Porous Silicon as a Sacrificial Material for Micromachining of Silicon Optical Platforms
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ABSTRACT Porous silicon obtained by the anodization of heavily doped n+-type silicon wafers was used as a sacrificial layer to micromachine silicon platforms. The effect of experimental parameters, such as the nature of the masking layer, current density and anodization time, on the geometry of the porous Si formed in patterned substrate is shown. Advantages of this method on the orientation-dependent chemical etching (ODCE), which is classically used for optical fiber alignment, are discussed.
INTRODUCTION Low-cost and low-power consuming optical modules are required for spread of opticalfiber communication and optical access networks. New technologies have been studied in order to realize a simple assembly process where a micromachined silicon wafer acted as an opticalbench. Optical component as single-mode fibers, laser diodes, lens and photodetectors, were hybridly integrated on the silicon platform with optical passive alignment by simple mechanical positioning and fixing. An optical fiber is aligned with a groove etched onto the silicon wafer and fixed with a lid. A classical method to achieve V-grooves is conventional photolithography and orientation-dependent chemical etching (ODCE) in a basic solution such as KOH [1] or a mixture
of ethylene diamine and pyrocatechol (EDP) [2]. The relative height between the fiber and the other component at which the fiber has to be connected is controlled by adjusting the width of the V-groove. However, difficulties and limitations in micromachining of silicon-platforms result from the high sensitivity of the ODCE to crystallographic orientations. Many of the disadvantages of ODCE can be avoided by using porous silicon as a sacrificial etching layer. Indeed porous silicon has many potential applications in micromachining technologies [3-4]. The formation of pores in silicon results from electrochemical etching in dilute hydrofluoric acid (HF) under anodic bias. The large specific area of porous silicon yields a high etch rate in dilute hydroxide solutions, which is desirable for a sacrificial etching layer. Sacrificial layer formation can be patterned by using a 1F-resistant masking layer delineated by standard photolithography [5]. In this paper results on the formation of porous silicon on patterned n+type Si substrate are presented. We show the effect of the anodization parameters and masking layer on the depth profile and we discuss the advantages of this method compared to the OCDE to fabricate grooves for fiber positioning and to micromachine silicon platforms.
EXPERIMENTAL N+-type heavily phosphorous-doped (5-12 mflcm) (100)-oriented silicon wafers were used to study the porous silicon formation on patterned substrates and then to fabricate grooves 373 Mat. Res. Soc. Symp. Proc. Vol. 486 ©1998 Materials Research Society
for optical fiber positioning. The mask consisted of parallel strips with a width of 8, 20 or 250 Rim. As masking material we used a commercially available photoresist (3 gm-thick) alone or with a Cr-Au (0.5 rim-thick) layer deposited by sputte
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