Fabrication of 3D charged particle trap using through-silicon vias etched by deep reactive ion etching

Positrons are charged antiparticles that annihilate upon contact with electrons or any form of matter. Two of the authors (Lynn and Weber) have proposed a new approach to store large numbers of charged particles such as positrons. The proposed structure consists of thousands of 0.1-mm-diameter holes etched through silicon wafers, which are then stacked axially to form cylindrical microtubes to store a large number of positrons. The authors used deep reactive ion etching on the wafers to create the through-holes with smooth and straight sidewalls. To produce the best possible etching results, they iteratively modified the etching parameters, including the chamber pressure, temperature, radio frequency power, inductively coupled plasma power, SF ₆/O₂ gas flow, and O₂ gas content. These iterations addressed the fabrication issues that were encountered, such as bowing, notching, undercutting, nonuniform etch rates across the wafer, and preferential etching along certain crystal planes. The trench widths were calibrated to match the etch rates between all the wafer features. The final results enabled the etching of through-wafer vias with straight sidewalls (89.9-90.1°), smooth surfaces, and high aspect ratios (≥5:1) with minimal defects. This paper describes the etching process used to produce these vias.