Title: Plasma processor apparatus and method, and antenna
Abstract: An antenna includes excitation terminals responsive to an RF source to supply an RF electromagnetic field to a plasma that processes a workpiece in a vacuum chamber. A matching network includes first and second portions respectively between the source and terminals and between the terminals and the antenna plasma excitation coil. In response to indications of impedance matching between the source and its load, currents flowing between (1) the first portion and the terminals and (2) the terminals and the coil are controlled so the latter exceeds the former. The indications control impedances of the first and second portions or the first portion impedance and the source frequency. The coil can include a transformer having a primary winding coupled to the excitation terminals and a multi-turn plasma excitation secondary winding.
Patent Number: 6,876,155 Issued on 04/05/2005 to Howald, et al.
| Inventors:
|
Howald; Arthur M. (Pleasanton, CA);
Kuthi; Andras (Thousand Oaks, CA)
|
| Assignee:
|
Lam Research Corporation (Fremont, CA)
|
| Appl. No.:
|
334063 |
| Filed:
|
December 31, 2002 |
| Current U.S. Class: |
315/111.51; 315/111.21; 118/723I; 118/723IR |
| Intern'l Class: |
H01J 007//24 |
| Field of Search: |
118/723 I,723 IR,723 E,723 MP
156/345,345.48
315/111.21,111.41,111.51,111.71,111.81
|
References Cited [Referenced By]
U.S. Patent Documents
| 5759280 | Jun., 1998 | Holland et al. | 118/723.
|
| 5800619 | Sep., 1998 | Holland et al. | 118/723.
|
| 6441555 | Aug., 2002 | Howald et al. | 315/111.
|
| 6531029 | Mar., 2003 | Ni et al. | 156/345.
|
| 6583572 | Jun., 2003 | Veltrop et al. | 315/111.
|
| 6741446 | May., 2004 | Ennis | 361/234.
|
Primary Examiner: Phan; Tho
Attorney, Agent or Firm: Lowe Hauptman & Berner, LLP
Parent Case Text
RELATION TO CO-PENDING APPLICATIONS
Certain aspects of the present application include subject matter disclosed
in the commonly assigned Howald et al. application Ser. Nos. 10/227,275
filed Aug. 26, 2002, (a continuation of U.S. Pat. No. 6,441,555) and
10/200,833 filed Jul. 22, 2002.
Claims
We claim:
1. A plasma processor antenna adapted to be driven by power from an AC
source, the antenna comprising first and second excitation terminals
adapted to be powered by power from the source, a plasma excitation coil
arranged to be energized by power coupled to said first and second
excitation terminals via circuitry outside the antenna, the antenna having
connections and reactances with values causing current flowing through the
excitation terminals via the circuitry outside the antenna to be less than
current flowing in a plasma excitation winding of the coil.
2. The antenna of claim 1 wherein the connections and reactances enable the
antenna and its load to be impedance matched or approximately impedance
matched to the source.
3. The antenna of claim 1 wherein the connections and reactances cause the
current adapted to flow in the plasma excitation winding to be maximized
or approximately maximized.
4. The antenna of claim 1 wherein the connections and reactances cause the
current flowing in the plasma excitation winding to be at least two times
the current flowing through the excitation terminals via the circuitry
outside the antenna.
5. The antenna of claim 1 wherein the source has at least one frequency in
a range of frequencies, the connections and the reactances causing a loop
of the antenna including the plasma excitation winding to have a resonant
frequency equal to or near a frequency in the range.
6. The antenna of claim 1 wherein the connections and the reactances cause
a loop of the antenna including the plasma excitation winding to have a
resonant frequency equal to or near a frequency in the range, the loop
including a first capacitor connected in series with the plasma excitation
winding.
7. The antenna of claim 6 wherein the first capacitor is connected in
series between first and second segments of the winding so the same
current flows through the first capacitor and the first and second
segments.
8. The antenna of claim 1 further including a first capacitor connected in
series with the coil between the first and second excitation terminals and
a second capacitor connected between the first and second excitation
terminals such that only some of the current flowing in the first
capacitor and the plasma excitation winding flows in the second capacitor.
9. The antenna of claim 8 wherein the first capacitor is connected between
one of the excitation terminals and one end of the winding so the same
current flows through the first capacitor and the winding.
10. The antenna of claim 8 wherein the first capacitor is connected in
series between first and second segments of the winding so the same
current flows through the first capacitor and the first and second
segments.
11. The antenna of claim 1 wherein the coil includes a transformer having a
primary winding and a plasma excitation secondary winding reactively
coupled with the primary winding.
12. The antenna of claim 11 wherein the primary winding has opposite ends
connected to the first and second excitation terminals.
13. The antenna of claim 12 wherein a coupling coefficient between the
windings is such that there is loose transformer coupling of the impedance
of the secondary winding to the primary winding.
14. The antenna of claim 13 wherein the coupling coefficient is in the
range of about 0.1 to 0.3.
15. The antenna of claim 11 wherein a coupling coefficient between the
windings is such that there is loose transformer coupling of the impedance
of the secondary winding to the primary winding.
16. The antenna of claim 11 wherein a capacitor is connected in series with
the plasma excitation secondary winding.
17. The antenna of claim 16 wherein the capacitor and plasma excitation
secondary winding are connected in a closed loop.
18. The antenna of claim 17 wherein the secondary winding includes plural
turns.
19. The antenna of claim 18 wherein the plural turns are in differing
planes adapted to be parallel to a coupling window of a chamber of the
processor.
20. The antenna of claim 19 wherein the turns are connected in series and
arranged so that current induced therein in response to AC current flowing
in the primary winding flows in a closed loop.
21. The antenna of claim 13 wherein the secondary winding includes plural
turns.
22. The antenna of claim 21 wherein the plural turns are concentric with an
axis of the coil, the primary winding including at least one further turn
that is concentric with the coil axis and is in a plane spatially parallel
to the plural turns of the secondary winding.
23. A plasma processor including the antenna of claim 1, the processor
including the AC source and a vacuum chamber having an interior arranged
to be responsive to electromagnetic fields derived by the antenna in
response excitation by the AC source.
24. Apparatus for processing a workpiece with a plasma comprising a vacuum
chamber arranged to process the workpiece with an AC plasma, an antenna
for supplying an AC electromagnetic field to the plasma, an AC plasma
excitation source, circuitry for coupling the AC source to excitation
terminals of the antenna, the antenna including a plasma excitation coil
for supplying a plasma exciting electromagnetic field to the plasma, and a
controller for causing current flowing between the coupling circuitry and
the excitation terminals to be less than current flowing in the coil.
25. The apparatus of claim 24 wherein the coupling circuitry includes a
matching network, a first portion of the matching network being coupled
between the source and the excitation terminals, the controller being
arranged for adjusting a variable impedance of the matching network so the
source output impedance is matched to the load of the source.
26. The apparatus of claim 25, wherein the source has a variable frequency,
the controller being arranged for adjusting the first variable impedance
and the source variable frequency to cause the current flowing between the
coupling circuitry and the excitation terminals to be less than current
flowing in the coil.
27. The apparatus of claim 26 wherein the controller includes a sensor
arrangement for deriving first and second indications respectively
associated with a desired frequency of the source and the impedance match
between the source and the load of the source, the controller being
arranged to respond to the indications derived by the sensor arrangement
to control the source frequency and the value of the first impedance to
provide the impedance match.
28. The apparatus of claim 27 wherein the sensor arrangement is arranged to
derive the first indication in response to an indication of maximum
current flowing in the coil.
29. The apparatus of claim 27 wherein the sensor arrangement is arranged to
derive the first and second indications in response to signals indicative
of power reflected from the load toward the source.
30. The apparatus of claim 26 wherein the controller includes a sensor
arrangement for deriving an indication of the impedance match between the
source and the load of the source, the controller being arranged to
respond to the impedance match indication derived by the sensor
arrangement to provide the impedance match and to control the source
frequency so the source frequency equals or nearly equals a resonant
frequency of a series circuit including a plasma excitation coil of the
antenna.
31. The apparatus of claim 25 wherein the second portion of the matching
network includes a second variable impedance, the controller being
arranged for adjusting the first variable impedance and the second
variable impedance to cause the current flowing between the coupling
circuitry and the excitation terminals to be less than current flowing in
the coil.
32. The apparatus of claim 31 wherein the controller arrangement includes a
sensor arrangement for deriving an indication of an impedance match
between the source and the load of the source, the controller being
arranged to respond to the impedance match indication derived by the
sensor to control the first and second impedances to attain an impedance
match.
33. The apparatus of claim 24 wherein the antenna includes a first
capacitor connected between the first and second excitation terminals and
a second capacitor connected in series with a plasma excitation winding of
the coil between the first and second excitation terminals, the first and
second capacitors being connected so different magnitude currents flow
therein.
34. The apparatus of claim 33 wherein the winding having the second
capacitor connected in series with it has a gap between a pair of
terminals, at least one terminal of the pair of terminals differing from
the excitation terminals, the second capacitor having opposite electrodes
respectively connected to the pair of terminals.
35. The apparatus of claim 34 wherein both terminals of the pair of
terminals differ from the first and second excitation terminals.
36. The apparatus of claim 24 wherein the coil includes a transformer
having primary and secondary windings, the primary winding being connected
to be driven by the coupling circuitry, the secondary winding being
arranged for coupling a plasma excitation electromagnetic field to the
plasma.
37. The apparatus of claim 36 further including a capacitor connected in
series with the secondary winding.
38. The apparatus of claim 37 wherein a coupling coefficient between the
windings is such that there is loose coupling of the impedance of the
secondary winding to the primary winding.
39. The apparatus of claim 38 wherein the coupling coefficient is in the
range of about 0.1 to 0.3.
40. The apparatus of claim 36 wherein the secondary winding includes plural
turns.
41. The apparatus of claim 40 wherein the plural turns are in differing
planes parallel to a coupling window of a chamber of the processor.
42. The apparatus of claim 40 wherein the turns are connected in series and
arranged so that current induced therein in response to AC current flowing
in the primary winding causes magnetic fluxes resulting from them to aid
in the plasma.
43. A method of operating a vacuum plasma processor having a vacuum chamber
including an AC plasma that processes a workpiece, the AC plasma being
excited by an antenna that supplies an AC electromagnetic field to the
plasma, the antenna being driven by power derived from an AC plasma
excitation source and coupled to first and second excitation terminals of
the antenna via coupling circuitry between the source and the terminals,
the method comprising causing the current flowing in a plasma excitation
coil of the antenna to exceed the current flowing between the coupling
circuitry and the antenna excitation terminals.
44. The method of claim 43 wherein the source has a variable frequency, the
causing step including adjusting the source variable frequency.
45. The method of claim 44 wherein the coupling circuitry includes a
portion of a matching network coupled between the source and the
excitation terminals, the causing step including adjusting a variable
impedance of the matching network portion so the source output impedance
is at least approximately matched to the load of the source.
46. The method of claim 45 further including deriving an indication of the
impedance match between the source and the load of the source, and
responding to the impedance match indication to adjust an impedance of the
matching network portion and the source frequency so the source impedance
is matched to the load of the source.
47. The method of claim 43 wherein the coupling circuitry includes first
and second portions of a matching network respectively coupled between the
source and the excitation terminals and between the excitation terminals
and the coil, the causing step including adjusting a first variable
impedance of the first portion and adjusting a second variable impedance
of the second portion.
48. The method of claim 47 further including deriving an indication of the
impedance match between the source and the load of the source, and
responding to the impedance match indication to adjust an impedance of the
impedances of the first and second portions so the source impedance is
matched to the load of the source.
Description
FIELD OF INVENTION
The present invention relates generally to plasma processor antennas and to
a method of and apparatus for operating plasma processors, and more
particularly, to a plasma processor antenna having components such that
the current flowing in a plasma excitation coil of the antenna exceeds the
current flowing in excitation terminals of the antenna. (Hereafter in this
document, current refers to RMS current or peak positive or negative
current during an AC cycle, unless otherwise noted.) Another aspect of the
invention relates to a plasma processor antenna having primary and
secondary windings.
BACKGROUND ART
A typical prior art workpiece processor, as illustrated in FIG. 1, includes
vacuum plasma processing chamber assembly 10, a first circuit 12 for
driving a planar excitation antenna 48 consisting of a coil for exciting
ionizable gas in chamber assembly to a plasma state, a second circuit 14
for applying RF bias to a workpiece holder in chamber assembly 10, and a
controller arrangement 16 responsive to sensors for various parameters
associated with chamber assembly 10 for deriving control signals for
devices affecting the plasma in chamber assembly 10. Controller 16
includes microprocessor 20 which responds to various sensors associated
with chamber assembly 10, as well as circuits 12 and 14, and signals from
operator input 22, which can be in the form, for example, of a keyboard.
Microprocessor 20 is coupled with memory system 24 including hard disk 26,
random access memory (RAM) 28 and read only memory (ROM) 30.
Microprocessor 20 responds to the various signals supplied to it to drive
display 32, which can be a typical computer monitor.
Hard disk 26 and ROM 30 store programs for controlling the operation of
microprocessor 20 and preset data associated with different recipes for
the processes performed in chamber assembly 10. The different recipes
concern gas species and flow rates applied to chamber assembly 10 during
different processes, the output power of AC sources included in circuits
12 and 14, the vacuum applied to the interior of chamber assembly 10, and
initial values of variable reactances included in matching networks of
circuits 12 and 14.
Plasma chamber assembly 10 includes chamber 40 having non-magnetic
cylindrical side wall 42 and non-magnetic base 44, both of which are
frequently metal and electrically grounded. Dielectric, typically quartz,
window 46 is fixedly positioned on the top edge of wall 42.
Wall 42, base 44 and window 46 are rigidly connected to each other by
suitable gaskets to enable a vacuum to be established within the interior
of chamber 40. Plasma excitation antenna 48 includes coil 49, that is
planar or dome shaped, and can be configured as disclosed in Ogle, U.S.
Pat. No. 4,948,458 or Holland et al., U.S. Pat. No. 5,759,280 or Holland
et al, U.S. Pat. No. 5,800,619 sits on or in very close proximity to the
upper face of window 46. Antenna 48 reactively supplies magnetic and
electric RF fields to the interior of chamber 40, to excite ionizable gas
in the chamber to a plasma, schematically illustrated in FIG. 1 by
reference numeral 50.
The upper face of base 44 carries holder (i.e. chuck) 52 for workpiece 54,
which is typically a circular semiconductor wafer, a rectangular
dielectric plate such as used in flat panel displays or a metal plate.
Workpiece holder 52 typically includes metal plate electrode 56 which
carries dielectric layer 58 and sits on dielectric layer 60, which is
carried by the upper face of base 44. A workpiece handling mechanism (not
shown) places workpiece 54 on the upper face of dielectric layer 58.
Workpiece 54 is cooled by supplying helium from a suitable source 62 to
the underside of dielectric layer 58 via conduit 64 and grooves (not
shown) in electrode 56. With workpiece 54 in place on dielectric layer 58,
d.c. source 66 supplies a suitable voltage through a switch (not shown) to
electrode 56 to clamp, i.e., chuck, workpiece 54 to holder 52.
With workpiece 54 secured in place on chuck 52, one or more ionizable gases
from one or more sources 68 flow into the interior of chamber 40 through
conduit 70 and port 72 in sidewall 42. For convenience, only one gas
source 68 is shown in FIG. 1. The interior of conduit 70 includes valve 74
and flow rate gauge 76 for respectively controlling the flow rate of gas
flowing through port 72 into chamber 40 and measuring the gas flow rate
through port 72. Valve 74 responds to a signal microprocessor 20 derives,
while gauge 76 supplies the microprocessor with an electric signal
indicative of the gas flow rate in conduit 70. Memory system 24 stores for
each recipe of each workpiece 54 processed in chamber 40 a signal
indicative of desired gas flow rate in conduit 70. Microprocessor 20
responds to the signal memory system 24 stores for desired flow rate and
the monitored flow rate signal gauge 76 derives to control valve 74
accordingly.
Vacuum pump 80, connected to port 82 in base 44 of chamber 40 by conduit
84, evacuates the interior of the chamber to a suitable pressure,
typically in the range of one to one hundred millitorr. Pressure gauge 86,
in the interior of chamber 40, supplies microprocessor 20 with a signal
indicative of the vacuum pressure in chamber 40.
Memory system 24 stores for each recipe a signal indicative of desired
vacuum pressure for the interior of chamber 40. Microprocessor 20 responds
to the stored desired pressure signal memory system 24 derives for each
recipe and an electric signal from pressure gauge 86 to supply an electric
signal to vacuum pump 80 to maintain the pressure in chamber 40 at the set
point or predetermined value for each recipe.
Optical spectrometer 90 monitors the optical emission of plasma 50 by
responding to optical energy emitted by the plasma and coupled to the
spectrometer via window 92 in side wall 42. Spectrometer 90 responds to
the optical energy emitted by plasma 50 to supply an electric signal to
microprocessor 20. Microprocessor 20 responds to the signal spectrometer
90 derives to detect an end point of the process (either etching or
deposition) that plasma 50 is performing on workpiece 54. Microprocessor
20 responds to the signal spectrometer 90 derives and a signal memory
system 24 stores indicative of a characteristic of the output of the
spectrometer associated with an end point to supply the memory with an
appropriate signal to indicate the recipe has been completed.
Microprocessor 20 then responds to signals from memory system 24 to stop
certain activities associated with the completed recipe and initiate a new
recipe on the workpiece previously processed in chamber 40 or commands
release of workpiece 54 from chuck 52 and transfer of a new workpiece to
the chuck, followed by instigation of another series of processing
recipes.
Excitation circuit 12 for driving coil 49 of antenna 48 includes constant
or variable frequency RF source 100 (see Barnes et al U.S. Pat. No.
5,892,198), typically having a frequency of 4.0.+-.10% MHz or 13.56.+-.10%
MHz. Source 100 drives variable gain power amplifier 102, typically having
an output power in the range between 100 and 3000 watts. Amplifier 102
typically has a 50 ohm output impedance all of which is resistive and none
of which is reactive. Hence, the impedance seen looking back into the
output terminals of amplifier 102 is typically represented by (50+j0)
ohms, and cable 106 is chosen to have a characteristic impedance of 50
ohms.
For any particular recipe, memory system 24 stores a signal for desired
output power of amplifier 112. Memory system 24 supplies the desired
output power of amplifier 102 to the amplifier by way of microprocessor
20. The output power of amplifier 102 can be controlled in an open loop
manner in response to the signals stored in memory system 24 or control of
the output power of amplifier 102 can be on a closed loop feedback basis,
as known in the art.
The output power of amplifier 102 drives coil 49 via cable 106 and matching
network 108. Matching network 108, configured as a "T," includes two
series legs including variable capacitors 112 and 116, as well as a shunt
leg including fixed capacitor 114. The antenna 48 includes excitation
terminals 122 and 124, respectively connected to (1) a first end of coil
49 and one electrode of capacitor 112 and (2) a second end of coil 49 and
a first electrode of series capacitor 126, having a grounded second
electrode; or terminal 124 can be connected directly to ground. The value
of capacitor 126 is preferably selected as described in the commonly
assigned, previously mentioned, Holland et al. '200 patent.
Electric motors 118 and 120, preferably of the step type, respond to
signals from microprocessor 20 to control the values of capacitors 112 and
116 in relatively small increments to maintain an impedance match between
the impedance seen by looking from the output terminals of amplifier 102
into cable 106 and by looking from cable 106 into the output terminals of
amplifier 102. Hence, for the previously described (50+j0) ohm output
impedance of amplifier 102 and 50 ohm characteristic impedance of cable
106, microprocessor 20 controls motors 118 and 120 so the impedance seen
looking from cable 106 into matching network 108 is as close as possible
to a matched impedance of (50+j0) ohm. Alternatively, microprocessor 20
controls the frequency of source 100 and the capacitance of capacitor 116
to achieve a matched impedance between the source and the load it drives.
As a result of a matched impedance being attained, the current flowing
through capacitors 112 and 126 and the leads connecting the capacitors to
terminals 122 and 124, is typically within a couple of percent of its very
high maximum value. The very high current in these leads has an adverse
effect on the uniformity of the density of plasma 50.
To control motors 118 and 120 or the frequency of source 100 and motor 120
to maintain matched conditions between the impedance seen looking into the
output terminals of amplifier 102 and the impedance amplifier 102 drives,
microprocessor 20 responds to signals from conventional sensor arrangement
104. The signals are indicative of the impedance seen looking from cable
106 into matching network 108; usually the signals represent the absolute
values of the current and voltage reflected toward the sensor from
capacitor 118, and the phase angle between the reflected current and
voltage. Alternatively, sensors are provided for deriving signals
indicative of the power that amplifier 102 supplies to its output
terminals and the power reflected by cable 106 back to the output of
amplifier 102. Microprocessor 20 responds, in one of several known
manners, to the sensed signals sensor arrangement 104 derives to control
motors 118 and 120 or the frequency of source 100 and motor 120 to attain
the matched condition.
Because of variations in conditions in the interior of chamber 40 which
affect plasma 50, the plasma has a variable impedance. The conditions are
aberrations in the flow rate and species of the gas flowing through port
72, aberrations in the pressure in chamber 40 and other factors. In
addition, noise is sometimes supplied to motors 118 and 120 causing the
motors to change the values of capacitors 112 and 116. All of these
factors affect the impedance reflected by the load including plasma 50
back to the output terminals of amplifier 102. Microprocessor 20 responds
to the output signals of sensor 104, to vary the values of capacitors 112
and 116 or the frequency of source 100, to maintain the impedance driven
by the output terminals of amplifier 102 matched to the output impedance
of the amplifier.
Circuit 14 for supplying RF bias to workpiece 54 via electrode 56 has a
construction somewhat similar to circuit 12. Circuit 14 includes constant
frequency RF source 130, typically having a frequency such as 400 kHz, 2.0
MHz or 13.56 MHz. The constant frequency output of source 130 drives
variable gain power amplifier 132, which in turn drives a cascaded
arrangement including directional coupler 134, cable 136 and matching
network 138. Matching network 138 includes a series leg comprising the
series combination of fixed inductor 140 and variable capacitor 142, as
well as a shunt leg including fixed inductor 144 and variable capacitor
146. Motors 148 and 150, which are preferably step motors, vary the values
of capacitors 142 and 146, respectively, in response to signals from
microprocessor 20.
Output terminal 152 of matching network 138 supplies an RF bias voltage to
electrode 56 by way of series coupling capacitor 154 which isolates
matching network 138 from the chucking voltage of d.c. source 66. The RF
energy circuit 14 applies to electrode 56 is capacitively coupled via
dielectric layer 48, workpiece 54 and a plasma sheath between the
workpiece and plasma to a portion of plasma 50 in close proximity with
chuck 52. The RF energy chuck 52 couples to plasma 50 establishes a d.c.
bias in the plasma; the d.c. bias typically has values between 50 and 1000
volts. The d.c. bias resulting from the RF energy circuit 14 applies to
chuck 52 accelerates ions in the plasma 50 to workpiece 54.
Microprocessor 20 responds to signals indicative of the impedance seen
looking from cable 136 into matching network 138, as derived by a known
sensor arrangement 139, to control motors 148 and 150 and the values of
capacitors 142 and 146 in a manner similar to that described supra with
regard to control of capacitors 112 and 116 of matching network 108.
For each process recipe, memory system 24 stores a set point signal for the
net power flowing from directional coupler 134 into cable 136. The net
power flowing from directional coupler 134 into cable 136 equals the
output power of amplifier 132 minus the power reflected from the load and
matching network 138 back through cable 136 to the terminals of
directional coupler 134 connected to cable 136. Memory system 28 supplies
the net power set point signal associated with circuit 14 to
microprocessor 20. Microprocessor 20 responds to the net power set point
signal associated with circuit 14 and the output signals that directional
coupler 134 supply to power sensor arrangement 141. Power sensor
arrangement 141 derives signals indicative of output power of amplifier
132 and power reflected by cable 136 back toward the output terminals of
amplifier 132.
FIG. 2 is a perspective view of an antenna consisting of a planar coil of
the type schematically illustrated in FIG. 6 of the previously mentioned
'619 patent and which has been incorporated as the coil of antenna 48 in
processors of the type illustrated in FIG. 1. The coil illustrated in FIG.
2 includes a single winding 160 including inner and outer concentric metal
turns 162 and 164, each of which has a square cross-section and is shaped
as a sector of a circle extending through an angle of approximately 340
degrees. Opposite ends of turns 162 and 164 respectively include
excitation terminals 166 and 168, respectively connected by metal posts
(i.e. current feeds) 170 and 172 to one electrode of capacitor 112 of
matching network 108 and to one electrode of capacitor 126; alternatively,
post 172 connects excitation terminal 168 to ground directly.
Consequently, the RF (i.e. AC) current which flows in posts 170 and 172 is
approximately equal to the RF current which flows in turns 162 and 164.
The ends of turns 162 and 164 remote from terminals 166 and 168 are
connected to each other by straight metal strut 174 that extends generally
radially between turns 162 and 164 and has the same cross-sectional
configuration as the turns.
The two turn coil of FIG. 2 differs from an ideal two turn coil which
consists of two coaxial circular loops having constant, equal amplitude RF
currents flowing therein throughout the length of each loop. Such an ideal
two turn coil would provide, to the plasma 50 in chamber 40, electric and
magnetic fields having complete cylindrical symmetry. The coil of FIG. 2,
as well as all practical coils that can be used as the coil of antenna 48,
has connections (such as strut 174 that connects turns 162 and 164)
between any loops or windings included in the coil, and current feed
points, such as excitation terminals 166 and 168 that connect posts 170
and 172 to turns 162 and 164. These connections prevent all practical
coils from having the complete cylindrical symmetry of the idealized coil.
The currents in the practical coil of FIG. 2 can be expressed as the sum of
the current in the ideal portions of the coil, i.e., turns 162 and 164,
plus the current in a hypothetical perturbation coil that includes
terminals 166 and 168, posts 170 and 172, and strut 174. The hypothetical
perturbation coil thus includes the effects of the current feeds formed by
posts 170 and 172, the "missing" sections of the loops formed by turns 162
and 164, as well as strut 174 which forms a connection between the loops
formed by turns 162 and 164. The current flowing in the hypothetical
perturbation coil, including the high current flowing in the current feeds
formed by posts 170 and 172, has a tendency to cause azimuthal asymmetry
in the magnetic field coupled by the coil to the plasma, resulting in
azimuthal asymmetry in the plasma density processing the workpiece.
One object of the present invention is to provide a new and improved plasma
processor including a plasma having a density with reduced azimuthal
asymmetry.
Another object of the present invention is to provide a new and improved
antenna arrangement for a plasma processor.
An added object is to provide a new and improved plasma processor antenna
arrangement for enabling the plasma of the processor to have density with
relatively low asymmetry.
An additional object of the present invention is to provide a new and
improved antenna arrangement for a plasma processor, wherein the antenna
arrangement is arranged so that the perturbing effects of RF feeds that
supply current to the antenna arrangement are reduced compared to a
typical prior art arrangement.
A further object of the present invention is to provide a new and improved
plasma processor wherein a relatively high current flows in a plasma
excitation coil of an antenna while a substantially lower amplitude
current flows in the leads connecting the antenna to circuitry which
drives the antenna.
SUMMARY OF THE INVENTION
One aspect of the invention relates to a plasma processor antenna adapted
to be driven by power from an AC source. The antenna comprises first and
second excitation terminals adapted to be powered by power from the source
and a plasma excitation coil arranged to be energized by power coupled to
the first and second excitation terminals via circuitry outside the
antenna. The antenna has connections and reactances with values causing
current flowing through the excitation terminals via the circuitry outside
the antenna to be less than current flowing in a plasma excitation winding
of the coil. The antenna excitation terminals are driven by power from the
AC source via the circuitry outside the antenna and supply power to
circuitry within the antenna.
Another aspect of the invention relates to an apparatus for processing a
workpiece with a plasma. The apparatus comprises a vacuum chamber arranged
to process the workpiece with an AC plasma, an antenna for supplying an AC
electromagnetic field to the plasma, and an AC plasma excitation source.
The antenna includes a plasma excitation coil for supplying a plasma
exciting electromagnetic field to the plasma. A controller causes current
flowing between coupling circuitry between the AC source and antenna
excitation terminals to be less than current flowing in the coil.
A further aspect of the invention relates to a method of operating a vacuum
plasma processor having a vacuum chamber including an AC plasma that
processes a workpiece. The AC plasma is excited by an antenna that
supplies an AC electromagnetic field to the plasma. The antenna is driven
by power derived from an AC plasma excitation source and coupled to first
and second excitation terminals of the antenna via coupling circuitry
between the source and the terminals. The method comprises causing the
current flowing in a plasma excitation coil of the antenna to exceed the
current flowing between the coupling circuitry and the antenna excitation
terminals.
Typically, the coupling circuitry includes a portion of a matching network
coupled between the source and the excitation terminals. The current
flowing in the coil is made to exceed the current flowing between the
coupling circuitry and the antenna excitation terminals by adjusting a
variable impedance of the matching network portion so the source output
impedance is at least approximately matched to the load of the source.
In one embodiment, the source has a variable frequency and the current
flowing in the coil is made to exceed the current flowing between the
coupling circuitry and the antenna excitation terminals by adjusting the
source variable frequency.
In another embodiment, the coupling circuitry includes first and second
portions of a matching network respectively coupled between the source and
the excitation terminals and between the excitation terminals and the
coil. The current flowing in the coil is made to exceed the current
flowing between the coupling circuitry and the antenna excitation
terminals by adjusting a first variable impedance of the first portion and
adjusting a second variable impedance of the second portion.
In preferred embodiments, an indication of the impedance match between the
source and the load of the source is derived. In response to the impedance
match indication, (1) the impedances of the first and second portions are
adjusted so the source impedance is matched to the load of the source, or
(2) the impedance of the first portion and the frequency of the source are
adjusted so the source impedance is matched to the load of the source.
Alternatively, both matching network impedances or the matching network
impedance in the first portion and the source frequency are adjusted to
cause the current in the coil to be maximized or approximately maximized.
Typically, the current flowing in the plasma excitation winding is at least
three times the current flowing through the excitation terminals via the
coupling circuitry outside the antenna.
In certain embodiments, a loop of the antenna including the plasma
excitation winding is resonant to a frequency in a range of frequencies of
the source. In this document, the excitation winding is considered to be
resonant to a frequency of the source when the impedances (at the
frequency of the source) of the load the source drives (including the
plasma excitation winding and the plasma) have values causing the current
in the excitation winding to have a maximum value for the source
frequency.
The antenna can include a first capacitor connected in series with the coil
between the first and second excitation terminals and a second capacitor
connected between the first and second excitation terminals such that only
some of the current flowing in the first capacitor and the plasma
excitation coil flows in the second capacitor. The first capacitor can be
connected between one of the excitation terminals and one end of the coil
so the same current flows through the first capacitor and the coil. The
first capacitor can alternatively be connected in series between first and
second segments of the winding so the same current flows through the first
capacitor and the first and second segments.
In other embodiments, the coil includes a transformer having a primary
winding and a plasma excitation secondary winding reactively coupled with
the primary winding. Typically, the primary winding has opposite ends
connected to the first and second excitation terminals. A coupling
coefficient between the windings is preferably such that there is loose
transformer coupling of the impedance of the secondary winding to the
primary winding; the coupling coefficient is usually in the range of about
0.1 to 0.3.
Preferably, at least one capacitor is connected in series with the plasma
excitation secondary winding to provide a closed loop that is resonant to
a frequency of the source.
Another aspect of the invention concerns an antenna for a plasma processor.
The antenna is adapted to be driven by power from an AC source for
exciting a plasma and comprises first and second excitation terminals, as
well as a coil including (1) a primary winding having opposite ends
respectively coupled with the first and second excitation terminals, and
(2) a secondary winding reactively coupled with the primary winding. A
capacitor has first and second opposite electrodes respectively connected
in series with the secondary winding.
A further aspect of the invention relates to an antenna for a plasma
processor. The antenna is adapted to be driven by power from an AC source
for exciting a plasma and comprises first and second excitation terminals,
as well as a coil including (1) a primary winding having opposite ends
respectively coupled with the first and second excitation terminals, and
(2) a secondary winding reactively coupled with the primary winding. The
secondary winding includes plural turns.
Preferably, the turns are in different parallel planes adapted to be
spatially parallel to a coupling window of a vacuum chamber of the
processor. The plural turns are also preferably concentric with an axis of
the coil. In such case, the primary winding includes at least one further
turn that is concentric with the coil axis and is in a plane spatially
parallel to the plural turns of the secondary winding and the secondary
winding includes multiple turns in each of the planes. The turns of the
secondary winding are preferably connected in series with each other and
arranged so AC current induced to them in response to excitation of the
primary winding flows in the same direction through half planes extending
from the axis through the turns.
The above and still further objects, features and advantages of the present
invention will become apparent upon consideration of the following
detailed descriptions of several specific embodiments thereof, especially
when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1, as previously described, is a schematic diagram of a prior art
vacuum plasma processor;
FIG. 2, as previously described, is a perspective view of an antenna coil
of the type which has been employed in the vacuum plasma processor of FIG.
1;
FIGS. 3-12 are schematic diagrams of different embodiments of drive
networks in combination with several antenna embodiments, in accordance
with preferred embodiments of the present invention;
FIG. 13 is a perspective view of the antenna included in the embodiments of
FIGS. 3, 4 and 8;
FIG. 14 is a perspective view of a the antenna included in the embodiments
of FIGS. 5 and 7;
FIG. 15 is a perspective view of an antenna including a transformer having
primary and secondary windings, as schematically illustrated in FIGS. 10
and 11; and
FIG. 16 is a schematic perspective view of an antenna including a
transformer having a primary winding and a four-turn secondary winding, as
schematically illustrated in FIG. 12.
DETAILED DESCRIPTION OF FIGS. 3-16
Reference is now made to the schematic diagram of FIG. 3 wherein fixed
frequency RF source 232 (typically having a frequency of 4.0 MHz or 13.56
MHz) is illustrated as having an output that drives cable 106, having an
output connected, via sensor 104, to one electrode of variable, series
connected capacitor 212 of matching network 211. A second electrode of
capacitor 212 is connected by current feed or post 213 to excitation
terminal 214 of antenna 215, including coil 216; details of antenna 215
are illustrated in FIG. 13. Antenna 215 replaces antenna 48, FIG. 1. A
first terminal of coil 216 is connected to grounded excitation terminal
220 via the series connection of ammeter 228 and fixed capacitor 426
(which is the equivalent of capacitor 126, FIG. 1). The impedance of
plasma 50 is indicated in FIG. 3 by the series impedance Z.sub.P (box 218)
between the first end of coil 216 and capacitor 426. Variable capacitor
223, preferably a semiconductor of the type that is electronically
controlled by a voltage applied to a control electrode thereof, is
connected in series with excitation terminal 214 and a second end of coil
216. Opposite electrodes of fixed capacitor 224 are respectively connected
to excitation terminals 214 and 220. Capacitors 212, 223 and 224 form
impedance matching network 211 similar to impedance matching network 108
of FIG. 1. However, capacitors 223 and 224 are relocated onto antenna 215
so that the relatively high current that flows in these capacitors does
not flow through rf feeds 213 and 222. In contrast, capacitors 112 and 114
of matching network 108 of the prior art are physically removed from
antenna 48 and the relatively high currents that flow through capacitors
112 and 114 also flow through the rf leads that connect capacitors 112 and
114 to antenna 48.
A controller including microprocessor 229 controls the values of capacitors
212 and 223 to achieve (1) maximum current in ammeter 228 and,
equivalently, (2) impedance matching between the output of source 210 and
the load it drives, at the input of cable 106. To achieve these results,
microprocessor 229 responds to the output of ammeter 228 or of sensor 104
to adjust (1) the capacitances of capacitors 212 and 223 until the output
impedance of source 232 and the impedance the source drives are matched,
or (2) until the current in coil 216, as measured by ammeter 228, is
maximized. In response to one of these criteria being achieved, usually in
an iterative manner, the current in the feeds is lower (typically about
one-half to one-fifth) than the current in branch 225, and problems
associated with high current flowing in feeds 213 and 222 discussed
previously are avoided. Some of the current in coil 216 also flows in
capacitor 224 and some in feeds 213 and 222. The phases of the
instantaneous currents flowing in branch 225, and feeds 213 and 222
differ, such that the phase of the instantaneous current in coil 216 is
about 90.degree. from the instantaneous currents in feeds 213 and 222.
Microprocessor 229 responds to indications of the current magnitude sensed
by ammeter 228 or the phase angle, voltage magnitude and current magnitude
as indicated by output signals of sensor 104. Microprocessor 229 responds
to the output signals of ammeter 228 or sensor 104 to control the
capacitances of capacitors 212 and 223 to maximize the current in coil 216
or so the impedance seen looking from cable 106 into sensor 104 equals the
characteristic impedance of cable 106, i.e., a match is attained. If
necessary, microprocessor 229 responds to the outputs of sensor 104 and
ammeter 228 to iteratively control the values of capacitors 212 and 223 so
that the current in coil 216 is maximized and the output impedance of
source 210 is matched to the load it drives. Maximizing or nearly
maximizing the current in coil 216 causes the electromagnetic field the
coil supplies to the plasma to be maximized.
Reference is now made to FIG. 4 of the drawing, a schematic diagram of an
embodiment which is similar to the embodiment of FIG. 3. FIG. 4 is the
same as FIG. 3 except that ammeter 228 is eliminated and microprocessor
230 replaces microprocessor 229. Microprocessor 230 includes a
conventional, prior art algorithm for controlling the impedances of a
matching network of a plasma processor. Microprocessor 230 thus can be the
same as microprocessor 20 of FIG. 1.
Microprocessor 230 controls capacitor 223 that is part of antenna 215 and
part of matching network 211 to achieve an impedance match between source
232 and the load it drives. Such an impedance match is accompanied by the
current in coil 216 being greater than the current flowing in feeds 213
and 222 from circuitry outside antenna 215. The coil current is typically
two to five times current in feeds 213 and 222.
When microprocessor 230 has controlled capacitors 212 and 233 to achieve
impedance matching of source 232 to the impedance it drives, the current
that flows in coil 216 is maximized. It can be shown that a match between
the output impedance of source 232 and the load the source drives is
achieved when the current in coil 216 (I.sub.c) is related to the current
flowing in feeds 213 and 222 (I.sub.IN) from circuitry outside antenna 215
in accordance with
##EQU1##
where
R.sub.0 =the characteristic impedance of cable 106, and
R.sub.P =the real part of impedance Z.sub.P of plasma 50 as coupled to coil
216.
Typical values of R.sub.0 and R.sub.P are respectively 50 ohms and 2-10
ohms, resulting in
##EQU2##
Hence, for the optimum impedance matching and maximum current in coil 216
(i.e., a resonant condition for the load that source 216 drives) the
current in coil 216 is typically about two to five times the current
flowing via feeds 213 and 224 through excitation terminals 214 and 220.
However, for some non-optimum conditions, e.g., there is a slight
impedance mismatch between source 232 and the load it drives or the
current flowing in coil 216 is somewhat less than the maximum current that
can flow in the coil for the frequency of source 232, the current in coil
216 is also about twice the current flowing through excitation terminals
214 and 220 via feeds 213 and 224, for values of plasma resistance less
than 10 ohms.
It can be shown that matching between the output impedance of source 232
and the load the source drives results when
##EQU3##
where
.omega.=2.pi..times.f
f=frequency of source 232
C.sub.1 =capacitance of capacitor 212
C.sub.2 =capacitance of capacitor 224 and
C.sub.3 =series capacitance of capacitors 223 and 426
L=inductance of coil 216
R.sub.P =real part of effective plasma impedance Z.sub.P (218), and
L.sub.P =inductance of effective plasma impedance Z.sub.P (218)
For typical values of: f=13.56 MHz, R.sub.0 =50 ohms, R.sub.P =5 ohms,
C.sub.2 =100 pf, L=2.2 .mu.H, and L.sub.P =-0.2 .mu.H, C.sub.1 =47 pf and
C.sub.3 =131 pf (by taking the positive value of the square root for the
values of C.sub.1 and C.sub.3).
Reference is now made to FIG. 5, which is the same as FIG. 4 except that
coil 216 and capacitor 223 are respectively replaced by coil 240 and
variable capacitor 242. The capacitances of capacitors 212 and 242 are
controlled by a microprocessor (not shown in FIG. 5) in the same way that
microprocessor 230 controls capacitors 212 and 223.
Coil 240 includes two series connected segments 244 and 246, having a gap
between them. The gap is defined by a pair of terminals 248 and 250 of
coil 240. Terminals 248 and 250 are respectively connected to opposite
electrodes of capacitor 242 so that capacitor 242 is connected in series
with segments 244 and 246. The gap between terminals 248 and 250 is
preferably located at a location in coil 240 which provides optimum
distribution of current and voltage along the length of the coil, as
described in U.S. Pat. No. 6,441,555.
Reference is now made to FIG. 6, which is the same as FIG. 5, except that
variable capacitor 242 is replaced by variable capacitor 602 and fixed
capacitor 604. Capacitors 212 and 602 are controlled by a microprocessor
(not shown in FIG. 6) in the same way that microprocessor 230 controls
capacitors 212 and 223.
Variable capacitor 602 is connected between terminal 214 and one end of
coil segment 244, the other end of which is connected to one electrode of
capacitor 604. A second electrode of capacitor 604 is connected to one end
of coil segment 246. The location of capacitor 604 in coil 240 is
typically different from the location of capacitor 242 because capacitor
602 changes the voltage and current distribution in coil 240.
Reference is now made to FIG. 7 of the drawing, which is the same as FIG. 5
except that variable frequency source 233 replaces fixed frequency source
232, microprocessor 231 replaces the microprocessor of FIG. 5 and fixed
capacitor 702 replaces variable capacitor 242. Variable frequency source
233 typically has a range of frequencies that is typically about .+-.10%
of the center frequency of the source. Microprocessor 231 responds to the
indications derived by sensor 104, to control (in a known manner) the
frequency of source 233 and the capacitance of capacitor 212 until the
microprocessor detects an impedance match between the source and the load
it drives. The combination of indications sensor 104 derives indicates the
impedance match between source 233 and the load it drives. As a result of
microprocessor 231 controlling the frequency of source 233 and the
capacitance of capacitor 212 to achieve the impedance match, plasma
impedance 218, capacitors 426 and 702, as well as coil segments 244 and
246, have impedances that cause the current in coil segments 244 and 246
to exceed the current flowing through feeds 213 and 222 from the circuitry
outside antenna 215; typically, the current in coil segments 244 and 246
is about two to five times the current in feeds 213 and 222 since
##EQU4##
and the values of R.sub.0 and R.sub.P are typically as previously stated.
It is to be understood that the frequency of source 233 can alternatively
be controlled in response to a signal that detects a maximum current in
branch 225, as described in connection with FIG. 3.
Reference is now made to FIG. 8 that is the same as FIG. 4 except that
variable frequency source 233 replaces fixed frequency source 232, a
microprocessor (not shown in FIG. 8) that is the same as microprocessor
231 replaces microprocessor 230 and fixed capacitor 802 replaces variable
capacitor 223. The microprocessor of FIG. 8 controls the frequency of
source 233 and the value of capacitor 212 as described in connection with
FIG. 7.
Reference is now made to FIG. 9, which is the same as FIG. 6, except that
variable frequency source 233 replaces fixed frequency source 232, a
microprocessor (not shown) that is the same as microprocessor 231 replaces
the microprocessor of FIG. 6, and fixed capacitor 902 replaces variable
capacitor 602. The microprocessor of FIG. 9 controls the frequency of
source 233 and the value of capacitor 212 as described in connection with
FIG. 7.
Reference is now made to FIG. 10 wherein the antenna of FIG. 4 is modified
to include air core transformer 1002 including primary winding 1004 and
secondary winding 1006 that drives the plasma load impedance 218 and is in
series with variable capacitor 1008. Opposite ends of primary winding 1004
are connected to excitation terminals 214 and 220, so that the primary
winding is in series with variable capacitor 212 of the matching network.
Primary winding 1004 can thus be considered as part of the matching
network. Fixed frequency source 232 is connected in series with cable 106,
variable capacitor 212, RF feeds 213 and 224, and winding 1004 so that
substantially the same current flows in winding 1004 as is derived by
source 232.
Secondary winding 1006, variable capacitor 1008 and plasma impedance 218
are in series in nearly resonant closed loop 1010. A microprocessor (not
shown) that is the same as microprocessor 230 responds to the output
signals of sensor 104 to control the capacitances 212 and 1008. The
control is such that there is an impedance match between source 232 and
the load the source drives. When the match occurs, loop 1010 has an
impedance with a resonant frequency that is nearly the same as the
frequency of the source, i.e., load 1010 is nearly resonant to the fixed
frequency of source 232.
To prevent the high amplitude, near resonant current that flows in loop
1010 from being coupled to primary winding 1004, as well as feeds 213 and
224, windings 1004 and 1006 are loosely coupled. The loose coupling
between windings 1004 and 1006 results in the impedance of loop 1010 that
is nearly resonant to the frequency of source 232 being coupled with a
large resistive component to winding 1004. Consequently, the current
flowing through feeds 213 and 224 has an amplitude that is much lower than
the current flowing in loop 1010. A typical coupling coefficient between
windings 1004 and 1006 to achieve the desired loose coupling effect and
provide a relatively efficient transfer of power from winding 1004 to
winding 1006 is in the range of about 0.1 to 0.3. This coefficient range
results in transformer 1002 having an efficiency of about 70 percent to 90
percent.
Reference is now made to FIG. 11, a modification of FIG. 10, wherein
variable frequency source 233 replaces fixed frequency source 232, a
microprocessor (not shown) configured the same as microprocessor 231
replaces the microprocessor of FIG. 10 and fixed capacitor 1102 replaces
variable capacitor 1008. Microprocessor 233 responds to the output signals
of sensor 104 to control the (1) value of capacitor 212 and frequency of
source 233 to provide an impedance match between source 232 and the load
it drives. When the match occurs, the frequency of source 233 and the
resonant frequency of loop 1010 are nearly the same.
Reference is now made to FIG. 12, which is the same as FIG. 11, except that
the antenna includes air core transformer 257 including primary winding
258 and secondary windings 346 and 348, both of which (1) are loosely
coupled with the primary winding, and (2) drive the plasma impedance 218.
As described in connection with FIG. 16, the inductive and capacitive
coupling between windings 346 and 348 and plasma impedance 218 is greater
than the inductive and capacitive coupling between winding 258 and the
plasma impedance. Secondary windings 346 and 348 are connected in series
with fixed capacitors 254 and 256 to form closed loop 2002. Capacitors 254
and 256 are located to provide a desired distribution of current and
voltage along the length of the closed loop 1202 formed by windings 346
and 348, as well as capacitors 254 and 256. The combined coupling
coefficient between winding 258 and windings 346 and 348 is approximately
in the 0.1 to 0.3 range. A microprocessor (not shown) that is the same as
microprocessor 231 responds to output signals of sensor 104 to control the
value of capacitor 212 and the frequency of source 233 to provide an
impedance match between source 232 and the load it drives. When the
impedance match is achieved, the source frequency and the resonant
frequency of loop 1202 are nearly the same.
It is to be understood that an ammeter can be connected to loops 1010 and
1202 to control the capacitance of capacitor 1008 (FIG. 10) and the
frequency of source 233 (FIGS. 11 and 12) so maximum current in loops 1010
and 1202 is achieved. Maximizing the current in loops 1010 and 1202
results from the impedances of the loop being resonant to the frequencies
of sources 232 or 233.
It is also to be understood that it is not necessary for the current in the
plasma excitation coil to be maximized or nearly resonant to the frequency
of the fixed or variable frequency sources of FIGS. 10-12 or for an exact
