Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method for producing
large size cadmium mercury telluride sputtering targets of
a homogeneous composition.
Cadmium mercury telluride, referred to as CMT here-
inafter, is a continuous series of ternary compounds having
the general formula of CdxHgl xTe wherein x has values of
between zero and one. Compounds exhibiting semi-conducting
properties have values of x in the range of about 0.14 to about
1. Semi-conducting compounds of CMT find application in the
solid-state electronics industry in, for example, infrared
detectors.
Presently, the most advanced type of material avail-
able for infrared detectors is linear array detector strips
made rom bulk single crystal material and measuring about
20 mm by 1.5 mm or less. These monolithic arrays are made
from bulk CMT and may contain up to 200 elements depending on
the homogeneity and size of the bulk CMT available. The manu-
facture of arrays with a higher number of elements is too
complex to be handled by methods normally used ~or connecting
the elements to the external electronics.
A simpler and potentially less expensive system
could be obtained by switching from a linear array to a focal
plane array which resembles, for instance, the solid-state,
charge coupled device (CCD) television camera operating in the
visible light range. The CCD approach makes use o~ a multi-
plexing function, i.e. the data from the focal plane array are
obtained in a multiplex form so that individual element leads
are not required with the focal plane system, whereas they are
required with linear arrays. A focal plane array, therefore,
makes it possible to use, for example, 1,000 or more elements,
resulting in much simpler scanning or no scanning at all while
retaining the high resolution and extreme sensitivity that are
required for sophisticated thermal imaging.
~o~
Although the feasihility of multiplexing CMT with a
silicon CCD has been demonstrated, there is presently no practical
method known whereby a focal plane array i~n CMT can be prepared
which possesses the required extreme homog,eneiky of composition
and the required electrical parameters. However, sputtering,
one of the thin-film techniques whereby a thin layer of C~lT is
deposited on a suitable substrate, such as for example silicon,
may make it possible to make focal plane arrays with the required
extreme homogeneity and a compatibility with multiplexing.
Sputtering is used to grow thin layers onto substrates
epitaxially,i.e., the crystal orientation of the substrate is
continued into the epitaxial layer, This is carried out in a
chamber which i8 maintained under a partial vacuum and in which
a sputtering target of the material to be deposite~ i~ mounted on
a water or air-cooled holder or backing plate. ~ heam o ions,
for example argon, from an RF generator or a glow-discharge gun,
is directed onto the target and causes sputtering of surface
material from the target. The liberated material deposits on one
or more suitable substrates arranged at a distance about the
sputtering target. The epitaxial layer deposited by sputtering
on the substrate has essentially the same composition as that of
the target. In the case of CMT it is essential that the targets
possess extreme homogeneity in composition,
Sputtering targets are used in a variety of siæes and
shapes, but the sizes of sputtering targets made of CMT are
limited because of the difficulty of preparing CMT which has the
required homogeneity. The methods for preparing CMT with a
homogeneous composition are usually processes involving crystalli-
zation and these methods are limited by the constrictions imposed
by the CdTe-HgTe phase diagram, viz., the large temperature
difference between the solidus and liquidus lines and the high
pressures involved at the higher values for x, the latter especi-
ally requiring sophisticated and consequently expensive equipment~
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For example, preparation of in~ots of CMT by the melt re-crystal- -
lization process involves temperatures of 700 and 800C at a
pressure of about 4000 KPa for CdxHgl xTe wherein x - 0.2 and of
800 and 950~C at 8000 KPa for CdxHgl xTe wherein x = 0.5. Conse-
quently, ingots so prepared have a diameter usually not larger than
about 15 mm and only portions of the ingots are of sufficiently
homogeneous composition to allow preparation of sputtering targets
such as are obtained by slicing CMT ingots perpendicular to the
axis of the ingot or by slicing strips of C~IT from the ingot along
the isocomposition lines. ~ence, the present limitation of the
size of linear array aetector strips of about 20 mm by 1.5 mm or
less~ unless mosaic patterns with complex lap-joint~ are used.
~ le have now found that CMT sputterin~ targ~3ts of rela~
tiv~ly large s~æ~, i.e., 9.~Ze~ larger than heretoEore possible,
can be made by size reduction o~ C~T to obtain finely divided C.~IT
and compression of the finely divided CMT into compacts of desired
large dimensions. Thus, C~IT sputtering targets of a desired
composition of crlT are prepared hy compacting finely divided C~lT
of similar composition and contained in a die under the application
of a compacting pressure suitable to produce a coherent compact of
CMT.
~ ccordin~ly, there is provided a method for the prepara-
tion of sputtering targets of cadmium mercury telluride of the
general formula CdxHgl xTe wherein x has values in the range of
about 0.14 to 0.60 which comprises the steps of preparing finely
divided cadmium mercury telluride of desired composition, said
finely divided cadmium mercury telluride having particle sizes all
less than 150 ~, mixing the particles of the finely clivided
cadmium mercury telluride to obtain a substantially even particle
size distribution, adding a predetermined amount of the mixed
42~
particles to a die of desired dimensions~ applying a compactiny
pressure to said amount to compact the finely divided cadmium
mercury telluride into a coherent compact having a density of
at least 97% of theoretical density~ releasing the pressure and
removing the compact having predetermined dimensions rom the
die, said predetermined amount of mixed particles being sufficient
to form said compact of predetermined dimensions.
According to a second embodiment of the invention,there
; 10 is provided a method si.milar to that of the first embodiment but
the values of x are restricted to the range of from about 0O14
up to about 0.20~the die is preheated to a temperature in the
rangé of about 100 to 300C and the compacting pressure is at
least about 400 MPa,
~ccording to a third embodiment of the invention~ there
is provided a method ~im~lar to that of the first embodiment,
but the values of x are restricted to the range of from about 0.20
to about 0.60, the die is preheated to a temperature of about
300C and the compacting pr~ssure is in the range of about 160 to
275 ~IPa.
According to other embodiments there are provided
Sputtering targets of coherent compacts of cadmium mercury tellu-
ride prepared according to the f~rst, second and third embodiment.
Finely divided CMT may be single or poly-crystalline
and may be prepared by size reduction of ingots or portions
thereof, of slices, or of other forms of CMT. Preferably, the
CMT has a composition wherein x has values in the range of about
0,14 to 0.60. The sources of the finely divided CMT should be
of homogeneous composition, but slight variations in composition
may be allowable as such variations tend to disappear, i.e.
average out! in the final composition of the compact. The particle
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sizes of the finely divided CMT should extend over a range of
sizes so that maximum density for the compacted CMT is obtained.
~article sizes of the finely divided CMT all less than 150 ~
(micron) are generally fine enough to ensure the required density
of the compact. The preferred range of palrticle sizes is from
150 to 44 ~. The size reduction is achieved by known methods
such as by grinding or crushing. If desired, the size reduction
-is performed under an inert or reducing at:mosphere, such as, for
example, an atmosphere of argon or hydrogen gas.
The finely divided CMT is thoroughly mixed to obtain a
substantially even particle size distribution. A predetermined
amount of the mixed CMT is added to a die of such form that a
compact with the desired dimensions will be o~tained. The CMT
i8 pre~erably ~dded at room temperature to avoid dcterioration
o~ the CMT. Th~ die is subjectecl to pressure usincJ a ~ui~able,
commercially available press. The die may be at room temperature
or may have been preheated before applying pressure. ~lso, the
die containing the C~T may be evacuated to a suitably low pressure
before applying compacting pressure. Preferahly, preheated,
evacuated dies are used. It :is to he understood that multi-
cavity dies can be used.
Although ~ood qu~lity compacts have been obtainecl with
dies at room temperature, as well as with dies that have not
been evacuated, the best results have been obtained by preheating
the die to a temperature of up to about 3noocr adding the pre-
determined amount of mixed, finely divided crlT at room temperature
to the preheated die, and evacuating the die with the contained
CMT to pressures of less than about 133 ~a absolute. The com-
pacting pressures applied to the die, i.e., to compact the mixed,
finely divided CMT~ should be sufficient to provide a coherent
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compact of high density and sufficient physical strength. ~hen
using dies at room temperature (about 20C), compacting pressures
of at least about 4Q0 ~IPa are required to ~produce compacts which
have a density of at least 97% of theoretical density. Preferably,
compacting pressures are in the range of about 400 to 1100 MPa
(about 30 to 80 tons per square inch). However, compacts so
produced show some cracking. We have found that, for CMT compo-
sitions wherein x is in the range of from about 0.14 up to about
0,20, when dies are preheated to a temperature in the range of
about 100 to 300C and evacuated to pressures of less than about
133 Pa absolute, compacting pressures in the preferred range
produce compacts which are substantially free of cracks and have
a density which i8 u5Ually higher th~n 98~ o theoretical densitys
the hl~her the temperatllre o the preheated die ancl ~he hlgher
the compacting pressure, the higher the density o the compact.
We have also found that, when finely divided CMT with a compo-
sition wherein x equals 0.20 or higher, i.e~ x = 0.20 to x = 0.~0,
is added at room temperature to preheated dies and the die is
evacuated prior to applying compacting pressure, the compacting
pressures are limited.
In the range of compositions wherein x has values in
the range of from about 0.20 to about 0.60, the die must be
preheated to a relatively high temperature in order to obtain
strong coherent compacts. The density of the compact increases
with increasing temperature, the best results being obtained
with dies preheated to about 300C, and with increasing compacting
pressure, the best results being obtained with compacting pressures
in the ranye of about 160 to 275 MPa (about 12 to 20 tons per
square inch). The compacts thus produced are substantially
free of cracks. At compacting pressures above ~bout 275 MPa
small cracks are present in the compacts when they are removed
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from the die and the cracking becomes progressively severe with
increasing pressure, i.e., cracks first develop laterally in planes
perpendicular to the axis of the compact and then radially and
finally the compact becomes incoherent.
In all cases, compacting pressures should be applied
for a period of time of not less than about one minute to produce
strong, coherent compacts. When CMT at room temperature is added
to a preheated die, a temperature equilibration period of about
one to three minutes should be allowed. The steps of equilibra-
ting, evacuating and applying pressure may be executed in succes-
sion or almost simultaneously. After the application o pressure
for the desired length of time, the pressure is released and the
compact is removed from the die Sintering of the compacts is
not necessary because the compacts have a density which i5 at
least 97% of theoretical density and in most cases higher than
98~, and possess the necessary physical strength. The compact,
as removed from the die, can be used as such for a sputtering
target, or if desired, may be cut, lapped and polished prior to
use as a sputtering target.
The invention will now be illustrated by the following
non-limitative examples.
Example 1
45 g of high purity, poly-crystalline Cdxrlgl xTe (x = O.lS)
powder at room temperature and having particle sizes all less
than 150 ~u were added to a 38 mm diameter die, which had been
preheated to a temperature of 2Q0C. The die was closed,
evacuated to a pressure of less than 133 Pa and subjected to a
compacting pressure of 690 MPa ~50 tons per square inch). ~fter
three minutes the pressure was released and the resulting disc
removed from the die. The compacted disc, measuring 38 mm in
diameter and 5 mm thick, was free of cracks, had smooth surfaces
and had a density of 99~ of theoretical density.
421
Exam~le 2
The test described in Example 1 was repeated but the die was
preheated to 100C. A compacted disc of the same dimensions,
free of cracks and having a density of 99% of theoretical density,
was obtained.
Example 3
The test described in ~xample 1 was repeated but the die ~Jas
preheated to a temperature of 50C. A compacted disc of the
same dimensions and having a density of 99% of theoretical
density was obtained. The compact showed a number of small
cracks, which did not affect the coherence of the compact.
Example 4
The test described in Example 1 was repeated but a 19 mm diameter
die preheated to 100C was used, the die was not evacuated pxior
to appl~cation of pressure and the pressure during compaction
was 940 MPa (68 tons per square inch). A compacted cylinder
measuring 19 mm in diameter and 20 mm thick, free of cracks and
having a density of 99% of theoretical density was obtained.
Example 5
The test described in Example 1 was repeated but with Cdxl~g 1 ~e
powder wherein x had a value of 0.55; the die was not pre-heated.
A disc of the same dimensions and a density of 98.5~ was obtained.
The disc showed some cracks.
It can be seen from Examples 1, 2 and 4, using dies
preheated to a temperature of at least 100C, that substantially
crack-free compacts of large diameter and thickness and having a
density of 99% of theoretical density can be made from finely
divided CMT wherein x is about 0.15. Examples 3 and 5 show
that, when x has a value above 0.20 or the dies is at a tempera-
ture below 100C, large compacts of high denisty can be obtained
but the compacts are not free of cracks.
Example 645 g portions at room temperatuxe of finely divided, high purity
poly-crystalline CMT where x had a value in the range of 0 a 20
to 0.60 were compacted at varying compacting pressures in a die
having a diameter of 38 mm which was preheated to a temperature
Of 300C and evacuated to less than 133 Pa absolute. The compact-
ing pressure was applied for a period of three minutes immediately
after closing and while evacuating the die, After removal from
the die~ the compacts, measuring 38 mm diameter and 5 mm thick,
were inspected and their density determined. The results are
given in Table I.
TADLE I
Value Compacting Den~ity as ~ Result o
of x vressure in MPa of theoretical X
.. _
0.20 165 97 free of cracks
0.20 207 98.0 free of cracks
0.20 234 98.5 free of cracks
0.20 276 99.3 free of cracks
20 0.20 345 - some lateral cracks
0.20 386 - some radial cracks
0.32 234 98.5 free of cxacks
0.32 276 99.5 some small cracks
0.40 276 99.5 some small cracks
0.60 207 98.2 free of cracks
0.60 276 99.2 some small cracks
~ s can be seen from the results of ~xample 6,compacts
free of cracks and having densities of at least 97~ of theore-
tical density can be made by compressing finely divided CMT
with x values in the range of 0.20 to 0.60 into forms of large
diameter and thickness using compacting pressures in ~he range
of 160 to 275 MPa and dies preheated to 300C.
11~(19L2~
It will be understood of course that modificationscan be made in the embodiment of the invention illustrated
and described herein without departing from the scope and
purview of the invention as defined by the appended claims.
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