Apparatus and method for automatic focusing of light using a fringe plate

摘要

The present invention provides an apparatus and method to automatically focus an incident beam of monochromatic parallel planar light on a work piece, using a fringe plate and a vision detector. The work piece may have a perfect mirror surface, or have a non-reflective patterned surface. The incident light beam is focused on the work piece and then reflected through a fringe plate forming an interference pattern on the vision detector. The incident light beam is then scanned or wobbled. The wobbling incident light beam forms a distinctively oscillating output signal from the vision detector when the light is focused on the work piece. When the light is not focused on the work piece a distinctive and more constant output signal is produced by the vision detector. For a given position of the work piece, by noting whether the output signal is rapidly changing or relatively constant, the invention can automatically determine whether or not the work piece is at the focal point of the incident light beam. This can be used to automatically focus the etching or writing laser on the chip or wafer in a laser etching system. The present invention can also be used to automatically determine the focal surface of an objective lens.