Automated photomask inspection apparatus

• US 6,363,166 B1
• Filed: 03/30/2000
• Issued: 03/26/2002
• Est. Priority Date: 08/22/1991
• Status: Expired due to Term

• Current Assignee: KLA-Tencor Corporation
• Original Assignee: KLA-Tencor Corporation
• Inventors: Fein, Michael E., Fu, Tao-Yi, Zywno, Marek, Wihl, Mark Joseph, Kvamme, Damon Floyd
• Primary Examiner(s): COUSO, YON JUNG
• Application Number:
  US09/539,672
• Time in Patent Office: 726 Days
• Field of Search: 382/141, 382/144, 382/145, 382/147, 382/148, 382/149, 364/468.28, 364/478.08, 348/126, 348/129, 356/394, 356/398
• US Class Current: 382/144
• CPC Class Codes: G01N 2021/95676 Masks, reticles, shadow masks
• CPC Class Codes: G01N 21/95607 using a comparative method
• CPC Class Codes: G03F 1/84 Inspecting

Abstract

An automated photomask inspection apparatus including an XY state (12) for transporting a substrate (14) under test in a serpentine path in an XY plane, an optical system (16) comprising a laser (30), a transmission light detector (34), a reflected light detector (36), optical elements defining reference beam paths and illuminating beam paths between the laser, the substrate and the detectors and an acousto-optical beam scanner (40, 42) for reciprocatingly scanning the illuminating and reference beams relative to the substrate surface, and an electronic control, analysis and display system for controlling the operation of the stage and optical system and for interpreting and storing the signals output by the detectors. The apparatus can operate in a die-to-die comparison mode or a die-to-database mode.

Claims

1. An optical inspection system, comprising:

a stage having a surface for carrying a substrate to be inspected such that said surface moves in at least two directions within an inspection plane;

a laser source for producing an illuminating light beam;

an autofocus mechanism based upon a monitoring of a cross-section of said light beam, including a plurality of optical elements for focusing said light beam to a pixel spot on the substrate, which is substantially unaffected by substrate patterns;

a beam deflector for scanning said light beam across said substrate;

a light detector for producing a signal representing intensity changes in said scanned light beam reflected from said substrate; and

electronic means for comparing said signal to a reference signal and identifying differences therebetween.

2. The optical inspection system of claim 1, wherein:

said plurality of optical elements includes a variable magnification lens system.

3. The optical inspection system of claim 1, wherein:

said beam deflector includes an acousto-optical deflector.

4. The optical inspection system of claim 1, wherein:

said stage moves in only two directions.

5. The optical inspection system of claim 1, further comprising:

means for sampling said signal.

6. The optical inspection system of claim 1, further comprising:

a beam splitter for reflecting part of said illuminating light beam to said autofocus mechanism.

25. The system of claim 1, wherein:

said intensity changes include phase shifts.

7. A method of optically inspecting a substrate, comprising:

providing said substrate on a stage for inspection;

moving said stage and said substrate in at least two directions within an inspection plane;

directing a laser beam toward said substrate;

focusing said laser beam to a pixel spot with a plurality of optical elements which provide an autofocus substantially unaffected by substrate patterns;

monitoring a cross-section of said focused laser beam to automatically maintain a focus of said focused laser beam;

scanning said focused laser beam across said substrate with a beam deflector;

detecting intensity changes of said scanned laser beam reflected from said substrate;

producing an electronic signal representing said intensity changes; and

comparing said electronic signal to a reference signal and identifying differences therebetween.

8. The method of optically inspecting a substrate of claim 7, wherein:

said focusing step is performed with a variable magnification lens system.

9. The method of optically inspecting a substrate of claim 7, wherein:

said beam deflector includes an acousto-optical deflector.

10. The method of optically inspecting a substrate of claim 7, further comprising:

moving said substrate on said stage in only two directions.

11. The method of optically inspecting a substrate of claim 7, further comprising:

sampling said electronic signal.

12. The method of optically inspecting a substrate of claim 7, further comprising:

reflecting a portion of said laser beam with a beam splitter to an autofocusing mechanism.

26. The method of claim 7, wherein:

said intensity changes include phase shifts.

13. An optical inspection system, comprising:

a stage for moving a substrate in a first direction;

a laser source for producing an illuminating light beam;

a plurality of optical elements for focusing said illuminating light beam on said substrate;

a beam deflector for scanning said illuminating beam across said substrate in a second direction perpendicular to said first direction;

a first light detector for producing a first signal representing changes in said scanned illuminating light beam transmitted through said substrate;

a second light detector for producing a second signal representing changes in said scanned illuminating light beam reflected from said substrate, said first and second light detectors being capable of functioning simultaneously; and

electronic means for comparing said first and second signals to a reference signal.

14. The optical inspection system of claim 13, further comprising:

a beam splitter for reflecting part of said illuminating light beam to an autofocus mechanism.

15. The optical inspection system of claim 13, wherein:

said beam deflector includes an acousto-optical deflector.

27. The system of claim 13, wherein:

said changes in said scanned illuminating light beam include phase shifts.

16. A method of optically inspecting a substrate, comprising:

moving said substrate in a first direction on a stage for inspection;

directing a laser beam toward said substrate;

focusing said laser beam with a plurality of optical elements;

scanning said laser beam in a second direction perpendicular to said first direction across said substrate with a beam deflector;

detecting any transmitted light changes of said scanned laser beam transmitted through said substrate;

detecting any reflected light changes of said scanned laser beam reflected from said substrate, said steps of detecting any transmitted light changes and detecting any reflected light changes capable of occurring simultaneously;

producing an electronic signal representing said any changes in said transmitted and reflected light; and

comparing said electronic signal to a reference signal.

17. The method of optically inspecting a substrate of claim 16, further comprising:

reflecting a portion of said laser beam with a beam splitter to an autofocusing mechanism.

18. The method of optically inspecting a substrate of claim 16, wherein:

said beam deflector is an acousto-optical deflector.

28. The method of claim 16, wherein:

said changes of said scanned laser beam include phase shifts.

19. An optical inspection system, comprising:

a non-rotatable stage with two degrees of freedom of movement for carrying a substrate;

a laser source for producing an illuminating light beam;

a plurality of optical elements for focusing said illuminating light beam to a pixel spot on said substrate;

an autofocus mechanism for maintaining the focus of said illuminating light beam, said autofocus mechanism being based upon monitoring of a cross-section of said illuminating light beam;

a beam deflector for scanning said illuminating light beam across said substrate;

a light detector for producing a signal representing intensity changes in said scanned light beam reflected from said substrate; and

electronic means for comparing said signal to a reference signal.

29. The system of claim 19, wherein:

said intensity changes include phase shifts.

20. A method of optically inspecting a substrate, comprising:

providing said substrate on a non-rotatable stage with two degrees of freedom of movement for inspection;

directing a laser beam toward said substrate;

focusing said laser beam to a pixel spot with a plurality of optical elements;

maintaining focus of said laser beam with an autofocus mechanism based upon monitoring of a cross-section of said focused laser beam;

scanning said laser beam across said substrate with a beam deflector;

detecting intensity changes of said scanned laser beam reflected from said substrate;

producing an electronic signal representing said intensity changes; and

comparing said electronic signal to a reference signal.

30. The method of claim 20, wherein:

said intensity changes include phase shifts.

21. An optical inspection system, comprising:

a stage for moving a substrate in a first direction;

a laser source for producing an illuminating light beam with real-time light intensity compensation;

a plurality of optical elements for focusing said illuminating light beam on said substrate;

an autofocus mechanism to maintain the focus of said illuminating light beam, said autofocus mechanism being based upon monitoring of a cross-section of said illuminating light beam;

a beam deflector for scanning said illuminating light beam across said substrate in a second direction perpendicular to said first direction;

a light detector for producing a signal representing any changes in said scanned illuminating light beam reflected from said substrate; and

electronic means for comparing said signal to a reference signal.

31. The system of claim 21, wherein:

said changes in said scanned illuminating light beam include phase shifts.

22. A method of optically inspecting a substrate, comprising:

moving said substrate in a first direction on a stage for inspection;

directing a laser beam with real-time light intensity compensation toward said substrate;

focusing said laser beam with a plurality of optical elements;

maintaining focus of said laser beam with an autofocus mechanism based upon monitoring of a cross-section of said focused laser beam;

scanning said laser beam in a second direction perpendicular to said first direction across said substrate with a beam deflector;

detecting changes of said scanned laser beam reflected from said substrate;

producing an electronic signal representing said changes; and

comparing said electronic signal to a reference signal.

32. The method of claim 22, wherein:

said changes of said scanned laser beam include phase shifts.

23. An optical inspection system, comprising:

a stage for moving a substrate in a first direction;

a laser source for producing a plurality of illuminating light beams;

a plurality of optical elements for focusing said plurality of illuminating light beams on said substrate;

a beam deflector for scanning said plurality of illuminating light beams across said substrate in a second direction perpendicular to said first direction;

a device using said plurality of scanned laser beams to measure phase shift;

a first light detector for producing a first signal representing changes in a scanned illuminating light beam transmitted through said substrate;

a second light detector for producing a second signal representing changes in a scanned illuminating light beam reflected from said substrate, said first and second light detectors being capable of functioning simultaneously; and

electronic means for comparing said first and second signals to a reference signal and identifying differences therebetween to identify defects in said substrate.

24. A method of optically inspecting a substrate, comprising:

moving said substrate in a first direction on a stage for inspection;

directing a plurality of laser beams toward said substrate;

focusing said plurality of said laser beams with a plurality of optical elements;

scanning said plurality of said laser beams in a second direction perpendicular to said first direction across said substrate with a beam deflector;

using said plurality of scanned laser beams to measure phase shift;

detecting any transmitted light changes of a scanned laser beam transmitted through said substrate;

detecting any reflected light changes of a scanned laser beam reflected from said substrate, said steps of detecting any transmitted light changes and detecting any reflected light changes capable of occurring simultaneously;

producing an electronic signal representing said any changes in said transmitted and reflected light; and

comparing said electronic signal to a reference signal and identifying differences therebetween to identify defects in said substrate.