High-resolution nonfluorescent imaging with structured illumination for patterned surface measurement

Abstract

Surface patterning is widely performed in various applications, and hence high-resolution, high-throughput, and nondestructive measurement of patterned surfaces is required. The current optical methods used for such measurements are constrained, in terms of spatial resolution, by the diffraction limit. In this study, structured illumination was employed for high-resolution wide-field imaging measurements. Structured illumination microscopy is well known as a fluorescent bio-imaging technique. For nonfluorescent imaging and industrial measurement applications, however, speckle noise, generated by the structured illumination pattern, environmental vibrations, and thermal drift, causes problems for the high-precision positioning and control of the illumination pattern. To solve these problems, we developed a structured illumination microscope that detects the phase of the structured illumination as a fringe signal by using an additional interferometric measurement system. In this system, a low-coherence light source is employed for illumination pattern generation and high-resolution imaging with low speckle noise. The constructed optical system consists of multiple Michelson interferometers that allow the manipulation of low-coherence interference fringes at the sample position and the detection of the phase via another sensor. Robust imaging can be implemented by feeding back the structured illumination phase through super-resolution image processing. We experimentally achieved high-resolution nonfluorescent imaging with low speckle noise and resolved patterned surfaces beyond the diffraction limit without high-precision positioning stages. Furthermore, the image reconstruction algorithm typically used in structured illumination microscopy was improved by adopting a Fourier ptychography approach. Thus, artifact reduction, as well as decreases in luminance and pitch errors, was achieved, and this result represents a step forward in the development of practical measurement methods for patterned surfaces.