Ž . Sensors and Actuators B 69 2000 314–319 www.elsevier.nlrlocatersensorb Structural and dimensional control in micromachined integrated solid state gas sensors D.G. Rickerby a, ) , N. Wachter a , M.C. Horrillo b , J. Gutierrez b , I. Gracia c , C. Cane c ¨ ´ ` ´ a Institute for Health and Consumer Protection, European Commission Joint Research Centre, T.P. 202, 21020 Ispra, Varese, Italy b Laboratorio de Sensores, Instituto de Fısica Aplicada, CSIC, Serrano 144, 28006 Madrid, Spain ´ c Centro Nacional de Microelectronica, IMB-CSIC, Campus UAB, 08193 Bellaterra, Spain ´ Received 16 October 1999; accepted 22 February 2000 Abstract The application of electron microscopy for quality control in the fabrication of solid state integrated tin oxide gas sensors is described. Among the parameters that influence the performance and mechanical stability of these devices are the thickness, grain size, porosity and Ž . point defect oxygen vacancy distribution of the active SnO layer and the intrinsic stresses and interfacial adhesion in the support layers. 2 The sputtered SnO films were highly crystalline with a columnar growth structure and equiaxed grains of mean diameter ;10 nm. 2 HREM studies revealed evidence of CSP defects, which act as traps for free carriers and can therefore be expected to influence the conductivity of the oxide layer. Increasing the thickness of the film from 300 to 600 nm lead to an increase in the sensitivity to low Ž . concentrations of NO ;1 ppm by up to a factor of two. Sensitivity is also affected by surface roughness and film porosity, which 2 increase the effective area on which gas molecules can be adsorbed. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas sensors; Micromachining; Tin oxide; Thin film; Electron microscopy 1. Introduction Solid state technology may be employed to fabricate integrated tin oxide gas sensors consisting of several indi- vidual microsensor elements on a single silicon chip. The advantages of applying microelectronic fabrication tech- nology in the manufacture of sensor devices include re- duced costs, small dimensions, low power consumption wx and the possibility of designing compact sensor arrays 1 . Furthermore, the use of silicon micromachining techniques allows thermally isolated structures to be fabricated, in order to minimize the heat dissipation from the sensor elements. However, in common with the manufacturing of other microelectronic devices, appropriate quality control procedures are necessary to ensure reproducibility and reliability. Electron microscopy is useful for this task and ) Corresponding author. Tel.: q39-332-785972; fax: q39-332-789434. Ž . E-mail address: david.rickerby@jrc.it D.G. Rickerby . is also a suitable technique for investigating the effect of the technological processes on the microstructure of the SnO film, which acts as the gas-sensing layer. The most 2 important parameters of interest are the grain diameter, growth structure, porosity and surface roughness of the oxide layer and the mechanical stresses present in the underlying layers. An additional factor that strongly affects the reliability of the device is the quality of the inter-layer adhesion, which is influenced by the choice of the mem- brane material. 2. Device fabrication An integrated sensor with an array of eight microsensor elements was designed using finite element analysis, to achieve reduced power consumption, and constructed us- wx ing microelectronic fabrication technology 1 . The sensor array is illustrated in Fig. 1; the sensing elements are deposited on thermally isolated membranes of two differ- ent sizes and with two different heater patterns. The active 0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0925-4005 00 00479-2