Potassium Hydroxide Etched Silicon Biomembrane Optimization and Characterization for Lung-on-a-Chip Applications

Organ-on-a-chip technology is heavily studied as an alternative to animal and cell culture models. Most of these studies are primarily focusing on the biological aspects. There is a need to understand the mechanical and morphological properties and functionality of critical components of such devices. Our group is developing a Lung-on-a-Chip device based on a novel biomembrane formed from porous silicon (PSi) that mimics the interstitial space between the epithelial and endothelial cells in vivo, with a thickness of approximately 1 µm. The membranes are fabricated through a process of chemical thinning p-type silicon wafers and electrochemical anodization to create the porous structure. Previously published results from our group on membranes that did not undergo the chemical thinning characterized with nanoindentation and scanning election microscopy show a correlation between the porosity of the PSi samples with their reduced modulus and thickness. This current work focuses on the development and refinement of the technique used to etch ultrathin (less than ~5μm) silicon membranes using a potassium hydroxide (KOH) etchant. Further, we show the results of a small parametric study by varying the formation conditions during the KOH etch. The specific formation conditions that affect the resultant membrane surface quality are etchant temperature, inclusion of a surfactant, and post-etch quench “rinse”. The membranes mechanical and morphological traits are characterized using scanning electron microscopy, atomic force microscopy, and nanoindentation to discern the surface quality of the silicon biomembrane, considering parameters like roughness, defect density, and uniformity. These aspects are correlated to the different KOH process parameters to find the optimal approach, as well as its potential relationship with the membrane's morphological features. In general, the surface quality and membrane thickness uniformity improves with decreasing etch temperature, inclusion of propanol as a surfactant, and employing a multiple stage hot rinse following the KOH.