U.S. patent application number 09/253424 was filed with the patent office on 2002-01-03 for constricted glow discharge plasma source.
Invention is credited to ANDERS, ANDRE.
Application Number | 20020000779 09/253424 |
Document ID | / |
Family ID | 26757059 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000779 |
Kind Code |
A1 |
ANDERS, ANDRE |
January 3, 2002 |
CONSTRICTED GLOW DISCHARGE PLASMA SOURCE
Abstract
A miniaturized construction and slit end orifice configurations
of a constricted glow discharge chamber and method are disclosed.
The polarity and geometry of the constricted glow discharge plasma
source is set so that the contamination and energy of the ions
discharged from the source are minimized. The several sources can
be mounted in parallel and in series to provide a sustained ultra
low source of ions in a homogeneous linear plasma stream with
contamination below practical detection limits. Other configuration
include a hollow chamber with an anode outside the chamber located
opposite its discharge constriction orifice. The constriction
orifice may be circular or a slit and can be aligned to form a
linear array for processing web substrates.
Inventors: |
ANDERS, ANDRE; (ALBANY,
CA) |
Correspondence
Address: |
LAWRENCE BERKELEY NATIONAL LABORATORY
PATENT DEPARTMENT
1 CYCLOTRON ROAD
MAIL STOP 90-1121
BERKELEY
CA
94720
|
Family ID: |
26757059 |
Appl. No.: |
09/253424 |
Filed: |
February 19, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09253424 |
Feb 19, 1999 |
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08711844 |
Sep 10, 1996 |
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6137231 |
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60075607 |
Feb 19, 1998 |
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Current U.S.
Class: |
315/111.21 ;
315/111.31 |
Current CPC
Class: |
H01J 37/32009 20130101;
H05H 1/4697 20210501; H05H 1/48 20130101 |
Class at
Publication: |
315/111.21 ;
315/111.31 |
International
Class: |
H05H 001/00 |
Claims
1. A source of plasma comprising: a gas supply tube having a gas
inlet constriction and an internal gas passage terminating at an
open first end, said tube acting as a cathode and being connected
to a DC power supply providing a negative voltage to said cathode;
an electrically non-conductive tube having an internal gas passage
connected with said open first end of said gas supply tube with
minimal gas leakage therebetween, said non-conductive tube
extending beyond said first open end of said tube to a closed end
of said tube, an electrically non-conducting end wall having a gas
discharge constriction passage therein, a metallic covering located
on the outside of and substantially in contact with an outside
surface of said electrically non-conducting tube and said end wall
and having an opening therein corresponding to the location of said
gas discharge constriction passage to allow gas flow therethrough,
wherein said metallic covering is connected to an electrical
ground, where the positive voltage of said DC power supply is
connected to said electrical ground; and a gas source connected to
feed gas to said gas supply tube.
2. The source of plasma as in claim 1, wherein said gas discharge
constriction passage is a substantially circular hole.
3. The source of plasma as in claim 1, wherein said gas discharge
constriction passage is a slit.
4. The source of plasma as in claim 2, further comprising a set of
one or more additional sources of plasma as in claim 2, wherein a
discharge direction from each of said sources of plasma is
approximately parallel and said gas discharge constriction passages
of each of said set of one or more additional sources are
substantially linearly aligned.
5. The source of plasma as in claim 3, further comprising a set of
one or more additional sources of plasma as in claim 3, wherein a
discharge direction from each of said sources of plasma is
approximately parallel and a set of longitudinal axes of said slits
forming said gas discharge constriction passages of each of said
set of one or more additional sources are substantially linearly
aligned.
6. The source of plasma as in claim 4, wherein said set of one or
more additional sources of plasma have a horizontal spacing between
adjacent sources gas discharge constriction passages that is set
individually between adjacent sources of plasma to provide a
particular plasma density profile across a width of a substrate
being processed which is facing said set of one or more additional
sources of plasma.
7. The source of plasma as in claim 5, wherein said set of one or
more additional sources of plasma have a horizontal spacing between
adjacent sources gas discharge constriction passages that is set
individually between adjacent sources of plasma to provide a
particular plasma density profile across a width of a substrate
being processed which is facing said set of one or more additional
sources of plasma.
8. The source of plasma as in claim 4, wherein said set of one or
more additional sources of plasma are disposed adjacent to a source
of metal molecules in a vacuum processing chamber, and opposite
from a substrate being processed in said chamber to enhance the
quality and speed of deposition of compound films on said
substrate.
9. The source of plasma as in claim 5, wherein said set of one or
more additional sources of plasma are disposed adjacent to a source
of metal molecules in a vacuum processing chamber, and opposite
from a substrate being processed in said chamber to enhance the
quality and speed of deposition of compound films on said
substrate.
10. The source of plasma as in claim 4, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
11. The source of plasma as in claim 5, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
12. The source of plasma as in claim 6, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
13. The source of plasma as in claim 7, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
14. The source of plasma as in claim 8, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
15. The source of plasma as in claim 9, where each of said set of
one or more sources of plasma is supplied power from a separately
controllable power source.
16. The source of plasma as in claim 4, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
17. The source of plasma as in claim 5, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
18. The source of plasma as in claim 6, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
19. The source of plasma as in claim 7, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
20. The source of plasma as in claim 8, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
21. The source of plasma as in claim 9, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
22. The source of plasma as in claim 10, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
23. The source of plasma as in claim 11, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
24. The source of plasma as in claim 12, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
25. The source of plasma as in claim 13, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
26. The source of plasma as in claim 14, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
27. The source of plasma as in claim 15, where each of said one or
more sources of plasma is supplied gas from a separately
controllable gas source.
28. A source of plasma comprising: a hollow enclosure having a gas
inlet opening and an electrically floating end plate having a gas
discharge constriction passage therein; a cathode surface being
exposed to an interior space of said enclosure, wherein said
cathode is connected to a DC power supply providing a negative
voltage to said cathode; an anode surface exposed to gas flow
emanating from said discharge constriction passage, wherein said
anode is connected to an electrical ground, where the positive
voltage of said DC power supply is connected to said electrical
ground; a gas source connected to feed gas to said gas inlet
opening.
29. The source of plasma as in claim 28, wherein said electrically
floating plate is metal.
30. A source of plasma comprising: a hollow enclosure having a gas
inlet opening and an electrically floating end plate having a gas
discharge constriction passage therein; a cathode surface being
exposed to an interior space of said enclosure, wherein said
cathode is connected to a DC power supply providing a negative
voltage to said cathode; an anode surface exposed to gas flow
emanating from said discharge constriction passage, wherein said
anode is connected to an anode potential near to but different from
said electrical ground, where the positive voltage of said DC power
supply is connected to said electrical ground; a gas source
connected to feed gas to said gas inlet opening.
31. The source of plasma as in claim 29, wherein said electrically
floating plate is connected to electrical ground through a 10 kilo
ohms or greater resistor.
32. The source of plasma as in claim 29, wherein said electrically
floating plate is connected to electrical ground through a 100 kilo
ohms or greater resistor.
33. The source of plasma as in claim 28, wherein said anode
includes an opening opposite the location of said gas discharge
constriction passage, larger than said gas discharge constriction
passage, beyond which a substrate to be processed is located.
34. The source of plasma as in claim 33, wherein said substrate is
substantially dielectric.
35. The source of plasma as in claim 1, wherein a magnetic field
surrounds a portion of the source of plasma.
36. The source of plasma as in claim 2, wherein a magnetic field
surrounds a portion of the source of plasma.
37. The source of plasma as in claim 3, wherein a magnetic field
surrounds a portion of the source of plasma.
38. The source of plasma as in claim 4, wherein a magnetic field
surrounds a portion of the source of plasma.
39. The source of plasma as in claim 5, wherein a magnetic field
surrounds a portion of the source of plasma.
40. The source of plasma as in claim 6, wherein a magnetic field
surrounds a portion of the source of plasma.
41. The source of plasma as in claim 7, wherein a magnetic field
surrounds a portion of the source of plasma.
42. The source of plasma as in claim 8, wherein a magnetic field
surrounds a portion of the source of plasma.
43. The source of plasma as in claim 9, wherein a magnetic field
surrounds a portion of the source of plasma.
44. The source of plasma as in claim 28, wherein a magnetic field
surrounds a portion of the source of plasma.
45. The source of plasma as in claim 29, wherein a magnetic field
surrounds a portion of the source of plasma.
46. The source of plasma as in claim 30, wherein a magnetic field
surrounds a portion of the source of plasma.
47. The source of plasma as in claim 31, wherein a magnetic field
surrounds a portion of the source of plasma.
48. The source of plasma as in claim 32, wherein a magnetic field
surrounds a portion of the source of plasma.
49. The source of plasma as in claim 33, wherein a magnetic field
surrounds a portion of the source of plasma.
50. The source of plasma as in claim 34, wherein a magnetic field
surrounds a portion of the source of plasma.
51. A source of plasma comprising: a hollow enclosure having a gas
inlet opening and a slot shaped gas discharge constriction passage;
a cathode surface being exposed to an interior space of said
enclosure, wherein said cathode is connected to a DC power supply
providing a negative voltage to said cathode; an anode surface
exposed to said discharge constriction passage, wherein said anode
is connected to an electrical ground, where the positive voltage of
said DC power supply is connected to said electrical ground; a gas
source connected to feed gas to said gas inlet opening.
52. The source of plasma as in claim 51, wherein said slot has a
length to width ratio of at least 50 to 1.
53. A method for producing a plasma from a plasma source comprising
the steps of: using a feed tube to supply gas and DC power to an
end of said feed tube which is connected to an insulating sleeve
connected to the end of said feed tube and extending beyond the end
of said feed tube; providing an inlet orifice requiring gas to pass
through said inlet orifice to flow into said feed tube; providing
an constriction orifice in an end wall of said insulating sleeve;
providing a metallic cover over an outside surface of an end of
said insulating sleeve at a location beyond and separated from a
location of said feed tube; connecting a negative voltage of a DC
power source to said feed tube; connecting said metallic cover to
an electrical ground which is connected to a positive voltage of a
DC power source; energizing said DC power source; and supplying gas
to said feed tube.
54. A method for producing a plasma comprising the steps of:
providing gas to a hollow chamber through an inlet orifice of a
cathode located at a feed end of the hollow chamber, where side
walls of said chamber are made of an electrically insulating
material and a discharge end of said chamber is a metal plate
having a constriction orifice therein; placing an anode at a
location outside of said hollow chamber opposite said metal plate;
and connecting said cathode to a negative voltage of a DC power
source and connecting a positive voltage of said DC power source to
said anode.
55. The method for producing a plasma as in claim 54, wherein said
anode is a substrate to which plasma is directed.
56. The method for producing a plasma as in claim 54, wherein said
metal plate is electrically connected to said positive voltage of
said DC power supply through a 10 kilo ohm or greater resistor.
57. The method for producing a plasma as in claim 54, wherein said
metal plate is electrically connected to said positive voltage of
said DC power supply through a 100 kilo ohm or greater
resistor.
57. The method for producing a plasma as in claim 54, wherein said
metal plate is electrically connected to said positive voltage of
said DC power supply through a 10 kilo ohm or greater resistor;
wherein said anode includes a hole opposite said constriction
orifice; and further comprising the step of: placing a substrate to
be processed on a side of said anode opposite from said hollow
chamber and opposite said hole in said anode.
58. The method for producing a plasma as in claim 54, wherein said
cons triction orifice is a slit.
59. The method of producing a plasma as in claim 54, further
comprising the step of: placing at least two hollow chambers
adjacent to one another; aligning a discharge direction of each of
their constriction orifices in an approximately linear pattern; and
arranging the spatial relationship between each of said at least
two hollow chambers to provide a homogeneous plasma flux on a
substrate placed opposite from said constriction orifices of said
at least two hollow chambers.
Description
RELATED APPLICATIONS
[0001] This application claims priority from provisional
application serial no. 60/075,607 filed on Feb. 19, 1998 and claims
priority as a continuation in part from application Ser. No.
08/711,844 filed on Sep. 10, 1996, pending.
FIELD OF THE INVENTION
[0002] This invention relates to structures and methods for
producing a linear array of streaming plasma with a very low level
of contamination and low energy ions in a vacuum processing
chamber, e.g., such as in those used to process compound films
(e.g., oxide films), to synthesize thin films on the surface of a
substrate.
BACKGROUND OF THE INVENTION
[0003] In a vacuum processing chamber ion sources are used to
change the properties of substrate surfaces. Gas is fed through an
electric field in a vacuum chamber to excite the gas to a plasma
state. The energized ions or excited neutrals (such as excited
atoms and disassociated molecules) of gas constituents bombard the
surface of the substrate. The effect that the ions have on the
surface is dependent on their atomic constituents and their energy
(in one example to treat a surface by providing active oxygen).
[0004] Low-energy, ultra-clean flows of plasmas are required, to
obtain a crystalline film with high crystal quality as required in
the case of gallium nitride (GaN). To avoid ion damage the ion
energy must not be too high, for instance, an ion with a kinetic
energy of 20 or more eV for GaN must be considered a high energy
ion. When an ion hits the surface during crystalline film growth
and its kinetic energy is high enough to displace atoms which are
already in place, a defect is created. So the materials grown with
energetic plasma streams tend to have lots of defects. The defects
create "carrier" densities for semiconductors such as GaN, leading
to the creation of a material with n-type doped properties. In
these cases it is difficult to obtain a p-type doped material. To
overcome this problem a source of low energy ions is needed.
[0005] A nitrogen plasma flow or low-energy ion beam (ion energy of
order 30-50 eV) is usually obtained by using a Kaufman ion source
or an Electron-Cyclotron-Resonance (ECR) plasma source.
[0006] The drawback of a Kaufman source is that a hot tungsten
filament is used as a cathode. The tungsten filament delivers large
quantities of electrons by thermionic emission, which sustain the
low-energy non-self-sustained arc discharge, but tungsten atoms
evaporate from the filament and can be found in the stream of
plasma and the growing films. This is not acceptable for instance
in the growth of GaN because the stream is not clean and the film
properties are altered by the impurities, and is not acceptable for
growth of oxide films, because the tungsten filament will oxidize
rapidly.
[0007] An ECR plasma source necessarily operates with a high
magnetic field to fulfill the resonance condition of microwave
frequency and electron cyclotron frequency. Typically, the standard
microwave frequency of 2.45 GHz is used, leading to a required
magnetic field of 875 Gauss. The gaseous microwave plasma is
produced in the region of the resonance magnetic field. The ions
gain kinetic energy when leaving the location of the high magnetic
field and streaming towards the substrate. When a plasma is made
this way there is a significant energetic component in the ion
energy distribution, i.e., ions having 30-50 eV of kinetic energy
are abundant. This energy is too high for the growth of a high
quality crystalline films. Although ECR plasma sources are cleaner
than Kaufman sources, ion damage is observed in growing films due
to the relatively high ion energy. One way to overcome this energy
problem is to bias the substrate electrically, to deflect the
energetic ions but in doing so the low energy ions are also
deflected and the growth rate decreases.
[0008] A better (cleaner) source of low-energy gaseous ions is
needed to deposit high quality thin films on substrates in both
research and commercial applications. This need includes not only
MBE-type but also IBAD-type deposition of thin films (MBE=molecular
beam epitaxy; that is film growth with reactive, activated gases;
IBAD=ion beam assisted deposition, that is film growth assisted by
the moderate kinetic energy of ions such as argon).
[0009] A plasma discharge chamber usable for an ion beam source,
electron beam source, and a spectral light source was introduced by
V. I. Miljevic and is described in several papers (Rev. Sci.
Instrum. 55 (1984) 931; Rev. Sci. Instrum 63 (1992) 2619) (also see
U.S. Pat. Nos. 4,871,918; 4,906,890). A preliminary explanation of
the working principle of the discharge is given in a paper
published in Plasma Sources Science & Technology, Vol.4. (1995)
p.571.
[0010] In one configuration as shown in FIG. 1, a gas flows through
a discharge chamber 20 which consists of a metal cathode 22
(grounded) and a metal anode 24 (positively biased) separated by a
Teflon insulator 26. A flange 28 holds the anode 24 in place and
seals it against the cathode flange using a series of O-rings 30.
By applying a sufficiently high voltage (500 V or more) to the
electrodes, a glow discharge ignites in the flowing gas. The gas is
introduced through an opening 40 in the cathode, and leaves the
source through a small aperture 38 in the anode. A high positive
voltage is applied to an extraction electrode 32 leading to
acceleration of the ions from the source 20 in the direction shown
by arrow 36. A high negative voltage would accelerate electrons,
turning the source into an electron beam source. An electromagnetic
coil 34 produces a magnetic field around the anode 24 focusing the
ions in an ion beam (whose direction is shown by the arrow 36)
departing from the discharge opening 38 in the anode 24. Gas
pressure supplied to the gas inlet 40 provides the motive force to
discharge the ions from the discharge chamber 20. The extraction
electrode 32 and magnetic coil 34 assist in accelerating and
focusing the ion discharge into a beam. The anode 24 is insulated
from the grounded cathode 22 and grounded support flange 28 by a
thin film of ceramic coating deposited on the respective mating
surfaces of the anode 24.
[0011] The feature which distinguishes this kind of discharge from
an ordinary glow discharge is the actual exposure of a very small
area of a large cross section anode facing the cathode, to the gas.
In the Miljevic configuration this effect is obtained by blocking
nearly all of the anode 24 by using an insulator 26, except for a
small discharge aperture. this discharge aperture forms a small
hollow anode, and Miljevic named the discharge "hollow-anode
discharge". Related research has found (Plasma Sources Science
& Technology, Vol. 4. (1995) p.571.) that a voltage drop
appears in front of the discharge opening, accelerating electrons
which gain enough energy to ionize the working gas through
inelastic collisions. A bright "anode plasma" forms in the anode
channel, and this plasma is blown out by the gas flow in the
channel due to the pressure gradient between the inside and the
outside of the source. The "anode plasma" does not form when there
is no blocking or covering such a large anode.
SUMMARY OF THE INVENTION
[0012] A structure and method according to the invention involves
using a special type of glow discharge plasma source, namely the
so-called "Constricted Glow Discharge Plasma Source". The
configuration of the prior art plasma source is adapted in a way
that it delivers a downstream gaseous plasma of low contamination
and very low kinetic energy, well-suited for the growth of
high-quality thin films. The source can operate in a wide range of
parameters, in particular it can also work at very low and very
high gas pressures. It has been found that the anode does not
necessarily need to form a small opening which is located next to
the blocking insulator. The "hollow anode discharge" is just one
possible configuration which makes use of a constriction element. A
configuration according to the invention includes a special type of
glow discharge characterized by a constriction between cathode and
anode. The inventor(s) named this type "constricted glow
discharge," and the derived downstream plasma source "constricted
glow discharge plasma source."
[0013] The constricted glow discharge plasma source includes a
discharge chamber where the potential of the cathode is below that
of the anode, and the potential of the anode is approximately the
same as the potential of the substrate being processed, thereby
minimizing the energy of ions reaching the substrate. Having the
substrate and anode at substantially the same potential, eliminates
the accelerative effect on ions or electrons that a biased
substrate would experience. Ion energy can be adjusted by biasing
the substrate or changing the plasma potential (changing the anode
potential). The configuration and method according to the invention
provides a simple construction and operation of a low energy plasma
source.
[0014] The source is simple, compact, and versatile. It can be used
with a large number of surface modification techniques and thin
film synthesis methods. Synthesis implies that some sort of
chemical reaction occurs while depositing plasma constituents. A
few applications which are already known include:
[0015] Growth of high-quality GaN thin films. GaN is a wide bandgap
semiconductor with a number of applications such as blue
light-emitting diodes, flat panel displays, and high temperature
electronics applications. The constricted glow discharge plasma
source has been shown to be a key element in achieving the required
film quality by an MBE-type growth.
[0016] Gas streams in all versions of the source are fed to the
discharge chamber. The electric field there causes the gas to
become partially ionized and leaves the source through an
constriction element located upstream of the anode.
[0017] The streaming plasma contains only low-energy ions (lower
than 20 eV) because (1) the anode is positive, therefore attracting
electrons but decelerating ions, and (2) a relatively dense,
collisional plasma is formed in the upstream vicinity of the
constriction element, in which energetic ions and neutral atoms
lose their energy by collisions. The requirements of low ion energy
(a few eV or less) for GaN film growth is therefore fulfilled.
[0018] The plasma stream from the source is clean because (1) no
filament or other hot parts are used, (2) material sputtered by the
ion bombardment from the cathode is - with a very high probability
- deposited inside the discharge chamber since the output aperture
of the constriction element is very small, (3) the source can be
built of material tolerable or desirable to the specific
application. For instance, a source has been construed where all
plasma-facing components are made of high-purity aluminum
(electrodes) and high-purity aluminum nitride ceramics (insulator
parts). Such a source makes growth of high-quality thin films, such
as GaN, possible.
[0019] It has been found that cathodes made from titanium show
excellent long term stability when operating with nitrogen. A TiN
film is formed on the surface of the cathode as a result of a
chemical reaction of the cathode with the activated plasma
nitrogen. Titanium nitride is sufficiently conductive to sustain
the cathode function, but does not form an electrically insulating
film as AIN eventually does.
[0020] Similarly, stainless steel (e.g., SS 304L) has also been
used successfully as a cathode with oxygen because it does not form
an insulating oxide film.
[0021] In general, a criterion for the selection of a cathode
material for a given gas is that this material, if it reacts
chemically with the gas, does not form an insulating film on the
cathode surface. However, it may well form a conducting film.
[0022] An optimization of the configuration according to the
invention, in particular the cathode shape and material adds new
features to the source:
[0023] operation at an even wider range of pressures,
[0024] operating with reactive gases such as oxygen,
[0025] operating with unusual gases such as water vapor (for
example it might be desirable to include a pre-defined amount of
water or its constituents in films),
[0026] increased plasma output and stability, and
[0027] higher power and improved cooling.
[0028] A configuration according to the invention includes a source
of plasma having a hollow enclosure with a gas inlet opening and a
gas discharge opening. A cathode surface is exposed to an interior
space of the enclosure and an anode surface is located downstream
of a constriction element, a gas is fed to the gas inlet opening
and plasma is emitted from the discharge opening of the
constriction element. The shape of the opening of constriction
element can be round or elongated (rectangular). In other
configurations the source is fluid cooled (liquid or gas) and/or
the cathode is insulated from the surrounding environment, so that
all exterior surfaces of the source are safely at ground potential.
The cathode can be a hollow configuration to increase its surface
area, and the inlet gas passage can include a small inlet orifice
to prevent a discharge in the gas feed line. The geometric
relationship between the substrate and the source may be fixed or
variable during processing.
[0029] In another configuration according to the invention, two
plasma chambers are constructed in series; the first chamber feeds
the second chamber with plasma. This increases the stability and
density of the plasma emitted and reduces the probability that a
high energy particle will be generated and leave the second chamber
through its discharge opening of the constriction element.
[0030] The plasma source cells can also be configured in parallel,
to provide several plasma streams toward a substrate.
[0031] The source can be constructed and operated in a way that the
anode is remote from the small discharge opening of the
constriction element. This can be done, for instance, by applying
the negative potential to the cathode as previously described but
keeping the other source parts (such as housing and the other metal
parts adjacent to the blocking insulator) electrically floating.
The anode can be a separate ring or tube located downstream of the
main body of the plasma source. The anode can be attached to the
main body (forming a unit) or detached from it. In an extreme case,
the substrate, substrate holder, and the chamber wall can function
as the anode, and no dedicated anode part is necessary. In all
cases, the discharge is constricted to a small area ("discharge
opening of the constriction element.") such that a voltage drop
forms at this flow constriction. The flow of current between the
interior cathode and the exterior anode is concentrated at the flow
constriction. This causes plasma production as described above, and
the plasma formed at the flow restriction is blown to the substrate
by the pressure gradient.
[0032] The source can operate with all kinds of gases. Besides
nitrogen, sources have been tested running with argon, air, water
vapor, ammonia and oxygen. The latter is important, for instance,
for the deposition of oxide films. Since the source can operate at
a high pressure typical for sputter deposition of thin films,
plasma-assisted sputter deposition becomes feasible by combining
one or several constricted glow discharge plasma sources with
magnetron sputtering and laser ablation facilities. This could have
great impact on the deposition of oxide films for controlling the
passage of sunlight and electrochromic windows and deposition of
high temperature superconducting films such as yttrium barium
copper oxide. The use of the source configuration according to the
invention with oxygen promotes the enhanced incorporation of oxygen
in the surface layer, as the concentration of oxygen is usually too
low in the original as-deposited films.
[0033] A source according to the invention is well suited to
produce a nitrogen plasma used for the MBE growth of gallium
nitride films on heated substrates such as sapphire and silicon
carbide.
[0034] Other gasses excited to a plasma state can be used with a
source according to the invention.
[0035] Operation of the source with an inert gas such as argon at
low pressures can be useful for IBAD thin film deposition.
[0036] The lifetime of constricted glow discharge plasma sources in
a configuration according to the invention is much longer than
sources operating with hot filaments. This is generally true but in
particular when operating with oxygen, because hot filaments bum
easily in oxidizing environments.
[0037] A method according to the invention includes the steps of:
feeding a gas into a hollow discharge cell, applying a DC voltage
to form a plasma in the discharge cell including a small
constriction area such as to provide a downstream plasma flow with
low energy ions to help synthesize nitride films, such as gallium
nitride, on suitable substrates such as AIN, sapphire, and SiC, and
to synthesize oxide films such as tungsten oxide films on glass or
other substrates, and to synthesize a yttrium barium copper oxide
film by exposing such films to an oxygen plasma to obtain an
increased concentration of oxygen atoms in the film, and to assist
the growth of thin films such as metal films by providing low
energy ions such as argon ions.
[0038] A device according to the invention provides for
[0039] (1) Plasma assisted deposition of oxide films on large area
substrates, including dielectric substrates such as glass and
plastics (webs).
[0040] In one case, the source runs with oxygen as the feed gas.
The oxide films can be: (a) indium tin oxide, a transparent,
conductive coating (part of multilayer electrochromic films
(variable optical transmission) or heatable glass for luxury car
windows); (b) ion conducting oxide films such as tungsten oxide
(part of the electrochromic multilayer structure); (c) solar
control films (such as zinc oxide films); and (d) anti-diffusion
barrier films such as aluminum oxide (used, for instance, on
plastic packaging for potato chips; the coatings prevent the
diffusion of water and oxygen thus help to keep the food
fresh).
[0041] (2) Plasma assisted deposition of nitride films.
[0042] The source can operate with all gases, and when used with
nitrogen, nitride 25 films can be deposited.
[0043] A most interesting and promising version of the Constricted
Plasma Source (CPS) is a linear array of miniaturized discharge
cells. The arrangement as with sources in parallel was only
recently reduced to practice because each source in the earlier
configuration was too large in diameter to allow the necessary
close spacing needed for a homogeneous plasma. Close spacing of the
sources (discharge cells) is mandatory to obtain acceptable plasma
homogeneity along the array.
[0044] A device according to the invention may include a plasma
source in which the constriction is not circular but narrow slit
(or slot). This allows the formation of a "linear" plasma. However,
when the slit becomes very long (say, several inches), the plasma
tends to concentrate at some part of the silt thus the output is
not homogeneous along the whole slit length. Combining the
pencil-sized source with the quasi-linear array as mentioned
earlier, a quasi-linear source with acceptable homogeneity could be
reduced to practice for the first time.
[0045] The term "quasi-linear CPS" refers to a multicell
Constricted Plasma Source whose discharge cells are aligned and
operate with constricted glow discharge cells with small circular
constrictions. The term "linear CPS" refers to a multicell
Constricted Plasma Source whose discharge cells are aligned and
operate with constricted glow discharge cells having slit-shaped
constrictions.
[0046] A device according to the invention includes an elegant and
compact small diameter source design. This miniaturized source can
be used in two ways. First, it can be used as a stand-alone device
for instance in the geometry required for ultra-high-vacuum
molecular-beam-epitaxy deposition facilities. Second, the small
diameter of a plasma source cell allows several of them to be
arranged closely spaced in a linear fashion, forming the
quasi-linear CPS, as discussed earlier.
[0047] The miniaturized discharge cell can also have a slit rather
than a circular hole as the constriction. The slit would be
relatively short (some mm) in this case because the overall
discharge cell is small. Many of these smaller slit sources can be
arranged to form a linear plasma source of arbitrary length.
[0048] The cells of a quasi-linear or linear CPS can be tuned to
improve homogeneity of the plasma process such as the deposition of
a compound film. Moreover, a desired or programmed output profile
can be obtained. A flat, homogenous and constant output profile is
one example of a possible desired output.
[0049] (1) The cells of the CPS can be operated independently from
each other by using
[0050] (a) independently controlled power which may, for instance,
be obtained by operating individual power supply modules in current
limit mode
[0051] (b) independently controlled individual gas feeds Methods
1(a) and 1(b) can be used together or independently to achieve a
desired output profile.
[0052] (2) The distance between individual source cells can be
adjusted and optimized for a desired profile. For instance, if a
homogeneous plasma profile is desired, the cells in the center of a
quasi-linear source are nearly spaced equally but the distance
between cells decreases the closer a cell is at one of the ends of
the arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 is a cross sectional schematic view of a prior art
hollow-anode ion beam source;
[0054] FIG. 2 is a cross sectional schematic view of a constricted
glow discharge plasma source with all exterior surfaces at ground
potential, in a configuration according to the invention;
[0055] FIG. 3 is an end view of the source as shown in FIG. 2;
[0056] FIG. 4 shows a cross sectional close-up view of a gas feed
passage of the type used for the source of FIG. 2 showing an
orifice in the passageway into the interior space of the source
formed by a plug positioned in the end of the passage;
[0057] FIG. 5 shows a cross sectional close-up view of the gas feed
passage of the type used for the source of FIG. 2 showing an
orifice in the passageway into the interior space of the source
formed by an insulating ring/disk between the cathode and the
member adjacent to it;
[0058] FIG. 6 is a cross sectional schematic view of a constricted
glow discharge plasma source with a hollow-cathode with multiple
constriction apertures and a magnetic field coil, according to the
invention;
[0059] FIG. 7 is a cross sectional schematic view of a constricted
glow discharge plasma source with a hollow-cathode with both ends
at ground potential, in a configuration according to the
invention;
[0060] FIG. 8 is a cross sectional schematic view of a constricted
glow discharge plasma source in a dual stage configuration
according to the invention;
[0061] FIG. 9 is a cross sectional schematic view of a
quasi-linear, constricted glow discharge plasma source with
hollow-cathodes according to the invention;
[0062] FIG. 10 is a cross sectional schematic view of a constricted
glow discharge plasma source where the cathode is inside a gas feed
chamber while the anode is outside the enclosure and is connected
to the wall of the chamber;
[0063] FIG. 11 shows a constricted glow discharge in a hollow-anode
configuration with a slit opening;
[0064] FIG. 12 shows a principal set up for coatings of dielectrics
such as webs with a linear constricted plasma source; the metal
comes from a metal source such as an evaporator or sputtering
source;
[0065] FIG. 13 shows a schematic cross sectional view of a
pencil-sized source according to the invention;
[0066] FIG. 14 shows the pencil sized plasma source of FIG. 13
connected to a gas feed source fitting;
[0067] FIG. 15 shows a side view showing the spacing of a linear
array of pencil sized plasma sources;
[0068] FIG. 16 shows schematic cross sectional view of a
constricted glow discharge cell with flat anode. The opening can be
a hole or slit, and the end plate containing the discharge
constriction can be made from an insulator material but is
preferably a metal;
[0069] FIG. 17 shows the constricted glow discharge of FIG. 16 with
an auxiliary glow discharge connection for improved reliability of
plasma ignition;
[0070] FIG. 18 shows constricted plasma source with auxiliary anode
for the treatment of dielectric substrates; and
[0071] FIG. 19 shows a constricted plasma source with a magnetic
field coil.
DETAILED DESCRIPTION
[0072] Structures and methods according to the invention include
several versions of constricted glow discharge plasma sources. In
contrast to Miljevic's high energy focused electron and ion (beam)
source, the anode is the most downstream electrode and kept at
ground potential to generate a flow of low-energy plasma.
[0073] As shown in FIGS. 2 and 3 the anode plate 50 is grounded
because the substrate (not shown) is usually grounded. This is to
avoid creating an electric field between the source and the
substrate which would accelerate the ions in the plasma. Having the
anode 50 grounded defines one boundary condition of the
configuration. Consequently, the cathode 52 has to be negative (a
potential less than ground). Therefore the cathode 52 cannot be
directly mounted to the chamber or to a holder connected to the
chamber because the chamber is always grounded (for safety
reasons). The cathode 52 has to be electrically insulated from the
surrounding metal parts 50 and 54.
[0074] In one configuration the outside surface of the body of
source is kept at ground, the same potential as the chamber, so
that all kinds of holders and clamps can be attached to the body
54. In the configuration of FIGS. 2 and 3, the source body 54 is
grounded and separated from the cathode 52 by a thin ceramic disk
56 (e.g., made of AlN approximately 0.25 mm thick) and by a
surrounding cylindrical sleeve 58 (preferably made of stainless
steel).
[0075] The anode 50 is separated from the edge of the hollow
cathode 52 by a ceramic disk 60. The ceramic disk 60 acts as an
anode insulating barrier covering all but a small portion of the
anode. A discharge opening 62 (0.05-1.0 mm) provides a passage
through the disk 60 and is the origin of a low energy plasma stream
(not shown).
[0076] At the discharge end of the source 48 the anode 50 is bolted
to the body 54. A magnetic member 64 can be positioned outside the
anode 50. The magnetic member 64 can be an electro-magnet or a
permanent magnet. The magnetic field will enhance the plasma
production at the discharge opening 62 and reduce the expansion of
the plasma as it exits the anode discharge opening 62. The cone 63
of the anode discharge opening 62 form a large angled (90 degree or
wider angle) nozzle at the end of the discharge passage 65 through
the anode 50. The shorter the discharge passage 65 and the wider
the discharge cone 63 the better the plasma production flowing in
the direction shown by an arrow 66.
[0077] The function of the magnet member 64 here is to magnetically
insulate the anode 50 to enhance the voltage drop and electric
sheaths thickness located at the constriction opening. This is
achieved by using an axial magnetic field, i.e. the magnetic field
lines are aligned with the axis of symmetry of the discharge
cell.
[0078] The gas inlet tube 70 provides gas to the interior hollow
space 72 of the body 54 and the center of the hollow cathode 52. As
gas travels through the inlet passage tube 70, a restriction to gas
flow is imposed by the hole 57 (0.05-1.0 mm in dia.) in the
insulating disk 56. The disk 56 can be configured as shown in FIG.
2, or in an alternate configuration as shown in FIG. 5. In FIG. 5
an inlet tube 70' closely approaches an insulating disk 56' such
that an inlet hole 57' acts as an orifice to establish a high
pressure on the upstream side to prevent plasma from forming in the
inlet tube 70'. According to Paschen's Law, the higher the gas
pressure the greater the voltage that will be needed to ionize the
gas. The separation of regions of high and low pressure by the
orifice 57' assures that ionization will only occur on the low
pressure side of the orifice and back burning of the feed passage
will thereby be prevented.
[0079] A similar function is performed by the configuration as
shown in FIG. 4. A gas inlet tube 70" is fitted with an orifice
plug 74 having an orifice 75 (0.05-1.0 mm in dia.) to create the
differential pressure required to prevent back burning. On the high
pressure side, electrons collide often elastically with gas
molecules and thus cannot gain sufficient energy between collisions
to allow them to ionize. Thus, plasma is not formed in the high
pressure region. This backburning situation does not occur in the
prior art as the polarity there is different.
[0080] The inlet end of the source 48 also includes a power supply
post 76 connected to the cathode 52 through an insulating sleeve
78. A DC power supply 80 supplies power to the post 76. The source
body 54, (preferably made of stainless steel) includes an annular
channel 55 enclosed by an annular channel-enclosing cylinder 67. A
water inlet tube 68 and a water outlet tube 69 (FIG. 3) circulate
water (or other cooling liquid) through the annular channel 55,
through a set of water cooling passages (e.g., 71) only one of
which can be seen in FIG. 2. The cathode temperature during
operation is estimated to be 100 degrees Celsius.
[0081] FIG. 6 shows a configuration according to the invention
having several hollow cathode spaces (e.g., 84) in the cathode 86.
Each hollow cathode space (e.g., 84) faces a constriction opening
(e.g., 88). As configured in FIG. 6 there are 6 constriction
openings generally equally spaced along the circumference of a
circle (a seventh constriction opening of the set is configured at
the center axis of the cathode and cathode is not shown for
clarity). Each constriction opening (e.g., 88) has a discharge cone
(e.g., 90). A magnetic member 92 is shown surrounding the outside
of an anode 94.
[0082] Gas for the plasma is supplied to a gas inlet 96 through a
gas inlet tube 98 through a grounded backing member 100 and a
ceramic (e.g., AlN or Al.sub.20.sub.3) insulating disk 102 into a
cathode inlet plenum 104. From the inlet plenum 104 each hollow
cathode space (e.g., 84) is connected to the plenum 104 by its own
connection passage (e.g., 106). The hollow space (e.g., 84)
increases the surface area of the cathode 86 exposed to the gas
flow and subject to the electric field between the cathode 86 and
the anode 94. An insulating constriction disk 108 having 6
constriction openings (or 7 if the unillustrated center
constriction opening is included) therein matching the location of
the constriction openings (e.g., 88) in the anode covers
substantially all of the anode 94. Power to the cathode is supplied
through a power connection 87. Plasma produced at the location of
the passages and constriction openings (e.g., 88) travels in the
direction of the arrows (e.g., 110).
[0083] When using a hollow cathode configuration as shown in FIG. 6
and 7, the source will operate more efficiently and with greater
stability (constricted glow discharge plasma source with hollow
cathode). The utilization of the hollow-cathode effect (i.e.,
electrons oscillate in the cathode cavity due to electrostatic
reflections from the cathode sheath, and thereby increase the
probability of colliding inelastically with gas molecules to create
ions as the cavity is repeatedly crossed) significantly increases
the generation of plasma over a configuration with a flat faced
cathode.
[0084] An advantage of using a hollow cathode is that the pressure
range in which the source can operate, is widened. Different
embodiments have been tested which operate in the very wide range
of 9.times.10.sup.-6 Torr to 0.5 Torr (pressure measured outside
the source, i.e. in the vacuum chamber).
[0085] FIG. 7 shows a configuration of a hollow cathode 114 for use
with a configuration according to the invention. The large cathode
area enhances the generation of a plasma so that a plasma can be
initiated and supported at lower voltages than if the surface area
of the cathode were smaller. The use of a lower voltage reduces the
energy of ions generated in the second chamber.
[0086] The small diameter (.about.0.5 to 1.0 mm) of the
constriction opening 118, further reduces the probability that high
energy ions will leave the source. A ceramic inlet tube 121
supplies gas to the hollow cathode space 116. The gas, in plasma
form, is discharged through the discharge hole 118. The hollow
cathode 114 is insulated from the back plate 120 by an insulating
disk 122 (in this configuration made of a ceramic material). The
anode 124 is separated from the hollow cathode 114 by an anode
insulating barrier 126, made of a ceramic material approx. 0.25 mm
thick and having a hole to match the constriction opening 118 of
the source. The surrounding wall 130 is a hollow ceramic cylinder
(preferably made of AlN). The hollow cathode 114 is powered by a
power supply connection 128.
[0087] In a constricted glow discharge plasma source configuration
as shown in FIG. 8, a second discharge chamber is added to the
"single-stage" constricted glow discharge ("dual-stage constricted
glow discharge plasma source"). Gas is supplied through a feed
passage 138 (here pictured as a ceramic material) to a first
chamber 144 through a 1st cathode 140. An electric field is created
in the first chamber 144 by the difference in potential between the
first cathode 140 and an intermediate anode/cathode electrode 142
which is insulated from the first chamber 144 by a first insulating
anode barrier (constriction element) 148 causes the gas injected
therein to form a plasma. The first chamber 144 includes an
intermediate constriction opening 146 which routes the plasma from
the first chamber 144 to the second chamber 150. The electric field
in the second chamber 150 (created by the difference in potential
between the intermediate anode/cathode electrode 142 and a second
anode 152) excites the gas in the second chamber 150 to form a
plasma. The second anode 152 is covered by a second insulating
constriction opening 154.
[0088] In this dual stage configuration, the additional (first)
chamber can be any source for a gas discharge (plasma), but a
constriction glow discharge chamber is preferred. The discharge
chambers are connected in such a way that the gas is first fed to
the additional chamber. The plasma from the first discharge chamber
(or cell) 144 streams into the main chamber 150 of the constriction
glow discharge, driven by a pressure gradient. The purposes of the
additional chamber are (1) to inject plasma into the main
constriction glow discharge (one may consider the first chamber as
a replacement of the filament cathode), (2) to provide a plasma
source to initiate the main discharge, and (3) to reduce the
anodecathode voltage drop in the main chamber and reduce the
associated cathode sputtering.
[0089] The benefit of the two inline chambers is that the
downstream chamber (i.e., 150) can burn at a lower voltage because
the plasma making is promoted by the plasma streaming in from the
first stage. The burning voltage between the cathode 142 and anode
152 in the final stage (e.g., 150) can be smaller than usual,
because this discharge is supported by a plasma input stream coming
from the first stage (e.g., 144).
[0090] There is also a process benefit, if problems are encountered
with contamination coming from a sputtered cathode material, caused
by the excessive sputtering of the cathode which depends strongly
on the magnitude of the energy of the ions hitting the cathode. The
magnitude of the energy can be reduced by reducing the burning
voltage. When this is done with a single stage source, the plasma
production will go down and it will eventually stop completely.
[0091] In the two stage configuration it is possible to keep the
plasma production alive because plasma is fed to the second chamber
(e.g., 150), so the voltage of the final stage 150 can still be
reduced and still have enough plasma to provide a reasonable
process rate. This configuration provides the benefit of reducing
contamination while being easier to operate and having a higher
output plasma density.
[0092] Constriction glow discharge plasma sources with a hollow
cathode can be built as single or dual-stage point sources or as
quasi-linear or multiple point sources as shown in FIG. 9.
[0093] Most sources are cylindrically shaped. The quasi linear
source as shown in FIG. 9 has a rectangular cross section.
[0094] FIG. 9 shows a quasi linear source would be here the design
of choice since large area substrates (window glass) are to be
treated.
[0095] FIG. 9 shows another version of the constricted glow
discharge plasma source, a quasi-linear constricted glow discharge
plasma source 158. It is characterized by using several individual
constricted glow discharge plasma source cells 160 which are
closely aligned in a row.
[0096] The source can behave even more like a linear source by
using constriction openings which are elongated ("slots") along the
row.
[0097] For convenience a common "quasi linear" anode 162 with
individual cathodes (e.g., 164) is used, separated from adjacent
cathodes and grounded members by a series of insulators 170a,b,c.
For stability reasons, each cathode (e.g., 164) has its individual
load resistor (e.g., 166) (or power supply unit operating in a
current mode), but conveniently a single power supply 168 can be
used. The advantage of a "quasi linear" source is that large
substrate areas can be homogeneously treated with the outwardly
streaming plasma by using a one-dimensional motion of the substrate
(perpendicular to the direction of line of the constriction
openings). The individual feed lines 174 to each chamber assure
equal burning at the discharge of each chamber's constriction
opening 176.
[0098] Typically the sources described above range from 1.5 to 4
inches in diameter and are about 3 inches in length.
[0099] As shown in FIG. 10, the source 180 and substrate 182 can be
stationary or can be moved as shown by the arrows 184 during
processing to process the full area of the substrate 182 although
the source 180 and substrate 182 in a stationary position can
deposit on a less than full area of the substrate being processed.
It is also possible to scan the plasma flow across the substrate by
magnetic field coils (not shown). The distance between the source
180 and the substrate 182 can be adjusted by bellows 188, while the
substrate 182 and substrate support 190 include mechanisms (the
details of are not shown but are understood by persons of ordinary
skill in the art) to move the substrate 182 in a predetermined
pattern. In this configuration only the cathode is powered by the
power supply 192 and the cathode container 196 is allowed to float
while a plasma 198 is allowed to stream from the constriction
opening 200 of the container 196 toward the substrate 182 and
substrate support 190 which are both grounded.
[0100] While the Figures show the plasma source to be mounted in a
horizontal configuration, a vertical or other angled mounting is
possible as the effect of gravity on the process, if any, is
negligible.
[0101] In one configuration, where the growth of gallium nitride is
intended, the distance between the source and the substrate is
approximately 4 to 5 inches. The pressure in the processing chamber
is in the low 10.sup.-4 Torr range, and all gas injected into the
processing chamber comes in through the plasma source. The gas is
pure nitrogen injected at a flow rate of approximately 5-50
sccm.
[0102] The output of the source can be increased by using an axial
magnetic field. Experiments show that this is true for sources with
a single constriction opening as well as with several openings.
[0103] All above described sources can be operated with an external
magnetic field which helps to stabilize the discharge and increases
the plasma output.
[0104] All above described sources can be built in such a way that
they are UHV (ultra high vacuum, i.e. residual pressure 10.sup.-9
Torr or less) compatible and made of the materials acceptable to
the specific application. This implies, for instance, to avoid
plastic materials and O-ring sealing; instead, only UHV metals such
as stainless steel, aluminum, and ceramics such as AlN and
Al.sub.20.sub.3 are used.
[0105] The cathodes of the sources shown in FIG. 2-10 are
indirectly cooled via heat conduction through thin insulating
ceramics such as AlN. It is principally possible to implement
direct cooling of the cathode by a cooling liquid such as water.
This is only necessary for very high power levels. The construction
with direct cathode cooling requires the use of electric breaks
separating the cathode potential (negative) from ground. Care must
also be exercised to keep the gas supply insulated from the power
source.
[0106] A constricted glow discharge plasma source has been used to
form lithium nitride on lithium battery electrodes by nitrogen
plasma immersion ion implantation to form a nitrogen-enriched
lithium surface.
[0107] Where a gallium nitride film is to be grown using a nitrogen
gas it is preferred that the substrate be a crystalline material.
The crystalline material may be such that it has a lattice constant
generally matched to that of the substrate such as SiC and bulk
GaN. Or the crystalline material may be such that it has a lattice
constant generally not matched with that of the substrate such as
sapphire.
[0108] Other nitride films might also be synthesized. It may be
possible to assist the growth of carbon nitrides using the
constricted plasma source.
[0109] A method according to the invention may include use of the
structure and method according to the invention while
simultaneously bombarding said substrate with a stream of material
for growth of film on the substrate. The source then acts as a low
energy ion beam assist to epitaxy deposition. In this configuration
the low energy beam reduces the likelihood of defect formation,
which higher energy ions have a high probability of producing.
[0110] A further development of the constricted glow discharge, and
combination of the discharge with extractor systems can lead to a
variety of ion and electron sources. Another way of obtaining
energetic particles (e.g. keV range of energy) is to bias the
source or substrate in order to obtain an accelerating electric
field; adjustments can also be done by magnetic fields. Thus the
source is also applicable to materials synthesis and modification
where energetic particles are required.
[0111] The Hollow-Anode Discharge is characterized by a small
constriction (usually a hole of about 1 mm diameter) with an
adjacent anode with an aligned hole (the hole is "hollow"--that's
why the name "hollow anode discharge.")
[0112] The hollow-anode discharge is used for materials processing
such as the epitaxial growth of gallium nitride. The hollow anode
plasma source can operate with all gases, including oxygen.
[0113] For some practical purposes of large-area coatings, at it
desirable to have a linear source (i.e., the plasma output comes
from a long slit rather than a hole), to move the to-be-coated
substrate perpendicular to the slit. As discussed above, a
quasi-linear source can be realized by aligning the output holes of
several sources (FIG. 9).
[0114] This solution is straight forward once a single-hole source
has been shown to produce the desired plasma. A single source can
also operate with several holes. However, it has been found
experimentally that the discharge will not utilize all holes in an
equal manner but operate preferentially with one or two holes.
[0115] With the need to implement an oxygen-plasma assisted
deposition process for the deposition of large-area conductive
transparent films (like indium tin oxide), solar control films
(like zinc oxide and tungsten oxide) on glass and plastics (webs),
and anti-diffusion barrier films (like aluminum oxide films) on
packing materials, the development of a linear source is
needed.
[0116] A linear source with a slit opening is shown in FIG. 11.
Extending the ideas and principles discussed above, a very thin
slit having a width of a fraction of a millimeter for both the
ceramic plates (blocking most of the anode area) and the adjacent
anode (FIG. 11) was built and aligned.
[0117] FIG. 11 shows a constricted glow discharge in a hollow-anode
configuration with a slit opening 200. A gas feed 204 extends
through a block cathode 202 to feed gas into an internal chamber
216 that is closed on the top by a top insulator 206 and on the
bottom by a bottom insulator 208. An insulating anode barrier with
slit 210 insulates the bulk of the anode with slit 212 from direct
exposure to the internal chamber 216. The arrow 214 shows the
direction of plasma flow through the slit.
[0118] FIG. 12 shows a web substrate 224 that is wound in the
direction shown by the arrow 230 from a bottom roller 226 to a top
roller 228 during processing. Facing the web substrate 224 is a
sputtering source or evaporator 220 adjacent to a linear
constricted plasma source 222 having a slit 223 facing the web
substrate 224. The linear constricted plasma source 222 can be
constructed, as space permits, from a constricted glow discharge
source 220 such as shown in FIG. 11.
[0119] FIG. 13 shows a "Pencil-size Constricted Plasma Source." It
is a novel design of the "constricted-plasma source" or
"hollow-anode plasma source" characterized by its extremely small
size (pencil size) and simple construction while maintaining the
output of older versions.
[0120] The new features of the pencil source are made possible
by
[0121] use of a metallic gas feed as a hollow cathode
[0122] integrated insulator body, in particular a low cost glass
(Pyrex) or quartz housing
[0123] metallized insulator housing acting as an anode
[0124] small housing diameter and constriction size.
[0125] The device can be made from high temperature materials which
are coated with noble metals such as gold, platinum and palladium.
Such sources can use oxygen as the gas supply. Tungsten will not
work with oxygen as it will oxidize, but will work with
nitrogen.
[0126] The re-designed source allows operation down to very low gas
pressures (1.times.10.sup.-5 Torr), thus making true MBE (molecular
beam epitaxy) conditions of thin film growth possible. The most
prominent applications is the growth of MBE gallium nitride. Other
applications include the plasma-assisted deposition of metal oxide
films for solar control, electrochromic devices ("smart windows"),
packaging materials, etc.
[0127] The difference between this and the previous versions are
that
[0128] the source can be made much cheaper (very simple, compact
construction) than any other earlier version or comparable
source.
[0129] due to its very small diameter, the source can be brought
closer to the substrate than any other source without obstructing
the flux of other reactive material such as gallium evaporated from
a Knudsen cell. As a result, the density of activated species
(atoms, ions, and excited molecules) is much higher, and the growth
rate can be higher.
[0130] the pencil-size source can be used as a module to be
assembled in large source structures such as a quasi-linear source.
For instance, a linear array with about one "pencil source" per
inch could be built to arbitrary length, allowing the processing
(like metal oxide deposition) on large-area substrates such as
window glass or webs (plastic film material).
[0131] FIG. 13 shows a pencil-sized plasma source having a metallic
gas feed hollow cathode 244 (one example having an outside diameter
of 0.25 inches (6.35 mm) with an inside diameter of 0.1875 inches
(4.76 mm) and being made of either stainless steel or titanium. The
choice of material depends on whether oxygen or nitrogen is used as
the gas to generate the plasma and avoid contamination of the
substrate being used. A glass housing 246 with a closed end
constriction 247 slips over the end of the hollow cathode 244 to
create structural support as well as an insulating barrier between
the hollow cathode 244 and a grounded coating or case 248 which has
been applied (for example by sputtering) or slipped over (as a
metallic tube) the end of the glass housing 246. The grounded
coating or casing 248 is connected to ground 254. A water jacket
252 surrounds to the feed end of the pencil size source 240 to
reduce operational temperatures. (In an example not shown, the
construction of the pencil size source has eliminated the use of a
water jacket such that cooling by radiation is sufficient to keep
the structure below a distortion temperature.) A water inlet 256
feeds the water jacket 52 and a water outlet 258 provides an outlet
for the circulating cooling water. While the end constriction 247
in the glass housing is pictured as a circulator opening, it may be
a narrow slit as well.
[0132] In practice, implementation of the pencil size source 240 as
shown in FIG. 13 requires that it be connected to a gas feed as
shown in FIG. 14 with a ceramic disk with orifice 262 located in
the gas feed at or near the location of a gas inlet flange or
fitting 260. The purpose of the ceramic disk with orifice 262 is to
prevent blow back (back burning) of the plasma which is generated
in the hollow cathode arrangement described. The gas supply tube is
fitted to the gas inlet flange or fitting 260 to provide a gas
source to the pencil size plasma source.
[0133] FIG. 15 shows an approximate variation in the spacing
between adjacent cells when using a pencil size plasma source. The
individual cells 270 through 282 in the linear array have their
discharge orifices linearly aligned to provide a cone of plasma
(for example 270a) which at a moderate distance from the source
combine to create a continuous substantially uniform plasma density
(flux) along the linear array. However, so that the array does not
have to be overly long when tracking relative to a substrate, the
spacing between cells (for example centering spacing 286, edge
spacing 288, and middle radius spacing 290) vary according to data
which are theoretically approximated by the assumption of an
asymtotic function (the change in the magnitude of flux density is
nearly negligible as the flux measurement approaches the center of
the substrate and drops off severely at the edge). The idea is to
provide uniform plasma density all the way to the edge of the
substrate without having to extend the plasma source too much
beyond or at all beyond the edge of the substrate at which the
plasma is being directed.
[0134] The ideal spacing distance between adjacent cells is a
continuously increasing and decreasing gradation, with wide spacing
and small changes in spacing at the center of the substrate track
and narrow spacing and large changes in spacing at the edge of the
array-with a gradual decrease in spacing towards the edge of the
substrate. Empirical measurements establish the exact spacing
needed to achieve the desired plasma flux profile. In general the
desired plasma flux profile is approximately uniform so that
variations in film thickness across a substrate does not vary more
than 2 to 5 percent of the total thickness from highest to
lowest.
[0135] There may be instances where a non-uniform flux is desired.
The ability to closely space the cells is easily achieved using the
pencil size plasma source described above, while such close spacing
would be difficult if the earlier discussed hollow plasma anode
source with its large body construction were used.
[0136] The precision of machining and assembly required to
successfully construct a slit opening of a hollow anode source is
difficult to achieve. A much better solution is to make use of
another version of the constricted glow discharge (a version which
is not a hollow-anode discharge). In this version, the slit can be
made of a conductive material (i.e., no blocking ceramic plate is
necessary), and the anode can be any close by electrode (like the
chamber wall, the substrate, or an auxiliary electrode structure).
In particular, the anode can be the chamber wall (ground) or a
conductive substrate, thus the anode is not hollow in this
configuration (FIG. 16).
[0137] The construction of the bare bones constricted glow
discharge cell which was described earlier, is shown in a
simplified form in FIG. 16. A gas feed 302 extends through a
cathode 304 to which power is supplied through a power supply cable
306. An insulator housing 308 separates the cathode 304 from a
metal flat end plate 310 having a hole or slit (constriction) 312
therein. Plasma flow from the constriction 312 towards the anode
318 which is connected to ground 320 is shown by an arrow 316.
[0138] It was found experimentally that when using a the
constricted glow discharge with a long slit, the plasma tended to
be concentrated in one or several spots along the slit, so the
plasma density is not homogeneous. Thus the re-configuration of a
constricted glow discharge to operate acceptably with a slit is
non-trivial. The wider and longer the slit, the higher will be the
flow of gas. The stable regime of operation (i.e., plasma appearing
substantially homogeneously along the slit) is related to the slit
dimensions and flow and pressure settings: the larger the slit
dimensions the higher must be the flow to maintain the pressure
needed for the discharge inside the source. However, there is a
contrary effect as well: a longer slit requires that the discharge
inside the source operate the at a lower pressure than with short
slit or hole (otherwise the discharge will contract and the plasma
will appear concentrated at only one or two locations of the slit).
Low pressure operation implies larger discharge cell dimensions
(which is consistent with a long slit). However, it was found that
homogeneous plasma production can be achieved if the source is
operated at sufficiently supply high voltage combined with a
suitable gas flow which depends on the slit geometry (length and
width). The dimensions of a slit are: length (for one discharge
cell) 5 mm-100 mm; width 0.1-2 mm; preferred (tested) size: length:
20-50 mm, width 0.2 mm (a length to width ratio in of at least 50
to 1 is preferred). For instance, one example of successful
operation (i.e. homogeneous plasma production) was achieved for a 2
cm slit length (width=1.0 mm) at 1 kV supplied voltage and a flow
rate of 100 sccm. In another experiment with 6.4 cm slit length
(width=0.20 mm), homogeneous plasma production was observed for a
voltage of 3 kV at a flow of 34 sccm.
[0139] The name "hollow-anode plasma source" is not appropriate
anymore since the anode is flat. This discharge is called
"constricted glow discharge" because the ionization effect is due
to an electric double sheath (layer) forming at the
constriction.
[0140] The transition from the cell of FIG. 11 to FIG. 16
represents not only a simplification in manufacturing the source
but also results in a more intense plasma (the measured ion
saturation current can be higher up to a factor 10, which can be
partly attributed to the different opening (nozzle) shape).
[0141] A problem appears when the anode is too remote from the
source body (say, more than 15 cm away: the discharge will not
ignite (at least not with a reasonable voltage of 1-2 kV). A
solution to the failure to ignite is to make the constriction from
conductive material (a metal such as stainless steel) and connect
the constriction to the anode potential (usually ground) via a
high-Ohm "starting" resistor. The resistance must be larger than
the external resistance of the discharge (which is about 10
k.OMEGA.) but small enough to allow the ignition of an auxiliary
glow discharge between the cathode and the constriction. Resistor
values between 50 k.OMEGA. and 1 M.OMEGA. have been successfully
tested. The main constricted glow discharge starts each time when a
starting resistor is used (FIG. 17) but does not start without
it.
[0142] FIG. 17 provides an enhanced initiation of the plasma of the
constricted glow discharge with flat anode 300 as shown in FIG. 16
by providing a cell end plate resistor to ground 328 (having a
value of 10 k-ohms to 2 M-ohms (greater than 10 k-ohms or greater
than 100 k-ohms)) to assist in initiating the plasma but having if
no effect once the plasma is initiated so that the plasma flow as
shown by the arrow 316 is unimpeded towards to anode 318. The power
source 324 is connected through a power resistor 326 (having the
external resistance of 10 k-ohms) to the cathode 304. Ground wire
connects the positive terminal of the power supply 324 to the
ground 320.
[0143] In the constricted glow discharges of FIGS. 16 and 17, the
substrate or a nearby chamber wall acts as the anode. This setup is
not suitable for large, dielectric (i.e., non-conductive)
substrates such as glass and plastic sheets (webs). Another setup,
shown in FIG. 18 was successfully tested for this situation: a
wide-slit auxiliary anode is closely placed downstream of the
constriction. For example, the 6.4 cm slit source was equipped with
an anode located 3 mm downstream, having a width of 6 mm. The
dimension of the anode is are not very critical, in contrast to the
constriction dimensions which are critical because they determine
the pressure difference inside vs. outside the source, flow speed,
and ionization processes.
[0144] FIG. 18 shows the enhanced configuration of the constricted
glow discharge with flat anode plasma source of FIG. 17 with a
different anode configuration. In this configuration, an anode 336
has a hole or slot matching the hole or slot 312 in the end of the
constricted end plate 310 such that acceleration of the plasma ions
takes place to create a plasma flow as shown by arrow 338 towards a
substrate 340 which may be a dielectric or any conductive or
non-conductive surface suitable for receiving a film coating.
[0145] For the deposition of oxide or nitride films, a flux of
metal vapor (ions or neutrals) is necessary, i.e., the substrate is
bombarded by both metal species and gas species (the latter from
the constricted plasma source); the substrate is usually moved
perpendicular to the slit to allow for large-area coatings for
example as shown in FIG. 12.
[0146] The plasma output of the linear constricted plasma source
can be considerably enhanced (by a factor of 2-3) by using an axial
magnetic field as shown in FIG. 19. A field strength of 10 mT is
sufficient to achieve a significant effect. The magnetic field can
be obtained using a electromagnetic coil (as shown in FIG. 19) or
can be formed by permanent magnets.
[0147] FIG. 19 shows the constricted glow discharge with flat anode
cell arrangement as shown in FIG. 18, the components inside the
dashed cloud shows similar components while the power supply is not
shown in FIG. 19. A magnet field coil 344 surrounds the chamber of
the constricted flow discharge cell to enhance the discharge of
plasma as shown by the plasma flow arrow 346 towards the substrate
being processed 340.
[0148] All other features described apply as well: the linear
constricted plasma source can be built as a dual-stage plasma
source, a grounded housing can be built around the source the
encapsulate all parts at high potential (cathode), the electrodes
can be water or gas-cooled, depending on the power level and
specific design (flange-mounted versus vacuum-mount). The cathode
can have a hollow shape utilizing the hollow-cathode effect such as
to allow stable operation in a wide range of pressures, included
relatively low chamber pressures (for instance, operation in the
10.sup.-5 Torr range). The source can be built in such a way that
it is ultrahigh vacuum (UHV) compatible by using suitable materials
(avoid plastics, rubber O-rings, etc., for instance).
[0149] The invention includes a method for producing a plasma from
a plasma source including the steps of: using a feed tube to supply
gas and DC power to an end of the feed tube which is connected to
an insulating sleeve connected to the end of the feed tube and
extending beyond the end of the feed tube; providing an inlet
orifice requiring gas to pass through the inlet orifice to flow
into the feed tube; providing an constriction orifice in an end
wall of the insulating sleeve; providing a metallic cover over an
outside surface of an end of the insulating sleeve at a location
beyond and separated from a location of the feed tube; connecting a
negative voltage of a DC power source to the feed tube; connecting
the metallic cover to an electrical ground which is connected to a
positive voltage of a DC power source; energizing the DC power
source; and supplying gas to the feed tube.
[0150] Alternate methods include the steps of: providing gas to a
hollow chamber through an inlet orifice of a cathode located at a
feed end of the hollow chamber, where side walls of the chamber are
made of an electrically insulating material and a discharge end of
the chamber is a metal plate having a constriction orifice therein;
placing an anode at a location outside of the hollow chamber
opposite the metal plate; and connecting the cathode to a negative
voltage of a DC power source and connecting a positive voltage of
the DC power source to the anode. The anode may be a substrate to
which plasma is directed. The metal plate may be electrically
connected to the positive voltage of the DC power supply through a
10 kilo ohm or greater resistor. The metal plate may be
electrically connected to the positive voltage of the DC power
supply through a 100 kilo ohm or greater resistor. The metal plate
may be electrically connected to the positive voltage of the DC
power supply through a 10 kilo ohm or greater resistor. The anode
may include a hole opposite the constriction orifice; and further
comprising the step of: placing a substrate to be processed on a
side of the anode opposite from the hollow chamber and opposite the
hole in the anode. The constriction orifice may be a slit.
Additional steps may include placing at least two hollow chambers
adjacent to one another; aligning a discharge direction of each of
their constriction orifices in an approximately linear pattern; and
arranging the spatial relationship between each of the at least two
hollow chambers to provide a homogeneous plasma flux on a substrate
placed opposite from the constriction orifices of the at least two
hollow chambers.
[0151] While the invention has been described with regards to
specific embodiments, those skilled in the art will recognize that
changes can be made in form and detail without departing from the
spirit and scope of the invention.
* * * * *