Object scanning scheme in wide-field low-coherence interferometry

摘要

Low-coherence interferometry which is usually combined with wide-field optical microscopy is a well known technique for surface profile measurement, micro step measurement and so on. One of the problems is that its axial measurement range is typically limited by its depths of field of imaging, which is determined by the numerical aperture of the objective lens and the central wavelengths of the light source. If a low-coherence interference fringe is far outside the depth of field, the measurement accuracy inevitably decreases, regardless of how well the reference mirror adjusted. To solve this problem and improve the axial measurement range of the low-coherence interferometry in this study, an object scanning measurement scheme involving a Linnik interferometer was developed. To calibrate the system in the proposed technique, image post-processing is performed for a well-conditioned state to ensure that a low-coherence interference fringe is generated within the depth-of-field. In this way three-dimensional objects with high-aspect-ratio structures can be scanned along the axial direction. During object scanning, this state is always monitored and corrected by adjusting the reference mirror. Using this scheme, the axial measurement range can be significantly increased up to the working distance of the objective lens without compromising the measurement accuracy. The working distance is typically longer than 10 mm, while the depth-of-field of the microscope is generally around 0.01 mm, although it varies depending on the imaging system. In this report, the experimental setup of an object scanning low-coherence interferometry is presented, a series of experimental verifications is described, and the results are discussed.