Nanoscale Freestanding Transmission Gratings for Space-based Mass Spectrometers
At a Glance
Carbon foils are used in time-of-flight space instrumentation to neutralize incident ions and to trigger start signals via secondary electron emission. If carbon foils were replaced by UV filtering nanogratings, mass spectrometers could be developed without the need for extensive light trap systems.
Focus of Research
Carbon foils, an enabling technology in plasma mass spectrometers, are used to ionize incident neutrals via charge exchange or to trigger timing signals in time-of-flight telescopes via secondary-electron emission. However, the mass resolution and overall effectiveness of these spectrometers is negatively affected by foil-induced scattering of both the ions' energy and trajectory. The utility of nanoscale freestanding transmission gratings, developed at the University of Michigan, is being further investigated. If successful, these gratings will function similarly to carbon foils relative to their ionization and secondary electron yield. The gratings will also be able to suppress EUV radiation and thus enable novel types of instrument architectures. The feasibility of using these gratings as an alternative to carbon foils is the primary focus of the investigation.
The UV filtering properties of nanoscale freestanding transmission gratings have been well documented. These nanogratings allow the passage of particles while reducing the transmission of UV, making them ideal for inclusion in space-based mass spectrometers. Work has been done at the University of Michigan to create silicon nanogratings with a structural grid made from built-in bulk silicon, as described in Mukherjee et al. (2009). The use of Silicon for the structural support eliminates differences in thermal expansion that could arise in gratings constructed from multiple materials. Gratings consisting of ~70 nm wide slits patterned 220 nm apart are etched through Silicon measuring 5 ± 0.5 µm µm thick. A silicon support structure 570 ± 10 µm thick is patterned as a 1-mm period grid under the nanograting. The geometric transparency of the nanogratings are g = 70/220 ≈ 30%. Both grid and grating are made from <100> P-type silicon semiconductor, boron-doped to a resistivity of 1-30 Ohm-cm.