Compensating porosity gradient to produce flat, micromachined porous silicon structures

Obtaining flat micromachined porous silicon structures is extremely challenging due to the myriad of factors affecting film stress in such structures. In this work, the relationships between the current control during anodization, average porosity, and residual stress within porous silicon thin films and micro-fabricated structures were investigated. Using a combination of electron microscopy, surface profilometry and reflectance spectroscopy, the optimum conditions to produce near zero residual stress in thin films and micro-fabricated structures were determined. The residual stress was adjusted by a continuous variation of the anodization current in order to achieve flat structures. The flattest released porous silicon microbeams were 2.3 mu m thick with a peak to valley height variation of only 72 nm over a length of 150 mu m. These were achieved by using an initial current density of 20 mA/cm(2) that was reduced down to 8 mA/cm(2) during anodization. By using this method of reducing the current during anodization, the inherent high porosity at the porous-silicon/silicon interface was reduced, which also enabled a 36% increase in the film thickness before film delamination compared with using a constant current. These results provide a pathway to fabricate thick, optically flat micromachined multi-layer filters using a single base material (silicon) for the structural and released layers.