ELSEVIER Sensors and Actuators A 62 (1997) 612-615 ACTU RS A PHYSICAL Silicon thermal microrelays with multiple switching states K. Hiltmann *, M. Ashauer, H. Sandmaier, W. Lang lnstitut fiir Mikro- und htJbrmationstechnik, W.-Schickard-Strafle lO, D-78052 Villingen-Schwenningen, Germany Abstract As possible approaches to reducing the cost of micromachined switches per switching function, two kinds of membrane switches with multiple contacts have been fabricated by bulk silicon micromachining and compared technically. These devices are tested as binary pressure sensors and additionally bonded to silicon heaters and filled with suitable fluid to work as theixnopneumatic relays. Compared with the single membrane closing an atTay of contacts, an array of small membranes is shown to be a favourable alternative. Thermopneumatics, on tbe other hand, are a promising means for actuating microrelays with loads above pure signal level. © 1997 Elsevier Science S.A. Keywords: Micromachinedswitches;Silicon; Thermal microrelays; Thermopneumatics 1. Introduction Switching devices, i.e., binary sensors or devices intended to switch currents, have been an attractive topic since the very beginnings of micromechanics in the 1970s. Since that time, a number of studies have been done on electrical switches as binary pressure [1-61, force [ 1,7], and acceleration [ 8-13 ] sensors as well as on microrelays [ 14-17], i.e., switches with integrated means of actuation. However, in spite of the seeming simplicity of a mechan- ical switching function in micmmachined elements, up to the present no commercial devices are known to the authors. This is mainly due to the fact that mechanical switches may demand structural elongations which are hard to realize with devices small enough for economical production, Compared with the cost of conventional technologies, micromachining then often fails to prove its economical advantage. In the case of hermetically sealed membrane switches, a great part of the chip surface is consumed for the chip bonding area and for wire bonding pads. A possibility for reduction of chip real estate per switching function thus lies in the design of devices with multiple contacts on a single chip. Even if these cannot be addressed individually, they can suit- ably act as digital sensors or, in combination with a means for actuation, as relays consecutively closing a series of contacts. Two variants of multiple membrane switches have been presented before: a series of contacts on a single membrane * Corresponding author. Tel.: +49 7721 943 236, Fax: +49 7721 943 21(1. E-mail: Hiltnmnn@Imit'Uni-Stuttgart'de 0924-4247/97/$17.00 © I997 Elsevier Science S,A. All rights reserved Pll S0924-4247 ( 97 ) 0 1608-7 as with Allen et al. [ 3,7 ] and an array of independent switches as described by Bower's group [4,5]. We have here com- pared both approaches and additionally connected a driving element for electrical actuation. For this latter, a thermopneumatic actuator was chosen [ 18] because of low driving voltage, large achievable elon- gations, and narrow range of working temperatures when used in a controlled mode. 2. Device layout The two basic variants, a single membrane with multiple contacts and an array of single membranes on a common substrate, are depicted in Figs. 1 and 2. To be able to run basic tests on several layouts with one actuation unit, chips were dimensioned comparatively large with edge lengths of 1.5 mm. Fig. 1. Membrane switch for multiple states. The Pyrex baseplate contains electrodes to be closed consecutively. Left: substrate metallization; right, cross section. Thicknesses enlarged 10 ×.