. A Novel Microfabricated Formaldehyde Gas Sensor with NiO Thin Film Chia-Yen Lee 1 , Che-Ming Chiang 2 , Po-Cheng Chou 3 , Lung-Ming Fu 4 , Che-Hsin Lin 5 1 Department of Mechanical and Automation Engineering, Da-Yeh University, Changhua, Taiwan 2 Department of Architecture, National Cheng-Kung University, Tainan, Taiwan 3 Department of Interior Design, Shu-Te University of Science and Technology, Kaohsiung, Taiwan 4 Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan 5 Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, chehsin@mail.nsysu.edu.tw Abstract - This paper presents a novel microfabricated formaldehyde gas sensor with enhanced sensitivity and detection resolution capabilities. The device comprises a silica microstructure suspended at a small distance above a glass substrate. A sputtered NiO thin film is used as the formaldehyde sensing layer. The gas sensor incorporates Pt heating resistors integrated with a micro hotplate to provide a heating function and utilizes Au interdigitated electrodes. When formaldehyde is present in the atmosphere, it is adsorbed by the sensing layer and causes a change in the electrical conductivity of the NiO film. Therefore, the measured resistance between the interdigitated electrodes changes correspondingly. The application of a voltage to the Pt heaters causes the temperature of the micro hotplate to increase, which in turn enhances the sensitivity of the sensor. The nanometer scale grain size of the sputtered oxide thin film is conducive to improving the sensitivity of the gas sensor. The experimental results indicate that the developed device has a sensitivity of 10 ohm ppm -1 at 300 o C and a detection capability of less than 1.0 ppm. I. INTRODUCTION Formaldehyde (HCHO) is a highly important commercial chemical due to its chemical activity, high purity and relative cheapness. However, various risk factors have been associated with this chemical recently. For example, it is a known carcinogen found in: (1) buildings as a result of condensation polymerizations used in building materials, paints, and carpets, (2) industrial chemical processes, and (3) pathology laboratories from the vapors of formalin solutions composed of 40 wt.% formaldehyde, 12.5 wt.% methanol and 47.5 wt.% water. It has been shown that throat and nose irritation can occur at formaldehyde levels as low as 0.08 ppm [1]. Hazardous formaldehyde levels in air are mandated on a “long-term exposure” basis (on average a 1-2 week- period). The NIOSH (National Institute for Occupational Safety and Health, USA) has established a permissible long- term exposure limit of 1 ppm [2]. Historically, the detection of formaldehyde has been achieved using a stringent semi-batch air-sampling procedure followed by a batch analysis of the sample. In this procedure, the air samples are collected through sampling tubes containing an XAD-2 adsorbent pre-coated with 2- hydroxymethyl piperidine. The samples are then desorbed with toluene and analyzed by gas chromatography (GC) using a nitrogen selective detector. This procedure is clearly incapable of providing immediate formaldehyde exposure information. Hence, several researchers have developed optical sensors for formaldehyde quantification applications [3, 4]. However, the associated optical arrangements are bulky and elaborate. Recently, emerging MEMS technologies and micromachining techniques have contributed significantly to the miniaturization of sensors and have permitted the development of a new generation of micro- scale sensing instrumentation. Many previous studies have discussed the development and application of MEMS technologies to the fabrication of micro-scale gas sensors incorporating NiO thin films as sensing elements [2, 5, 6]. The conductivity of a NiO thin film is changed by the adsorption of, and subsequent reaction with, formaldehyde at elevated temperatures [2]. NiO thin films demonstrate high sensitivity, high linearity and high stability in formaldehyde sensing applications. It has been shown that interdigitated electrodes (IDE) deposited on gas sensor structures increase the electric signal sensitivity of the micro-device [5]. A micromachined hotplate with integrated Si heaters has been proposed to enhance the response and selectivity of gas sensors fabricated on Si substrates [7]. The effects of humidity can be reduced by incorporating heating devices within the gas sensors. Therefore, this study develops a gas sensor with a silica hotplate suspended at a small distance above a glass substrate. The device employs a NiO sensing film and incorporates Pt micro-heaters and IDEs for electric signal measurement. The electrical properties of the sensing film vary as the formaldehyde concentration in the air changes, and these variations enable the precise value of the formaldehyde concentration to be determined. Developing a sensor with favorable characteristics (high sensitivity, low detection limit, good linearity, and fast response time) is challenging. Recently, Dirksen et al. [2] fabricated NiO thin-film formaldehyde gas sensors using a dipping process with a nickel acetylacetonate solution for an alumina substrate. A 0.5 µm NiO thin film was formed. The conductivity of this film was found to change as the formaldehyde concentration was varied for temperatures ranging from 400-600 o C. At SIcon/05 – Sensors for Industry Conference Houston, Texas, USA, 8-10 February 2005 Copyright 2005 by ISA - The Instrumentation, Systems and Automation Society. Presented at Sicon/05, 8-10 February 2005, Houston, Texas; http://www.isa.org 1