Micro-structured electron accelerator for the mobile gas ionization sensor technology

Abstract

Mobile and economically priced gas monitoring and warning systems will become increasingly important for civil security, such as in fire brigade operations in undefined hazardous environments (Daum et al., 2006). Normally, photoionization detectors (PIDs) are used for the detection of gases. Hereby, the principle is based upon the ionization of the measured gas by photons, which are generated by a high-energetic gas discharge lamp with energy of 10–11 eV. Besides the detrimental unspecific gas detection because of the ionization of all gases with ionization potential (IP) below the provided photon energy, sensors also have a short lifetime combined with a high cost (http://www.intlsensor.com/pdf/photoionization.pdf). This can be remedied by the concept of an electronic supported photoionization detector (ePID; Zimmer et al., 2012) consisting of a durable UV-LED with an above-positioned electron accelerator chip manufactured on a glass substrate by planar technology. Photoelectrons are extracted by UV illumination out of the bottom electrode and will be accelerated to an energy matching the ionization potential of the gas by a downstream acceleration grid. Thereby, the stable honeycomb-structured grid acts as a porous separator between the evacuated electron acceleration path due to nm scaling and the ionization area of the detector. To enhance the emitting area yielding a higher photoelectron current, the grid structure almost levitates, realized by the use of compatible planar technological processes such as reactive ion etching (RIE) and isotropic wet etching of sacrificial layers, which will be explained in detail in this paper. Furthermore, the tunability of the grid's acceleration voltage would enable a substance-specific determination of the gas composition, where the ionization of the analytes is clearly performed by photoelectrons instead of photons.