Capacitive pressure sensors and switches fabricated using strain compensated SiGeB S. Chatzandroulis a, * , S. Koliopoulou a , D. Goustouridis a , D. Tsoukalas b a Institute of Microelectronics, NCSR ‘Demokritos’, 15310 Aghia Paraskevi, Greece b Department of Applied Sciences, National Technical University of Athens, 15780 Zografou, Greece Available online 3 February 2006 Abstract The fabrication of capacitive pressure sensors and pressure switches using a 2.4 lm thick strain compensated heavily boron doped SiGeB diaphragm is presented. The process relies on the silicon fusion bonding of two silicon wafers to seal the pressure sensor cavity and construct the device. Both rectangular and circular type pressure sensors and pressure switches have successfully been fabricated using this process. Results are presented of a capacitive type sensor operating in the medical pressure regime (0–300 mmHg) with a sen- sitivity to pressure of 1.5 fF/mmHg or 305 ppm/mmHg, and of a pressure switch operating in the 3–8 bar pressure range and are thus suitable for a number of industrial and automotive applications. The current flowing through the switch when biased at 2 V jumps over six orders of magnitude when its two plates come in contact at a well defined pressure threshold. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Pressure sensor; Pressure switch; Silicon germanium; Strain compensation; Capacitive sensor 1. Introduction Heavy doping of silicon combined with wet etching in alkaline solutions or EDP (ethylenediamine, pyrocatechol and water) is commonly used in the fabrication of silicon micromechanical systems [1], taking advantage of the abrupt fall of silicon etch rate upon exposure to highly doped regions [2,3]. However, the incorporation of B atoms in the silicon lattice results in high tensile stress due to the small radius of B. This internal tensile stress leads, in turn, to the significant degradation of the pressure sensitivity of heavily boron doped membranes when com- pared to their stress free counterparts, according to a scal- ing theory in [4]. Thus compensating for the induced stress in the sensor diaphragm is highly desirable if device perfor- mance and reliability is to be improved. Previous efforts towards that goal employed the deposition of silicon oxide or nitride coatings [5]. Another way to alleviate the internal tensile stress due to boron doping is the introduction of an element, like Ge, in the lattice with larger covalent radius than silicon in order to compensate for the contraction of the lattice due to the smaller covalent radius of the boron atom [6,7]. Such strain compensated Si 1ÀxÀy Ge x B y layers have been shown to exhibit excellent characteristics and high selectivities, simi- lar to those exhibited by simple heavily boron doped layers when used as etch stops in wet chemical etching solutions [8]. In this work we take advantage of these properties to fabricate capacitive type pressure sensors and pressure switches using a strain compensated Si 1ÀxÀy Ge x B y epitax- ial layer and silicon fusion bonding. A schematic view of the sensor we have developed is depicted in Fig. 1. The device consists of a cavity etched in a thick wet oxide, a fixed electrode and a flexible elec- trode. When pressure is applied the flexible electrode deflects towards the fixed electrode and the device capaci- tance changes. In capacitive type pressure sensors it is this change that is of interest. Rather in pressure switches the value of pressure at which the flexible electrode will touch the fixed electrode is the important parameter. 0167-9317/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2006.01.048 * Corresponding author. Tel.: +30 210 6503271; fax: +30 210 651 1723. E-mail address: stavros@imel.demokritos.gr (S. Chatzandroulis). www.elsevier.com/locate/mee Microelectronic Engineering 83 (2006) 1209–1211