Active Autofocus Control Using Source Dithering Technique Based on Fibre-optic Confocal Principle

Abstract

A significant process for highly precise measurement in 2-D inspection and 3D surface metrology is accurate and consistent imaging of target being measured. Dithering locking method, one of widely-used focus-locking method, has been only applied to objective modulation type which cause physical influence and image distortion. To overcome this, we have developed an actively locked focus-tracking system based around source dithering technique using fiber-optic confocal principle. Further, the whole measurement system can be simplified through both fiber confocal microscope design with reciprocity and fiber source modulation. It is verified that the proposed focus-tracking system has sub-pm focusing stability and accuracy. By testing focus tracking performance on a LCD panel substrate with a few pm-sized features, the applicability to optical profiler with vision inspection was confirmed.