Surface Imaging Technique by an Optically Trapped Microsphere in Air Condition

Abstract

Recent advancement of fabrication technologies enables nanoscale surface patterning on microstructures, which requires multi-scale measurements for the determination of their geometric dimensions and surface texture in the same coordinate system. Previously, we have developed a micro-coordinate measuring machine with a laser-trapped microprobe that uses an optically trapped microsphere as a surface detecting sensor. In this paper, we propose a surface imaging technique using the optically trapped microsphere as a micro-lens, i.e., the surface imaging system is integrated into the coordinate measuring system. The optically trapped microsphere with a diameter of 8 μm was brought close to a measured surface to image the surface underneath it. Because there is an unknown gap between the microsphere and surface, the focal plane of the imaging system had to be adjusted to the measured surface by the developed imaging lens system. With the microsphere-based imaging system, an optical diffractive grating of sub-micrometer periodic structure with 417 nm pitch and 50 nm depth was successfully imaged and the defect on the grating was detected. Thus, we verified that development of an imaging system based on the optically trapped microsphere for multi-scale evaluation systems can be accomplished.