U.S. patent application number 12/098781 was filed with the patent office on 2009-01-01 for apparatus for plasma processing a substrate and a method thereof.
This patent application is currently assigned to Varian Semiconductor Equipment Associates, Inc.. Invention is credited to Bernard G. Lindsay, Timothy J. Miller, Vikram Singh.
Application Number | 20090001890 12/098781 |
Document ID | / |
Family ID | 40159571 |
Filed Date | 2009-01-01 |
United States Patent
Application |
20090001890 |
Kind Code |
A1 |
Singh; Vikram ; et
al. |
January 1, 2009 |
Apparatus for Plasma Processing a Substrate and a Method
Thereof
Abstract
An apparatus for processing a substrate includes a pulsed power
supply that generates a waveform having a first period with a first
power level and a second period with a second power level. A plasma
source generates a first plasma during the first period and a
second plasma during the second period. The first plasma may have
higher plasma density than the second plasma. A bias voltage power
supply generates a bias voltage waveform at an output that is
electrically connected to a platen which supports a substrate. The
bias voltage waveform having a first voltage and a second voltage
may be coupled to the substrate. The first voltage may have more
negative potential than the second voltage.
Inventors: |
Singh; Vikram; (North
Andover, MA) ; Miller; Timothy J.; (Ipswich, MA)
; Lindsay; Bernard G.; (Danvers, MA) |
Correspondence
Address: |
VARIAN SEMICONDUCTOR EQUIPMENT ASSC., INC.
35 DORY RD.
GLOUCESTER
MA
01930-2297
US
|
Assignee: |
Varian Semiconductor Equipment
Associates, Inc.
Gloucester
MA
|
Family ID: |
40159571 |
Appl. No.: |
12/098781 |
Filed: |
April 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11771190 |
Jun 29, 2007 |
|
|
|
12098781 |
|
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Current U.S.
Class: |
315/111.21 |
Current CPC
Class: |
H01J 37/321 20130101;
H01J 37/32165 20130101; H01J 37/32412 20130101; H01L 21/32136
20130101; C23C 14/345 20130101 |
Class at
Publication: |
315/111.21 |
International
Class: |
H05H 1/46 20060101
H05H001/46 |
Claims
1. An apparatus comprising: a processing chamber; a plasma source
proximate to the processing chamber; an RF power unit configured to
output to the plasma source an RF waveform having a first power
level during a first period and a second power level during a
second period, the first power level being less than the second
power level, the plasma source being configured to generate first
plasma during the first period and second plasma during the second
period; a substrate being supported by a platen; and a bias voltage
unit being configured to output to the substrate a bias voltage
waveform having a first voltage during the first period and a
second voltage, the first voltage having more negative potential
than the second voltage.
2. The apparatus of claim 1, wherein the bias voltage unit being
further configured to output the first voltage during the second
period.
3. The apparatus of claim 1, wherein the bias voltage unit being
further configured to output the second voltage during the first
period.
4. The apparatus of claim 1, wherein the bias voltage waveform
includes a first bias voltage pulse width and a second bias voltage
pulse width, the first voltage width extending from the first
period to second periods.
5. The apparatus of claim 4, wherein the second voltage width
extends from the first period to the second period.
6. The apparatus of claim 1, wherein the second plasma has higher
plasma density than the first plasma.
7. The apparatus of claim 6, wherein each of the first and second
plasma comprises ions and electrons.
8. The apparatus of claim 7, wherein number of ions and electrons
of the second plasma is greater than number of ions and electrons
of the first plasma.
9. The apparatus of claim 7, wherein ions of the first plasma are
attracted to the substrate applied with first voltage and electrons
of the second plasma are attracted to the substrate applied with
second voltage.
10. The apparatus of claim 1, wherein each of first plasma and
second plasma comprises a plurality of charged particles, and
wherein the substrate is treated with the charged particles of the
first and second plasma.
11. The apparatus of claim 10, wherein the charged particles of the
first and second plasma treating the substrate have opposite
charge.
12. A method of processing a substrate comprising: inputting to a
plasma source a current waveform having a first amplitude during a
first period and a second amplitude during a second period to
generate a first plasma during the first period and a second plasma
during the second period, the second amplitude being greater than
the first amplitude; coupling a bias voltage waveform having a
first voltage and a second voltage to a substrate, the first
voltage having more negative potential than the second voltage; and
synchronizing the current waveform and the bias voltage waveform
such that the first voltage is coupled to the substrate during the
first period.
13. The method of claim 12, further comprising synchronizing the
current waveform and the bias voltage waveform such that the second
voltage is coupled to the substrate during the first period.
14. The method of claim 12, wherein the first plasma has lower
plasma density than the second plasma.
15. The method of claim 14, further comprising synchronizing the
current waveform input and the bias voltage waveform input such
that the second voltage is coupled to the substrate during
generation of the second plasma.
16. The method of claim 12, wherein each of the first plasma and
second plasma comprises ions and electrons.
17. The method of claim 16, further comprising attracting ions of
the first plasma to the substrate.
18. The method of claim 17, further comprising attracting electrons
of the second plasma to the substrate.
19. A method comprising: applying current to a plasma source
proximate to a processing chamber during a first period to generate
a first plasma; increasing the amplitude of the current applied to
the plasma source during a second period to generate a second
plasma; and coupling a first voltage of a voltage waveform having
the first voltage and a second voltage to a substrate, the first
voltage having greater negative potential than the second
voltage.
20. The method of claim 19, wherein the first voltage is coupled to
the substrate during the first period.
21. The method of claim 19, wherein the first voltage is coupled to
the substrate during the first period.
22. The method of claim 20, further comprising applying the second
voltage to the substrate during the second period.
23. The method of claim 19, further comprising coupling the first
voltage to the substrate during the first and second periods.
24. The method of claim 19, wherein the first plasma has higher
plasma density that the second plasma.
25. The method of claim 18, further comprising: attracting ions of
the first plasma to the substrate and attracting electrons of the
second plasma to the substrate.
Description
RELATED APPLICATION SECTION
[0001] This application is a continuation application and claims
priority to U.S. patent application Ser. No. 11/771,190, filed Jun.
29, 2007, entitled "Plasma Doping with Enhanced Charge
Neutralization." The entire specification of U.S. patent
application Ser. No. 11/771,190 is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] Plasma processing has been widely used in the semiconductor
and other industries for many decades. Plasma processing is used
for tasks such as cleaning, etching, milling, and deposition. More
recently, plasma processing has been used for doping. Plasma doping
is sometimes referred to as PLAD or plasma immersion ion
implantation (PIII). Plasma doping systems have been developed to
meet the doping requirements of some modern electronic and optical
devices.
[0003] Plasma doping is fundamentally different from conventional
beam-line ion implantation systems that accelerate ions with an
electric field and then filter the ions according to their
mass-to-charge ratio to select the desired ions for implantation.
In contrast, plasma doping systems immerse the target in a plasma
containing dopant ions and bias the target with a series of
negative voltage pulses. The electric field within the plasma
sheath accelerates ions toward the target thereby implanting the
ions into the target surface.
[0004] Plasma doping systems for the semiconductor industry
generally require a very high degree of process control.
Conventional beam-line ion implantation systems that are widely
used in the semiconductor industry have excellent process control
and also excellent run-to-run uniformity. Conventional beam-line
ion implantation systems provide highly uniform doping across the
entire surface of state-of-the art semiconductor substrates.
[0005] In general, the process control of plasma doping systems is
not as good as conventional beam-line ion implantation systems. In
many plasma doping systems, charge tends to accumulate on the
substrate being plasma doped. This charge build-up can result in
the development of a relatively high potential voltage on the
substrate that can cause doping non-uniformities, arcing, and
device damage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The invention, in accordance with preferred and exemplary
embodiments, together with further advantages thereof, is more
particularly described in the following detailed description, taken
in conjunction with the accompanying drawings. The drawings are not
necessarily to scale, emphasis instead generally being placed upon
illustrating principles of the invention.
[0007] FIG. 1 illustrates a plasma doping system with charge
neutralization according to the present invention.
[0008] FIG. 2A illustrates a prior art waveform generated by the RF
source having a single amplitude that can cause charge accumulation
on the substrate under some conditions.
[0009] FIG. 2B illustrates a waveform generated by the bias voltage
supply that applies a negative voltage to the substrate during
plasma doping to attract ions in the plasma.
[0010] FIG. 3A illustrates a waveform generated by the RF source
according to the present invention that has multiple amplitudes for
at least partially neutralizing charge accumulation on the
substrate.
[0011] FIG. 3B illustrates a waveform generated by the bias voltage
supply according to the present invention that applies a negative
voltage to the substrate during plasma doping to attract ions.
[0012] FIG. 3C illustrates a waveform generated by the bias voltage
supply according to the present invention that applies a negative
voltage to the substrate during plasma doping to attract ions and
that applies a positive voltage to the substrate after plasma
doping is terminated to assist in neutralizing charge on the
substrate.
[0013] FIGS. 4A-C illustrates a waveform generated by the RF source
and waveforms generated by the bias voltage supply according to the
present invention that are similar to the waveforms described in
connection with FIGS. 3A-3C, but that are displaced in time so as
to plasma dope with both the first and the second power level
P.sub.RF1, P.sub.RF2.
[0014] FIGS. 5A-C illustrate a waveform generated by the RF source
with a variable frequency and waveforms generated by the bias
voltage supply according to another embodiment of the present
invention.
DETAILED DESCRIPTION
[0015] Reference in the specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. The
appearances of the phrase "in one embodiment" in various places in
the specification are not necessarily all referring to the same
embodiment.
[0016] It should be understood that the individual steps of the
methods of the present invention may be performed in any order
and/or simultaneously as long as the invention remains operable.
Furthermore, it should be understood that the apparatus and methods
of the present invention can include any number or all of the
described embodiments as long as the invention remains
operable.
[0017] The present teachings will now be described in more detail
with reference to exemplary embodiments thereof as shown in the
accompanying drawings. While the present teachings are described in
conjunction with various embodiments and examples, it is not
intended that the present teachings be limited to such embodiments.
On the contrary, the present teachings encompass various
alternatives, modifications and equivalents, as will be appreciated
by those of skill in the art. Those of ordinary skill in the art
having access to the teachings herein will recognize additional
implementations, modifications, and embodiments, as well as other
fields of use, which are within the scope of the present disclosure
as described herein. For example, it should be understood that the
methods for neutralizing charge in a plasma doping system according
to the present invention can be used with any type of plasma
source.
[0018] Many plasma doping systems operate in a pulsed mode of
operation where a series of pulses is applied to the plasma source
to generate a pulsed plasma. Also, a series of pulses can be
applied to the substrate being plasma doped during the on-periods
of the plasma source pulses to bias the substrate to attract ions
for implantation. In the pulsed mode of operation, charge tends to
accumulate on the substrate being plasma doped during the on-period
of the plasma source pulses. When the duty cycle of the plasma
source pulses is relatively low (i.e. less than about 25%), the
charge tends to be efficiently neutralized by electrons in the
plasma.
[0019] However, there is currently a need to perform plasma doping
in a pulsed mode of operation with relatively high duty cycles
(i.e. duty cycles above about a 25%). Such higher duty cycles are
necessary to achieve the desired throughputs and to maintain doping
levels that are required for some modern devices. For example, it
is desirable to perform poly gate doping and counter doping of some
state-of-the art devices by plasma doping with a duty cycle greater
than 25%.
[0020] As the duty cycle is increased above about 25%, there is a
shorter period of time where the charge on the substrate being
plasma doped can be neutralized during the pulse-off period of the
plasma source. Consequently, charge accumulation or charge build up
can occur on the substrate being plasma doped, which results in the
development of a relatively high potential voltage on the substrate
being plasma doped that can cause doping non-uniformities, arcing,
and device damage. For example, thin gate dielectrics can be easily
damaged by excess charge build up.
[0021] The present invention relates to methods and apparatus for
neutralizing charge during plasma doping. The method and apparatus
of the present invention allow implants to be performed at higher
duty cycles by reducing the probability of damage caused by
charging effects. In particular, a plasma doping apparatus
according to the present invention includes a RF power supply that
varies the RF power applied to the plasma source to at least
partially neutralize charge accumulation during plasma doping. In
addition, the bias voltage to the substrate being plasma doped can
be varied to at least partially neutralize charge accumulation.
Furthermore, the relative timing of the RF power pulses applied to
the plasma source and the bias voltage applied to the substrate
being plasma doped can be varied to at least partially neutralize
charge accumulation.
[0022] More specifically, a plasma implantation system according to
the present invention includes a RF power supply that varies the RF
power applied to the plasma source to at least partially neutralize
charge accumulation during plasma doping. In various embodiments
single or multiple RF power supplies are used to independently
power the plasma source and the bias the substrate being plasma
doped so as to at least partially neutralize charge during plasma
doping. Also, in various embodiments, the RF power applied to the
plasma source and the bias voltage applied to the substrate during
plasma doping are applied at relative times to at least partially
neutralize charge during plasma doping.
[0023] In addition to neutralizing charge, the method and apparatus
of the present invention can precisely control at least one of the
power to the RF source and the bias applied to the substrate during
periods where the plasma doping is terminated (i.e. pulse-off
period) in order to improve the retained dose. The resulting
improvement in retained dose will help to reduce implant time and
thus will increase throughput. Also, in addition to neutralizing
charge, the method and apparatus of the present invention can
precisely control at least one of the power to the RF source and
the bias applied to the substrate during periods where the plasma
doping is terminated in order to achieve knock-on type implant
mechanisms that obtain better sidewall coverage.
[0024] FIG. 1 illustrates a plasma doping system 100 with charge
neutralization according to the present invention. It should be
understood that this is only one of many possible designs plasma
doping systems that can perform ion implantation with charge
neutralization according to the present invention. The plasma
doping system 100 includes an inductively coupled plasma source 101
having both a planar and a helical RF coil and a conductive top
section. A similar RF inductively coupled plasma source is
described in U.S. patent application Ser. No. 10/905,172, filed on
Dec. 20, 2004, entitled "RF Plasma Source with Conductive Top
Section," which is assigned to the present assignee. The entire
specification of U.S. patent application Ser. No. 10/905,172 is
incorporated herein by reference. The plasma source 101 shown in
the plasma doping system 100 is well suited for plasma doping
applications because it can provide a highly uniform ion flux and
the source also efficiently dissipates heat generated by secondary
electron emissions.
[0025] More specifically, the plasma doping system 100 includes a
plasma chamber 102 that contains a process gas supplied by an
external gas source 104. The external gas source 104, which is
coupled to the plasma chamber 102 through a proportional valve 106,
supplies the process gas to the chamber 102. In some embodiments, a
gas baffle is used to disperse the gas into the plasma source 101.
A pressure gauge 108 measures the pressure inside the chamber 102.
An exhaust port 110 in the chamber 102 is coupled to a vacuum pump
112 that evacuates the chamber 102. An exhaust valve 114 controls
the exhaust conductance through the exhaust port 110.
[0026] A gas pressure controller 116 is electrically connected to
the proportional valve 106, the pressure gauge 108, and the exhaust
valve 114. The gas pressure controller 116 maintains the desired
pressure in the plasma chamber 102 by controlling the exhaust
conductance and the process gas flow rate in a feedback loop that
is responsive to the pressure gauge 108. The exhaust conductance is
controlled with the exhaust valve 114. The process gas flow rate is
controlled with the proportional valve 106.
[0027] In some embodiments, a ratio control of trace gas species is
provided to the process gas by a mass flow meter that is coupled
in-line with the process gas that provides the primary dopant
species. Also, in some embodiments, a separate gas injection means
is used for in-situ conditioning species. Furthermore, in some
embodiments, a multi-port gas injection means is used to provide
gases that cause neutral chemistry effects that result in across
substrate variations.
[0028] The chamber 102 has a chamber top 118 including a first
section 120 formed of a dielectric material that extends in a
generally horizontal direction. A second section 122 of the chamber
top 118 is formed of a dielectric material that extends a height
from the first section 120 in a generally vertical direction. The
first and second sections 120, 122 are sometimes referred to herein
generally as the dielectric window. It should be understood that
there are numerous variations of the chamber top 118. For example,
the first section 120 can be formed of a dielectric material that
extends in a generally curved direction so that the first and
second sections 120, 122 are not orthogonal as described in U.S.
patent application Ser. No. 10/905,172, which is incorporated
herein by reference. In other embodiment, the chamber top 118
includes only a planer surface.
[0029] The shape and dimensions of the first and the second
sections 120, 122 can be selected to achieve a certain performance.
For example, one skilled in the art will understand that the
dimensions of the first and the second sections 120, 122 of the
chamber top 118 can be chosen to improve the uniformity of plasmas.
In one embodiment, a ratio of the height of the second section 122
in the vertical direction to the length across the second section
122 in the horizontal direction is adjusted to achieve a more
uniform plasma. For example, in one particular embodiment, the
ratio of the height of the second section 122 in the vertical
direction to the length across the second section 122 in the
horizontal direction is in the range of 1.5 to 5.5.
[0030] The dielectric materials in the first and second sections
120, 122 provide a medium for transferring the RF power from the RF
antenna to a plasma inside the chamber 102. In one embodiment, the
dielectric material used to form the first and second sections 120,
122 is a high purity ceramic material that is chemically resistant
to the process gases and that has good thermal properties. For
example, in some embodiments, the dielectric material is 99.6%
Al.sub.2O.sub.3 or AlN. In other embodiments, the dielectric
material is Yittria and YAG.
[0031] A lid 124 of the chamber top 118 is formed of a conductive
material that extends a length across the second section 122 in the
horizontal direction. In many embodiments, the conductivity of the
material used to form the lid 124 is high enough to dissipate the
heat load and to minimize charging effects that results from
secondary electron emission. Typically, the conductive material
used to form the lid 124 is chemically resistant to the process
gases. In some embodiments, the conductive material is aluminum or
silicon.
[0032] The lid 124 can be coupled to the second section 122 with a
halogen resistant O-ring made of fluorocarbon polymer, such as an
O-ring formed of Chemrz and/or Kalrex materials. The lid 124 is
typically mounted to the second section 122 in a manner that
minimizes compression on the second section 122, but that provides
enough compression to seal the lid 124 to the second section. In
some operating modes, the lid 124 is RF and DC grounded as shown in
FIG. 1.
[0033] In some embodiments, the chamber 102 includes a liner 125
that is positioned to prevent or greatly reduce metal contamination
by providing line-of-site shielding of the inside of the plasma
chamber 102 from metal sputtered by ions in the plasma striking the
inside metal walls of the plasma chamber 102. Such liners are
described in U.S. patent application Ser. No. 11,623,739, filed
Jan. 16, 2007, entitled "Plasma Source with Liner for Reducing
Metal Contamination," which is assigned to the present assignee.
The entire specification of U.S. patent application Ser. No.
11/623,739 is incorporated herein by reference.
[0034] In various embodiments, the liner can be a one-piece or
unitary plasma chamber liner, or a segmented plasma chamber liner.
In many embodiments, the plasma chamber liner 125 is formed of a
metal base material, such as aluminum. In these embodiments, at
least the inner surface 125' of the plasma chamber liner 125
includes a hard coating material that prevents sputtering of the
plasma chamber liner base material.
[0035] Some plasma doping processes generate a considerable amount
of non-uniformly distributed heat on the inner surfaces of the
plasma source 101 because of secondary electron emissions. In some
embodiments, the plasma chamber liner 125 is a temperature
controlled plasma chamber liner 125. In addition, in some
embodiments, the lid 124 comprises a cooling system that regulates
the temperature of the lid 124 and surrounding area in order to
dissipate the heat load generated during processing. The cooling
system can be a fluid cooling system that includes cooling passages
in the lid 124 that circulate a liquid coolant from a coolant
source.
[0036] A RF antenna is positioned proximate to at least one of the
first section 120 and the second section 122 of the chamber top
118. The plasma source 101 in FIG. 1 illustrates two separate RF
antennas that are electrically isolated from one another. However,
in other embodiments, the two separate RF antennas are electrically
connected. In the embodiment shown in FIG. 1, a planar coil RF
antenna 126 (sometimes called a planar antenna or a horizontal
antenna) having a plurality of turns is positioned adjacent to the
first section 120 of the chamber top 118. In addition, a helical
coil RF antenna 128 (sometimes called a helical antenna or a
vertical antenna) having a plurality of turns surrounds the second
section 122 of the chamber top 118.
[0037] In some embodiments, at least one of the planar coil RF
antenna 126 and the helical coil RF antenna 128 is terminated with
a capacitor 129 that reduces the effective antenna coil voltage.
The term "effective antenna coil voltage" is defined herein to mean
the voltage drop across the RF antennas 126, 128. In other words,
the effective coil voltage is the voltage "seen by the ions" or
equivalently the voltage experienced by the ions in the plasma.
[0038] Also, in some embodiments, at least one of the planar coil
RF antenna 126 and the helical coil RF antenna 128 includes a
dielectric layer 134 that has a relatively low dielectric constant
compared to the dielectric constant of the Al.sub.20.sub.3
dielectric window material. The relatively low dielectric constant
dielectric layer 134 effectively forms a capacitive voltage divider
that also reduces the effective antenna coil voltage. In addition,
in some embodiments, at least one of the planar coil RF antenna 126
and the helical coil RF antenna 128 includes a Faraday shield 136
that also reduces the effective antenna coil voltage.
[0039] A RF source 130, such as a RF power supply, is electrically
connected to at least one of the planar coil RF antenna 126 and
helical coil RF antenna 128. In many embodiments, the RF source 130
is coupled to the RF antennas 126, 128 by an impedance matching
network 132 that matches the output impedance of the RF source 130
to the impedance of the RF antennas 126, 128 in order to maximize
the power transferred from the RF source 130 to the RF antennas
126, 128. Dashed lines from the output of the impedance matching
network 132 to the planar coil RF antenna 126 and the helical coil
RF antenna 128 are shown to indicate that electrical connections
can be made from the output of the impedance matching network 132
to either or both of the planar coil RF antenna 126 and the helical
coil RF antenna 128.
[0040] In some embodiments, at least one of the planar coil RF
antenna 126 and the helical coil RF antenna 128 is formed such that
it can be liquid cooled. Cooling at least one of the planar coil RF
antenna 126 and the helical coil RF antenna 128 will reduce
temperature gradients caused by the RF power propagating in the RF
antennas 126, 128.
[0041] In some embodiments, the plasma source 101 includes a plasma
igniter 138. Numerous types of plasma igniters can be used with the
plasma source 101. In one embodiment, the plasma igniter 138
includes a reservoir 140 of strike gas, which is a highly-ionizable
gas, such as argon (Ar), which assists in igniting the plasma. The
reservoir 140 is coupled to the plasma chamber 102 with a high
conductance gas connection. A burst valve 142 isolates the
reservoir 140 from the process chamber 102. In another embodiment,
a strike gas source is plumbed directly to the burst valve 142
using a low conductance gas connection. In some embodiments, a
portion of the reservoir 140 is separated by a limited conductance
orifice or metering valve that provides a steady flow rate of
strike gas after the initial high-flow-rate burst.
[0042] A platen 144 is positioned in the process chamber 102 a
height below the top section 118 of the plasma source 101. The
platen 144 holds a substrate 146 for plasma doping. In many
embodiments, the substrate 146 is electrically connected to the
platen 144. In the embodiment shown in FIG. 1, the platen 144 is
parallel to the plasma source 101. However, in one embodiment of
the present invention, the platen 144 is tilted with respect to the
plasma source 101.
[0043] A platen 144 is used to support a substrate 146 or other
workpieces for processing. In some embodiments, the platen 144 is
mechanically coupled to a movable stage that translates, scans, or
oscillates the substrate 146 in at least one direction. In one
embodiment, the movable stage is a dither generator or an
oscillator that dithers or oscillates the substrate 146. The
translation, dithering, and/or oscillation motions can reduce or
eliminate shadowing effects and can improve the uniformity of the
ion beam flux impacting the surface of the substrate 146.
[0044] A bias voltage power supply 148 is electrically connected to
the platen 144. The bias voltage power supply 148 is used to bias
the platen 144 and the substrate 146 so that dopant ions in the
plasma are extracted from the plasma and impact the substrate 146.
The bias voltage power supply 148 can be a DC power supply, a
pulsed power supply, or a RF power supply. In plasma doping
apparatus according the present invention, the bias voltage power
supply 148 has an output that is independent of the output of the
RF source 130 that powers at least one of the planar coil RF
antenna 126 and helical coil RF antenna 128. However, the bias
voltage power supply 148 and the RF source 130 can physically be
the same power supply as long as the bias voltage output is
independent of the RF source output.
[0045] A controller 152 is used to control the RF power supply 130
and the bias voltage power supply 148 to generate a plasma and to
bias the substrate 146 so as to at least partially neutralize
charge accumulation during plasma doping according to the present
invention. The controller 152 can be part of the power supplies
130, 148 or can be a separate controller that is electrically
connected to control inputs of the power supplies 130, 148. The
controller 152 controls the RF power supply 130 so that pulses are
applied to either or both of the planar coil RF antenna 126 and the
helical coil RF antenna 128 with at least two different amplitudes.
Also, the controller 152 controls the RF power supply 130 and the
bias voltage power supply 148 so that the pulses are applied to
either or both of the planar coil RF antenna 126 and the helical
coil RF antenna 128 and to the substrate at relative times that at
least partially neutralize charge accumulation during plasma doping
according to the present invention.
[0046] One skilled in the art will appreciate that the there are
many different possible variations of the plasma source 101 that
can be used with the features of the present invention. See for
example, the descriptions of the plasma sources in U.S. patent
application Ser. No. 10/908,009, filed Apr. 25, 2005, entitled
"Tilted Plasma Doping." Also see the descriptions of the plasma
sources in U.S. patent application Ser. No. 11/163,303, filed Oct.
13, 2005, entitled "Conformal Doping Apparatus and Method." Also
see the descriptions of the plasma sources in U.S. patent
application Ser. No. 11/163,307, filed Oct. 13, 2005, entitled
"Conformal Doping Apparatus and Method." In addition, see the
descriptions of the plasma sources in U.S. patent application Ser.
No. 11/566,418, filed Dec. 4, 2006, entitled "Plasma Doping with
Electronically Controllable implant Angle." The entire
specification of U.S. patent application Ser. Nos. 10/908,009,
11/163,303, 11/163,307 and 11/566,418 are herein incorporated by
reference.
[0047] In operation, the controller 152 instructs the RF source 130
to generate RF currents that propagate in at least one of the RF
antennas 126 and 128. That is, at least one of the planar coil RF
antenna 126 and the helical coil RF antenna 128 is an active
antenna. The term "active antenna" is herein defined as an antenna
that is driven directly by a power supply. In many embodiments of
the plasma doping apparatus of the present invention, the RF source
130 operates in a pulsed mode. However, the RF source 130 can also
operate in the continuous mode.
[0048] In some embodiments, one of the planar coil antenna 126 and
the helical coil antenna 128 is a parasitic antenna. The term
"parasitic antenna" is defined herein to mean an antenna that is in
electromagnetic communication with an active antenna, but that is
not directly connected to a power supply. In other words, a
parasitic antenna is not directly excited by a power supply, but
rather is excited by an active antenna, which in the present
invention is one of the planar coil antenna 126 and the helical
coil antenna 128 powered by the RF source 130. In some embodiments
of the invention, one end of the parasitic antenna is electrically
connected to ground potential in order to provide antenna tuning
capabilities. In this embodiment, the parasitic antenna includes a
coil adjuster 150 that is used to change the effective number of
turns in the parasitic antenna coil. Numerous different types of
coil adjusters, such as a metal short, can be used.
[0049] The RF currents in the RF antennas 126, 128 then induce RF
currents into the chamber 102. The RF currents in the chamber 102
excite and ionize the process gas so as to generate a plasma in the
chamber 102. The plasma chamber liner 125 shields metal sputtered
by ions in the plasma from reaching the substrate 146.
[0050] The controller 152 also instructs the bias voltage power
supply 148 to bias the substrate 146 with a negative voltage that
attract ions in the plasma towards the substrate 146. During the
negative voltage pulses, the electric field within the plasma
sheath accelerates ions toward the substrate 146 which implants the
ions into the surface of the substrate 146. In some embodiments, a
grid is used to extract ions in the plasma towards the substrate
146.
[0051] When the RF source 130 and the bias voltage power supply 148
are operated in the pulse mode under some processing conditions,
such as with relatively high duty cycles, charge can accumulate on
the substrate 146. Charge accumulation can result in the
development of a relatively high potential voltage on the substrate
146 being plasma doped that can cause doping non-uniformities,
arcing, and device damage.
[0052] FIG. 2A illustrates a prior art waveform 200 generated by
the RF source 130 having a single amplitude that can cause charge
accumulation on the substrate 146 under some conditions. The
waveform 200 is at ground potential until the plasma is generated
with a pulse having a power level P.sub.RF 202. The power level
P.sub.RF 202 is chosen to be suitable for plasma doping. The pulse
terminates after the pulse period T.sub.P 204 and then returns to
ground potential. The waveform then periodically repeats.
[0053] FIG. 2B illustrates a waveform 250 generated by the bias
voltage supply 148 according to the present invention that applies
a negative voltage 252 to the substrate 146 during plasma doping to
attract ions in the plasma. The negative voltage 252 is applied
during the period T.sub.1 254 when the waveform 200 generated by
the RF source 130 has a power equal to the power level P.sub.RF
202. The waveform 200 is at ground potential during the period
T.sub.2 256 when the plasma doping is terminated. At relatively
high duty cycles (i.e. greater than about 25%), charge tends to
accumulate on the substrate 146 during the pulse period T.sub.1 254
when the waveform 250 generated by the RF source 130 has a power
equal to the power level P.sub.RF 202.
[0054] The methods and apparatus of the present invention allow
plasma doping implants to be performed at higher duty cycles by
reducing the probability of damage caused by charging effects.
There are numerous methods according to the present invention to
power the plasma source 101 and to bias the substrate 146 being
process to at least partially neutralize charge accumulation on the
substrate 146.
[0055] FIG. 3A illustrates a waveform 300 generated by the RF
source 130 according to the present invention that has multiple
amplitudes for at least partially neutralizing charge accumulation
on the substrate 146. The waveform 300 is pulsed and has a first
302 and second power level 304 indicated in the figure as P.sub.RF1
and P.sub.RF2, respectively. However, it should be understood that
waveforms with more than two amplitudes can be used in the methods
of the present invention to at least partially neutralize charge
accumulation on the substrate 146. It should also be understood
that the waveforms may or may not have discrete amplitudes. For
example, the waveforms can be continuously changing. That is, in
some embodiments, the waveforms can ramp (i.e. have positive and
negative slopes) linearly or nonlinearly.
[0056] The first power level P.sub.RF1 302 is chosen to provide
enough RF power to at least partially neutralize charge
accumulation on the substrate 146 when the substrate 146 is not
biased for plasma doping. The second power level P.sub.RF2 304 is
chosen to be suitable for plasma doping. In various embodiments,
the waveform 300 generated by the RF source 130 including the first
and second power levels P.sub.RF1 302, P.sub.RF2 304 is applied to
one or both of the planar coil RF antenna 126 and the helical coil
RF antenna 128 (see FIG. 1). In one specific embodiment, the
waveform 300 generated by the RF source 130 is applied to one of
the planar coil RF antenna 126 and the helical coil RF antenna 128
when it is at the first power levels P.sub.RF1 and is applied to
the other of the planar coil RF antenna 126 and the helical coil RF
antenna 128 when it is at the second power levels P.sub.RF2. In
another specific embodiment, the waveform 300 generated by the RF
source 130 is applied to one of the planar coil RF antenna 126 and
the helical coil RF antenna 128 when it has a first frequency and
is applied to the other of the planar coil RF antenna 126 and the
helical coil RF antenna 128 when it has a second frequency that is
different from the first frequency as described in connection with
FIGS. 5A-5C.
[0057] The waveform 300 shown in FIG. 3A indicates that the first
power level P.sub.RF1 302 is greater than the second power level
P.sub.RF2 304. However, in other embodiments, the first power level
P.sub.RF1 302 is less than the second power level P.sub.RF2 304.
Also, in some embodiments, the waveform 300 includes a third power
level that is zero or some relatively low power level when the
substrate 146 is not biased for plasma doping.
[0058] The waveform 300 also indicates a first pulse period
T.sub.P1 306 corresponding to the time period were the waveform 300
has a power equal to the first power level P.sub.RF1 302 and a
second pulse period T.sub.P2 308 corresponding to the time period
were the waveform has a power equal to the second power level
P.sub.RF2 304. The total multi-amplitude pulse period for the
waveform 300 T.sub.Total 310 is the combination of the first pulse
period T.sub.P1 306 and the second pulse period T.sub.P2 308. For
example, in one embodiment, the first and second pulse periods
T.sub.P1 306, T.sub.P2 308 are both in the range of 30-500 .mu.s
and the total pulse period T.sub.Total 310 is in the range of 60
.mu.s-1 ms. In other embodiments, the total pulse period
T.sub.Total 310 can be on order of 1 ms or greater.
[0059] FIG. 3A indicates that the frequency of the waveform 300
during the first pulse period T.sub.P1 306 is the same as the
frequency of the waveform 300 during the second pulse period
T.sub.P2 308. However, it should be understood that in various
embodiments, the frequency of the waveform 300 during the first
pulse period T.sub.P1 306 can be different from the frequency of
the waveform 300 during the second pulse period T.sub.P2 308 as
described in connection with FIGS. 5A-5C. In addition, the
frequency of the waveform can be changed within at least one of the
first and the second pulse periods T.sub.P1, 306, T.sub.P2,
308.
[0060] Thus, in some embodiments, the waveform 300 includes both
multiple frequencies and multiple amplitudes that are chosen to at
least partially neutralize charge accumulation during plasma
doping. In addition, in some embodiments, the waveform 300 includes
both multiple frequencies and multiple amplitudes that are chosen
to improve the retained dose as described herein. Furthermore, in
some embodiments, the waveform 300 includes both multiple
frequencies and multiple amplitudes that are chosen to assist in
creating knock-on implants as described herein.
[0061] FIG. 3B illustrates a waveform 350 generated by the bias
voltage supply 148 according to the present invention that applies
a negative voltage 352 to the substrate 146 during plasma doping to
attract ions. The negative voltage 352 is applied during the second
pulse period T.sub.P2 308 when the waveform 350 generated by the RF
source 130 has a power equal to the second power level P.sub.RF2
304. The waveform 350 is at ground potential during the first pulse
period T.sub.P1 306 when the plasma doping is terminated and the
waveform 300 has a power equal to the first power level P.sub.RF1
302.
[0062] Applying a waveform to the plasma source 101 with two
different power levels where the first power level P.sub.RF1 302 is
applied by the RF source 130 during the period 306 T.sub.P1 306
when the waveform 350 generated by the bias voltage supply 148 is
at ground potential will assist in neutralizing charge accumulated
on the substrate 146. Electrons in the corresponding plasma will
neutralize at least some of the charge accumulated on the substrate
146.
[0063] FIG. 3C illustrates a waveform 360 generated by the bias
voltage supply 148 according to the present invention that applies
a negative voltage 362 to the substrate 146 during plasma doping to
attract ions and that applies a positive voltage 364 to the
substrate 146 after plasma doping is terminated to assist in
neutralizing charge on the substrate 146. The negative voltage 362
is applied during the second pulse period T.sub.P2 308 when the
waveform 300 generated by the RF source 130 has a power equal to
the second power level P.sub.RF2 304. The waveform 360 is at a
positive potential during the first pulse period T.sub.P1 306 when
the waveform 300 generated by the RF source 130 has a power equal
to the first power level P.sub.RF1 302.
[0064] Applying a waveform to the plasma source 101 with two
different power levels where the first power level P.sub.RF1 302 is
applied by the RF source 130 during the first period 306 T.sub.P1
306 when the waveform 360 generated by the bias voltage supply 148
is at a positive potential will assist in neutralizing charge
accumulated on the substrate 146. Electrons in the corresponding
plasma will neutralize at least some of the charge accumulated on
the substrate 146. In addition, the positive voltage 364 applied
the substrate 146 will also neutralize at least some of the charge
accumulated on the substrate 146.
[0065] FIGS. 4A-C illustrate a waveform 400 generated by the RF
source 130 and waveforms 402, 404 generated by the bias voltage
supply 148 according to the present invention that are similar to
the waveforms 300, 350, and 360 described in connection with FIGS.
3A-3C, but that are displaced in time relative to the waveforms
300, 350, and 360 so as to plasma dope with both the first and the
second power level P.sub.RF1 302, P.sub.RF2 304. Changing the power
generated by the RF source 130 during plasma doping allows the user
to more precisely control the amount of charge that is accumulating
on the surface of the substrate 146 during plasma doping. For
example, increasing the power near the end of the second pulse
period T.sub.P2 308 will assist in neutralizing at least some of
the charge accumulated on the substrate 146.
[0066] FIGS. 5A-C illustrate a waveform 500 generated by the RF
source 130 with a variable frequency and waveforms 502, 504
generated by the bias voltage supply 148 according to another
embodiment of the present invention. The waveform 500 is similar to
the waveforms 300, 400 described in connection with FIGS. 3 and 4.
However, the RF powers in the first and second pulse periods
T.sub.P1 306, T.sub.P2 308 are the same, but the frequencies are
different. Changing the frequency of the waveform 500 changes the
ion/electron density and, therefore, changes the charge
neutralization efficiency.
[0067] Thus, in one embodiment, the frequency of the waveform 500
in the first pulse period T.sub.P1 306 is different from the
frequency of the waveform 500 in the second pulse period T.sub.P2
308 and these frequencies are chosen to at least partially
neutralize charge accumulation during plasma doping. The waveforms
502, 504 are similar to the waveforms 350 and 360 that were
described in connection with FIG. 3. However, in other embodiments,
the waveforms 502, 504 are displaced in time relative to the
waveform 500, similar to the waveforms 402, 404 that were described
in connection with FIG. 4.
[0068] In addition, in one aspect of the present invention, at
least one of the multiple power levels generated by the RF source
130, the frequency of the waveform 500 in at least one of the first
and second pulse periods T.sub.P1 306, T.sub.P2 308, and the
relative timing of the waveform 500 with respect to the waveforms
generated by the bias voltage supply 148 are chosen to improve the
retained dose on the substrate 146. For example, generating
multiple power levels with the RF source 130 where one power is
generated by the RF source 130 when the bias voltage is at ground
potential allows the user to use less power during plasma doping
because some plasma doping will occur between negative bias voltage
steps. Using less power during plasma doping will result in less
deposition and, therefore, a higher retained dose. The operating
pressure, gas flow rates, type of dilution gas, and plasma source
power can also be selected to improve the retained dose.
[0069] In addition, in one aspect of the present invention, at
least one of the multiple power levels generated by the RF source
130, the frequency of the waveform 500 in at least one of the first
and second pulse periods T.sub.P1 306, T.sub.P2 308, and the
relative timing of the waveform 500 with respect to the waveforms
generated by the bias voltage supply 148 are chosen to obtain
better sidewall coverage. For example, waveforms can be generated
by the RF source 130 with multiple power levels, multiple
frequencies, and with certain relative timings with respect to the
waveforms generated by the bias voltage supply 148 so as to create
knock-on implants. The term "knock-on implant" is defined herein as
a recoil implantation where a non-dopant species is implanted
through the surface layers of the substrate 146 to drive the dopant
material into the substrate 146.
[0070] The non-dopant species used for the knock-on implant can be
a benign species. For example, inert ions, such as He, Ne, Ar, Kr
and Xe, can be formed from an inert feed gas. In some embodiments,
the mass of the inert ions is chosen to be similar to a mass of the
desired dopant ions. The RF source 130 generates a RF power level
that directs the inert ions towards the substrate 146 with a
sufficient energy to physically knock the deposited dopant material
into both the planar and nonplanar features of the substrate 146
upon impact. Also, the operating pressure, gas flow rate, plasma
source power, gas dilution, and duty cycle of pulsed bias supply
can be chosen to enhance knock-on implants.
[0071] One skilled in the art will appreciate that waveforms
generated by the RF source 130 according to the present invention
can have both multiple amplitudes and multiple frequencies and can
have various relative timings with respect to the waveforms
generated by the bias voltage supply 148. In fact, there are an
almost infinite number of possible waveforms with multiple power
levels and multiple frequencies that can be generated by the RF
source 130 and relative timing with respect to the waveforms
generated by the bias voltage supply 148 that will at least
partially neutralize charge according to the present invention. In
addition, the retained dose can be improved by generating waveforms
with the RF source 130 with multiple power levels, multiple
frequencies, and relative timings with respect to the waveforms
generated by the bias voltage supply 148. Furthermore, knock-on
implants can be enhanced by generating waveforms with the RF source
130 with multiple power levels, multiple frequencies, and relative
timings with respect to the waveforms generated by the bias voltage
supply 148. These waveforms can also have many different duty
cycles.
[0072] It should be understood that the methods for charge
neutralization according to the present invention can be used with
numerous other types of plasma doping apparatus. For example, the
methods for charge neutralization can be used with plasma doping
apparatus that have inductively coupled plasma (ICP) sources,
helicon resonator plasma sources, microwave plasma sources, ECR
plasma source, and capacitive coupled plasma sources. In fact, any
type of plasma source that can be operated in a pulsed mode can be
used to perform the methods of the present invention.
EQUIVALENTS
[0073] While the present teachings are described in conjunction
with various embodiments and examples, it is not intended that the
present teachings be limited to such embodiments. On the contrary,
the present teachings encompass various alternatives, modifications
and equivalents, as will be appreciated by those of skill in the
art, may be made therein without departing from the spirit and
scope of the invention.
* * * * *