Enhanced sensitivity automated photomask inspection system

• US 6,282,309 B1
• Filed: 05/29/1998
• Issued: 08/28/2001
• Est. Priority Date: 05/29/1998
• Status: Expired due to Term

• Current Assignee: KLA-Tencor Corporation (KLA Corporation)
• Original Assignee: KLA-Tencor Corporation (KLA Corporation)
• Inventors: Emery, David G
• Primary Examiner(s): Mancuso, Joseph
• Assistant Examiner(s): Bali, Vikkram
• Application Number:
  US09/087,318
• Time in Patent Office: 1,187 Days
• Field of Search: 382/141, 382/143, 382/144, 382/145-152, 356/237.2, 356/237.3, 356/237.4, 348/86, 348/87, 348/125, 348/126, 702/57, 702/58
• US Class Current: 382/145
• CPC Class Codes: G01N 21/95607 using a comparative method
• CPC Class Codes: G06T 2207/30148 Semiconductor; IC; Wafer
• CPC Class Codes: G06T 7/001 using an image reference approach

Abstract

A method and apparatus for inspecting patterned transmissive substrates, such as photomasks and particularly embedded phase shift photomasks, for unwanted particles and features occurring on the transmissive as well as pattern defects. A transmissive substrate is illuminated by a laser through an optical system comprised of a laser scanning system, individual transmitted and reflected light collection optics and detectors collect and generate signals representative of the light transmitted and reflected by the substrate as the substrate is scanned repeatedly in one axis in a serpentine pattern by a laser beam which is focused on the patterned substrate surface. The defect identification of the substrate is performed using only those transmitted and reflected light signals, and other signals derived from them, such as greyscale representations and image features. Defect identification is performed using a pattern inspection algorithm by comparing image feature representations of the present substrate with an idealized representation thereof.

Claims

1. A system for inspecting a substrate, comprising:

a T-R remapper for receiving transmitted and reflected images of said substrate, comprising;

a setup mechanism for converting transmitted and reflected images of a baseline specimen substrate into a signal representative of a greyscale depiction of said baseline specimen substrate, said baseline specimen greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions; and

an evaluation mechanism for converting transmitted and reflected images of a present specimen substrate into a signal representative of a greyscale depiction of said present specimen substrate, said present specimen greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions.

2. The system of claim 1, further comprising a present specimen greyscale remapper for remapping present specimen greyscale data received from the T-R remapper into image features.

3. The system of claim 2, further comprising a baseline greyscale remapper for remapping greyscale data of the baseline specimen into image features.

4. The system of claim 3, further comprising an aligner/comparator for aligning the image feature representations of the baseline and present specimens and comparing the two aligned image feature representations.

5. The system of claim 4, wherein said aligner/comparator also performs an interpolation between image feature representations of the baseline and present specimens.

6. The system of claim 1, further comprising:

an illumination system to provide an illumination beam through a path to a point on said substrate and a pattern located thereon;

a transmission detector coaxially aligned with said path to detect a transmitted portion of said illumination beam through said substrate and to provide a signal representative of said detected transmitted portion of said illumination beam to the T-R remapper; and

a reflection detector to detect a portion of said illumination beam reflected from said illuminated point on said substrate and said pattern thereon coaxially along said path of said illumination beam and to provide a signal representative of said detected reflected portion of said illumination beam.

7. The system of claim 6, further comprising:

a comparator to compare signals from said transmission and reflection detectors and provide a comparison value of those signals; and

a processor to determine whether said comparison value is within a predetermined range.

8. A method for inspecting a substrate, comprising the steps of:

detecting transmitted and reflected images for a baseline specimen substrate;

converting said transmitted and reflected baseline specimen images to a greyscale representation of said baseline specimen substrate;

converting said greyscale representation into an image feature representation of said baseline specimen substrate;

detecting transmitted and reflected images for a present specimen substrate;

converting said transmitted and reflected present specimen images to a greyscale representation of said present specimen substrate;

converting said greyscale representation into an image feature representation of said present specimen substrate;

aligning image features of said baseline specimen and present specimen; and

comparing image features of said baseline specimen and present specimen for defects on the present specimen.

9. The method of claim 8, further comprising the step of interpolating data between said aligning and comparing steps.

10. The method of claim 8, wherein:

said greyscale representation of said baseline specimen substrate provides a single greyscale value for each point along the scan axis under all conditions; and

said greyscale representation of said present specimen substrate provides a single greyscale value for each point along the scan axis under all conditions.

11. An apparatus for inspecting a photomask, comprising:

a remapper for receiving transmitted and reflected images of said photomask and remapping said images into a greyscale representation of said photomask;

a transformer for transforming said greyscale representation into an image feature representation of said photomask; and

a comparison tool for comparing the image feature representation with a desired photomask representation.

12. The apparatus of claim 11, wherein said remapper comprises:

a setup mechanism for translating transmitted and reflected images for said desired semiconductor representation into a signal representative of a greyscale depiction of said desired representation, said desired greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions; and

an evaluation mechanism for converting transmitted and reflected images of a present specimen substrate into a signal representative of a greyscale depiction of said present specimen substrate, said present specimen greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions.

13. The apparatus of claim 12, wherein said transformer comprises a present specimen transformer, and further comprising a desired representation transformer for transforming said desired greyscale depiction into a desired image feature representation.

14. The apparatus of claim 13, wherein said comparison tool aligns the image feature representations of the desired representation and present specimen and compares the two aligned image feature representations.

15. The apparatus of claim 14, wherein said aligner/comparator also performs an interpolation between image feature representations of the desired representation and present specimen.

16. The apparatus of claim 11, further comprising:

an illumination system to provide an illumination beam through a path to a point on said substrate and a pattern located thereon;

a transmission detector coaxially aligned with said path to detect a transmitted portion of said illumination beam through said substrate and to provide a signal representative of said detected transmitted portion of said illumination beam to the remapper; and

a reflection detector to detect a portion of said illumination beam reflected from said illuminated point on said substrate and said pattern thereon coaxially along said path of said illumination beam and to provide a signal representative of said detected reflected portion of said illumination beam.

17. The apparatus of claim 16, further comprising:

a comparator to compare signals from said transmission and reflection detectors and provide a comparison value of those signals; and

a processor to determine whether said comparison value is within a predetermined range.

18. A system for inspecting a specimen, comprising:

means for mapping transmitted and reflected images of said specimen to a greyscale representation of the specimen;

means for mapping the greyscale representation into a image feature representation of the specimen; and

means for comparing the image feature representation of the specimen to an idealized specimen representation.

19. The system of claim 18, wherein said image mapping means convert transmitted and reflected images of said specimen into a signal representative of a greyscale depiction of said specimen, said greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions.

20. The system of claim 18, wherein said comparing means align the image feature representation of the specimen with the idealized specimen and compare the image feature representation of the specimen with the idealized specimen.

21. The system of claim 20, wherein said comparing means also interpolates between image feature representations of the specimen a point on said substrate and a pattern located thereon;

a transmission detector coaxially aligned with said path to detect a transmitted portion of said illumination beam through said substrate and to provide a signal representative of said detected transmitted portion of said illumination beam to the mapping means; and

a reflection detector to detect a portion of said illumination beam reflected from said illuminated point on said substrate and said pattern thereon coaxially along said path of said illumination beam and to provide a signal representative of said detected reflected portion of said illumination beam.

22. A system for inspecting a specimen, comprising:

a T-R remapper for receiving transmitted and reflected images of said specimen, comprising;

a setup mechanism for converting transmitted and reflected images of a baseline specimen substrate into a signal representative of a greyscale depiction of said baseline specimen substrate, said baseline specimen greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions; and

an evaluation mechanism for converting transmitted and reflected images of a present specimen substrate into a signal representative of a greyscale depiction of said present specimen substrate, said present specimen greyscale depiction providing a single ordinate value for any point along the abscissa under all conditions, and comparing said baseline specimen greyscale depiction with said present specimen greyscale depiction to determine irregularities contained in said specimen.