Study of Porosity Gradient in Released Porous Silicon Microstructures

• Published in: 2018 Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD) pp. 1 - 4
• Main Authors: Afandi, Yaman, Parish, Giacinta, Keating, Adrian
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.12.2018
• Subjects: Australia; Current density; Hafnium; Microstructure; Scanning electron microscopy; Stress
• ISSN: 2377-5505
• DOI: 10.1109/COMMAD.2018.8715242

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 \Delta\mathrm{I}=10 & 12 mA/cm 2 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 \Delta \mathrm{I}=10 mA/cm 2 (where \mathrm{I}_{\mathrm{initial}}=20 mA/cm 2 and \mathrm{I}_{\mathrm{final}}=10 mA/cm 2 ) 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.