U.S. patent number 3,892,650 [Application Number 05/319,409] was granted by the patent office on 1975-07-01 for chemical sputtering purification process.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Jerome J. Cuomo, Walter W. Molzen, Jr..
United States Patent |
3,892,650 |
Cuomo , et al. |
July 1, 1975 |
Chemical sputtering purification process
Abstract
A process for purifying the diffuse sputtering region of a
sputtering system by providing therein a readily disproportionated
active vapor species which decomposes therein to form an active
getterer of undesirable reactive gases, such as desorbed and source
sputtering gases present in the system. In one example, silane
mixed with argon decomposes in the diffuse sputtering region to
form films of silicon and compounds thereof throughout the
sputtering chamber, which silicon acts to chemically getter the
reactive gases present. Using an ultra pure vanadium target, films
of vanadium are produced having bulk superconducting and
resistivity properties.
Inventors: |
Cuomo; Jerome J. (Bronx,
NY), Molzen, Jr.; Walter W. (Patterson, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23242142 |
Appl.
No.: |
05/319,409 |
Filed: |
December 29, 1972 |
Current U.S.
Class: |
204/192.15;
148/DIG.6; 148/DIG.158; 204/298.07; 148/DIG.60 |
Current CPC
Class: |
C23C
14/34 (20130101); C23C 14/345 (20130101); C23C
14/564 (20130101); Y10S 148/06 (20130101); Y10S
148/006 (20130101); Y10S 148/158 (20130101) |
Current International
Class: |
C23C
14/56 (20060101); C23C 14/34 (20060101); C23c
015/00 () |
Field of
Search: |
;204/192 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Getter Sputtering for the Preparation of Thin Films of
Superconducting Elements and Compounds", H. C. Theuerer et al.,
Journal of Applied Physics, Vol. 35, No. 3, (2 parts-part 1) pp.
554-555, March 1964. .
"Superconductive Films Made by Protected Sputtering of Tantalum or
Niobium", Journal of Applied Physics, Vol. 33, No. 5, p. 1898,
1962, by Frerichs..
|
Primary Examiner: Vertiz; Oscar R.
Assistant Examiner: Langel; Wayne A.
Attorney, Agent or Firm: Jordan; John A.
Claims
What is claimed is:
1. A process for purifying a sputtering chamber, comprising the
steps of:
procuring an active vapor species selected from the group
consisting of SiH.sub.4, TiCl.sub.4, WF.sub.6, and UF.sub.6 which
active vapor species is readily disproportionated by the plasma
generated within the diffuse sputtering region of said sputtering
chamber during sputtering;
positioning a substrate and source material in said sputtering
chamber so that said source material may be sputter deposited upon
said substrate;
evacuating said sputtering chamber;
introducing into said sputtering chamber an inert sputtering
atmosphere including the procured active vapor species mixed
therewith in an amount of from 0.1 to 10% by volume of said
atmosphere; and
generating a plasma from said atmosphere including generating a
diffuse plasma in said sputtering region around and outside the
active deposition region between said source material and said
substrate by applying an RF source of sputtering power to said
substrate and source material under conditions such that said
procured active vapor species is decomposed therein by said diffuse
plasma before reaching said active deposition region to thereby
form an active gettering species to coat the interior of said
chamber and thereby reduce the outgassing rate thereof.
2. The process as set forth in claim 1 wherein the said step of
evacuating comprises evacuating said chamber to a level of
approximately 10.sup.-.sup.7 Torr or less.
3. The process as set forth in claim 2 wherein said inert
sputtering atmosphere is argon.
4. The process as set forth in claim 3 wherein said source material
is selected from the group consisting of vanadium, niobium,
tantalum, iron, cobalt, nickel, scandium, yttrium, and the
lanthanide series.
5. The process as set forth in claim 1 wherein said source material
is selected from the group consisting of vanadium and niobium.
6. The process as set forth in claim 5 wherein the said step of
introducing into said chamber comprises introducing said inert
sputtering atmosphere to a pressure between 10.sup.-.sup.4 and
10.sup.-.sup.1 Torr.
7. A process for sputtering high purity materials, comprising the
steps of:
positioning within a sputtering chamber a target material and
substrate upon which said target material is to be sputtered;
evacuating said sputtering chamber;
providing an RF sputtering potential between said target and
substrate sufficient to effect an ionization within the deposition
region between target and substrate to produce target material
deposition upon said substrate at a rate between approximately
5A/sec and 50A/sec;
procuring an active vapor species selected from the group
consisting of SiH.sub.4, TiCl.sub.4, WF.sub.6 and UF.sub.6 which
active vapor species will be readily disproportionated in the
diffuse plasma produced within said chamber outside said deposition
region; and
introducing into said chamber an inert sputtering atmosphere of
argon having mixed therewith from 0.1 to 10% by volume of said
procured active vapor species, said inert sputtering atmosphere of
argon becoming ionized therein through stimulation by said RF
sputtering potential to produce said diffuse plasma around and
outside said deposition region under conditions such that said
procured active vapor species is decomposed before reaching said
deposition region so as to form an active gettering species to
thereby reduce the outgassing rate required to maintain a given
level of purity.
8. The process as set forth in claim 7 wherein said step of
evacuating comprises evacuating said sputtering chamber to a
pressure level of approximately 10.sup.-.sup.7 Torr or less for a
relatively short period of time.
9. The process as set forth in claim 8 wherein after said step of
evacuating said sputtering chamber, said sputtering chamber is
back-filled with said inert sputtering atmosphere of argon having
mixed therewith said procured active vapor species to a pressure
level between 10.sup.-.sup.4 and 10.sup.-.sup.1 Torr.
10. The process as set forth in claim 7 wherein said inert
sputtering atmosphere of argon is introduced into said chamber to a
pressure level of between 10.sup.-.sup.4 and 10.sup.-.sup.1
Torr.
11. The process as set forth in claim 10 wherein said step of
introducing into said chamber said inert sputtering atmosphere of
argon having mixed therewith from 0.1 to 10% by volume of said
procured active vapor species comprises premixing said procured
active vapor species with said inert sputtering atmosphere of argon
and admitting the mixture thereof into the evacuated sputtering
chamber to said pressure level.
12. The process as set forth in claim 7 wherein said target
material is selected from the group consisting of vanadium and
niobium and wherein said procured active vapor species is
SiH.sub.4.
13. The process as set forth in claim 7 wherein said target
material is selected from the group consisting of vanadium and
niobium and wherein said procured active vapor species is
TiCl.sub.4.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to sputtering processes. More
particularly, the present invention relates to a chemical
sputtering purification process, whereby high purity materials,
such as films of vanadium or niobium, may be produced.
2. Description of the Prior Art
In the process of sputtering pure materials, various techniques are
known to aid in attempting to achieve high purity levels. Achieving
high purity materials through sputtering depends, to great extent,
upon system preconditioning, substrate bias, and general care in
choosing gas and target material purity. In particular, it has been
shown that pure, and in some cases ultra pure, materials can be
sputtered by presputtering with gaseous materials that are active
getterers of the undesirable reactive gases present in the
sputtering system, and by bias sputtering. Bias sputtering provides
a bombardment mechanism of the substrates, which mechanism has been
described as a "scrubbing" of the depositing surface which acts to
desorb loosely held species. Another approach to obtaining high
purity materials using the sputtering process, involves the ultra
high vacuum techniques of system baking and prepump down pressures
of about 1 .times. 10.sup.-.sup.10 Torr.
Still another approach to obtaining high purity materials using the
sputtering process involves "getter sputtering" techniques. Getter
sputtering techniques were developed to eliminate the need for
ultra high vaccum systems. Such techniques are described, for
example, in an article entitled, "Getter Sputtering for the
Preparation of Thin Films of Superconducting Elements and
Compounds," by H. C. Theuerer et al., Journal of Applied Physics,
Vol. 35, No. 3, (2 parts - part 1) pp. 554-5, March, 1964. Another
approach which obviates the need of ultra high vacuum techniques,
involves a form of back-sputtering operation. An arrangement for
carrying out this latter type of sputtering is described by Rudolf
Frerichs in an article entitled, "Superconductive Films Made by
Protected Sputtering of Tantalum or Niobium," Journal of Applied
Physics, Vol. 33, No. 5, p. 1898, 1962.
The difficulties and disadvantages in prior art techniques used to
obtain high purity materials in sputtering systems resides in the
fact that prolonged and sometimes elaborate preconditioning
processes are required to prepare the sputtering environment. Still
other prior art techniques for obtaining high purity films require
somewhat elaborate and cumbersome sputtering apparatus.
Accordingly, in the prior art of sputtering high purity materials,
considerable steps were taken to reduce the undesirable reactive
species from the sputtering environment. The need to reduce or
eliminate reactive species from the sputtering environment is
particularly acute where efforts are being made to sputter one or
more of the more chemically active elements, such as vanadium,
niobium, scandium, and the like.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, there
is provided a relatively simple and effective process for purifying
the sputtering environment. More particularly, in accordance with
the present invention, a process is provided for chemically
purifying the sputtering environment by providing within the
diffuse sputtering region an active vapor species which acts to
chemically getter undesirable reactive species therein, such as
reactive gases and the like. Any active vapor species which is
easily disproportionated, and which is chemicaly reactive with at
least one undesirable species present in the sputtering
environment, may be employed. A relatively small percentage of the
active vapor species is mixed with the inert sputtering environment
species employed. For example, concentrations of the active vapor
species ranging between 0.1 and 10% may be used, depending upon the
relative sputtering rate employed. Typical sputtering pressures
range between 1 .times. 10.sup.-.sup.4 to 1 .times. 10.sup.-.sup.1
Torr. The active vapor species is decomposed in the sputtering
environment to form an active gettering species which in turn acts
to getter desorbed and source sputtering gas contaminants in the
sputtering environment. The mild diffuse plasma region existing
around and outside of the material deposition region acts to
readily decompose, react and deposit on the walls, the active vapor
species present in the system, prior to its reaching this
deposition region.
In a specific example, silane (SiH.sub.4) is employed as the active
vapor species in an argon environment, for sputtering films of
vanadium, and the like. The silane decomposes to form an active
silicon species, which, in turn, acts as a getterer.
It is, therefore, an object of the present invention to provide an
improved sputtering process.
It is a further object of the present invention to provide a
relatively simple technique for purifying sputtering
environments.
It is yet a further object of the present invention to provide a
relatively simple process for purifying sputtering environments,
whereby pure materials may readily be sputtered.
It is yet still a further object of the present invention to
provide a chemical sputtering purification process.
It is still another object of the present invention to provide a
sputtering process whereby relatively simple and conventional
sputtering apparatus which reach conventional vacuum levels may be
employed to fabricate high purity materials, by introducing a
chemical gettering agent into the sputtering environment.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of the preferred embodiments of the invention as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE is a schematic representation of a sputtering
system, to be used in the description of the process and apparatus
therefor, in accordance with the principles of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The schematic representation of the FIGURE depicts an RF sputtering
system, typical of those that may be employed in practicing the
present invention. As shown in the FIGURE, RF source 1 is coupled
to target holder assembly holder 3, via impedance matching network
5 and capacitor 7. Impedance matching network 5 is employed to
match the impedance of the system to the impedance of source 1.
As is known to those skilled in the art, it is possible to control,
to some degree, the purity of sputtered films by applying a bias
voltage to the substrate upon which the target is to be sputtered.
Accordingly, there is provided, in the particular sputtering system
shown in the FIGURE, a typical biasing network comprising a.c.
impedance elements 9, 11, and 13. These elements act to insure that
an appropriately selected bias is applied on substrate 15. In this
regard, it should be recognized that relatively simple d.c. bias
techniques may be employed where sputtering is to occur upon a
substrate exhibiting relatively good conductive characteristics.
However, on the other hand, where the substrate to be employed does
not exhibit relatively good conductive characteristics, a.c.
biasing methods may be employed, as shown in the RF system of the
FIGURE.
Although there are any of a variety of a.c. circuit techniques that
may be employed to bias the substrate used for sputtering, the RF
driven biasing arrangement shown has been found to be particularly
effective. As can be seen, inductor 9 is coupled at one end point
thereof to the node between impedance matching network 5 and
capacitor 7, and at an intermediate point thereof, to movable wiper
arm 17. The midpoint of conductor 9 is grounded, while the remote
end 19 is left floating, with wiper arm 17 acting to provide a
complete path, the impedance of which varies in accordance with the
wiper arm position.
As shown in the FIGURE, wiper arm 17 is coupled to variable
capacitor 11 and variable inductor 13, with the latter element
being coupled to capacitor 21. Also shown in the FIGURE is meter 23
arranged to be coupled to the junction of capacitor 21 and anodic
substrate holder 25, via conductor 27. This meter is employed to
measure half the peak-to-peak or applied d.c. voltage on the anodic
substrate holder. In this regard, inductor 27 and capacitor 29 are
employed to isolate the meter from the RF substrate biasing
network.
It should be understood that although a specific sputtering
arrangement and attendant particular biasing network have been
shown, any of a variety of sputtering arrangements and biasing
networks, and the like, may be employed in practicing the present
invention. In the particular RF substrate biasing network shown,
the grounded center tap inductor 9 acts to invert the RF voltage,
so that when the RF signal from source 1 is positive, for example,
the voltage between the grounded center tap and the floating end is
negative. In this regard, wiper arm 17 may be varied selectively to
vary the amplitude of the inverted voltage. Variable capacitor 11
and variable inductor 13 are employed to selectively tune the
biasing network arrangement. Accordingly, it can be seen that the
degree of phase shift may be varied over 360.degree. by varying
capacitor 11 and inductor 13. With such an arrangement, both the
amplitude and phase of the RF substrate bias may readily be
adjusted. The significance of being able to readily adjust phase
and amplitude will be appreciated when it is recognized that the RF
load, i.e., the impedance of the sputtering system, varies in
accordance with the parameters of the sputtering system.
Accordingly, where it is desirable to obtain a maximum or
peak-to-peak voltage on the substrate, for a given wiper arm
setting, the tuning circuit may be adjusted to resonance, whereby
matching between the impedance of the sputtering system and
inductor 9 is readily obtained.
It should be understood that, although not a part of the present
invention, the practical advantage of employing the RF driven
substrate biasing arrangement shown resides in the fact that a more
controllable biasing level on the substrate is more accurately and
readily obtained and adjusted, independent of target potential. In
this regard, it should be appreciated that the purpose of substrate
biasing is to provide a mechanism for substrate sputtering, whereby
over a portion of the RF operating cycle, bombardment of the
substrate occurs. Although the substrate biasing technique and
resultant bombardment of the target are not essential to the
practice of the present invention, such a technique does aid in
reducing impurities and effecting a redistribution of atoms, such
that the loosely bonded atoms sputtered thereon are removed.
Typically, in this regard, voltage values for the system shown may
involve, for example, a target voltage of approximately 1000 volts,
and an RF driven substrate bias of up to several hundred or more
volts.
Substrate 15, shown in the FIGURE mounted on substrate holder 25,
may be any of a variety of substrate materials, as is well known to
those skilled in the art. In accordance with the principles of the
present invention, target 26 may be any of the variety of high
purity target materials desired to be sputtered onto substrate 15.
Target holder assembly 3, which is in conductive and thermal
contact with target 26, may be water cooled with the water entering
and exiting in accordance with the arrows shown at the top of the
assembly. Surrounding both the cathodic target holder assembly 3
and pedestal portion 51 of anodic substrate holder 25, are a pair
of Helmholtz coils 31. In this regard, coils 31 act to provide a
magnetic field of approximately 30-80 gauss perpendicular to the
plane of target 26 and substrate 15. The function of this magnetic
field is to increase the concentration of electrons in the
sputtering region, so that the sputtering efficiency will be
increased. Moreover, the magnetic field also acts to increase, to
some extent, the bias on the substrate. Grounded shield 33, around
target 26, acts to limit and focus the sputtering of target 26 to
the central portion thereof.
The pair of ceramic sleeves 37 and 39 act to insulate the cathodic
target holder assembly 3 from the metal sputtering chamber 41 and
housing portion 35a of mount 35, respectively. In order to maintain
substrate 15 at the desired temperature, a heating assembly 43 is
provided. To maintain the area surrounding the substrate holder 25
cool, cooling coils, such as those shown at 45, may readily be
employed. To facilitate presputtering steps, a shutter arrangement
47 is provided to be movably positionable between substrate 15 and
target 26. As can be seen in the FIGURE, turning assembly 49,
external to chamber 41, acts to accomodate removal of the shutter
from the region between substrate 15 and target 26. As shown, the
pedestal portion 51 of the anodic substrate holder is mounted upon
insulation 53, so as to thereby electrically isolate the substrate
holder assembly from metal chamber 41.
To aid in obtaining high purity sputtered materials, sputtering
chamber 41 is equipped with a titanium sublimation pump 55,
surrounded by a liquid nitrogen shroud 57. As is known to those
skilled in the art, the sublimation pump acts to getter active
species, such as oxygen and oxygen-bearing compounds, and the like,
from within the chamber onto the surface of the cryogenically
cooled drum 59 of the pump, before sputtering begins. Titanium
filament 61 may be energized via an electrical source coupled to
the external wires extending therefrom. Typically, port 63 may be
used to pass the high purity gas employed, through the titanium
pump and into the sputtering chamber. By passing the high purity
gas through the titanium pump, the gas becomes even further
purified.
THE PROCESS
In accordance with the principles of the present invention, any of
a variety of high purity target materials may be used as the target
26. Because the process of the present invention acts quite simply
and effectively to provide a high purity environment, some of the
more reactive target materials may be employed to sputter high
purity thin films, and the like, on to the substrate. For example,
vanadium, niobium, tantalum, and compositions thereof, may readily
be sputtered in accordance with the process of the present
invention. Likewise, iron, cobalt, nickel, scandium, yttrium, and
the lanthanide series may readily be sputtered. In addition, the
actinide series, and the like, may be sputtered, in accordance with
the process of the present invention. The difficulties typically
encountered in sputtering these materials, are well known to those
skilled in the art.
Typically, the system employed in accordance with the principles of
the present invention is prepared for a sputtering operation by
initially prepumping the sputtering chamber 41 down to a pressure
of from 2.0 to 8.0 .times. 10.sup.-.sup.7 Torr, with substrate 15,
upon which sputtering is to occur, being maintained at the desired
substrate operating temperature. The prepumping may be achieved by
any of a variety of conventional pumping arrangements. Accordingly,
for the sake of simplicity, the pumping operation has only been
shown by the legend "vacuum" at the lower right hand portion of
chamber 41.
It should be appreciated that by employing the active vapor species
to provide a chemical getterer in accordance with the present
invention, as will be described in more detail hereinafter, the
prepumping operation to prepare the system for high purity
sputtering is simplified. It should be noted in this regard, that
in order to achieve the level of purity obtained by the process of
the present invention, it is normally necessary to maintain a
higher vacuum level for a longer period of time. Thus, in the
present invention, to prepare the system for sputtering, it is only
necessary that the system be prepumped to 10.sup.-.sup.6 to
10.sup.-.sup.7 Torr, at most, and this vacuum level be maintained
for a matter of a few minutes. To achieve the same level of purity
without the active vapor species provided by the present invention,
it is necessary to prepump to a level of around 10.sup.-.sup.8
Torr, and maintain this level for relatively long periods of time,
i.e., tens of minutes. The reason for the latter vacuum constraints
will be understood when it is recognized that in order to clean up
the chamber, it is necessary that the vacuum system employed exceed
the outgassing rate and act to provide a net reduction of the level
of impurity in the system. On the other hand, the method in
accordance with the principles of the present invention acts to
reduce the outgassing itself and, accordingly, obviates the need
for total dependence on the vacuum system for cleaning the
sputtering chamber.
During the prepumping operation, titanium filament 61 acts in
combination with the cryogenically cooled evaporated film on drum
59 to getter active species, such as oxygen and oxygen-bearing
compounds from within the system. After the system has been
sufficiently pumped down and maintained at the desired vacuum
level, the system is next back filled with a high purity argon
having mixed therein an active vapor species, in accordance with
the present invention. The high purity argon with active vapor
species may, if desired, be admitted via port 63. With this gas
mixture admitted into the system via port 63, it is passed through
the titanium sublimation pumping arrangement 55 whereby oxygen,
oxygen-bearing compounds, nitrogen, and the like, may be removed
from the argon mixture.
It should be noted that when the argon mixture is admitted into the
sputtering chamber through the titanium sublimation pumping
arrangement, a certain amount of the active vapor species mixed
therewith, may be gettered out. Accordingly, whether the argon
mixture is passed through the titanium sublimation pumping
arrangement, depends upon the particular application employed. It
might be preferable in certain applications that the argon mixture
be admitted into the system via another port, which will not act to
pass the gas through the titanium sublimation pump. Thus, where a
high purity argon having mixed therewith silane, for example, is
employed, it might be preferred to admit the gas into the chamber,
in accordance with this latter approach.
In accordance with the preferred mode of practicing the invention,
then, a high purity inert sputtering gas species having mixed
therewith selected active vapor species is admitted into the
sputtering chamber, via port 64, until a pressure of approximately
10.sup.-.sup.2 Torr is reached. In this regard, it should be noted
that the argon and active vapor species may be entered into the
sputtering chamber either separately, or in a premixed state. Thus,
port 64 could be employed to first enter the argon and then the
active vapor species, or vice versa. On the other hand, separate
ports could be used for each gas. However, in a preferred mode, a
single port, such as 64, is employed to enter a premixture of argon
and the active vapor species from a single container.
It should be understood, however, the significance of the present
invention does not reside in the exact manner by which the active
vapor species is entered into the sputtering chamber, but rather in
the fact that the active vapor species is present in the sputtering
chamber. In this regard, it should be further understood that the
active vapor species is selected, in accordance with the particular
application. The conditions for selecting the active vapor species
are that it exhibit a chemical reactivity with at least one of the
undesirable species present in the system, and that it be readily
disproportionated, particularly as pertains to being readily
disproportionated by way of a relatively weak plasma. In addition
to being readily disproportionated and chemically reactive with at
least one of the undesired species, the active vapor species must
be present in concentrations sufficiently low so as to not be
allowed to penetrate the region between the sputtering electrodes,
and yet sufficiently high so as to provide adequate gettering. In
this regard, where the sputtering rate is, on the one hand,
approximately 5 A/sec. the active vapor species may be present in
amounts as low as 0.1%. On the other hand, where the sputtering
rate is approximately 50 A/sec., the active vapor species may be
present in concentrations as high as 10%.
It should be understood, that the function of the active vapor
species is to decompose and provide an active gettering species,
which is available for gettering the desorbed and source sputtering
gasses in the system. For example, where vanadium is being
sputtered onto a substrate, a mixture of argon and silane may be
employed. As the silane enters the sputtering chamber, it
decomposes to form an active silicon species which acts to getter
desorbed and source sputtering gases in the chamber. In this
regard, the silane decomposes before it reaches the active film
growth area, since it is readily activated by the mild diffuse
plasma region existing around and outside of the film deposition
area, between the sputtering electrodes. As the silane decomposes,
the resultant silicon and reacted silicon species deposit upon the
internal surfaces of the sputtering chamber. Accordingly,
substantially the entire internal surface of the sputtering chamber
becomes coated with a thin film of silicon and compounds thereof.
The silicon in this manner, then, acts to react with and trap the
desorbed and source sputtering gases, thereby substantially
reducing the outgassing rate.
Prior to actual sputtering, it may be desired to perform a
presputtering operation in the argon and active vapor species
mixture. Although the presputtering step is not essential in
accordance with the process of the present invention, in the
preferred mode it might be desirable to utilize this process so as
to insure the utmost purity within the system. It should be noted
that the system may be energized with an RF source sufficient in
power to deliver in the neighborhood of around 8 watts/cm.sup.2 to
the target. Where a typical target of roughly 10 cm.sup.2 is
employed, roughly 100 watts of power would be sufficient to provide
effective sputtering. However, it is clear that the amount of power
required to be delivered to the system is not critical, and varies
in accordance with the particular sputtering parameters employed in
a given application.
In the presputtering operation, the shutter arrangement (which is
grounded via the walls of chamber 41) is first positioned between
the target and the substrate by knob 49, so as to obstruct
deposition onto the substrate, as shown in the FIGURE, when the
system power is turned on. Thus, with the argon and active vapor
species mixture present in the chamber and the system power turned
on, a plasma is generated and the high purity target is sputtered
upon shutter 47. This presputtering time may range from 5 to 30
minutes.
After the target is presputtered on to the shutter 47, the system
may then be prepared for sputter cleaning substrate 15. However,
sputter cleaning substrate 15 is a matter of choice, and is not an
essential operation. To sputter clean substrate 15, adjustments are
made in the biasing network so as to produce a d.c. level bias on
substrate 15. A d.c. bias level of approximately 150 volts has been
found satisfactory. With shutter 47 in conductive contact with
shield 33, which shield is grounded via the walls of mount 35 and
chamber 47, sputter cleaning of the substrate is effected for 5 to
10 minutes.
After presputtering, the system is ready to sputter any of a
variety of materials, as hereinabove mentioned. It has been found,
for example, that in sputtering vanadium from an ultra pure
vanadium target, using a mixture of argon and from 0.1 to 10%
silane, pure films of vanadium are produced having bulk
superconducting and resistivity properties. The process was
repeated with niobium having, however, the addition of a few
percentage of oxygen to test the process. Similar results of pure
films were obtained.
In the above processes, the silane decomposed to form a film of
silicon compounds, deposited over the internal surface of the
sputtering chamber. However, in the region of interest between the
sputtering electrodes, no silicon was detectable. The absence of
silicon within the sputtering region between targets 26 and
substrate 15 is due to the fact that the argon based plasma
produced therebetween, particularly at the periphery thereof, acts
to quickly break down the silane before it has an opportunity to
significantly penetrate this plasma zone. This latter fact is
evidenced by the relatively rapid gradation in silicon deposits on
the substrate across the barrier wall of the plasma, whereby the
silicon deposits vanish rather markedly. On the other hand, silicon
deposits exist rather uniformly throughout the various surfaces,
internal to chamber 21. For example, the outer surface of the walls
of mount 35, and the inner surface of the vertical walls of the
chamber through which ports 63 and 64 penetrate, are generally
uniformly coated with silicon, or silicon compounds.
It should be appreciated that although silane is known in the art
for purposes of sputtering silicon layers upon a substrate, silane
has not been employed as an active vapor species to provide a
gettering agent effectively operative within the sputtering
environment to reduce outgassing, and the like. Reference is made
to U.S. Pat. No. 3,647,663 for a description of the manner by which
silane may typically be employed to fabricate, for example, layers
of silicon oxide on a substrate.
Although silane has been found to be a particularly effective vapor
species, it should be understood as hereinabove mentioned, that any
of a variety of vapor species may, as readily be employed. For
example, titantium tetrachloride (TiCl.sub.4) may be mixed with
argon, for example, to provide an active gettering mechanism for
removing impurities in the fabrication of ultra pure materials.
Titanium tetrachloride would readily decompose in the sputtering
chamber into a titanium gettering species, whereby water vapor,
nitrogen, oxygen, carbon and the like would be gettered.
Likewise, tungsten hexafluoride (WF.sub.6) may be employed as the
active vapor species in accordance with the principles of the
present invention. In addition, uranium hexafluoride (UF.sub.6)
could be employed as the active vapor species, in accordance with
the present invention. Each of these species, mixed in
concentration of from 0.1 to 10% in argon would readily be
disproportionated in the sputtering environment to provide an
active gettering species (W and U) for gettering water vapor,
nitrogen, oxygen, carbon, and the like, during the sputtering of
any of a variety of ultra pure materials.
It is evident that, in accordance with the principles of the
present invention, an active gettering species is effectively
provided in the sputtering environment to reduce the outgassing
rate, whereby a purification level is obtained therein without
prolonged preconditioning and presputtering periods of time. In
addition, this purification level is maintained during the
sputtering process with a minimum of system resources. As a result
of employing such techniques, relatively high purity materials are
obtained with a minimum of steps.
Typically, preconditioning periods for prepumping and presputtering
of from 5 to 15 minutes are sufficient to provide high purity
materials. Moreover, pressure ranges from 1 .times. 10.sup.-.sup.1
to 1 .times. 10.sup.-.sup.4 Torr are sufficient to produce the high
purity materials, using a conventional sputtering system
configuration.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
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