U.S. patent application number 10/199086 was filed with the patent office on 2002-12-26 for multi-anode device and methods for sputter deposition.
Invention is credited to Burton, Clive H., Pratt, Rodney, Samson, Frank.
Application Number | 20020195332 10/199086 |
Document ID | / |
Family ID | 24423508 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020195332 |
Kind Code |
A1 |
Burton, Clive H. ; et
al. |
December 26, 2002 |
Multi-anode device and methods for sputter deposition
Abstract
A method and apparatus for vacuum coating plural articles
employs a drum work holder configuration and a sputter source with
a plurality of individually controlled anodes for effectively
providing uniform coatings on articles disposed at different
locations on the drum work holder. A small number of measured
process parameters are used to control a small number of process
variable to improve coating uniformity from batch to batch.
Inventors: |
Burton, Clive H.; (Novato,
CA) ; Pratt, Rodney; (Crafers, AU) ; Samson,
Frank; (Macclesfield, AU) |
Correspondence
Address: |
Samuel C. Miller III
BURNS, DOANE, SWECKER & MATHIS, L.L.P
P.O. Box 404
Alexandria
VA
22313-1404
US
|
Family ID: |
24423508 |
Appl. No.: |
10/199086 |
Filed: |
July 22, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10199086 |
Jul 22, 2002 |
|
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09605401 |
Jun 28, 2000 |
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6440280 |
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Current U.S.
Class: |
204/192.13 ;
204/192.26; 204/298.03; 204/298.16; 700/266 |
Current CPC
Class: |
C23C 14/542 20130101;
H01J 37/3405 20130101; C23C 14/0078 20130101; C23C 14/354 20130101;
C23C 14/505 20130101; H01J 37/3438 20130101 |
Class at
Publication: |
204/192.13 ;
204/192.26; 204/298.03; 204/298.16; 700/266 |
International
Class: |
C23C 014/00; C23C
014/32; C25B 009/00; C25B 011/00; C25B 013/00; G05B 021/00 |
Claims
We claim:
1. A method for controlling the application of thin coatings
applied to a substrate in a vacuum sputtering system, comprising:
measuring an electrical parameter and a pressure parameter of the
sputtering system during a sputtering run and producing measurement
signals indicative of the parameters; producing control signals
responsive to said measurement signals based on a rule set; and
using said control signals to adjust in real time at least one
process gas flow rate while sputtering.
2. The method of claim 1, wherein the electrical parameter measured
is a cathode voltage and the pressure parameter measured is total
system pressure.
3. The method of claim 2, wherein the sputtering performed is
reactive sputtering and the process gas flow rates that are
controlled are a reactive gas flow rate and a non-reactive gas flow
rate.
4. The method of claim 1, wherein the control signals are
determined by a fuzzy-logic computation employing the electrical
parameter and the pressure parameter as inputs.
5. The method of claim 1, wherein the control signals are
determined from a computerized look-up table based on the
electrical parameter and the pressure parameter as inputs.
6. The method of claim 5, wherein the look-up table employs the
following rule base:
3 Cathode Voltage Total System Pressure Flow Adjustment Low High
Decrease non-reactive gas Low Low Increase reactive gas High High
Decrease reactive gas High Low Increase non-reactive gas OK High
Decrease non-reactive gas OK Low Increase non-reactive gas High OK
Decrease reactive gas Low OK Increase reactive gas
7. The method of claim 5, wherein the look-up table employs a
decision structure for adjusting the at least one process gas flow
rate based on a categorization of the electrical parameter and a
categorization of the pressure parameter.
8. The method of claim 6, wherein the non-reactive gas is argon and
the reactive gas is oxygen.
9. A method for controlling the application of a coating to a
substrate in a coating system, comprising: measuring a first system
parameter and a second system parameter of the coating system
during a coating run and producing measurement signals indicative
of the system parameters; producing control signals responsive to
said measurement signals based on a fuzzy-logic rule set; and using
said control signals to control in real time at least one process
variable of the coating system while applying a coating.
10. The method of claim 9, wherein applying the coating comprises
sputtering, wherein the first system parameter measured is a
cathode voltage of a sputtering source, and wherein the second
system parameter measured is a gas pressure.
11. The method of claim 10, wherein the at least one process
variable comprises a flow rate of a first gas and a flow rate of a
second gas.
12. The method of claim 9, wherein the control signals are
determined by a fuzzy-logic computation based upon the first system
parameter and the second system parameter as inputs.
13. The method of claim 9, wherein the control signals are
determined from a computerized look-up table based on the first
system parameter and the second system parameter as inputs.
14. The method of claim 9, wherein the at least one process
variable comprises a gas partial pressure.
15. A control system for controlling the application of a coating
to a substrate in a coating system, comprising: a fuzzy-logic
controller; and an interface coupled to the fuzzy-logic controller,
wherein the interface is configured to receive first and second
measurement signals corresponding to first and second measured
system parameters, respectively, of a coating system during a
coating run, wherein the fuzzy-logic controller is configured to
receive the first and second measurement signals from the interface
and to produce control signals responsive to said first and second
measurement signals based on a fuzzy-logic rule set, and wherein
the interface is configured to provide the control signals produced
by the fuzzy-logic controller to the coating system to control at
least one process variable of the coating system during application
of a coating.
16. The control system of claim 15, wherein the fuzzy-logic
controller is configured to produce the control signals using a
cathode voltage of a sputtering source as the first measured system
parameter and using a total system pressure as the second measured
system parameter.
17. The control system of claim 16, wherein the at least one
process variable comprises a flow rate of a first gas and a flow
rate of a second gas.
18. The control system of claim 15, wherein the fuzzy-logic
controller is configured to produce the control signals using a
fuzzy-logic computation based upon the first measurement signal
corresponding to the first measured system parameter and the second
measurement signal corresponding to the second measured system
parameter as inputs.
19. The control system of claim 15, wherein the fuzzy-logic
controller is configured to produce the control signals using a
computerized look-up table based on the first measurement signal
corresponding to the first measured system parameter and the second
measurement signal corresponding to the second measured system
parameter as inputs.
20. The control system of claim 15, wherein the at least one
process variable comprises a gas partial pressure.
Description
[0001] This application is a divisional of copending U.S. patent
application Ser. No. 09/605,401, "Multi-Anode Device and Methods
for Sputter Deposition", filed on Jun. 28, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to methods and apparatus for
sputter coating articles, and especially, for reactive sputter
coating of plastic ophthalmic lens elements using a sputter source
with multiple anodes. As used herein, lens elements include,
according to context, edged lenses, semi-finished lenses and lens
blanks. Also included are wafers for forming laminate lenses or
wafer blanks therefor. Ophthalmic uses of the lens elements include
uses in eyeglasses, goggles and sunglasses.
BACKGROUND AND OBJECTS OF THE INVENTION
[0003] Many ophthalmic lenses produced today are made from a single
plastic body or laminated plastic wafers. The plastic material may
include thermoplastic material such as polycarbonate or thermoset
material such as diallyl glycol carbonate types, e.g. CR-39 (PPG
Industries). The material may also be a cross linkable polymeric
casting composition such as described in U.S. Pat. No. 5,502,139 to
Toh et al. and assigned to applicant.
[0004] Ophthalmic lens elements are frequently coated to achieve
special properties. Anti-reflection coatings improve the
transmittance of visible light and the cosmetic appearance of the
lenses. Reflective and absorptive coatings may be employed in sun
lenses to reduce light transmittance to the eye, to protect the eye
from UV radiation and/or to impart cosmetic colorations to the
lens. Coatings may also provide other beneficial properties such as
increased hardness and scratch resistance and anti-static
properties.
[0005] Desirable lens coatings may be created by applying single or
multiple layers of metal, metal oxides or semi-metal oxides to
surfaces of the lens element. Such materials include oxides of
silicon, zirconium, titanium, niobium and tantalum. Metal and
semi-metal nitrides are also used. Examples of such multilayer
coatings are given, for example, in U.S. Pat. No. 5,719,705 to
Machol entitled "Anti-static Anti-reflection Coatings", assigned to
applicant. Interference filter coatings for sunglasses are
disclosed, for example, in U.S. Pat. No. 2,758,510 to Auwarter.
Other lens coatings are disclosed in International Application WO
99/21048 to Yip, et al., which is hereby incorporated by
reference.
[0006] Various methods are disclosed in the prior art for applying
metal and semi-metal oxide coatings to ophthalmic lenses. Such
coatings have traditionally been deposited by means of thermal
evaporation, and more recently, by electron-beam (e-beam)
evaporation and reactive sputtering. Evaporations are typically
carried out at vacuums better than 10E-5 Torr. Ritter et al. U.S.
Pat. No. 4,172,156, for example, discloses e-beam evaporation in an
oxygen atmosphere of Cr and Si to form coating layers on a plastic
lens. The use of reactive sputter deposition to form various oxide
layers on lens elements is disclosed in the above-mentioned '705
patent to Machol.
[0007] Reactive sputtering in general is a conventional technique
often used, for example, in providing thin oxide coatings for such
items as semiconductor wafers or glass lamp reflectors. Examples of
various conventional vacuum deposition systems for the formation of
coatings by reactive sputtering are disclosed in the following
patents: U.S. Pat. No. 5,616,224 to Boling; U.S. Pat. No. 4,851,095
to Scobey et al.; U.S. Pat. No. 4,591,418 to Snyder; U.S. Pat. No.
4,420,385 to Hartsough; British Patent Application GB 2,180,262 to
Wort et al.; Japanese Kokai No. 62-284076 to Ito; and German Patent
No. 123,714 to Heisig et al.
[0008] The coating of plastic lenses in spinning drum coaters by
means of sputtering technology, including DC reactive sputtering,
is a relatively recent development. A conventional drum vacuum
coating system used for this purpose is shown in FIG. 1. The system
includes a vacuum chamber 11, which contains a hollow workpiece
holder or drum 12. Lens elements, such as lens 13 are arranged in
columns on an external surface of the drum 12. A coating applicator
14 is located near a wall of the vacuum chamber adjacent the drum
12. The coating applicator 14 may comprise a combination of
magnetron sputtering sources and microwave plasma generators with a
reactive gas supply such as disclosed in U.S. Pat. No. 5,616,224 to
Boling, which is hereby incorporated by reference. Power is
delivered to the coating applicator 14 by one or more power
supplies (not shown) via an electrical lead assembly 17. A
reversing power supply for arc suppression such as disclosed in
U.S. Pat. No. 5,616,224 to Boling may be included. A sputtering gas
is introduced into the vacuum chamber through gas-supply plumbing
built into the coating applicator or through a separate port (not
shown) on the vacuum chamber 11. The sputtering gas is controlled
by a gas controller (not shown) and may be an inert gas such as
argon or a reactive gas mixture such as argon/oxygen or
argon/nitrogen.
[0009] The vacuum chamber 11 is evacuated by vacuum pumps (not
shown) attached to a pumping plenum 15. A cryopumping surface,
known as a Meissner trap, is conventionally provided in the form of
cryocoils 16 in the plenum 15. A coolant with a temperature well
below the freezing point of water flows through the cryocoils 16,
allowing the Meissner trap to remove water vapor from the vacuum
chamber 11. A Meissner trap may be advantageously configured in the
vacuum chamber 11 rather than in the pumping plenum 15 to improve
the cryopumping of water vapor. Such a configuration is especially
useful when coating plastic lens elements because plastic lens
elements have a tendency to outgas substantially more water vapor
than conventional glass lenses, as disclosed in U.S. Pat. No.
6,258,218, hereby incorporated by reference.
[0010] A drum vacuum coating system with an elongated magnetron
sputter source 14 such as that illustrated in FIG. 1 provides a
convenient means of coating numerous lens elements or other
articles. However, Applicants have observed that such systems
typically do not produce uniformly thick coatings on multiple
articles disposed in a given column of the drum 12 due to the
variation in sputter rate along the length of the sputter source.
In other words, an article or lens element positioned near the top
of a given column may not receive a coating of the same thickness
as an article or lens element positioned near the center of that
column.
[0011] Several methods directed toward improving coating uniformity
of sputtered films have been disclosed in the prior art. U.S. Pat.
No. 5,645,699 issued to Sieck discloses a system comprising two
cylindrical magnetron sputter sources, each with an anode
substantially spanning the length of the sputter source, wherein
the placement of a third anode between the two sputter sources has
improved coating uniformity. U.S. Pat. No. 4,849,087 issued to
Meyer discloses the use of multiple gas nozzles distributed along
the length of a sputter source to deliver varying amounts of an
argon/oxygen gas mixture to local regions of the plasma above the
sputter target (cathode). Individual resistance probes disposed
along the width of the substrate measure the local resistance of
the coating and provide feedback signals to adjust the gas flow
through the various nozzles to maintain uniform electrical
resistance in various regions of the coating. While this approach
provides control of the electrical resistance of the coating, it
does not necessarily provide control of the coating thickness, a
quantity of importance for optical coatings.
[0012] U.S. Pat. Nos. 5,487,821 and 5,683,558 to Sieck et al.
disclose the use of "wire brush" anodes in conjunction with
magnetron sputter sources and indicate that the wire-brush point
density of an anode may be adjusted along the length of the sputter
source to affect the uniformity of the deposited film. U.S. Pat.
No. 5,616,225 issued to Sieck et al. discloses the use of wire
brush anodes and the use of multiple anodes in conjunction with a
single magnetron sputter cathode for coating substrates (especially
large substrates) wherein the anode voltages may be individually
controlled. The '225 patent indicates that this control may be
utilized to improve the thickness uniformity of the deposited
coating. The disclosure in the '225 patent, however, does not
address controlling the thickness uniformity of reactive coatings
deposited on large numbers of individual lens elements using an
elongated magnetron sputter source in a drum vacuum coating
system.
[0013] A need still exists to provide drum vacuum deposition
systems for high volume production of individual articles, such as
plastic lens elements, while ensuring a high degree of control over
the thickness and composition of the coatings.
[0014] Accordingly, it is an object of the present invention to
improve the degree of control over the thickness and composition of
thin metal and semi-metal oxide coatings deposited on multiple
articles, particularly plastic lenses, disposed on a rotatable
holder in a vacuum coating system.
[0015] It is another object of the present invention to provide a
multi-anode sputter source adapted to the geometry of a cylindrical
drum vacuum coating system for depositing coatings on numerous
plastic parts.
[0016] It is another object of the present invention to provide an
apparatus for depositing a high quality coating on large numbers of
plastic lens elements in a system which is relatively inexpensive
to construct and operate.
[0017] These and other objects and features of the present
invention will be apparent from the written description and
drawings presented herein.
SUMMARY OF THE INVENTION
[0018] A drum vacuum coating system with an elongated magnetron
sputter source provides a convenient means of coating numerous lens
elements or other articles located on a rotatable drum. However,
Applicants have observed that such systems typically suffer from
the inability to produce coatings of uniform thickness on multiple
articles disposed in various locations on the drum due to
variations in the sputter rate along the length of the sputter
source. Applicants have determined that providing an elongated
sputter source with multiple anodes, wherein the currents to the
anodes may be individually controlled or controlled in pairs,
allows the deposition of coatings of substantially uniform
thickness on multiple articles regardless of their position on the
drum. The thickness uniformity is acceptable for thin optical
coatings on ophthalmic lens elements.
[0019] A preferred embodiment of the present invention is a method
and apparatus for sputter coating a plurality of articles such as
plastic lens elements. The system includes a vacuum chamber, a
rotatable cylindrical holder for holding the plurality of articles,
and at least one sputter source that is elongated along a
lengthwise direction and that has a cathode and a plurality of
anodes. The sputter source is disposed with its lengthwise
direction parallel to a rotation axis of the cylindrical holder,
and the anodes are disposed adjacent the cathode substantially
along at least one line parallel to the lengthwise direction. The
articles may be disposed in a predetermined pattern on the
cylindrical holder, and the anodes may be disposed in positions
corresponding to positions of the articles disposed on the
cylindrical drum. Additionally, the articles may be arranged in
columns and rows on the cylindrical holder, the columns being
parallel to the rotation axis of the cylindrical holder, and the
anodes may be configured in pairs, each anode pair being aligned
with a row on the cylindrical holder. The sputter source may be a
planar magnetron sputter source.
[0020] A cathode power supply provides a negative voltage to the
cathode, and a separate anode power supply with a plurality of
channels provides anode currents to the anodes. Alternatively, the
cathode power supply and the anode power supply may be provided in
a single unit. The anodes may be configured in pairs, and each
anode pair may be powered by a separate channel. In addition, the
anode currents may be adjusted in a manner to produce coatings of
increased uniformity of thickness on articles positioned in
different locations on the cylindrical holder. In one embodiment,
the same amount of current is provided to each active anode pair by
the controlled power supply.
[0021] The length of the sputter source may range from twenty
inches to sixty inches, though approximately forty inches is a
preferred length. The number of pairs of anodes may range from six
pairs to fifteen pairs. Eight or nine pairs of anodes are
preferred. In addition, it is preferred that the length of each
anode is approximately the same as the diameter (height) of the
surface of article to be coated. The height and diameter of the
drum may be approximately forty inches, and the drum may hold
approximately 200 to 400 articles.
[0022] In order to prevent the buildup of dielectric material on
the electrically active anodes each anode of the sputter source may
be configured as an electrically conducting bar having a recessed
slot, the slot being oriented such that the opening of the slot is
directed away from the cathode. In this manner, anodes are provided
with interior surfaces that remain substantially free of dielectric
coatings, ensuring good electrical conduction between the anodes
and the sputter plasma. Alternatively, the anodes may be configured
as wire-brush anodes, wherein each anode comprises a plurality of
electrically conducting wires emanating from an electrically
conducting support member. This configuration similarly provides
anode surfaces that remain substantially free of dielectric coating
deposits at the root of the brush.
[0023] In another embodiment, the apparatus may be used to carry
out reactive DC sputtering of a thin coating, which may comprise a
dielectric layer deposited onto surfaces of plural articles such as
plastic lens elements. In this case, the apparatus again includes a
vacuum chamber and a sputtering source that is elongated in a
lengthwise direction and that has a cathode and a plurality of
anodes, the anodes being arranged in pairs. The apparatus also
includes a rotatable article holder located in the vacuum chamber
that rotates the plural articles past the sputtering source, the
articles being arranged in a predetermined pattern on the article
holder. The article holder may be a hollow drum rotated about its
central axis. In addition, the apparatus includes a source of
reactive gas, such as oxygen or nitrogen. An elongated plasma
applicator, such as a microwave plasma generator, is also provided
adjacent to the sputtering source for producing a plasma to
facilitate the reaction of the reactive gas with material sputtered
from the sputtering source. The sputter source may be a planar
magnetron sputter source.
[0024] The articles may be arranged in columns and rows on the
article holder, each anode pair being aligned with a row on the
cylindrical holder. The reactive sputtering may comprise the
sputtering of a metal or semi-metal utilizing a sputtering gas that
contains oxygen in order to produce metal-oxide or semi-metal-oxide
coatings. A sputtering gas comprising nitrogen may also be used to
produce nitride coatings. In addition, a second sputtering source
may be provided adjacent to the first sputtering source or adjacent
to a plasma applicator for sputtering a metal or semi-metal
different than that sputtered from the first sputtering source. In
this manner, multiple coatings with different indexes of refraction
may be provided on articles such as plastic lens elements.
[0025] The apparatus may further comprise a controller that
receives an input signal corresponding to a small number of
measurable process variables (for example, two) and which controls
a small number of process variables (for example, two) in response
thereto. In a preferred embodiment, the measurable process
variables are cathode voltage and total gas pressure; the
controlled variables are a first flow rate for a first gas and a
second flow rate for a second gas. A purpose of the controller is
to maintain batch-to-batch uniformity of coating thickness and
coating composition. The controller may be used in roll coating or
the coating of discrete articles such as lens elements as described
below.
[0026] Batch to batch (run to run) stability of deposition rates of
the sputtered materials may vary due to a number of causes even
when the sputter plasma is held at a constant power dissipation.
These causes includes: historical effects on the target (such as
oxide coverage and oxygen implantation), target cleaning, tooling
cleaning, chamber cleaning, length of time the chamber is opened
for loading, unloading or servicing and consequent coverage of
chamber and tooling with absorbed layers of water vapor, thickness
of deposited materials on tooling and chamber, type of plastic
constituting the lens substrates and their degree of water uptake,
gas leaks, improper calibration of partial pressure gauges and/or
other means of measuring partial pressures such as Optical Gas
Controllers. Applicants have determined that, at constant sputter
plasma power or constant sputter cathode current, more stable
deposition rates may be obtained by manually adjusting two directly
controllable process variables (in a preferred embodiment, the flow
rates of the buffer gas (usually Argon) and the reactive gas
(usually oxygen)) according to a set of rules based on observations
of two measured input parameters (in a preferred embodiment,
cathode voltage and total pressure). The rules may be experiential
rules based on an expert's understanding of the operation of a
particular sputtering system. These rules may be embedded in the
fuzzy logic control system. For example input parameters may
include classifying three levels of cathode voltage and total
pressure: "LOW", "OK" and "HIGH". The output parameters for
adjusting the operation of the system may include three
classifications for the flow rates of the buffer gas and the flow
rate of the reactive gas, namely: "INCREASE", "HOLD" and
"DECREASE". The fuzzy logic determination may be implemented in
control signals for opening or closing buffer gas and reactive gas
valves by a precise amount.
[0027] In another embodiment, a multi-anode device for use in
sputter deposition is provided. Note that the multi-anode device
may be used separately from the above described gas control
techniques. However, both techniques may be advantageously used to
achieve a high degree of coating thickness uniformity.
[0028] The multi-anode device comprises a cathode and a plurality
of anodes located in predetermined positions adjacent to first and
second opposing sides of the cathode. The anodes disposed at each
of the first and second opposing sides of the cathode are
configured in a regular arrangement to substantially minimize or
eliminate gaps between adjacent anodes. Further, a different
current may be applied to each anode. Such a configuration may be
desirable for facilitating uniformity of the sputter plasma. In
particular, the anodes at a given side of the cathode may be
configured in a sawtooth arrangement, wherein an end of one anode
is disposed closer to the cathode than an end of the adjacent
anode. The anodes at opposing sides of the cathode may be arranged
identically or in a mirror image arrangement. The anodes may also
be arranged in an essentially linear fashion wherein the ends of
the anodes approach each other closely but are shielded from one
another by interposing an electrical and/or physical barrier
between said ends. In another embodiment, the ends of adjacent
anodes may be purposely separated from, for example, 1 to 5 inches
to modify electrical coupling of the anodes with one another
through the plasma.
[0029] In addition, each anode of the multi-anode device may be
configured as an electrically conducting bar having a recessed
slot, the slot being oriented such that the opening of the slot is
directed away from the cathode. Alternatively, the anodes may be
configured as wire-brush anodes, wherein each anode comprises a
plurality of electrically conducting wires emanating from an
electrically conducting support member.
[0030] In another embodiment, a method for sputter coating a
plurality of articles is provided. A vacuum chamber is provided
having at least one sputter source with a plurality of anodes
configured in pairs and having an article holder that rotates about
an axis. Plural articles are located in a predetermined pattern on
a radially outward facing surface of the article holder, and the
chamber is pumped down. A sputtering gas of the desired
composition, flow and pressure is provided, and the article holder
is rotated relative to the sputter source. Sputter coating is
carried out on the radially outward facing surfaces of the articles
while controlling current to each pair of anodes separately.
[0031] The articles may be arranged in columns and rows on the
outward facing surface of the article holder, and the rows may be
aligned with pairs of anodes. The drum may be continuously or
sequentially rotated relative to the sputter source or sputter
sources while voltages are applied to the cathode and anodes to
cause material to be sputtered from the sputtering target onto the
radially outward facing surfaces of the articles. Further, the
currents to the anodes may be adjusted in a manner to provide a
uniform coating to the articles at all positions on the article
holder. In addition, the sputter coating may comprise a reactive DC
process in which sputtered material reacts with reactant gas to
form a dielectric layer. The dielectric layer may comprise at least
one layer of a metal oxide or semi-metal oxide. The articles may be
plastic lens elements, and the sputter source may be a planar
magnetron sputter source. In one embodiment of the invention the
individual currents to be applied to each anode pair for each
target material, in order to adjust uniformity, are determined
before each coating batch by spectral measurements or otherwise on
articles or witness samples coated during prior batches.
[0032] The foregoing has been provided as a convenient summary of
aspects of the invention. The invention intended to be protected
is, however, defined by the claims and equivalents thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a pictorial view in partial phantom of a system
known in the prior art for vacuum coating plural plastic lens
elements;
[0034] FIG. 2 is a pictorial view in partial phantom of drum vacuum
coating system according to one embodiment of the present
invention;
[0035] FIG. 3 is a pictorial view of a multi-anode planar magnetron
sputter source such as that shown in FIG. 2;
[0036] FIG. 4(a) is an illustration in side view of a multi-anode
sputter source according to the present invention with anode pairs
configured in zones corresponding to article positions on the drum
holder;
[0037] FIG. 4(b) is an illustration of an inside view of a portion
of a multi-anode sputter source showing controlled current sources
and anode shielding of a preferred embodiment of the present
invention;
[0038] FIG. 5(a) is an illustration in side view of an anode
configuration according to another embodiment of the present
invention;
[0039] FIG. 5(b) is an illustration in side view of another anode
configuration different than that shown in FIG. 5(a);
[0040] FIG. 5(c) is an illustration in side view of an individual
anode for the embodiments illustrated in FIGS. 5(a) and 5(b);
[0041] FIG. 5(d) is a pictorial view of an individual anode having
a slot for maintaining an uncoated anode surface;
[0042] FIG. 6(a) is an illustration in side view of a wire-brush
configuration of multiple anodes for a planar magnetron sputter
source according to another embodiment of the present
invention;
[0043] FIG. 6(b) is an illustration in plan view of an individual
wire brush anode for the embodiment illustrated in FIG. 6(a);
[0044] FIG. 6(c) is a cross-sectional view of a wire brush anode
for the embodiment illustrated in FIG. 6(a);
[0045] FIG. 7(a) is a pictorial illustration of a multi-channel
high-current power supply for use with a multi-anode sputter
source;
[0046] FIG. 7(b) is a pictorial illustration of an optional
computer controller for use with the power supply illustrated FIG.
7(a);
[0047] FIG. 8 is a block diagram illustrating a fuzzy-logic control
method according to the present invention;
[0048] FIG. 9 is a flow diagram illustrating a fuzzy logic
technique of a preferred embodiment of the present invention;
[0049] FIGS. 10(a) and 10(b) are plots of class membership as a
function of cathode voltage and pressure, respectively, for a
system presented as an example embodying teachings of the present
invention;
[0050] FIG. 11 presents details of the calculations performed in an
illustrative Example of the implementation of techniques of the
present invention;
[0051] FIGS. 12(a) and (b) are depictions of three dimensional
control surfaces usable in an embodiment of the present
invention.
DETAILED DESCRIPTION
[0052] The disclosed embodiments address the need for effective
control over the thickness of a deposited coating, particularly in
systems containing drum workpiece holders and elongated magnetron
sputter sources for coating numerous plastic lens elements.
[0053] A first embodiment according to the present invention is a
drum vacuum coating system 20 incorporating an elongated, planar
magnetron sputter source 30 with a target (cathode) 32 and multiple
anodes 33-1 through 33-N where N is the total number of anodes as
illustrated in FIG. 2. The system 20 includes a vacuum chamber 21
with a pumping plenum 25 attached to vacuum pumps (not shown) for
the purpose of evacuating the chamber 21. The chamber 21 contains a
hollow, rotating drum 22 shown in partial phantom. Articles 23,
such as plastic lens elements may be arranged in columns and rows
on an external surface of the drum 22. The multi-anode sputter
source 30 (described further below) is located near a wall of the
vacuum chamber adjacent the drum 22. A hinged door (not shown) or a
detachable top plate (not shown) may be used to provide access to
the vacuum chamber 21 for loading articles 23 onto the drum 22.
[0054] A sputtering gas is introduced into the vacuum chamber
through gas-supply plumbing built into the sputter source 30 or
through a separate port 24 to the vacuum chamber 21. The sputtering
gas is controlled by a mass-flow controller (not shown) and may be
an inert gas such as argon or a reactive gas mixture such as
argon/oxygen or argon/nitrogen for carrying out DC reactive
sputtering.
[0055] A Meissner trap in the form of cryocoils 26 is optionally
provided at the base of the chamber 21 and is especially beneficial
when the articles to be coated are plastic lens elements, which may
outgas water vapor at a substantial rate. A similar Meissner trap
(not shown) may optionally be provided at the top of the chamber. A
coolant with a temperature well below the freezing point of water
flows through the cryocoils 26, allowing the Meissner trap to
remove water vapor from the vacuum chamber 21, improving the
quality of the coatings as disclosed in U.S. Pat. No.
6,258,218.
[0056] Power is delivered to the multi-anode sputter source 30 by
one or more power supplies (not shown) with optional computer
control via an electrical lead assembly 27. As illustrated in FIG.
3, a negative cathode voltage -V.sub.C is applied to the target,
and anode currents I.sub.A1 through I.sub.AN are individually
applied to the anodes. By controlling the voltages to the anodes
individually, the spatial distribution of the plasma may be
controlled to a large extent, thus allowing control of the rate of
ejection of the target atoms from different regions of the target
32 defined by the positions of the anodes. Thus, individual control
of the anode currents provides additional control over the
thickness of coatings deposited on articles 23 positioned in
proximity to the anodes. Arc suppression may be employed.
[0057] As shown in FIG. 2, plasma applicators 28 and 29, such as
those described in U.S. Pat. No. 5,616,224 to Boling, may
optionally be placed at each side of the sputter source 30 to
provide additional sources of plasma that diffuse into the region
above the target 32 of the sputter source 30. The additional plasma
provided by the plasma applicators 28 and 29 may provide reduced
arcing, enhanced deposition rate, and enhanced reaction of freshly
deposited metal species in DC reactive sputtering. Power is
delivered to the plasma applicators 28 and 29 by one or more power
supplies (not shown) via electrical lead assemblies (not
shown).
[0058] The coating process is carried out by first placing a
collection of articles 23 in columns and rows on a radially outward
facing surface of the drum 22. The vacuum chamber 21 is then pumped
down to a desired base pressure, and a sputtering gas of the
desired composition, flow and pressure is provided. The drum 22 is
then rotated relative to the sputter source 30, and voltages are
applied to the target (cathode) 32 and to the anodes to cause
material to be sputtered from the sputtering target 32 onto the
radially outward facing surfaces of the articles 23. The currents
to the anodes are adjusted in a manner to provide a uniform coating
to the articles at all positions on the drum 22. As mentioned
above, the coating process may comprise a reactive DC process in
which sputtered material reacts with reactant gas to form a
dielectric coating. The drum 22 may be rotated continuously during
the sputtering process, for example, at about 90 rpm.
[0059] Optionally, one plasma applicator rather than two plasma
applicators as illustrated in FIG. 2 may be disposed adjacent to
the multi-anode sputter source 30. Further, a configuration
comprising two or more multi-anode sputter sources rather than one
multi-anode sputter source 30 (as illustrated in FIG. 2) may be
utilized. In a preferred embodiment, a plasma applicator may be
disposed adjacent two multi-anode sputter sources, so that a
substrate to be coated first encounters a first sputter source,
then a second sputter source and finally the microwave applicator.
Only one sputter source may be active during a "pass" made by the
substrate.
[0060] Advantageously, first and second multi-anode sputter sources
may include targets of different metal and/or semi-metal materials
to form sequential coatings of diverse metals, semi-metals, and
their oxides or nitrides on the lens elements, the coatings having
different indices of refractions. Layers are built up by repeatedly
rotating the lens elements on the drum past the sputter sources.
For example, the system may be used to apply a multi-layer oxide
coating to columns of lens elements whose radially outwardly facing
optical surfaces have been treated with a hard coat. A five-layer
coating comprising alternating layers of silicon oxide and
zirconium oxide, the silicon oxide layers being outermost and
innermost, is one such example.
[0061] The outer cylindrical face of the drum 22 is typically 2 to
9 inches from the target surfaces of the sputter sources. The drum
itself may be on the order of 40 inches in diameter and 40 inches
high and carry hundreds of articles 23 on its outer surface.
Correspondingly, the sputter sources and plasma applicators (if
present) may be on the order of 40 inches in length. A preferred
number of lens elements disposed on the drum 22 is 200 to 450
articles 23.
[0062] The sputter source 30 illustrated in FIG. 2 will now be
described in more detail with reference to FIG. 3. The sputter
source 30 comprises a body 31, a target (cathode) 32, and a
collection of anodes 33-1 through 33-N where N is the total number
of anodes. The anodes may be configured as electrically conductive
bars and may be made of aluminum, aluminum alloys, copper, copper
alloys, stainless steel, other alloys, or other conductive
materials. Individual electrical connections are made to the anodes
and to the target (cathode) 32 using individual electrical leads of
the lead assembly 27. Alternatively, separate electrical lead
assemblies for the cathode and for the anodes may be used to
provide the electrical connections. In the particular example shown
in FIG. 3, there are eighteen anodes (N=18); however, it should be
understood that the sputter source 30 may comprise a larger or
smaller number of anodes. The body 31 encloses an assembly of
permanent magnets (not shown) that substantially confine the plasma
to an oval shaped racetrack 34. Such magnet configurations are well
known in the art.
[0063] The body 31 and the target 32 of the sputter source 30
illustrated in FIG. 3 are substantially elongated and rectangular
in shape. In addition the anodes may be disposed symmetrically in
pairs along opposing elongated sides of the sputter source body 31
with one anode of an anode pair being disposed at one side of the
target and the other anode of the pair being disposed at the
opposing side of the target with both anodes positioned an equal
distance from either end of the sputter source body 31. For
example, in FIG. 3, opposing anodes 33-1 and 33-10 comprise an
anode pair, opposing anodes 33-2 and 33-11 comprise an anode pair,
and so forth. Insulating members (not shown) are provided between
the anodes and the body 31 to provide electrical isolation of the
anodes from the body 31. The body 31 (which would normally be the
anode in the absence of the multi-anodes) will usually be attached
to the chamber at ground potential. However this body rapidly
becomes coated with insulating dielectric materials in reactive
sputter coating so that the surface exposed to the plasma may float
at a potential above ground potential and therefore lead to the
well known wandering anode problem which contributes to non-uniform
sputter deposition.
[0064] In a preferred embodiment according to FIGS. 2 and 3, the
anodes of the sputter source 30 are configured to correspond to the
placement and size of the articles 23 to be coated. Articles 23,
such as plastic lens elements, are disposed uniformly on the drum
22 and may be arranged in vertical columns and horizontal rows. The
elongated sputter source 30 is positioned vertically (parallel to
the rotation axis of the drum 22) as shown in FIG. 2. The number of
anode pairs may be equal to the number of rows of articles 23
disposed on the drum 22. The anodes may be uniformly and
symmetrically arranged in pairs at opposing sides of the target 32
wherein the separation between anodes of an anode pair is the same
for all anode pairs. Such a configuration is schematically
illustrated in FIG. 4, which shows the projection of a column of
articles 23 superimposed on a projection of the surface of the
sputter target 32. In addition, pairs of anodes and rows of
articles 23 are centered in zones 4-1 through 4-9 separated
vertically by horizontal dashed lines as illustrated in FIG. 4. It
should be understood that the column of articles 23 illustrated in
FIG. 4 is displaced a distance away from the sputter target 32.
[0065] In addition, according to this embodiment and as indicated
in FIG. 4, the end regions of the sputter target 32 that encompass
the curved portions of the race track 34 are positioned in end
zones 4-10 and 4-11 that do not contain articles 23. It is
advantageous to exclude articles 23 from the end zones 4-10 and
4-11 because the racetrack 34 has a different geometrical
configuration in the end zones compared to the inner zones as
illustrated in FIG. 4. Though not shown, anode pairs may also be
provided in end zones 4-10 and 4-11 to provide control of the
plasma in these zones as well.
[0066] The configuration illustrated in FIG. 4 is characterized by
nine pairs of anodes and nine rows of articles 23. It should be
understood, however, that larger or smaller number of anode pairs
and rows of articles 23 may be utilized. Configurations utilizing
sputter sources ranging from twenty to sixty inches in length
having six to fifteen anode pairs are advantageous. The use of
sputter sources approximately forty inches in length having from
six or ten anode pairs is preferred. It is also preferred that
anode pairs in the same zone be connected together electrically and
that they be supplied by means of a voltage limited constant
current source for each zone.
[0067] As shown in the configurations illustrated in FIG. 4, the
anodes may be configured with a length that is approximately equal
to the vertical size of the articles 23. Such a configuration is
advantageous for controlling the thickness uniformity of the
coating deposited on a given article 23 as well as for controlling
the coating thickness uniformity between articles 23. Anodes
substantially shorter than the zone length may be desirable under
some process conditions to electrically decouple them from one
another.
[0068] The operation of the sputter source 30 will now be described
in more detail with reference to FIG. 4. A simple version of the
control scheme is shown in FIG. 4(a) wherein the variable resistors
41 are connected to a single positive potential source 42. Anode
33-1 is connected to anode 33-10 and then via the variable resistor
41 for zone 4-1 to the source 42. Similarly anode 33-2 is connected
to anode 33-10 and then via the resistor 41 for zone 4-2 to the
source 42 and so on.
[0069] The invention may be further understood by means of an
example with reference to FIG. 4. Consider a 40-inch long by 5-inch
wide silicon target 32 mounted on the multi-anode sputter source
30. With each variable resistor 41 initially set to zero ohms, all
anodes 33-1 through 33-18 will be at ground potential. A voltage of
-500 volts is applied to the target (cathode) 32 resulting in a
total current delivered to the target of approximately 18.0 amperes
when the source is energized in an argon/oxygen atmosphere at a
pressure of a few millitorr. The anodes are controlled in pairs via
the variable resistors such that anodes 33-1 and 33-10 form a pair,
anodes 33-9 and 33-18 form a pair, and so forth. Given the target
voltage and current noted above, each anode at ground potential
draws approximately 1.0 Amps of current; each pair of anodes,
therefore, draws approximately 2.0 Amps.
[0070] For simplicity we will assume that, in the absence of the
source 42, with all variable resistors 41 set at zero, the multi
anodes 33 are effectively connected to ground potential so that the
anode current per sector is 2 Amps if they are all the same. The
plasma impedance per sector will therefore be 250 ohms. Suppose we
now install the source 42 and set it at +20 volts and set all the
resistors 41 to 10 ohms. Assume the anodes 33-1 through 33-18 are
not covered by dielectric coatings whereas the vacuum chamber walls
in the vicinity of the sputter cathodes (and in the vicinity of the
microwave applicator if present) are coated with insulating
dielectrics and therefore will not act as effective ground points.
The cathode current (composed of electrons in the plasma) will
therefore preferentially flow through the anodes 33-1 through 33-18
to the positive potential VA. It is still desirable to have a
current flow of 2 Amps through each anode pair, the anode potential
of each of the multi anodes will still be zero and the system will,
to first order, operate as before. A major influence on the coating
rate on articles on the work holder opposite those zones is the
plasma density in them which, to a measurable and significant
extent, is controlled by the electron current passing from cathode
to anode in that zone.
[0071] Alternatively the positive supply 42 may be dispensed with
and the common terminals of all variable resistors 41 connected to
a good ground potential. This scheme will only work if the anodes
33-1 through 33-18 are effectively uncoated by dielectric whilst
the chamber in the vicinity of the sputter magnetrons is
substantially coated with electrically insulating dielectrics and
therefore does not provide a good ground potential. Under these
circumstances the variable resistors 41 can be used to adjust the
anode currents up to the point where the anode potential becomes
more negative than the effective ground plane of the nearby
dielectrically coated chamber walls.
[0072] To provide more certainty than either of the previously
discussed schemes for controlling the anode currents for each zone,
it is desirable to provide separate constant current power supplies
for each anode pair. These supplies may be set to a given voltage
limit and a constant current appropriate to each zone in order to
provide the desired uniformity profile Such an arrangement is shown
in FIG. 4(b) which also illustrates a number of other preferred
details of operation. In that Figure the anodes 33 are illustrated
as wire brushes which have several advantages as previously noted.
The constant current power supplies are indicated as 11 through 13
and these may be controlled by either digital or analog means but
in either case it is preferred that their outputs be adjustable
under computer control. It may occur that the potentials developed
on the anodes in an effort to control current may rise to levels
sufficiently different on adjacent anodes that a substantial
current would flow between them. This is because they are to some
extent bathed in a plasma which aids such electrical conduction
irrespective of the fact that they are, when not bathed in a plasma
at least, electrically insulated from the metal mounting bar on
which they are mounted via ceramic insulators or the like. To
obviate this possibility, it is desirable to install shields such
as those illustrated as S12, S23 and S34 and so on between anode
33-1 and 33-2 and between 33-2 and 33-3 and so on. As these and the
mounting bar will ordinarily become coated with insulating
materials they will act as both a physical and an electrical
barrier to the passage of current between adjacent anodes. The
shields S12, S23 and S34 may be made of electrically insulated
dielectric material.
[0073] In the embodiment illustrated in FIG. 3, adjacent anodes of
the multi-anode sputter source 30 are separated by gaps 35.
However, it may be advantageous to minimize such gaps in order to
facilitate spatial uniformity of the plasma. Accordingly, another
embodiment of the present invention minimizes the gaps between
adjacent anodes as illustrated in side view in FIG. 5(a).
[0074] In another embodiment, adjacent ends of anodes in a line may
be separated by a gap (for example a gap of 1 to 3 inches) or every
other anode in the line removed to reduce undesired coupling
between the anodes through the plasma.
[0075] FIG. 5(a) schematically illustrates a multi-anode sputter
source 50 similar to that illustrated in FIGS. 3 and 4 but with a
different anode configuration to minimize gaps between adjacent
anodes. Anodes 53-1 through 53-N are uniformly and symmetrically
disposed in pairs on opposing sides of a sputter target 52 in zones
5-1 through 5-10 separated by horizontal dashed lines. The
individual anodes are attached to insulating mounting blocks 51
such that one end of each anode is disposed closer to the sputter
target 52 than the other end of that anode. Further, except for
mounting blocks 51 disposed near the ends of the sputter target 52,
each mounting block 51 supports the ends of two anodes separated by
a gap 55 in the horizontal direction. The anodes disposed at a
given side of the sputter target 52 are thus positioned in a
sawtooth-shaped arrangement as illustrated in FIG. 5(a). An
identical anode configuration is disposed at the opposing side of
the sputter target 52.
[0076] Unlike the embodiment illustrated in FIGS. 3 and 4 which
possesses a gap in the vertical direction between the anodes, the
embodiment illustrated in FIG. 5(a) possesses a gap in the
horizontal direction with a minimal gap or no gap in the vertical
direction. Thus in the vertical direction along the length of the
sputter target 52, an anode surface is provided at substantially
all vertical positions. Such an anode configuration may be
desirable for facilitating spatial uniformity of the sputter source
plasma.
[0077] An alternative anode configuration that possesses a minimal
gap or no gap in the vertical direction is illustrated in FIG.
5(b). In this configuration, the anodes are disposed in a regular,
sawtooth-shaped arrangement similar to that shown in FIG. 5(a).
However, in the configuration shown in FIG. 5(b), anodes at
opposing sides of the sputter target 52 are disposed in a
mirror-image configuration.
[0078] A magnified illustration of one anode and two mounting
blocks 51 is shown in FIG. 5(c). Each mounting block has an array
of mounting means 56 for mounting an anode to the block via
fastening means (not shown) at the bottom surface of the anode. For
example the mounting means 56 may be holes extending through the
mounting block 51, and the fastening means (not shown) may be bolts
extending from the bottom surface of the anode. However, the
mounting means 56 and the attaching means (not shown) are not to be
limited to these examples.
[0079] In a preferred embodiment, an anode configuration such as
that shown in either FIG. 5(a) or FIG. 5(b) is provided with anodes
possessing recessed slots 58 as illustrated in pictorial view in
FIG. 5(d) to maintain anode surfaces that remain substantially free
of coating deposits. In FIG. 5(d), an anode is illustrated with a
recessed slot 58 disposed at an angle with the opening of the slot
pointing away from the sputter target 52. Such recessed slots
provide interior anode surfaces that receive less coating deposits
during sputter deposition due to shadowing provided by the outer
surfaces of the anode. This is especially beneficial in the
reactive sputtering of dielectric materials (oxides and nitrides,
for example) to ensure good electrical conduction between the
anodes and the plasma. Also shown in FIG. 5(d) are bolts 59 as a
particular example of fastening means extending from the bottom
surface of the anode. The bolts not only provide a means of
fastening the anode to the mounting block (not shown), but also
provide a means of making electrical contact to the anode.
[0080] In another embodiment of the present invention, the
multi-anode sputter source is configured with wire-brush anodes as
illustrated in side view in FIGS. 6(a)-6(c). In FIG. 6(a) a
sawtooth configuration of wire-brush anodes 63-1 through 63-9 for
placement along one elongated side of a sputter source (not shown)
is illustrated. It should be understood that an identical
configuration or a mirror image configuration of anodes (not shown)
is disposed at the opposing side of the sputter source (not shown)
in a manner such as that illustrated in FIG. 5(a) or FIG. 5(b).
[0081] A magnified side view of anode 63-1 is shown in FIG. 6(b).
FIG. 6(c) shows a cross-sectional view of anode 63-1, taken along
plane A-A wherein a collection of individual metal wires 68 extends
radially from a central electrically conducting support member 69.
The central support 69 and the metal wires 68 may be formed from a
variety of materials including, but not limited to, copper, brass,
stainless steel, tungsten, and other electrical conductors. The
density of wires 68 promotes effective shadowing of individual
wires 68, thus allowing the surfaces of numerous wires 68 to remain
substantially free of coating deposits. This is especially
beneficial in the reactive sputtering of dielectric materials
(oxides and nitrides, for example) to ensure good electrical
conduction between the anodes and the plasma. The copper containing
brush wires have the advantage that they tend to form conductive
oxides. Sharp points on the ends of the wire lead to high electric
field strengths which lead to breakdown of any oxides deposited on
them.
[0082] The wire-brush anodes 63-1 through 63-9 are shown as being
disposed below the mounting blocks 61 in FIG. 6(a). However, it
should be understood that the wire-brush anodes may be attached to
the tops of the mounting blocks 61 in a manner similar to that
shown in FIGS. 5(a) and 5(b) that minimizes or eliminates vertical
gaps between adjacent anodes. Alternatively, the wire-brush anodes
may by configured in a manner similar to that shown in FIGS. 3 and
4, wherein vertical gaps exist between adjacent anodes.
[0083] The embodiments above have been described with reference to
an elongated planar magnetron sputter source. However, it should be
understood that the invention disclosed herein may be practiced in
conjunction with a rotating cylindrical magnetron sputter source as
well. In this case, the multiple-anode configurations illustrated
in FIGS. 2-6 are applied to a rotating cylindrical magnetron
sputter source.
[0084] The electrical control of the anodes discussed previously
with reference to FIG. 4 utilized variable resistors electrically
connected between the anode and an electrical ground. However,
control of the anodes may also be achieved with a multi-channel
high-current power supply such as the power supply 70 illustrated
in FIG. 7(a).
[0085] The multi-channel high-current power supply 70 illustrated
in FIG. 7(a) comprises a housing 711, a power supply cord 712 that
delivers power to the power supply 70, a main power switch 720 that
controls the power to the power supply, and ten channels that each
control the voltage and current delivered to a pair of anodes of
the sputter source 30. If desired, a channel of the power supply
may be used to control a single anode rather than a pair of anodes.
Preferably, the power supply 70 is rack mountable in a conventional
electrical mounting rack.
[0086] Each channel of the power supply 70 comprises a voltage
readout 713, a current readout 714, a voltage control member 715, a
current control member 716, and a channel power switch 717 to
control power to an individual channel. Preferably the readouts 713
and 714 are digital LED or LCD readouts. The control members 715
and 716 may be analog or digital potentiometers.
[0087] Each channel of the power supply 70 also comprises a
manual/remote switch 718 that allows an operator select between
manual control of the voltage and current using control members 715
and 716 or computer control of the voltage and current using an
optional computerized controller 750 shown in FIG. 7(b). The
controller 750 may be connected to the power supply 70 via
interfaces at the rear of the controller 750 and power supply (not
shown) using a detachable connecting line 708.
[0088] The controller 750 illustrated in FIG. 7(b) comprises a CPU
751, a display 752, and a keyboard 753 connected to the CPU via a
connecting line 754. The CPU 751 further comprises interfaces (not
shown) at the rear of the CPU for sending and receiving signals to
and from the multi-channel power supply 70 and other equipment (not
shown) of the drum vacuum coating system 20. For example, the
controller 750 may control mass flow controllers (not shown) and
receive signals from pressure measurement and voltage measurement
equipment (not shown) for controlling the pressure, flow and ratios
of sputtering gases such as argon, oxygen, nitrogen, and
others.
[0089] The controller 750 may also receive signals from coating
thickness monitors such optical thickness monitors disposed in each
coating zone shown in FIGS. 4 and 5 to provide real-time feedback
control of the anode voltages. Such real-time feedback obtained
from optical coating thickness monitors provides an advantageous
method for maintaining coating thickness uniformity among the
various coating zones illustrated in FIGS. 4 and 5. The controller
750 may also be used to monitor and control the cathode power
supply (not shown). One purpose of such control would be to adjust
the total current from the cathode power supply to match the total
current applied to the anodes by means of the constant current
power supplies. Detachable signal lines 755 may be used to pass
electrical signals between the controller 750 and such
equipment.
[0090] As noted above a real-time feedback method utilizing signals
from optical coating thickness monitors may be used to provide
real-time control of the anode currents via the controller 750 and
the multi-channel high-current power supply 70 shown in FIGS. 7(a)
and 7(b). Alternatively, post-deposition coating-thickness
measurements may be carried out on articles coated during a given
batch run to provide information that may be used to adjust the
currents to anode pairs in subsequent runs. This latter approach
provides a method for achieving coating-thickness uniformity that
is simple to implement and that does not require a computerized
controller 750.
[0091] There are a number of subtleties to be considered in the
implementation of the multi anode scheme of control. Firstly it is
usual that the sputter power supply potential is switched to about
100 volts positive for a brief period on a cyclic basis e.g. for
about 20 microseconds every 200 microseconds. The purpose of this
is to discharge the islands of metal oxide on the target which
would otherwise build up charge to the point where they would lead
to an electrical breakdown of the oxide film and consequently cause
a localized arc which has various deleterious effects. In a
conventional system there is a difficulty with this technique in
that, as soon as the sputter power supply begins to reverse its
polarity, the sputter plasma dies and there are few electrons left
to neutralize the positive charge on the islands of oxide.
Furthermore, the few electrons which are left to do this duty are
bottled up in the magnetic field of the sputter magnetron and are
not at liberty to neutralize charge on the islands of oxide outside
the magnetic bottle where they are most needed.
[0092] In a known conventional system (the MicroDyne.TM. System)
the existence of the microwave supported plasma nearby is supposed
to overcome these difficulties in two ways. During the polarity
reversal of the sputter power supply the microwave supported plasma
is still operational and is able to supply the electrons needed to
neutralize the charged metal oxide islands. Furthermore, the
electrons from the microwave plasma are outside the magnetic bottle
of the sputter magnetron and are therefore able to reach said
islands without difficulty.
[0093] However, during the polarity reversal of the sputter power
supply, the constant current generators attached to the anodes are
trying to gather their allotted requirement of electron current
which is typically on the order of two or a few amperes per anode
pair. The dying sputter plasma will not be able to supply this
electron current during said polarity reversal and the constant
current power supplies will attempt to draw it from the
surroundings, that is from the microwave plasma if present. In any
case the positive potential on these constant current power
supplies is likely to rise until they reach their voltage limit at
which point they will draw whatever electron current is available
to them at that voltage limit and which may be significantly below
the constant current set point. This is of no consequence to the
overall operation of the system with one important proviso: that
the constant current power supplies stay in or instantly return to
their constant current mode when the sputter power supply ceases
its brief cyclic polarity reversal.
[0094] The response of the system to an arc also bears
consideration. When an arc occurs as described previously or due to
other causes, modern sputter power supplies respond very quickly by
lowering the cathode voltage from a few hundred volts negative
potential to near zero volts thus quenching the arc. The sputter
power supply voltage may be kept near zero for some milliseconds to
ensure the arc does not reoccur when the cathode voltage is
returned to normal. During this arc-quenching period the multi
anode constant current power supplies are again going to scour the
surrounding volume for electrons and behave in the same manner as
previously described when the sputter plasma dies during brief
cyclic polarity reversals of the sputter power supply. However, in
this case the impact on the microwave plasma will be of longer
duration and the drain of electrons from it may be evidenced by a
longer period in which the microwave power source is mismatched to
the plasma. The microwave power reflected from the plasma may have
deleterious effects on said microwave power source and these need
to be monitored and guarded against if necessary. At the cost of
additional complication it may be deemed necessary or desirable to
shut down the constant current power supplies to the multi anodes
during periods of sputter power supply polarity reversal or arc
quenching.
[0095] Process Variable Monitoring and Control
[0096] In another preferred embodiment, a controller monitors
process variables and controls process parameters to improve
batch-to-batch uniformity of coating thickness and coating
composition.
[0097] Reactive sputtering systems have a large number of process
variables including the following:
1 (1) oxygen partial pressure (2) argon partial pressure (3) total
system pressure (4) water vapor partial pressure (5) pump down time
to base pressure (6) cathode voltage (7) kilowatt hours of use on
each target since new (8) kilowatt hours on each target since last
target cleaning (9) number of minutes of door open between runs
(10) calibration drifts and inaccuracies of vacuum gauges (11)
contamination of gauges especially of the Optical Gas Controller
(12) test usage of targets for deposition of thick single layers
(13) time since last shield clean (14) lens substrate type (15)
lens hardcoat type (16) substrate pretreatment (17) lens form
[0098] Many of these process variables are directly or indirectly
controllable. The interrelationships and dependencies of the
variables are complicated. Applicants have determined that the
cathode voltage and total system pressure are sensitive indicators
of the operating point and thus of deposition rate. Applicants have
also determined that it is possible to take an expert system
operator's knowledge of DC reactive sputter stabilization and
effectively use it in a Fuzzy Logic Control System for reactive
sputter deposition.
[0099] Work with such a system indicates fuzzy logic control can
improve batch to batch stability by a factor of almost 5 using just
two inputs and adjusting just two outputs. The two inputs used were
Cathode Voltage and Total Pressure as measured on an MKS Baratron.
The latter happens to be one of the very few instruments used in a
vacuum measurement which has very acceptable long term drift
performance.
[0100] The fuzzy logic can effectively encode the knowledge of an
expert operator in systems which preferably have just a few
important input variables and likewise just a few output controls.
As the above list indicates, there are actually very many input
variables to the deposition process. However, as discussed below, a
few key measured parameters and controllable output variables can
be identified.
[0101] FIG. 8 illustrates a fuzzy logic system of a preferred
embodiment of the present invention. The fuzzy-logic controller is
preferably used to control the system. The anode currents are
controlled utilizing the power supply 70 with or without the
controller 710 as described above.
[0102] As shown in FIG. 8, input signals corresponding to the
magnitude of the target (cathode) voltage and the total sputtering
gas pressure are monitored by the fuzzy logic controller 800.
Depending upon the magnitudes of the total gas pressure (X) and
cathode voltage (Y), the cathode voltage and total pressure signals
are categorized by the fuzzy-logic algorithm as high, medium, or
low values.
[0103] The fuzzy-logic algorithm then applies predetermined rules
based on these categorizations to provide output signals to mass
flow controllers to adjust the flow rates of sputtering gases A and
B. For example, if the cathode-voltage signal is monitored (or set)
as a "high" value, and the total pressure signal is monitored (or
set) as a "medium" value, output signals corresponding to
particular values of flow rates are directed to the mass flow
controller that controls the flow of process gases A and B. The
desired flow rates are determined by the fuzzy-logic controller
according to rule-based algorithm. Input signals corresponding to
different categorizations of cathode voltage and total pressure may
yield different output signals resulting in different flow rates
for the process gases (reactive and non-reactive) A and B. In this
manner, batch-to-batch uniformity of coating composition and
coating thickness is maintained. The fuzzy-logic rule-based
algorithm may also be employed in the computer controller 710
rather than in a separate fuzzy logic controller 800.
[0104] It may be necessary to have some form of averaging process
applied to the inputs (cathode voltage and total pressure). The
averaging period is tied to the natural time constants of the
measurement equipment, gas supply systems and the sputter process
itself. An adaptive control system may be needed which in effect
operates like a servo system based on first and second derivatives
of the error signal where the error signal is, for instance, the
difference between the instantaneous value of an input parameter
and its average value as formed by the averaging process. The
effect of this is to allow the fuzzy logic system to quickly reach
a point near the desired operating point and then settle closer to
it on the basis of longer term averages.
[0105] Because the process environment changes rapidly, the system
employs deterministic real-time control in order to guarantee
process stability. LabVIEW RT (a product of National Instruments
Co.) may be used as the software platform to run the fuzzy logic
control routine. The software is run on a PXI-7030/6030E real-time
data acquisition card. Besides the two analog I/O signals required
for fuzzy control, the 6030E also required additional analog
signals for process monitoring and additional digital signals from
an existing Programmable Logic Controller (PLC) which controlled
sequencing of the batch. LabVIEW RT running on the Windows NT host
PC was used for the user interface, data presentation, saving data
to file, additional SCADA functions via GPIB and Ethernet, and
editing the fuzzy control strategy. A Transmission Control Protocol
(TCP) communication link is used to send fuzzy control strategies
and commands to, and receive process data from, the LabVIEW program
running on the real-time card. By separating the data and process
control layers from the user interface and presentation layers, the
process control algorithm can run reliably with deterministic
timing. It is not affected by the CPU requirements of Windows NT or
other programs running in Windows. LabVIEW RT made this
possible.
[0106] LabVIEW RT allows the fuzzy logic control algorithms to be
run on a real-time data acquisition card with an embedded
processor. Exemplary system requirements are:
[0107] Real-Time Deterministic Control
[0108] Fuzzy Logic Controller
[0109] Analog Signal Interface for multiple Inputs and Outputs
[0110] Digital Signal Interface for Acquiring from a Programmable
Logic Controller (PLC)
[0111] GPIB interface for Instrument Communication
[0112] Ethernet Interface for Communication to a Vacuum Control
System
[0113] The National Instruments PXI platform may be used as the PC
architecture for the system.
[0114] The Fuzzy Logic Toolkit for LabVIEW may be used as a basis
for a customized fuzzy logic editor. This editor allows the
operator to easily toggle between the membership functions of the
inputs and outputs, as well as edit the rulebases for both outputs.
The amount of time required for the operator to precisely tune to
the control algorithm is reduced. As the system or product
requirements change, the algorithm can be readjusted to account for
these changes.
[0115] Also, using the 3D graphing capabilities of LabVIEW 5.1,
control surface plots can be created from the fuzzy logic
controllers for the operator to view. These plots can be rotated in
3 dimensions for careful inspection (see FIGS. 12(a) and 12(b). The
operator can zoom in on regions of interest when tuning the
system.
[0116] Even though the inputs of the system are defined with
imprecise terms like High, Low, and OK, the fuzzy logic system is
actually completely deterministic. For any set of input values
there is a unique set of outputs. The fuzzy controller allows
intuitive knowledge about a process to be realized in a
controller.
[0117] The described fuzzy logic system uses only two out of the
many possible input parameters and uses only two outputs of a
number of possible control outputs. The particular functions and
rule strengths etc. will be a function of the vacuum system; the
materials being deposited, etc. An implementation and example of
use of the techniques in a particular system will now be
discussed.
[0118] Fuzzy Logic Process
[0119] FIG. 9 is a flow diagram illustrating a preferred fuzzy
logic technique of the present invention. In the indicated first
step 810, the inputs are "Fuzzified", that is, classified into
their membership of fuzzy sets with linguistic definitions such as
"low", "OK" and "high". The classification of Cathode Voltage and
Total Pressure in the low, OK and high categories is shown in FIG.
10(a) and 10(b), respectively. More specifically, FIG. 10(a) shows
the cathode voltage membership function for reactive sputtering of
Niobium to form Niobia. FIG. 10(b) shows the corresponding
membership function for total pressure.
[0120] Evaluation of Rules
[0121] As a second step 812, a Rule Table is employed (based on
expert's knowledge or experimentation) which tells the system "what
to do" with control settings for all possible category combinations
of the input membership functions.
[0122] The Rule Table for this example is shown below in Table
I.
2 TABLE I Sputter Voltage Sputter Pressure Flow Adjustment Low High
Argon DECREASE Low Low Oxygen INCREASE High High Oxygen DECREASE
High Low Argon INCREASE OK High Argon DECREASE OK Low Argon
INCREASE High OK Oxygen DECREASE Low OK Oxygen INCREASE
[0123] Note that all other possible combinations of inputs not
listed in Table I result in a HOLD condition for both Oxygen and
Argon.
[0124] Table I entries are linguistically defined in a "fuzzy"
manner, i.e. as "decrease", "Hold", "Increase". The rules can be
determined from experience or based on an understanding of the
physics of the system.
[0125] Defuzzification
[0126] In the third step, 814, the linguistic "what to do"
instructions are "Defuzzified", i.e. converted to actual numbers
which change the control settings (in the example, those for only
Oxygen and Argon pressures).
EXAMPLE
[0127] The following is an example of a fuzzy logic control process
calculation. In the example, we shall assume that the instantaneous
operating point has drifted to a Cathode Voltage=370 Volts (Line A
of FIG. 10(a)) and a Total Pressure of 8.82 mBar (line B of FIG.
10(b)). From the graphs of FIG. 10(a), one may obtain a Voltage
Membership 0.75 "LO" and 0.25 "OK". From the Pressure Membership
Functions of FIG. 10(b) one may obtain the pressure membership
values of 0.30 "OK and 0.70 "HI. (Note that these membership
functions happen to add up to 1.00 in each case but they do not in
general need to). These membership function values are shown in the
top left hand box of FIG. 11 which will be used to track the
calculation of the output for oxygen flow (i.e. percent valve
opening of a piezoelectric valve).
[0128] Next, the membership of the output sets (DECRease, HOLD,
INCRease) for oxygen is calculated. Two Rules are used to do this.
The appropriate First Rule applied to the Pressure and Voltage
membership functions has been found to be Rule
Strength--MINimum.
[0129] That is, as illustrated in the box of FIG. 11 labeled "Rule
Strength MIN". The output membership is determined for "Pressure
OK" and "Voltage LO"=MIN (0.30, 0.75)=0.30 and so on for the other
eight values in this box.
[0130] The Second Rule which has been found to apply to the result
of the first rule is MAXimum, as illustrated in the box labeled
"Rule Strength=MAX". The resultant value for the membership
function of HOLD is the maximum of all HOLD values=MAX (0.70, 0.00,
0.25, 0.25, 0.00)=0.70. Likewise the resultant value for the
membership function of DECRease=MAX (0.30, 0.00)=0.30 and for
INCRease=MAX (0.00, 0.00)=0.00. The "fuzzy" membership values of
DECRease, HOLD and INCRease are next converted to "crisp" values
which can be sent to an oxygen flow controller as a percentage of
fully open of a piezoelectric valve or mass flow controller. We
have chosen to do this by first constructing the membership
defuzzification functions illustrated in the graph at the bottom of
FIG. 11 and then applying a "Center of Gravity" calculation to
"crisipfy" the output value as a percentage opening value. The
"Center of Gravity" calculation can be envisaged as a balancing
about the fulcrum point of 70% of the (DECR) area and 30% of the
(HOLD) area to yield an output of (say) 81% valve opening.
[0131] It should be understood that the foregoing examples are
adapted to a particular sputter deposition system and material and
would probably require some modification for use with other systems
and materials.
[0132] The controller should operate in real time to keep the
operating point stable during the deposition of each layer. It is
desirable that the measurement cycle time of the control system be
less than 50 msec.
[0133] As noted above, once all the rules and rule strengths of a
Fuzzy Logic Controller are established, the relationship between
the inputs and the outputs are completely deterministic and can in
principle and in practice be replaced by a lookup table which gives
the appropriate output values (oxygen and argon settings in our
case) for any given combination of input parameters (cathode
voltage and total pressure as in the case of the foregoing
example).
[0134] Indeed FIG. 12 indicates the type of three dimensional
control surface that one can generate for oxygen flow and argon
control versus the two above mentioned input parameters. The fuzzy
logic algorithms can be used to provide a somewhat intuitive
framework for developing rules and membership functions. It is
relatively easy, with such fuzzy controls, to make intuitively
understandable changes to the "low", "OK", "high" input regimes and
the "DECR"ease, "HOLD", "INCR"ease regimes of the outputs. This
would be much more difficult using just a lookup table. However, it
may be noted that, once the lookup table has in effect been
established in the development phase, it may well be easier to
implement such a lookup table.
[0135] The instant invention has been described with respect to
particular preferred embodiments and examples. The invention to be
protected, however, is intended to be defined by the literal
language of the claims and the equivalents thereof.
* * * * *