Method of thickness measurement for self-supporting film with double chromatic confocal probes

With the randomness and immeasurability properties of zero point, the conventional self-supporting film thickness measurement model must calibrate the distance between two chromatic confocal sensors using a standard part whose thickness needs to be measured by other methods in advance. The measurement performance is easily disturbed by the calibration process, and by the accuracy of sample thickness or its uniformity. In order to overcome these limitations, a new thickness measurement model was developed by adding an auxiliary transparent film in the initial position of the dispersion field. The lower plane of the reference film is not only applied as the zero point of the first sensor but also can be measured by another sensor, whose value is equal to the sensor distance. Theoretical analysis and simulation showed that the proposed method does not change the linear relationship of the displacement coefficient. In order to verify the proposed measurement model, a laboratory thickness measurement system was developed based on two commercial chromatic confocal sensors with a displacement accuracy less than 0.2 µm. A set of self-supporting film was measured using the proposed system, the traditional method, and the reference system. These experiments indicated that the standard deviation of the calibration results of the sensor distance based on the proposed method was reduced to 0.1 µm, which can be concluded that its stability was improved significantly compared to the conventional model. In addition, the proposed method was able to achieve a measurement accuracy of 0.4 µm, which can demonstrate its efficiency and practicability.