A Parallel Differential Confocal Method for Highly Precise Surface Height Measurements

A parallel differential confocal method (PDCM) is proposed to obtain rapid surface height measurements with nanoscale precision with a conventional optical microscope. In contrast to existing differential surface profiling methods where multiple sensors are required and either pixel-by-pixel horizontal or layer-by-layer vertical scanning is necessary, the PDCM uses only a single sensor without scanning. It uses parallel pinholes to generate confocal images Ia and Ib at an equal distance but in opposite directions away from the focal plane. Then the method generates differential intensity signals by subtracting these two images, and then employs these signals to determine the heights of the object surface based on precalibrated curves. The precision and relative standard error of the proposed method were 1 nm and ≤ 0.2%, respectively. Its efficiency was 13 fps for $2018 \times 512$ horizontal locations. Due to its advantages in efficiency and precision, it can be considered as one of the ideal online surface height profiling methods for industrial metrological applications.