Microtechnologies for Discharge-based Sensors.

Abstract

Microdischarge-based sensors are known to offer advantages such as the ability to operate at temperature extremes and to provide large output signals that do not require local amplification. This work is primarily directed at the design and microfabrication of pressure sensors that use differential microdischarge currents. Two approaches are evaluated. The first uses a common anode and reference cathode located on a glass substrate, whereas a sensing cathode is located on an opposing silicon diaphragm that is deflected by applied pressure. Leads are transferred by electroplated through-glass vias. The second uses a common cathode and reference anode located on a silicon substrate, whereas a sensing anode is located on a thin film diaphragm that deflects under applied pressure. Leads are transferred by through-wafer isolated bulk-silicon lead transfer (TWIST). Fabricated sensors with 200-µm diameter have footprints as small as 300×300 µm2, and volume of ≈0.01 mm3, which is 150× smaller than prior work. The fractional differential current (I1-I2)/(I1+I2) increases monotonically from -0.7 to 0.2 as external pressure increases from 1 atm to 8 atm. The TWIST process can also be used to fabricate ultra-miniature capacitive pressure sensors with backside contacts that minimize the form factor and allow stacking of the sensor on interface electronics. A sensor with a 100-µm diameter diaphragm measures 150×150 µm2 in size. Fabricated sensors with thicknesses of 3 µm (C100t3) and 5 µm (C100t5) have dynamic ranges of 20 MPa and 50 MPa, respectively. Pressure responses in the non-contact mode and the contact mode are 3.1 fF/MPa, 5.3 fF/MPa for C100t3, and 1.6 fF/MPa, 1.6 fF/Ma for C100t5, respectively. This thesis also describes a preliminary exploration of options to initiate microdischarges using scavenged energy – in this case from mechanical impact. A miniature high voltage generator is formed by connecting multiple electrode pairs in series on a single PZT element. This strategy amplifies voltage roughly in proportion to the electrode pair count; a three electrode-pair device is used to successfully initiate microdischarges with peak voltages exceeding 1.35 kV.