Sensors and Actuators B 119 (2006) 159–166 Micrometric patterning process of sol–gel SnO 2 , In 2 O 3 and WO 3 thin ﬁlm for gas sensing applications: Towards silicon technology integration L. Francioso a,b,∗ , M. Russo a , A.M. Taurino a , P. Siciliano a a Institute for Microelectronic and Microsystems (CNR-IMM), Unit of Lecce Via Monteroni, University Campus, 73100 Lecce, Italy b Department of Engineering for Innovation, University of Lecce, Via per Arnesano, 73100 Lecce, Italy Received 8 August 2005; received in revised form 9 November 2005; accepted 1 December 2005 Available online 19 January 2006 Abstract Gas sensors research moves nowadays towards new applications of traditional metal oxide gas sensors into new ﬁelds like food quality, security applications and combustion control. Besides an implementation of this kind of transducers requires often a reduction of sensor dimensions, introduction of mature silicon technology substrates and fabrication methods. This work focuses on results obtained about implementation of reliable, selective, subtractive processes to perform high resolution patterning of different metal oxide ﬁlms, deposited by cheap sol–gel technology onto oxidized silicon wafer or provided with thin layers of sputtered silicon nitride. Main innovations in this paper deal with an experimental evaluation of different methods to perform etching processes for three different sol–gel metal oxide materials deposited on standard dielectric layers, and evaluation of photoresist mask chemical inertness. © 2005 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Metal oxide ﬁlm; Wet etching; High resolution patterning; Silicon technology 1. Introduction From last two decades, scientiﬁc research into gas sensors ﬁeld, knows an high growth in results and quality of devices developed and tested. Well known materials used in metal oxide gas sensors are typically SnO 2 , ZnO, TiO 2 and WO 3 , typically as thin of thick ﬁlm or as innovative nanostructures like nanobelts and nanowires. All those materials are n-type semiconductors which present, under normal atmospheric conditions and typical working tem- peratures from 200 to 400 ◦ C range, an electron depleted sur- face. Electron depletion at the surface is due to adsorption of atmospheric oxygen mainly as O 2 - or O 2 species which tie up electronic carriers. The electron depleted surface is highly gas sensitive: reducing gases like CO or H 2 react with the sur- face removing the chemisorbed oxygen such that the depletion region decreases, oxidizing gases like NO 2 cause an increase of the depletion region. The microscopic changes in the depletion ∗ Corresponding author. E-mail address: luca.francioso@le.imm.cnr.it (L. Francioso). region strongly inﬂuence the electrical properties. Measure- ments of changes in the electric conductivity therefore often serve as sensor signal [1]. Several methods have been reported for the preparation of tin oxide ﬁlms such as sputtering [2,3], evaporation [4], sol–gel [5–7], spray pirolysis, and chemical-vapor deposition. Use of a sol–gel solution for depositing tin-oxide ﬁlms seems attractive due to high porosity and uniform surface features of the sensitive layer. Sol–gel technology among different methods to deposit thin ﬁlm of metal oxide sensitive layer, is characterized to be a very cheap chemical route that allows also an easy control of synthesis process and doping procedure. Different research groups, involved in preparation of sol–gel thin ﬁlm gas sensors, have to solve often the problem of sensitive ﬁlm patterning, because sol–gel route has intrinsic drawbacks as regards to microfabrication compatibility like: a) substrate must be as ﬂat as possible to obtain higher ﬁlm thickness control; b) spun ﬁlm before calcination dissolves easily in contact with organic solvent and strong acids. Attempts to realize a soft 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.12.006