U.S. patent number 3,661,747 [Application Number 04/848,935] was granted by the patent office on 1972-05-09 for method for etching thin film materials by direct cathodic back sputtering.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Peter A. Byrnes, Jr., Martin P. Lepselter.
United States Patent |
3,661,747 |
Byrnes, Jr. , et
al. |
May 9, 1972 |
METHOD FOR ETCHING THIN FILM MATERIALS BY DIRECT CATHODIC BACK
SPUTTERING
Abstract
Thin layers of material including dielectric films are etched or
cleaned by placing them in a low pressure gas ambient, forming a
plasma in the ambient, and establishing a periodic voltage between
the layers and the plasma. One important application of the process
is the formation of metal silicide contacts through small windows
in a dielectric layer protecting the silicon surface. In one
application, the metal is sputtered onto the exposed silicon at the
same time that the surface is subjected to ion bombardment. The
sputtering and etching rates are adjusted so that some of the
sputtered metal reacts with the silicon upon impact and the
unreacted metal is etched away.
Inventors: |
Byrnes, Jr.; Peter A.
(Bridgewater Township, Somerset County, NJ), Lepselter; Martin
P. (New Providence, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
25304659 |
Appl.
No.: |
04/848,935 |
Filed: |
August 11, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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607203 |
Jan 4, 1966 |
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Current U.S.
Class: |
204/192.23;
257/E21.226; 204/192.3; 257/E21.252; 257/E21.507; 257/E21.165 |
Current CPC
Class: |
H01L
21/31116 (20130101); H01L 21/02046 (20130101); C23C
14/35 (20130101); H01L 21/28518 (20130101); C23C
14/022 (20130101); H01L 21/76897 (20130101); H01L
2924/3011 (20130101) |
Current International
Class: |
C23C
14/02 (20060101); H01L 21/60 (20060101); H01L
21/311 (20060101); H01L 21/02 (20060101); H01L
21/285 (20060101); H01L 21/306 (20060101); C23C
14/35 (20060101); C23c 015/00 () |
Field of
Search: |
;204/192 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Davidse, "Theory & Practive of RF Sputtering" Vacuum Vol. 17,
No. 3, (1966), pg. 145 .
Maissel et al. "Thin Films Deposited by Bias Sputtering" J. of App.
Phy., 1965.
|
Primary Examiner: Mack; John H.
Assistant Examiner: Kanter; Sidney S.
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of the copending
application, Ser. No. 607,203, filed Jan. 4, 1966 by P. A. Byrnes,
Jr. and M. P. Lepselter.
Claims
What is claimed is:
1. A method for forming a metal silicide layer into a silicon
substrate and wherein the silicon substrate is covered with a
protective dielectric layer except for exposed regions where the
silicide layer is to be formed comprising the steps of:
cleaning the exposed regions of the silicon substrate in a low
pressure noble gas ambient by positive ion bombardment;
sputtering a metal into the exposed regions to form a metal
silicide layer and simultaneously back sputtering the silicon
substrate by bombardment of noble gas ions to remove excess metal
from the dielectric layer at the same time that the metal silicide
layer is being formed; and
adjusting the sputtering and back sputtering rates so that the
metal deposits only in the exposed regions thus forming the
silicide layer.
2. The method of claim 1 wherein the metal is selected from the
group consisting of titanium, zirconium, hafnium, nickel,
palladium, platinum, ruthenium, rhodium, osmium, and iridium.
3. The method of claim 1 wherein the noble gas is argon.
Description
BACKGROUND OF THE INVENTION
This invention relates to cleaning or etching by cathodic back
sputtering. More specifically, it concerns a method for cleaning or
etching which utilizes a periodic voltage to extract bombarding
ions from a plasma.
Since the dimensions of microelectronic devices and circuits are
measured in thousandths of an inch, precise methods of etching and
cleaning are of considerable importance in their fabrication.
Various techniques, including chemical etching and indirect back
sputtering have been adapted for use in microelectronic fabrication
processes, but these methods have important limitations.
Chemical etching, such as is described by Schlabach and Rider in
Printed and Integrated Circuitry (1963) at p. 83 et seq. is one
commonly used etching or cleaning method. However, there are at
least three difficulties associated with this approach. First,
because the eroding action proceeds at a different rate as the
etching depth increases, chemical etching generally produces cross
sections that are either undercut or slop-sided rather than
rectilinear. Second, since different materials react differently to
the same etchant, chemical etching becomes a time-consuming,
multi-step process when it is necessary to etch through several
layers of different materials; and, third, chemical etching is not
useful with certain materials, such as iridium and rhodium, because
of their high resistivity to chemical action.
A second approach to etching and cleaning, which is of particular
value in the fabrication of microelectronic circuits and devices,
is positive ion bombardment. When positive ions, moving at high
velocities, collide with the surface of a workpiece, they remove
material from it. Thus, for example, the workpiece may be etched or
cleaned by placing it on a cathode in a low pressure noble gaseous
ambient and applying a high constant voltage between the cathode
and an anode. Ions, which are formed by collisions between
electrons accelerated from the cathode and noble gas atoms, are
accelerated toward the cathode where they bombard the workpiece
surface.
Present methods utilizing ion bombardment are, however, of limited
utility in the etching or cleaning of workpieces containing thin
films of dielectric materials or other materials which cannot be
subjected to high, constant voltages without damage. Typically, the
voltages required to form ions and obtain etching are in excess of
the breakdown voltage of thin dielectric films. And while the
problem of breakdown may be overcome by placing an insulating layer
between the workpiece and the cathode, (see M. P. Lepselter, U.S.
Pat. No. 3,271,286, dated Sept. 6, 1966) the efficiency of the
process is greatly reduced by the presence of an insulator because
the electric field must be bent around it. As a result, the etching
is produced by bombardment of stray ions from the surrounding field
rather than by ions attracted directly to the workpiece, and is
less satisfactory.
The method for etching or cleaning, in accordance with the present
invention, comprises, in brief, the steps of placing the workpiece
in a low pressure noble gas ambient, forming a plasma in the
ambient, and effecting the bombardment of the surface by ions drawn
from the plasma by the establishment of a periodic voltage between
the workpiece and the plasma. This method is particularly useful in
the subsequent formation of an atomically clean bond between a
surface of the workpiece and some other material.
When it is desired to deposit a layer of material on the surface of
a workpiece, the formation of an atomically clean bond is important
because of greater adherence and greatly increased uniformity in
its electrical properties. One difficulty typically encountered in
depositing a layer of material on a metal or semiconductor
workpiece surface is the formation of an unwanted, thin, oxide-like
surface layer on the workpiece surface prior to deposition.
Typically, such a surface layer forms immediately after cleaning
when the substrate is exposed to air, and forms even at moderate
vacuums so low as 10.sup.-.sup.6 torr. The presence of the surface
layer prevents uniform reaction between the deposited material and
the workpiece because the reaction must take place through randomly
distributed voids in the surface layer. But when the intervening
surface layer is eliminated, more uniform depositions may be
obtained. The result is to produce more uniform ohmic and
rectifying contacts. (See Kahng & Lepselter, Planar Epitaxial
Silicon Schottky Barrier Diodes, Bell System Technical Journal
44:1525, 1965)
While the advantages of atomically clean bonds are known,
previously devised methods of forming them are of only limited
utility in the fabrication of microelectronic devices. One method
of forming such a bond is cleaving a sample of material in a vacuum
and depositing a layer of material on the freshly cleaved surface.
It is, however, impractical to cleave the surface of a workpiece
containing layers of material as thin as those typically
encountered in microelectronics. A second method is vacuum heating
a sample of material to a very high temperature and then depositing
a layer of material on it. A difficulty with this approach,
however, is that the intense heat pits oxide layers and destroys
the properties of delicate junctions.
In accordance with an important use of the invention, an atomically
clean bond is formed by cleaning the workpiece surface utilizing
ionic bombardment and then depositing a layer of material upon the
freshly cleaned surface.
The invention may now be described in greater detail by reference
to the accompanying drawings wherein:
FIG. 1 is a cross section of a typical workpiece to be etched or
cleaned in accordance with the invention;
FIGS. 2, 5, 6 and 8 show various forms of apparatus which may be
used for the practice of the invention;
FIG. 3 is a graph illustrating features of a preferred voltage
waveform suitable for application between the workpiece and the
plasma;
FIG. 4 is a graph of the ion-induced etching of a typical material
as a function of the voltage between the workpiece and the plasma;
and
FIG. 7 is a typical workpiece upon which an atomically clean bond
is to be formed in accordance with the invention.
Similar reference characters are applied to similar elements
throughout all the drawings.
In FIG. 1 is shown a cross section of a typical workpiece.
Typically, it comprises a relatively thick substrate 10 such as,
for example, a silicon wafer, supporting a thin film of material
11. The thin film typically comprises several layers which have
been deposited in succession upon the surface. These layers may be
dielectrics, semiconductors, conductors or even ferrites. Also
shown is a masking layer 12 which may be formed by known
techniques, such as the well-known photo-resist method, to protect
areas of the surface which are not to be etched. In typical
applications the thin film 11 is a few ten-thousandths of an inch
thick or smaller with the thickness of the individual layers being
a few hundred-thousandths of an inch.
Reference is now made to FIG. 2 which is a schematic illustration
of apparatus used to practice the invented method.
The workpiece, along with an electron-emitting filament or cathode
20 and an anode 21, is placed within a vacuum chamber 22. In one
typical arrangement the filament was placed approximately five
inches from the anode and the workpiece was positioned two inches
from the filament-anode line. The chamber is also provided with
apparatus (not shown) for evacuating it and injecting a suitable
gaseous ambient into it. Since a plasma 23 is to be formed between
the filament and the anode, structure for containing a plasma and,
incidentally, for supporting the workpiece in proper relationship
to the plasma, is advantageously provided. In the apparatus of FIG.
2, a quartz container 24, having suitably located apertures 25, 26,
and 27 for the workpiece, filament and anode, respectively, is
provided for this purpose. Additionally, apparatus for establishing
a longitudinal magnetic field along the filament-anode direction,
such as, for example, ring magnets 28, are provided in order to
control the plasma. Terminals extending through chamber 22 are also
provided for connecting the filament and the anode to external
power supplies (not shown) and for connecting the workpiece to a
periodic voltage power supply 31.
Using the above-described apparatus, a workpiece is etched in
accordance with the invention, by the steps of evacuating the
chamber, introducing a noble gas ambient, forming a plasma between
the filament and the anode, and applying an appropriate periodic
voltage between the workpiece and the plasma, as is now explained
in greater detail hereinbelow.
With the various members in place, the workpiece is surrounded with
a noble gas ambient which can be ionized to form a plasma. While
argon is typically used, any noble gas will work. The reason a
noble gas is used is to prevent unwanted chemical reactions between
the ambient and the thin film. Ambient pressures between about
one-half micron and several hundred microns have been found to be
useful. As an exception to the ordinary practice of using ambients
comprised solely of noble gases, it has been found that the etching
rate of oxidizable metals, such as titanium, for example, can be
controlled by "bleeding" oxygen into the ambient. Typically the
partial pressure of the oxygen is about 1 to 5 percent that of the
noble gas. The oxide layer which forms on the metal etches more
slowly than the pure metal, thus slowing the etching rate. Other
non-noble gases which react with the particular material of the
thin film surface to form slowly etching compounds can be similarly
used.
A plasma 23 is formed in the ambient between the filament 20 and
the anode 21 by heating the filament by a current derived from an
external power supply and by applying a constant voltage between
the filament and the anode as provided by another external power
supply. The plasma is formed by collisions between electrons
emitted from the heated filament and atoms of the gaseous ambient.
Ring magnets 28, which establish a longitudinal magnetic field, are
used to control the shape of the plasma.
Etching of the thin films 11 is produced by applying an alternating
voltage derived from an external power source 31, between the
workpiece and the filament-anode system. When the workpiece is at a
negative potential with respect to the plasma, positive ions 29 are
drawn out of the plasma and toward the workpiece where they bombard
the workpiece and produce etching and/or cleaning of the exposed
surface area.
As indicated above, the workpiece may include a dielectric material
as one of the layers of the thin film. When this is so, extra care
must be given to the nature and parameters of the alternative
voltage that is used, and to the details of the electrical circuit
through which it is applied, in order that no significant average
voltage is built up across the dielectric layer. Basically, the
problem arises due to the fact that an electron is much more mobile
than an ion. This means that there is a much greater flow of
electrons to the workpiece, when the latter is at a positive
potential relative to the plasma, as compared to the flow of ions
when the workpiece is at a negative potential relative to the
plasma. As a consequence of this disparity in the mobilities of an
ion and an electron, an alternating voltage having an average value
of zero, such as a simple sinusoidal wave, produces a net average
voltage across the dielectric film.
When it appears that the resulting voltage buildup may be
dangerously near the dielectric breakdown voltage, one or both of
the following precautions are advantageously taken. The first of
these precautions is to include a blocking capacitor in series with
the workpiece. The blocking capacitor can take the form of a second
dielectric layer 30 included between the workpiece and the
connection to the periodic voltage supply as shown in FIG. 2. The
inclusion in the circuit of a blocking capacitor results in a
sharing of the average voltage buildup between the two dielectrics.
In particular, if the capacitance of the capacitor formed by the
second dielectric 30 is smaller than the capacitance of the
capacitor formed by the dielectric layer, a greater proportion of
the average voltage appears across dielectric 30.
The second precaution that can be taken for reducing the voltage
buildup across the dielectric layer is to use an alternating
voltage that has a low average negative voltage. This has the
effect of compensating for the lower ion mobility.
FIG. 3 illustrates a typical voltage waveform which has a negative
average value and is suitable for use in connection with the
invention. The periodic voltage shown comprises a train of negative
pulses having a pulse peak, V.sub.max, a pulse duration, t.sub.1,
and a period, T. So long as the average voltage is less than a few
hundred volts, typical dielectric layers are not damaged. It is
understood, however, that the maximum permissible average in any
instance depends upon the nature of the dielectric material and its
thickness. A particularly advantageous negative pulse voltage is
obtained when the pulse peak is chosen with reference to the yield
curve of the material to be etched so that the amount of etching is
maximized.
FIG. 4 illustrates a typical etching yield curve. The Y-axis
indicates the yield of atoms etched per bombarding ion, while the
V-axis indicates the ion energy in volts. The curve is typified by
three regions. As ion energy is increased, the curve passes through
a first region of low yield until the ion energy is increased
beyond a first knee (knee 1). This low yield region is followed by
a second region of rapidly increasing yield until a second knee
(knee 2) is reached. Beyond knee 2 is an upper plateau region in
which the yield continues to increase but at a much slower rate.
Thus, one can maximize the amount of etching per unit of average
voltage by choosing a peak voltage for the negative pulse that is
approximately equal to the voltage at a low end of the upper
plateau of the yield curve, i.e., near knee 2.
In order to minimize the output voltage requirements of the
periodic voltage supply 31, the impedance of the workpiece is
advantageously made small. Since this impedance is primarily
capacitive when a dielectric layer is present, it has been found
advantageous to use a high frequency voltage source. In practice,
it has been found that the range of frequencies between 100
kilocycles and 10 megacycles is satisfactory.
An idea of the relative magnitudes of the parameters appropriate
for use in accordance with this embodiment of the invention may be
obtained by consideration of one set of parameters used in a
typical operation.
In one example, a 2,000 angstrom thickness was removed from a film
of SiO.sub.2 disposed on a one inch diameter slice of silicon in
ten minutes. The ambient used in this example was argon at a
pressure of 10 microns. The voltage applied between the filament
and the anode was 50 volts. A 0.02 microfarad blocking capacitor
was placed in series with the workpiece, and the waveform applied
between the capacitor and ground (the workpiece being between the
two) was a negative pulse train having a peak voltage of
approximately -1,500 volts at frequency of 150 kilocycles per
second.
It is understood that the example described above is merely
intended to be illustrative. Many other combinations of parameters
within the previously described ranges have been successfully
employed; the exact combination being dependent upon the particular
requirements of each case.
FIG. 5 is a schematic illustration of an alternative apparatus
which can be used for the invention. The apparatus differs from
that shown in FIG. 2 chiefly in that the filament and the anode of
FIG. 2 have been eliminated and in that a grounded metal electrode
40 has been added. In this apparatus, the workpiece is supported on
an insulating base 42 in a vacuum chamber 22. A quartz cylinder 24
surrounding the workpiece is also supported by the insulator, and a
grounded metal electrode is placed across the opposite end of the
cylinder. A ring magnet 41 is placed around the cylinder 24 above
the workpiece.
In this apparatus, a plasma 23 is formed by collisions between
excited electrons and the ambient gas atoms rather than by
collisions involving filament-emitted electrons, as in the
apparatus of FIG. 2. When the gas is at a sufficiently high
pressure, the application of a periodic voltage between electrode
40 and the workpiece causes sufficient movement of the electrons to
form a plasma. However, the plasma may advantageously be formed at
a lower pressure by the addition of magnets for concentrating the
electron movement. Thus, for example, in FIG. 5, ring magnet 41 can
be used to establish a magnetic field in the direction of the
electric field. Such a magnetic field concentrates the electron
movement and the resulting plasma within the central region of the
ring. An advantage in forming the plasma in this manner is its
simplicity. The need for a separate electron source is eliminated,
and a more nearly uniform plasma is generated. Etching or cleaning
can be carried out in substantially the same manner as described
previously.
In the discussion thus far, cleaning and etching of a workpiece
have been considered. As previously mentioned, an important
application of the invention is in the formation of an atomically
clean bond between a workpiece surface and another material.
The present invention is particularly suited to dealing with the
problem of unwanted surface layers and is especially useful where
the workpiece contains one or more dielectric layers. In accordance
with this use of the invention, the surface layer is removed and
prevented from reforming prior to deposition. This is done by
depositing the material either at the same time the surface layer
is being removed, or immediately afterward, but before the
workpiece is exposed to air. Known techniques such as, for example,
sputtering or vacuum evaporation can be used to deposit the
material on the cleaned surface.
FIG. 6 illustrates apparatus which can be used to atomically clean
a workpiece surface and deposit a layer of material onto it. The
apparatus differs from that shown in FIG. 5 chiefly in that a
sputtering electrode 50 of the material to be deposited is placed
in the quartz container 24 facing the workpiece. The sputtering
electrode is connected to its own separate power supply, which can
be either alternating or direct current.
Cleaning of the workpiece is accomplished by ionic bombardment in
the manner described above. Material is sputtered onto the
workpiece from the sputtering electrode 50 either immediately after
the cleaning process or, advantageously, at the same time that the
cleaning is taking place. The advantages of simultaneous cleaning
and sputtering are that there is no time for a surface film to form
and that the more loosely bound atoms of sputtered material are
knocked off the workpiece while the more tightly bound atoms
remain. The result is a denser and more strongly adherent
contact.
One use for this atomically clean bonding process is the formation
of a metal-semiconductor contact. FIG. 7 illustrates a typical
workpiece comprising a semiconductor substrate 60 such as silicon
upon which there is shown disposed a passivating film 61 such as
silicon dioxide and a masking layer 62 such as photo-resist or a
metal having a high sputtering threshold.
An atomically clean contact is formed by first back sputtering the
workpiece so that the silicon substrate is exposed and then
sputtering a contact metal such as platinum onto the workpiece.
Back sputtering can be carried out at a reduced rate while the
sputtering is taking place in order to achieve a cleaner, more
adherent bond. By either controlling the energy with which the
sputtered atoms reach the silicon or heating the silicon substrate
(heater not shown), an atomically clean ohmic contact or barrier
layer of metal silicide may be obtained.
When it is desired to clean the workpiece and sputter the material
to be deposited in separate steps, the substrate is first cleaned
by grounding the sputtering electrode and applying a periodic
voltage to the workpiece. After the workpiece is cleaned, it is
disconnected from the power supply or grounded, and an alternating
current or a direct current negative voltage is applied between the
sputtering electrode and ground to sputter metal onto the
workpiece. A separate heating step is usually required to produce a
metal silicide by sintering. An advantage of the technique is that
both the cleaning and the metal deposition take place without
breaking vacuum in the chamber.
The heating step can be eliminated by both etching the workpiece
and sputtering metal simultaneously. The sputtered metal is ionized
as it passes through the plasma and then accelerated toward the
workpiece by a negative voltage. (If the workpiece contains no
dielectric layers, a d.c. negative voltage can be used. However, in
the usual case, a protective layer will be present and a periodic
voltage having a negative average value is used.) The metal ions
are thus given sufficient energy to penetrate through surface
barriers both chemical and physical and into the lattice structure
of the silicon. In addition, the impact of the ions--both of metal
and of the noble gas--provide sufficient energy at the interaction
region of the silicon surface that much of the sputtered metal
reacts immediately with the silicon. By properly adjusting the rate
of sputtering and etching, any metal which does not react can be
immediately etched away, while the silicide builds up because it is
more etch resistant than the loosely bound, unreacted metal. Thus,
this process has two additional advantages: first, no separate
heating step is required -- saving time and reducing stress in the
workpiece; and, second, no separate step is required to etch excess
metal from the surface of the workpiece.
More subtle advantages also accrue from the use of this process.
Certain metals, such as zirconium, rhodium and palladium, are very
difficult to use in forming metal silicides by prior art
techniques. Zirconium, when deposited in films of 200 angstroms or
more, usually cracks and peels when sintered. Rhodium cannot
generally be chemically etched to remove the unreacted metal, and
palladium usually forms a multiphase structure having nonuniform
electrical properties. However, when the process just described is
used, excellent silicide contacts are formed using any of these
metals. In the case of zirconium, sufficient metal is implanted
into the silicon to overcome whatever surface barrier prevents the
formation of good contacts, while in the case of palladium a single
phase silicide is formed. The use of sputter etching simultaneously
with the formation of rhodium silicide is clearly advantageous.
These processes and their advantages will become clearer by
reference to FIG. 8, which illustrates an alternative apparatus for
forming atomically clean bonds or contacts. This apparatus is
substantially the same as that described previously except that a
titanium cathode 81 has been added and adaptations have been made
so that the workpiece 84 can be moved from a position beneath the
titanium cathode to a position beneath the cathode of the metal to
be deposited. This adaptation is accomplished by disposing the
workpiece on graphite cathode 80 on the outer circumference of a
rotatable conductive disc 83. Electrodes 50 and 81 are electrically
coupled to 5,000 volt d.c. power supplies, and the graphite cathode
80 is electrically coupled to an 800 Khz oscillator with a zero to
minus 5,000 volt peak-to-peak output, advantageously through spring
contact 82 so that the potential may be kept on the workpiece
during 360.degree. of rotation.
The workpiece, with contact windows previously etched in the
protective oxide, is placed on the graphite and positioned so that
it is beneath neither of the metal cathodes. The chamber is then
evacuated to a pressure of 10.sup.-.sup.7 torr and argon is bled in
at a rate sufficient to cause the pressure in the chamber to
stabilize at approximately 10 microns of argon.
The titanium cathode is then sputtered using a potential of 5,000
volts and a current density of 1 milliampere per square inch for
about 10 minutes to be certain that the titanium surface is clean
and is subsequently kept sputtering throughout the process to act
as a getter. After this preliminary step, the cathode of the metal
to be used in forming the metal silicide (henceforth referred to as
the depositing cathode) is sputtered using a potential of 5,000
volts d.c. and a current density of approximately one milliampere
per square inch. When this electrode has been cleaned, a negative
rf voltage having peak-to-peak value of about 4,000 volts is
applied to the graphite electrode 82. When the workpiece has been
back sputtered in this manner for about one minute to remove
residual oxide in the contact areas, it is rotated into position
under the depositing cathode. The energy coupled to the silicon
surface by bombardment of noble gas ions and the impact of
sputtered metal ions is sufficient to cause a reaction between the
metal and the silicon. For depositing platinum, it was found that
rf voltages between 3,000 volts and 4,500 volts, and preferably
between 4,000 and 4,300 volts, produced excellent platinum silicide
contacts. Voltages below 3,000 volts result merely in the
deposition of a platinum film on the silicon; and voltages above
4,500 volts produce a coarse platinum silicide contact. With these
voltages, any unreacted metal which deposits on the slice is
immediately sputtered off. The intermetallic compound formed by the
metal and silicon is also removed by the back sputtering but since
a two-to-one increase in volume occurs during its formation, a net
gain results at a rate of approximately 35A per minute.
After an elapsed time commensurate with the thickness of metal
silicide desired, usually in the order of 10 minutes, the d.c.
potential is removed from the metal cathode and the slice is back
sputtered for an additional 30 seconds to ensure a clean oxide
surface. The slice is then positioned under the titanium for 15
minutes and then the platinum for 15 minutes to build up a
titanium-platinum overlay. The workpiece is then removed from the
chamber, and the remaining steps in the standard Ti-Pt-Au overlay
beam lead process followed, if desired.
An X-ray analysis of platinum silicide contacts formed in this
manner shows it to be of the same phase and of the same preferred
crystal orientation as platinum silicide formed by the standard
sintering process. In addition, X-ray measurements of the stresses
in the silicon show that they are only 25 percent of those
developed by the standard process. Finally, it may be noted that
Schottky barrier diodes fabricated in this manner have more nearly
ideal electrical characteristics.
The metal silicide contacts most advantageously produced by the
technique of this invention include TiSi, ZrSi, HfSi, NiSi and the
silicides of the six platinum group metals. Sputtering voltages in
the range of 1 kv to 10 kv are appropriate for spontaneous
formation of these materials.
It is understood that the above-described arrangements are simply
illustrative of the many possible specific embodiments which can
represent applications of the principles of the invention. Numerous
and varied other arrangements can readily be devised in accordance
with these principles by those skilled in the art without departing
from the spirit and scope of the invention.
* * * * *