Microcantilever Based Potentiometric Sensors For Harsh Environment Applications

Microcantilever based sensors have been under intense research focus for more than a decade due several advantages including high sensitivity, quick response, low cost, and ability to be miniaturized in small module. The microcantilever surface is usually functionalized (coated with an appropriate selective layer) to facilitate adsorption of targeted molecules which changes the resonant frequency or the static bending of the cantilever. However, cantilever coating suffers from non-uniformity, inability to tune the sensitivity and the problem of replacing the cantilever for sensing different gases. In this work a novel highly sensitive microcantilever based potentiometric detection technique for molecular sensing is demonstrated. The technique is based on surface work function (SWF) changes of sensing layers due to molecular adsorption phenomenon. It does not require functionalization of the cantilever itself, instead a ground electrode is functionalized which also acts as the electrode for the capacitive interaction for the microcantilever. A minimum detectable SWF change of \u3c 0.1 meV using an Atomic Force Microscope based setup was reported where trace amount of H2 as low as 8 ppm was sensed using platinum and 600 ppb NO2 was sensed using large area In2O3 and SnO2 thin films demonstrating the efficacy of the technique. The sensitivity towards NO2 increased significantly (60 ppb NO2 was sensed) when nanostructured graphite (NG), which has increased adsorption sites was used as sensing layers. The issue of selectivity was addressed using simultaneous SWF and conductance measurements on NG. The SWF and conductance changes have been found to be uncorrelated for different analyte molecules resulting in unique gradients that can be used as two-dimensional signatures gradient (2DSG) for molecular identification. NO2 showed 2DGS of ~75 meV/ % change while volatile organic compounds like acetone, ammonia and methanol showed negative 2DGS of -110, -45 and -13 meV/ % respectively. Separate potentiometric experiments on 6H-SiC epilayers reveal that NO2 is responsible for surface electron affinity change of the semi-insulating epilayer and change in SWF of ~150 meV was recorded. Finally, GaN microcantilever based potentiometric sensor with embedded AlGaN/GaN HFET was designed and fabricated targeting harsh environment operation. The piezoresistive and piezoelectric properties of AlGaN/GaN heterostructure is highly attractive for harsh environment applications of microelectromechanical systems (MEMS) sensors. This is because it can cause large variation in 2-dimensional electron gas (2DEG) at the interface with mechanical strain and also sustain harsh environments. From bending experiments on the GaN microcantilevers the transverse gauge factor was found out to be -38 and -21 for dc and ac drain current measurements respectively. In addition, under ultra violet illumination the transverse gauge factor reduced to -13 indicating the presence of trap related effect in the piezo-response of these cantilevers. We found out that under different conditions the gauge factor can vary from -13 to 860