Tech Briefs Magazine - September 2023 - PIT-8

Chamber Clean Endpoint Detection
Foreline
Gas
gas present or 2) using the intensity of
light in a region where the gas absorbs
and another region where it is does not
absorb. The absorbance is wavelength
dependent with spectral characteristics
that are unique to each gas. Since
the measured absorbance changes with
the concentration of the gas, the signal
from an infrared detector can be used
for endpoint determination. The sensitivity
of infrared absorption measurements
ensures the detection of the low
concentrations that exist under endpoint
conditions.
Throttle
Valve
Figure 1 shows the basic components
of an infrared absorbance-based endFigure
2. Typical installation configuration of the Process Sense Detector (left in the photograph) or
IGA-T End Point Detector (right in the photograph).
Process Sense Detector IGA-T
Technology
Measurement
Readings
Gas Measured
NDIR
Integrated absorbance
(single absorbance value)
One gas
SiF4 or WF6
Scanning NDIR (Tunable Filter
Spectroscopy)
Absorbance spectrum
One or two gases
SiF4 or WF6 or CO2 or SiF4 + CO2
Other configurations possible
Detection Limit
SiF4: 1.5 mTorr
WF6: 2 mTorr
SiF4: 0.4 mTorr
WF6: 0.8 mTorr
CO2: 0.4-0.6 mTorr depending on
configuration
Outputs
Advantages
Analog output
Low cost
Analog and serial outputs; includes
self-diagnostic features
Minimal drift, does not need to be
re-zeroed often, high sensitivity
Table 1. Comparison of Process Sense Detector and IGA-T Detector.
8
point detector. A broadband infrared
light beam created by an IR source
traverses a gas cell through which the
sample gas is flowing. Its intensity is reduced
with the reduction dependent on
the concentration of the gas present.
The beam goes through a filter that selects
those wavelengths specific to the
gas being measured after which it impinges
on a detector that measures the
light intensity. Periodically, a " zero " signal
is collected while flowing inert gases
such as N2 or Ar or with the gas cell under
high vacuum. This step eliminates
instrument effects and hardware drift
and ensures that the value determined
for the absorbance depends only on the
sample gas.
MKS Instruments offers two infrared-based
chamber clean endpoint
detectors: the Process Sense™ NDIR
End Point Detector and the T-Series
IR Gas Analyzer (IGA-T). Both instruments
have similar gas cells, IR emitter
sources, and IR detectors. However,
the Process Sense™ Detector employs
Non-Dispersive Infrared (NDIR) filter
optics, while the IGA-T uses Tunable
Filter Spectroscopy (TFSTM)-based
optics.
The Process Sense NDIR End Point
Detector is a low-cost SiF4 or WF6 detector
specifically designed for high
volume chamber clean endpoint detection
applications. The Process Sense
Detector measures the concentration
of etch by-products in the chamber exhaust
flows with a sensitivity down to
mTorr. This low detection limit makes
it well-suited for chamber clean applications
in silicon oxide, silicon nitride,
polysilicon, and other silane or TEOSbased
CVD processes.
The Process Sense Detector employs
two filters, one that transmits only the
bandpass specific to the sample gas
Photonics & Imaging Technology, September 2023

Tech Briefs Magazine - September 2023

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