Novel capacitive microstructures for chemical and biological sensors using liquid crystals

This dissertation focused on the investigation and design of optimum capacitive microstructures for chemical and biological sensors using liquid crystals. The microstructures for environmental, chemical, and biological detection were verified and demonstrated by both optical and capacitive transductions simultaneously. The capacitive transduction technique in liquid crystals exhibits a change in capacitance dependent on orientational response to a change in the liquid crystal molecules. Three types of microfabricated capacitive structures were modeled, designed, fabricated and tested according to their applications: monitoring, sensing, and improved sensing.;A parallel plate capacitive structure for monitoring was designed and fabricated with thin film photo-lithography to monitor the orientational transition of liquid crystal molecules by external forces such as an electric field or chemical reactions.;The second structure, practical sensor implementation utilizing interdigitated electrodes having a thin electrode layer, was fabricated and tested to be used as liquid crystal sensors. The ability of this interdigitated capacitive (IDC) microstructure involving the capacitive technique to identify and track the average molecular distortion has been investigated and experimentally verified. However, the IDC structure having a thin electrode layer (typically ≅ 40 nm thick) had low capacitance efficiency depending on the orientational transition of liquid crystals due to the fringing field effect. For this reason, the change via the fringing field capacitance was relatively smaller than the change via transverse field capacitance.;In order to improve the sensing mechanism from the second structure, a pulse micro-electroplating technique was introduced. The IDC microstructure by using an electroplating method (typically ≅ 10 mum thick electrodes) allowed for an open surface and used a transverse field effect with liquid crystal films. An electroplated IDC microstructure was developed and tested by the detection of vapor-phase chemical analytes based on liquid crystal sensors to change the orientational response of the liquid crystal molecules and simultaneously demonstrated by the optical sensing as well as capacitive sensing. Both simulated and experimental results confirmed that the electroplated IDC microstructure using the liquid crystals showed high capacitance efficiency and proved promising for a wide range of sensor applications.;Finally, the capacitive technique to monitor the functionality of anchoring energy in liquid crystal alignment layers was introduced. Sensing changes in the anchoring energy of a liquid crystal film can significantly improve sensitivity of liquid crystal sensors. All fabrication and testing of these microstructures was done at Nano and Micro Devices Center (NMDC) at the University of Alabama in Huntsville.