Devices, materials and fabrication processes for integrated micro-systems

This dissertation investigates the fundamental scientific and engineering principles and techniques involved in the design, microfabrication and characterization of a prototype catalytic microreaction system for methanol fuel reforming. It specifically concentrates on a micromembrane reactor for hydrogen gas separation and shift reaction in the microreaction system. In addition, this work addresses thin film electronic materials and devices critical for temperature sensing applications when non-silicon substrates are utilized.; The field of building miniature fuel processors for in situ hydrogen production is of particular interest in compact fuel cells, which are currently being considered as alternative energy sources for high-end portable devices such as cellular phones and laptops. This work explores the use of a micro-reactor to produce hydrogen by the reaction of methanol with water. The part that performs selective hydrogen permeation, one of the important components of this micro-reactor, is the main focus in this work. The novelty of this work is that it integrates for the first time in the same structure the hydrogen separator as well as the water gas shift reactor. Unwanted carbon monoxide produced earlier in the steam reformer gets converted into hydrogen in presence of copper at about 200°C in the shift reactor. The hydrogen separating palladium based micromembrane is successfully fabricated. It is supported by a perforated copper film that also acts as catalyst for the shift reaction. This work details the design, materials and processes for successfully fabricating the micromembrane. It evaluates the membrane for its mechanical strength and hydrogen permselectivity.; The microreaction system mentioned above is currently being fabricated on a silicon substrate. But ultimately a chemical microreactor may utilize a substrate different from silicon. Substrates such as glass or stainless steel may be used for respectively low or high thermal conductivity depending upon the application. To this end, this work explores the polysilicon thin film device technology, since it is compatible with substrates other than silicon. Lateral polysilicon thin film diodes and polysilicon thin film transistors (TFTs) can be used in the electronics that control the functioning of the microreaction system. Lateral polysilicon diodes, for example, can act as thermal sensors, while polysilicon TFTs can be used in the CMOS control circuitry. This work specifically evaluates the performance of lateral polysilicon diodes as a function of fabrication conditions such as the polysilicon grain size, the type and amount of doping in their lightly doped region. It then gives a method to use the diodes as temperature sensors in the microreaction systems. It investigates a novel use of series combination of the polysilicon diodes to obtain reduced reverse current and improved performance as temperature sensors.