Surface ionisation gas detection: Vertical versus planar readout modes

The work is concerned with the micro-miniaturisation of surface ionisation (SI) gas sensing devices. MEMS microheaters, originally designed for the heating and readout of metal oxide (MOX) gas sensing layers, have been configured to observe SI gas sensor signals. We show that this can be performed in two distinctly different ways. In the first, vertical mode, one of the interdigital platinum (Pt) electrodes on top of the dielectric heater membrane is used as an ion emitting layer while a flat-plate counter electrode, positioned at a short distance above the emitting Pt electrode, is used for the ion current readout. In the second, planar mode, one of the two Pt interdigital electrodes is used as an ion emitter while the second serves as an ion collector. We show that both modes of readout feature ionisation efficiencies orders of magnitude larger than our previously investigated thin-film, flat plate devices. We attribute this first fact to the field enhancement that occurs at the sharp edges of Pt interdigital electrodes. Both modes of readout, however, differ considerably with regard to gas selectivity: whereas in the vertical readout mode a relatively high level of amine selectivity is observed, only a broad-range selectivity is observed in the planar mode. We conclude from this latter observation that the amine-selectivity, which is typical of SI devices, only arises when the surface-adsorbate bond needs to be broken, i.e. whenever analyte ions are forced across an air gap.