Study of Porosity Gradient in Released Porous Silicon Microstructures

Porosity gradients are a key factor limiting the application of porous silicon (PS) in micromachined structures. In this work, the in-depth porosity profile of PS was studied by measuring the average porosity for samples grown at both constant current density and varied current density; the latter designed to compensate vertical porosity gradient effects. For constant current density, the results showed that the porosity increases from the top surface toward the Si/PS interface. This result is applicable to samples less than 4-μm thick. The varied current density investigations with current density reduced over the duration of anodisation indicated that two recipes at ΔI=10 & 12 mA/cm
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resulted in a relatively low porosity gradient and low tensile residual stress. Using these two recipes, released MEMS-based PS microstructures were fabricated. The results showed that the recipe using a current density variation of ΔI=10 mA/cm
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(where Iinitial=20 mA/cm
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and Ifinal=10 mA/cm
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) was the best approach to produce microstructures with a peak to valley (PV) flatness of 200 nm. This study provides a pathway to create released, MEMS-based resonant cavity optical devices using only a single material (Si) platform.