Automatic photomask inspection system and apparatus

• US 4,347,001 A
• Filed: 04/03/1979
• Issued: 08/31/1982
• Est. Priority Date: 04/03/1978
• Status: Expired due to Term

• Current Assignee: KLA Instruments, Corporation
• Original Assignee: KLA Instruments, Corporation
• Inventors: Levy, Kenneth, Sandland, Paul
• Primary Examiner(s): ROSENBERGER, RICHARD A
• Application Number:
  US06/026,784
• Time in Patent Office: 1,246 Days
• Field of Search: 356/71, 356/138, 356/123, 356/388, 356/394, 356/390, 356/398, 356/399, 356/400
• US Class Current: 356/398
• CPC Class Codes: G01N 21/95607 using a comparative method
• CPC Class Codes: G03F 1/84 Inspecting

Abstract

Optical inspection apparatus for detecting differences between two dies in a photomask and including a carriage for supporting the objects to be inspected and for simultaneously moving such objects along an inspection path, an illuminator for illuminating corresponding portions of the objects as they are moved along the inspection path, electro-optical detectors for individually inspecting the illuminated portions and for developing first and second electrical signals respectively corresponding thereto, electronic memories for storing the first and second electrical signals, a computer for scanning the memories and for electronically aligning a readout of the first signal relative to a readout of the second signal, and a comparator for comparing the electronically aligned signals and for indicating any differences therebetween.

Claims

1. Optical inspection apparatus for detecting differences between two like objects, comprising:

carriage means for supporting the objects to be inspected and for simultaneously moving such objects along an inspection path;

illuminator means for illuminating corresponding portions of said objects as they are moved along said path;

electro-optical means for individually inspecting said illuminated portions and for developing first and second electrical signals respectively corresponding thereto, said electro-optical means including separate optical systems for respectively collecting light from said corresponding portions of the inspected objects, and each said system including photosensitive detector means responsive to the collected light and operative to generate electrical signals commensurate therewith, each said separate optical system further including a movable member, an objective lens carried by said movable member for collimating light rays received from one of said objects, a variable focal length lens assembly for receiving the collimated light rays and focusing them onto an image plane at said detector means, an adjustable attachment means mounting said objective lens to the said movable member, focus monitoring means for determining the position of said objective lens relative to the inspected object and for developing an adjustment signal, and adjustment means responsive to said adjustment signal and operative to adjust said attachment means so as to maintain said objective in a focused position relative to the inspected object, the movable member of one of said separate optical systems being mechanically coupled to the movable member of the other of said separate optical systems such that movement of one of said members in one direction causes a like movement of the other of said members in the opposite direction, said focus monitoring means including a laser for passing a narrow beam of light through at least a portion of said separate optical system at an angle relative to the optical axis thereof and onto a peripheral spot of the inspected object, and means disposed along said optical axis for detecting the position of said spot relative to said focused position and for developing said adjustment signal commensurate therewith, said focus monitoring means further including an aperture disposed within the path of the incident laser beam so as to cause the beam spot to have a predetermined configuration which permits unambiguous detection of out-of-focus images caused by an object surface having varying reflectivity;

memory means for storing said first and second electrical signals;

means for scanning said memory means and for electronically aligning a readout of said first signal relative to a readout of said second signal; and

means for comparing the electronically aligned signals and for indicating any difference therebetween.

2. Optical inspection apparatus as recited in claim 1 and further including beam-splitting means disposed along the optical axis of said system for coupling a portion of the imaged light into binocular means.

3. In an optical inspection apparatus including carriage means for transporting two like objects along an inspection path, means for illuminating corresponding portions of the objects, and electro-optical means for individually inspecting the illuminated portions, an improved electro-optical means comprising:

first and second separate optical systems for respectively collecting light from the corresponding portions of the inspected objects, each said separate optical system including a movable member, an objective lens carried by said movable member for collimating light rays received from one of said objects, a variable focal length lens assembly for receiving the collimated light rays and focusing them onto an image plane, said movable member being mechanically coupled to the movable member of the other of said separate optical system such that movement of one of said members in one direction causes a like movement of the other of said members in the opposite direction, each said separate optical system further including an adjustable attachment means mounting said objective lens to its corresponding movable member, focus monitoring means for determining the position of said objective lens relative to the inspected object and for developing an adjustment signal, and adjustment means responsive to said adjustment signal and operative to adjust said attachment means so as to maintain said objective lens in a focused position relative to the inspected object, said focus monitoring means including a laser for passing a narrow beam of light through at least a portion of said separate optical system and onto a peripheral spot of the inspected object, and means disposed along the optical axis of said system for detecting the position of said spot relative to a predetermined focused position and for developing said adjustment signal commensurate therewith, said focus monitoring means further including an aperture disposed within the path of the incident laser beam so as to cause the beam spot to have a predetermined configuration which permits unambiguous detection of out-of-focus images caused by an object surface having varying reflectivity; and

photosensitive detector means disposed at said image plane for receiving the focused light rays and operative to generate electrical signals commensurate therewith.