Linearity calibration and uncertainty evaluation for capacitance displacement sensor

As the linearity of capacitance sensor in adjusting a lithographic projection objective could not meet the requirement of the adjustment mechanism for accuracy standard, a method to improve the measuring linearity of the capacitance sensor was proposed. A piezoelectric actuator was used to provide displacement feeding and a higher precise laser length interferometer was used to provide displacement feedback to ensure the accuracy of moving control. The fitting factors getting from the high order curve fitting were used to calibrate the linearity of the capacitance sensor online. Then, the uncertainties of the environment, installation mechanism and control were analyzed and evaluated to meet the accuracy of the linearity measurement. Finally, calibration experiments for the sensor were carried out. The experiment results indicate that the calibration method of the linearity proposed reduces the influence of the error on the measurement results. After calibration, the linearity of capacitance sensor increases from 0.04714% to 0.00484%, nearly an order of magnitude. Moreover, the capacitance sensor has high linearity repeatability, the deviation of repeatability is 0.38 nm, and the combined uncertainty of the linearity in the whole stroke is 5.70 nm, which meets the displacement control accuracy of the lithographic projection objective. ©, 2015, Chinese Academy of Sciences. All right reserved.