Porous Silicon as a Sacrificial Material for Microstructures Fabrication
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M. MOREL, M. LE BERRE, V. LYSENKO, G. DELHOMME, A. DITTMAR, D. BARBIER L.P.M.-INSA LYON, UMR CNRS 5511, 20 Av. Einstein, 69621 Villeurbanne, France e-mail: [email protected]
ABSTRACT Porous silicon (PS) is generated by electrochemical etching in hydrofluoric acid (HF). Recently porous silicon has been applied to micromachining and micro-devices as an alternate material, this material being used as a sacrificial layer. This technology competes with conventional techniques like surface and bulk micromachining regarding its speed, simplicity and reduced costs. A wide range of microstructures and free-standing structures can be fabricated with a large freedom of design in relation to the isotropic behavior of the etching. A sacrificial layer may be realized fast over varying thickness (PS formation rate 45 Vim/h compared to silicon bulk micromachining rate 20 jIm/h for KOH etching). This contribution is devoted to the materials aspects of patterning and processing: we will show how basic microstructures (trenches, polysilicon cantilevers, polysilicon free-standing membranes) may be fabricated using a very simple process based on a single photolithography. The important points are the choice of the mask, porous silicon properties as a function of its formation parameters and the choice of the solution removing the sacrificial layer. The morphology and porosity of the porous silicon layers are indeed mainly determined by the electrolyte composition and by the current density for a given substrate type. Optimized conditions (HF 15% and 80 mA/cm 2) lead us to an appropriate porous silicon. Finally the applicability of this technology for various microsensors will be underlined.
1. INTRODUCTION The implementation of porous silicon (PS) in microsystem technologies as a sacrificial layer is very attractive: there is indeed a wide range of realizable patterns that may be obtained according to various doping and anodization conditions and to the relative easy removal of the PS. Among others, this technology has been used for anemometers, bolometers [1] and accelerometers [2]. The main advantages of this technology compared to bulk or surface micromachining are the following: - the large freedom of design as there is no limitation in the pattern geometry due to the substrate crystallographic orientation, - the cavity depth can be varied from close to the surface to several hundreds of jIm which is impossible in conventional surface micromachining. - the process is fast and may be made at reduced costs.
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Mat. Res. Soc. Symp. Proc. Vol. 605 ©2000 Materials Research Society
2. EXPERIMENTAL DETAILS In this study the self-standing structures are made of polysilicon. P-type silicon wafers are used as the starting material. The polysilicon layer is also used as a mask for defining selected areas where PS will be produced. This allows a reduction of the number of processing steps compared to previous studies [3]. The single photolithography defines both the selected areas where the PS will be formed and the shape of the free
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