U.S. patent application number 13/066721 was filed with the patent office on 2011-09-15 for electron-assisted deposition.
Invention is credited to Donald Bennett Hilliard.
Application Number | 20110220491 13/066721 |
Document ID | / |
Family ID | 27760577 |
Filed Date | 2011-09-15 |
United States Patent
Application |
20110220491 |
Kind Code |
A1 |
Hilliard; Donald Bennett |
September 15, 2011 |
Electron-assisted deposition
Abstract
Previous limitations in utilizing energetic vapor deposition
means are addressed through the introduction of a novel means of
vapor deposition, namely, an Electron-Assisted Deposition (EAD)
process and apparatus. The EAD mode of film growth disclosed herein
is generally achieved by, first, forming a magnetic field that
possesses field lines that intersect electrically non-grounded
first and second surfaces, wherein at least one surface is a
workpiece, thereby forming a magnetic trap between first and second
surfaces; second, introducing a high flux of electrons axially into
the magnetic field existing between the first and second surfaces,
so that the electrons form an electron-saturated space-charge in
the space adjacent to the substrate, wherein plasma interactions
with the substrate are substantially avoided, and modification of
film growth processes is provided predominantly by electron--rather
than plasma--bombardment.
Inventors: |
Hilliard; Donald Bennett;
(Tucson, AZ) |
Family ID: |
27760577 |
Appl. No.: |
13/066721 |
Filed: |
April 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10375938 |
Feb 26, 2003 |
7931787 |
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13066721 |
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60359965 |
Feb 26, 2002 |
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Current U.S.
Class: |
204/192.12 ;
204/298.16; 204/298.17 |
Current CPC
Class: |
C23C 14/0089 20130101;
H01J 37/3408 20130101; C23C 14/355 20130101; C23C 14/0068
20130101 |
Class at
Publication: |
204/192.12 ;
204/298.17; 204/298.16 |
International
Class: |
C23C 14/35 20060101
C23C014/35 |
Claims
1. A process for forming a film on a substrate, comprising: a.
providing a vacuum chamber; b. positioning the substrate within the
vacuum chamber, the substrate providing a first surface for forming
the film thereon, the first surface electrically floating; c.
providing a vapor source within the vacuum chamber, the source for
directing vapor toward the substrate, so that the film is formed on
the first surface; d. providing a second surface, the second
surface having a negative potential; e. providing a magnetic field,
the magnetic field comprising field lines that intersect the first
surface and the second surface, the magnetic field having a central
field region corresponding to a relatively high magnetic field;
and, f. providing an electron source adjacent the central region,
the electron source providing a sufficiently high flux of electrons
into the central field region, so that the electrons expand into
the remaining magnetic field in such a way as to decrease average
energy of ions bombarding the substrate, relative to an ion energy
produced at a lower electron flux.
2. A sputtering apparatus for forming a film on a substrate,
comprising: a. a vacuum chamber; b. a sputtering cathode structure,
the cathode structure having a magnetic field means for providing a
magnetic field with substantial symmetry about an axis of symmetry,
the sputtering cathode structure positioned in the chamber; c.
means for positioning the substrate within the chamber, the
substrate providing a first surface for forming the film thereon,
the first surface electrically floating, the first surface
intersecting the magnetic field of the cathode structure so that an
electron trap is formed between the substrate and the magnetron;
and, d. an electron source, the electron source positioned for
providing a flux of electrons into the magnetic field substantially
along the axis of the magnetic field, the flux sufficiently large
to provide an electron-saturated field adjacent to the first
surface.
3. A vapor deposition apparatus utilized in combination with a
rotatable cylinder for depositing material onto a substrate,
comprising: a. a vacuum chamber; b. a vapor source disposed in the
vacuum chamber, the vapor source positioned so as to direct a vapor
towards the rotatable cylinder, the vapor containing constituents
for forming the material; c. magnetic field assembly means
providing a magnetic field, the magnetic field possessing field
lines that intersect the cylindrical surface; d. an electron
source, the electron source positioned for providing a flux of
electrons axially into the magnetic field.
Description
[0001] This application is related to and claims the priority date
of U.S. Provisional Application Ser. No. 60/359,965, filed Feb. 26,
2002, as well as U.S. patent application Ser. No. 10/375,938, filed
Feb. 26, 2003, now issuing.
FIELD OF THE INVENTION
[0002] The present invention relates generally to processes and
equipment utilized in the vapor deposition of thin films, and, in
particular, a vapor deposition apparatus that provides modification
of the film growth process through bombardment of the depositing
film with a heavy electron flux. In its first preferred embodiment,
the disclosed invention comprises a sputtering apparatus that
provides an effective electron trap for implementing what may be
termed as electron-assisted deposition (EAD). Potential
applications exist in the fabrication of integrated circuits,
optical elements, optoelectronic devices, and other thin or thick
film structures in which thin film growth can benefit from
non-equilibrium or activating growth conditions, but for which
heavy ion bombardment is undesirable.
DESCRIPTION OF THE RELATED ART
[0003] Deposition methods utilizing energetic means that deliver
energetic particles to the growth front of a depositing film have
been favored in thin film deposition, since such energetic means
are capable of achieving well-adhering, stoichiometric, and dense
layers of compound materials. Such energetic means will typically
be in the form of either an ion-assisted deposition (IAD) process,
or one of various deposition processes wherein energetic
interactions with a depositing film are essentially that of a weak
plasma. However, use of such energetic means incur additional
challenges.
[0004] Delineating the effects of plasma-generated species in the
growth of a non-metallic thin film continues to be difficult, since
there are only indirect and unreliable means available to ascertain
the behavior of charge and energetic species at the growth
interface of an electrically floating surface. This difficulty is
also due, in part, to the fact that such electronegative species as
oxygen are quite prone to becoming negatively ionized.
FIELD OF THE INVENTION
[0005] The present invention relates generally to processes and
equipment utilized in the vapor deposition of thin films, and, in
particular, a vapor deposition apparatus that provides modification
of the film growth process through bombardment of the depositing
film with a heavy electron flux. In its first preferred embodiment,
the disclosed invention comprises a sputtering apparatus that
provides an effective electron trap for implementing what may be
termed as electron-assisted deposition (EAD). Potential
applications exist in the fabrication of integrated circuits,
optical elements, optoelectronic devices, and other thin or thick
film structures in which thin film growth can benefit from
non-equilibrium or activating growth conditions, but for which
heavy ion bombardment is undesirable.
DESCRIPTION OF THE RELATED ART
[0006] Deposition methods utilizing energetic means that deliver
energetic particles to the growth front of a depositing film have
been favored in thin film deposition, since such energetic means
are capable of achieving well-adhering, stoichiometric, and dense
layers of compound materials. Such energetic means will typically
be in the form of either an ion-assisted deposition (IAD) process,
or one of various deposition processes wherein energetic
interactions with a depositing film are essentially that of a weak
plasma. However, use of such energetic means incur additional
challenges.
[0007] Delineating the effects of plasma-generated species in the
growth of a non-metallic thin film continues to be difficult, since
there are only indirect and unreliable means available to ascertain
the behavior of charge and energetic species at the growth
interface of an electrically floating surface. This difficulty is
also due, in part, to the fact that such electronegative species as
oxygen are quite prone to becoming negatively ionized.
[0008] A question that has not been sufficiently addressed in the
vapor deposition art is whether or not it is possible to produce a
practical deposition environment in which an electron gas is made
to impinge on the electrically floating surface of a growing film
for modification of growth behavior; this, without simultaneously
producing positive ion bombardment to equilibrate the arrival rate
of positive and negative charge. If such a scenario were realized,
it would then form a basis for an electron-assisted deposition
(EAD) process.
[0009] To distinguish an EAD process from a plasma-assisted, or
ion-assisted deposition process, we must be able to clearly
demonstrate electron-dominated bombardment of the growing film.
Also, if EAD is to be a meaningful term, we must be able to clearly
differentiate the results of the EAD process from that of either
plasma-assisted growth or an ion-assisted deposition (IAD) process.
There is not found, in the vapor deposition literature, any account
of another research group reporting what one may claim as a
definitive EAD process. Potential reports of an EAD process do not
satisfy the above conditions, in that they fail to demonstrate a
verifiable method for eliminating the ion contribution, nor do they
demonstrate a resulting growth process that may be distinguished
from that of low energy plasma bombardment.
[0010] The term "electron-assisted deposition" has been used
frequently in the past to describe film growth wherein a hot
filament, or other electron source, is used in conjunction with
chemical vapor deposition (CVD) of a material, typically diamond.
This usage was introduced by Sawabe and Inuzuka (Thin Solid Films,
vol. 137, pp.89-99, 1986) for referring to such a CVD process, and
has become commonplace. However, while the usage of such terms as
"electron-assisted chemical vapor deposition" does recognize
functionality of electrons in electron-activated film growth
processes, such processes are performed at much higher gas
pressures than those allowed to attain the environment of an
electron gas--rather than a weak plasma--found in the present
invention. Such unconfined, higher pressure, electron emission will
only cause an electrically floating surface to become negatively
charged until positive and negative charge arrival are
equilibrated, in accordance with the congruence principle, which is
a standard plasma interaction. That the electron component of the
weak plasma formed in these CVD processes is recognized as
essential does not provide a physical basis for eliminating the
positive ion bombardment--or its effect--that is also a required
component of such processes, nor has there been offered any
suggestion of how such positive ion bombardment might be
eliminated.
[0011] Some recent reports by Inoue et al are based upon
experimental efforts in "electron-beam assisted deposition" of
CeO.sub.2 on silicon, by way of installing a secondary thermionic
filament near the substrate, in an existing e-beam evaporation
system. These prior accounts of electron-beam assisted deposition
resulted in the observation of a lowered epitaxial temperature for
the growing oxide film [Inoue, 1997 #132; Inoue, 1999 #133; Inoue,
2000 #129]. However, such effects as a lowering of the required
epitaxial growth temperature are not distinguishable from the
effects readily obtained with low-energy ion bombardment, nor is
there cited a mechanism by which the bombardment of the insulating
film would not result in the equilibration of positive ion and
negative charge arrival at the insulated portions (corresponding to
the desired insulating film) of the substrate.
[0012] The electron bombardment of any electrically floating
surface in a deposition process will tend to develop weak
plasma-like interactions, at least in so far as the floating
surface will electrically self-bias until the arrival rate of
positive ions and negative charge at the floating surface
equilibrates, in accordance with the "congruence assumption" used
in plasma analysis. As such, even relatively sparse creation of
bombarding gas/vapor ions will result in equilibration of positive
and negative charge flux to an electrically floating substrate
surface, wherein ion bombardment will easily dominate growth
behavior of a depositing film, since the typical gas/vapor atoms in
any deposition process will be 1,800 to 100,000 times the mass of
the electrons. Thus, while many deposition methods are known to
bombard the substrate surface with electrons, energetic gas-phase
interactions with a depositing non-metallic film will normally be
in the form of such plasma bombardment.
[0013] It therefore remains problematic to positively identify an
electron-assisted, as opposed to a plasma-assisted, process.
Positive identification would require observation of growth
behavior that is not also characteristic of an ion-assisted or
plasma-assisted process. Clearly, verifying a substantial
bombarding electron current by grounding it through the growing
film is of little practical use for nonmetallic films, nor would it
isolate the effects of bombardment from the (normally unwanted)
effects of electron conduction through the growing film. In
addition, it should be understood that providing a grounded
substrate does not entail that the surface of an insulating film,
deposited on the substrate, is grounded.
[0014] Alternatively, the bombardment of many floating dielectric
surfaces with high energy electrons, typically in the KeV range,
can obviate the need of charge compensation at the surface, due to
the secondary electron coefficient of many dielectric materials
rising above unity at high primary electron energies. However, such
high electron energies would normally be impractical to implement
over large substrates, or in a practical deposition environment.
Such high electrical fields--in the many kVolt range--are used in
vacuum web processes for the curing of organic films, wherein the
dielectric film is supported by a grounded substrate for achieving
the required field; however, this is a post-deposition process, as
opposed to the assisted film growth processes that are addressed in
the present invention.
[0015] In the sputtering art, injection of electrons into the
volume between the cathode and substrate has commonly been
performed in sputtering configurations referred to generally as
"triode" and "tetrode" sputtering. These latter sputtering methods
utilize an electron source--usually a thermionic filament--for
injecting electrons into the deposition space of, basically, a
diode sputtering arrangement, wherein the sputtering plasma is not
magnetically confined. The injection of electrons in these devices
allows for the establishment of higher ionization rates, higher
plasma densities, and lower operating pressures.
[0016] In the magnetron sputtering art, the introduction of
additional electrons into the deposition space was performed in
U.S. Pat. No. 4,588,490, by Cuomo et al, wherein it is noted that
an auxiliary electron source may be effectively utilized to
increase the plasma density in a magnetron sputtering process. The
emission from a hollow cathode is found effective for increasing
ionization of the gas when the hollow cathode emits electrons
within the magnetic trap of the magnetron sputter source. These
findings have been similarly realized in subsequent magnetron
sources.
[0017] In more recent years, there have been reports of injecting
electrons into the field of an unbalanced magnetron (UBM), as
introduced by Savvides and Window. However, these previous reports
were again based on attempts to increase ionization and enhance
plasma bombardment of coatings, such as nitride tool coatings, that
benefit from the ion-bombardment of on-axis "Type II" UBM's. As is
consistent with the prior art, the term "Type II" UBM will herein
refer to magnetron sputter sources that possess an outer magnet
assembly that produces an excess of magnetic field lines, relative
to that of an inner magnet assembly. These excess field lines will
terminate at chamber surfaces other than those of the cathode,
wherein some field lines may terminate at a substrate placed
opposite the cathode, so that a magnetic "trap" will channel plasma
toward the substrate. The resulting magnetic trap is known to
create plasma interactions with the substrate, so that substantial
ion bombardment of the substrate is achieved.
[0018] Penfold has found that such a magnetic trap may also be
realized with inner and outer magnetic assemblies possessing
matched (i.e., "balanced") field strengths, and that as long as the
placement of grounding surfaces dictates termination of the
magnetron's magnetic field lines at the substrate, such plasma
bombardment may be achieved. In any case, the term, "UBM", will
herein refer to magnetron sputtering apparatuses that provide
magnetic field lines that intersect the substrate, so that ion
bombardment of the substrate is realized. Such a UBM apparatus may
comprise a single magnetron source, or, as discussed in the prior
art, a combination of sources that provides a contiguous magnetic
trap. A more complete description of the UBM operational
characteristics and design may be found in Penfold's exposition in
The Handbook of Thin Film Process Technology (Glocker, ed.):
[0019] In prior art accounts of electron injection into a UBM,
high-temperature, polycrystalline (including textured
polycrystalline) coatings, such as diamond-like carbon (DLC),
chromium nitride, hafnium nitride, and other such tribological
coatings benefit from "low energy" ion bombardment of tens to
hundreds of eV. However, such ion energies, as described in these
prior accounts, are incompatible with the requirements of many
damage-sensitive thin film applications, wherein ion bombardment
energy in excess of interatomic bond strengths--one to several
eV--can cause such undesirable effects as crystalline defects,
mechanical stress, or resputtering of a volatile film constituent.
Damage-sensitive thin film applications, including those of
epitaxial thin film growth, or those of various multi-component
compounds that possess an easily re-sputtered or volatilized
component, are therefore not compatible with such processes that
provide energetic ion bombardment. Hence, one will not find, in the
prior art, on-axis "Type II" UBM sputter deposition being
successfully used for such applications.
[0020] Further elaboration of UBM design has resulted in the
convention of referring to the magnetic fields of such devices as
either "open" or "closed". Generally, magnetic fields in which the
field lines do not terminate at a pole piece are referred to as
open fields, in that charged particles may be easily lost at
whichever surface the open field lines terminate.
[0021] The term "magnetic assembly" will be used herein to refer to
any structure disposed to provide a magnetic pole (north or south)
for magnetic field lines that traverse open space.
[0022] As such, a magnetic assembly may comprise a permanent
magnet, an electromagnet, or any combination of the same. The
terms, "substrate", and "workpiece", will herein refer to the
article on which material is being deposited.
[0023] Consistent with the terminology of the prior art, the term
"on-axis" deposition will herein refer to deposition configurations
wherein at least a portion of the substrate or fixture can be, at
some point during deposition, intersected by a depositing vapor
flux that is substantially parallel to the deposition axis. In
sputtering, on-axis sputtering may be easily distinguished from
90-degree off-axis sputtering, wherein the substrate is placed
completely outside of the path of such on-axis vapor flux.
SUMMARY OF THE INVENTION
[0024] The previously cited limitations in utilizing energetic
vapor deposition means, as recognized in the present invention, are
addressed through the introduction of a novel means of vapor
deposition, namely, an Electron-Assisted Deposition (EAD) process
and apparatus. The EAD mode of film growth disclosed herein is
generally achieved by, first, forming a magnetic field that
possesses field lines that intersect electrically non-grounded
first and second surfaces, wherein at least one surface is a
workpiece, thereby forming a magnetic trap between first and second
surfaces; second, introducing a high flux of electrons into the
magnetic field existing between the first and second surfaces, so
that the electrons form an electron-saturated space charge, wherein
formation of an ion sheath with damaging ion bombardment at the
substrate is substantially avoided.
[0025] It is found in the present invention that, by floating the
substrate surface electrically, and introducing a sufficient
electron flux into the center region of the magnetic field, the
magnetic field lines that intersect the substrate will be
effectively "closed" at the substrate, even though the magnetic
trap might be considered "open" by previous conventions. Electron
space-charge within the magnetic field can then be rendered
sufficiently dominant to preclude the formation of a plasma sheath
at the substrate, so that damaging plasma-type bombardment--namely
by ions--of the growing film is substantially avoided.
[0026] In the present invention, damaging ion bombardment of the
growing film is found absent even under the highly negative
substrate self-bias potentials, greater than -200 VDC, imposed by
the bombarding electrons of the invention. The requirements of the
"congruence assumption" are obviated by allowing charge
compensation at the electrically floating substrate to occur, not
through the equal arrival rate of positive ions and negative
charge, but through allowing the substrate to bias negatively, and
to stabilize at a point where electron departure from the substrate
becomes equal to the electron arrival rate. This establishment of
electron flux from the substrate is thought to occur primarily
through primary/secondary electron exchange, wherein the magnetic
trap effectively enforces a state of unity in the secondary
electron coefficient of the depositing film. Alternatively, the
point of stabilization of electron flux to and from the substrate
surface may also occur, or be modified, through field emission,
photoemission, or simply by conducting a relatively small current
of electrons from the substrate fixture.
[0027] In the preferred embodiment, the effective closure of the
magnetic trap coincides with a relatively high (typically greater
than --100 VDC) self-bias potential at the floating substrate,
which thereby provides an electrostatic mirror at what would
normally be the open end of the magnetic field. Contrary to
previous reports of magnetically aided deposition processes, the
magnetically trapped electrons in the present invention do not
produce a plasma sheath at the substrate, nor may the primary role
of the electrons in film modification be characterized by the
ionization of gas atoms for subsequent ion bombardment of the
film/substrate. Also, in the present invention, a much more
negative substrate potential does not produce an increased
bombardment effect by positive ions, as previously reported.
Rather, a significant increase in negative potential of the
substrate, in the present invention, marks a transition to an
electron-dominated space-charge in the magnetic trap, such that
electron--not ion--bombardment is the dominant mechanism modifying
film growth.
[0028] As a result of this EAD operational mode, increasing the
negative self-bias of the substrate surface does not result in an
increased ion bombardment effect in the present invention, because
the bias of the substrate is not an applied acceleration potential,
but rather, a reflection of the local electron-saturated field
within the magnetic trap. By the formation of an efficient electron
trap, and subsequently saturating the trap with electronic charge,
a damaging plasma sheath is unable to form at the growing film.
Thus, the disclosed mode of operation is described herein as a
"saturation" of the magnetic field with electrons. The term
"saturation" may be defined variously in this context, but will be
defined here as a condition producing a sufficient effect to
obviate the formation of plasma sheaths that produce damaging ion
energies.
[0029] The practical effects of this EAD process may be contrasted
with previous ion-assisted, or plasma-assisted, thin film
deposition processes in that the damaging effects of ion
bombardment are eliminated. Another practical result of the EAD
deposition process is that it allows, for the first time,
metallic-mode reactive sputtering of such complex compositions as
the multicomponent ferroelectics and High-T.sub.c materials. The
metallic-mode reactive sputter deposition of such complex materials
is enabled by the activation-limited nature of film growth in the
EAD mode, wherein the dominating mechanism of electron bombardment
activates surface reactions and substantially increases effective
sticking coefficients of the more volatile vapor constituents,
thereby obtaining the desired material at the substrate. In this
way, required oxygen--or other reactive gas--pressures are much
lower, so that poisoning of a metal sputtering target in the
process can be substantially avoided, and the sputtering target can
be operated well into the metallic-mode region of the target
hysteresis curve. At the same time, re-sputtering of the growing
film, due to the formation of negative ions of the reactive gas at
the target surface, is also substantially eliminated.
[0030] As will be further supported, the environment of the
depositing film demonstrates the earlier suggested criteria for
establishing an EAD process: first, there is verifiable energetic
bombardment of the growing film that is clearly dominated by
electron, rather than ion, current; and, second, the effects of the
energetic bombardment are clearly differentiated from the effects
of ion, or plasma, bombardment.
[0031] Some of the damage-sensitive films that could benefit from
various embodiments of the disclosed deposition process and
apparatus, in general, include those deposited epitaxially on a
single crystal substrate, or those multicomponent films containing
easily re-sputtered elements. The term "epitaxy" will refer herein,
as elsewhere, to the growth of a crystalline material on a
single-crystalline substrate, wherein the grown material is
uniformly aligned to the substrate's crystal lattice in its three
axes.
[0032] In accordance with the first preferred embodiments, the
present invention is initially disclosed as a modification of
previous "Type II" unbalanced magnetron apparatuses and processes.
The EAD process is accomplished in this first embodiment by
reversing the operational characteristics that have, up to the
present invention, been attributed to an unbalanced "Type II"
magnetron, as previously discussed. In contrast to prior art UBM
processes, an objective of the present invention is not to maximize
ionization of the working gas near the substrate, neither is it to
form plasma-like characteristics at the substrate. In fact, these
prior art objectives are contrary to the operation of the present
invention.
[0033] In direct contrast to the established behavior of "Type II"
magnetrons in on-axis deposition, wherein the substrate is heavily
bombarded with energetic ions, thin film growth in the present
invention is characterized instead by heavy electron bombardment.
The electron bombardment of the disclosed invention does not cause
the substrate damage that is associated with the former ion
bombardment, and accordingly allows on-axis deposition of damage
sensitive films. In particular, the present invention also
comprises the introduction of a on-axis "Type II" UBM process and
apparatus for depositing such damage sensitive materials as the
lead and bismuth containing perovskite oxides; e.g., the lead
zirconate titanate (PZT) family, or layered perovskites such as
strontium bismuth tantalate (SBT).
[0034] While the EAD operational mode is disclosed, in its first
preferred embodiment, as a sputtering process, the novel aspects of
the invention can be applied to a variety of so-called "physical
vapor deposition" techniques. In fact, the use of an
electron-saturated magnetic trap, which terminates at the
workpiece, may be utilized to similar advantages in conjunction
with almost any low-pressure (typically, the lower transition-flow
pressure region) vapor deposition technique. The disclosed
invention is seen to be particularly useful in the reactive
deposition of compound materials. Furthermore, the disclosed
source, in one embodiment, may be utilized within a cluster of such
sources, so that problems normally associated with processing large
substrates may be overcome, since a plurality of both electron
sources and reactive gas injection sources may then be interspersed
between the sources for homogenous treatment of the substrate.
[0035] Because a substantial flux of electrons must be introduced
into the magnetic field to achieve the disclosed EAD mode, the
point of introduction is significant, as electrons introduced in
less favorable locations will result in a significant fraction
grounding on various chamber elements, rather than interacting with
the substrate. As such, the electrons are preferably introduced at
a location in the magnetic trap that requires the maximum number of
magnetic field lines, or magnetic field surfaces, to be traversed
before the electrons are allowed to escape the trap.
[0036] Because the diffusion of electrons across magnetic field
lines is determined primarily by electron space-charge/field
interactions, rather than by ambipolar plasma interactions,
homogeneous electron bombardment does not require that the
electrons be equally delivered to all magnetic field lines. As
opposed to plasma-type interactions wherein the space charge is
much closer to neutral, the negative and mutually repulsive
interactions of the electron-dominated space charge will result in
the electrons diffusing across magnetic field lines to the outer
perimeter of the magnetic trap, therein finding a path to ground. A
practical advantageous result is much more uniform bombardment of
the substrate with electrons, relative to the non-uniformity of
plasma bombardment in prior UBM sources. Another practical and
advantageous result is that the disclosed invention provides for a
spatially homogenous deposition, which is in contrast to the
inhomogeneous distribution characteristic of previous, especially
reactive, "Type II" UBM processes.
[0037] Another advantage of the disclosed EAD process is that the
substrate temperature required for the growth of the desired
material phase can be significantly lower than that required in
other deposition processes. This is due to the highly
non-equilibrium nature of the EAD process, where electron
bombardment of the surface may create material phases
characteristic of much higher temperatures than that of the bulk
substrate. For instance, desired phases of Group IV B metal oxides,
such as ZrO.sub.2, may be formed on silicon at bulk substrate
temperatures below 600 C, thereby avoiding formation of an
interfacial amorphous layer at the oxide/silicon interface. The
highly activating, non-equilibrium character of the disclosed
invention also makes it particularly suitable for material
processes, such as GaN or diamond deposition, that benefit from
such high activation.
[0038] Other objects, advantages and novel features of the
invention will become apparent from the following description
thereof.
BRIEF DESCRIPTION OF DRAWINGS
[0039] FIG. 1 is a sectional side view of a prior art "Type II"
unbalanced magnetron, taken along a plane containing the axis of
circular symmetry (14).
[0040] FIG. 2 is a sectional view of the disclosed sputtering
apparatus in the first preferred embodiment, taken along a plane
containing the axis of circular symmetry (14).
[0041] FIG. 3 is an enlarged sectional view of a magnetically
trapped, hollow cathode electron source, in another preferred
embodiment of the disclosed vapor deposition apparatus.
[0042] FIG. 4 is a top planar view of a circular magnetron cathode
of the present invention, demonstrating preferred placement of the
integrated electron sources.
[0043] FIG. 5 is the typical on-axis deposition characteristics of
a prior art "Type II" unbalanced magnetron. A top view of the
coated wafer is presented with spatially correlated profiles. The
upper profile is relative percentage (in arbitrary units) of
bismuth in the deposited film as a function of the radial location
on the wafer. The lower profile is the radial thickness
distribution (in arbitrary units) of the deposited film.
[0044] FIG. 6 is the deposition characteristics of the sputtering
apparatus of the first preferred embodiment. A top view of the
coated wafer is presented with spatially correlated profiles. The
upper profile is relative percentage (in arbitrary units) of
bismuth in the deposited film as a function of the radial location
on the wafer. The lower profile is the radial thickness
distribution (in arbitrary units) of the deposited film.
[0045] FIG. 7 is the reactive deposition characteristics of the
disclosed sputtering apparatus in a normally reducing environment,
wherein activation-limited film growth by the electron-assisted
deposition process is demonstrated.
[0046] FIG. 8 is an enlarged and detailed portion of the sectional
side view of the invention as disclosed in the preferred
embodiments of FIG. 2, showing the alternative utilization of
low-valence metals for production of excess electrons.
[0047] FIG. 9 is a perspective view of an alternative magnetic
field configuration in the invention. FIG. 9 also serves to
illustrate the use of an ultraviolet source for photoemission of
electrons.
[0048] FIG. 10 is a perspective view of an alternative embodiment
of the disclosed vapor deposition apparatus and process, wherein a
modified hollow magnetron source is disclosed.
[0049] FIG. 11 is a sectional side view of an alternative
embodiment of the disclosed vapor deposition apparatus and process,
wherein a modified effusion cell is disclosed.
[0050] FIG. 12 is a perspective view of an alternative embodiment
of the disclosed vapor deposition apparatus, utilizing multiple
sources in a single deposition process.
[0051] FIG. 13 is the top view of an alternative configuration for
placement of pole-pieces and electron sources in FIG. 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] The following description and FIGS. 1-13 of the drawings
depict various embodiments of the present invention. The
embodiments set forth herein are provided to convey the scope of
the invention to those skilled in the art. While the invention will
be described in conjunction with the preferred embodiments, various
alternative embodiments to the structures and processes illustrated
herein may be employed without departing from the principles of the
invention described herein.
[0053] The most historically prominent example of a magnetic field
used in the growth environment of a vapor deposition chamber is in
the case the unbalanced magnetron (UBM). A circular unbalanced
magnetron (UBM) source of the prior art is shown in FIG. 1, which
is configured according to the principles of a "Type II" UBM, as
introduced by Savvides and Window. The sectional view of FIG. 1 is
taken along the axis of circular symmetry (14). The cathode
structure of the magnetron comprises a sputtering target (22) that
is fastened to a cathode housing (11). The prior art UBM sputter
cathode, as with magnetron cathodes in general, possesses an inner
magnetic pole piece (23) and a peripheral outer magnetic pole
piece(s) (24) that are oppositely poled with respect to the
sputtering target (22), thereby creating a set of magnetic field
lines that terminate only at the cathode structure, forming a
closed trap for confining a sputtering discharge over the target
(22). The magnetic pole pieces (23) and (24) typically utilize
permanent magnets, electro-magnets, or various combinations of
ferrous pole pieces and such magnets. The outer pole piece(s) (24)
may comprise a single monolithic circular magnet, or several
segmented magnets. Frequently a magnetic shunt plate (26) is
employed, so that in the case of the "Type II" UBM of FIG. 1, the
weaker inner magnetic pole piece (23) may comprise only an iron
pole piece. The cathode housing (11) is typically electrically
isolated, by an insulator (15), from a grounded sputter shield
(16), which is typically utilized to prevent formation of unwanted
discharges and to provide specific grounding characteristics for
operation.
[0054] It may be noted, in FIG. 1, that the number of magnetic
field lines emanating into the deposition space from the outer
magnetic pole piece(s) (24) of the magnetron cathode is greater
than the number of field lines emanating from the inner magnetic
pole piece (23). Thus, some of the excess field lines will not be
"closed" by terminating only at the other magnetic pole piece, but
will instead intersect other surfaces within the deposition
chamber. As those field lines that intersect the substrate are of
primary interest in the present disclosure, only such lines are
represented in the accompanying figures, where the
substrate/fixture assembly (10) is located, substantially, in an
on-axis position with respect to the UBM sputter source. The UBM
trap region (2) of prior art UBM sources, whether "open" or
"closed" has been used for the efficient ionization of gas/vapor
atoms for ion bombardment of the substrate/fixture assembly (10).
In this sense, the UBM magnetic trap (2) is used similarly to the
magnetron discharge trap (3) of the magnetron discharge, as well as
magnetic traps used in other deposition methods, such as activated
reactive evaporation (ARE) processes, in that the desired and
achieved effect is increased ionization of gas (or vapor)
constituents. The magnetic field lines of the magnetron discharge
trap (3) are separated from the field lines of the UBM trap region
(2) by the zero-flux surface (5), which is the abstracted surface
representing the division between the magnetron trap (3) and the
EAD trap (2), and across of which exists no magnetic flux
(Penfold). Note that the various magnetic field lines and zero-flux
line become three dimensional surfaces when translated about the
axis of circular symmetry (14). In the case of "Type II" UBM's, the
grounded sputter shield (16) can not exist within the UBM trap
region (2), since this would ground out the electrons and
associated ambipolar plasma traveling along the intercepted field
lines of the trap.
[0055] As is uniformly represented in the prior art of such "Type
II" UBM devices, the confining nature of the extending field lines
in FIG. 1 provides for formation of a bombarding plasma (6) in the
on-axis space adjacent to the substrate/fixture assembly (10), to
the extent that heavy ion bombardment of the depositing film is
provided. It has been noted that the plasma (6) is highly
non-uniform, and hence, produces a highly non-uniform deposition
profile. This non-uniformity of the plasma bombardment is further
exaggerated by the presence of electronegative species, such as
oxygen, that are required in many reactive deposition processes,
and may be produced and accelerated at the vicinity of the
electrically negative surface of the sputtering target (22).
[0056] The substrate/fixture assembly (10) may incorporate a
heating means for sustaining the substrate at elevated temperatures
during deposition. As discussed in the prior art, the substrate
assembly may incorporate various magnetic, ferromagnetic, and other
means for additionally shaping the magnetic field produced by the
unbalanced magnetron. Instead of the simple, planar substrate
assembly of FIG. 1, substrate and fixturing may also be configured
into various planetary and conveyer schemes for standard purposes
of increased uniformity and efficiency.
[0057] Because the prior art UBM sources typified in FIG. 1 are
most frequently utilized for the development of wear-resistant
coatings, such as nitrides, carbides, and diamond-like carbon
(DLC), reaction of the process gas with the sputtering target is
normally viewed as acceptable. As such, sputtering gas and reactive
gas are typically injected together, using generic injection means
(12).
[0058] A first preferred embodiment of the disclosed deposition
apparatus, in FIG. 2, utilizes a plasma sputtering source that
incorporates elements of the conventional "Type II" UBM variety, as
provided in the prior art in FIG. 1. However, because it operates
according to fundamentally different principles, namely, those of
an EAD process, than in the case of previous UBM sputtering
apparatuses, the apparatus disclosed in FIG. 2 will herein be
referred to as an UBM-EAD apparatus.
[0059] The EAD trap region (25) will refer, in the present
disclosure, to a space containing magnetic field lines that
generally terminate at the substrate/fixture assembly (10), and, in
the case of the UBM-EAD apparatus, may be, similarly to FIG. 1,
differentiated from the magnetron discharge trap (3) by the
zero-flux surface (5). The field lines of the EAD trap region (25)
indicate a magnetic field for trapping electrons to the degree that
a state of saturation is achieved. The substrate/fixture assembly
(10) in FIG. 2 is electrically floating, so that it may be
electrically biased, and, in the preferred embodiment, is typically
allowed to self-bias. Grounded chamber surfaces such as the
grounded chamber elements (18) and grounded magnetron shield (16)
at the periphery of the EAD trap provide the true anode for the EAD
process and, as such, no grounded surfaces exist within the EAD
trap region (25). As a result of this arrangement, electron current
must flow across field lines to exit the EAD trap at its perimeter,
so as to find the grounded surfaces. For the EAD mode to be
properly established, the EAD trap must be homogeneously saturated
with electron charge, or else growth conditions will revert back to
those of plasma-type interactions, in that energetic ion
bombardment of the substrate takes place. Because the mutually
repulsive electrons flow from the center to the edge of the EAD
trap, it is important that sufficient electron flux is introduced
into the center region of the EAD trap, where it is intersected by
the central axis of symmetry (14) in FIG. 2, so that this central
region becomes saturated. Because the homogeneity of the EAD trap
environment is not dependent on effects of ambipolar diffusion, but
rather, on the mutually repulsive electron space charge,
introducing the electrons in this center region will then result in
electron space charge homogeneously filling the EAD trap as the
electrons diffuse to the grounded surfaces (18, 16).
[0060] This latter condition of homogenous saturation is provided
when the electrons, in FIG. 2, are emitted into the innermost field
lines of the EAD trap (25), which correspond to the axis of
circular symmetry (14) in FIG. 2. Injecting a flux of electrons
into the central region of the EAD trap is accomplished, in the
embodiments of FIG. 2, by integrating an electron source (30) into
the magnetron cathode, namely, at the cusp regions of the
magnetron, trap (3). Thus, an electron source (30) is disposed so
as to emit a high flux of electrons into field lines that are
either tangential to, or within, the zero-flux surface (5). The
most straightforward means of achieving this latter condition is to
inject a high electron flux into the central cusp region (4) that
corresponds to the termination of the magnetron discharge trap (3)
at the cathode center. To this end, the inner magnetic pole piece
(23) of the prior art magnetron in, FIG. 1, is replaced with a
similarly poled inner magnetic assembly (27), in FIG. 2, that
incorporates the electron source (30). It may be noted that the
magnetic assemblies (27,29) in FIG. 2 comprise part of the exposed
area of the magnetron cathode, so that electrons may be injected
into the EAD trap. As a result of this geometry, the target (22) in
FIG. 2 is constructed so as to accommodate electron sources (30),
so that the exposed target area is limited only to those areas
corresponding to the region of sputter erosion (13).
[0061] In the first preferred embodiment of the present invention,
an electron source, powered by power source "P2" via connection (8)
in FIG. 2, is integrated into the inner magnetic assembly (27), so
that electrons are emitted into the field lines of the magnetron
discharge trap (3). Alternatively, electron sources may also inject
electrons into the central region of the EAD trap by placing such
sources near the intersection between the zero-flux surface (5) and
the magnetron cathode. The precise point of intersection of the
zero-flux surface (5) and the cathode will depend upon various
parameters, including target material and dimensions, magnetic
design, etc. However, the points of intersection will typically lie
along the inner edge of the outer magnetic assembly (29). The
electron source(s) (30) may accordingly be, additionally or
alternatively, integrated into this portion of the outer magnetic
assembly (29), in FIG. 2, where the outer magnetic assembly would
be similarly poled to the outer magnetic pole pieces(s) (24) in
FIG. 1. In the present disclosure, therefore, reference to such
magnetic assemblies (27,29) will refer to those magnetic assemblies
that can incorporate an electron source (30).
[0062] The embodiments of FIG. 2 provide for separate electrical
connections (7), (8), and (9), for the sputter cathode power "P1",
inner electron source power "P2", and outer electron source power
"P3", respectively. In some cases, such as when the electron
sources utilize thermionic emitters, it may be possible for both
electron sources and magnetron cathode to operate at the same
voltage, so that they may be electrically connected, and the
electron source connections, (8) and (9), provide only a biased
current for heating the thermionic element of the electron
sources.
[0063] It is normally preferable that the integrated electron
source (30) be integrated into the inner magnetic assembly (27) of
the UBM-EAD apparatus in FIG. 2. This central placement, first of
all, will normally allow greater flexibility in the electron source
design, as such things as efficient water cooling, electrical
isolation, and uniform emission into the deposition environment,
are more easily implemented. In addition, the weaker inner magnetic
assembly (27) may, in some cases, comprise only a ferromagnetic
material, which does not produce any additional magnetic flux, but
rather, serves to appropriately shape the magnetic field. The
desired interaction of the magnetic field with the electron source
will vary with the specific choice of electron source. In general
though, it may be seen that, in FIGS. 2-3 of the preferred
embodiment, the electrons will interact with the magnetron
discharge trap (3) at the cathode surface before proceeding into
the EAD trap region (25).
[0064] Because the embodiments of FIG. 2 are particularly
well-suited for reactive deposition processes, wherein reactive
gases or vapors are used to provide a constituent of a deposited
compound material, a preferred arrangement of reactive gas
introduction means is provided by the reactive gas injection means
(21). The sputtering gas is provided at the UBM-EAD source by an
integrated injection line (19). Such a gas distribution geometry is
useful for preventing reactive gases that are prone to forming
negative ions from contacting the sputtering target.
[0065] The integrated electron sources (30), in FIG. 2, may
comprise one or more of the conventional electron emission means,
such as a thermionic filament, a field emitter, (typically UV)
light on a photoemitting surface, or one of various hollow cathode
designs. However, the hollow cathode is preferred for its ability
to produce very large electron currents. In FIG. 3, the structure
housed by components (37), (38), and (39) are representative of the
magnetic structure/electron source that may be used as either the
inner magnetic assembly (27) and integrated electron source (30) or
the outer magnetic assembly (29) and integrated electron source
(30) in FIG. 2. An alternative embodiment of the invention is also
provided in the embodiments of FIG. 3, in that, rather than
providing the EAD magnetic trap (25) by the excess magnetic flux of
the outer magnetic assembly of magnetron sputter source, as in the
case of the UBM-EAD source in FIG. 2, one may also implement the
EAD trap through use of a magnetron with a very strong inner magnet
which provides flux lines that intersect the substrate. Once again,
the grounded chamber elements (18) indicate that the electrons must
exit the EAD trap before finding ground.
[0066] An effective means of introducing a high flux of electrons
into a sputtering environment is through the use of hollow cathode
devices, such as is taught by Cuomo et al, in U.S. Pat. No.
4,588,490, and included here by reference. It is noted, however,
that the latter invention instructs placement of the hollow cathode
at a position that is sufficiently removed from the target surface
(more than 2-3 Larmor radii) so as not to interfere with the drift
currents. This restriction is overcome to significant advantage in
the present invention. By integrating hollow cathode sources within
the magnetron sputtering cathode structure itself, one may still
emit electrons into the closed magnetron field lines that sustain
the sputtering discharge without impeding drift currents.
[0067] The hollow cathode (30) of FIG. 3 is of the variety that
possesses a constricted emitting aperture for increased retention
of electrons, or, in some hollow cathodes, increased retention of
the discharge gas. The restricted aperture is also deemed
preferable for avoiding redeposition of material from the
sputtering target (22), on the hollow cathode interior. While the
electron injection means may vary considerably, the housing
structure of FIG. 3 defines an electron source aperture (34) for
emitting electrons into the deposition environment. The housing
piece (38) may then provide a bias voltage with respect to the
hollow cathode (30). It may be preferable to introduce the
sputtering gas through the hollow cathode, via feedthrough (31),
which provides a conduit for both gas supply (33) and electrical
power supply (8) to the hollow cathode. As will be discussed
further, the electron source(s) for achieving the presently
disclosed EAD process may be placed at a variety of positions in
the magnetic field(s). The effectiveness of a particular hollow
cathode used will depend, in part, upon its position within the
magnetic field, the precise deposition process at hand, the process
gases used, etc. In the case that the electron source of the
disclosed apparatus is chosen to utilize a thermionically emitting
surface, water cooling lines (32) are integrated into the housing
to prevent undue heating of the surrounding structure.
[0068] FIG. 4 is a top view of the magnetron cathode, which
comprises target (22), inner magnetic assembly (27), and outer
magnetic assembly (29), with cross-hatched area indicating the
sputtered region in FIG. 2. A suggested placement of electron
source apertures (34) is also indicated in FIG. 4. The
substantially unsputtered regions of the magnetron cathode surface
may correspond to a continuation of the target material, or
alternatively, to modular and electrically isolated pieces of the
cathode assembly, which, as in the top view of FIG. 4, often has a
circular aspect.
[0069] It is an option that the integrated electron sources be
incorporated within the peripheral magnet assembly (29) in FIG. 2.
It should be noted that this latter arrangement must provide
electrons into the innermost magnetic lines emitting from the
peripheral magnets, if electrons are to be coupled to the central
on-axis region. As a result of this requirement, the electron
sources apertures (34), when integrated into the outer magnetic
assembly (29) should be placed so as to be adjacent to,
intersecting, or within the circular intersection of the zero-flux
surface (5) with the sputtering target (22). Of course, if the
electron source is part of the target, or is fabricated from the
target material, such limited sputtering bombardment is of little
consequence. Because the outside edge of the erosion groove of the
target will typically lie somewhat inside the inside edge of the
peripheral magnet pole piece(s) (24), it is advisable, in this
alternative embodiment, to place the electron source directly over
the inside edge of the peripheral magnet assembly.
[0070] If the electron sources are not positioned so as to
introduce electrons into the inner-most field lines that correspond
to either the magnetron trap (3) or those lines tangential to the
zero-flux surface (5), then the EAD condition may be expected to be
compromised, with resulting ion bombardment and deposition
inhomogeneity at the substrate. This requires, when injecting
electrons with the outer magnetic assembly (29), that electrons be
introduced close to the erosion track (13) of the sputtering
target. Various theoretical means have been suggested for
estimating the width of the erosion profile in magnetron sputtering
sources; for example, Penfold estimates that the sputter-eroded
area corresponds to the region of the magnetron trap (3) as defined
in FIGS. 1-2. On the other hand, Lieberman estimates the erosion
profile dimensions, more restrictively, as a relationship between
electron gyration radii and the radii of curvature of the magnetron
trap's field lines. Practically speaking, though, the perimeter of
the erosion pattern will be found to vary considerably, depending
on a variety of factors including the original thickness of the
target, relative flux of inner and outer magnets, relative magnet
height, the target material, etc., so that the erosion profile may
begin well inside, at, or over the inside perimeter of the outer
magnet assembly (29). Accordingly, the precise placement of
electron source apertures (34) will depend on the geometry of the
cathode and magnetic field, but the peripheral electron sources in
the outer magnetic assembly (29) should, in general, be placed as
close to the erosion pattern as is reasonably possible.
[0071] The precise electron current required will depend, to a
great extent, on many factors of the specific process at hand,
including the electrical properties of the deposited material, the
deposition rate, the strength of the magnetic field, the size of
the source, the precise partial and total pressures of the process,
the ionization cross-sections of the gases used, and the physical
location of the electron source(s). Generally, though, the required
electron current may be expected to be in excess of 100 mA for even
small EAD electron traps. Electron sources may be placed in any
location that is appropriate for the specific chamber design used.
Separate electron source modules may also be readily implemented
for providing electrons to the magnetic field of the magnetron
source in a configuration similar to previous magnetron deposition
configurations, and may provide adequate electron injection for the
purposes of the present invention.
[0072] Either permanent magnets or electromagnetic coils may
provide the magnetic field. The substrate/fixture assembly (10), in
FIG. 2, may also incorporate a material selectively composed to
further shape or strengthen the magnetic field of the EAD trap
(25). While the sputtering process and apparatus of the invention
is disclosed, in its preferred embodiment, as a D.C. (direct
current) sputtering process, it may be possible to utilize any of a
variety of power sources for powering the sputtering discharge,
while still maintaining EAD conditions. As is used generally in the
art of sputter deposition, various embodiments of the disclosed
invention might be powered by normal D.C., pulsed D.C., modulated
D.C., low frequency R.F., high frequency R.F., or microwave powers;
though, the use of high frequency power may incur difficulties due
to efficient electron heating. The source may also be arranged with
other sources using a bi-polar powering scheme, as has been
popularly discussed and used in recent years. Likewise, the flux of
deposited vapor may also be pulsed or modulated.
[0073] Experimental results by which the disclosed deposition
process is characterized as a true electron-assisted deposition
(EAD) process differentiate the disclosed invention from
plasma-assisted or ion-assisted deposition (IAD) processes of the
prior art. This is demonstrated, in FIGS. 5-7, through experiments
conducted in the sputter deposition of a multicomponent material
that is very sensitive to ion bombardment and consequent
re-sputtering of a volatile constituent from the growing film.
Because of the high volatility and relatively low-magnitude
heat-of-oxidation of bismuth, ferroelectric strontium bismuth
tantalate (SBT) provided a suitable material for such a study. Any
appreciable ion bombardment of the substrate will result in the
deposition of bismuth-deficient films. The re-sputtering of Bi
atoms from sputter-deposited SBT films is especially problematic at
the elevated substrate temperatures, above 400 .degree. C., where
the thin film depositions of FIGS. 5-7 took place. At these higher
substrate temperatures, Bismuth is known to easily revolatilize
from the growing SBT film. The strontium bismuth tantalate films in
FIGS. 5-7 were deposited at a substrate temperature of 450 .degree.
C. The deposition environments of both the prior art, in FIG. 5,
and of the present invention, in FIGS. 6-7, are revealed in the
spatial distribution of the resultant wafer surfaces, which are
accompanied by both thickness and composition profiles in FIGS.
5-6. In FIGS. 5-7, the dimension of "TARGET DIAMETER" indicates
roughly the area of the target that is within the inner diameter of
the sputter shield (16). In one sense, the plasma sputtering
apparatus of the first preferred embodiment may be described as a
modification of the "Type II" UBM's of the prior art. However,
these previous UBM sources are characterized by dramatically
different operating characteristics. The deposition characteristics
of prior art unbalanced magnetrons of the general "Type II"
magnetic field geometry, utilized in the preferred embodiment, are
represented in the various embodiments of FIG. 5, wherein typical
results of substantially on-axis sputtering with such prior art UBM
sources are displayed. A primary characteristic that is attributed
to these prior art "Type II" UBM's is heavy plasma bombardment,
which may be witnessed, in FIG. 5, in the almost complete depletion
of the easily resputtered bismuth atoms from the film in the
on-axis sputter region (43, 44), this, with a self-bias potential
of -27 VDC for accelerating positive ions in the ion sheath. One
may note that outside of the on-axis region corresponding to the
UBM trap (2) in FIG. 1, the resputtering of the volatile component
from the growing film is significantly less, so that the off-axis
region (42) of the deposited film in FIG. 5 contains significantly
more bismuth.
[0074] A second general characteristic attributed to these prior
art on-axis deposition processes is a very non-uniform deposition
rate with respect to radial distance from the target's central
axis. Such non-uniformity in UBM deposition is typically found to
be significantly greater than in the case of "balanced" magnetrons,
due to inhomogeneity in the UBM plasma environment. These
characteristics of resputtering the deposited film and a
non-uniform rate of deposition are represented in the two profiles
of FIG. 5.
[0075] In particular, a characteristic that occurs in oxide
deposition processes of the prior art UBM sources, generally
represented in FIG. 5, is the development of significant fluxes of
negative oxygen ions. These negative ions contribute significantly
to the formation of beam-like plasma bombardment of the substrate.
The central dip (48) in the thickness profile (47) in FIG. 5 is not
due to the erosion profile of the sputtering target, but is due
rather to such beam-like bombardment and resputtering of the
substrate. This bombardment may be witnessed as well in the
equivalent region of the composition profile (46), wherein the
relatively volatile bismuth is substantially absent from the film
composition. The resputtering evident in the central region (44) in
FIG. 5 is frequently even more severe in such processes, such that,
rather than material deposition, sputter etching of the substrate
occurs.
[0076] Such heavy plasma bombardment, and the accompanying spatial
inhomogeneity of both growth rate and composition, found in
previous on-axis "Type II" UBM processes, has precluded their use
in such high tolerance, damage-sensitive applications, such as in
the growth of epitaxial films, or multicomponent oxides such as
ferroelectrics, piezoelectrics, pyroelectrics, or High-T.sub.a
superconductors.
[0077] The operational characteristics of the sputtering source of
the preferred embodiment are represented in FIG. 6, wherein the
resultant film properties may be contrasted to those of FIG. 5. Of
primary importance is the relationship between the percentage of Bi
incorporated into the deposited film, and the introduction of
excess electrons into the on-axis deposition environment. The
re-sputtering of Bi atoms from the growing film is effectively
reversed from the case of the prior art, in FIG. 5. In fact, as
witnessed in the to composition profile (53), it is noted that the
Bi percentage in the on-axis region (57) of the deposited film, in
FIG. 6, is actually increased over the less bombarded areas of the
wafer's edge region (49). It is understood that the incorporation
of the volatile Bi metal atoms in the on-axis region (57) is
actually increased over that of a balanced magnetron process. In
this way, the composition of the source material is more faithfully
reproduced in the deposited film.
[0078] It should also be noted that the electrically floating
self-bias, during deposition, of the prior art in FIG. 5, was -27
V, whereas that of FIG. 6 was -180 V. Substrate self-biases higher
than -200 V have also been witnessed. Such a large increase in the
substrate self-bias would be interpreted, in the prior art, as a
commensurate increase in the energy of positive ions that bombard
the substrate. With any reasonable, or previously recorded plasma
potential, such a -180 V bias at the substrate would result in
heavy ion bombardment, of energy more than sufficient to resputter
the bismuth in FIGS. 10-11. However these self-bias voltages are
not acceleration potentials with respect to an adjacent plasma
potential. This may be explained by the fact that such assumptions
of conventional plasmas, such as those of ambipolar diffusion, are
not adequate to describe the presently disclosed EAD process. As
such, the local environment of the substrate may be better
described in terms of a magnetically confined electron gas.
[0079] It is also noted that the increased self-bias voltages
witnessed in the disclosed invention are not due to the capacitive
effects that are reported in pulsed or alternating-current plasma
discharges, as have been witnessed in prior art sputtering
experiments. Instead, the increased self-bias potentials possible
in the present invention may be obtained utilizing only true,
unswitched, D.C. power.
[0080] Whereas, for purposes of disclosing the invention, the
substrates in FIGS. 5-7 were insulated from ground and allowed to
electrically float at the self-bias potential incurred by the EAD
process, it is not necessary for the substrate to float at the
self-bias potential. Various electrical biases might be applied to
the substrate for promoting certain types of growth; insofar as
damaging ion bombardment is not created at the growing film.
[0081] An additional important development, in FIG. 6, of the
disclosed EAD process is the improved homogeneity of growth rate as
a function of radial distance from the target center, as witnessed
in both the composition profile (53) and thickness profile (54). It
may be noted that, despite any radial electric field that may exist
within the on-axis area (51), the deposition characteristics are
nonetheless spatially uniform in this area, with the primary
transition in deposition characteristics occurring in the edge
region (50) of the EAD trap region (25), which marks the region of
the magnetic field where electrons trapped along magnetic field
lines, in FIG. 2, begin to be grounded out by the sputter shield
(16).
[0082] The utility of increased activation of the growth interface,
in the disclosed electron-assisted deposition process and
apparatus, is further revealed in the establishment of an
activation-limited growth process, as is illustrated in FIG. 7.
Whereas the growth profiles in FIGS. 5-6 are obtained from
deposition environments utilizing sufficient partial pressures of
oxygen to deposit fully oxidized films over the entire substrate
fixture, the deposition environment of the SBT film in FIG. 7 is
characterized by an unusually low flow rate and partial pressure of
oxygen. FIG. 7, which is a fused silica substrate coated with the
disclosed deposition process, using an unusually low reactive gas
(oxygen) flow and partial pressure (<0.25 sccm and <0.1
mTorr), with a total pressure of 2 mTorr. This low oxygen partial
pressure and flow rate is inadequate for depositing a
stoichiometric oxide film in the same chamber configuration using a
prior art sputtering source for reactive metal target sputtering.
The film deposited by the prior art sputtering source is
essentially identical to the outer region (56) of FIG. 7, which is
completely opaque, indicating a severely reduced oxygen content.
Using the same chamber conditions and deposition rate with the EAD
process and apparatus of the present invention, the resulting
deposited film, in FIG. 7, displays a well oxidized, transparent
region (55), which corresponds to the on-axis trap region (25)
above the target in FIG. 2 of the invention. It is noted that the
oxygen is actually injected to the substrate at the location of
peripheral gas injection means (21), in FIG. 2. Thus, although the
oxygen is admitted, and most abundant, at the periphery of the
substrate, the region of the substrate that is oxidized corresponds
to the inner region that is exposed to the high electron flux.
[0083] In terms of thin film growth, the domination of surface
chemistry in determining the growth mode in this EAD process
further blurs any fundamental delineation between metallic-mode
reactive sputtering processes and plasma-enhanced, low-pressure
CVD. At the point that complexed precursor gases are used in such
an EAD process, such delineation will be even less distinct. From a
CVD point-of-view, the EAD process disclosed cannot be viewed in
the traditional sense of either an "adsorption-limited" or a
"diffusion-limited" process, as the limiting mechanism for film
growth is actually the absence of electron bombardment. Therefore
we have found the positive identification of an electron-assisted
deposition process. That positive identification is found in the
observation of an activation-limited growth mode, wherein high
negative potentials at the substrate do not produce the
corresponding re-sputtering action of positive ions accelerated
through a plasma sheath. Bismuth and oxygen remain on the
substrate, in FIG. 7, only when the growth front of the film is
activated by electron bombardment, so that the probability of
forming the more stable bismuth oxide is dramatically
increased.
[0084] Thus, as is also suggested in the embodiments of FIGS. 6
& 7, the effective sticking coefficient of a volatile metal in
a multicomponent film is significantly increased, due to electron
assistance, by more readily reacting to form a less volatile
compound. Because of this electron activation, the stoichiometry of
the metal target is substantially replicated in the composition of
the deposited compound film, as the various sticking coefficients
of the disparate elements become more equalized under the electron
activation. In other words, the presence of the magnetically
trapped electrons of the invention also effectively increases the
sticking coefficient of volatile elements. Such electron activation
of the growth front thus allows for the resultant film composition
to be less dependent on the particular local chemistry of the
underlying substrate, so that substrates with spatially
heterogeneous properties may be coated more uniformly.
[0085] This latter point is especially apparent in FIG. 7, due to
the compositional profile of the deposited film, which once again
demonstrates that the inner transparent region contains the bismuth
content required in the SBT phase, whereas the reduced outer region
(56) of the deposited film is a reduced combination of tantalum and
strontium sub-oxides. Thus, a conclusion of FIG. 7 is that the EAD
environment in the on-axis environment of FIG. 7 has effectively
increased the oxygen-gettering of bismuth to exceed that of
tantalum in the un-assisted outer region.
[0086] Another conclusion that may be drawn from FIG. 7 is that
less reactive gas is required in the EAD environment to achieve a
fully reacted film, and thus, less of the reacted gas may be made
available to poison a metal sputtering target used for depositing
the film. Because of this ability to run in a metallic mode wherein
only the deposit at the substrate is non-metallic, the disclosed
apparatus and process provide a means of achieving a reproducible
and stable metallic-mode process. Such a stable metallic-mode
process, combined with the improved replication of the target
composition, is highly desirable for deposition of multicomponent
oxide materials from a single metal target. This ability to
replicate the target stoichiometry is, in part, due to the
prevention of high energy negative ions being formed through
contact of the reactive gas with the target surface.
[0087] Various means for providing high electron flux into the EAD
trap may be utilized in place of the modular electron sources
previously discussed. In some cases, such as those in FIG. 8, in
which the sputtering target (22) contains certain group I or group
II metals, the secondary electron coefficient of the target surface
can be high enough so that a slightly oxidized, or otherwise
poisoned, portion of the target becomes a very efficient electron
emitter. Such an efficient electron-emitting surface will develop
when the target is a metallic target containing a group I or group
II metal, and a reactive gas such as oxygen (or fluorine, or
nitrogen) is used to form a compound film. In the case of these
highly reactive metals, even a metallic mode sputtering process
will result in oxidized metal atoms (17) forming at the relatively
unsputtered regions of the target. Because these less sputtered
regions will also correspond to the magnetic cusp regions discussed
above, such poisoned surfaces become an excellent means of emitting
secondary electrons into the magnetic field, even though the target
is operated in metallic mode, wherein the eroded area, as indicated
by sputtered region (13) in FIG. 2 & 8, and by the
cross-hatched area in FIG. 4, remains metallic. Additional target
poisoning, as practiced in the disclosed invention, will also
produce efficient electron emission from the sputtered portion of
the target's surface, to substantially the same effect.
[0088] It is well known that metal oxides frequently possess
secondary electron coefficients significantly higher than that of
the corresponding metal. It is also known that certain group I and
group II metals, such as magnesium, form metal oxides of unusually
high secondary electron coefficients. While there are large
discrepancies between measurements of these coefficients, it has
been found that the coefficient of MgO can increase markedly for a
vacuum-cleaved surface. It has been discovered in the present
invention that such group I and group II metal oxides as MgO, BaO
and SrO can produce an unexpectedly large flux of electrons when
existing at the surface of a metal target, in the plasma
environment of the sputtering source. The magnitude of the
resultant electron flux may be witnessed to establish the EAD mode
of operation disclosed herein. It may be noted that emission of
electrons from the sputter-eroded area of the target (13) will also
provide electrons centrally to the trap region (25).
[0089] While some oxidation of the target surface may be found
useful in creating secondary electrons, it is not necessary that
the sputtered portion of the target surface be "poisoned" in the
traditional sense. That is to say, very little oxidation need occur
at the target to substantially increase electron emission. As such,
the deposition rate may still correspond to the high-rate,
un-poisoned, region of the sputtering target's hysteresis curve.
Also, because the target is, in FIG. 8, preferably an intermetallic
including a Group I or Group II metal, oxidation of discrete atoms
of the one component will not result in the sputter yield of the
component being reduced to that of its corresponding oxide, such as
would occur in phase segregated targets. In particular, targets
comprising certain intermetallics that include such group I and II
metals as Cs, Sr, Mg, La, Be, Ba, and Rb, can be found to be ionic
enough in character to provide the necessary high-y emission of the
EAD process of FIG. 8.
[0090] What is required to sufficiently enhance electron emission
into the trap region will depend on the particular materials and
processes used. While the coupling of electrons into the magnetic
field may be accomplished by any appropriate means, it is noted
that the operation of the disclosed vapor deposition apparatus in
the low pressures and selected gas-flow geometries of the preferred
embodiments, below 3 mtorr and in metallic mode, allows for more
liberty in the selection of certain electron sources. This liberty
is provided by the ability to maintain the immediate environment of
the electron source at a lower partial pressure of oxygen, or other
damaging reactive gas, used in forming the deposited compound.
[0091] For example, thermionic electron emission, which requires a
heated surface, is usually impractical in partial pressures of
oxygen greater than 10.sup.4 torr. However, because of the enabling
of a very metallic mode of reactive deposition, a thermionic
surface at the sputtering cathode may be operated with a
substantially longer lifetime and more stable performance. Thus,
because of the efficient use of reactive gas at the substrate,
provided by the EAD characteristics, electron sources that would be
otherwise difficult to use in reactive processes may be readily
implemented in the disclosed apparatus and process. Using chamber
pressures below 3 mtorr, in the present EAD process, is also
preferred for substantially avoiding the production of high-energy
reflected neutrals and other plasma species that may damage the
growing film.
[0092] While the present disclosure has taught the invention, in
FIGS. 2-8, within the context of an unbalanced magnetron apparatus,
the disclosed EAD apparatus and process may be implemented in
conjunction with a variety of vapor deposition approaches without
departing from its principles or scope. The alternative embodiments
of FIG. 9 serve to provide alternative means of implementing the
disclosed deposition apparatus and device. As previously noted, the
electron source means of the present invention may be provided by
any of a variety electron source means. The use of ultraviolet
radiation incident on the target itself, as disclosed in U.S. Pat.
No. 4,664,769 to Cuomo, and included here by reference, may also
comprise the electron source means of the present invention, as in
FIG. 9.
[0093] Accordingly, an ultraviolet radiation source (59) may be
directed at a negatively-biased metallic surface (58) that
comprises one of the first and second surfaces that terminates the
EAD trap region (25). In the embodiments of FIG. 9, this biased
metallic surface 5, (58) is biased by a power supply, `P`, which
provides the electrons that are photo-emitted into the EAD trap.
While this biased surface (58) may also contain means for providing
a vapor to deposit on the substrate/fixture (10), a depositing
vapor is also provided by an external vapor source (60), which is
external to the EAD trap (25). This external vapor source may
comprise any of a variety of prior art vapor sources, such as a
kaufmann ion source, cluster-ion source, fast atomic beam source,
evaporation source, etc.
[0094] While many of the benefits of the present invention may be
realized in a wide range of operating pressures, the benefits of
the invention are best realized, in its preferred embodiment, with
pressures approaching or within the collisionless sheath pressure
region, typically less than 3 millitorr. This preferred pressure
range best allows operation of an unpoisoned target, and, using the
recommended gas geometry, avoids production of damaging negative
ions. In addition, this lower pressure region also appears to
reduce damage due to high-energy reflected neutrals.
[0095] In addition to the substantially planar sputtering apparatus
of the preferred embodiments, as disclosed in the previous
discussion, the EAD process may also be implemented into other,
reactive or non-reactive, physical vapor deposition processes. For
instance, the magnet and sputtering target configuration may
alternatively be that of a linear magnetron or a hollow magnetron.
While the case of a linear planar magnetron may be easily
extrapolated from the case of the circular planar magnetron, a
modified hollow magnetron geometry, FIG. 10, allows several unique
advantages. One such advantage exists in the utilization of
substantially the same magnetic field vectors for both guiding of
the electrons and confinement of the sputtering discharge at the
cylindrical target surface (75). The cylindrical target surface may
that of any of a variety of hollow magnetron designs that provide a
magnetron trap region (3) that is compatible with the EAD trap of
FIG. 10. As such, the first and second terminating surfaces in FIG.
10 may both be a substrate/fixture assembly (10), which each
incorporate means--such as large magnets--for providing the
opposite poles of the EAD trap. Also, the electrons of FIG. 10 may
be provided into the magnetic field by one or more electron
emitting apertures (34) located in either of the surfaces that
terminate the EAD trap (25). The electron sources may comprise any
of those previously discussed.
[0096] The novel features of the present invention provide
advantages in a variety of vacuum deposition processes, which may
incorporate any compatible means of condensing a vapor onto a
workpiece. As such, the EAD trap (25) may be utilized in
conjunction with a generic central vapor source (81) that may be
electrically biased to prevent electrons from exiting but may also
utilize any of the vapor sources of the prior art that are
compatible with the low transition-flow and molecular-flow
pressures needed to achieve the EAD conditions. Such vapor sources
might include those of various e-beam and boat evaporation methods,
as well as the various vapor injection sources utilized in
low-pressure CVD. For example, rather than using a plasma sputter
source, one could implement the same features of the disclosed
magnetically confined electron source into an evaporation process.
Such a configuration, in FIG. 11, is illustrated herein using an
effusion cell (83). As in the previous drawings, only the magnetic
field lines intersecting the substrate are shown.
[0097] It will be noted that the invention need not be implemented
as a symmetrical arrangement, since a vapor source may introduce a
depositing vapor from a location and direction different from that
of the EAD trap. For example, a second effusion source (82) is
place asymmetrically to the EAD trap (25) in FIG. 11. Similarly to
the metallic mode sputtering process described in conjunction with
FIG. 2, the EAD apparatus of FIG. 11 may be implemented so as to
allow lower oxygen (or other reactive gas) pressures, thereby
preventing oxidation of the source material (84).
[0098] While the preferred embodiment of the disclosed deposition
process utilizes a magnetically confined sputtering plasma and a
sputtering target, the requirements of the condensable vapor source
could also be satisfied by a rather wide variety of sources. In
this latter sense, the vapor source of FIG. 11 could comprise any
means of providing a vapor, as long as such means does not
interfere with the operation of the EAD trap. Such a vapor might
comprise a pure elemental vapor, or a complexed precursor gas that
is to be disassociated at the substrate.
[0099] The vapor depoition processs and apparatus of the preferred
embodiments, FIGS. 1-11, may be used alone, in sequence with other
processing steps, or combined with other deposition or etching
tools in a single process. One such possibility is to arrange
several of the sputter deposition sources disclosed in FIGS. 2-7 in
a single deposition station. The latter arrangement would be well
suited for coating large articles, such as larger wafers. In the
geometry of FIG. 12, seven of the disclosed magnetron cathodes
(90), which would each comprise the magnetron cathode elements
described in FIG. 2, are placed in an interconnected hexagonal
arrangement. The interstitial areas between the cathodes may then
house additional electron and/or reactive gas emission means in
interstitial modules (91). Because there should be no grounded
surfaces present in the EAD trap (25) of FIG. 12, these
interstitial modules are negatively biased with the cathode
potential. As a result, these interstitial modules provide an
effective means of manipulating electron and gas flows within the
larger EAD trap (25a) that results from several smaller EAD traps
being joined together in the manner of FIG. 12. Such an arrangement
may provide advantages in large-area deposition equipment. Also,
because of the low total pressures and partial pressures of oxygen
use, ballistic oxygen atoms may be emitted toward the
substrate/fixture assembly (10) from one of these interstitial
modules, while the target remains shadowed and, hence, in a
relatively metallic state. One may also, or alternatively,
incorporate additional electron sources (30) into the
substrate/fixture assembly for saturation of the EAD trap.
[0100] In addition, the geometry of FIG. 12 avoids the difficulty
and expense incurred in producing homogenous, high-tolerance single
targets for such large deposition areas; an especially important
advantage in view of the more recent sets of exotic metals and
ceramics being investigated for thin film materials.
[0101] A novel arrangement of magnetic pole pieces, in the top view
of FIG. 13, allows for an alternative hexagonated arrangement of
sputtering magnetron discharges, wherein, rather than the separate
magnetron cathodes of FIG. 12, a single cathode housing (98)
incorporates an arrangement of outer magnetic pole piece(s)
(24)--all the "S" pieces--that provides separate magnetron traps
and EAD traps, each similar to that depicted in FIG. 2, except that
the plurality of EAD traps, provided by the arrangement of FIG. 13,
is, similarly to FIG. 12, sufficiently interconnected to mutually
trap electrons in the resulting larger EAD trap. For example,
electrons injected toward the substrate from the particular
interstitial module (91a) will be trapped within the assembly of
EAD traps. The inner magnetic pole pieces (23) may then be simple
magnets as in the prior art magnetron of FIG. 1, and electrons can
still homogeneously saturate the larger EAD trap by being injected
outside of the zero-flux surface (5), indicated by its intersection
with the cathode surface, of each individual magnetic trap. Of
course, electrons may also be injected at, or inside the zero-flux
surface, as discussed in the previous embodiments.
[0102] In FIG. 13, electron sources are, as in FIG. 12, located in
the interstitial modules (91) at the interstices of the hexagonal
arrangement. In this embodiment, some interstitial modules may,
again, also house, separately or in combination with an electron
source, gas introduction means for introducing either a sputtering
or reactive gas. This magnetic pole arrangement, in FIG. 13, allows
for sputtering from segmented targets, with outer diameters
coinciding roughly with the zero-flux surfaces (5); or,
alternatively, a large-area target, with outer diameter coinciding
with that of the cathode housing (98). The latter large area target
would, as in previous embodiments, provide holes through which the
interstitial modules could inject electrons and/or gases into the
EAD trap. The embodiments of FIGS. 12-13 are particularly useful
for uniformly introducing gas/electrons over a large-area
substrate. The embodiments of FIGS. 12-13 also avoid some of the
deposition non-uniformity inherent in using a large target with a
single erosion track. Also, while both the substrate and sources in
FIGS. 12-13 are planar, The principles of the invention are easily
extended to non-planar substrate, as well as to non-planar
arrangements of the vapor sources.
[0103] While the magnet configurations in of the various magnetron
embodiments serve to demonstrate the versatility of the disclosed
process, many variations of the magnet configuration and resultant
magnetic field may be realized without departing from the teachings
or spirit of the invention. Additional chamber elements may also be
introduced to additionally modify the magnetic field of the
apparatus. Modification of the magnetic field shape and
distribution may be accomplished through use of additional magnets,
ferromagnetic materials, as well as materials selected for other
practical magnetic characteristics, such as a magnetic coercivity,
permeability, saturation, etc. As such, desirable modification of
the magnetic field may also be achieved by selecting appropriate
materials for constructing the chamber elements already
discussed.
[0104] As is described in relation to FIG. 9, in certain
embodiments of the invention, the vapor source may deliver
condensable material to the substrate by delivering vapor across
the field lines of the magnetic electron trap, so that the vapor
source is external to the effective trap region. An additional
embodiment utilizing this approach, in FIG. 14, wherein, as in
previous embodiments, electrically floating structures providing
the magnetic field are maintained separate from the substrate
supporting structure.
[0105] As mentioned previously, the principles of the present
invention may be extended to a variety of vapor source geometries.
In this case of a linear source, a straightforward approach is to
position a plurality of the integrated electron source/magnetic
assembly, such as in FIG. 3, in a row, thereby forming an effective
line source. In FIG. 14, the `X` indicates an axis normal to the
plane of the figure, indicating the long axis of the line source
and the substrate fixture, which is represented in this embodiment
as a rotating drum.
[0106] In this configuration, the integrated electron
source/magnetic assembly of FIG. 3 is used with an opposing pole
piece for defining a magnetic trap region that conveniently
accommodates the geometrical restraints of such processes.
Intersection of the magnetic field with the substrate surface is
rendered symmetric about the major axis of the cylindrical surface,
so that the deposition may be easily made uniform with respect to
the cylindrical surface. As in previous embodiments, a vapor source
(60) may be used alone, or in conjunction with reactive gas
injection means (21) for reactively depositing material.
INDUSTRIAL APPLICABILITY
[0107] Because the present invention discloses a fundamentally new
method of thin film deposition, it may be used in a broad range of
applications. In addition to the damage sensitive films previously
discussed, the disclosed deposition process may be used for a
variety of optical materials applications. For example, the
deposition of optical filters in telecommunications may benefit
from the invention through its ability to deposit fully dense films
without induced stress from high-energy ion bombardment. In
addition, application-specific optical waveguide materials that may
typically suffer from phase segregation or from precipitation of a
doped ion can also benefit from the highly non-equilibrium nature
of the disclosed deposition process.
[0108] In addition, the electron activation of the growth front is
highly advantageous for the formation of more covalent, relatively
low-melting temperature compounds, wherein complete reaction of the
components is highly sensitive to processing conditions. Examples
of such compounds would be many of the narrow bandgap materials
used in photovoltaic research and infrared detection, such as
cadmium sulfide, copper indium selenide, copper indium gallium
selenide, indium phosphide, and the like.
[0109] The reactive gases used in the disclosed invention may be
any elemental or molecular gas, or vapor, that may be introduced at
the substrate for forming a solid film. As such the deposited
material may be any of a wide variety of materials, including, but
not limited to, solid compounds of oxides, nitrides, fluorides,
carbides, borides, phosphides, selenides, sulfides, or tellurides.
High temperature materials, such as SiC, III-V nitride compounds,
and diamond-like materials will also benefit from the highly
activating EAD environment of the present invention.
[0110] The ability of the invention to increase the sticking
coefficient of relatively volatile, less reactive metals, such as
bismuth, lead, tin, zinc, cadmium, and antimony, is useful in the
stoichiometric deposition of many multicomponent materials. Such
materials include many ferroelectric, piezoelectrics,
pyroelectrics, conducting oxides, and narrow bandgap materials.
[0111] Complex compounds, such as the ferroelectric and
superconducting perovskites are seen as materials that may be
effectively and reproducibly formed with the disclosed process. For
example, the ferroelectric perovskite, strontium bismuth tantalate
(SBT) may be deposited by co-sputtering two target
materials--separately or in a mosaic target--comprising an Sr-Bi
intermetallic and the Ta metal, wherein the Sr in the intermetallic
provides the high electron flux required for the EAD process, as
described above. Similarly, barium strontium titanate (BST) may be
deposited from a target material that includes a solid solution of
Ba and Sr, as these two metals have a continuous solid solution
range. Unlike previous sputtering processes, the enhanced
activation of oxygen at the depositing film, due to the present EAD
process, allows for the oxygen population near the sputtering
targets to be maintained sufficiently low to allow metallic mode
sputtering.
[0112] It should also be noted that the deposition processes and
apparatus of the invention provide an excellent method for lowering
the required substrate temperature for achieving epitaxial growth
or the formation of a particular material phase. For example,
because of this capacity to reduce the epitaxial temperature, the
sputtering process disclosed would not require the normally high
substrate temperatures of 775-800 Celsius to deposit epitaxial
layers of CeO.sub.2 or silicon.
[0113] Also, such operating conditions of the disclosed EAD process
can be useful in forming certain high-temperature materials, such
as gallium nitride, which have a tendency of being deposited in a
reduced form. Such non-equilibrium characteristics will also be
useful in depositing thin films of various metastable material
phases, such as diamond or Zr.sub.3N.sub.4. Similarly, various
materials in the GaAlInN phase space can be deposited with
stoichiometric nitrogen incorporation, so that intrinsic
semiconductor material may be formed, or, as in the case of
Zr.sub.3N.sub.4, even a nitrogen over-stoichiometry may be
achieved.
[0114] Also, it is noted that, while a best attempt has been made
to explain the novel operating characteristics of the disclosed
vapor deposition process and apparatus, the purpose of the present
disclosure is to demonstrate the practical aspects of operating the
present invention. Accordingly, although the present invention has
been described in detail with reference to the embodiments shown in
the drawing, it is not intended that the invention be restricted to
such embodiments. It will be apparent to one practiced in the art
that various departures from the foregoing description and drawing
may be made without departure from the scope or spirit of the
invention.
* * * * *