has been made as
shown in Fig. 1 (10). Of course, this etch rate analysis
assumes that the native a l u m i n u m oxide coating is
the sole cause of the slow initiation step in a l u m i n u m
etching. As indicated previously (10), this assumption
is not entirely correct, since w a t e r vapor or oxygen
in the etching c h a m b e r also i n h i b i t a l u m i n u m etching (8, 9, 12). Nevertheless, such an analysis allows
comparisons to be made b e t w e e n the ability of different etch gases to minimize or control oxygen or water
vapor effects, and to etch native a l u m i n u m oxide.
c
"6
n.,.
uJ
0.t
0.01
i
2.40
l
I
2.80
i A-'---I
3.20
I / T x I0 -3 (~
Fig. 1. Etch rate of alumimzm and "oxide" (composed of oxygen
and water vapor scavenging plus native aluminum oxide etching)
films in CCI4 as a function of electrode temperature on anodized
aluminum electrodes.
Electrochemical Society Active Member.
Key words: aluminum plasma etching, boron trichloride plasma,
"
carbon tetrachloride plasma, oxygen scavenging, water vapor
scavenging,
851
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852
$. E~ectrochem. Sac.: S O L I D - S T A T E SCIENCE AND TECHNOLOGY
sputtered aluminum oxide, since scavenging effects
should be similar in both cases.
The etch rate data of Fig~ 1 also indicate that the
activation energies for aluminum and "oxide" etching
are nearly equal (,-,0.2 eV/mole). The meaning of the
equality of the activation energies is not clear, since
the "oxide" etch rate is composed of oxygen and water
vapor scavenging, along with native aluminum oxide
etching. All that can be said at present, is that under
the conditions used in this study, the overall inhibition
period displays an activation energy of ,~0.2 eV/mole.
A few comments can be made in regard to the portion of the inhibition period due to native aluminum
oxide etching. Atomic chlorine appears to be the
etchant for aluminum, while a chlorocarbon species is
believed to be the p r i m a r y etchant for aluminum oxide
(2, 6). Indeed, this is in agreement with results in our
system as well as others (2), that demonstrate the
inability of C12 plasmas to effectively and uniformly
etch aluminum. However, if substrate bombardment is
enhanced, as in the case of RIE, aluminum oxide can
be etched successfully in Ar/CI~ atmospheres (6, 13).
BCI~ etching.--An Arrhenius plot of the etch rate
of aluminum and "oxide" films in BC13 using anodized
aluminum electrodes, is shown in Fig. 2. Unlike the
results obtained with CC14, BCl~ displays little t e m perature dependence of the etch rates-activation energy ,-,0.02 eV/mole for aluminum, and ~0.03 eV/mole
for "oxide." These observations suggest basic differences in the rate-controlling steps for BCls vs. CC14
plasma etching of aluminum.
BCI3 was also used ~6 etch sputtered aluminum o x ide. At a substrate temperature of 100~ the etch rate
was ,~0.01 nm/sec, which is a factor of two lower
than that determined via our analysis on "oxide" films.
Discussion
One of the p r i m a r y differences between CC14 and
BCl~ etching of aluminum involves the ability of CC14
to form unsaturated chlorocarbons, and thus polymer
films in a glow discharge. With CC14, and an active
surface as stainless steel (SS), chlorine atom recombination, chlorocarbon recombination, and reaction of
chlorine with the electrode material are all likely
T (~
I00
150 125
I0.0
i
~
75
1
50
i
I
I
Aluminum
ID
1.0
--
A
A
IlL
E
e0
EE
r
0
IJ.I
0.1
"Oxide"
Ap~Z 1981
processes, and contribute to the concentration of unsaturated chlorocarbons and thus to polymerization
(10). Under the high pressure and low power conditions utilized in our previous study (i0), polymerization of CCI4 on SS electrodes was difficult to avoid,
although at lower pressures, where ion and electr0n
energies are higher, polymerization can be minimized,
and etching can proceed, even with low power densities. For instance, at 0.I Tort, both BCI3 and CCI4
exhibit aluminum etch rates approximately a factor of
two lower with SS than with anodized aluminum
electrodes.
Since boron does not form the same type of polymer
structures as does carbon, no polymer is observed on
the samples or on the electrodes w i t h BCla using SS
or anodized aluminum electrodes. Boron compounds
(possibly a combination of BC18 and boron oxychlorides) do condense or adsorb on the bell j a r walls with
anodized aluminum or SS electrodes, because when
the etch chamber is vented to the atmosphere, a white
deposit (B (OH)3) forms immediately. Such deposits
are never observed with CCI4.
Aluminum oxide etching and oxygen scavenging.-Under the present plasma conditions, the etch rate of
sputtered aluminum oxide is an order of magnitude
higher with CC14 than with BCI~ as discussed p r e viously. Thus, if native aluminum oxide etching is the
p r i m a r y cause of the inhibition period for aluminum
etching, the inhibition period should be shorter for
CC14 than for BCls, at least to the extent that the etch
rate of native aluminum oxide can be equated to that
of sputtered aluminum oxide. Indeed, the inhibition
period is slightly shorter for CC14 etching than for
BCI8 etching, at higher temperatures (100~
but the
difference is less than expected on the basis of sputtered aluminum oxide etch rates. However, the inhibition period is always more variable with CC14 than
with BC13.
In order to c o m p a r e the abilities of CC14 and BC1s
to scavenge ox~]gen -and water vapor from the chamber
atmosphere, and thereby to estimate the relative importance of scavenging in the inhibition period, the
effluent gases from 1% mixtures of oxygen in CC14
and in BC13 were monitored with a UTI Model 100 C
quadrupole mass spectrometer. Figure 3a demonstrates
that when a plasma is ignited in the O2/CC14 mixture,
the intensity of the O2 + peak decreases as expected,
due to the reaction of carbon-chlorine fragments with
O2, but O2 is still present in the plasma effluent. However, when an O2/BC13 plasma is struck, the intensity
of the O2 + peak goes to zero, and remains there until
the plasma is extinguished (Fig. 3b). Further, with
BC18, the O2 + peak intensity is zero with an 02 content in the inlet of approximately 5 volume percent.
In addition, the H20 + peak intensity decreases when
a plasma is ignited in either a BC18 or a CC14 plasma,
but does not go to zero in either case. Nevertheless,
the inhibition time is more reproducible in a BC1s than
in a CC14 plasma. These results indicate that BCI~ is a
more efficient oxygen and possibly w a t e r - v a p o r
scavenger than is CC14. As a result, etch reproducibility
should be improved with BCls compared to CC14, since
etching should not be as critically dependent upon
residual oxygen and water vapor in the etch chamber.
Indeed, such conclusions have been demonstrated in
our system, and these studies are consistent with the
effects of oxygen and water vapor on CC14 etching
(8, 9, 12).
o.ol
I
2.40
1
.....
I....
2.80
I
I
&20
I / T x 10-3 (~
Fig. 2. Etch rate of aluminum and "oxide" (composed of oxygen
and water vapor scavenging plus native aluminum oxide etching)
films in BCI3 as a function of electrode temperature on anodized
aluminum electrodes.
Additional evidence for the scavenging differences
between BC13 and CC14 arises from consideration of
the reflected power observed in CC14 and BC18 plasmas.
The reflected power for a CC14 plasma is shown as a
function of time in Fig. 4. A p p r o x i m a t e l y 1 rain is
required from plasma ignition until the reflected power
reaches a minimum value. However, only 15 sec is
required when BC18 is used (Fig. 4). The exact time
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VoL 128, No. 4
ALUMINUM ETCH RATES
I
I
Plasma
On
~
9
a.
Plasma
Off
r
"s
n-
I
2
0
Time (rain)
I
b.
~9
Plasma
"-
Plasma
On
Off
e-
2
0
I
Start
2
T i m e (rain)
Fig. 3. (a) Relative 02 + peak intensity in a 1% 02/99% CCI4
plasma as a function of discharge time. (b) Relative 02 + peak intensity in a 1% 02/99% BCI3 plasma as a function of discharge
time.
r e q u i r e d to reach a m i n i m u m varies s l i g h t l y ( a p p a r e n t l y due to w a t e r c o n c e n t r a t i o n ) ; however, the t r e n d
is a l w a y s the same. F u r t h e r , t h e m i n i m u m reflected
p o w e r is less w i t h BC18 t h a n xvith CC14, a n d this obs e r v a t i o n a p p a r e n t l y depends u p o n w a t e r c o n c e n t r a tion also. These results are consistent w i t h the mass
s p e c t r o m e t e r studies shown in Fig. 3, in t h a t o x y g e n
is still p r e s e n t in the p l a s m a effluent w h e n a p l a s m a
is ignited in a 1% OJCC14 mixture, b u t is absent w i t h
a 1% O2/BC13 m i x t u r e . Also, Fig. 3 a n d 4 d e m o n s t r a t e
a m o r e r a p i d change in O2 + p e a k i n t e n s i t y and r e flected power, respectively, for BCI8 as c o m p a r e d to
CCL~. Thus t h e reflected p o w e r m a y indicate th4 portion
of the inhibition p e r i o d due to o x y g e n and w a t e r v a p o r
scavenging. In fact, in a l u m i n u m etch runs, the a l u -
I
I0 -
I
I
I
I 0 0 W Forward Power 0.1 Torr Pressure
~
a.
15
--
o
4
-
LCI3~
0
0
1
I
I
l
2
:3
T i m e (min)
4
Fig. 4. Reflected power in BCI3 and CCI4 plasmas as a function
of discharge time.
853
m i n u m has not b e g u n to etch at the p o i n t in time
w h e r e the reflected p o w e r reaches a minimum. Rather,
it begins to etch a f t e r a p p r o x i m a t e l y one a d d i t i o n a l
m i n u t e of p l a s m a exposure.
The above results are consistent w i t h discharge i m p e d a n c e monitoring of CCl4 plasmas d u r i n g a l u m i n u m
etching using RIE (14). In that study, a g r a d u a l d e crease (over -~1 rain) of electrode voltage was o b s e r v e d a f t e r ignition of the CC14 plasma. Thus, i t is
quite l i k e l y t h a t o x y g e n o r w a t e r v a p o r scavenging
was in p a r t responsible for the i m p e d a n c e changes o b served at the s t a r t of the etch process.
Since BC13 is a r e a s o n a b l y efficient s c a v e n g e r of
o x y g e n and w a t e r vapor, one can assume t h a t the
inhibition p e r i o d for BC18 etching is p r i m a r i l y due to
n a t i v e a l u m i n u m oxide etching. However, the etch
ra~e estimate from a l u m i n u m samples is ~ 2 t i m e s
h i g h e r than t h a t o b s e r v e d for s p u t t e r e d a l u m i n u m
oxide. This d i s c r e p a n c y is no d o u b t r e l a t e d to the
h e a v i l y h y d r a t e d n a t u r e of n a t i v e a l u m i n u m oxide
surfaces (15). Since BCls reacts r e a d i l y w i t h water,
the native oxide is p r o b a b l y m o r e reactive t o w a r d
BCI~ than is s p u t t e r e d a l u m i n u m oxide.
CC14, on the o t h e r hand, displays an "oxide" etch
rate that is ~ 2 times l o w e r t h a n that of s p u t t e r e d a l u m i n u m oxide. Again, this is p r o b a b l y due to the high
moisture content of the n a t i v e oxide surface, since
CC14 plasmas are not efficient o x y g e n and w a t e r v a p o r
scavengers, and, in fact, these species t e n d to i n h i b i t
a l u m i n u m etching. Currently, it is not clear w h y CC14
plasmas can etch s p u t t e r e d a l u m i n u m oxide m o r e
r a p i d l y t h a n BC18 plasmas. However, such results m a y
be r e l a t e d to differences in the p l a s m a and electrode
potentials b e t w e e n CC14 and BC13, or to different concentrations of e t c h a n t species g e n e r a t e d b y the two
etch gases.
Chemistry Of BCls and CCI4 etching.--Etch r a t e
studies w i t h BCI8 and CCI~ have i n d i c a t e d t h a t the
a l u m i n u m etch r a t e is h i g h e r on anodized a l u m i n u m
than on S S electrodes. This difference m a y be p a r t l y
due to higher r e c o m b i n a t i o n a n d / o r reaction r a t e of
chlorine free radicals, which have been proposed to
be the etchant species for aluminum, on SS surfaces.
Such conclusions are consistent w i t h studies of fluorine
a t o m r e c o m b i n a t i o n on steel vs. a l u m i n a surfaces (16).
These l a t t e r studies have shown that steel is a b e t t e r
r e c o m b i n a t i o n surface for fluorine atoms t h a n is a l u mina.
A n o t h e r consideration w i t h r e g a r d to electrodes involves the effect of o x y g e n from the electrode m a t e r i a l (10, 23). Oxygen could p r o m o t e the r e m o v a l of
boron from the a l u m i n u m surface if boron o x y c h l o rides are formed, and in addition can g e n e r a t e a d d i tional chlorine atoms b y reaction with b o r o n - c h l o r i n e
fragments. Thus, enhanced a l u m i n u m etch rates m a y
be expected.
Comparison of Fig. 1 and 2 suggests t h a t u n d e r the
p l a s m a conditions i n v e s t i g a t e d in this study, basic
differences exist in the r a t e - l i m i t i n g steps for BC18
vs. CC14 etching of a l u m i n u m . A possible r e a c t i o n seq u e n c e for the etching of a l u m i n u m in BCI~ p l a s m a s
can be w r i t t e n as follows
BCl3(g) -5 e--> B C l 2 ( g ) -5 C l ( g ) -{- e
[1]
C1 (g) -* C1 (ads)
[2]
Cl(ads) + AI ~ AlCl.(ads)
[3]
AlCl(ads) -~ AlCl(g)
[4]
The dissociation of BCI3 via electron collisions into
BC12 has been indicated previously, since rf glow discharge dissociation of BCI3 represents a high yield
pathway for the formation of B2CI4 (17). Also, the
adsorption of chlorine gas and/or chlorine atoms onto
clean aluminum surfaces has been reported (18).
Clearly, gas phase species other than Cl2 or C1 might
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854
J. E$ectrochem. Soc.: S O L I D - S T A T E S C I E N C E A N D T E C H N O L O G Y
adsorb onto a l u m i n u m surfaces (for instance, CClz or
B C I j , b u t for the p u r p o s e of the following discussion,
these possibilities need not be considered. A1C1 has
been o b s e r v e d b y emission spectroscopy of CCI~
p l a s m a s (19). Of course, A I C l ( a d s ) can f u r t h e r r e act on the a l u m i n u m surface, the electrode surface,
or in the gas phase, to f o r m A1CI~ a n d / o r A1CI~ as the
final reaction product. The above m e c h a n i s m (Eq. [1][4]) is in principle s i m i l a r to t h a t proposed for t h e
etching of silicon in CF4 p l a s m a s (20). The p r e s e n t
mechanism, however, considers the adsorption of C1
atoms on the a l u m i n u m surface, as w e l l as the possib i l i t y of desorption of various a l u m i n u m - c h l o r i n e
f r a g m e n t s in a d d i t i o n to A1Clz. Indeed, f r a g m e n t d e sorption has been d e m o n s t r a t e d for the etching of
silicon w i t h fluorine-containing gases (21).
Since reactions [2]-[4] a r e surface reactions, a t e m p e r a t u r e dependence is expected for these steps. On
the other hand, reaction [1] r e p r e s e n t s a gas phase
generation of chlorine free radicals, which is i n d e p e n d e n t of s u b s t r a t e t e m p e r a t u r e . Thus, the r a t e d e t e r m i n i n g step for BC18 etching of a l u m i n u m on
anodized a l u m i n u m or stainless steel electrodes is
p r o b a b l y the gas phase generation of etchant species.
Such conclusions are consistent w i t h previous a l u m i n u m etch studies using BC13 on stainless steel
electrodes (2).
A s i m i l a r reaction sequence for CC14 etching can be
w r i t t e n b y r e p l a c i n g reaction [1] w i t h [ l a ]
C C h ( g ) + e--> CCIs(g) + C l ( g ) § e
[la]
Indeed, mass spectrometric studies have shown that
CC13 is p r e v a l e n t in CC14 discharges (22). Since a
s u b s t r a t e t e m p e r a t u r e dependence is o b s e r v e d for
CC14, one (or more) surface reactions of the type
[2]-[4] control the etch rate.
Reactions similar to [1]-[4] can be w r i t t e n for the
etching of native a l u m i n u m oxide. However, the composition of this l a y e r is uncertain, and so no specific
reactions are proposed. Nevertheless, if it is assumed
that oxygen assists the r e m o v a l of boron or carbon
species, the chlorine is left to react with aluminum.
Indeed, carbon oxides and oxychlorides are w e l l
known, and g a s - p h a s e oxychlorides of boron h a v e been
r e p o r t e d (24). These considerations a r e consistent
with t h e i m p r o v e d etching characteristic of CC14 and
BC13 on anodized a l u m i n u m electrodes c o m p a r e d to
stainless steel electrodes. On stainless steel, an efficient
means of r e m o v i n g carbon and boron is not a v a i l a b l e
after the native a l u m i n u m oxide coating has been r e moved. However, w i t h anodized a l u m i n u m electrodes,
small concentrations of oxygen are p r e s e n t to assist
r e m o v a l of carbon (23) or boron species b u t the concentrations are sufficiently low to p r e v e n t inhibition of
a l u m i n u m etching by a l u m i n u m oxide formation.
The scavenging studies discussed above indicate t h a t
the r e l a t i v e r e a c t i v i t y of oxygen with a l u m i n u m vs.
CCI.~ or BCI~ is an i m p o r t a n t aspect of a l u m i n u m
p l a s m a etching. A p p a r e n t l y , in the competition for
o x y g e n b e t w e e n BClx and AI, BCI~ is the p r e f e r r e d
reactant, whereas w i t h CClz, a l u m i n u m m a y be the
b e t t e r scavenger. Certainly, the etch r a t e of a l u m i n u m
samples decreased when oxygen was a d d e d to CC14
(10), and our analysis indicated that both the "oxide"
and the a l u m i n u m etch rates decreased. However,
small additions ( ~ 1 % ) of oxygen to BC13 resulted in
a decrease in the "oxide" etch rate, but an increase in
the a l u m i n u m etch rate, such that the overall etch rate
increased. H i g h e r o x y g e n concentrations r e s u l t in
boron oxide deposition. These results are consistent
with previous studies which r e p o r t an increase in o v e r all a l u m i n u m etch rate w i t h oxygen addition to BC18
plasmas (7).
In CCl~ etching, o x y g e n reacts w i t h a l u m i n u m or
carbon species, while with BC13, o x y g e n reacts p r e f e r e n t i a l l y with boron to form oxides or oxychlorides,
thus g e n e r a t i n g a d d i t i o n a l chlorine atoms, which en-
April I981
hance the a l u m i n u m etch rate. This l a t t e r reaction
is analogous to t h a t o b s e r v e d w h e n o x y g e n is a d d e d
to CF4 d u r i n g silicon etching (25).
If CC14 is a b e t t e r etchant for a l u m i n u m films than
is BC13, and if BCI~ is a b e t t e r scavenger for o x y g e n
and w a t e r v a p o r than is CC14, then an i m p r o v e d etcha n t for a l u m i n u m m a y be some m i x t u r e of the two.
Indeed, p r e l i m i n a r y studies in our l a b o r a t o r y indicate
that such m i x t u r e s d i s p l a y higher etch rates and b e t t e r
r e p r o d u c i b i l i t y t h a n e i t h e r CC14 or BC13 individually.
However, boron residues m a y p r e c l u d e their usefulness.
Summary
The etch rates of a l u m i n u m and " n a t i v e a l u m i n u m
oxide" (composed of n a t i v e a l u m i n u m oxide etching
and o x y g e n and w a t e r v a p o r scavenging) in CC14 and
BC13 plasmas were i n v e s t i g a t e d as functions of s u b strate t e m p e r a t u r e in a p a r a l l e l plate p l a s m a etcher.
O x y g e n and w a t e r v a p o r scavenging a p p e a r e d to be
the most i m p o r t a n t process in initiating a l u m i n u m
etching in CC14 plasmas, while a l u m i n u m oxide etching was the p r i m a r y factor in the inhibition p e r i o d
w i t h BC18 plasmas. These results suggest a chemical
competition for oxygen or w a t e r v a p o r b e t w e e n a l u m i n u m and either b o r o n - c h l o r i n e or c a r b o n - c h l o r i n e
fragments, w i t h boron compounds being the b e t t e r
scavengers.
The etch rates of a l u m i n u m in CC14 p l a s m a s on
anodized a l u m i n u m electrodes w e r e critically d e p e n d ent upon t e m p e r a t u r e , indicating t h a t surface reactions
w e r e r a t e controlling. However, BC13 etching on
anodized a l u m i n u m or on SS electrodes showed little
or no t e m p e r a t u r e dependence, suggesting that the
r a t e - c o n t r o l l i n g s t e p ( s ) involved gas phase g e n e r a tion of etching species.
The etch rate of a l u m i n u m using BC18 was d e p e n d e n t upon the electrode material, w i t h the etch
r a t e being g r e a t e r b y a factor of 2 w i t h anodized a l u m i n u m than w i t h SS electrodes. The differences o b served are believed to be due to increased chlorine
atom r e c o m b i n a t i o n on SS electrodes a n d / o r to the
ease of surface boron r e m o v a l and a d d i t i o n a l chlorine
atom g e n e r a t i o n w h e n o x y g e n is a v a i l a b l e f r o m the
electrode material.
Acknowledgments
The authors w o u l d like to t h a n k Mr. G. A u at
A m d a h l Corporation for s u p p l y i n g the s p u t t e r e d a l u m i n u m films used in this study. This m a t e r i a l is based
upon w o r k s u p p o r t e d by the N a t i o n a l Science F o u n d a tion u n d e r G r a n t ENG-7812236.
Manuscript s u b m i t t e d Sept. 8, 1980; r e v i s e d m a n u script received Nov. 17, 1980.
A n y discussion of this p a p e r will a p p e a r in a Discussion Section to be published in the D e c e m b e r 1981
JOURNAL. A l l discussions for the D e c e m b e r 1981 Discussion Section should be s u b m i t t e d b y Aug. 1, 1981.
Publication costs of this article were assisted by the
University of California.
REFERENCES
1. A. R. Reinberg, in "Etching for P a t t e r n Definition," H. G. Hughes and M. J. Rand, Editors,
p. 91, The Electrochemical Society Softbound
Proceedings Series, Princeton, N.J. (1976).
2. R. G. Poulsen, H. Nentwich, and S. Ingrey, in " P r o ceedings of the I n t e r n a t i o n a l Electron Devices
Meeting," p. 205, Washington, D.C. (1976).
3. T. O. H e r n d o n and R. L. Burke, Interface '77 K o d a k
Microe~ectronics Symposium, Monterey, California, 1977.
4. R. Reichelderfer, D. Vogel, and R. L. Bersin, A b stract 151, p. 414, The Electrochemical Society
E x t e n d e d Abstracts, Atlanta, Georgia, Oct. 9-14,
1977.
5. A. Yasuoka, H. Nagata, H. Harada, and T. Enomoto,
IPC Technical Report, 1977.
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"CoL 128, No. 4
ALUMINUM ETCH RATES
6. P. M. Schaible, W. C. Metzger, and J. P. Anderson,
J. Vac. Sci. Technol., 15, 334 (1978).
7. M. N a k a m u r a , M. Itoga, a n d Y. Ban, A b s t r a c t 114,
p. 298, The E l e c t r o c h e m i c a l Society E x t e n d e d
Abstracts, St. Louis, Missouri, M a y 11-16, 1980.
8. T. Y. Fok, A b s t r a c t 115, p. 301, The Electrochemical
Society E x t e n d e d Abstracts, St. Louis, Missouri,
M a y 11-16, 1980.
9. D. K. R a n a d i v e a n d D. L. Losee, A b s t r a c t 116, p.
304, The Electrochemical Society E x t e n d e d A b stracts, St. Louis, Missouri, M a y 11-16, 1980.
10. K. T o k u n a g a and D. W. Hess, This Journal, 127, 928
(1980).
11. R. L. Bersin, Solid State Technol., p. 117, (April,
1978).
12. R. Le Claire, ibid., p. 139, (April, 1979).
13. N. Heiman, V. Minkiewicz, and B. Chapman, J. Vac.
Sci. Technol., 17, 731 (1980).
14. K. U k a i and K. Hanazawa, ibid., 16, 385 (1979).
15. M. S. H u n t e r and P. Fowle, This Journal, 103, 482
(1956).
855
16. P. C. N o r d i n e a n d J. D. Le Grange, A I A A Journal,
14, 644 (1976).
17. T. Wortik, R. Moore, and H. J. Schlesinger, J. Am.
Chem. Soc., 71, 3265 (1949).
18. T. Smith, Surf. Sci., 32, 527 (1972).
19. B. J. Curtis and H. J. B r u n n e r , This Journal, 125,
829 (1978).
20. H. F. Winters, J. Appl. Phys., 49, 5165 (1978).
21. V. M. D o n n e l l y and D. L. F l a m m , A b s t r a c t 123, p.
323, The Electrochemical Society E x t e n d e d A b stracts, St. Louis, Missouri, M a y 11-16, 1980.
22. C. M. M e l l i a r - S m i t h and C. J. Mogab, in "Thin
F i l m Processes," J. L. Vossen and W. Kern, E d i tors, p. 497, A c a d e m i c Press, New Y o r k (1978).
23. J. W. Coburn, H. F. Winters, and T. J. Chuang,
J. Appl. Phys., 48, 3532 (1977).
24. J. Blauer a n d M. F a r b e r , J. Chem. Phys., 39, 158
(1963).
25. C. M. M e l l i a r - S m i t h and C. J. Mogab, in "Thin
F i l m Processes," J. L. Vossen and W. Kern, E d i tors, p. 505, A c a d e m i c Press, New Y o r k (1978).
Reduction of Grain Boundary
Effects in Indium Phosphide Films by Nitridation
T. L. Chu* and Shirley S. Chu*
Southern Methodist University, Dallas, Texas 75275
C. L Lin* and Y. C. Tzeng*
Poly Solar Incorporated, Garland, Texas 75041
and L. L. Kazmerski and P. J. Ireland
Solar Energy ReseaTch Institute, Golden, Colorado 80401
ABSTRACT
I n d i u m phosphide, a direct gap semiconductor w i t h a r o o m t e m p e r a t u r e
e n e r g y gap of 1.35 eV, is a p r o m i s i n g photovoltaic m a t e r i a l for t h i n film d e vices. Thin films of i n d i u m phosphide have been deposited on t u n g s t e n - c o a t e d
g r a p h i t e s u b s t r a t e s by the reaction of indium, h y d r o g e n chloride, a n d phosphine in a gas flow system. W i t h o u t i n t e n t i o n a l doping, the deposited films
a r e n - t y p e w i t h a c a r r i e r concentration of about 10 z7 cm -3. S c h o t t k y b a r r i e r s
p r e p a r e d from these films have been found to e x h i b i t low rectification ratios,
high d a r k currents, and poor photovoltaic response due to the shunting effects
of g r a i n boundaries. The grain b o u n d a r y effects can be p a r t i a l l y r e d u c e d b y
h e a t - t r e a t m e n t in an a m m o n i a atmosphere, converting the surface of grains
into i n d i u m nitride. The f o r m a t i o n of i n d i u m n i t r i d e has been verified b y
A u g e r electron spectroscopy and x - r a y photoelectron spectroscopy.
Direct gap semiconductors w i t h b a n d g a p e n e r g y of
1.3-1.8 eV a r e most promising m a t e r i a l s for thin film
photovoltaic devices. Because of t h e i r sharp optical
absorption edges and large absorption coefficients, solar
r a d i a t i o n w i t h e n e r g y in excess of the b a n d g a p e n e r g y
a r e essentially all a b s o r b e d w i t h i n 2-3 #m of the surface. Thus, the g r a i n size and m i n o r i t y c a r r i e r diffusion
l e n g t h r e q u i r e m e n t s a r e less stringent t h a n those r e q u i r e d of i n d i r e c t - g a p materials. A m o n g I I I - V compounds, g a l l i u m arsenide w i t h a r o o m t e m p e r a t u r e ene r g y gap of 1.43 eV a n d i n d i u m phosphide w i t h a
b a n d g a p e n e r g y of 1.35 eV a r e considered as most
promising. F o r example, g a l l i u m arsenide films d e posited on g r a p h i t e or t u n g s t e n - c o a t e d g r a p h i t e s u b strates b y the h a l i d e process have an a v e r a g e g r a i n
size of about 10 #m, and l a r g e a r e a (10 cm 2) MOS solar
cells have been f a b r i c a t e d from this t y p e of p o l y c r y s talline films (1, 2). The AM1 efficiency of thin film
gallium arsenide solar cells is h i g h e r t h a n 8%; while
* Electrochemical Society Active Member.
Key words: semiconductor, photovoltaic, thin films.
their s h o r t - c i r c u i t c u r r e n t density is about 80% of t h a t
of single c r y s t a l l i n e g a l l i u m arsenide solar cells, their
o p e n - c i r c u i t voltage is c o n s i d e r a b l y lower. I n d i u m
phosphide films have also been deposited on foreign
substrates such as carbon and m o l y b d e n u m , and small
area (3 m m 2) thin film solar cells of the configuration
n - C d S / p - I n P / p + - G a A s / c a r b o n fabricated. A l t h o u g h
the p h o t o c u r r e n t from thin film cells is s i m i l a r to t h a t
from single c r y s t a l l i n e C d S / I n P cells, thin film cells
have c o n s i d e r a b l y l o w e r o p e n - c i r c u i t voltage and fill
factor, resulting in a conversion efficiency of about
5.7%.
The c o n s i d e r a b l y l o w e r o p e n - c i r c u i t voltage in thin
film cells is due p r e s u m a b l y to the high s a t u r a t i o n
c u r r e n t which arises f r o m the poor g r a i n s t r u c t u r e or
grair~ b o u n d a r y effects. In this work, i n d i u m phosphide
films have been deposited on t u n g s t e n - c o a t e d g r a p h i t e
substrates b y the halide process. S c h o t t k y b a r r i e r d e vices p r e p a r e d from these films exhibit excessive d a r k
currents due to the shunting effects of grain b o u n d aries. The grain b o u n d a r y effects can be r e d u c e d s u b -
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