Background of the Invention
Recently, there has been considerable interest in the use of particle
monitoring systems within semiconductor processing equipment. Monitoring the level
of particulates present in semiconductor processing equipment can provide useful
information regarding the processing of semiconductor wafers. For example, elevated
particulate levels in the exhaust line of a plasma deposition system may be associated
with certain contamination problems that can cause reduced yields. As such, a particle
monitoring system may be useful either as a tool for process development or as
an in situ process monitoring tool. One application of a particle monitoring
system in semiconductor processing equipment is described in U.S. Patent No. 5,083,865
entitled "Particle Monitor System and Method," owned by the assignee of the present
invention, the disclosure of which is hereby incorporated by reference.
Most existing particle monitoring systems are based on light scattering.
When such systems are installed in semiconductor processing equipment, a beam of
light is directed into the exhaust line where some fraction of the input light
scatters from particulate matter flowing within the exhaust line. An optical detector
is disposed in the exhaust line so that only light scattered by the particulate
matter is collected by the detector. The output signal from the detector corresponds
to relative particulate levels within the exhaust line. One problem with particle
monitoring systems based on light scattering is that such systems generally can
only detect particles larger than 0.1 µm (microns) and, more practically, larger
than 0.3 µm (microns).
U.S. Patent No. 3,787,746 to Atac describes a system useful for detecting
the passage of energetic charged particles of the sort produced by radioactive
decay reactions. The Atac detector includes a pair of electrodes biased to an electric
field below breakdown and immersed in a special gaseous medium. When a high-velocity
charged particle passes between the electrodes of the detector, the charged particle
causes a portion of the gaseous medium to become ionized. If the particle is sufficiently
energetic to ionize the gaseous medium and to produce an electrical discharge,
a sustained current passes between the electrodes of the detector. This current
can be measured in an attached circuit to signal the passage of the charged particle.
Systems such as that described in the Atac patent are not suitable for detection
of particulate matter in the exhaust line of semiconductor processing equipment.
This is because the velocity of particulate matter in the exhaust line of the reactor
is too low to ionize gas. Thus, particulate matter flowing in the exhaust line
of semiconductor processing equipment would not produce a signal in a detector
such as that described in the Atac patent.
Summary of the Preferred Embodiments
The invention is defined in the independent claims 1, 9, 16 and 23, respectively.
Particular embodiments of the invention are set out in the dependent claims.
In accordance with the present invention, a particle monitoring system
includes a first and a second electrode separated by a gap. A voltage supply coupled
to the first and second electrodes generates an electric field near breakdown between
the first and the second electrodes. A current monitoring circuit is also coupled
to the first and the second electrodes so that the passage of a charged particle
near the first and the second electrodes causes a transient current to be detected
by the current monitoring circuit.
Brief Description of the Drawings
FIG. 1 is a schematic representation of a particle monitoring system
in accordance with the present invention.
FIG. 2 is a graph of the current-voltage characteristics of the FIG.
FIG. 3 is a schematic illustration of a semiconductor processing
system incorporating a particle monitoring system in accordance with the present
FIG. 4 is a schematic illustration of another embodiment of a particle
FIG. 5 illustrates a modification of the FIG. 4 system.
FIG. 6 is a schematic illustration of a particle monitoring system
in accordance with the present invention.
FIG. 7 is a schematic illustration of a particle monitoring system
incorporating a plurality of discrete electrodes.
FIG. 8 is a schematic illustration of another embodiment of the particle
monitoring system of the present invention.
Detailed Description of the Preferred Embodiments
The present invention relates to a particle monitoring system for
detecting the passage of charged particles between two or more electrodes. The
present invention also relates to a method of detecting particles. In preferred
embodiments of the present invention, a pair of electrodes is disposed within the
exhaust line of a semiconductor processing system. Charged particles passing between
the electrodes distort an electric field maintained between the electrodes and
cause a transient current to flow in a circuit coupled to the electrodes. High
speed, high sensitivity current monitoring circuitry detects the transient current
pulse. Current transients associated with the passage of charged particles may
be correlated with the presence of particulate matter within, for example, the
exhaust line of a semiconductor processing system.
In preferred embodiments of the present invention, an electric field
is maintained between the electrodes of the particle monitoring system so that
the electrode field is near to the point at which breakdown occurs. Current flows
in the circuit coupled to the electrodes when the near-breakdown field between
the electrodes is distorted by the passage of charged particle. As used herein,
breakdown refers to the phenomena whereby the electric field between two points
separated by a gap increases to a sufficient level so that an electric current
begins to flow between the two points. Typically, the two points are electrodes
and the gap between the electrodes may be filled by air or other gas at atmospheric,
above atmospheric, or subatmospheric pressures. The particular electric potential
that must be reached for an appreciable current to start flowing is referred to
herein as the breakdown field. Breakdown is characterized by a sharp increase in
the amount of current flowing between the electrodes in response to relatively
little change in applied voltage. As is well known in the art, the voltage at which
breakdown occurs is a complicated function of electrode shape, electrode geometry,
and electrode separation, as well as a variety of other factors. For example, the
pressure of the gas filling the electrode gap affects breakdown in accordance with
Paschen's Law. Because of the many factors affecting breakdown, it is often most
practical to determine breakdown characteristics empirically.
A simplified geometry for practicing the present invention is shown
in FIG. 1. In this embodiment, a first electrode 10 is disposed adjacent to a
second electrode 12 along the flow path of charged particulate matter such as charged
particle 14. Although it is not indicated in the figure, these electrodes are preferably
mounted in the exhaust line of a semiconductor processing system. In this embodiment,
the electrodes are rigidly mounted in fixed relationship to one another and are
electrically insulated from the walls of the exhaust line. The first electrode
10 is charged positively with respect to the second electrode 12 by a high voltage
power supply 16. Typically, the high voltage power supply 16 is a DC power supply
that is capable of supplying a voltage substantially above the particular breakdown
voltage and is adjustable to establish the desired field between the electrodes.
An electric field nearly at the breakdown field for the particular ambient environment
is established between the two electrodes. As used herein, an electric field is
"near" the breakdown field if a positive or negative perturbation in the electrode
field will cause the current flowing in the circuit to increase or decrease more
sharply than would happen if the electrodes were biased at a point well above or
below breakdown. In accordance with this definition, a field "near" the breakdown
field has a slightly different meaning for different electrode configurations and
differently charged particles. Here again, the appropriate operating potential
is best determined empirically. When an electrically charged particle such as charged
particle 14 passes between or near the two electrodes, the electric field between
the two electrodes is distorted. The distortion in the electrode field is accompanied
by the flow of current in the electric circuit coupled to the two electrodes. Pulse
current monitor 18 detects the transient current signal induced by the particulate
matter. Typically, this transient signal will be sharp, as determined by the flow
velocity of the particulate matter within the reduced pressure environment of the
exhaust line of a semiconductor processing system. Thus, the signal characteristic
of a charged particle is readily distinguishable from slowly varying noise signals.
FIG. 2 is a graph schematically illustrating the current-voltage
characteristics of the FIG. 1 system. It is generally preferred that the electric
field between the two electrodes be established at or near the most highly sloped
portions of this curve. In other words, the electric field between the electrodes
should be almost at breakdown. This may be observed experimentally by biasing the
two electrodes up to the point where a small amount current flows between the two
electrodes. At atmospheric pressure, the breakdown field of air is approximately
13,000 V/cm. The operating pressure in the exhaust line of semiconductor processing
equipment is on the order of a few millitorr, and the corresponding breakdown
field is substantially lower than that at atmospheric pressure. In practice, the
breakdown voltage can be determined experimentally by biasing the electrodes until
a current starts to flow between the electrodes. The allowable separation between
electrodes is largely a function of what power supply is being used to create the
electric field. It is desirable to separate the electrodes so that the electrode
field samples a large area. However, this separation is limited by the requirement
that a field near breakdown be maintained. Typically, an electrode gap on the order
of one centimeter is readily achieved.
By operating the particle monitoring system of the present invention
at the steepest part of the FIG. 2 current-voltage curve, the passage of a charged
particle can be detected as a brief perturbation in the small current flowing between
the two electrodes. Thus, the present particle monitoring system does not require
ionization or the formation of a continuous current path between the electrodes.
Rather, the present invention senses the distortion in the electrode field associated
with the passage of a charged particle. Because a sufficiently high electric field
is maintained between the electrodes, a charged particle passing through the electric
field region creates an easily measured distortion in the electric field. This
distortion in the electric field is accompanied by a flow of electrical charge
in the circuit coupled to the electrodes. If this current is sufficiently large,
it can be detected by a current meter wired in series with the electrodes.
A particularly preferred embodiment of the present invention is illustrated
in FIG. 3, which shows a particle monitoring system in accordance with the present
invention disposed along the exhaust line of a semiconductor processing system.
Generally speaking, a semiconductor processing system may include an input gas
line 20, a processing chamber 22 that holds the semiconductor wafer 24 during processing,
an exhaust line 26, and a vacuum pump 28. FIG. 3 shows the FIG. 1 particle monitoring
system partially enclosed in a special line adapter 27 disposed along the exhaust
line 26. In operation, particulate matter will be withdrawn from the processing
chamber 22 via the exhaust line 26 by means of the vacuum pump 28. As the particulate
matter is withdrawn from the processing chamber, the particulate matter will pass
by the electrodes 10 and 12 of the particle monitoring system, and the passage
of charged particles may be detected in the pulse current monitor 18.
One common semiconductor processing step is plasma etching, which
is used during various stages of semiconductor device lithography. The particle
monitoring system of the present invention can be used to monitor the progression
of that process or it can be used to determine whether the processing chamber 22
and wafer 24 have been sufficiently cleaned after the completion of the etching
process to continue with other processing steps. If the FIG. 3 system were used
for plasma etching, the plasma etch would be established in a glow discharge region
(not shown) and would be provided to the processing chamber 22 through input gas
line 20. The plasma etch would then interact with the wafer 24, removing the desired
material, which would then be extracted from the processing chamber 22 as particulate
matter. Because the etching process is performed by a highly ionized plasma etch,
the particulate matter travelling through the exhaust line 26 will be charged.
Thus, the particle monitoring system will register a high level of particulate
matter during the etching process. After the plasma etching process has been completed,
particulate matter will continue to be withdrawn from the surface of the wafer
24 and from the walls of the processing chamber 22. It may be useful to monitor
the level of particulate matter passing by electrodes 10 and 12 to determine when
sufficient particulate matter has been withdrawn. The residual particulate matter
may be a measure of the cleanliness of the processing chamber and might provide
information regarding whether it is advisable to begin further processing steps.
FIG. 1 shows one electrode geometry that may be used in an embodiment
of the present invention. In the FIG. 1 embodiment, two needle electrodes within
the exhaust line of a reactor are disposed in relation to each other so that a
gap exists between the tips of the two electrodes. In general, the requirements
for the electrodes of the present invention are similar to the requirements for
electrodes for use in corona chargers. Corona chargers are well known for use in
electrophotographic printing and other non-contact charging applications. The electrode
material is chosen to be rigid and should be durable so as to minimize electrode
erosion in the reactive environment of the exhaust line. For example, silicon carbide
is a preferred electrode material.
In the FIG. 1 geometry, only those particles that pass through the
high electric field region created by the electrodes are detected. The interaction
between the charged particle and the electrode field must be of a sufficient magnitude
to be detected. This requirement places a constraint on both the level of charge
present on the particulate matter and on the magnitude of the electric field necessary
to detect a particle. The amount of charge present on particulates within the exhaust
line will generally be controlled by the basic processing technique (i.e., plasma
etching or deposition versus a thermal process) and by whether any precharging
is employed for the particle detector. The electric field created by the electrodes
falls off more rapidly than the field of a single charge because one electrode
is charged positively and the other is charged negatively. In fact, the electric
field produced by the electrodes has the field characteristics of an electric dipole
at sufficiently great distances. Thus, the two needle electrode particle monitoring
system of FIG. 1 has a limited range, and depending on the electrode separation
and the size of the exhaust line, may not sample all of the particles flowing in
the exhaust line.
The limited sampling associated with the first particle monitoring
system geometry is acceptable for a variety of applications. For example, if only
a fraction of the total particulate concentration need be sampled for a process
control measurement, the simplicity and mechanical stability of the FIG. 1 geometry
would most likely be preferred. In other circumstances, a higher level of detection
may be necessary, and other detection geometries may be employed. For example,
in the alternate embodiment shown in FIG. 4, a plate 30 with a substantially circular
orifice 32 may be disposed across the cross section of an exhaust line. Plate 30
forms one electrode of the particle monitoring system and the limited flow orifice
directs all particulate matter through the plate's orifice. The plate 30 may be,
for example, metallic or it may be metal coated with a ceramic such as silicon
carbide. A needle electrode 34 disposed near the center of the plate orifice 32
serves as the second electrode for the detector. Needle electrode 34 is formed
of any material suitable for a corona charging electrode, such as silicon carbide.
In practice, it may be preferred to place the supports and the wiring for the
needle electrode downstream from the orifice to limit the disturbances to the exhaust
line flow. The point of needle electrode 34 is then disposed at, or just below,
the plane of the plate electrode 30. This is, however, only a practical consideration
and the electrode 34 will function in the particle monitoring system whether it
is disposed above or below the plate 30.
As with the FIG. 1 geometry, a high electric field must be maintained
between the plate electrode and the needle electrode. This may be done in a variety
of ways but, because it will often be preferable to maintain the potential of the
plate electrode at or near the potential of the exhaust line wall, it may be preferable
to maintain the plate electrode at or near electrical ground while maintaining
a charge on the needle electrode. Under special circumstances, it might be useful
to maintain electrical contact between the plate electrode and the walls of the
exhaust line. This will not generally be possible because the walls of the exhaust
line could act as a noise source, interfering with the current-based detection
scheme of the present invention. If the exhaust line wall is not a significant
noise source, then the plate electrode can be maintained at the potential of the
exhaust line wall.
Electrical forces between the plate electrode 30 and the needle electrode
34 introduce a mechanical instability into the FIG. 4 geometry. Accordingly, it
is necessary to maintain the needle electrode in a fixed structural relation to
the orifice of the plate electrode. In this embodiment, a highly insulating ceramic
spacer may be used to fix the relationship between the needle electrode and the
orifice of the plate electrode. As is shown in FIG. 5, the ceramic spacer 36 may
be rigidly fixed across a diameter of the orifice. Preferably, the ceramic spacer
is thin so that it does not substantially disturb the exhaust line flow and is
sufficiently thick in the dimension parallel to the exhaust line flow to maintain
the needle electrode in a substantially fixed position with respect to the orifice
in the plate. By maintaining the electrodes in a fixed relationship, noise signals
created by variations in the relative positions of the electrodes are minimized.
Minimizing the variations in the relative positions of the electrodes
is preferred but may not be essential to the performance of the present invention.
The noise signals associated with such movements vary slowly in comparison to the
signals associated with the passage of a charged particle. By associating only
transient signals with the presence of particulate matter, the present particle
detector's sensitivity to low frequency noise sources is minimized.
In other embodiments, the needle electrode may be replaced by a thin
wire running through the plate's orifice where the wire is supported both above
and below the plane of the plate electrode. Such an embodiment may be more mechanically
stable than the simplified geometry shown in FIG. 4.
The limiting orifice of the FIG. 4 embodiment allows this particle
monitoring system geometry to detect a greater fraction of the total particulate
matter flowing in the exhaust line than will be detected using the two needle geometry
of FIG. 1. Thus, in certain circumstances requiring high levels of particle detection,
the FIG. 4 geometry may be preferred to the FIG. 1 geometry, because it is likely
that a greater fraction of the total particles will be detected.
In other circumstances, the FIG. 4 geometry may be undesirable because
the reduced flow cross section may cause back pressuring problems in the exhaust
line of the semiconductor processing equipment. As is well known in the art, back
pressure in the exhaust line of semiconductor processing equipment may produce
undesirable contamination or other problems. Accordingly, it may be desirable to
retain the high detection efficiency of the FIG. 4 embodiment in a less flow-restrictive
geometry. From a circuit point of view, the functional portion of the plate electrode
of the FIG. 4 geometry lies along the edge of the orifice. Thus, the plate electrode
may be replaced by a wire loop, such as that shown in the particle monitoring system
geometry of FIG. 6.
In this geometry, a substantially circular loop 38 serves as one
electrode and the needle electrode 34 is disposed near the center of the loop.
Charged particulate matter is detected as it passes between the loop electrode
and the needle electrode. As with the FIG. 4 embodiment, the electrodes of the
FIG. 6 particle monitoring system may be maintained in fixed relationship by a
ceramic spacer. Depending on the rigidity of the loop material, two or more ceramic
spacers, formed for example in an "X" configuration, may be necessary to maintain
the relationship between the loop electrode 38 and the needle electrode 34. Also
like the FIG. 4 device, the needle electrode in the FIG. 6 geometry might be replaced
by a wire running through the loop electrode and supported on either side of the
A different embodiment of the FIG. 6 configuration is schematically
shown in FIG. 7. An array 40 of loop electrodes is disposed across the cross section
of the exhaust line of a semiconductor processing system (not shown). A corresponding
needle electrode 34 is disposed near the center of, and in fixed relation to, each
of the associated loop electrodes 38. This electrode array 40 may be wired in parallel
to minimize the complexity of the support circuitry or each loop/needle pair may
be wired individually. Such a wiring configuration allows not only better spatial
resolution across the exhaust line cross section, but also allows better time resolution
if a number of particles enter different portions of the detection grid at closely
In practice, it may be preferable to maintain all of the loop electrodes
and all of the needle electrodes in fixed relation using a single ceramic grid
Still another embodiment of the present invention is illustrated in
FIG. 8. Two wires "a" and "b" are placed across the exhaust line 50 of the processing
equipment so that they are parallel and spaced at substantially equal intervals.
The two wires are arranged so that they alternately cross the exhaust line to produce
the interwoven structure shown. In this embodiment, the "a" wire may comprise the
negative electrode and the "b" electrode may comprise the positive electrode. It
is preferred to take care to insulate or otherwise electrically isolate the two
wires "a" and "b." Charged particulate matter is detected as it passes between
any two strands of the electrode wires.
When the particle monitoring system of the present invention is used
in conjunction with semiconductor processing equipment in which plasma is used
for processing, particulate matter in the exhaust line will be charged. Typically,
particulate matter in the exhaust line of plasma-based processing equipment will
carry a charge having a magnitude of approximately 10&sup4; times the electron
charge. In such processing equipment, particulate matter carries a sufficient charge
to be detected using the present invention. On the other hand, in processing equipment
that uses thermal processes, exhausted particulate matter will normally not be
charged and it may be necessary to charge neutral particulates prior to their entry
into the particle detector. Neutral charge particles may be charged by placing
a precharging unit upstream from the electrodes of the particle monitoring system.
Such a precharger would be necessary to charge particulate matter produced by
thermal processes such as tungsten CVD or thermal oxidation.
Suitable precharging may be accomplished by, for example, corona
charging between parallel plates (field charging) or by charging particles using
a radioactive source (diffusion charging). Corona charging is typically more effective
on large particles. Radioactive charging is necessary to ensure charging of small
particles. The amount of radioactive material necessary for diffusion charging
is sufficiently small so as to be practical for most systems. Typically, less radioactive
material will be necessary than is found in a common home smoke detector.
Often, not only will particulate matter on the order of one tenth
of a micron be present in the exhaust line of a piece of semiconductor processing
equipment, there will also be free electrons or other small charged particles.
Electrons or other small particulate matter can act as a noise source in the present
system. In such instances, it may be desirable to remove the less massive of the
charged particles from the exhaust stream before the particles reach the electrodes
of the particle detector. A precipitator may be used to remove very light charged
particles from the exhaust line, so that a greater fraction of the particulates
detected are of at least a certain size. Such a precipitator may be formed, for
example, by placing plates adjacent to either wall of the exhaust line upstream
from the particle detector. An electric field is placed across these plates so
that the small particulates are collected on the plates while heavier particles
pass through. Typically, an electric field on the order of 5 V/cm will be sufficient
to separate out the small particulate matter from the heavier particulate matter.
For processing equipment which does not produce charged particulates, a precharger
is placed upstream from the precipitator. Thus, neutral particles would pass through
the precharger before entering the precipitator. The more massive particles pass
through the precipitator and are detected by passing through the electric field
of the particle detector.
While the present invention has been described with reference to
specific preferred embodiments thereof, it will be understood by those skilled
in this art that various changes may be made without departing from the true spirit
and scope of the invention. In addition, many modifications may be made to adapt
the invention to a given situation without departing from its essential teachings.