Sensors and Actuators B 42 (1997) 67–79 A capacitance-type chemical sensor that employs VAPO-5, MnAPO-5 and MAPO-36 molecular sieves as the dielectric phase Kenneth J. Balkus Jr. a, *, Laura J. Ball a , Mary E. Gimon-Kinsel a , J. Mark Anthony b , Bruce E. Gnade b a Department of Chemistry, Uniersity of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083 -0688, USA b Materials Science Laboratory, Semiconductor Research and Deelopment, Texas Instruments, Dallas, TX 75265, USA Received 7 December 1995; received in revised form 14 April 1997; accepted 18 April 1997 Abstract We have developed a chemical sensor based on a capacitor design that utilizes molecular sieves as the dielectric phase. In this study, thin films of vanadium aluminum phosphate (VAPO-5), the isostructural manganese aluminum phosphate (MnAPO-5) and magnesium aluminum phosphate (MAPO-36) were deposited onto titanium nitride-coated silicon wafers by laser ablation. The molecular sieve films function as the dielectric phase and were patterned with Au/Pd to complete the capacitor. The molecular sieve-based sensors exhibited capacitance changes upon exposure to CO 2 , CO, N 2 and H 2 O at room temperature. Significant responses to CO 2 and CO were registered at levels as low as 50 and 100 ppm, respectively. © 1997 Elsevier Science S.A. Keywords: Sensor; Capacitance; Laser ablation; Molecular sieves 1. Introduction We have previously reported the preparation of ca- pacitance type chemical sensors that utilize molecular sieves as the dielectric phase [1,2]. Fig. 1 shows a schematic diagram of this device. The fabrication of this sensor depends on the deposition of a thin (150– 250 nm), continuous film of the molecular sieve onto a titanium nitride (TiN) substrate. The deposition has been accomplished using pulsed laser ablation of a molecular sieve target followed by post hydrothermal treatment of the resulting thin film [1–4]. The TiN serves as one electrode and a patterned series of Au/Pd contacts acts as the other electrode. A voltage is applied across the electrodes and the capacitance is measured. When the device is exposed to different gases, the capacitance changes due to the physical interactions of the gas molecules with the molecular sieve film. We have previously observed changes as great as four orders of magnitude above the background for polar molecules such as carbon monoxide [5]. These results represent a significant advancement for molecular sieve- based chemical sensors. Other electronic devices that employ zeolites as the active matrix have been reported to only measure capacitance changes in the picofarad range [6–10]. Related mass sensitive devices [11–15] based on zeolite molecular sieves have not been shown to discriminate between analytes of similar size and shape such as CO and CO 2 . In contrast, we have shown that sensors based on the design in Fig. 1 can readily distinguish between small molecules such as H 2 O, N 2 , CO and CO 2 [1–4]. Ultimately, we hope to construct a device composed of an array of different molecular sieve capacitors where each molecular sieve will exhibit a signature response (capacitance change) to each gas. The identifi- cation and quantification of a particular molecule in complex mixtures such as combustion gases may be achieved by a combination of molecular sieves. Our effort towards this objective requires the evaluation of different molecular sieves in our capacitance device. In the case of CoAPO-5 we found the incorporation of Co 2 + into the AlPO 4 -5 framework dramatically altered the observed capacitance changes for different gases [5]. For example, the measured capacitance ratio for CO/ CO 2 was 120 for CoAPO-5 but only 5 for AlPO 4 -5 [1,5]. Thus, metal substituted aluminum phosphates * Corresponding author. Tel.: +1 972 8832659; fax: +1 972 8832925; e-mail: balkus@utdallas.edu 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S09 2 5 -4 005(97)00 1 89 - 5