A microscale plasma transistor capable of high speed switching was manufactured using microfabrication techniques and operated using microplasma discharges. Such a device has feature sizes on the order of 25 μm, is robust against spikes in power, high temperatures, as well as electromagnetic interference and capable of low cost production through microfabrication. In this work two aspects of the Microscale Plasma Device development were investigated; (1) the microplasma properties and (2) the manufacturing of the MPD. To study the required plasma discharges and develop them to become better suited for the task at hand, Direct Current (DC) plasma discharge characteristics were investigated under pressures from atmospheric to 1.65 MPa while varying current from 0.1 mA to 4.5 mA and gap length from 5 μm up to 250 μm. This testing was carried out in a high pressure test chamber fitted with a micrometer for variable electrode spacing and gas inlets and outlets for varying the working gas and pressure. Voltage and current measurements along with microscopic imaging and optical emission spectroscopy were taken for discharges in the varying gases to determine discharge V-I characteristics, sizes and temperatures. Data gathered was used to understand the microdischarge characteristics and tailor the plasma for application in the microtransistor. Discharges with diameters as small as 7 μm have been achieved in Nitrogen and 16.7 μm in Helium by operating at low current, 0.5mA, and at pressures of 1.65 MPa and 0.34 MPa respectively. Paschen type, Pd, scaling is observed both in breakdown voltages, steady state voltage, and discharge size from 0.1 MPa to 1.65 MPa. The discharges operated in a normal glow regime with near constant normalized current densities and relatively flat voltage current characteristics indicating normal glow discharge behavior in the microplasmas. During device development small diameter metal wires, 25 μm and 50 μm, were used to carry out experiments on the microscale device to aid in electrode and substrate material selection. Robust electrode and substrate materials were required to withstand high discharge temperatures in order to prolong the device life time. As material selection was narrowed down microfabrication techniques were utilized to achieve smaller electrode geometry and electrode gap spacing with greater consistency. Photolithography coupled with thin film deposition using metal evaporation and electroplating resulted in electrodes with features and electrode gap spacing on the order of 25 μm and film thicknesses of 35 μm. Aluminum, Nickel, Copper and Titanium were deposited onto Alumina (Al_(2)O_(3)) and Ceria Stabilized Zirconia (CSZ) ceramic substrates, where only the latter two electrode materials proved to be robust enough to withstand the discharge temperatures. Superficial oxide layers on the electrode surfaces provided a protective coating, prolonging electrode lifetime in atmospheric air from 360 seconds to greater than 400 seconds without metal evaporation occurring. The oxide coating also acted as a dielectric barrier and increasing the resistance of the electrode surfaces to the order of 300 MΩ in some cases thus acting as a ballast resistance, compensating for stray capacitance and helped stabilize the plasma discharge.
- Staack, David Associate Professor