Large-Scale Fabrication of Ordered Silicon Nanotip Arrays Used for Gas Ionization in Ion Mobility Spectrometers

The 9/11 events have led to an increase in the request for sensors and sensor systems that can detect rapidly, efficiently, and at moderate cost trace explosives and a whole range of toxic substances at diverse control points, e.g., at airports and inside air conditioning systems in aircraft and public buildings. To date, the security screening instruments of choice are ion mobility spectrometers (IMS), which are basically time-of-flight mass spectrometers (Sielemann, 1999 and Stach, 1997). Such instruments allow for the detection of explosives, chemical warfare agents, and illicit drugs. Widespread adoption of the IMS technology in civilian security screening applications, for instance, at airports, has been hindered due to the fact that state-of-the-art spectrometers employ radioactive ion sources. We report on fabrication and measurements of large-scale-ordered silicon nanotip arrays, used to replace the radioactive source for IMS gas ionization. Surface ionization mechanisms on the platinum-coated silicon surface can be significantly increased compared to flat structures due to the strong field enhancement at the tips. We will show measurements of the ion current of planar surfaces compared to microstructured surfaces as well as a photoelectrochemical etching process that allows to etch flat tips with a low aspect ratio as well as long tips with high aspect ratios with exact control about the tip profile.