Note: Descriptions are shown in the official language in which they were submitted.
26, ~ ~113~3
This invention relates to electrically conductive
cadmlum stannate films (Cd2SnO4) useful as transparent elec-
trodes for electrochromic eyeglasses, window dlsplays, liquid
crystal displays, solar cells, heat-reflective coatings for
solar energy conversion devices and the like. It relates
particularly to an improved reliable and economical deposi-
tion technique for cadmium stannate films.
Heretofore, electrically conductive Cd2SnO4 films
have been prepared by deposition from hot-pressed Cd2SnO4
powder targets using radio-frequency (RF) sputtering as shown
by A. J. Nozik, U.S. ~o. ~811J95~. For that technique, the
cadmium stannate powder has to be synthesized first by firing
mixtures of CdO and SnO2. The powder is then hot-pressed to
i form a ceramic target plate and bonded to a backing plate
suitable for mounting into a RF sputtering unit,
The hot-pressing of powders is complicated and
time consuming and requires expensive equipment. Indeedg if
target sizes exceeding 30 cm in diameter are needed, the
equipment costæ become prohibitlve. The bonding of the cer-
amic target to a backing plate i8 also complicated and ex-
pensive since the bond must provide for good thermal conduc-
tivity 90 that heat generated during the sputtering process
at the target surface can be readily removed by water cooling
the backing plate.
Even with water cooling hot-pressed ceramic target
plates cannot be used for sputtering at high RF powers. High
RF powers heat the target surface to temperatures which cause
thermal stresses in ceramic plates and eventually destroy
them, thus requiring frequent costly replacements. Limita-
~0 tions on the permissible RF power, however, restricts the
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film deposition rate. merefore, sputtering from ceramic targets is a
slow, uneconomical process.
In accordance with the present invention, it has now been
discovered that target plates having the composition Cd2SnO4 are not
required for the sputter deposition of Cd2SnO4 films. Thus, it has now
been found that cadmium stannate can also be formed when a target plate
is used consisting of a cadmium/tin metal alloy. The sputter deposition
of binary metal oxides from metal targets in an oxygen plasma is a
known technique and is generally referred to as reactive sputtering.
Thus, the preparation of & 2 films by reactive sputtering has been
shown by E. Giani and R. Kelly, A Study of SnO2 Thin Films Formed by
Sputtering and by Anodizing, Journal of the Electrochemical Society,
Vol. 121, No. 3, pages 394 - 399. Hcwever, as far as is known, the
successful reactive sputtering of ternary oxides containing two differ-
ent metals has not yet been reported.
Thus, the sputtering from an alloy metal target, such as
a cadmium-tin target, would not be expected to produce a Cd2SnO4 film,
but most probably a film comprised of a mixture of one or both of the metals
per _, and/or one or more of the oxides of the metals. Surprisingly,
however, we have found that Cd2SnO4 films can be produced by sputter-
ing from cadmium~tin metal targets in an oxygen plasma, as discussed
hereinafter.
~ccordingly, the present invention provides -che method for
preparing cadmium stannate films by RF or DC sputtering, the improvement
which oomprises sputtering the cadmium stannate from a cadmium-tin metal
alloy target having a mol ratio of cadmium to tin of from about 1.7:1 to
about 2.5:1 in an oxygen plasma.
The advantages of replacing Cd2SnO4 ceramic target plates by
Cd/Sn metal alloy plates are manifold. First, the metal targets can be
easily manufactured by simply melting the required am~unts of cadmium and
tin metal together and casting the melt into plates. Expensive hot-
pressing is not
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needed. Also, the metal alloy plates can be machined into
any size or shape. The superior mechanical strength of metal
over ceramic plates makes handling easy and destruction by
thermal stresses does not occur. Moreover, the high thermal
conductivit~ of metals, compared to ceramics, enables easy
heat removal from the target surface during sputtering so
that the metal plate can be bonded to the backing plate with
readily available conductive epoxies. Complicated and costly
bonding processes are not required.
Another advantage of econo~ical significance is
that metal targets are electrically conductive so that sput-
tering at radio-frequencies is not necessary. Rather, direct
current (DC) sputtering can be applied. Power supplies for
DC sputtering are less expensive than for RF sputtering,
thus requiring lower capital costs.
Finally, since the mechanical properties of metal
targets do not place a restriction on the power level util-
ized during sputtering high deposition rates are possible.
This improves the economics of the sputterin~ process by
shortening the overall preparation times.
The cadmium to tin ratio in the metal target plates
is not very critical. For the compound Cd2SnO4 a Cd/Sn mol
ratio of 2:1 would be expected to be best, however, equally
good film properties (high electrical conductivity and high
optical transmission) are obtained fram targets which are
deficient in cadmium. The Cd/Sn mol ratio can be a~ low as
1.7:1 and still excellent transparent conductor properties
are obtained. On the other hand~ a Cd/Sn ratio as high as
2.5:1 may suitably be used.
The plasma gas is preferably pure oxygen. However,
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acceptable electrical and optical film properties are ob-
tained if mixtures of oxygen and argon or other inert gases
such as nitrogen are used.
An important step in utilizing cadmium/tin metal
targets for the sputter deposition of Cd2SnO~ films is the
condition1ng of the target surface. Thus~ films prepared
from a newly made target plate will contain an excess of CdO
as a second phase resulting in non-reproducible electrical
and optical properties even after a post-deposition heat
treatment. The reason for this is a preferential sputtering
of the cadmium component from the target. Continuous sput-
tering leading to the removal of 1-10 ~m target surface brings
the surface to equilibrium after which reproducible trans-
parent conductor properties are obtained.
The following examples will serve to illustrate
the invention.
~xam~le 1
A five inch diameter target plate consisting of a
cadmium/tin metal alloy in the 2:1 mol ratio was installed
in an 18 inch diameter vacuum chamber and sputtered in an
oxygen plasma (8 ~u pressure) for 90 minutes onto a shutter
in order to condition and equil1brate the target surface.
After this, four 1" x 1" x 1 mm silica substrates were placed
under the target plate at a target substrate distance of 2
inches and coated with Cd2SnO4 at 8 ~ oxygen pressure and
900 watts RF power. After 20 minutes the samples were re-
moved from the sputter unit and heat treated at 650C. in an
Ar/CdS atmosphere. The re ulting films were optically clear
with 2.8 ohm/square electrical sheet resistance and 84~ lu-
minous transmittance.
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Example 2
The same target as in example 1 was used to coat a
1.5" x 1" substrate consisting of Corning 7059 glass. The
sputtering time was five minutes at 900 watts RF power. Af-
ter heat treatment the sample film had 13.6 ohm/square sheet
resistance and 87% luminous transmittance.
Exam~le 3
The above cadmium/tin alloy target was used to
coat four 1" x 1" x 1 mm silica substrates. After 30 min-
utes deposition time and post-deposition heat treatment (20
minutes at 650C. in Ar/Cds) the films had 1.8 ohm/square
electrical sheet resistance and 82~ luminous transmittance.
Equivalent result~ to those shown in the foregoing
exampleR are obtained by the use of DC rather than RF sput-
tering technique.
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