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ABSTRACT A fully C-MOS compatible process for bulk silicon micromachining using porous silicon technology and front-side lithography is developed. The process is based on the use of porous silicon as a sacrificial layer for the fabrication of deep cavities into monocrystalline silicon, so as to avoid back side lithography. Cavities as deep as several hundreds of micrometers are produced with very smooth surface and sidewalls. The process is used to produce : a) suspended monocrystalline silicon membranes, b) free standing polysilicon membranes in the form of bridges or cantilevers with lateral dimensions from a few gtms to several hundreds of ýtms. Important applications to silicon integrated devices as sensors, actuators, detectors etc., are foreseen. I.

INTRODUCTION

Porous silicon is a material which has been intensively studied since 1990 [1] after the discovery that it can emit light very efficiently at room temperature. Foreseen applications were in silicon integrated optoelectronics. However, due to its easy fabrication by electrochemistry in a C-MOS compatible way, other interesting applications are foreseen. In this work we use porous

silicon as a sacrificial layer for bulk silicon micromachining with potential applications in sensor fabrication. C-MOS compatible chemicals were used in all processing steps concerning also the dissolution of the porous silicon sacrificial layer. KOH, which was used by other authors [2,3], has been avoided. Two different processes will be described in this paper. The first one is used for the formation of polycrystalline silicon membranes in the form of bridges or cantilevers suspended on deep cavities into silicon and the second one is used for the fabrication of free standing monocrystalline silicon membranes. The process is described in detail below. II.

PROCESSING FOR MEMBRANE AND CANTILEVER FABRICATION

11.1

Polycrystalline silicon membranes and cantilevers

The process for producing free standing polycrystalline silicon membranes and cantilevers has been described in detail elsewhere [4,5]. Porous silicon is produced by electrochemical dissolution of monocrystalline silicon on selected areas of the silicon wafer. As masking material for the local electrochemical reaction we use a thin layer of undoped polycrystalline silicon on top of a thin silicon dioxide layer. This bilayer was found to be a perfect mask for anodization of silicon in an ethanoic HF solution. The use of the silicon dioxide layer under polysilicon revealed to be necessary in order to avoid initiation of porous silicon formation under the mask, due to a small current passing through polysilicon [4]. The HF 249 Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society

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(a)E (b)j

(C)m M Silicon

U Porous

Polysilicon

Figure 1 : Process flow for polysilicon membrane formation (a) Porous formation in selected areas (b) Polysilicon deposition and patterning (c) Porous removal. concentration in the solution was 60%. Under the above conditions, very thick porous sili