Sensors and Actuators B 188 (2013) 286–292 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/ locate/snb Surface ionisation gas detection: Vertical versus planar readout modes Angelika Hackner ∗ , Benoit Bouxin, Gerhard Müller EADS Innovation Works, D-81663 München, Germany a r t i c l e i n f o Article history: Received 8 March 2013 Received in revised form 27 May 2013 Accepted 26 June 2013 Available online xxx Keywords: Surface ionisation Gas detection Platinum Interdigital electrodes Microheater MEMS a b s t r a c t 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Metal oxide (MOX) gas sensors are widely employed for gas monitoring and alarm applications. Other applications are in the field of electronic nose devices [1–6]. For operation, MOX gas sen- sors need to be heated to temperatures in the range from 200 to 500 ◦ C. In the past two decades a great deal of work had been devoted to reduce the heating power consumption of such devices [7–10]. Whereas early MOX sensors on ceramic heater substrates consumed heating powers in the order of 1 W per sensor element [11,12], this power consumption could be decreased in the order of tens of milli-Watts using silicon MEMS technologies [7–10]. The latest development in this quest for even lower power consump- tions are MOX devices based on single nanowires (SNW). In such extremely miniaturised devices the current flow, that is needed to interrogate the SNW devices, is able to produce a self-heating effect that is large enough to heat the nanowires into the range of tem- peratures where conventional MOX gas sensors are being operated [13]. In this latter kind of SNW devices the power consumption can be reduced into the level of micro-Watts; i.e. into a range that can ∗ Corresponding author. Tel.: +49 089 60726450; fax: +49 089 60724001. E-mail address: angelika.hackner@eads.net (A. Hackner). be directly supplied by energy harvesting from the environment, i.e. without using batteries [14]. The common working principle of all such sensors are oxidising or reducing surface interactions which alter the concentration of electronic charge carriers in the sub-surface region [1–6,15,16]. In such resistive response (RES) sensors the sensor signal is a change in the in-plane electronic conductivity of the sensing layers. More recently we have shown that MOX sensing layers can also give rise to a surface ionisation (SI) response [17–19]. Surface ionisation is a form of gas response that relies on the adsorption of gas or vapour molecules on heated solid surfaces, the transfer of a valence elec- tron from the adsorbed analytes to the adsorbent solid and the extraction of the ionic surface species towards a collector elec- trode positioned at a short distance from the ion-emitting surface [20–22]. A striking feature of the SI process is its high sensitivity and selectivity towards hydrocarbons with amine functional groups. This latter property has recently been exploited for realising sensi- tive, selective and low-complexity sensors for amphetamine-type illicit drugs [23,24]. In the present work we are concerned with the miniaturisation of SI gas sensors using silicon MEMS technologies. In particular, we should like to show that it is possible to produce efficient SI gas sensors from commercially available silicon microheaters, which had been designed to realise conventional RES gas sensors [25]. In the work presented below we show that silicon microheaters fitted 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.06.091