U.S. patent application number 13/313078 was filed with the patent office on 2013-06-13 for apparatus and method for charge neutralization during processing of a workpiece.
This patent application is currently assigned to Varian Semiconductor Equipment Associates, INC.. The applicant listed for this patent is Daniel Distaso, Peter F. Kurunczi, Christopher J. Leavitt, Timothy J. Miller. Invention is credited to Daniel Distaso, Peter F. Kurunczi, Christopher J. Leavitt, Timothy J. Miller.
Application Number | 20130146790 13/313078 |
Document ID | / |
Family ID | 47326373 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130146790 |
Kind Code |
A1 |
Kurunczi; Peter F. ; et
al. |
June 13, 2013 |
APPARATUS AND METHOD FOR CHARGE NEUTRALIZATION DURING PROCESSING OF
A WORKPIECE
Abstract
A processing system may include a plasma source for providing a
plasma and a workpiece holder arranged to receive ions from the
plasma. The processing system may further include a pulsed bias
circuit electrically coupled to the plasma source and operable to
switch a bias voltage supplied to the plasma source between a high
voltage state in which the plasma source is biased positively with
respect to ground and a low voltage state in which the plasma
source is biased negatively with respect to the ground.
Inventors: |
Kurunczi; Peter F.;
(Cambridge, MA) ; Leavitt; Christopher J.;
(Gloucester, MA) ; Distaso; Daniel; (Merrimac,
MA) ; Miller; Timothy J.; (Ipswich, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kurunczi; Peter F.
Leavitt; Christopher J.
Distaso; Daniel
Miller; Timothy J. |
Cambridge
Gloucester
Merrimac
Ipswich |
MA
MA
MA
MA |
US
US
US
US |
|
|
Assignee: |
Varian Semiconductor Equipment
Associates, INC.
Gloucester
MA
|
Family ID: |
47326373 |
Appl. No.: |
13/313078 |
Filed: |
December 7, 2011 |
Current U.S.
Class: |
250/492.3 ;
315/111.21 |
Current CPC
Class: |
H01J 37/32412 20130101;
G21K 1/14 20130101; H01J 37/32422 20130101; H01J 37/026 20130101;
H01J 37/32146 20130101; H01J 2237/0044 20130101; H01J 37/3171
20130101 |
Class at
Publication: |
250/492.3 ;
315/111.21 |
International
Class: |
G21K 5/08 20060101
G21K005/08; H05H 1/24 20060101 H05H001/24 |
Claims
1. A processing system, comprising: a plasma source for providing a
plasma, a workpiece holder arranged to receive ions from the
plasma; and a pulsed bias circuit electrically coupled to the
plasma source, the pulsed bias circuit operable to switch a bias
voltage supplied to the plasma source between a high voltage state
in which the plasma source is biased positively with respect to
ground and a low voltage state in which the plasma source is biased
negatively with respect to the ground.
2. The processing system of claim 1, further comprising an
extraction plate disposed between the plasma source and the
workpiece holder, the extraction plate arranged to define an ion
beam during the high voltage state.
3. The processing system of claim 1, wherein the pulsed bias
circuit comprises a high voltage circuit that includes a capacitor
for discharging current into the plasma source when the bias
voltage is switched to the high voltage state.
4. The processing system of claim 3, the high voltage circuit
comprising a high voltage power supply to supply a positive voltage
and high voltage switch operable to alternately connect and
disconnect the high voltage power supply to the plasma source.
5. The processing system of claim 4, comprising a low voltage
circuit that includes: a low voltage power supply to output a
negative voltage; and a low voltage switch operable to alternately
connect and disconnect the low voltage supply to the plasma
source.
6. The processing system of claim 3, the high voltage state defined
by the plasma source having a potential of greater than 100 V
positive with respect to the workpiece holder.
7. The processing system of claim 1, wherein the pulsed bias
circuit comprises a low voltage circuit that includes a capacitor
for discharging current from the plasma source when the bias
voltage is switched to the low voltage state.
8. The processing system of claim 7, the low voltage state defined
by the plasma source having a potential of between about 2 V and
100 V negative with respect to the workpiece holder.
9. The processing system of claim 1, further comprising a
controller to direct the pulsed bias circuit to output a voltage
pulse train comprising alternating portions of the high voltage
state and low voltage state, the controller operable to vary a duty
cycle of the voltage pulse train.
10. A method of treating a workpiece in a processing system,
comprising: igniting a plasma using a plasma source; grounding a
workpiece holder to receive ions from the plasma; and applying a
voltage pulse train to the plasma source, the voltage pulse train
comprising a high voltage portion in which the plasma source is
biased positively with respect to ground and a low voltage portion
in which the plasma source is biased negatively with respect to
ground.
11. The method of claim 10, comprising directing ions through an
extraction plate disposed between the plasma source and the
workpiece holder, wherein the ions define an ion beam comprising
ions incident over an angular range toward the workpiece
holder.
12. The method of claim 10, comprising discharging current into the
plasma source from a capacitor when the bias voltage is switched to
a high voltage state.
13. The method of claim 10, comprising discharging current from the
plasma source into a capacitor when the bias voltage is switched to
a low voltage state.
14. The method of claim 10, comprising setting a plasma source
potential in the high voltage state of greater than 100 V positive
with respect to the workpiece holder.
15. The method of claim 10, comprising setting a plasma source
potential in the low voltage state of between about 2 V and 100 V
negative with respect to the workpiece holder.
16. The method of claim 10, further comprising: monitoring a charge
at the workpiece after the voltage pulse train is applied to the
plasma source; and setting a second voltage pulse train by changing
one or more of a set voltage in the high voltage state, a set
voltage in the low voltage state, a duration of the high voltage
state, and a duration of the low voltage state in response to the
charge.
17. An ion implantation system comprising: a plasma source; a
movable workpiece holder coupled to ground to receive ions from a
plasma supplied by the plasma source while moving the workpiece
holder; an extraction plate disposed between the plasma source and
the workpiece holder and arranged to direct ions over a range of
angles toward the workpiece holder; and a pulse bias circuit
operable to switch a bias voltage supplied to the plasma source and
the workpiece holder between a high voltage state in which the
plasma source is biased positively with respect to ground and a low
voltage state in which the plasma source is biased negatively with
respect to ground.
18. The ion implantation system of claim 17, wherein the pulsed
bias circuit comprises: a high voltage circuit that includes a
capacitor for discharging current into the plasma source when the
bias voltage is switched to the high voltage state; and a low
voltage circuit that includes a capacitor for discharging current
from the plasma source when the bias voltage is switched to the low
voltage state.
19. The ion implantation system of claim 18, the high voltage state
defined by the plasma source having a potential of greater than 100
V positive with respect to ground, and the low voltage state
defined by the plasma source having a potential of between about 2
V and 100 V negative with respect to ground.
Description
FIELD
[0001] This invention relates to ion treatment of workpieces and,
more particularly, to a method and apparatus for charge
neutralization during ion treatment of workpiece.
BACKGROUND
[0002] Ion beam and plasma processing of workpieces (substrates)
may be performed for a variety of purposes including for ion
implantation, surface texturing, and etching of a surface. Ion
implantation in particular is a standard technique for introducing
property-altering impurities into substrates. A desired impurity
material is ionized in an plasma source, the ions are accelerated
to form an ion beam of prescribed energy, and the ion beam is
directed at the surface of the substrate. The energetic ions in the
beam penetrate into the sub-surface of the substrate material and
are embedded into the crystalline lattice of the substrate material
to form a region of desired conductivity or material property.
[0003] One challenge for ion beam processing is the need to
dissipate charge at a workpiece, which may occur during ion
implantation of a workpiece because ions impinging on a substrate
by nature carry charge. In the case of ion beams that comprise
positive ions, positive charge may build up on the workpiece after
exposure to an ion beam. In order for this charge to be dissipated,
the workpiece holder may be grounded, thereby providing a
conductive path for conducting the charge from the workpiece
surface. However, if a workpiece itself is a poor conductor or an
electrical insulator, the charge on the workpiece surface may have
no conductive path to ground, thereby preventing the charge from
being dissipated.
[0004] Neutralization of charge that builds up on a workpiece
surface due to exposure to an ion beam may also be accomplished by
providing charged species of opposite polarity to the charge on the
workpiece. In typical known ion implantation systems that employ
pulsed ion implantation using positive ions, including plasma
immersion ion implantation, a plasma may be established proximate a
workpiece holder and a periodic bias may be applied in pulses
between the plasma and workpiece holder. During "on" periods
positive ions may be attracted to the workpiece by providing a bias
between the plasma and workpiece holder, wherein the potential at
the workpiece holder is negative with respect to the plasma. At the
same time, electrons in the plasma may be repelled from the
workpiece holder due to its relatively negative potential with
respect to the plasma. During "off" periods when the implantation
system no longer sets the workpiece holder at a negative potential
with respect to the plasma, electrons may drift towards the
workpiece. However, the flux of electrons during these "off"
periods may be insufficient to neutralize the surface of the
workpiece and excessive positive charge may remain.
[0005] FIG. 1a illustrates a voltage pulse train 100 that includes
a series of "on" periods 102 interrupted by "off" periods 104.
During the "on" periods 102 a positive high voltage may be applied
to a plasma source, while the workpiece is grounded, thereby
setting the workpiece at a high negative potential (voltage) with
respect to the plasma. Accordingly, positive ions may be attracted
to the workpiece at a high energy of about 10 kV in the example
shown in FIG. 1a. During the "off" periods 104, when the DC voltage
of the plasma source is nominally at ground potential, in principle
the voltage between plasma and workpiece is about zero. Accordingly
some electrons may drift out of a plasma and towards a workpiece
during the "off" periods 104, thereby tending to neutralize the
workpiece.
[0006] FIG. 2 provides an illustration of circuitry 202 that may be
used to create the voltage pulse train 100. As depicted in FIG. 2,
a plasma source 210 is coupled to the circuitry 202 to provide
pulsed ion beams 214 to workpiece holder 212. The circuitry 202
includes a high voltage power supply 204, and a high voltage switch
206 that can connect or disconnect the high voltage power supply
204 to the plasma source 210. When a plasma is created in the
plasma source 210, the plasma source 210 may be biased to a high
positive potential, such as +10 kV illustrated in FIG. 1a, by
closing high voltage switch 206. This high positive potential
serves to extract positive ions 214 from plasma source 210 and
accelerate the positive ions 214 toward workpiece holder. When high
voltage switch 206 is open and second switch 208 is closed, the
plasma source is grounded and positive ions are no longer attracted
toward workpiece holder 212. Accordingly, by alternating between
configurations in which one switch of switches 206, 208 is open,
and the other closed, pulsed ion beams 214 may be created during
"on" periods 202.
[0007] While the circuitry 202 may produce a waveform generally as
shown by voltage pulse train, an actual voltage waveform may differ
from a desired waveform where the voltage is zero during "off"
periods. For example, the high voltage switch 206 and second switch
208 may have an internal impedance that results in a small voltage
drop. Thus, during the "off" periods 104 in which the plasma source
is connected through second switch 208 to ground, the small
internal impedance of second switch 208 may result in plasma source
210 not being directly grounded, but rather floating at a potential
that may be several volts above zero. As illustrated in FIG. 1b,
which shows a more expanded view of one "off" period 104, the
plasma source may actually acquire a potential up to several volts
positive due to the internal impedance. During the "off" periods
104, the resulting positive bias of plasma source 210 with respect
to workpiece holder 212 impedes the flow of electrons to workpiece
holder 212, since the workpiece holder potential is several volts
more negative than the plasma source potential. Thus, during these
"off" periods the flux of electrons from the plasma with sufficient
initial energy to overcome the negative potential of the workpiece
may be insufficient to neutralize the surface of the workpiece,
such that excessive positive charge may remain.
[0008] In view of the above, it will be appreciated that it may be
useful to provide improvements for neutralization of charge in
systems that provide charged species of a predominant polarity,
such as ion beam systems.
SUMMARY
[0009] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended as an aid in determining the scope of the
claimed subject matter.
[0010] In one embodiment, a plasma processing system includes a
plasma source for providing a plasma, a workpiece holder arranged
to receive ions from the plasma, and a pulsed bias circuit operable
to switch a bias voltage supplied between the plasma source and
workpiece holder, between a high voltage state in which the plasma
source is biased positively with respect to the workpiece and a low
voltage state in which the plasma source is biased negatively with
respect to the workpiece.
[0011] In another embodiment, a method of treating a workpiece in a
processing system, comprises igniting a plasma using a plasma
source, providing a workpiece to receive ions from the plasma
source, and applying a voltage pulse train to the plasma source,
the voltage pulse train comprising a high voltage state in which
the plasma source is biased positively with respect to the
workpiece and a low voltage state in which the plasma source is
biased negatively with respect to the workpiece.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For a better understanding of the present disclosure,
reference is made to the accompanying drawings, which are
incorporated herein by reference and in which:
[0013] FIG. 1a illustrates a conventional voltage pulse train;
[0014] FIG. 1b shows an expanded view of the voltage pulse train of
FIG. 1a;
[0015] FIG. 2 provides an illustration of circuitry that may be
used to create the voltage pulse train of FIGS. 1a, 1b;
[0016] FIG. 3 is a block diagram that depicts a processing system
consistent with present embodiments;
[0017] FIG. 4 depicts one embodiment of a pulsed bias circuit;
[0018] FIG. 5 depicts one mode of operation of an exemplary pulsed
bias circuit;
[0019] FIG. 6 specifically depicts another mode of operation of an
exemplary pulsed bias circuit;
[0020] FIG. 7 provides a composite illustration of multiple modes
of operation of an exemplary pulsed bias circuit;
[0021] FIG. 8a depicts one embodiment of a voltage pulse train;
[0022] FIG. 8b depicts another embodiment of a voltage pulse train;
and
[0023] FIG. 8c depicts a further embodiment of a voltage pulse
train.
DETAILED DESCRIPTION
[0024] Embodiments of a system and method are described herein in
connection with ion processing of workpieces (substrates). In
various embodiments, this system can be used with, for example,
semiconductor substrates, bit-patterned media, solid-state
batteries, or flat panels, or other substrates. Thus, the invention
is not limited to the specific embodiments described below.
[0025] In various embodiments, a processing system includes a
plasma source and switch circuitry for providing pulsed biasing of
the plasma source with respect to a workpiece holder. The switch
circuitry may provide a pulsed biasing that provides alternating
pulses of ion beams and electrons to a workpiece (holder) in
various embodiments. By providing a novel pulsed biasing circuit
arrangement, a workpiece exposed to ions by pulsed ion processing
of positive ions can be effectively neutralized by electrons
supplied during off periods of the pulsed implantation process. In
the discussion to follow, reference in the text and FIGs. may be
made to a workpiece holder without explicit reference to a
workpiece. However, it is to be understood that, unless otherwise
indicated, it is implicit that a workpiece may be present in
scenarios or arrangements that merely depict or describe a
workpiece holder.
[0026] In various embodiments, plasma based ion implantation
systems may employ plasma source to generate a plasma and separate
circuitry to control biasing between the plasma and workpiece. In
addition to use in plasma immersion ion implantation systems, the
present embodiments may be employed in processing systems that
situate an extraction plate between plasma and workpiece in order
to provide a controllable ion beam having a unique set of
properties.
[0027] FIG. 3 is a block diagram that depicts a processing system
that provides ions at multiple angles to a workpiece. The
processing system 10 includes a plasma source 12, an extraction
plate 14 (or sheath engineering plate), and a process chamber 16. A
gas source 18 is connected to the plasma chamber 16. The plasma
source 12 or other components of the processing system 10 also may
be connected to a pump (not shown), such as a turbo pump. In
various embodiments, the plasma source 12 may be an RF plasma
source, inductively-coupled plasma (ICP) source, indirectly heated
cathode (IHC), helicon, glow discharge source, or other plasma
sources known to those skilled in the art. However, in the example
shown in FIG. 3, the plasma source 12 may be an RF plasma source
that includes an RF generator 20, an RF matching network 22, and
antenna 23. The plasma source 12 is surrounded by an enclosure 24
and an insulator 26 separates the enclosure 24 from the plasma
chamber 16. As illustrated, the workpiece holder 28 may be
grounded.
[0028] The extraction plate 14 is used to extract ions 30 for
implantation into a workpiece (substrate) 40, which may be
grounded, as illustrated. The extraction plate 14 may be cooled.
The plasma source 12 may be biased and a bias circuit as described
below may be provided to provide a continuous or pulsed bias to the
plasma source 12to attract the ions 30. The extraction plate 14 may
have at least one aperture 34, through which ions 30 are provided
to the workpiece 40.
[0029] An ion beam extracted from a plasma using processing system
10 may be used to simultaneously provide to workpieces 40 ions 30
over a range of angles if desired without requiring complicated
masking or lithography procedures. This ability to create a wide
angular distribution of ions facilitates processing of workpieces
having three dimensional features where it may be desirable to
simultaneously provide ions incident on the features from different
directions. Moreover, the exact angular distribution of ions 30
that are provided to workpiece 40 may be established according to a
specific set of ion beam optics conditions (parameters) in
processing system 10. Parameters that may affect the angular
distribution of ions 30 include the shape and size of aperture 34,
the implantation voltage, spacing between extraction plate 14 and
workpiece 40, and plasma density. Thus, a specific set of
parameters may establish a specific ion angular distribution of
ions 30.
[0030] Processing system 10 also includes a pulsed bias circuit 42
that may provide pulses of voltage to created pulses of charged
particles directed to workpiece(s) 40, as detailed below.
Consistent with the present embodiments, the pulsed bias circuit 42
may produce a voltage waveform that provides a pulsed ion beam to
workpiece during "on" periods of a pulse, as well as electrons that
serve to neutralize workpiece 40 during "off" periods of the pulse.
Also illustrated in FIG. 3 is a controller 44 that may set control
signals to control operation of the pulsed bias circuit 42, as
described below. In some embodiments, a mechanism 46, such as a
movable stage, may be provided to move workpiece holder 28 along
one or more mutually orthogonal directions 48, 50, 52. For example
the position of workpiece holder 28 may be changed along the
direction 50 to adjust the separation between extraction plate 14
and workpiece holder 28. The workpiece holder 28 may also be
scanned along direction 48 and/or direction 52 while exposed to
ions 30 to provide coverage over desired regions of workpiece(s)
40.
[0031] FIG. 4 depicts one embodiment 400 of the pulsed bias circuit
42 generally depicted at FIG. 3. The pulsed bias circuit 400 may be
coupled to the controller 44 as illustrated to receive control
signals that direct operation of the pulsed bias circuit 400. An
output 402 of pulsed bias circuit 400 is coupled to the plasma
source 12 to control voltage applied between plasma source 12 and
workpiece holder 28. As depicted in FIG. 4, the workpiece holder 28
may be connected to ground so that a voltage difference between the
workpiece holder 28 and plasma source 12 is equivalent to the
voltage set by pulsed bias circuit 400.
[0032] Included in pulsed bias circuit 400 is a high voltage
circuit 404 and low voltage circuit 406. The high voltage circuit
404 may include, for example, a high voltage power supply 408, high
voltage switch 412, and capacitor 416 and the low voltage circuit
406 may include a low voltage supply 410, low voltage switch 414,
and capacitor 418.
[0033] Consistent with the present embodiments, the high voltage
circuit 404 may set the voltage of plasma source 12 to be at a high
positive potential with respect to workpiece holder 28. As
illustrated, the high voltage circuit 404 includes a high voltage
(HV) power supply 408, which may output a positive voltage of about
100 V or more, and in particular may output a positive voltage of
500 V to 50 kV. Accordingly, when the high voltage supply 408 is
connected to plasma source 12, via high voltage switch 412, the
plasma source 12 may attain a potential of about +100V to +50 kV,
causing positive ions extracted from plasma source 12 to be
accelerated towards workpiece holder 28 at energies ranging from
100 eV to 50 keV for singly charged ions. As detailed below, the
high voltage circuit may 404 may operate to intermittently
electrically connect and disconnect the high voltage supply 408
from plasma source 12, thereby pulsing the plasma source 12
intermittently at high voltage, which may drive pulses of ions
toward the workpiece holder 28.
[0034] Consistent with the present embodiments, the low voltage
circuit 406 may set the voltage of plasma source 12 at a small
negative potential with respect to workpiece holder 28. As
illustrated, the low voltage circuit 406 includes a low voltage
supply 410, which may output a negative voltage of about -2 V or
more, and in particular may output a negative voltage of -2 V to
-100 V. Accordingly, when the low voltage supply 410 is connected
to plasma source 12, via low voltage switch 414, the plasma source
may attain a potential of about -2 V to -100 V, preventing ions in
plasma source 12 from accelerating towards workpiece holder 28,
while at the same time accelerating electrons in plasma source 12
toward the workpiece holder 28 at low energies, that is, energies
less than or equal to about 100 eV. As detailed below, the low
voltage circuit 406 may operate to intermittently electrically
connect and disconnect the low voltage supply 410 from plasma
source 12, thereby pulsing the plasma source 12 intermittently at a
small negative voltage, which may drive electrons (not shown) in
pulses toward the workpiece holder 28.
[0035] Referring now to FIG. 5, there is shown one mode of
operation of pulsed bias circuit 400 that directs positive ions
toward workpiece holder 28. In the scenario depicted in FIG. 5, the
high voltage switch 412 is closed, thereby coupling the high
voltage supply 408 to plasma source 12. In the operation mode
depicted in FIG. 5, the low voltage switch 414 is open, thereby
decoupling the low voltage supply 410 from the plasma source 12. In
this scenario, plasma source 12 may charge up to a positive
potential (voltage) supplied by high voltage supply 408.
Accordingly, ions 420 may be accelerated towards workpiece holder
28. In various embodiments of a plasma source 12, the ions 420 may
form a beam, such as beam 430, which includes ions directed to the
workpiece holder 28 over a range of angles. In accordance with
various embodiments, the controller 44 may direct the high voltage
switch 412 to open and close periodically in order to provide ions
420 as pulses of ions to workpiece holder 28.
[0036] In order to prevent excessive positive charge from building
up on workpiece holder 28, the pulsed bias circuit 400 may be
intermittently switched to a second mode of operation, as
illustrated in FIG. 6. FIG. 6 depicts a scenario in which the high
voltage switch is open, thereby disconnecting the high voltage
supply 408 from the plasma source 12. In addition, the low voltage
switch 414 is closed, which connects the low voltage supply 410 to
the plasma source 12, and thus may provide a small negative bias to
the plasma source 12 as discussed above. In this scenario, the
plasma source may attain a negative voltage set by the low voltage
supply 410, causing electrons 422 to accelerate from the plasma
source 12 and impinge upon workpiece holder 28, which may be
grounded as shown.
[0037] In various embodiments, the controller 44 may direct the
pulsed bias circuit 400 to alternate between the configurations
depicted in FIGS. 5 and 6 so that a series of pulses of positive
voltage (>100 V) are applied to plasma source 12, which are
interspersed with a series of pulses of negative voltage (-100 V to
about -2V). Thus, the pulsed bias circuit may alternately close
high voltage switch 412 while simultaneously opening low voltage
switch 414, and open high voltage switch 412 while closing low
voltage switch 414. This may produce a series of pulses of ions 420
that are interspersed with pulses of electrons 422, both species
being accelerated from the plasma source and incident on the
workpiece holder, as illustrated in FIG. 7.
[0038] FIG. 7 provides an illustration of processing of a workpiece
(not explicitly shown) by alternating pulses of ions 420 and
electrons 422. As noted, the ions 420 may be accelerated when high
voltage switch 412 is closed (and low voltage switch 414 open),
while the electrons 422 may be accelerated while the high voltage
switch 412 is open (and low voltage switch 414 closed). Thus,
although multiple pulses of ions 420 and electrons 422 are depicted
in the view of FIG. 7, it will be readily appreciated that at any
given instant only a pulse of ions 420 or electrons 422 may be
incident upon the workpiece holder 28.
[0039] By alternating pulses of ions 420 and electrons 422, a
processing system, such as processing system 10, operating in
conjunction with the pulsed bias circuit 400, may effectively
process a workpiece (see workpieces 40 in FIG. 3) located on
workpiece holder 28 with a dose of ions 420 while preventing
excessive positive charge buildup. In various embodiments the
duration of the pulses of electrons 422 and/or ions 420 may be on
the order of a few milliseconds or less, and in particular may be
on the order of about 1 .mu.s to 10 ms. Depending upon one or more
factors, including the dose of positive charge placed upon the
workpiece holder 28 by ions 420, the negative voltage set by low
voltage supply 410, and the duration in which the low voltage
switch 414 remains closed, the pulse of electrons 422 may
neutralize some or all of any positive charge on the workpiece
holder between each dose of ions 420, as discussed further below
with respect to FIG. 8.
[0040] In order to ensure effective switching back and forth
between "on" states in which the high voltage switch 412 is closed,
and "off" states in which the high voltage switch 412 is open, the
pulsed bias circuit also includes a high voltage circuit capacitor
416 and low voltage circuit capacitor 418. The high voltage circuit
capacitor 416 may store charge that can be discharged in a rapid
pulse into plasma source 12 when high voltage switch 412 is closed.
In this manner, the voltage may build up rapidly in plasma source
12, facilitating the ability of the high voltage supply 408 to
rapidly bring the plasma source to the actual voltage set by the
high voltage supply 408. This may be especially useful in
processing systems in which the components of plasma source 12 are
relatively massive and require substantial charge to adjust the
potential to the desired voltage.
[0041] On the other hand, the low voltage circuit capacitor 418 may
facilitate discharge of the plasma source 12 when the low voltage
switch 414 is closed and the high voltage switch is open. Because a
relatively large charge may be present on components of plasma
source 12 during an "on" period, in order to effectively establish
in a timely manner the negative voltage set by low voltage supply
410, it may be advantageous to quickly discharge plasma source 12
components.
[0042] The actual voltage attained by plasma source 12 during the
"off" periods may differ from the voltage set by low voltage supply
410. Thus, due to internal circuit impedance, the plasma source 12
may remain at a potential that is slightly positive with respect to
the nominal voltage, as discussed above. Accordingly, the present
embodiments may adjust for this by setting the low voltage supply
410 to a greater negative voltage than actually desired for plasma
source 12. For example, if it is desired to provide electrons with
an energy of about 5 eV when incident on a workpiece 40, the low
voltage supply 410 may be set at -10 V to account for a +5 V offset
that may exist between the set voltage and voltage attained on
plasma source 12. In this manner the potential of the plasma source
12 may actually reach about -5V, thereby accelerating negatively
charged electrons through a field of 5 V between the plasma source
12 and ground potential at the workpiece holder 28.
[0043] In various embodiments, the pulse bias circuit 400 may
adjust "on" periods, "off" periods, and the negative voltage set
during "off" periods, among other factors, to optimize processing
of a workpiece. For example, a workpiece may be processed using a
first voltage pulse train that includes components such as a first
"on" period, first "off" period, and first negative voltage, after
which the accumulated charge at the workpiece may be monitored. Any
of the aforementioned components of the voltage pulse train may be
adjusted based upon the monitored accumulated charge. FIG. 8a
depicts one embodiment of a voltage pulse train 802 that may be
output by a pulsed bias circuit, such as pulsed bias circuit 400.
The voltage pulse train 802 includes "on" portions 804 in which a
positive bias is output, which may correspond to a positive bias
applied to a plasma source. The "on" portions 804 are interspersed
by "off" portions 806, in which a small negative bias voltage is
output. In this example, the duty cycle of "on" portions 806 may be
50%, meaning that the "on" portions and "off" portions 806 are of
equal duration. As noted above, the pulsed bias circuit 400 may
include capacitors that facilitate charging and discharging of a
plasma source 12, so that the voltage pulse train 802 may roughly,
though not necessarily identically, correspond to the actual
voltage variation on plasma source 12. Thus, the voltage at plasma
source 12 may constitute a series of square wave pulses whose tops
correspond to a high positive voltage, and whose troughs correspond
to a small negative voltage. When subject to a 50% duty cycle, a
workpiece may accordingly be exposed to ion processing for one half
of the total processing time, and may be exposed to electrons
during one half of the total processing time. Although it may be
desirable to increase the duty cycle in order to complete a desired
ion treatment more rapidly, increasing the percentage of time that
a workpiece is exposed to ions may result in excessive positive
charge building up on the workpiece. Accordingly, under some
processing conditions, the positive charge developed during "on"
portions 804 may be incompletely neutralized during "off" portions
806.
[0044] In order to address this situation, the negative voltage
applied during "off" portions 806 may be increased to attract a
higher flux of electrons from a plasma source, thereby providing a
faster neutralization of positive charge on a workpiece. However,
this should be balanced with possible damage the increased electron
energy may cause if the workpiece is a semiconductor substrate for
building electronic devices.
[0045] FIG. 8b illustrates another way of increasing the
neutralization of a workpiece. In the embodiment of FIG. 8b, a
voltage pulse train 812 comprises a series of "on" portions 814
interspersed by "off" portions 816 whose duration is longer than
that of "on" portions 814. Thus, the high voltage pulse duty cycle
is lower than in the scenario of FIG. 8a, thereby affording
relatively more time for electrons to neutralize positive charge on
a workpiece during the "off" portions 816, without having to
increase electron energy.
[0046] FIG. 8c depicts another embodiment in which the voltage
pulse train 822 may comprise the same duty cycle (50%) as in FIG.
8a thereby providing the same workpiece throughput as in FIG. 8a by
maintaining the same proportion of ion processing time to total
processing time. However, the duration of "on" portions 824 and
"off" portions 826 is longer than in the corresponding portions of
voltage pulse train 802 because the frequency of the voltage pulse
train 822 is lower than the frequency of the other voltage pulse
train 802. The longer "off" portions 826, in particular, may be
more effective in neutralizing positive charge than the "off"
portions 806, even though the proportion of time of the "off"
portions 806 and 826 is the same. This may result from the fact
that a decay time, shown by a voltage decay curve 830, which may be
on the order of hundreds to thousands of nanoseconds, is needed for
the plasma source to attain a desired negative potential after
switching from a high positive voltage. This therefore shortens the
actual time t.sub.negative where the plasma source is at the
desired negative potential and electrons are accelerated toward a
workpiece during "off" portions 826, as shown in FIG. 8a. In the
case of FIG. 8c, t.sub.negative may occupy a greater proportion of
the "off" portion 826 as compared to the proportion of the desired
"off" portion 806, where voltage decay curve 830 consumes a larger
fraction of the total "off" portion 806.
[0047] However, if the duration of the "off" portion in which a
negative voltage is applied to the plasma source is sufficiently
long, a workpiece 40 may develop a negative charge due to excessive
exposure to electrons, after which electrons may no longer by
accelerated from the plasma source. Thus, depending on various
other operating parameters, including plasma power, gas pressure
and plasma source voltage, an optimum range for the duration of
"off" portion of a voltage pulse train may be defined.
[0048] The methods described herein may be automated by, for
example, tangibly embodying a program of instructions upon a
computer readable storage media capable of being read by machine
capable of executing the instructions. A general purpose computer
is one example of such a machine. A non-limiting exemplary list of
appropriate storage media well known in the art includes such
devices as a readable or writeable CD, flash memory chips (e.g.,
thumb drives), various magnetic storage media, and the like.
[0049] In particular, steps for providing voltage pulse trains to a
plasma source may be performed at least partially by a combination
of an electronic processor, computer readable memory, and/or
computer readable program. The computer memory may be further
configured to receive, display and store process history
information associated with operation of a plasma system and as
exemplified by the stored voltage values.
[0050] The present disclosure is not to be limited in scope by the
specific embodiments described herein. Indeed, other various
embodiments of and modifications to the present disclosure, in
addition to those described herein, will be apparent to those of
ordinary skill in the art from the foregoing description and
accompanying drawings. Thus, such other embodiments and
modifications are intended to fall within the scope of the present
disclosure. Furthermore, although the present disclosure has been
described herein in the context of a particular implementation in a
particular environment for a particular purpose, those of ordinary
skill in the art will recognize that its usefulness is not limited
thereto and that the present disclosure may be beneficially
implemented in any number of environments for any number of
purposes. Thus, the claims set forth below should be construed in
view of the full breadth and spirit of the present disclosure as
described herein.
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