Hybrid Polymer/Ultrathin Porous Nanocrystalline Silicon Membranes System for Flow-through Chemical Vapor and Gas Detecti
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Hybrid Polymer/Ultrathin Porous Nanocrystalline Silicon Membranes System for Flowthrough Chemical Vapor and Gas Detection Maryna Kavalenka1, David Fang1, Christopher C. Striemer1,3, James L. McGrath2 and Philippe M. Fauchet1,2 1 Electrical and Computer Engineering, University of Rochester, Rochester, New York, USA 2 Biomedical Engineering, University of Rochester, Rochester, New York, USA 3 SimPore, Inc., West Henrietta, NY, USA ABSTRACT Here we discuss a novel capacitive-type chemical sensor structure that uses recently discovered porous nanocrystalline silicon (pnc-Si) membranes [1] covered with metal as the capacitor plates while a polymer layer sandwiched between them serves as the sensing layer for solvent vapor detection. Pnc-Si is new ultrathin (15 nm) membrane material with pore sizes ranging from 5 to 50 nm and porosities from < 0.1 to 15 % that is fabricated using standard silicon semiconductor processing techniques. We present a study of pnc-Si membranes as a platform for such a sensor. The degree of swelling and the reversibility of the polymer/pnc-Si membrane system exposed to analyte-containing vapors are observed using optical and electrical techniques. INTRODUCTION Highly sensitive sensors capable of chemical vapor and gas detection are an increasing demand in industry, medicine and environmental monitoring applications. Polymer based sensors are attractive because of their low cost and the variety of different polymers available for sensing applications. The polymer materials are used as responsive coatings as they are able to selectively absorb different molecules, which results in polymer swelling or a change in its electrical properties. The transducer elements for polymer-based sensors include chemiresistors that measure resistance changes [2], chemicapacitors that detect changes in dielectric properties or polymer layer thickness [3-6], and mechanical oscillators that respond to mass changes [7,8]. Capacitivetype polymer based sensors can measure the response of a polymer film as a function of gas or vapor concentration and are the most preferable type of electrical sensors because of good response times, low-cost, low-power and relative ease of fabrication [3-6]. In such sensors, the polymer is used as a selective dielectric layer in a capacitor whose capacitance increases (decreases) as the analyte is absorbed into the polymer to change its permittivity (thickness). The capacitive sensors have two geometries. The first geometry is based on Interdigitated Electrodes (IDE) [4, 5]. The sensor consists of two comb-shaped metal electrodes deposited on a substrate and a polymer deposited on top of them. The second type is a parallel-plate sensor that consists of a polymer layer sandwiched between two electrodes. The top electrode in the parallelplate geometry must be porous for the analyte to reach to the sensing polymer. Etching voids in a top metal layer is challenging because metal etchants would destroy the polymer deposited under metal layer. One solution it is to create a parall
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