MEMS monocrystalline-silicon based thermal devices for chemical and microfluidic applications

MEMS monocrystalline-silicon based thermal devices for chemical and microfluidic applications

Mihailovic, M.

Sarro, P.M. (promotor)

Electrical Engineering, Mathematics and Computer Science

Microelectronics & Computer Engineering

2011-06-29

This thesis explores the employment of monocrystalline silicon in microsystems as an active material for different thermal functions, such as heat generation and heat transfer by conduction. In chapter 1 applications that need thermal micro devices, micro heaters and micro heat exchangers, are briefly introduced. The shortcomings of commonly used materials are listed, and monocrystalline silicon is identified as an appropriate choice for several thermal micro devices. Chapter 2 briefly presents the basic theory on resistive heating and heat transfer (by conduction, convection and radiation) and how they relate to the devices and structures presented in the following chapters. Chapter 3 summarizes the temperature dependence of the electrical and thermal properties of monocrystalline silicon in a wide temperature range. Thermal conductivity of silicon places silicon among good thermal conductors at room temperature and even better at cryogenic temperatures. In spite of the declining value of thermal conductivity at higher temperatures, silicon is still among the better thermal conductors compared to other materials typically employed in microsystem fabrication. The temperature dependence of the electrical resistivity is in chapter 3 both derived theoretically and confirmed experimentally on specially prepared samples that used differently-doped silicon layers of the same thickness. Electrical resistivity of silicon first increases with temperature, and after reaching the peak value (at so-called intrinsic temperature) it decreases with further temperature increase. This intrinsic temperature is also dependent on the doping level. Such doped silicon layers can be employed in sensors and specifically in heaters with sensing capability. Chapter 4 introduces a micro-hotplate heater capable of reaching temperatures up to 800 °C. The used materials are epitaxial Si for the heater and TiSi/TiN layer stack for the interconnects, which provides compatibility with CMOS technology. Differently doped silicon layers of different thicknesses were released by bulk micromachining to fabricate free-standing structures. The temperature dependent resistance of silicon was used for temperature monitoring. A silicon-based micro-evaporator is introduced in chapter 5. This is a cooling device intended for dealing with high heat fluxes caused by intensive local heating (e.g. by electrical power dissipation or exothermal chemical reaction). The aim of this micro-evaporator is to achieve a maximum cooling capacity and operation stability at very small liquid flow rates (in the order of 1-5 ml/h). Four different proposed fin-channel structures, with high aspect ratio channels (10 µm and 20 µm wide, 100 µm deep) are sealed with silicon or glass by wafer bonding and tested with de-ionized water as coolant. Silicon fins enhance the heat transfer to the coolant. The embedded bulk silicon heater mimics external heat sources and, at the same time, acts as temperature sensor. Measured absorbed power fluxes were up to 3 W (which corresponds to 40 W/cm2 for a heater footprint of 2.7 mm × 2.7 mm), for a fluid flow of 5 ml/h. Optimizations of the fin-channel structure and removing material by etching a cavity for thermal insulation led to a more stable operation in a broader range of set-point conditions. Chapter 6 presents a miniaturized resistojet thruster device with an integrated thin-film heater, capable of delivering thrusts in the micronewton–millinewton range. Such devices can be applied for fine attitude control of nano-satellites. Miniaturized resistojet comprises a microchannel (width: 50 µm, height: 150 µm, length: 2 cm) and a nozzle throat narrowed to 10 µm. Both channel and the nozzle were etched in silicon and sealed by anodic bonding to glass. In this device, silicon acts as a heat spreader from the integrated aluminum heater to the propellant flow inside the etched microchannel, to reduce propellant consumption. Based on the pressure measurements, calculated thrust is in 20-960 µN range, which complies with the desired range, and a 30% reduction of propellant consumption is observed when propellant flow is heated from room temperature to 350 °C. Reducing the propellant consumption is essential as the propellant storage mass and volume are very limited on-board. Finally, concluding remarks are given in chapter 7, together with recommendations for further research.

MEMS
monocrystalline silicon
bulk micromachining
wafer bonding
resistive heating
two-phase flow
micro-evaporation
micro-propulsion
high-temperature resistance measurements

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doctoral thesis

(c) 2011 Mihailovic, M.