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Structure Design and Reliability Evaluation of the Elasto-Optical Sensor for the Space Charge Ellipsometry Detection Method

To support the development of the space charge ellipsometry detection method, this article focuses on the elasto-optical sensor as the core of the measurement system. Oriented to low elastic modulus and high thermomechanical stability, the modification method of SU-8 photoresist as the sensor material was proposed. The molecular dynamics simulation pointed out that the SU-8 system with 70% cross-linking degree showed higher thermomechanical stability, and the average elastic modulus was 3.7459 GPa with a mean square error of 0.1669. Then, the refractive index response of different structure sensors under picosecond elastic waves was evaluated based on the finite-element simulation. It was indicated that, compared to the applied elastic wave, the square and circular structure sensors showed measurement distortion and delay, while the hexagon structure sensor benefited from the superior mechanical stability, which could achieve the real-time response. Furthermore, the sensor was prepared based on the photolithography principle, and the ellipsometry measurement platform was constructed to carry out the experimental selection and reliability evaluation of the sensor. It was found that, compared with the square and circular structure, the hexagon structure sensor could achieve the measurements of elastic waves with kilohertz repetition and femtosecond pulsewidth. In contrast to the pulsed electro-acoustic (PEA) piezoelectric sensing module, the measurement signal of hexagon structure sensor was at the same millivolt level, while the measurement waveform was smoother and less distorted.