PECVD a-C:H films for STW resonant devices G. Cicala a, * , P. Bruno a , A. Dragone b , A.M. Losacco c , C. Sadun d , A. Generosi e a Istituto di Metodologie Inorganiche e dei Plasmi (IMIP-CNR) Sezione di Bari Via G. Amendola 122/D 70126 Bari, Italy b Dipartimento di Ingegneria Elettrotecnica ed Elettronica Politecnico di Bari, via Orabona 4, 70126 Bari, Italy c Centro Laser, S.P. Casamassima km 3 70010 Valenzano (Bari), Italy d Dipartimento di Chimica Universita’ di Roma bLa SapienzaQ P.le Aldo Moro 5, Roma, Italy e Istituto di Struttura della Materia (ISM-CNR) Via del Fosso del Cavaliere 100, 00133 Roma, Italy Available online 29 December 2004 Abstract Plasma polymeric hydrogenated amorphous carbon (a-C:H) coatings have been found to be a promising material for use in surface transverse acoustic wave (STW) resonator employed as humidity sensor. The tailoring of coating properties has been easily obtained by variation of CH 4 percentage in CH 4 –Ar mixture that enables to tune the response of polymer-coated STW resonator. Specifically, the optimum sensing characteristics of the STW resonator has been exhibited by coating obtained at 50% of CH 4 percentage. The correlation between the device sensitivity and the material properties, as determined by small angle X-ray scattering (SAXS), X-ray reflectivity (XRR), ellipsometry and Fourier infrared spectroscopy (FTIR) measurements, permits us to highlight the sensing mechanism of the active a-C:H coatings. The porosity has been considered the main effect on the enhancement of the sensor response. D 2004 Elsevier B.V. All rights reserved. Keywords: PECVD; Polymeric a-C:H films; STW device; Humidity sensor 1. Introduction Abundant literature exists on several and different kinds of humidity sensors based on the measurements of the change of a particular physical parameter. The present paper deals with a novel humidity sensor based on a plasmapol- ymer-coated surface transverse acoustic wave (STW) resonant device that relies on a mass change of polymer coating upon the exposure to the relative humidity. In these last years the micro/nanoporous materials such as porous silicon [1] and polymers [2–7] have attracted a lot of interest owing to their potential application in humidity sensor. In our earlier paper [5], polymeric coatings (such as hexamethyldisiloxane, HMDSO, and a-C:H) have been employed and tested on STW resonator. HMDSO film has been already utilized and its sensitivity has been linked to the higher oxygen in the polymer (i.e. to its hydrophilicity) [3], whereas polymeric a-C:H film has been used for the first time and it has been shown that this material is appropriate for humidity sensing. Amorphous carbon films, produced by radio frequency Plasma Enhanced Chemical Vapor Deposition (r.f. PECVD) technique, have been widely and deeply investigated ([8] and refs. therein). It is well known that their amorphous matrix is a three-dimensional network composed of trigonally and tetragonally coordinated carbon atoms and voids ([9] and refs. therein). Specifically, the porous structure of polymer- like hydrogenated amorphous carbon (a-C:H) films is due to the low growth temperature (RT) that (a) promotes the high incorporation of hydrogen and (b) inhibits the mobility of adsorbed species at the surface as well as in its bulk leading to the void formation. The porosity extent also depends on the deposition rate of the film, at higher deposition rate indeed the growth precursors stick to the film surface where they land and do not have time to rearrange before the arrival of other species. Therefore, the void amount depends on the process parameters. Obviously, the porous structure limits the 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.156 * Corresponding author. Tel.: +39 080 592 9518; fax: +39 080 592 9520. E-mail address: grazia.cicala@ba.imip.cnr.it (G. Cicala). Thin Solid Films 482 (2005) 264 – 269 www.elsevier.com/locate/tsf