Automated photomask inspection apparatus and method

• US 5,563,702 A
• Filed: 07/13/1994
• Issued: 10/08/1996
• Est. Priority Date: 08/22/1991
• Status: Expired due to Term

• Current Assignee: KLA Instruments, Corporation
• Original Assignee: KLA Instruments, Corporation
• Inventors: Saidin, Zain K., Emery, David G., Wihl, Mark J., Fein, Michael E., Fu, Tao-Yi, Zywno, Marek, Kvamme, Damon F.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US08/274,310
• Time in Patent Office: 818 Days
• Field of Search: 356/73, 356/237
• US Class Current: 356/73
• 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

A method and apparatus for inspecting patterned transmissive substrates, such as photomasks, for unwanted particles and features occurring on the transmissive, opaque portions and at the transition regions of the opaque and transmissive portions of the substrate. 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 the second derivative of each of them. The actual defect identification is then performed by comparing combinations of at least two of those measured and derived signals. Simultaneously, defect and particle inspection using the same measured transmitted and reflected light signals. Additionally, phase shift and line widths on the substrate can also be performed simultaneously using the same transmitted light signal that is collected for defect analysis.

Claims

1. An inspection system to inspect a substrate for unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said inspection system comprising:

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

a transmission detector coaxially aligned with said path to detect a transmitted portion of said illumination beam through said substrate from an illuminated point on said top of said substrate and to provide a signal representative of said detected transmitted portion of said illumination beam;

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;

a comparator to compare said signals, with each other, from said transmission and reflection detectors developed by illumination of the same point on said top surface of said substrate and said pattern thereon to provide a comparison value of those signals;

a first memory to store expected values of comparison values from said comparator; and

a processor coupled to said comparator and said first memory to determine if said comparison value is an expected value and to generate a report when an unexpected value is determined.

2. An inspection system as in claim 1 wherein:

said inspection system further includes a translational stage to provide translational motion between said top surface of said substrate and said illumination beam;

said comparator further compares signals, with each other, from said transmission and reflection detectors to provide a comparison value for each point on said top surface of said substrate and said pattern thereon; and

said processor further determines if one or more of said comparison values has an expected value and generates a report when at least one unexpected value is determined.

3. An inspection system as in claim 1 wherein:

said inspection system further comprises a second memory to store the signal from one of said transmission and reflection detectors; and

said comparator receives one of said transmission and reflection signals for comparison from said second memory and the other one of said transmission and reflection signals from the corresponding one of said transmission and reflection detectors from the same point on the surface and said pattern thereon of said substrate when both signals are available.

4. An inspection system as in claim 2 wherein:

said inspection system further comprises a second memory to store the signal from one of said transmission and reflection detectors and a designation of the corresponding illuminated point on said top surface of said substrate and said pattern thereon for each stored signal; and

said comparator receives one of said transmission and reflection signals for comparison from said second memory and the other one of said transmission and reflection signals from the corresponding one of said transmission and reflection detectors from the same point on the surface and said pattern thereon of said substrate when both signals are available.

5. An inspection system as in claim 1 wherein:

said inspection system further comprises a second memory to store a plurality of signal pairs comprising one signal from each of said transmission and reflection detectors resulting from the illumination of the top surface of a plurality of substrates of the same design to generate a range of expected values for each of said signals from each of said transmission and reflection detectors for each point for substrates of the same design; and

said processor is coupled to said second memory to determine if the transmission and reflection signals for each point on the substrate being inspected are within said range of expected values.

6. An inspection system as in claim 2 wherein:

said inspection system further comprises a second memory to store a family of a plurality of signal pairs comprising one signal from each of said transmission and reflection detectors resulting form the illumination of the top surface of a plurality of substrates of the same design to generate a range of expected values for each of said signals from said transmission and reflection detectors each point for substrates of the same design; and

said processor is coupled to said second memory to determine if the transmission and reflection signals for each point on the substrate being inspected are within said range of expected values.

7. An inspection system as in claim 2 wherein:

said inspection system further comprises;

a second memory to systematically store values from one of said transmission and reflection detectors and the corresponding location of the corresponding illuminated point on the top surface of the substrate for each corresponding illuminated point on the top surface of the substrate; and

a function generator disposed to receive stored values from said second memory to generate a first specific function from the signal values stored in said second memory; and

said comparator further compares signals from two or more of said transmission detector, reflection detector, and said function generator.

8. An inspection system as in claim 7 wherein:

said inspection system further comprises;

a third memory to systematically store values from the other one of said transmission and reflection detectors and the corresponding location of the corresponding illuminated point on the top surface of the substrate for each corresponding illuminated point on the top surface of the substrate; and

said function generator is also disposed to receive stored values from said third memory to generate a second specific function from the signal values stored in said third memory.

9. An inspection system as in claim 7 wherein said first specific function is a second derivative.

10. An inspection system as in claim 8 wherein said first specific function is a second derivative.

11. An inspection system as in claim 8 wherein said second specific function is a second derivative.

12. An inspection system as in claim 1 wherein:

said first memory further includes stored therein potential unwanted particle and feature types and a range of comparison values for each unwanted particle and feature type; and

said processor, when an unexpected comparison value is obtained, further obtains unwanted particle and feature type information corresponding to said unexpected comparison value from said first memory and includes said unwanted particle and feature type information in said report.

13. An inspection system as in claim 2 wherein:

said first memory further includes stored therein potential unwanted particle and feature types and a range of comparison values for each of said unwanted particle and feature type; and

said processor, when an unexpected comparison value is obtained, further obtains unwanted particle and feature type information corresponding to said unexpected comparison values from said first memory and includes said unwanted particle and feature type information in said report.

14. An inspection system as in claim 13 wherein:

said translation stage further generates a position signal that corresponds to the location of said illumination point on the top surface of said substrate;

said comparator is further coupled to said translational stage to receive said positional signals that are provided together with said comparison values; and

said processor, when an unexpected comparison value is obtained, further decodes said positional signals and provides top surface of said substrate location information for each unwanted particle and feature in said report.

15. An inspection system as in claim 13 wherein:

said translation stage further generates a position signal that corresponds to the location of said illumination point on the top surface of said substrate; and

said processor is further coupled to said translational stage to receive said positional signals and, when an unexpected comparison value is obtained, further decodes said positional signals and provides top surface of said substrate location information for each unwanted particle and feature in said report.

16. An inspection system as in claim 1 wherein said first memory includes a tolerance value for said stored expected values.

17. An inspection system as in claim 1 wherein each of said transmission detector and reflection detector are image scanners.

18. An inspection system as in claim 1 wherein said processor includes a neural network to process the signals from each of said transmission and reflection detectors.

19. A method for inspecting a substrate for unwanted particles and features, said substrate having a top and a bottom surface with a pattern of a opaque material on said top surface, said method including the steps of:

a. directing an illumination beam through a path to a point on the top surface of said substrate and said pattern thereon;

b. detecting, in coaxial alignment with said illumination beam of step a., a transmitted portion of said illumination beam through said substrate;

c. generating a signal representative of said detected transmitted portion of said illumination beam of step b.;

d. detecting, coaxially along said path of step a., a reflected portion of said illumination beam from said top surface of said substrate and said pattern thereon;

e. generating a signal representative of said detected reflected portion of said illumination beam of step d.;

f. generating a comparison value of said signals, with respect to each other, from steps c. and e.;

g. storing expected comparison values; and

h. generating a report when the comparison value of step f. does not correspond to an expected comparison value stored in step g.

20. A method as in claim 19 further including the steps of:

i. translating said substrate to a next position; and

j. repeating steps a. through i. for each point of interest on said substrate.

21. A method as in claim 19 wherein:

said method further includes the following step between steps e. and f.;

k. storing the signal from one of steps c. and e.; and

said step f. generates said comparison value from the signal stored in step k. and the unstored one of the signals from steps c. and e. from the same point on the surface of said substrate when both signals are available.

22. A method as in claim 20 wherein:

said method further includes the following step between steps e. and f.;

k. storing the signal from one of steps c. and e. for each point illuminated on the top surface of said substrate; and

said step f. generates said comparison value for each point on said substrate from the signal stored in step k. for that point and the unstored one of the signals from steps c. and e. for the same point on the substrate when both signals are available.

23. A method as in claim 19 wherein:

step g. further includes the step of;

m. storing at least one potential unwanted particle and feature type and a range of unexpected comparison values that corresponds to that unwanted particle and feature type; and

step h. further includes the step of;

n. including unwanted particle and feature type information from step m. in the generated report.

24. A method as in claim 20 wherein:

step g. further includes the step of;

o. storing at least one potential unwanted particle and feature type and a range of unexpected comparison values that corresponds to that unwanted particle and feature type; and

step h. further includes the step of;

p. including unwanted particle and feature type information from step o. in the generated report.

25. A method as in claim 20 wherein:

step i. includes the step of;

q. generating a position signal that corresponds to the point on said substrate being illuminated in step a.;

step f. includes the step of;

r. identifying the position signal from step q. that corresponds to each generated comparison value; and

step h. includes the step of;

s. including in said report the corresponding point on said substrate from step r. where an unwanted particle and feature was detected.

26. A method as in claim 20 wherein:

step i. includes the step of;

t. generating a position signal that corresponds to the point on said substrate being illuminated in step a.; and

step h. includes the step of;

u. including in said report the corresponding point on said substrate from step t. where an unwanted particle and feature was detected.

27. A method as in claim 19 wherein step g. also includes the storing of a tolerance for the stored expected comparison values.

28. A method for inspecting a substrate for unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said method including the steps of:

a. selecting a substrate of the type to be inspected;

b. directing an illumination beam through a path to a point on the top surface of said substrate and said pattern thereon;

c. detecting, in coaxial alignment with said illumination beam of step b., a transmitted portion of said illumination beam through said substrate;

d. generating a signal representative of said detected transmitted portion of said illumination beam of step c.;

e. detecting, coaxially along said path of step b., a reflected portion of said illumination beam from said top surface of said substrate;

f. generating a signal representative of said detected reflected portion of said illumination beam of step e.;

g. generating an expected comparison value of said signals from steps d. and f. with respect to each other;

h. storing said expected comparison value of step g.;

i. repeating steps a. through h. for a selected number of different substrates of the type to be inspected;

j. selecting a particular substrate to be inspected;

k. directing an illumination beam through a path substantially normal to a point on the top surface of said substrate of step j.;

l. detecting, in coaxial alignment with said illumination beam of step k., a transmitted portion of said illumination beam through said substrate of step j.;

m. generating a signal representative of said detected transmitted portion of said illumination beam of step l.;

n. detecting, coaxially along said path of step k., a reflected portion of said illumination beam from said top surface of said substrate of step j. and said pattern thereon;

o. generating a signal representative of said detected reflected portion of said illumination beam of step n.;

p. generating a comparison value of said signals, with respect to each other, from steps m. and o.; and

q. generating a report when the comparison value of step p. does not correspond to an expected comparison value stored in step h.

29. A method as in claim 28 wherein step g. also generates a tolerance value for said comparison value.

30. A method for inspecting a substrate for unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said method including the steps of:

a. selecting a substrate of the type to be inspected;

b. directing an illumination beam through a path to a point on the top surface of said substrate and said pattern thereon;

c. detecting, in coaxial alignment with said illumination beam of step b., a transmitted portion of said illumination beam through said substrate;

d. generating a signal representative of said detected transmitted portion of said illumination beam of step c.;

e. detecting, coaxially along said path of step b., a reflected portion of said illumination beam from said top surface of said substrate and said pattern thereon;

f. generating a signal representative of said detected reflected portion of said illumination beam of step e.;

g. generating an expected comparison value of said signals from steps d. and f. with respect to each other;

h. storing said expected comparison value of step g.;

i. translating said substrate to a next position; and

j. repeating steps a. through i. for each point of interest on said substrate of step a.;

k. repeating steps a. through j. for a selected number of different substrates of the type to be inspected;

l. selecting a particular substrate to be inspected;

m. directing an illumination beam through a path substantially normal to a point on the top surface of said substrate of step l. and said pattern thereon;

n. detecting, in coaxial alignment with said illumination beam of step m., a transmitted portion of said illumination beam through said substrate of step l.;

o. generating a signal representative of said detected transmitted portion of said illumination beam of step n.;

p. detecting, coaxially along said path of step m., a reflected portion of said illumination beam from said top surface of said substrate of step l. and said pattern thereon;

q. generating a signal representative of said detected reflected portion of said illumination beam of step p.;

r. generating a comparison value of said signals, with respect to each other, from steps o. and q.;

s. storing said comparison value of step r.;

t. translating said substrate to a next position;

u. repeating steps m. through s. for each point of interest on said substrate of step l.;

v. generating a report when any comparison value for a particular point on said substrate of step l. stored by of step s. does not correspond to an expected comparison value stored in step h. for the same point on substrates of the same type.

31. A method as in claim 30 wherein step g. also generates a tolerance value for said expected comparison value.

32. A method for inspecting a substrate for unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said method including the steps of:

a. directing an illumination beam through a path to a point on the top surface of said substrate and said pattern thereon;

b. detecting, in coaxial alignment with said illumination beam of step a., a transmitted portion of said illumination beam through said substrate;

c. generating a signal representative of said detected transmitted portion of said illumination beam of step b.;

d. detecting, coaxially along said path of step a., a reflected portion of said illumination beam from said top surface of said substrate and said pattern thereon;

e. generating a signal representative of said detected reflected portion of said illumination beam of step d.;

f. storing the generated signals of steps c. and e. together with the location of the point illuminated in step a.;

g. translating said substrate to a next position; and

h. repeating steps a. through f. for each point of interest on said substrate and said pattern thereon;

i. generating a signal corresponding to a first selected function value of the signal values for one of the stored transmission and reflection signals of step f. for each point of interest on the substrate being inspected;

j. storing the signal values generated in step i. together with the corresponding point on the surface of said substrate;

k. generating expected comparison values for each combination of the three values stored in steps f. and j. for each point of interest on a substrate of the type being inspected;

l. generating a comparison values of two or more of said signal values from steps f. and j., with respect to each other, for each point of interest on the substrate being inspected;

m. generating a report when the comparison value of step

l. for any point of interest on the substrate being inspected does not correspond to an expected comparison values stored in step l. for that point on the surface of substrates of the same type.

33. A method as in claim 32 wherein step i. further includes:

n. generating a signal corresponding to a second selected function value of the signal values for the other one of the stored transmission and reflection signals of step f. for each point of interest on the substrate being inspected.

34. A method as in claim 32 wherein said first selected function of step i. is a second derivative.

35. A method as in claim 33 wherein said first selected function of step i. is a second derivative.

36. A method as in claim 33 wherein said second selected function of step n. is a second derivative.

37. A method as in claim 32 wherein step k. also generates a tolerance value for said expected comparison values.

38. An inspection system to simultaneously inspect a substrate for defects, and unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said inspection system comprising:

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

a transmission detector coaxially aligned with said path to detect a transmitted portion of said illumination beam through said substrate from an illuminated point on said top of said substrate and to provide a signal representative of said detected transmitted portion of said illumination beam;

a reflection detector adjacent said top surface of said substrate to detect a portion of said illumination beam reflected from said illuminated point on said substrate coaxially along said path of said illumination beam and to provide a signal representative of said detected reflected portion of said illumination beam;

a first comparator coupled to each of said transmission and reflection detectors to compare said signals from each of said detectors, with each other, developed by illumination of the same point on said top surface of said substrate to provide a first comparison value of those signals;

a memory having expected values of said first comparison value from said first comparator stored therein;

a database containing the value of a pair of expected signals from said transmission and reflection detectors for a point being inspected for the particular type of substrate being inspected;

a second comparator coupled to each of said transmission and reflection detectors and said database to compare a value of each of said signals from said transmission and reflection detectors with the values of the expected pair of transmission and reflection detector signals from said database to identify the presence of a defect at the point of inspection on said substrate; and

a processor coupled to said first and second comparators and said first memory to determine when said first comparison value is an unexpected value and to identify the type of unwanted particle or feature when an unexpected value is determined.

39. An inspection system as in claim 38 wherein:

said inspection system further includes a translational stage having said substrate mounted thereon to provide translational motion between said top surface of said substrate and said illumination beam;

said first comparator further compares signals, with each other, from said transmission and reflection detectors at each point on the top surface of said substrate to provide a first comparison value for each point on said top surface of said substrate;

said database stores a pair of expected values of signals from said transmission and reflection detectors together with positional data for each point being inspected on the top surface of said substrate, said stored values and corresponding positional data is for the particular type of substrate being inspected;

said second comparator further compares said signals from said transmission and reflection detectors from each point on the top surface of said substrate with the values of the expected pair of transmission and reflection detector signals from said second memory for each point on the top surface of said substrate to identify the presence of defects at all points on the top surface on said substrate; and

said processor further determines if one or more of said first comparison values has an unexpected value and generates a report when at least one unexpected value is determined identifying the type of each unwanted particle or feature corresponding to each of said first comparison values.

40. A method for simultaneously inspecting a substrate for defects, and unwanted particles and features, said substrate having a top and a bottom surface with a pattern of opaque material on said top surface, said method including the steps of:

a. directing an illumination beam through a path to a point on the top surface of said substrate;

b. detecting, in coaxial alignment with said illumination beam of step a., a transmitted portion of said illumination beam through said substrate;

c. generating a signal value representative of said detected transmitted portion of said illumination beam of step b.;

d. detecting, coaxially along said path of step a., a reflected portion of said illumination beam from said top surface of said substrate and said pattern thereon;

e. generating a signal value representative of said detected reflected portion of said illumination beam of step d.;

f. generating a first comparison value of said signal values, with respect to each other, from steps c. and e.;

g. storing expected first comparison values;

h. storing a pair of expected values of said signal values representative of said detected transmission and reflected portions of said illumination for the particular type of substrate being inspected;

i. individually comparing each of said signal values of steps c. and e. to the expected value of each of said signal values of step h.;

j. generating a report when the comparison value of step f. does not correspond to an expected comparison value stored in step g. and also if either of the signals does not agree with an expected value in step i.

41. A method as in claim 40 wherein:

said method further includes the step of;

k. providing motion relative to the top surface of said substrate and said directed illumination beam of step a.;

said step h. further includes storing a pair of expected values of said signal values representative of said detected transmission and reflected portions of said illumination and corresponding positional data for each point illuminated on the top surface the particular type of substrate being inspected; and

said method further includes the said step of;

l. repeating steps a. through f. and i. through j. for each point illuminated on the top surface of said substrate.