Microfabrication of Microdischarge-Based Sensors and Actuators.

Abstract

For sensors and actuators that use microdischarges, microfabrication technology provides both portability and ease of integration into microsystems. This thesis investigates two types of microfabricated devices: sputter ion pumps (SIP) to control on-chip vacuum, and miniature radiation detectors for sensing beta particles, gamma rays or neutrons. The SIP utilizes lithographically micromachined Penning cell array to ignite a plasma at pressures as low as 1.5 µTorr. The system pressure is reduced from 1 Torr to <10 mTorr. By reducing the interelectrode distance, the plasma is ignited as low as 400-600 V, compared to >2000 V for commercial devices. The resulting power consumption is 100-250 mW. The overall pump volume is 0.2 cm3. A microfabricated neutron detector, operating in the Geiger Muller regime, utilizes electrodes that are lithographically micromachined from 50-µm thick stainless steel #304 foil. The cathode is coated with 2.9-µm thick layer of Gd on one side to convert thermal neutrons into fast electrons and gamma rays, which are then detected by ionization of the fill gas (Ar). Three electrodes are stacked in a cathode-anode-cathode arrangement, separated by 70-µm thick polyamide spacers, and assembled within a commercial TO-5 package. For a 90 µCi 252Cf neutron source, placed at a distance of 10 cm from the detector, the total neutron count rate with an applied voltage of 285 V is typically 8.7 counts per minute (cpm). Detector dead time is measured as 5.3 ms. The device is operated at lower voltages with a reduced volume and can detect beta particles, gamma rays and neutrons, when compared to commercial devices which operate at >900 V, have a higher detector volume (>100 mm3) and can only detect a subset of the listed radiation. Finally, this thesis describes a new architecture for microdischarge based radiation detectors intended to enhance stability, improve sensitivity and reduce dead time. The device stability can be improved through use of an asymmetric electric field between the anode and cathode. Detector sensitivity can be improved through use of stacked cathodes as it increases radiation interaction probability. The device dead time can be improved by having multiple detectors operate in parallel.