Automated photomask inspection apparatus

• US 5,572,598 A
• Filed: 02/25/1994
• Issued: 11/05/1996
• Est. Priority Date: 08/22/1991
• Status: Expired due to Term

• Current Assignee: KLA Instruments, Corporation
• Original Assignee: KLA Instruments, Corporation
• Inventors: Wihl, Mark J., Fein, Michael E., Fu, Tao-Yi, Zywno, Marek, Kvamme, Damon F.
• Primary Examiner(s): COUSO, YON JUNG
• Application Number:
  US08/202,868
• Time in Patent Office: 984 Days
• Field of Search: 356/394, 356/398, 356/389, 356/237, 356/73, 358/505, 358/506, 358/509, 358/510, 358/511, 358/514, 310/12, 310/90.5, 250/201.2, 250/201.4, 382/144, 382/145, 382/149
• 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 for inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers, comprising:

stage means for carrying a substrate to be inspected such that a surface of said substrate moves in a particular manner within an inspection plane;

laser means for providing a pixel illuminating beam of light;

optical means defining a first optical axis intersecting said inspection plane and along which said pixel illuminating beam of light is initially passed, said optical means including a variable magnification subsystem for focusing said beam of light to a pixel defining spot on the substrate to be inspected;

beam deflecting means disposed along said first optical axis and operative to deflect said beam of light in oscillatory fashion whereby said pixel defining spot is caused to sweep across the surface of said substrate from one side to another of the path traced by the intersection of said optical axis with said substrate as said substrate is moved in said particular manner, and in a direction transverse to said path, as the substrate is carried along said path, the limits of deflection of said beam of light from one side of said path to the other defining the width of a scanning swath over a care area of the substrate including at least a portion of one of said objects;

light detecting means for detecting changes in the intensity of said beam of light caused by its intersection with the inspected substrate as said beam of light is either transmitted or reflected by said substrate, said light detecting means being responsive to the detected changes in intensity and operative to develop scan signals corresponding thereto, said light detecting means including sampling means for sampling said scan signals to produce pixel sample signals; and

electronic means for comparing said pixel sample signals to corresponding reference signals whereby differences therebetween may be used to identify defects in the inspected substrate.

5. An optical inspection system as recited in any one of claims 1 through 4 wherein said stage means is constrained to move with no more than two degrees of freedom, and is driven by x-direction and y-direction inputs generated by said electronic means.

6. An optical inspection system as recited in claim 5 wherein said stage means is an X/Y air bearing stage driven by linear motors including stators and sliders separated by air bearing means, the stators being configured to preload the air bearings of each slider in at least two directions.

7. An optical inspection system as recited in any one of claims 1 through 4 wherein said laser means is selected to have a predetermined wavelength of substantially the same wavelength as a light source to be later used in association with the inspected substrates during wafer processing.

8. An optical inspection system as recited in any one of claims 1 through 4 wherein said optical means includes means for rotating the direction of deflection of said beam of light so that it has a desired relationship to said predetermined path.

9. An optical inspection system as recited in any one of claims 1 through 4 wherein said optical means also includes means defining a second optical axis along which said beam of light may be passed, and having a portion thereof in common with a portion of said first optical axis, the optical path lengths of said first and second optical axes being substantially equal so that the two axes experience substantially identical thermal variation, said second optical axis including a tilted mirror for reflecting the beam of light back along said second optical axis and onto said light detecting means for interferometric comparison with light reflected back along said first optical axis from the substrate under inspection.

10. An optical inspection system as recited in any one of claims 1 through 4 wherein said light-detecting means includes a first detector for detecting the intensity of light reflected from said substrate and for generating first scan signals commensurate therewith.

11. An optical inspection system as recited in claim 10 wherein said light detecting means further includes a second detector for detecting the intensity of light from said beam passing through the substrate under inspection and for generating second scan signals commensurate therewith.

12. An optical inspection system as recited in claim 11 wherein said electronic means compares said first and second scan signals to first and second reference signals and determines both the existence and type of any defect encountered.

13. An optical inspection system as recited in any one of claims 1 through 4 wherein said light detecting means includes a transmitted light detector for detecting the intensity of light from said beam of light passing through the substrate under inspection and for generating corresponding scan signals.

14. An optical inspection system as recited in any one of claims 1 through 4 wherein said light detecting means includes means for monitoring the intensity of said beam of light and for generating an output which may be used by said electronic means to provide beam intensity variation correction to said scan signals.

15. An optical inspection system as recited in any one of claims 1 through 4 wherein said beam deflecting means includes an acousto-optic modulator for causing said beam of light to be deflected back and forth over a relatively small angle, said angle being the factor which determines the width of said scanning swath.

16. An optical inspection system as recited in any one of claims 1 through 3 and further comprising database means containing ideal data corresponding to the substrate under inspection, said data being selectively read out to generate said reference signals.

17. An optical inspection system as recited in any one of claims 1 through 3 and further comprising means for storing said scan signals, the stored signals being subsequently used to provide said reference signals to which presently scanned signals may be compared.

18. An optical inspection system as recited in any one of claims 1 through 4 wherein said light detecting means includes a first detector for detecting light transmitted through the substrate being inspected and a second detector for simultaneously detecting light reflected from the substrate being inspected, said first and second detectors respectively generating first and second signals from which said electronic means can determine both the existence of a defect and the type of defect detected.

19. An optical inspection system as recited in any one of claims 1 through 4 and further comprising means disposed along a second optical axis intersecting said first optical axis and operative to develop a beam of light for interfering with light reflected from the substrate under inspection to develop an interference beam for detection by said light detecting means such that the intensity of the interference beam detected by said light detecting means may be used by said electronic means to determine variations in the height of the inspected surface material above a particular surface of the inspected substrate.

20. An optical inspection system as recited in any one of claims 1 through 4 and further comprising means defining an auto-focus optical axis intersecting said first optical axis and including means for introducing astigmatism in the beam reflected from the surface of the inspected substrate, and auto-focus detecting means for measuring the shape of the astigmatized beam and determining therefrom the degree of focus of the beam on the substrate, said auto-focus detecting means generating correction signals for input to said electronic means.

21. An optical inspection system as recited in claim 20 and further comprising means for splitting said auto-focus optical axis into first and second branches having oppositely polarized anamorphous elements which distort the shape of the light beam passed therethrough, and wherein said auto-focus detecting means includes a first image shape detector associated with said first branch and a second image shape detector associated with said second branch, the outputs of said image shape detectors being used by said electronic means to determine whether said beam of light is in focus at said inspection plane, and if not in focus to determine the degree and direction in which the beam is out of focus.

22. An optical inspection system as recited in claim 21 wherein said first and second image shape detectors are appropriately oriented quadrature detectors.

23. An optical inspection system as recited in any one of claims 1 through 4 wherein said electronic means measures the magnitudes of said scan signals corresponding to predetermined grid points on the surface of the substrate under inspection and then uses such measurements to determine the surface dimensions of substrate features.

24. An optical inspection system as recited in claim 17 wherein said light-detecting means includes a first detector for detecting the intensity of light reflected from the substrate under inspection and for generating first scan signals commensurate therewith.

25. An optical inspection system as recited in claim 24 wherein said light detecting means further includes a second detector for detecting the intensity of light from said beam passing through the substrate under inspection and for generating second scan signals commensurate therewith.

26. An optical inspection system as recited in claim 25 wherein said electronic means compares said first and second scan signals to corresponding first and second reference signals and determines both the existence and type of any defect encountered.

27. An optical inspection system as recited in claim 26 wherein said light detecting means includes means for monitoring the intensity of said beam of light and generating an output which may be used by said electronic means to provide beam intensity variation correction to said scan signals.

28. An optical inspection system as recited in claim 27 and further comprising means defining an auto-focus optical axis intersecting said first optical axis and including means for introducing astigmatism in the beam reflected from the surface of the inspected substrate, and auto-focus detecting means for measuring the shape of the astigmatized beam and determining therefrom the degree of focus of the beam on the substrate, said auto-focus detecting means generating correction signals for input to said electronic means.

29. An optical inspection system as recited in claim 28 and further comprising means for splitting said auto-focus optical axis into first and second branches having oppositely polarized anamorphous elements which distort the shape of the light beam passed therethrough, and wherein said auto-focus detecting means includes a first image shape detector associated with said first branch and a second image shape detector associated with said second branch, the outputs of said image shape detectors being used by said electronic means to determine whether said beam of light is in focus at said inspection plane, and if not in focus to determine the degree and direction in which the beam is out of focus.

30. An optical inspection system as recited in claim 29 wherein said first and second image shape detectors are appropriately oriented quadrature detectors.

31. An optical inspection system as recited in claim 30 wherein said electronic means measures the magnitude of said scan signals corresponding to predetermined grid points on the surface of the substrate under inspection and then uses such measurements to determine the surface dimensions of substrate features.

32. An optical inspection system as recited in claim 31 wherein said stage means is an X/Y air bearing stage driven by linear motors including stators and sliders separated by air bearing means, the stators being configured to preload the air bearings of each slider in at least two directions.

33. An optical inspection system as recited in claim 32 wherein said optical means includes means to rotate the direction of deflection of said beam of light so that it has a desired relationship to said predetermined path.

34. An optical inspection system as recited in claim 16 wherein said light-detecting means includes a first detector for detecting the intensity of light reflected from said substrate and generating first scan signals commensurate therewith.

35. An optical inspection system as recited in claim 34 wherein said light detecting means further includes a second detector for detecting the intensity of light from said beam passing through the substrate under inspection and for generating second scan signals commensurate therewith.

36. An optical inspection system as recited in claim 35 wherein said electronic means compares said first and second scan signals to first and second reference signals and determines both the existence and type of any defect encountered.

37. An optical inspection system as recited in claim 36 and further comprising means defining an auto-focus optical axis intersecting said first optical axis and including means for introducing astigmatism in the beam reflected from the surface of the inspected substrate, and auto-focus detecting means for measuring the shape of the astigmatized beam and determining therefrom the degree of focus of the beam on the substrate, said auto-focus detecting means generating correction signals for input to said electronic means.

38. An optical inspection system as recited in claim 37 and further comprising means for splitting said auto-focus optical axis into first and second branches having oppositely polarized anamorphous elements which distort the shape of the light beam passed therethrough, and wherein said auto-focus detecting means includes a first image shape detector associated with said first branch and a second image shape detector associated with said second branch, the outputs of said image shape detectors being used by said electronic means to determine whether said beam of light is in focus at said inspection plane, and if not in focus to determine the degree and direction in which the beam is out of focus.

39. An optical inspection system as recited in claim 38 wherein said electronic means measures the magnitude of said scan signals corresponding to predetermined grid points on the surface of the substrate under inspection and then uses such measurements to determine the surface dimensions of substrate features.

40. An optical inspection system as recited in claim 39 wherein said stage means is an X/Y air bearing stage driven by linear motors including stators and sliders separated by air bearing means, the stators being configured to preload the air bearings of each slider in at least two directions.

41. An optical inspection system as recited in any one of claims 1 through 4 wherein said light detecting means includes a transmission detector disposed along said first optical axis on the side of said inspection plane opposite the side including said laser means and operative to detect transmitted light passing through an inspected substrate, and wherein said optical means further includes means defining a second optical axis along which said beam of light may be passed, said second optical axis not intersecting said inspected substrate but having at least a portion thereof in common with said first optical axis and intersecting said transmission detector whereby interferometric comparison between said beam of light and said transmitted light may be conducted.

42. An optical inspection system as recited in claim 41 wherein said optical means also includes means defining a third optical axis along which said beam of light may be passed, and having a portion thereof in common with a portion of said first optical axis, the optical path lengths of said second and third optical axes being substantially equal so that the two axes experience substantially identical thermal variation, said third optical axis including a tilted mirror for spatially shifting and reflecting the spatially shifted beam of light back along said third optical axis and onto said reflection detector for interferometric comparison with light reflected along said first optical axis from the inspected substrate.

43. An optical inspection system as recited in claim 11 and further including circuit means responsive to said first and second scan signals and operative to develop gated signals for input to said electronic means to indicate defects in the inspected substrate as a function of the phase error between the light reflected from the surface of the substrate and the light passing through the inspected substrate.

44. An optical inspection system as recited in claim 43 wherein said circuit means includes:

an analog phase detector for comparing said first scan signal to a reference signal to develop an analog signal proportional to the phase difference therebetween,

means for converting said analog signal to a digital signal, and

encoder means gated by said second scan signal and operative to generate said gated signals.

2. An optical inspection system for inspecting objects formed on substrates such as photomasks, reticles, phase shift masks and semiconductor wafers, comprising:

stage means for carrying a substrate to be inspected such that a surface of said substrate moves in a particular manner within an inspection plane;

laser means for providing a pixel illuminating beam of light;

optical means defining a first optical axis intersecting said inspection plane and along which said pixel illuminating beam of light is initially passed, said optical means being operative to focus said beam of light to illuminate a pixel defining spot on the substrate to be inspected, the spot size determining at least one dimension of a pixel of the substrate;

beam deflecting means disposed along said first optical axis and operative to deflect said beam of light in oscillatory fashion whereby said pixel defining spot is caused to sweep across the surface of said substrate from one side to another of a path traced by the intersection of said optical axis with said substrate as said substrate is moved in said particular manner, and in a direction transverse to said path as the substrate is carried along said path, the limits of deflection of said beam of light from one side of said path to the other defining the width of a scanning swath over care areas of the substrate including at least a portion of one of said objects;

light detecting means for detecting changes in the intensity of said beam of light caused by its intersection with pixel areas of the inspected substrate as said beam of light is either transmitted or reflected by said substrate, said light detecting means being responsive to the detected changes in intensity and operative to develop scan signals corresponding thereto;

electronic means for comparing said scan signals to corresponding reference signals whereby differences therebetween may be used to identify defects in the inspected substrate;

means for recording the substrate locations of said defects;

means for subsequently causing said stage means to reposition and hold the inspected substrate at a previously recorded location of a selected defect;

oscillatory reflective means disposed along said optical axis for causing a portion of said optical axis to sweep back and forth along a portion of said path in cooperation with said beam deflecting means whereby a selected segment of a swath is repetitively scanned and said light detecting means continuously generates display signals representative of the selected defect; and

display means responsive to said display signals and operative to provide a visual display of substrate area covered by the swath segment and including the selected defect.

3. An optical inspection system for inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers, comprising:

laser means for providing a pixel illuminating beam of light;

optical means defining a first optical axis along which said pixel illuminating beam of light is to be passed;

stage means for moving a substrate to be inspected such that a surface thereof is carried within an inspection plane intersected by said optical axis and such that the point of intersection traces a serpentine path over at least a portion of the substrate including at least one of the objects to be inspected;

beam deflecting means disposed along said first optical axis and operative to deflect said pixel illuminating beam of light in oscillatory fashion such that said beam of light sweeps back and forth across said path from one side to another and in a direction generally transverse to said path such that, as the substrate is carried, the deflection of said beam of light from one side of said path to the other defines a scanning swath over the portion of the substrate;

light detecting means for detecting changes in the intensity of said beam of light caused by pixel areas of the inspected substrate illuminated by said beam of light as said beam of light is either transmitted or reflected by said substrate, said light, detecting means being responsive to the detected changes in intensity and operative to develop scan signals corresponding thereto; and

electronic means for comparing said scan signals to corresponding reference signals whereby differences therebetween may be used to identify defects in the inspected substrate, and for recording the location of said defects on said substrate.

4. An optical inspection system for inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers including a plurality of supposedly identical patterned objects disposed in an ordered array on a surface thereof, comprising:

laser means for providing a pixel illuminating beam of light;

optical means defining a first optical axis along which said pixel illuminating beam of light is to be initially passed;

stage means for moving a substrate to be inspected such that a surface thereof moves within an inspection plane intersected by said optical axis and such that as the substrate is moved the point of intersection traces a serpentine path over a portion of the substrate surface including at least one of the objects;

beam deflecting means disposed along said first optical axis and operative to deflect said beam of light in oscillatory fashion such that said beam of light sweeps back and forth across said path from one side to another and in a direction generally transverse to said path such that, as the substrate is carried, the deflection of said beam of light from one side of said path to the other defines a scanning swath across the portion of the substrate;

light detecting means for detecting changes in the intensity of said beam of light caused by pixel areas of the inspected substrate illuminated by said beam of light as said beam of light is either transmitted or reflected by said substrate, said light detecting means being responsive to the detected changes in intensity and operative to develop scan signals corresponding thereto;

electronic means for storing first scan signals developed as said scanning swath passes over a first of the patterned objects and for comparing the stored first scan signals to second scan signals developed as the scanning swath passes over a second of the patterned objects whereby differences therebetween may be used to identify defects in the inspected substrate; and

means for recording the locations of said defects on said substrate.

45. A method of inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers, comprising the steps of:

transporting a substrate such that a surface thereof to be inspected moves within an inspection plane;

providing means defining a first optical axis intersecting said inspection plane;

directing a beam of light along said first optical axis and focusing said beam of light to illuminate a spot of a selected size on the substrate to be inspected;

sweeping said beam of light in oscillatory fashion such that the illuminated spot moves from one side to another of a path defined by the intersection of the optical axis with the substrate surface as said substrate surface is moved within said inspection plane whereby care areas of the substrate including at least one of said objects are scanned in swaths the width of which is determined by the beam sweep limits;

detecting changes in the intensity of said beam of light caused by its intersection with pixel areas of the inspected substrate as said beam of light is either transmitted or reflected by said substrate, and developing scan signals corresponding thereto;

sampling said scan signals to produce pixel sample signals; and

comparing said pixel sample signals to corresponding reference signals and using differences therebetween to identify defects in the inspected substrate.

46. A method of inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers, comprising the steps of:

transporting a substrate to be inspected such that a surface thereof moves within an inspection plane;

providing a pixel illuminating beam of light;

providing optical means defining a first optical axis intersecting said inspection plane, the intersection of said first optical axis and said substrate surface describing an inspection path across said substrate as said substrate is moved;

directing said pixel illuminating beam of light along said optical axis;

selectively focusing said beam of light to illuminate a spot on the substrate surface to be inspected, the spot size determining at least one dimension of a pixel of the substrate;

causing said beam of light to sweep in oscillatory fashion across the surface of said substrate from one side to another of said path and in a direction generally transverse to said path as the substrate is carried along said path, the limits of deflection of said beam of light from one side of said path to the other defining the width of a scanning swath over a care area of the substrate including at least a portion of one of said objects;

detecting changes in the intensity of said beam of light caused by its intersection with pixel areas of the inspected substrate as said beam of light is either transmitted or reflected by said substrate, and developing scan signals corresponding thereto;

sampling said scan signals to produce pixel sample signals; and

comparing said pixel sample signals to corresponding reference signals whereby differences therebetween may be used to identify defects in the inspected substrate.

47. A method of inspecting objects formed on substrates such as photomasks, reticles, phase shift masks and semiconductor wafers, comprising the steps of:

transporting a substrate to be inspected such that a surface thereof moves within an inspection plane;

using a laser means to provide a pixel illuminating beam of light;

providing an optical means defining a first optical axis intersecting said inspection plane, the intersection of said first optical axis and said substrate surface describing an inspection path across said substrate as said substrate is moved;

directing said pixel illuminating beam of light along said first optical axis;

focusing said beam of light to illuminate a pixel defining spot on the substrate to be inspected, the spot size determining at least one dimension of a pixel of the substrate;

causing said beam of light to be deflected in oscillatory fashion such that said spot moves across the surface of said substrate from one side of said path to another and in a direction transverse to said path as the substrate is moved, the limits of deflection of said beam of light from one side of said path to the other defining the width of a scanning swath over care areas of the substrate including at least a portion of one of said objects;

detecting changes in the intensity of said beam of light caused by its intersection with pixel areas of the inspected substrate as said beam of light is either transmitted or reflected by said substrate, and developing scan signals corresponding thereto;

comparing said scan signals to corresponding reference signals and using differences therebetween to identify defects in the inspected substrate;

recording the locations of said defects on said substrate;

subsequently causing said stage means to reposition the inspected substrate such that a previously recorded location of a particular defect is intersected by said optical axis;

stopping said stage means to hold the repositioned substrate in place;

sweeping a portion of said optical axis back and forth along said path in a direction orthogonal to the deflection caused by said beam deflecting means so as to cause said light detecting means to continuously generate display signals representative of the substrate surface area in the immediate vicinity of said selected defect; and

using said display signals to provide a visual display of the substrate surface including the selected defect.

48. A method of inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers, comprising the steps of:

using a laser means to provide a pixel illuminating beam of light;

providing optical means defining a first optical axis along which said pixel illuminating beam of light is passed to illuminate a single pixel area of a substrate to be inspected;

moving the substrate to be inspected within an inspection plane intersected by said first optical axis such that the point of intersection traces a serpentine path over at least a portion of the substrate including at least one of the objects to be inspected;

deflecting said beam of light in oscillatory fashion such that the illuminated pixel area moves from one side of said path to another and in a direction generally transverse thereto and such that, as the substrate is carried, the deflection of said beam of light from one side of said path to the other defines a scanning swath over the portion of the substrate;

detecting changes in the intensity of said beam of light caused by pixel areas of the inspected substrate illuminated by said beam of light, as said beam of light is either transmitted or reflected by said substrate, and developing scan signals corresponding thereto; and

comparing said scan signals to corresponding reference signals whereby differences therebetween may be used to identify defects in the inspected substrate, and recording the location of said defects on said substrate.

49. A method of inspecting objects formed on substrates selected from the group consisting of photomasks, reticles, phase shift masks and semiconductor wafers including a plurality of supposedly identical patterned objects disposed in an ordered array on a surface thereof, comprising the steps of:

using a laser means to provide a pixel illuminating beam of light;

using an optical means to define a first optical axis along which said pixel illuminating beam of light is passed;

using a stage means to move a substrate to be inspected within an inspection plane intersected by said optical axis such that the point of intersection traces a serpentine path over at least a portion of the substrate including at least one of the objects to be inspected;

deflecting said beam of light in oscillatory fashion to illuminate pixels on a first one side of said path and then another, and in a direction transverse to said path such that, as the substrate is carried along said path, the deflection of said beam of light from one side of said path to the other defines a scanning swath over the portion of the substrate;

detecting changes in the intensity of said beam of light caused by pixel areas of the inspected substrate illuminated by said beam of light as said beam of light is either transmitted or reflected by said substrate, and developing scan signals corresponding thereto;

storing first scan signals developed as said scanning swath passes over a first of the patterned objects and comparing the stored first scan signals to second scan signals developed as the scanning swath passes over a second of the patterned objects, and using differences therebetween to identify defects in the inspected substrate; and

recording the locations of said defects on said substrate.