U.S. patent application number 09/821753 was filed with the patent office on 2002-10-03 for plasma processing method and apparatus with control of plasma excitation power.
This patent application is currently assigned to Lam Research Corporation. Invention is credited to Huang, Chung-Ho, Jiang, Weinan, Lin, Frank Y., Ni, Tuqiang.
Application Number | 20020139477 09/821753 |
Document ID | / |
Family ID | 25234217 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020139477 |
Kind Code |
A1 |
Ni, Tuqiang ; et
al. |
October 3, 2002 |
Plasma processing method and apparatus with control of plasma
excitation power
Abstract
The amount of RF power supplied to a plasma in a vacuum plasma
processing chamber is gradually changed on a preprogrammed basis in
response to signals stored in a computer memory. The computer
memory stores signals so that other processing chamber parameters
(pressure, gas species and gas flow rates) remain constant while
the gradual change occurs. The stored signals enable rounded
corners, instead of sharp edges, to be etched, e.g., at an
intersection of a trench wall and base.
Inventors: |
Ni, Tuqiang; (Fremont,
CA) ; Lin, Frank Y.; (Fremont, CA) ; Huang,
Chung-Ho; (Fremont, CA) ; Jiang, Weinan; (San
Jose, CA) |
Correspondence
Address: |
LOWE HAUPTMAN GILMAN & BERNER, LLP
Suite 310
1700 Diagonal Road
Alexandria
VA
22314
US
|
Assignee: |
Lam Research Corporation
|
Family ID: |
25234217 |
Appl. No.: |
09/821753 |
Filed: |
March 30, 2001 |
Current U.S.
Class: |
156/345.44 ;
118/723E; 216/67 |
Current CPC
Class: |
H01J 37/3299 20130101;
H01J 37/32082 20130101; H01J 37/32174 20130101 |
Class at
Publication: |
156/345.44 ;
216/67; 118/723.00E |
International
Class: |
B44C 001/22; C23F
001/00; C23C 016/503 |
Claims
We claim:
1. A method of processing a workpiece in a vacuum plasma processor
chamber wherein a gas species is converted into an AC plasma, the
vacuum chamber being subject to operating at different pressures
while the workpiece is being processed, the gas species being
subject to flowing into the chamber at different flow rates while
the workpiece is being processed, comprising, gradually changing on
a pre-programmed basis, the amount of AC power supplied to the
plasma during processing of the workpiece.
2. The method of claim 1 wherein the gradual power change occurs
while no change is made in (a) the species, (b) the pressure or (c)
the flow rate.
3. The method of claim 1 wherein the AC power is supplied by an
electrode coupling an AC electric field to plasma in the
chamber.
4. The method of claim 3 wherein the electrode is responsive to an
AC power source that supplies RF bias voltage to the electrode, the
electrode being on a holder for the workpiece.
5. The method of claim 3 wherein the electrode is responsive to an
AC power source that supplies RF plasma excitation voltage to the
electrode, the electrode responding to the RF voltage to supply RF
electric field to the plasma to excite the gas to the plasma.
6. The method of claim 3 wherein the AC power is supplied by a coil
coupling an RF plasma excitation electromagnetic field to the
chamber.
7. The method of claim 1 wherein a gradual transition in the shape
of material in the workpiece being processed occurs in response to
the gradual power change.
8. The method of claim 7 wherein the species is ionized into a
plasma that etches the material, the gradual power change and the
species being such that the material is shaped to have a rounded
corner in response to changes in the ionized plasma etchant
resulting from the gradual power change.
9. The method of claim 8 wherein the etching, which occurs in
response to changes in the ionized plasma etchant resulting from
the gradual power change, forms a trench wall including the rounded
corner.
10. The method of claim 9 wherein the rounded corner is at an
intersection of a wall and a base of a trench.
11. The method of claim 7 wherein the rounded corner is at an
intersection of a wall and a surface intersecting the wall, the
surface extending generally at right angles to the wall.
12. The method of claim 1 wherein the gradual change includes steps
having power changes no greater than about several watts, the power
remaining at a constant wattage for no more than about 1
second.
13. The method of claim 12 wherein the power steps are a few
milliwatts and remain at a constant power for about 1
millisecond.
14. A vacuum plasma processor for processing a workpiece in a
vacuum plasma processor chamber wherein a gas species is converted
into an AC plasma comprising a reactive element for supplying an
electric field to plasma in the chamber, and an electric source for
supplying gradually changing amounts of power on a preprogrammed
basis to the reactive element.
15. The processor of claim 14 further including a controller for
causing the source to supply the gradually changing amounts of
power on the preprogrammed basis to the reactive element while a
single workpiece is being processed.
16. The processor of claim 15 wherein the controller is arranged
for (a) controlling (i) a gas species adapted to flow into the
chamber, (ii) the pressure in the vacuum chamber, and (iii) the
flow rates of the gas species, and (b) maintaining the constant (I)
the gas species, (ii) the gas species flow rate and (iii) the
chamber pressure while the plasma power is gradually changing on
the preprogrammed basis.
17. A computer program for controlling a computer for controlling
processing of a workpiece in a vacuum plasma processor chamber
wherein a gas species is converted into an AC plasma, the computer
program storing signals causing (a) the vacuum chamber to operate
at different pressures while the workpiece is being processed, (b)
control of the gas species type and the flow rates thereof into the
chamber while the workpiece is being processed, (c) the amount of
AC power applied to the plasma while the workpiece is being
processed; the stored signal for the amount of applied AC power
causing gradual preprogrammed changes in the amount of AC power
supplied to the plasma during processing of the workpiece.
18. The program of claim 17 wherein the stored signal causes the
gradual power change to occur while no change is made in (a) the
species, (b) the pressure or (c) the flow rate.
19. The program of claim 17 wherein the stored signal causing
gradual power change causes a gradual transition in the shape of
material in the workpiece being processed in response to the
gradual power change.
20. The program of claim 19 wherein the stored signal controls
etchant species supplied to the chamber while the workpiece is
being processed and the gradual power transition so as to cause the
workpiece to be etched to have a rounded corner.
21. The program of claim 20 wherein the stored signal controls
etchant species supplied to the chamber while the workpiece is
being processed and the gradual power transition so as to cause the
workpiece to be etched to have a trench wall including the rounded
corner.
22. The program of claim 21 wherein the rounded corner is at an
intersection of a wall and a base of a trench.
Description
FIELD OF INVENTION
[0001] The present invention relates generally to vacuum plasma
processors for processing workpieces on a workpiece holder and more
particularly to a method of and apparatus for gradually changing,
on a preprogrammed basis, power an AC plasma excitation source
supplies to plasma in a vacuum processor chamber.
BACKGROUND ART
[0002] Vacuum processors for processing a workpiece (i.e., etching
materials from or depositing materials onto the workpiece)
typically include first and second ports respectively connected to
a vacuum pump and one or more gas sources. The gas is excited to a
plasma in the chamber by an electric source including a reactance
responsive to a first AC source, typically an RF or microwave
source. A first matching network is usually connected between the
first AC source and the reactance for exciting the plasma. If the
source is an RF source, the reactance is either a coil for
supplying magnetic and electric fields to the chamber interior via
a dielectric window or a parallel plate capacitive arrangement for
supplying an electric field to the chamber interior.
[0003] The workpiece, which is typically a semiconductor wafer or a
dielectric sheet or a metal plate, is clamped in place on a
workpiece holder, i.e., chuck, that frequently includes an
electrode covered by a dielectric. DC voltage is typically applied
to the electrode to provide an electrostatic clamping force to hold
the workpiece in situ on the holder. The workpiece is usually
cooled by applying a coolant agent, such as helium, to a recess in
the chuck and by applying a liquid to conduits in the chuck. To
accelerate ions in the plasma to the workpiece, a second AC source
is connected to the electrode by way of a matching network. Each
matching network includes a pair of variable reactances having
values that are varied by motors, typically step motors.
[0004] Sensors for electric parameters associated with the plasma,
as coupled to the excitation reactance and as coupled to the chuck
electrode, derive signals which assist in controlling the values of
the variable reactances. Pressure and flow rate transducers
respectively in the chamber and in a line supplying gas to the
second port derive signals which assist in controlling the vacuum
pressure in the chamber and the flow rate of gas flowing into the
chamber through the second port.
[0005] A controller, including a microprocessor and a memory system
including a hard drive, random access memory (RAM) and a read only
memory (ROM), responds to the signals derived by the transducers
and signals from an operator input console to produce signals for
controlling the variable reactances, output power of the two AC
sources, the vacuum pressure in the chamber and the flow rate of
gases supplied to the chamber through the second port. The memory
system stores several recipes, each in the form of signals
representing various parameters controlling the deposition and
etching of the workpieces for differing situations. The parameters
of each recipe are, inter alia, gas species to be supplied to the
chamber, flow rates of the species, vacuum pressure in the chamber
and output powers of the two AC sources. Each recipe can include
other parameters, such as time for carrying out each recipe step.
The controller responds to the parameters of the recipe to control
valves for the flow of the gases into the chamber, the chamber
pressure, as well as the output power of the first and second AC
sources. During processing, the controller controls the reactances
of the first and second matching networks so that there is an
efficient transfer of power between the first and second AC sources
and the loads they drive so the impedances seen looking into the
output terminals of the first and second sources are substantially
equal to the impedances the first and second sources respectively
see by looking from their output terminals into cables connected to
the first and second matching networks.
[0006] Typically, a recipe change has been marked by step, i.e.
sudden, changes in at least one of (1) gas flow rate, (2) chamber
pressure, (3) power supplied to a plasma excitation coil, (4) the
gas species flowing into the chamber, and (5) power supplied to (a)
an electrode, such as bottom electrode on which the workpiece is
mounted or a top electrode for exciting a gas to a plasma, or (b)
an RF plasma excitation coil. These step changes result in sharp
demarcations between layers etched from the workpiece or deposited
on the workpiece. For example, the step changes during etching of a
trench in a workpiece, e.g., a silicon substrate, result in sharp
corners between a wall and base of the trench. Such step changes
also frequently result in sharp corners at a boundary between a
trench wall and a layer at the top of the trench. Such sharp
corners can make it difficult to fill the trench during subsequent
operations and have other known disadvantages, such as causing
stress related defects and/or electrical leakage.
[0007] One method of addressing the problem which has resulted in
somewhat smooth transitions when certain recipe changes are made
involves adding dilutants, such as argon or helium, or passivation
gases, such as oxygen, on a transient basis, to gases flowing into
the processing chamber during a process recipe step occurs.
However, there are disadvantages in transiently adding dilutant
and/or passivation gases to the processing chamber. Because of the
relatively large volume of a typical plasma processing chamber, a
significant amount of time, up to ten seconds, is required to purge
"old" gas from a line coupling gas from a gas source into the
chamber. As a result, there are substantial increases in workpiece
processing time, to reduce chamber efficiency and decrease
workpiece throughput. In addition, changing the gas species on a
transient basis results in a change in plasma impedance. The change
in plasma impedance adversely affects the ability of the matching
network between the electric source and the coil and/or electrode
to provide an efficient transfer of power between the source or
sources and the driven loads. In addition, the time for the new
gas, i.e., the dilutant or passivation gas, to flow into the
chamber is likely to vary as a function of gas line length between
the chamber and the gas source. As a result, precise control of the
processing step is difficult to achieve and/or recipe processing
steps must be customized for the different gas line lengths between
the different gas sources and the chamber.
[0008] Chen et al, U.S. Pat. No. 5,807,789 discloses a method of
operating a plasma processor to form in a semiconductor workpiece a
shallow trench with a tapered profile and round corners. Such a
shallow trench is formed during successive recipe steps. During a
first step the plasma power and chamber pressure are respectively
relatively high and low. During the next steps, the plasma power
and chamber pressure are respectively decreased and increased. The
process continues in this way for at least one additional step.
[0009] In a particular etching embodiment Chen et al discloses, the
plural gas species applied to the chamber remain the same and at
constant flow rates while power supplied to the plasma is reduced
in three steps, each of which occurs simultaneously with an
increase in chamber pressure. During a first step, which lasts for
eight seconds, the power supplied to a plasma excitation reactance
and chamber pressure are respectively 800 watts and 50 millitorr.
At the beginning of a second eight second step, the supplied power
is reduced suddenly from 800 watts to 750 watts while chamber
pressure is increased suddenly to 80 millitorr. At the beginning of
a third 46 second step, supplied power is suddenly reduced further
to 650 watts while chamber pressure is suddenly increased to 100
millitorr.
[0010] The aforementioned process suffers from similar problems to
the previously mentioned problems associated with adding dilutants
because of the substantial time required to change pressure in the
relatively large volume vacuum chamber. In addition, the sudden
power changes frequently do not enable the corners to be rounded to
the desired extent.
[0011] It is, accordingly, an object of the invention to provide a
new and improved method of and apparatus for operating a vacuum
plasma processing chamber.
[0012] An additional object of the invention is to provide a new
and improved method of and apparatus for operating a vacuum plasma
processing chamber in such a manner that sharp corners on processed
workpieces are avoided.
[0013] Another object of the invention is to provide a new and
improved method of and apparatus for controlling a vacuum plasma
processor such that changes in a processing recipe are performed in
a manner which avoids sharp corners on a processed workpiece.
[0014] Still another object of the invention is to provide a new
and improved method of and apparatus for processing a workpiece in
a vacuum plasma workpiece processor so that changes during a recipe
are performed in such a way as to avoid sharp corners in a
processed workpiece and wherein processor throughput is relatively
high.
[0015] Still another object of the invention is to provide a new
and improved method of and apparatus for controlling processing of
workpieces in a vacuum plasma processor, wherein changes in steps
of a recipe are performed in such a way that sharp corners of the
workpiece are avoided, without substantial impedance mismatches
occurring between one or more sources driving reactive components
which supply power to processing gas in the chamber.
SUMMARY OF THE INVENTION
[0016] In accordance with the present invention, AC power supplied
to a plasma in a vacuum plasma workpiece processing chamber is
controlled on a preprogrammed basis so there are gradual changes in
the amount of AC power supplied to the plasma during processing of
one workpiece. Preferably, the gradual power change occurs while no
change is made in (a) the gas species flowing into the chamber, (b)
the chamber pressure or (c) the gas species flow rates. The AC
power can be supplied to the chamber by an upper or lower chamber
electrode coupling an AC electric field to gas in the chamber or a
coil coupling an AC electromagnetic field to the chamber gas. The
gradual power change is typically such that it causes a gradual
transition in the shape of material in the processed workpiece.
[0017] In one preferred embodiment, a gas species is ionized into a
plasma that etches the material and the preprogrammed gradual power
change and the species are such that the material is shaped so a
rounded corner is formed in the material as a result of the
etching. In one specific application, the etching forms a trench
wall including the rounded corner, which in one embodiment is at an
intersection of a wall and a base of a trench.
[0018] The gradual change is typically performed in response to a
computer program storing steps having (1) power changes in the
range of a few milliwatts to less than 5% of the maximum output
power of a source (e.g., if a source has a maximum output power of
3 kW, the maximum power change is 15 watts), and (2) durations in
the range of about 1 millisecond to about 1 second. Steps having
power changes greater than about 5% of maximum output power are too
steep to provide the desired control over the plasma to achieve the
desired workpiece shapes and steps lasting longer than about 1
second do not have adequate temporal resolution to achieve the
desired workpiece shapes.
[0019] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed description of several specific embodiments
thereof, especially when taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWING
[0020] FIG. 1 is a block diagram of a typical vacuum plasma
processor and controller capable of performing the present
invention;
[0021] FIG. 2 is a waveform of power versus time that can be
applied to the coil or electrode of the apparatus illustrated in
FIG. 1, wherein power increases gradually in an upwardly ramping
manner;
[0022] FIG. 3 is a waveform similar to the waveform of FIG. 2,
wherein the power ramps downwardly;
[0023] FIG. 4 is a waveform of power versus time that can be
applied to the coil and/or electrode of FIG. 1, wherein the
waveform is derived from experimental data;
[0024] FIG. 5 is a schematic diagram of a cross section of an
illustrative semiconductor wafer prior to etching; and
[0025] FIG. 6 is an schematic diagram of the wafer illustrated in
FIG. 5 after it has been etched in accordance with a specific
embodiment of the present invention.
DETAILED DESCRIPTION OF THE DRAWING
[0026] The workpiece processor illustrated in FIG. 1 includes
vacuum plasma processing chamber assembly 10, a first circuit 12
for driving a reactance for exciting ionizable gas in chamber
assembly 10 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.
[0027] 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.
[0028] Plasma chamber assembly 10 includes chamber 40 having metal,
non-magnetic cylindrical side wall 42 and metal, non-magnetic base
44, both of which are 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. Planar plasma excitation coil 48, for
example, as configured in Ogle, U.S. Pat. No. 4,948,458 or Holland
et al., U.S. Pat. No. 5,759,280, sits on or in very close proximity
to the upper face of window 46. Coil 48, an electric reactance,
reactively supplies magnetic and electric AC fields usually at an
RF frequency such as 13.56 MHz, to the interior of chamber 40, to
excite ionizable gas in the chamber to plasma, schematically
illustrated in FIG. 1 by reference numeral 50. It is to be
understood that for the purposes of the present invention, coil 48
can be replaced with a powered or grounded electrode that extends
parallel to electrode 56 and can be located in chamber 40.
[0029] 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. Chuck holder 52 typically includes metal plate 56
that forms an electrode (a reactive element). Electrode 56 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 and by
supplying a liquid from a suitable source (not shown) to conduits
(not shown) in chuck 52. With workpiece 54 in place on dielectric
layer 58, DC source 66 supplies a suitable voltage through a switch
(not shown) to electrode 56 to clamp, i.e., chuck, workpiece 54 to
chuck 52.
[0030] 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, but it is
to be understood that usually there are several gas sources of
different species, e.g. etchants, such as SF.sub.6, CH.sub.4,
C.sub.12 and HBr, dilutants such as Ar or He, and O.sub.2 as a
passivation gas. 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 step 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.
[0031] 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
step 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
step 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
step.
[0032] 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 that 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 that the recipe step has been
completed. Microprocessor 20 then responds to signals from memory
system 24 to stop certain activities associated with the completed
recipe step and initiate a new recipe step on the workpiece being
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 recipe processing steps.
[0033] Excitation circuit 12 for driving coil 48 includes constant
frequency RF source 100, having a constant output power and
typically having a frequency of 13.56 MHz. Source 100 drives power
amplifier 102, having an electronically controlled power gain, so
that the amplifier response time is on the order of a few
microseconds or less, i.e., the output power of amplifier 102
changes from a first value to a second value in a few microseconds
or less. The output power of amplifier 102 is 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.
[0034] For any particular recipe, memory system 24 stores a signal
for desired output powers of amplifier 102. 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 is also gradually
dynamically changed as a function of time as preprogrammed changes
in a recipe step are ordered by memory system 24. The preprogrammed
dynamic changes in the output power are stored in memory system 24
and control the power gain of amplifier 102.
[0035] The output power of amplifier 102 drives coil 48 via cable
106 and matching network 108. Matching network 108, typically
configured as a "T," includes two series legs including variable
capacitors 112 and 116, as well as a shunt leg including fixed
capacitor 114. Coil 48 includes input and output terminals 122 and
124, respectively connected to one electrode of capacitor 112 and
to a first electrode of series capacitor 126, having a grounded
second electrode. The value of capacitor 126 is preferably selected
as described in the commonly assigned, previously mentioned,
Holland et al. patent.
[0036] 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 (50+j0) ohms.
[0037] To control motors 118 and 120 to maintain a matched
condition for the impedance seen looking into the output terminals
of amplifier 132 and the impedance amplifier 132 drives,
microprocessor 20 responds to signals from conventional sensor
arrangement 104 indicative of the impedance seen looking from cable
106 into matching network 108. Alternatively, sensors can be
provided for deriving signals indicative of the power amplifier 102
supplies to its output terminals and the power reflected by
matching network 108 back to cable 106. Microprocessor 20 responds,
in one of several known manners, to the sensed signals that sensor
arrangement 104 derives to control motors 118 and 120 to attain the
matched condition.
[0038] 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, having a
constant output power and typically having a frequency such as 400
KHz, 2.0 MHz or 13.56 MHz. The output of source 130 drives
electronically controlled variable gain power amplifier 132, having
the same characteristics as amplifier 102. Amplifier 132 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.
[0039] 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 DC 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 that
chuck 52 couples to plasma 50 establishes a DC. bias in the plasma;
the DC bias typically has values between 50 and 1000 volts. The DC
bias resulting from the RF energy circuit 14 applies to electrode
52 accelerates ions in plasma 50 to workpiece 54.
[0040] 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.
[0041] For each process recipe step, memory system 24 stores set
point signals for the net power coupled by directional coupler 134
to cable 136. The net power coupled by directional coupler 134 to
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 24 supplies the net power set point signal
associated with circuit 14 to microprocessor 20. Microprocessor 34
also responds to output signals directional coupler 134 supplies 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.
[0042] Microprocessor 20 responds to the set points and measured
signals sensor arrangement 141 derives, which measured signals are
indicative of the output power of amplifier 132 and the power
reflected back to amplifier, to control the power gain of amplifier
132. The output power of amplifier 132 is also gradually
dynamically changed as a function of time as changes in a recipe
are ordered by memory systems 24. The dynamic changes in the output
power are stored in memory system 24 and control the power gain of
amplifier 132.
[0043] One of the elements of memory system 24, typically read-only
memory 30, stores preprogrammed values for controlling the output
power of amplifier 102 and/or 132 during a step of the recipe of
plasma 50 processing workpiece 54. The preprogrammed values thereby
control the amount of power coil 48 and/or electrode 56 supply to
the plasma 50 in chamber 40 to enable the power that coil 48 and/or
electrode 56 supplies to the plasma to change gradually as a
function of time in accordance with a preprogrammed predetermined
function, such as the mathematical functions 170 and 172
illustrated in FIGS. 2 and 3 or the empirical function 174
illustrated in FIG. 4. Functions 170 and 172 of FIGS. 2 and 3 are
respectively upwardly and downwardly directed substantially
continuous, gradual linear ramping functions.
[0044] The preprogrammed values for controlling the output power of
amplifier 102 and/or 132 that read-only memory 30 stores are, in
actuality, a series of relatively small incremental steps, each of
which usually has the same value. The incremental steps are such
that the output power of amplifier 102 and/or 132 changes suddenly
at the beginning of each step, by a small value, in the range of
about 1 milliwatt to less than 5% of the maximum output power of
amplifier 102 and/or 132 (e.g., if the maximum output power of
amplifier 102 is 3000 watts, the maximum change in a step of the
output power of amplifier is 15 watts). Each step usually has the
same relatively short duration, typically between one millisecond
and one second during which the output power of amplifier 102
and/or 132 remains constant. Steps longer than one second will not
usually provide the desired rounding effect previously discussed in
this document. A series of steps in the foregoing ranges provides
substantially continuous and gradual variations in power supplied
to coil 48 and electrode 56 and therefore the power supplied to
plasma 50.
[0045] When it is desired to change the output power of amplifier
102 and/or 132 in accordance with a preprogrammed function, such as
those illustrated in any of FIGS. 2-4, the stored program in hard
disk 26 periodically reads stored numeric values in read-only
memory 30 indicative of the gain amplifier settings which provide
the desired output power of the amplifier. Microprocessor 20
responds to the values read from read-only memory 30 to control the
gain of at least one of amplifiers 102 and 132 and thereby vary the
power at least one of coil 48 and electrode 56 supplies to plasma
50.
[0046] For purposes of explanation, assume that the functions of
FIGS. 2, 3 and 4 represent the power RF source 30 and variable gain
amplifier 132 supply to electrode 56. Prior to the beginning of
ramping function 170, at time T1, memory system 24 and
microprocessor 20 set the gain of amplifier 132 so that electrode
56 supplies constant power P1 to plasma 50. During a recipe step of
interest, memory system 24 and microprocessor 20 control the gain
of amplifier 132 to increase the power supplied to electrode 56 as
indicated by linear, upwardly directed, gradually increasing and
substantially continuous ramping function 170. Ramping function 170
continues until the recipe step has been completed at time T2.
Thereafter, memory system 24 and microprocessor 20 maintain the
gain of amplifier 132 constant so that electrode 56 supplies
constant power P2 to plasma 50. Memory system 24 and microprocessor
20 control the gain of amplifier 132 and the power electrode 56
supplies to plasma 50 to gradually and substantially continuously
decrease the plasma power along ramping function 172. Ramping
function 172 extends from a constant value P2, at time T1, to a
constant value P1, at time T2. The power decrease from P2 to P1 is
performed in the same manner described for upwardly directed
ramping function 170.
[0047] The slopes of ramping functions 170 and 172 are determined
by the magnitude and duration of each step change in the gain of
amplifier 132. Typically, the magnitude and duration of each step
change in the gain of amplifier 132 for a particular recipe change
are the same; it is to be understood, however, that different step
changes in the gain of amplifier 132 for a particular recipe change
can have different magnitude and duration values.
[0048] Function 174 of FIG. 4 is empirically derived and results
from a series of experiments performed on test workpieces 54 under
various RF power settings and from measurements of profile angles
under these various power settings. It is to be understood that
function 174 of FIG. 4 is merely for illustrative purposes and that
many different empirically derived functions can be employed, as
necessary. The particular function 174 varies between a constant
power level P3 to a higher constant power level P4, which
respectively subsist prior to time T3 and subsequent to time T4,
the temporal boundaries of function 174. Function 174 decreases
slowly from power level P3, then increases at a faster rate to a
value above power level P4 and then returns to power level P4 at
time T4.
[0049] Reference is now made to FIGS. 5 and 6 of the drawing,
respectively schematic drawings of an illustrative semiconductor
structure prior and subsequent to etching operations in accordance
with one embodiment of the present invention. The pre-etch
structure of FIG. 5 includes silicon substrate 202 having a top
face coated by thin film silicon oxide layer 204, typically having
a thickness of 150 angstroms, which in turn is covered by a thin
film silicon nitride layer 206, typically having a thickness of
1600 angstroms. Layer 206 is coated with an epitaxial bottom
anti-reflective coating 208, typically having a thickness of 570
angstroms, in turn covered by two spaced photoresist strips
210.
[0050] The structure of FIG. 5 is initially processed by reducing
the height of photoresist strips 210 to form truncated photoresist
strips 212. Photoresist strips 210 are reduced in height by
supplying a typical photoresist etchant from gas sources 68 to the
interior of chamber 40 under the control of a program hard disk 26
stores and which is read to microprocessor 20. Simultaneously, the
program that hard disk 26 stores and microprocessor 20 cause the
power supplied to coil 48 and electrode 56 and the vacuum in
chamber 40 to remain constant. Next, the program that hard disk 26
stores causes bottom anti-reflective coating 28 to be etched by
opening valves 74 connected to hydrogen bromide (HBr) and oxygen
(O.sub.2) sources 68 so that the flow ratio of these gases is 75 to
22. At the same time, disk 26 and microprocessor 20 control vacuum
pump 80 so the pressure in chamber 40 is 3 millitorr.
Simultaneously, hard disk 26 causes the gains of amplifiers 102 and
132 to be such that coil 40 is supplied with 500 watts of RF power
at 13.56 MHz while electrode 56 is supplied with 178 watts of RF
power at 13.56 MHz; the 178 watts of RF power supplied to electrode
56 causes a DC bias of -200V to be established on the electrode.
Optical spectrometer 90 detects when the etching end point of layer
208 occurs. Microprocessor 20 and memory system 24 respond to the
signal from optical spectrometer 90 to cause over etching of layer
208 by 30 percent, a result achieved by not changing the etching
parameters in chamber 40.
[0051] When the over etch has been completed, microprocessor 20 and
memory system 24 cause silicon nitride layer 206 to be etched in a
somewhat similar matter to that described in connection with
coating 208 until optical spectrometer 90 detects an etch end
point. Etching of layer 206 is in response to a suitable mixture of
fluorine-based etchants while the chamber 40 pressure is 10
millitorr, the RF power supplied to coil 48 is 1000 watts and the
RF power supplied to electrode 56 is 155 watts, resulting in the
electrode having a DC bias of -70V.
[0052] Microprocessor 20 and memory system 24 then cause silicon
nitride layer 206 to be over etched for 10 seconds. The over etch
is performed by causing a suitable mixture of fluorine-based
etchants argon and oxygen to flow from sources 68 into chamber 40,
while the chamber is maintained at a pressure of 7 millitorr, and
1400 and 400 watts RF power are respectively applied to coil 48 and
electrode 56. The application of 400 watts RF power to electrode 56
results in the electrode being at a DC voltage of -145 V.
[0053] Microprocessor 20 and memory system 24 then cause a
breakthrough of silicon oxide layer 204 by causing 100 sccm
Cl.sub.2 to be applied for five seconds from gas sources 68 to
chamber 40, while 500 and 120 watts are respectively applied to
coil 48 and electrode 56.
[0054] Microprocessor 20 and memory system 24 then cause the main
etch operation for shallow trench isolation of silicon substrate
202 to be performed. The main etch operation is performed for 65
seconds in response to a suitable mixture of HBr/Cl.sub.2/O.sub.2
flowing from gas sources 68 to chamber 40. During the 65 seconds,
vacuum pump 80 maintains the pressure in chamber 40 at 15
millitorr, the output of amplifier 102 supplies coil 48 with 1000
watts and amplifier 132 supplies electrode 56 with 235 watts so
that the DC bias voltage of electrode 56 is -320V.
[0055] Upon completion of the 65 second main etch operation, the
silicon in substrate 202 is at the location indicated by point 212,
FIG. 7, slightly above the trench final base 214. The final etch
operation of silicon substrate 202 between point 212 and base 214
is performed in such a manner as to achieve rounded edges 216
between point 212 and base 214.
[0056] To this end, microprocessor 20 and memory system 24 perform
the final etch operation for 15 seconds. During the 15 second final
etch operation, vacuum pump 80 maintains the pressure in chamber 40
constant at 10 millitorr, amplifier 102 maintains the power
supplied to coil 48 constant at 100 watts and a suitable mixture of
HBr/O.sub.2 constantly flows from sources 68 into chamber 40, while
the power that amplifier 132 supplies to electrode 56 gradually
decreases from 200 to 100 watts. The gradual decrease in the power
that amplifier 132 supplies to electrode 56 is in 15,000 steps,
each having a duration of 1.0 millisecond and an amplitude of 6.667
milliwatts. After base 214 has been reached, the etchant gases are
purged from the chamber while the chamber pressure remains constant
as does the power supplied to coil 48 and electrode 56. Then
workpiece 54 can be removed from chamber 40 for further
processing.
[0057] If it is desired to provide a rounded corner, that is a
gradual transition, between the top portion of silicon substrate
202 and a layer deposited thereon, the power applied to electrode
56 and/or coil 48 can be similarly varied while the gas species and
the flow rates thereof into chamber 40 are maintained constant,
simultaneously with the pressure in chamber 40 remaining constant.
Gradual transitions in deposited layers can also be provided by
gradually and substantially continuously varying the power applied
to coil 48 and/or electrode 56, while maintaining constant the gas
species, the flow rates thereof and the pressure in chamber 40.
Because of the fast response times of amplifiers 102 and 132, the
changes in the gains of these amplifiers almost instantaneously
change the characteristics of plasma 50 to provide relatively high
throughput processing of workpieces 54 and more accurate control of
workpiece processing during a recipe step change than can be
provided by varying a parameter such as gas flow rate or chamber
pressure.
[0058] While there have been described and illustrated specific
embodiments of the invention, it will be clear that variations in
the details of the embodiments specifically illustrated and
described may be made without departing from the true spirit and
scope of the invention as defined in the appended claims. For
example, the variable gain features provided by amplifiers 102 and
132 can be incorporated directly into RF sources 100 and 30,
respectively.
* * * * *