PyroMEMS: Improved Fabrication Methods and Ignition Reliability

Abstract

Generating large force/displacement actuators at the millimeter and sub-millimeter scales remains an important challenge for microsystems engineers. One promising solution is to integrate high-energy density solid propellant fuels in microsystems (pyroMEMS) to generate high-pressure, high-temperature combustion gases. However, integrating combustible materials within microsystems is non-trivial: solid propellant fuels are generally integrated manually into the device during back-end processing leading to large variability in the device performance. The goal of this thesis was to improve pyroMEMS reliability and performance. Reliability is particularly important for single-use devices –such as pyroMEMS– that cannot be tested before use. This was accomplished by simplifying their fabrication process and developing experimentally-validated models to improve device performance. First, a solid propellant fuel –and associated deposition technique– was specifically formulated for use in pyroMEMS devices. The fuel used was a mixture of potassium 4,6-dinitrobenzofuroxan (K-DNBF) and binder. The propellant combustion was modeled using a chemical equilibrium code and validated experimentally by measuring the in situ pressure and temperature inside a pyroMEMS test device. We recorded combustion temperatures of 1300±160 °C and overpressures of up to 60±5 bar. These results were in good agreement with the values obtained from simulations. This represents the first such measurements ever carried out in pyroMEMS devices. Two complementary pyroMEMS igniters were developed in this thesis: a drop-coated bottom-side igniter and a top-side shadow-mask evaporated igniter. The bottom-side igniter concept was based on the peripheral heating and ignition of fuel droplets to obtain smooth, steady combustion without fuel peeling or ejection. The overall fabrication process was highly simplified and extremely robust –over 350 igniters were successfully fabricated and tested. An investigation of igniter performance based on substrate material, igniter layout, fuel binder content and input power was performed. Semi-analytical models were developed and successfully predicted the ignition behavior of the igniters. The top-side pyroMEMS igniter was fabricated by direct deposition of a metal igniter on the fuel surface via offset shadow mask evaporation. In this way, reliable ignition was ensured by the intimate thermal contact between the fuel and igniter. The ignition concept was successfully demonstrated on fuel drops and over top full fuel chambers without fuel degradation or ignition. Lastly, pyroMEMS balloon actuators for use in automated drug delivery systems and microfluidic actuators were designed, fabricated and tested. A foil-level fabrication technique using low-cost polymer materials and additive fabrication techniques was developed. The fabrication was compatible with cleanroom-free processing, thus facilitating solid propellant fuel integration. Balloon actuators were successfully inflated yielding membranes displacements of between 5 and 7.5 mm (for an initial membrane diameter of 5 mm). A semi-analytical model was developed that successfully predicted the balloon displacement. This work represents an important step in developing simple, large-scale fabrication techniques that minimize manual back-end processing in order to improve device reliability.