Sensors and Actuators B 186 (2013) 180–185 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journal h om epage: www.elsevier.com/ locate/snb Microfabrication of MOS H 2 sensors based on Pd-gate deposited by pulsed laser ablation M. Crivellari a , M. Mattevi b , A. Picciotto a,∗ , P. Bellutti a , A. Collini a , L. Torrisi c , F. Caridi d , S. Gennaro e , A. Gasparotto b a MTLab, Fondazione Bruno Kessler, Via Sommarive 18, 38123 Povo, Trento, Italy b Department of Chemistry, Padova University and INSTM, Via Marzolo 1, 35131 Padova, Italy c Department of Physics, Messina University, Ctr. Papardo Sperone, 31, 98166 S. Agata, Messina, Italy d Science Faculty, Messina University, Ctr. Papardo Sperone, 31, 98166 S. Agata, Messina, Italy e MiNALab, Fondazione Bruno Kessler, Via Sommarive 18, 38123 Povo, Trento, Italy a r t i c l e i n f o Article history: Received 30 October 2012 Received in revised form 17 April 2013 Accepted 30 May 2013 Available online 10 June 2013 Keywords: Laser ablation MOS gas sensors Microfabrication H2 detection a b s t r a c t A Nd:YAG high power 532 nm pulsed laser was employed to deposit, a thin layer (30 nm) of palladium on the gate of a microfabricated gas sensor (MOS capacitor). The functional performances of the MOS device were investigated by characterizing the capacitance (C–t, C–V) variations occurring upon H 2 exposure, as a function of the gas concentration. A comparison in terms of sensor response versus concentration and recovery time was also performed with sensors in which the Pd layer was deposited by metal evaporation, maintaining all the parameters of the microfabrication process constant and by changing the operative working temperature. An improved performance was observed for the laser deposited Pd layer with respect to the evaporated one, an effect ascribed to the presence of Pd atoms into the underlying silicon oxide layer. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The physical and electrical properties of palladium-gate MOS (metal-oxide-semiconductor) gas sensors have been widely inves- tigated in the last years [1–4] due to their attractive functional performances and for the potential large-scale development of this technology. The interest on Pd-based MOS sensing devices is largely related to the detection of hydrogen, a promising energy vector whose use rises, however, serious issues regarding its safe manip- ulation and storage. As a consequence, the development of reliable and miniatur- ized gas sensors with high sensitivity and quick response/recovery times toward H 2 detection is of outstanding importance. It is well known that the capacitance of a MOS device changes when the sensor is exposed to hydrogen, causing a variation in the palladium work function [5]. This change can be observed by measuring the capacitance versus voltage (C–V) curve of the MOS structure, which shows a shift of the flat band potential upon interaction with H 2 [6]. From a microscopic point of view, because of catalytic activity of palladium, hydrogen molecules are dissociated on the metal gate and hydrogen atoms subsequently diffuse through the underlying ∗ Corresponding author. Tel.: +39 0461 314425; fax: +39 0461 302040. E-mail address: picciotto@fbk.eu (A. Picciotto). silicon dioxide layer, thus reaching the Si/SiO 2 interface [7]. In the present work, the use of a pulsed laser deposition (PLD) technique is proposed for the preparation of Pd-based H 2 gas sensors with improved performances. Compared to another method, such as thermal evaporation, PLD allows a better control of the film properties, such as stoichiometry, roughness, grain size, crystallinity and porosity that can be finely modified tuning the laser wavelength, pulse intensity, pulse width, substrate nature, etc. [8–10]. It is shown that MOS devices prepared by PLD display a sensitivity toward hydrogen appreciably higher than in the case of evaporated Pd films. 2. Experimental The fabrication of the MOS devices investigated in the present work was performed on 4 inches, p-type Si 〈1 0 0〉 wafers, according to the scheme displayed in Fig. 1. After a wet-oxidation of the silicon substrate to grow a 150 nm thick SiO 2 , an area of 150 m × 150 m was defined by lithography and the oxide was dry etched. Then wafers were oxidized in dry oxygen to obtain an oxide thickness of 36 nm. Subsequently, an aluminum (1% silicon) layer of 1.2 m was deposited by sputtering and metal strips were defined around the thin oxide layer to be used for both Pd adhesion layer and electrical contact pads. 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.096