Measurement of Thickness of High-Resistivity Substrate By Photoconduction Enhanced Capacitance Displacement Sensor

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
T. H. Lee (National Central University, Dept. of Mechanical Engineering) and M. C. Lin (National Central University)
Capacitive displacement sensors can offer non-contact, extreme resolution, and absolute accuracy thickness measurements. However, for general capacitive displacenent sensors, if the resistivity of a target substrate is within 105-107 ohm-cm, a measurement uncertainty will occur in the thickness measurement. The measnurement uncentration may be several microon meters. The common solution is to adjust the resistivity to be outside the range that causes the measurement uncertainty by implanting dopants and followed by an activation anneal, but this will unavoidably lead to changes in the material properties and morphology. Here, we exploit the photoconductive effect to generate sufficient high amount of electron-hole pairs, thereby temporarily decreasing the resistivity and thus enabling the capacitive displacement sensor to accurately measure the thickness at nanoscale resolution. After the measurement is complete, the resistivity of the substrate will return to its original status. The photoconductive effect can be simply induced via laser irradiation at the sensing point, which narrows or eliminates the gap in the measurement range of capacitive sensors to include high-resistivity substrates.