By Alan Petrillo, COMSOL
Semiconductor manufacturing, particle physics research, and other valuable processes occur in high-vacuum or ultra-high-vacuum (HV/UHV) conditions. To help develop a better ionization gauge for measuring pressure in HV/UHV environments, instrument manufacturer INFICON of Liechtenstein used multiphysics modeling to test and refine its impressive new design. The Ion Reference Gauge 080 (IRG080), shown in Figure 1, resulted from an international project coordinated by the European Metrology Programme for Innovation and Research (EMPIR) to develop a better tool for quantifying pressure in HV/UHV environments.
“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,” said Martin Wüest, head of sensor technology at INFICON.
The most commonly used HV/UHV pressure-measuring tool is a Bayard–Alpert hot-filament ionization gauge placed inside the vacuum chamber. The heart of this instrument consists of the filament (or hot cathode), the grid, and the 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 electrons flowing toward and within the grid will collide with any free-floating gas molecules circulating in the vacuum chamber. Electrons that collide with gas molecules 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.”
While the operational principles of these devices are sound, their calibration is too easily compromised by routine use and handling. 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% — an unacceptably wide range of variation.
The project team chose INFICON’s IE514 extractor-type gauge as the current best practice for ionization gauge design. Francesco Scuderi, an INFICON engineer specializing in simulation, used the COMSOL Multiphysics software to model the IE514.
“After constructing the model geometry and mesh, we set boundary conditions for our simulation,” said Scuderi. “We are looking to express the coupled relationship of electron emissions and filament temperature, which will vary from approximately 1400 to 2000° C across the length of the filament. This variation thermionically affects the distribution of electrons and the paths they will follow.” (Figure 2)
Next, the model was used to calculate the percentage of electrons that collide with gas particles. From there, ray tracing of the resulting ions was performed, tracing the ions’ paths toward the collector (Figure 3).
“We can then compare the quantity of circulating electrons with the number of ions and their positions. From this, we can extrapolate a value for ion current in the collector and then compute the sensitivity factor,” said Scuderi.
INFICON’s model did an impressive job of generating simulated values that closely aligned with test results from the benchmark prototype. This enabled the team to observe how changes to the modeled design affected key metrics, including ionization energy, the paths of electrons and ions, emission and transmission current, and sensitivity.
The end product of INFICON’s design process, the IRG080, incorporates many components as existing Bayard–Alpert gauges, but key parts look quite different. For example, the new design’s filament is a solid suspended disc, not a thin wire. The grid is no longer a delicate wire cage but made from stronger-formed metal parts. The collector now consists of two components: a single pin or rod that attracts ions and a solid metal ring that helps direct electron flow away from the collector and toward a Faraday cup. This arrangement, refined through ray tracing simulation, improves accuracy by better separating the paths of ions and electrons (Figure 4).
INFICON built 13 prototypes that underwent evaluation by the project consortium. Testing showed that the IRG080 achieved the goal of reducing measurement uncertainty to below 1%. Regarding sensitivity, the IRG080 performed eight times better than the benchmark.
Of course, this success was not the team’s alone. INFICON benefited from its partners’ insights and support, and in turn, the broader scientific and manufacturing community will benefit from more consistent measurements of HV/UHV conditions.
Visit COMSOL’s user story gallery to read the full case study, Elevating the performance of ionization gauges with simulation.