IE83492B1 - Fault classification in a plasma process chamber - Google Patents
Fault classification in a plasma process chamberInfo
- Publication number
- IE83492B1 IE83492B1 IE2002/0107A IE20020107A IE83492B1 IE 83492 B1 IE83492 B1 IE 83492B1 IE 2002/0107 A IE2002/0107 A IE 2002/0107A IE 20020107 A IE20020107 A IE 20020107A IE 83492 B1 IE83492 B1 IE 83492B1
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- IE
- Ireland
- Prior art keywords
- baseline
- chamber
- plasma
- magnitudes
- fault
- Prior art date
Links
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Description
PATENTS ACT, 1992
2002/0107
FAULT CLASSIFICATION IN A PLASMA PROCESS CHAMBER
SCIENTIFIC SYSTEMS RESEARCH LIMITED
FAULT CLASSIFICATION IN A PLASMA PROCESS CHAMBER
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The present invention relates to a method of fault
classification in a plasma process chamber powered by an RF
SOLIICE .
2. PRIOR ART
Many thin film processes use plasma processes to facilitate
the rapid and accurate fabrication of minute structures with
desired properties. Plasma processes include the deposition
and etching of insulators, conductors and semiconductors on
a substrate, for example, a silicon wafer. The plasma
process usually involves placing the substrate in a vacuum
chamber, introducing process gases and applying radio-
frequency (RF) power, typically 0.1 to 200MHz, to create a
plasma. The plasma consists of ions, electrons, radical gas
species and neutral gas, all of which permit the desired
reaction to proceed. The plasma reaction has many inputs,
including RF power, gas type and flow rates, chamber
pressure, substrate and wall temperatures, chamber wall
conditions, electrode spacing, and so on.
Control of the complex plasma process is the key to improved
manufacturing, i.e. to have accurate and repeatable
processing the plasma itself should be repeatable.
Unfortunately there are few schemes in existence for direct
plasma monitoring and control. It is more usual to monitor
or control process inputs such as gas flow, power output
from RF generator, chamber pressure or temperature, etc ,
using statistical process control charts. since
However,
the plasma process depends directly on the plasma
parameters, measuring these indirect variables is generally
not sufficient.
In the ideal production scenario, the plasma chamber
operates continually, only stopping for scheduled
maintenance. However, because of the complexity of the
process, faults do occur, resulting in unscheduled tool
downtime. To maximize productivity, these faults need to be
repaired as rapidly as possible. Faults generally appear
either as aborts on process set points or as out of control
product metrology, which includes fails in parameters such
as etch rate on test wafers as well as device yield drops.
Because of the dearth in tool control and monitoring, faults
are often addressed using a cause and effect approach. For
example, during production monitor checks it may be that
there is an out—of—control etch—rate. The technician will
consider all inputs (causes) which affect etch rate,
including plasma power, pressure, gas flows, cathode
position and others. Generally there will be few clues as to
which has changed so all may have to be checked. This is
costly in terms of labour and lost production time.
Furthermore, the drive to return the process tool to
production as rapidly as possible may involve unnecessary
parts replacement, again incurring significant cost.
The RF powered plasma represents a non—linear complex load
in electrical terms. This results in the generation of
harmonics of the RF driving signal. known
These harmonics,
as Fourier components, are very sensitive to changes both in
the plasma process and the process parameters.
It is generally accepted that monitoring the Fourier
components of the RF power signal provides a useful way to
monitor the plasma process. These components are a more
direct measurement of the plasma process since they are more
directly related to fundamental plasma parameters.
It is known to use an RF sensor to monitor and control RF
plasmas by measuring the Fourier components of voltage and
current. The sensor can be used in closed or open loop
control, as for example, in etch end—point control or as in-
situ monitoring of the plasma process. In either case the
plasma can be terminated when one or more of the RF Fourier
components reaches pre—determined limits.
Unfortunately, when problems occur in the plasma reactor due
to undesired changes in input parameters, such as changes in
gas flow rate, the RF sensor as previously described can
only determine that a fault has occurred, i.e., it cannot
determine the exact fault mechanism or faulty component.
There is a need, therefore, for an improved method for fault
identification on plasma processing chambers.
SUMARY OF THE INVENTION
Accordingly, the present invention provides a method of
fault classification in a plasma process chamber powered by
an RF source, comprising the steps of:
a) running a plurality of baseline processes of
different types on the chamber,
(b) in respect of each said baseline process,
determining the magnitudes of a plurality of Fourier
components of delivered RF power and storing the magnitudes
as reference data for that baseline process, and
c) when a fault is to be classified, repeating at least
one of the said baseline process types according to a
predetermined decision tree to classify the fault by
comparing the current magnitudes of the said Fourier
components with the corresponding reference data.
In the present specification a baseline process is a plasma
process with pre—determined values for the process input
parameters. It is therefore a datum from which inferences
regarding the plasma process may be drawn.
An embodiment of the invention involves the maintenance of a
set of baseline impedance fingerprints for a given plasma
chamber. This set includes an impedance fingerprint of the
chamber running a typical process recipe. Also included is
an impedance fingerprint of the chamber alone, without any
plasma. Also included is a fingerprint of the plasma
chamber running an inert gas plasma. All of these impedance
fingerprints includes a set of electrical signals associated
with the RF power delivery. They may include RF fundamental
and Fourier components of voltage, current and phase and
derived impedance.
This set of impedance fingerprints is recorded and
maintained regularly. When a fault occurs on the chamber, a
systematic approach to fault finding is employed. Each
individual impedance fingerprint is retaken as necessary and
compared to each of the baseline impedance fingerprints.
The approach allows the user to classify the fault as either
a hardware component, or a process component.
The invention therefore provides a technique that allows an
operator to diagnose the cause of a fault, or at least
eliminate a large number of probable causes, which would
otherwise prove costly to investigate.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described, by way
of example, with reference to the accompanying drawings, in
which:
Fig. 1 depicts a typical plasma process chamber;
Fig. 2 shows a baseline impedance fingerprint;
Fig. 3 shows a baseline impedance fingerprint against a
fault impedance fingerprint;
Fig. 4 is a flow diagram of the method in accordance with
the present embodiment; and
Fig.
shows how the method of Fig. 4 is applied to chamber
difference resolution.
DETAILED DESCRIPTION OF A PREFERRED EMODIMENT
Fig. 1 shows a typical plasma process reactor. It includes
a plasma chamber l containing a wafer or substrate 2 to be
processed. A plasma is established and maintained within
the chamber by an RF power source 3. This source generally
has real oumutimpedance which must undergo a transformation
to match that of the complex plasma load. This is done via
match network 4. Power is coupled to the plasma chamber,
typically by capacitive coupling, through an electrode 8.
However, the invention also applies to systems that have
more than one capacitive electrode, those that are
inductively coupled or transformer coupled, helical/helicon
wave systems and electron—cyclotron resonance systems.
Process gases are admitted through gas inlet 7 and the
chamber is maintained at a desired process pressure by
removing process gases and by—products through gas exhaust
line l0 using pump ll. The plasma permits effective
manufacture of for example, semiconductor devices. Gases
such as C12, used to etch silicon and metal, for example,
are converted into reactive and ionized species. Etching of
the very fine geometry used to fabricate semiconductor
devices is made possible by the reactive gases, ions and
electrons of the plasma.
The wafer is processed according to some recipe, which is
controlled by the chamber operator. This recipe includes
input parameter settings such as process gas types and flow
rates, chamber pressure, substrate/wall temperatures, RF
power settings on one or more power generators, recipe time,
inter—electrode spacing, etc. This is the case for all
plasma processing tools, such as etch, deposition, etc. The
wafer will undergo very many plasma process steps before
completion. Each step contributes to the overall product
yield; a fault at any one step may destroy potential
product.
Referring again to Fig. 1, an RF sensor 5 is used to measure
the voltage and current of the RF electrical power signal in
the complex post—match electrical line. A Fourier Transform
is performed in data collection electronics 6 using a
sampling technique which extracts the Fourier components of
the voltage and current and the phase angle between these
vectors. This data sampling should have sufficiently high
resolution to determine Fourier components (in this
embodiment the first five including the fundamental)
(90dB)
across
a very large dynamic range with phase resolution of
up to 0.00l degree. Suitable techniques for high resolution
sampling and measurement of Fourier components are described
in US Patent 5,808,415. The output of the data collection
electronics 6 is connected to a controller 12 which may be a
computer or other system which uses the signals to yield
information about and/or control the plasma process.
The Fourier components are very sensitive to plasma events.
Fig. 2 shows the magnitudes of voltage, current and phase of
the delivered RF power at the fundamental and four higher
harmonics for a typical plasma process. Such a set of data
is referred to herein as an "impedance fingerprint". In the
present embodiment, three different baseline plasma
processes are run on the chamber 1 using a test wafer in
each case and, in respect of each such process, the
corresponding impedance fingerprint is determined and stored
as reference data for that process by the electronics 6 and
controller 12.
Firstly, a baseline process is run using a typical process
recipe. In particular, the same gases are included as those
used in the production run for which the chamber is
currently being used or intended to be used. The exact flow
rates are not important, only that the baseline process
always uses the same flow rates each time it is run. This
is referred to as the full process baseline. In the case of
an oxide etch process, for example, this may involve running
plasma power (which may include a plurality of power
sources), maintaining the chamber at a selected process
pressure, and running gases such as 02, CHF3, CF4 etc. All
process inputs are fixed for this baseline. Secondly, a
baseline process is run using an inert gas plasma only, such
as helium. This is referred to as the inert plasma
baseline. Finally, a baseline process is run in which very
low power is delivered to an evacuated chamber so that no
plasma ignites. This is referred to as the plasma—less
chamber baseline. In each case the respective baseline
impedance fingerprint is determined and stored as reference
data. The test wafer may be a polysilicon wafer in each
case, but the baseline processes may be carried out with
other substrates or indeed no substrate.
A wafer fabrication process typically involves running
entire batches of wafers with similar plasma process recipes
to ensure reliable volume production. If the plasma process
on each wafer is the same, then the measured Fourier
components will reflect this. Any change in the plasma
process will be registered by change(s) in the Fourier
components. Fig. 3 shows a typical impedance fingerprint
taken following a fault condition compared to the baseline
impedance fingerprint taken under normal conditions. A
change is apparent.
The use of the baseline impedance fingerprints for fault
classification will be described with reference to the
decision tree shown in Fig. 4. The starting point is the
running of the three different baseline processes on the
plasma chamber 1 and the storage of their respective
impedance fingerprints, step 20, as described above. During
a subsequent production run the plasma process is monitored
for faults, steps 22 and 24. If a fault is detected, then
the full process baseline is repeated and the corresponding
The new
impedance fingerprint determined, step 26.
fingerprint is compared with the original full process
fingerprint taken at step 20 by comparing the current
magnitudes of the Fourier components with their original
values, step 28. If no significant change is detected, the
plasma has not changed, and the test wafers or metrology
tool are determined to be at fault, step 30, and the
production process is resumed, step 32. In this connection,
a change is significant if it is outside the normal variance
of the baseline repeated several times in a healthy chamber
condition. If any one component is outside its normal
variance (typically using 3~sigma, which represents a 98%
confidence limit) the baseline is considered to have
changed.
If a significant change is detected at step 28, the plasma-
less chamber baseline is repeated and the corresponding
impedance fingerprint determined, step 34, and compared to
the original plasma—less chamber baseline fingerprint, step
. If a significant change is registered, then the chamber
hardware is inferred to have changed, step 38. By reference
to the electrical design of the chamber, step 42, the fault
can be traced and fixed, step 32. Note that other inputs
such as process power, gas flows, pressure, etc , are not
suspects so need not be tested, saving considerable time.
If no significant change is registered at step 36, the
hardware is inferred to be good, and rather the plasma
impedance is inferred to have changed, step 44. The load
power is calculated as IVCos®, step 46, and compared to its
step 48. If a significant change
original baseline value,
is recorded, the match unit or RF power generator are
inferred to have changed, step 50. If the power is
unchanged at step 48, the inert plasma baseline is repeated
and the corresponding impedance fingerprint determined, step
52, and compared to the original inert plasma baseline
fingerprint,
step 54. If a significant change is recorded,
it is inferred that the plasma has changed, step 56, and
chamber pressure,
checked,
vacuum integrity, inert gas flow are
step 58. If no significant change in inert
impedance fingerprint is recorded then it is inferred one of
the reactive gas components has changed,
step 60, and they
are individually checked, step 62.
Recording each of the impedance fingerprints is very rapid,
so that the user can very quickly determine what class of
fault has occurred,
thereby eliminating a host of possible
fault conditions which would otherwise need to be checked.
The method can also be used as a health check for RF plasma
chambers following any scheduled downtime, for example
preventative maintenance (PM) cycles. In many cases, the
qualification of the chamber prior to hand—over to
production is delayed by problems introduced during the PM.
Thus, the three initial baseline processes would be
performed before the scheduled downtime, and then the method
of Fig. 4 performed after the downtime to detect and
classify any faults.
The method of Fig. 4 can also be used to compare two
different chambers to solve inter—chamber differences. This
can be a very costly issue in the production environment.
Ideally, all plasma tools running the same process should
yield identically. However, small differences from tool to
tool can mean different yield from each tool. In worst
cases, processes have to be customised for individual
chambers. Again, because of the dearth in control
monitors, these differences cannot readily be solved.
By using the method described herein, the user can determine
in what class the differences lie. For example, hardware
differences will show up when comparing the plasma—less
impedance of chambers.
Fig. 5 shows a comparison of four
plasma chambers, where real impedance V/I Cos®, imaginary
impedance V/I Sin® and phase, Q, are displayed for each
chamber. In this case chamber #2 has a different yield and
the problem is apparent as a different plasma impedance. In
this case, this difference was traced to hardware rather
than process inputs, and a fix was rapidly found.
To use the method of Fig. 4 for comparing two different
chambers, step 20 is carried out on one of two chambers to
be compared and steps 26 onwards are performed on the second
of the two chambers to be compared. Steps 22 and 24 are not
applicable.
It is to be understood that the impedance fingerprints are
not limited to the measurement of 15 Fourier components as
described. Any number can be used, provided that there is a
sufficient number of independent components to adequately
classify the plurality of process inputs.
It should also be understood that a test wafer may not be
necessary. The baseline processes may be run on an
alternative substrate or with no substrate. Furthermore, it
is also possible to run the baseline processes on a product
wafer.
It should also be understood that the entire process may or
may not be automated in software. The technique as
described is performed in step—wise fashion. However, it is
possible to automate the entire routine, from collecting the
initial baseline data to reporting faults.
Although the embodiment described in fig 1 is that of a
capacitively coupled or Reactive Ion Etch (RTE)
configuration, the invention can be used in any RF plasma
configuration. Also, although described for a semiconductor
process, the invention can be applied to any plasma process,
including the fabrication of flat panel displays, optical
components, memory devices and any other process utilising
plasma.
The invention is not limited to the embodiment described
herein which may be modified or varied without departing
from the scope of the invention.
Claims (1)
- CLAIMS 1. A method of fault classification in a plasma process chamber powered by an RF source, comprising the steps of: a) running a plurality of baseline processes of different types on the chamber, (b) in respect of each said baseline process, determining the magnitudes of a plurality of Fourier components of delivered RF power and storing the magnitudes as reference data for that baseline process, and c) when a fault is to be classified, repeating at least one of the said baseline process types according to a predetermined decision tree to classify the fault by comparing the current magnitudes of the said Fourier components with the corresponding reference data. and 2. A method as claimed in claim 1, wherein steps (a) (b) are performed prior to a production run, wherein the method further comprises monitoring the chamber for faults during the production run, and wherein step (c) is performed upon detection of a fault during the production run. 3. A method as claimed in claim l, wherein steps (a) and (b) are performed prior to scheduled downtime of the chamber and step (c) is performed after the scheduled downtime and prior to a production run. 4. A method as claimed in claim 1, wherein the baseline processes of different types comprise a first baseline process including the same gases as those used in a l5 production run for which the chamber is used, a second baseline process running an inert gas plasma, and a third baseline process running at sufficiently low power that no plasma ignites. F J. A method as claimed in claim 1, wherein the Fourier components are those of the voltage, current and phase of the delivered RF power. 6. A method as claimed in claim 1, wherein each baseline process is carried out on a test substrate. 7. A method as claimed in claim 1, wherein each baseline process is carried out on a product wafer. wherein each baseline 8. A method as claimed in claim l, process is run in the absence of a substrate. 9. A method of comparing two plasma process chambers powered by an RF source, comprising the steps of: a) running a plurality of baseline processes of different types on one of the chambers, b) in respect of each said baseline process, determining the magnitudes of a plurality of Fourier components of delivered RF power and storing the magnitudes as reference data for that baseline process, c) running at least one of the said baseline process types on the other chamber according to a predetermined decision tree to classify any differences between the l6 chambers by comparing the current magnitudes of the said Fourier components with the corresponding reference data. l0. A computer—readable storage medium bearing program code adapted in execution on a computer to perform the following steps on a plasma process chamber powered by an RF source: a) run a plurality of baseline processes of different types on the chamber, (b) in respect of each said baseline process, determine the magnitudes of a plurality of Fourier components of delivered RF power and store the magnitudes as reference data for that baseline process, and c) when a fault on the chamber is to be classified, repeat at least one of the said baseline process types according to a predetermined decision tree to classify the fault by comparing the current magnitudes of the said Fourier components with the corresponding reference data. F. R. KELLY & CO., AGENTS FOR THE APPLICANTS
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2002/0107A IE83492B1 (en) | 2002-02-13 | Fault classification in a plasma process chamber |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
IE2002/0107A IE83492B1 (en) | 2002-02-13 | Fault classification in a plasma process chamber |
Publications (2)
Publication Number | Publication Date |
---|---|
IE20020107A1 IE20020107A1 (en) | 2003-08-20 |
IE83492B1 true IE83492B1 (en) | 2004-06-30 |
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