Cryogenic deep reactive ion etching of silicon micro and nanostructures

Abstract

This thesis focuses on cryogenic deep reactive ion etching (DRIE) and presents how it can be applied to the fabrication of silicon micro- and nanostructures that have applications in microfluidics and micromechanics. The cryogenic DRIE process relies on inductively coupled SF6/O2 plasma at temperatures below -100 °C. Low etching temperatures can cause some photoresist materials to crack, but Al2O3 has been shown to be a very well-suited masking material for cryogenic etching. The anisotropy of the etching process is enhanced by a thin passivation layer on sidewalls that prevents lateral etching. The main parameters that are used to adjust the thickness of the passivation layer are the process temperature and the O2 flow. Under adequate conditions vertical sidewalls are obtained, whereas passivation layers that are too thin result in negatively tapered sidewall slopes. Under conditions where a passivation layer is not formed, at higher temperatures and/or without oxygen flow, the etching profiles are isotropic. On the other hand, too high oxygen flow results in over passivation. Under conditions where the sidewall is slightly over passivated, its slopes are positively tapered, while more pronounced over passivation results in the formation of black silicon (or silicon nanograss, silicon nanoturf or columnar microstructures). Typically, vertical sidewall profiles are desirable. However, this thesis shall also demonstrate the usefulness of under and over passivation regimes. Here, highly anisotropic etching conditions are utilized to create trenches with vertical sidewalls, fluidic channels with regular micropillar arrays, and high aspect ratio silicon nanopillars. An isotropic etching process is utilized during the release of aluminum heaters fabricated on top of perforated free-standing Al2O3 membranes and silicon dioxide coated thermal silicon actuators. The fabrication process of three-dimensional sharp electrospray ionization (ESI) tips takes advantage of etching conditions that result in negatively tapered sidewalls. A self-feeding ESI interface for mass spectrometry (MS) is fabricated by combining a lidless micropillar filled channel with a sharp tip. Two approaches to the fabrication of silicon nanopillars are presented, both of which are suitable for wafer-scale manufacturing. One method combines silica nanoparticles with a highly anisotropic DRIE step, while the other method relies on highly over passivating conditions in a maskless DRIE process. Due to a large surface area and efficient light absorption in UV-range, silicon nanopillar structured surfaces are utilized as sample plates in laser desorption/ionization (LDI) MS. The wetting of nanopillar structured silicon surfaces is also studied. Fluoropolymer coated nanopillar structured surfaces have a contact angle of more than 170° and are ultrahydrophobic, whereas oxidized nanostructured surfaces are completely wetting. The accurate patterning of both completely wetting and ultrahydrophobic areas side by side allows complex droplet shapes and droplet splitters to be tailored.