Tech Briefs Magazine - October 2023 - Comsol-21A

SIMULATION-BASED PRODUCT DEVELOPMENT
FIGURE 3 INFICON's IE514 physical gauge.
D » DETERMINING GAS
ENSITY THROUGH
IONIZATION
" There are almost no high-tech products
that do not involve a vacuum process, "
says Martin Wüest, head of sensor
technology at INFICON.
The term " vacuum " can describe a
theoretical absolute of absence, but the
emptiness of an actual space is usually a
matter of degree. Measuring different
degrees of vacuum calls for various
methods for determining pressure levels.
" At near-atmospheric pressures, you
can use a capacitive diaphragm gauge, "
says Wüest. " At middle vacuum, you
can measure heat transfer occurring via
convection. " Neither of these approaches
is effective at HV or UHV pressure levels.
" At HV/UHV pressures, there are not
enough particles to force a diaphragm to
move, nor are we able to reliably measure
heat transfer. This is where we use
ionization to determine gas density and
corresponding pressure, " Wüest explains.
The most commonly used HV/UHV
pressure-measuring tool is a Bayard-
Alpert hot-filament ionization gauge
(Figure 2), which is placed inside the
vacuum chamber. This instrument
includes a filament, grid, and ion
collector. Its operation starts with
supplying low-voltage electric current
to the filament, causing it to heat up.
As the filament becomes hotter, it
emits electrons that are attracted to
the grid, which is supplied with higher
voltage. Some of the electrons flowing
toward and within the grid will collide
with free-floating gas molecules that
are circulating in the vacuum chamber.
Electrons that collide with gas molecules
will form ions that flow toward the
collector. This measurable ion current in
the collector will be proportional to the
density of gas molecules in the chamber.
" We can then convert density to
pressure, according to the ideal gas law, "
Wüest said. " Pressure will be proportional
to the ion current divided by the electron
current, divided by a sensitivity factor
that is adjusted depending on what gas is
in the chamber. "
S » EXISTING GAUGES ARE
ENSITIVE TO HEAT AND
ROUGH HANDLING
Something to note about Bayard-
Alpert ionization gauges is that their
calibration is easily compromised by
routine handling.
" A typical ionization gauge contains
fine metal structures that are held in
spring-loaded tension, " says Wüest.
" Each time you use the device, you
heat the filament to between 1200
and 2000°C. That affects the metal in
the spring and can distort the shape of
the filament. This changes the starting
location of the electron flow and the
paths the electrons follow. "
Along with their sensitivity to heat,
the core components of a Bayard-Alpert
gauge can become easily misaligned. This
can introduce measurement uncertainty
of 10 to 20%. " Most vacuum chamber
systems are overbuilt as a result, " Wüest
says, and the need for frequent gauge
recalibration also wastes development
time and money.
FIGURE 4 IE514 simulation results
showing the filament temperature (top)
and the electric potential surrounding
the grid structure (bottom).
S » BUILDING A
IMULATION MODEL OF
A BENCHMARK DESIGN
The project team set a measurement
uncertainty target of 1% or less for when
the gauge is used to detect nitrogen gas.
Another goal was to eliminate the need to
recalibrate gas sensitivity factors for each
gauge and gas species being detected.
The new design's performance needed
to be unaffected by minor shocks and
reproducible by multiple manufacturers.
To achieve these goals, the project
team first dedicated itself to studying
HV/UHV measurement. Its research
encompassed a broad review of 260
relevant studies. After completing their
review, the project partners selected
one design that incorporates current
best practice for ionization gauge
design: INFICON's IE514 extractor-type
COMSOL NEWS 21a
Tech Briefs Magazine - October 2023

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