(The) correction of distortions due to different scanning velocity and variation of light intensity in confocal laser scanning microscope

Abstract

In a confocal scanning laser microscope, the detector pinhole is confocal with the illuminated spot on the specimen, and rejects light from objects that are not in the focal plane. This results in images that out-of-focus blur is essentially absent from confocal images, resulting in increased contrast and signal-to-noise ratio in the final images. This enables minute defects and contamination on wafers to be detected quickly and reliably in semiconductor quality measurement devices. Because beam scanning microscopes are fast, and has high resolution, we use beam scanning microscope with scanners as acousto-optic deflectors. But in scanning with acousto-optic deflectors there was a ``speed down`` in vicinity of maximum deflected angle in spite of linear input as driving voltage, that is, there is some difference in scanning velocity on lateral direction that affects resolution. Non-linearity in AO scanning results from the way it is controlled. The typical non-linearity of the frequency control is ± 5 % in VCO driver. In addition the lateral, tilting error in optical system along incident beam’s path have not any influence on scanning velocity in practical point. The input control driving voltage can be obtained for a constant angular velocity equation. If we can correct distortions due to different scanning velocity by driving this calculated voltage, we can expand the scanning range and there will be some improvements in resolution. It’s known that the image extinction is due to the diffraction efficiency of acousto-optic deflectors and the waviness of detection. But the alignment errors and finite size of pinhole make the image extinct. Additionally the tilting on stage and variable refractive index give rise to the fatal distortion of an image. To overcome this DC offset which results from AO diffraction and optical alignment error, the cut off frequency must be known in experiment. The filtering is done about this frequency and the filtered response is t...