Microfabrication of a Silicon Turbopump with Embedded Thermal Isolation for a Rankine MEMS Heat Engine

Abstract

This paper presents the fabrication of a silicon MEMS microturbopump with embedded thick thermal isolation, covering the key building blocks to incorporate thermally insulated zones in a multiple wafer stack, and the resulting device. Thermal insulation is a roadblock for the implementation of heat engines at the MEMS scale. To implement the Rankine cycle on a chip, insulation is required to prevent boiling of water in the pump while the turbine is exposed to high temperature vapour flows. To prevent heat from conducting through the silicon rotor, thick glass insulation was embedded into the rotor structure using glass molding and planarization. This composite rotor approach allows us to leverage the capabilities of silicon DRIE, while creating over 100 micron thick glass insulation between the turbine and the pump. Also, in-plane thermal insulation is required between the central pump zone and the surrounding fluid film bearings, fed with steam. This was accomplished by etching deep trenches in silicon to form an array of thin walls, followed by a thermal oxidation step that consumes the silicon walls and closes the trenches to form a deep monolithic oxide zone. These techniques were successfully implemented for the fabrication of a complete MEMS turbopump, which consists of a stack of 4 silicon wafers and one glass wafer, and a total of 18 lithography/etch steps.