U.S. patent number 3,849,276 [Application Number 05/125,993] was granted by the patent office on 1974-11-19 for process for forming reactive layers whose thickness is independent of time.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to James H. Greiner.
United States Patent |
3,849,276 |
Greiner |
November 19, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
PROCESS FOR FORMING REACTIVE LAYERS WHOSE THICKNESS IS INDEPENDENT
OF TIME
Abstract
A method for forming reactive layers on materials where the
final thickness of the reactive layer is independent of time after
an initial time period in which the final thickness is attained.
Films are grown by a process, such as oxidation, and are removed
simultaneously by a process, such as sputtering. The parameters of
the growth process and removal process are initially fixed so that
at a desired final thickness, the growth rate is equal to the
removal rate. When this steady-state equilibrium is reached, the
thickness of the reactive layer will remain constant, independent
of time. The reactive layer can be grown to the desired thickness
by this method, or a thicker layer can be reduced to a lesser final
thickness by this method. Very small (less than 50A) films of
excellent quality can be made.
Inventors: |
Greiner; James H. (Millwood,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
22422437 |
Appl.
No.: |
05/125,993 |
Filed: |
March 19, 1971 |
Current U.S.
Class: |
204/192.35;
204/192.32; 204/298.26; 257/E21.293; 257/E21.25; 257/E21.283;
257/E21.29; 257/E21.266 |
Current CPC
Class: |
H01L
21/02233 (20130101); H01L 21/31105 (20130101); H01L
39/24 (20130101); H01L 21/3185 (20130101); H01L
21/02175 (20130101); H01L 23/29 (20130101); H01L
21/31683 (20130101); H01L 27/00 (20130101); H01L
21/314 (20130101); H01L 21/31654 (20130101); H01L
23/291 (20130101); H01L 21/02274 (20130101); C23C
8/36 (20130101); H01L 2924/0002 (20130101); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
H01L
21/316 (20060101); H01L 21/311 (20060101); C23C
8/06 (20060101); H01L 23/28 (20060101); H01L
21/02 (20060101); H01L 27/00 (20060101); H01L
39/24 (20060101); H01L 21/314 (20060101); H01L
21/318 (20060101); H01L 23/29 (20060101); C23C
8/36 (20060101); C23c 013/00 (); C23c 015/00 () |
Field of
Search: |
;204/192 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Wernick et al., Surface Treatment of Aluminum, pg. 345-348, 1964.
.
O'Hanlon, "Plasma Anodization of Metals and Semiconductors,"
Journal of Vacuum Science and Technology, Vol. 7, p. 330 (1969).
.
Scharfe, MSEE Thesis, University of Minnesota (1966). .
Schroen, "Physics of Preparation of Josephson Barriers," Jour. of
Applied Physics, Vol. 39, p. 2671 (May 1968). .
Miles et al., "The Formation of Metal Oxide Films Using Gaseous and
Solid Electrolytes," J.E.C.S., Vol. 110, p. 1240 (Dec. 1963). .
Worledge et al., "Controlled Oxidation of Tantalum and Aluminum in
a Radio-Frequency-Exited Glow Discharge," British Journal of
Applied Physics, Vol. 18, p. 1337 (1967)..
|
Primary Examiner: Mack; John H.
Assistant Examiner: Langel; Wayne A.
Attorney, Agent or Firm: Stanland; Jackson E.
Claims
What is claimed is:
1. A method for forming reactive layers of controlled thickness on
base materials, said reactive layers being combinations of the base
material and at least one constituent which reacts with said base
material, comprising:
bringing said reactive constituent to said base material in a
substantially vacuum environment for reaction thereon to grow said
reactive layer at a first rate;
removing said reactive layer simultaneously with said growth, the
removal of said reactive layer occurring at a seond rate;
adjusting said first and second rates to be equal for a desired
final thickness of said reactive layer;
continuing said growth and said removal at least until said first
and second rates of growth and removal respectively are equal, at
which time a final thickness of said reactive layer is
achieved.
2. The method of claim 1, where said base material is a metal, and
said reactive constituent is oxygen.
3. The method of claim 1, where said base material is selected from
the group consisting essentially of metals and semiconductors, and
said reactive constituents are selected from the group consisting
of oxygen, nitrogen, hydrogen, sulfur, H.sub.2 S , and
CH.sub.4.
4. The method of claim 1, where said first rate is dependent on the
thickness of said grown reactive layer and said second rate is
substantially independent of the thickness of said reactive layer,
said first and seond rates tending to equalize as the thickness of
said reactive layer changes.
5. The method of claim 1, wherein said reactive layer is initially
present on said base material, and said second rate is initially
greater than said first rate, said rates then varying relatively
until a final thickness of said reactive layer is reached at which
said first and second rates are equal.
6. The method of claim 1, where said reactive layer grows by a
vapor deposition process.
7. A method of forming reactive layers on base material, said
layers having a thickness independent of time after a final
thickness is achieved, said method comprising:
placing said base material in a system having a cathode and an
anode located in a chamber in which a vacuum can be established,
said base material having the electrical potential of said
cathode;
introducing a reactive gas species into said chamber, said species
being combined with said base material to grow said reactive layer
thereon when a voltage is established between said cathode and said
anode to provide a gaseous plasma between said cathode and said
anode, said reactive layer being removed by the incidence of
gaseous particles from said plasma as said growth occurs;
adjusting the rate of growth of said layer and the rate of removal
of said layer to establish a steady state equilibrium at which said
rates are substantially equal, at the final desired thickness of
said reactive layer.
8. The method of claim 7, where said base material is selected from
the group consisting of metals and semiconductors, and said
reactive species is selected from the group consisting of oxygen,
nitrogen, hydrogen, sulfur, CH.sub.4, H.sub.2 S, and combinations
thereof.
9. The method of claim 7, where said base material is a metal and
said reactive species is oxygen.
10. The method of claim 7, where a plurality of reactive species
are introduced into said chamber, thereby resulting in a reactive
layer of variable composition.
11. The method of claim 7, where an inert gas is also present in
said gaseous plasma.
12. The method of claim 7, where said voltage is an RF voltage
applied between said cathode and said anode.
13. A method of producing films of controllable thickness on a
substrate, comprising the steps of:
depositing gaseous particles of film material on said substrate,
said deposition occurring at a first rate;
removing said film material from said substrate during said
deposition, said removal occurring at a second rate which is
independently adjustable with respect to said first rate;
adjusting said first and second rates to provide steady state
equilibrium conditions at which said rates will be equal;
continuing said deposition and said removal at least until steady
state equilibrium is attained.
14. The method of claim 13, wherein said first rate is dependent on
the thickness of said deposited material.
15. The method of claim 13, including the further step of
depositing a layer onto said film material after said steady state
equilibrium is attained.
16. The method of claim 13, where said steady state equilibrium is
attained at a film thickness less than about 100A.
17. The method of claim 13, where said removal occurs by a vapor
process.
18. A method of forming reactive layers on substrates, comprising
the steps of:
placing said substrate in a vacuum environment housing a cathode
and an anode, said substrate being located on the surface of said
cathode;
introducing a gaseous ambient between said cathode and said anode,
said ambient containing a species which reacts with said substrate
to form reactive layers thereon;
establishing a voltage between said cathode and said anode to
create an energetic plasma about said substrate whereby said
species from said plasma strikes said substrate to form a reactive
layer thereon having a first growth rate, said reactive layer also
being removed at a second rate by the incidence of particles from
said plasma, the growth rate and the removal rate varying
relatively to one another as the thickness of said reactive layer
changes;
adjusting said first and second rates to be equal at a desired
final thickness of said reactive layer;
continuing said growth and said removal at least until said first
rate is substantially equal to said second rate.
19. The method of claim 18, where said reactive layer formation
occurs by oxidation and the removal of said reactive layer occurs
by sputtering.
20. The method of claim 18, where said substrate is selected from
the group consisting of metals and semiconductors, and said ambient
includes a plurality of gaseous species, at least one of said
species reacting with said substrate to form said reactive
layer.
21. The method of claim 18, where said gaseous ambient contains at
least one gaseous species selected from the group consisting of
oxygen, nitrogen, sulfur, hydrogen, CH.sub.4 and H.sub.2 S.
22. A method of forming a multi-layer structure comprising the
steps of:
placing a substrate in a vacuum environment containing a cathode
and an anode, and a source of a first material;
coating said substrate with said first material to form a base
layer;
introducing a gaseous ambient between said cathode and said anode,
said ambient containing gaseous species which react with said base
layer to form a reactive layer thereon;
producing an energetic plasma about said base layer by applying a
voltage between said cathode and said anode, said base layer being
at said cathode potential, said gaseous species in said plasma
striking said base layer and forming a reactive layer on said base
layer at a first rate of formation, said reactive layer being
removed by the incidence of particles from said plasma, said
removal occurring as said reactive layer is being formed and at a
second rate of removal;
adjusting the relative rates of formation and removal to provide a
steady state equilibrium at which said formation rate will be
substantially equal to said removal rate;
continuing said formation and said removal at least until said
steady state equilibrium is achieved, at which time said reactive
layer will have a desired final thickness;
coating said reactive layer with an electrically conductive
material to form a counter layer thereon.
23. The method of claim 22, wherein said base layer and said
counter layer are metals and said reactive layer is an oxide.
24. The method of claim 22, wherein said voltage is an RF potential
and said substrate and base layer are located on said cathode.
25. The method of claim 22, where said reactive gas species
includes at least one of the group consisting of oxygen, nitrogen,
hydrogen, sulfur, CH.sub.4, and H.sub.2 S.
26. A method for forming reactive layers of controlled thickness on
base materials, said reactive layers being combinations of the base
material and at least one constituent which reacts with said base
material, comprising:
bringing said reactive constituent to said base material for
reaction thereon to grow said reactive layer at a first rate;
removing said reactive layer by a sputtering process which occurs
simultaneously with said growth, the removal of said reactive layer
occurring at a second rate;
adjusting said first and second rates to be equal for a desired
final thickness of said reactive layer;
continuing said growth and said removal at least until said first
and second rates of growth and removal respectively are equal, at
which time a final thickness of said reactive layer is
achieved.
27. A method of producing films of controllable thickness on a
substrate, comprising the steps of:
depositing particles of film material by oxidation on said
substrate, said deposition occurring at a first rate;
removing said film material from said substrate by sputtering
during said deposition, said removal occurring at a second rate
which is independently adjustable with respect to said first
rate;
adjusting said first and second rates to provide steady state
equilibrium conditions at which said rates will be equal;
continuing said deposition and said removal at least until said
steady state equilibrium is attained.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for forming reactive layers of
desired thicknesses, and specifically to a method for forming
reactive layers having reproducibly accurate final thicknesses
which remain constant even if the method is continued further.
2. Description of the Prior Art
Reactive layers, such as oxides, nitrides, sulfides, etc. are
useful in the production of many devices. For instance, Josephson
junction devices use very thin (less than 50A) reactive layers as
tunnel barriers between two superconducting electrodes. Generally,
the base electrode is oxidized to form the tunnel barrier. Another
device using reactive layers is a field effect device having an
oxide insulator below its gate electrode.
Reactive layers are defined generally as layers formed on base
materials and are combinations of the base material and other
reactive materials. For instance, if lead is oxidized by oxygen
species located adjacent to the lead substrate, a reactive layer of
lead oxide will be formed on the lead substrate.
Many processes have been used for depositing reactive layers on
metals and semiconductors. A review paper treating these processes
if J. F. O'Hanlon, Journal of Vacuum Science and Technology, Vol.
7, No. 2, Page 330 (1969). This paper notes that plasma anodization
has been used to produce oxide films where the samples to be
anodized are located between the cathode and the anode. Both RF and
DC discharges have been used.
The following references discuss many aspects of production of
reactive films, some of which are very thin for particular use in
Josephson junctions. These references describe thermal oxidation,
evaporation, sputtering, and anodization.
W. schroen, Journal of Applied Physics, Vol. 39, No. 6, May 1968,
Page 2,671.
J. l. miles and P. H. Smith, Journal of the Electro-chemical
Society, Dec. 1963, Page 1,240.
L. d. locker and L. P. Skolnick, Applied Physics Letters, Vol. 12,
No. 11, 1 June 1968, Page 396.
G. a. jennings and W. McNeill, Applied Physics Letters, Vol. 12,
No. 2, 15 Jan. 1968, Page 25.
P. l. worledge and D. White, British Journal of Applied Physics,
Vol. 18, 1967, Page 1,337.
R. i. nazarova, Russian Journal of Physiochemistry, Vol. 36, No. 5,
May 1962, Page 522.
M. scharfe, "The Gaseous Anodization of Tantalum," Thesis for MSEE,
University of Minnesota, July 1966.
In the production of reactive layers, it is important that the
thickness of the formed layers be reproducibly obtained over many
cycles of the process. This is especially true if the films to be
produced are very thin, as is the case with Josephson devices.
Since the tunneling current in Josephson devices varies
exponentially with the tunnel barrier thickness, small variations
in thickness from one device to another can lead to serious
problems when arrays are to be fabricated.
Another problem that occurs frequently in prior art methods for
formation of reactive layers involves impurity incorporation in the
growing film. Often, the substrate on which the reactive layer is
to be produced is placed in the glow discharge between a cathode
and an anode. Sputtering from the cathode occurs and often the
cathode material finds its way to the target, even if shielding is
used.
It is also important that the quality of the reactive film be good.
It should be a very dense film having no pinholes or impurities
therein which may cause problems such as electrical shorting in
devices incorporating these films. In prior processes, ultra clean
systems are required and the quality of the grown films was
difficult to evaluate over a period of deposition ruins.
Prior methods for forming reactive layers do not provide good
control of the final thickness of the layer. Generally the process
is stopped after a certain amount of time, knowing that the
thickness of the film would have an approximate value after a
certain amount of time had passed. However, formation of very thin
films (less than 50A) is difficult when the only control over
thickness is the amount of time during which the process is allowed
to continue. For instance, water vapor may be present in the system
and may contribute additional oxygen ions. If an oxide film is
being formed, the initial oxidation rate may be very high, so that
the film achieves a thickness greater than that desired before the
process can be terminated. In this situation, the quality of the
film is apt to be inferior, in addition to its being too thick. The
prior art methods do not have good thickness control, since the
primary technique for obtaining thickness control is to slow the
growth rate and then turn off the process after a period of time
calculated to achieve a desired thickness.
Accordingly, it is a primary object of this invention to provide a
process for growing reactive layers which is time independent.
Another object of this invention is to provide an improved process
for growth of reactive layers, in which layers of any desired final
thickness can be grown.
Still another object of this invention is to provide a process for
growth of reactive layers in which the thickness of the grown
layers is reproducibly constant during each deposition.
A further object of this invention is to provide a process for
formation of reactive layers, where the thickness of the layers is
uniform throughout the layer.
A still further object of this invention is to provide a time
independent process for formation of reactive layers, even if the
initial thickness of the reactive layer is greater than that
desired.
Another object of this invention is to provide an improved process
for formation of reactive layers which is time independent and
which produces high quality reactive layers.
Still another object of this invention is to provide an improved
process for formation of reactive layers which is time independent
and easily controlled to provide reactive layers of any desired
thickness, regardless of the cleanliness of the process environment
system and the substrate used.
SUMMARY OF THE INVENTION
This method forms reactive layers on any substrates, such as
semiconductors or metals. The layers formed include oxides,
nitrides, sulfides, etc. A reactive species is brought into contact
with the substrate and the reactive layer is formed thereon. While
the layer is forming, another process occurs simultaneously which
removes the layer. After a period of time, a steady state
equilibrium is produced at which the growth rate is equal to the
removal rate. At this thickness, the reactive layers will maintain
a constant thickness, independent of time.
In the particular case of an oxide film, a low power RF oxygen
discharge can be used to form the oxide layer. A substrate is
affixed to the cathode in the discharge and consequently will be
both oxidized and sputtered during the process. If the oxidizing
rate is initially larger than the sputtering rate, an oxide film
will form. The oxide rate decreases as the oxide thickness
increases because it is intrinisically a diffusion limited process.
However, the oxide sputtering rate is independent of the thickness
of the bombarded oxide. Therefore, if the oxidation rate is
initially higher than the sputtering rate the oxide layer thickness
will increase until the rate of oxidation becomes equal to the
sputtering rate, at which time a steady state oxide thickness is
attained.
Conversely, if an oxide film is present and the sputtering rate is
initially higher than the oxidation rate, the oxide thickness
decreases until the oxidation and sputtering rates become equal, or
until the oxide is removed. The final thickness of the oxide layer
will depend upon the equalization of the sputtering rate and the
oxidation rate, both of which can be initially established to yield
the desired final thickness at equilibrium.
As another alternative, an oxide layer can be reduced by a species
such as hydrogen, which is present in the glow discharge. This will
lead to production of a pure base material having a desired
thickness established by the system parameters. For instance, a
film of lead oxide can be reduced by bombardment of hydrogen ions,
to produce a pure lead layer on the lead oxide, the pure lead layer
having a constant final thickness determined by the system
parameters.
This process is particularly attractive for formation of very thin
films of constant thickness. Films less than 100A can easily be
produced merely by setting the system parameters, which include (1)
the temperature the substrate, (2) the RF power, (3) the pressure
of the reactive species, and (4) the composition of the gaseous
discharge. Some of these parameters affect the removal process
(sputtering), while others have a greater effect on the growth
process (oxidation). These can be set initially, and no dependency
on other factors, such as impurity levels, is present. Once a
steady state equilibrium is reached, the film thickness will remain
constant and the process can be run for any length of time in order
to provide high purity films having constant thickness.
In the preferred system, the substrates are placed on the cathode
and an RF glow discharge is established between the cathode and an
anode. The reactive species is introduced into the chamber
containing the cathode and the anode, and a discharge is maintained
by application of RF power between the cathode and the anode. The
system parameters mentioned above are adjusted so that, at a
desired final thickness, the deposition rate and the removal rate
will be equal. Since the sample is placed on the cathode,
deposition of cathode material on the sample can be eliminated,
which is not the case if the substrate were placed in the glow
dischage between the cathode and the anode. In addition to the
contamination problem, systems in which the substrate is placed
between the cathode and the anode produce films of varying
thickness, since some sputtering may occur between the cathode and
the substrate. In those systems, the final thickness is generally
limited by the geometry of the system. Consequently, a process in
which the substrate is placed on the cathode avoids these problems
and yields films of constant thickness having minimal impurity
contamination, and in which there is no deposit other than that
from the intended source.
Since the solid state equilibrium is reached quickly in this
process, good quality films are produced. This is a high energy
process in which ions are propelled at the target, in contrast to
the production of reactive layers by thermal processes. Further,
since the reactive film is being removed at the same time it is
formed, new reactive material is being constantly established. This
means that any impure initial layer will be removed and, as the
system becomes more clean, the quality of the grown layer will
improve.
This method has another advantage in that the composition of the
reactive film can be tailored by varying the composition of the
discharge ambient. Regardless of the thickness required, chemically
different reactive layers may be produced by incorporating
different amounts of the reactive species in the gaseous
ambient.
Because the substrate is placed on the cathode, there is nothing to
perturb the discharge existing between the cathode and the anode.
This leads to a process having greater control, resulting in
uniform films of reproducible thickness. Since an equilibrium
process is described, constant thickness independent of time can be
produced continually. For manufacture of large arrays of devices
and for reactive layer formations on a continuing basis, no danger
exists that the thickness will change from one system to another
and from one formation run to another. Thus the process is
especially attractive for use in production lines where countless
numbers of devices using reactive layers are produced.
Another advantage in the present method is that initial cleaning
steps are not required. In conventional processes, a long initial
discharge time is required to clean the system before deposition
onto the substrate. With the present system impurities such as
H.sub.2 O do not influence the quality of the final reactive layer.
If the layer initially grows too thick due to the impurities, it
will settle back to its desired final thickness when the rate of
growth is equal to the rate of removal. Also, the initially
produced impure film will be removed during the formation process,
with the result that the film produced after steady state
equilibrium is achieved will be very pure.
Although DC discharges can be used, RF discharges are preferred for
growth of the reactive layers. Discharges can be sustained at lower
pressures when RF power is used, as opposed to the higher pressures
needed to sustain the discharge when DC power is used. This makes
it more easy to remove adsorbed particles on the reactive layer
before deposition of a subsequent layer. For instance, if an oxide
layer is produced on a base electrode and it is desired to produce
a counter electrode on this oxide layer, removal of adsorbed oxygen
atoms from the reactive layer before deposition of the counter
electrode is more simple if an RF discharge had been used to form
the reactive layer.
The prior art does not teach that a growth process can be balanced
with a removal process to form reactive layers having any desired
thickness, where the final thickness is independent of time. While
the art does suggest that terminal thicknesses can be the result of
various processes, nowhere is it suggested that any desired
thickness can be achieved by initially selecting system parameters
to force competing processes to be equal at that thickness. It has
also been known that processes such as oxidation produce films of
limiting thickness, in this case due to the diffusion limited
nature of the oxidation process. However, it is believed that
applicant is the first to teach that competing processes should be
used to produce films of any desired thickness, and that it makes
no difference whether the film is grown from zero thickness or is
reduced from a thickness greater than that desired. The process
enables reproducible formation of films having any desired
thickness, totally independent of the time of formation once steady
state equilibrium is achieved. Of course, the time period for
achieving steady state equilibrium will vary depending upon the
thickness desired, and the initial setting of the system
parameters.
While sputtering and deposition within the sputtering apparatus is
a particularly useful embodiment, other growth and removal
processes can be used according to this invention. For instance,
the removal process can be achieved by an apparatus separate from
the growth process. An example of this is the use of an ion beam to
etch away the depositing reactive layer. Another combination of
growth and removal processes may be a liquid anodization growth
process (for example to oxidize a substrate), in which the
electrolyte dissolves some of the grown layer into solution,
thereby removing it. Consequently, it is apparent that the growth
and removal processes can be separate, independently adjustable
processes using the same or separate means for performing those
processes. Also, the rates of the growth and removal processes can
be varied at any time during the formation of the reactive layer or
they can initially be set to reach steady state equilibrium at some
thickness of the reactive layer without further adjustment.
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 DRAWINGS
FIG. 1 is an illustration of an RF sputtering apparatus suitable
for carrying out the present method.
FIG. 2A is a plot of thickness of the formed reactive layer as a
function of time for a growth process, in this case oxidation.
FIG. 2B is a plot of the thickness of the reactive layer as a
function of time for a removal process, in this case
sputtering.
FIG. 2C is a plot of the rates of the growth process (oxidation)
and the removal process (sputtering) as a function of time.
FIGs. 3A, 3B, and 3C illustrate the growth of a reactive layer on a
substrate with time, the reactive layer being conveniently shown as
an oxide.
FIGS. 4A, 4B and 4C show the production of a pure layer from a
reactive layer, where the reactive layer is conveniently shown as
an oxide and the pure layer is conveniently shown as a metal
forming a constituent of the oxide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
This method employs simultaneously occuring growth processes and
removal processes in order to obtain good quality, reproducibly
uniform reactive layers on various substrates. In particular, the
method will be described by describing the formation of a
controllable oxide layer on a substrate. It should be understood
that other reactive layers, such as nitrides, sulfides, etc. can be
formed on a base material (substrate). Whatever the species
comprising the reactive layer, growth of the layer will occur at
the same time that a portion of the newly grown layer is removed.
In this way, the quality of the material grown will be good.
Because the growth process and the removal process are
independently controllable, the rates of each process can be made
equal at any thickness.
For the specific case of an oxide reactive layer, a low power RF
oxygen discharge can be used to form the reactive layer. The
underlying substrate is fixed to the cathode in the discharge and
is both oxidized and sputtered. In this case the growth process is
oxidation while the removal process is sputtering. If the oxidizing
rate is initially larger than the sputtering rate, an oxide film
will form. The oxidizing rate decreases as the oxide thickness
increases because oxidation is intrinsically a diffusion-limited
process. However, the oxide sputtering rate is independent of the
thickness of the bombarded oxide. Therefore, if the oxidation rate
is initially higher than the sputtering rate, the oxide thickness
will increase until the rate of oxidation becomes equal to the rate
of sputtering, at which time a steady state oxide thickness is
obtained. This thickness will remain constant independent of time
even if the method is continued.
Conversely, if an oxide film is initially present and the
sputtering rate is initially larger than the oxidation rate, the
oxide thickness will decrease until the oxidation rate and the
sputtering rate become equal, at which time a steady-state
equilibrium will be achieved. With an applied RF voltage,
substantial equality of rates means that over a full cycle of
applied RF voltage the oxidation and sputtering cancel one another.
It should be understood that within each half cycle of applied RF
power, either oxidation or sputtering may dominate; however, after
a full cycle, the amount grown is the same as the amount removed.
Using this method, very small oxide layers 20-50A in thickness have
been formed. Of course, the final thickness of the film can be
adjusted by appropriately varying the system parameters. These
parameters include the oxygen pressure, the RF power, the
composition of the discharge, and the substrate temperature. The
rate of sputtering and the rate of oxidation are independently
controllable using these parameters; therefore, any desired final
thickness can be achieved.
FIG. 1 shows an apparatus used for oxidation and sputtering to form
reactive layers. This conventional aputtering apparatus comprises a
glass chamber 10 having metal top and bottom plates 11A and 11B. RF
cathode 12 is held by collar 13 which insulates it from top plate
11A. Cathode 12 is water-cooled, having an inlet pipe 14 and an
outlet pipe 16. The cathode is connected to an RF voltage source
which provides an RF discharge at a power density of 0.03 to 2
watts/cm.sup.2.
The cathode is surrounded by a grounded shield 18 which protects
against unwanted bombardment of the cathode. The substrates 19,
onto which the reactive layer is to be formed, are placed in a
substrate holder 20 which is fastened to the cathode 12 by set
screws 22. In a typical case, the cathode is copper and the
substrate holder is aluminum, although other materials are often
used.
Top and bottom metal plates 11A, 11B are grounded and serve as the
anode. Connected into the chamber 10 by a valve 24 is a vacuum
system comprising a freon-trapped oil defusion pump 25 for
providing the desired vacuum. Also connected into the system is a
manually controlled leak valve 26 for introduction of the reactive
gas species. Valve 26 is connected to a source 27 of reactive gas
species (and other gases) and the amount of any gas introduced into
chamber 10 is monitored. If desired, the cathode can be surrounded
by an aluminum ring which is used for cleaning during an initial DC
discharge.
Also included in chamber 10 is a source 28 of material which can be
deposited onto substrates 19, or onto formed reactive layers.
Insulating material 30 provides electrical isolation from base
plate 11B. Located above source 28 is a movable shutter 32
connected to a turning knob 34. Shutter 32 prevents sputtering from
source 28 when located above source 28.
The apparatus described is a conventional RF sputtering apparatus;
however, the substrates are placed on the cathode rather than being
located between the cathode and the anode somewhere in the
discharge. In a typical operation, the substrates are initially
cleaned by sputter-etching, for instance at a power density of 0.8
watts/cm.sup.2 at an argon pressure of 4 .times. 10.sup..sup.-3
Torr. The argon leak into chamber 10 is balanced by the throttled
diffusion pump 25 in order to maintain constant pressure. Typical
cleaning times used to remove any residue oxides, photoresistive
particles, or other contaminants vary from approximately 1-5
minutes.
Immediately after the sputter-etching cleaning steps, the argon in
chamber 10 is removed and is replaced by oxygen or by an
argon-oxygen mixture. The reactive oxide layer is formed on
substrate 19 by maintaining an RF discharge at a power density of
approximately 0.03-0.1 watts/cm.sup.2 for 10 to 20 minutes. In this
particular example, a reactive oxide layer having a thickness less
than 50A is produced.
The argon in the gas mixture during formation of the reactive layer
is ionized and bombards the cathode sputtering away portions of the
oxide formed there by the oxygen gas species present in the
chamber. If no argon is present, the oxygen gas species themselves
will perform the sputtering function, but the sputtering rate will
be less than that which occurs when argon is also present in the
discharge.
In order to account for the oxide growth, consideration must be
given to the sputtering and oxidation processes which occur at the
cathode. Most likely, the sputtering occurs in the usual manner as
a result of positive ion bombardment while the oxidation is thought
to be dependent on the presence of negative oxygen ions. The oxygen
discharge is sustained by driving the cathode at a frequency of
13.56 MHz. This results in the usual ion sheath that is visible as
a dark space near the cathode surface. The cathode surface
potential varies with time having a peak negative value nearly as
large as the peak amplitude of the applied potential. Sputtering
occurs when positive ions from the sheath are drawn toward the
cathode by the negative potential. It is likely that the negative
ions for oxide growth are extracted from the plasma or created at
the oxide-gas interface by electron attachment. The negative bias
resulting from the electron accumulation at the oxide-gas interface
may enhance oxidation by providing an additional driving force for
diffusion of cations through the oxide. In addition, high energy
neutrals and implanted oxygen ions may be important in the
sputtering and oxidation processes.
Although the sputtering and oxidation mechanisms have not been
identified in detail, insight can be obtained regarding the
dependence of oxide thickness on process parameters by assuming
that the sputtering and oxidation rates are additive. Accordingly,
the rate of oxide thickness change is
dx/dt = dx/dt oxidation - dx/dt sputter , 1
where x is the oxide thickness.
In thermal oxidation, the oxidation rate is generally found to be a
decreasing function of oxide thickness. For very thin oxide films,
a direct logarithmic growth with time is frequently found. Miles
and Smith (referenced previously) have reported a logarithmic oxide
growth with time on vacuum deposited A1 films exposed (without any
external applied potential) to a dc oxygen glow discharge. If, in
the present process, the oxidation is assumed to follow a
logarithmic law, the oxidation rate may be represented by the
expression Ke.sup..sup.-x/x , where K and x.sub.o are oxidation
parameters which depend on factors such as pressure and
temperature. In addition, if the sputtering rate is assumed to be a
constant (R) when an oxide is present, the Eq. (1) reduces to
dx/dt = Ke.sup..sup.-x/x - R for x > 0 2
It is interesting to note from Eq. (2), that a balance between the
sputtering and oxidation will occur when dx/dt = 0, implying steady
state type of saturation in contrast to thermal or dc glow
discharge oxidation. The associated steady-state oxide thickness
is
x.sub.L = x.sub.o ln (K/R) , 3
hereafter referred to as the limiting oxide thickness. From Eq. (3)
it follows that it should be possible to obtain different limiting
oxide thicknesses by varying the oxidation and sputtering
parameters, e.g., oxygen pressure, RF input power and substrate
temperature.
This is consistent with laboratory development in which the
limiting oxide thickness was found to increase with oxygen
pressure, indicating that the oxidation rate increases more rapidly
with oxygen pressure than does the sputtering rate. The oxide
thickness could be further adjusted by using an oxygen-argon gas
mixture which increased the sputtering rate and decreased the
oxidation rate as compared to 100 percent oxygen. The sputtering
rate increases with RF power due to an increase in the ion density
and ion energy. Laboratory results indicate that the sputtering
rate has a stronger dependence than the oxidation rate on the RF
power. Since the energy of the bombarding ions is large compared to
kT (k is the Boltzman constant, and T is substrate temperature)
little change in sputtering rate occurs for increasing target
temperature. However, the oxidation is a diffusion dependent
process and is temperature dependent.
If Eq. (2) is integrated, the dependence of oxide thickness of time
is obtained, namely,
x = x.sub.o 1n [K/R - (K/R - e .sup.x /x ) e .sup..sup.-Rt/x ] for
x > 0 , 4
where x.sub.i is the initial oxide thickness. Eq. (4) reduces to
(3) when e.sup..sup.-Rt/x approaches zero. It should be noted that
the limiting oxide thickness might be less than, equal to, or
greater than the initial oxide thickness depending on the amplitude
of the initial oxidation rate as compared to the sputtering
rate.
An effective time constant, t.sub.o = x.sub.o /R can be estimated
from Eq. (4). From thermal oxidation data and from oxidation in a
dc oxygen glow discharge, x.sub.o is estimated to be in the range
of 1.5 to 3.5A. The sputtering rate is about 0.1A/sec for a power
density of 0.1 watts/cm.sup.2 for oxygen pressures in the
10.sup..sup.-2 Torr. range. Hence, the effective time constant is
estimated to be about a minute.
The system parameters which can be varied to affect the
sputtering/oxidation processes are (1) substrate temperature, (2)
RF power, (3) reactive gas species, (4) composition of the gaseous
discharge.
If the substrate temperature is varied, the oxidation rate is
changed although the sputtering rate is not appreciably affected.
Since oxidation is diffusion limited, increasing the substrate
temperature will increase the oxidation rate. As the temperature
decreases, the oxidation rate will also decrease.
Sputtering rate is dependent on RF power. As the voltage on the
cathode increases, there will be a high efficiency of ionization of
oxygen and the sputtering rate will increase. While variation of
the RF power may also vary oxidation rate, the primary change is to
the sputtering rate. Since the voltage on the cathode determines
the ion energy striking the cathode, this is a direct way to
increase sputtering.
The reactive gas pressure in the discharge (assume that it is
oxygen) directly affects the growth process. As the pressure of
oxygen in the discharge increases, more oxygen ions will be
produced per unit time so the oxidation rate will increase at a
constant RF power. However, it is possible that sputtering will
increase also since more oxygen ions are present. It has been found
that the thickness of films increases when the reactive gas species
pressure increases, which indicates that the oxidation (growth)
process is primarily increased by varying the reactive gas species
pressure.
Varying the composition of the discharge can vary either the
oxidation or the sputtering rates, or both. For instance, if
additional oxygen is added to the discharge, the oxidation rate
will increase. Conversely, if argon is added to the discharge, the
sputtering rate will increase.
FIGS. 2A-2C show the growth process, removal process, and the rate
of change in these processes. In these figures, it is assumed that
the reactive layer to be produced is an oxide layer, although it
should be understood that other reactive layers can also be
produced by corresponding processes.
In FIG. 2A, the thickness of a reactive layer in the presence of an
oxidation process is plotted against time. It is seen that a
terminal thickness is eventually achieved, since oxidation is
primarily a diffusion limited process.
In FIG. 2B, the thickness of a reactive film removed by a
sputtering process is plotted against time. Since sputtering
removes the atoms from the reactive layer linearly with time, the
plot is a linearly decreasing curve.
FIG. 2C shows a plot of the growth process rate and the removal
process rate, for the growth process of FIG. 2A and the removal
process of FIG. 2B. The oxidation rate curve decreases
exponentially with time, resulting in a substantially constant rate
after a sufficient amount of time has passed. The sputtering rate
is constant since the curve of the sputtering process (FIG. 2B) is
linear. At a time T.sub.0, the oxidation rate and the sputtering
rate are equal. At this time, the reactive layer has achieved its
final thickness and from then on will maintain this thickness
independent of time. This cross-over point of the growth and
removal rate curves is determined by the desired final thickness. A
particular amount of time T.sub.0 will be required to get to this
cross-over point, but from then on the reactive layer will maintain
a constant thickness with time. During the applied RF voltage
cycle, growth of the reactive layer will predominate during
one-half of each cycle, while sputtering (removal) will predominate
during the other one-half cycle. Consequently, the average change
over a cycle is zero.
FIGS. 3A-3C demonstrate the production of a reactive layer by this
process. In FIG. 3A, a substrate having height h is shown at time
T.sub.1. This substrate is placed in the apparatus of FIG. 1, and
the method as described occurs. Assuming that the reactive layer is
to be an oxide, an oxide having a terminal thickness X.sub.L is
produced after a time period (T.sub.2 -T.sub.1) at the end of which
the oxidation rate is equal to the sputtering rate. The new
thickness of the oxide/substrate is now (h+.DELTA.), due to
expansion when the substrate is oxidized.
At a later time T.sub.3, the oxide has the same final thickness
X.sub.L, but the overall thickness of the oxide and the underlying
substrate is reduced to a value less than h. As the process
continues from time T.sub.2, additional oxide and substrate atoms
are sputtered away although the oxide thickness remains constant,
since the oxidation rate and the sputtering rate are equal, and
remain constant in magnitude.
FIGS. 4A-4C show the effect of a reduction process in accordance
with the method of the present invention. In this process, an
initial oxide layer exists at time T.sub.1, as shown in FIG. 4A. It
is desired to reduce a portion of this oxide layer to leave a layer
of pure constituents of the oxide. In FIG. 4B, this is shown as a
metal for illustration purposes. For instance, the oxide of FIG. 4A
can be lead oxide which is to be reduced into an oxide layer and a
pure lead layer.
If the oxide layer of FIG. 4A is a substrate in the apparatus of
FIG. 1, and if the reactive gas is hydrogen, the incidence of
hydrogen ions on the substrate oxide will reduce the oxide, leaving
a pure lead layer on the lead oxide. Again, the rate of hydrogen
deposition and the rate of sputtering from the oxide surface is
balanced to produce a pure metal layer of predetermined thickness
which remains constant with time, after a steady state equilibrium
has been reached at time T.sub.2.
In FIG. 4C, the reduction process has continued, but the thickness
of the metal layer remains X.sub.L, since the growth and removal
processes are equal, and have constant magnitudes. However, the
thickness of the oxide-metal combination is less than h.
In the described method, the substrate can be any material
including metals and semiconductors. The reactive layers can also
be a variety of materials including oxides, nitrides, sulfides, and
semiconductors, etc. For instance, if the reactive gas species is
H.sub.2 S and lead is the substrate, a lead sulfide layer of
controlled thickness will be formed. Lead sulfide is a
semiconductor material, so this illustrates an example of the
production of a controlled thickness of a semiconductor
material.
The method is particularly useful in semiconductor technology and
in the fabrication of Josephson junction devices. Josephson devices
require very thin tunnel barriers having reproducibly uniform
thicknesses, and this process is particularly useful. By proper
adjustment of the system parameters, a controlled tunnel barrier of
any thickness can be produced. The same system can then be used to
deposit a counter electrode merely by introducing a source of
material to be deposited on the previously produced tunnel barrier.
For example, a lead evaporation source 28 can be incorporated in
the chamber 10 of FIG. 1. The substrate could be niobium 6,000A in
thickness deposited by RF sputtering in argon at a pressure of
10.sup..sup.-2 Torr. and at a rate of approximately 400A/minute
onto glass substrates held at 670.degree.K. During deposition, the
substrate support is grounded. To fabricate the tunnel barrier, the
niobium films are affixed to an RF cathode 12 and the
above-described method is undertaken to produce an oxide tunnel
barrier. The lead counter electrode is then evaporated onto the
tunnel barrier.
As stated previously, the operation is not limited to metals or to
an oxygen glow discharge. If other gas species or mixtures are
used, (for instance H.sub.2, N.sub.2 H.sub.2 CH.sub.4, etc.), many
other reactive layers can be made. The formation of uniform, dense,
short-free, reproducible reactive layers on base materials is
achieved in many technologies other than Josephson tunneling
devices. For example, semiconductor technology, MOS, and FET
structures utilize reactive layers.
Controlled multilayer materials are also obtained by this process,
since a reactive gas species can be altered as the depositions
proceed. In addition, compositional or different structural forms
of various reactive layers can be controlled by varying the
reactive conditions, such as temperature, pressure, gas species,
ion energy, etc.
What has been described is an improved process for forming reactive
layers in which a growth process and a removal process occur
simultaneously to offset one another. The rates for each of these
processes are independently adjustable and a steady-state
equilibrium in which the rates are equal determines the final
thickness. In a particular embodiment, oxidation and RF sputtering
provide a particularly good working example. However, it should be
understood that the principle of this invention includes methods
where the growth process is not oxidation and the removal process
is not sputtering, as was explained previously.
Films of any kind can be made through the use of competing growth
and removal processes to provide films of reproducible final
thickness. The film material is deposited on a substrate by any
known process (such as evaporation, sputtering, plating, etc.) and
a portion of the deposited film is removed simultaneously by any
suitable process (such as ion beam, electron beam, sputtering,
etc.). The final thickness of the film will be determined by
conditions at which the rate of deposition is equal to the rate of
removal. When the rates are held equal, the film thickness will
remain the same, even if further deposition and removal occurs.
Thus the dependence of film thickness on time, which is present
when only deposition is used to grow a film, is eliminated as a
variable in this process.
* * * * *