Specular surface deformation measurement based on projected-speckle deflectometry with digital image correlation

Traditional deflectometry techniques suffer from the issue of out-of-focus image acquisition since the camera is pointed directly at the specular surface, and the image quality is greatly disturbed by ambient light. In this study, the projected-speckle deflectometry based on the digital image correlation (DIC) technique is proposed. The projection speckle pattern is reflected by specular surface onto the screen, at which the camera is oriented directly. The deformation of specular sample is represented by the speckle pattern variation, and the camera collects changed images to further calculate slope and deformation. Quantitative deformation experiments are employed to validate the measurement accuracy of the system, and the average error of the five measured values is within 4.55%. Wafer measurement experiments are carried out to compare deformed results with those of the laser interferometer and optical 3D metrology, and the effects of projector vibration, electronic noise and out-of-plane movements are analyzed in detail. The root mean square (RMS) of measured deformation resolution can be as low as 0.11μm. With high measurement efficiency, this method avoids the problem of out-of-focus image acquisition due to excessive surface deformation and reduces the interference of ambient light.