Browse Prior Art Database

Focus Detection Method for an Optical Instrument

IP.com Disclosure Number: IPCOM000035127D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 3 page(s) / 82K

Publishing Venue

IBM

Related People

Kirk, CP: AUTHOR

Abstract

An algorithm has been proposed to improve focusing procedures of optical measuring equipment so as to minimize ambiguous results evident with existing techniques. The suggested method uses the existing hardware of the optical line-width measuring tool along with a stop in the illuminator. (Image Omitted)

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Focus Detection Method for an Optical Instrument

An algorithm has been proposed to improve focusing procedures of optical measuring equipment so as to minimize ambiguous results evident with existing techniques. The suggested method uses the existing hardware of the optical line-width measuring tool along with a stop in the illuminator.

(Image Omitted)

Optical microscopes are used for measuring geometries on integrated circuit wafers. In many of the instruments it is necessary to locate an object in the focal plane of an imaging lens and the object must be positioned with great accuracy. Focus detection is often performed by moving the object through the focus to find the position producing the sharpest image. Where illumination is partially or fully spatially coherent and there is a phase difference between light returning from different parts of the object, the sharpest image will not necessarily occur when the object lies in the focal plane, thereby causing a focusing error.

(Image Omitted)

The proposal suggests the use of off-axis illumination to generate a series of image intensity profiles as the object is moved through focus. The apparent position of the object as indicated by the image profile is recorded through focus with the illumination placed on one side of the optical axis, and then on the other. When positioned relative to the imaging lens such that the two illumination conditions produce images of a line which appear to be in the same lateral position, the object is in focus.

In the schematic illustration an illumination source 1 (Fig. 1) fills an aperture 2 which lies in the back focal plane of an imaging lens 3. A line object...