Single crystal silicon as a macro-world structural material : application to compact, lightweight high pressure vessels

Abstract

Single crystal silicon has promising inherent structural properties which are attractive for weight sensitive applications. Single crystal silicon, however, is a brittle material which makes the usable strength that can be obtained from silicon devices dependent on flaws or cracks that can arise during fabrication in a sample that begins relatively free of defects. This research explores the use of micro-machined, single crystal silicon (Si) for high-strength macro-scale application and determines its practical advantages when compared to conventional approaches. The major contributions of this thesis includes evaluation of wafer-scale silicon for macro-world applications, identification of cellular pressure vessels as a promising wafer-scale silicon device, identification of design criteria for brittle pressure vessels and quantified metrics for a competitive silicon pressure vessel design, identification of DRIE failure mechanisms, elucidation of the role of wafer-level strength variation related to surface morphology variation in wafer-scale structures, and identification of promising strength recovery techniques and promising avenues to explore for further strength improvements. High pressure vessels for aerospace applications with volumes of 10's of cc's were designed under the premise that the superior strength-to-density ratio of Si and post- processing strength recovering techniques can compensate for fabrication technology limitations that constrain the vessel shape and for the brittle nature of the material. A combination of literature review, analysis, and numerical simulations suggest that there are single crystal silicon fabrication technology compromised pressure vessel designs that can have lower structural mass to pressurant mass ratios than conventional pressure vessel designs at design pressures above about 6000 psi. A honeycomb geometry offered the best results of those studied. The honeycomb silicon pressure vessel offers the possibility of integrated micro-valves, regulators and sensors. Such vessels would be useful for nano and pico satellites and for launch vehicles. Fabrication processes and strength recovering techniques were explored experimentally to understand and improve the usable strength of microfabricated single crystal silicon macro-structures. It was found that silicon strength will vary across a DRIE etched wafer as a result of submicron sidewall roughness variation independent 3 of larger geometric parameter variation. Secondary anisotropic SF6 plasma etching and surface migration were shown to be a promising combination for recovery of microfabricated silicon strength while oxidation and oxide removal offer less promise. Long SF6 smoothing etches revealed significantly varied sidewall roughness from that hidden by overhanging silicon which affects correlations of surface morphology to device strength. An analytical model was developed that correlates silicon strength data found experimentally to the silicon pressure vessel design. The model predicts that reasonable improvements to usable silicon strength need to be made to make a silicon pressure vessel design competitive with conventional designs. Further work on strength uniformity across wafer and strength recovery is recommended to improve the usable strength of wafer-scale, DRIE single crystal silicon devices.