Method of and apparatus for detecting foreign substances

• US 5,225,886 A
• Filed: 09/18/1990
• Issued: 07/06/1993
• Est. Priority Date: 09/18/1989
• Status: Expired due to Term

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Ohshima, Yoshimasa, Koizumi, Mitsuyoshi
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US07/584,120
• Time in Patent Office: 1,022 Days
• Field of Search: 356/237, 356/446, 356/336, 356/337, 356/338, 356/445
• US Class Current: 356/237.4
• CPC Class Codes: G01N 21/94 Investigating contamination, e.g. dust G01N21/85 takes precedence

Abstract

Disclosed herein are method of and apparatus for detecting a foreign substance on an object by illuminating said object, detecting via an optical system light reflected from said object and detecting said foreign substance in distinction from a background. The object is exposed to first illumination such that light reflected from said background is suppressed but light reflected from said foreign substance is highlighted relative to the light reflected from said background. The light reflected from said object as a result of said first illumination is detected, thereby obtaining a first detection signal. The object is exposed to second illumination such that light reflected from said background is free from suppression relative to the light reflected from said substance as a result of said second illumination. The light reflected from said object as a result of said second illumination is detected, thereby obtaining a second detection signal. Foreign-substance-highlighting processing is performed on said first detection signal by using said second detection signal, whereby said substance is detected.

Claims

1. A method of detecting a foreign substance on a patterned object having a plane surface and a raised pattern formed on said plane surface by illuminating said patterned object and detecting via an optical system light reflected by said patterned object to detect said foreign substance in distinction from said raised pattern, comprising the steps of:

applying oblique illumination onto an oblong area of the plane surface of said patterned object having said raised pattern formed thereon in an oblique direction relative to an optical axis of said optical system, and applying vertical illumination vertically onto the oblong area of the plane surface of said patterned object along the optical axis of said optical system;

receiving, via said optical system, light from said oblique illumination reflected and scattered from the oblong area and light from said vertical illumination reflected and scattered from the oblong area, separating the received scattered lights from the received reflected lights by shutting off the received reflected lights, and independently detecting the separated scattered lights with one-dimensional photodetecting means to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination, wherein the oblong area is large enough to contain an image of an entire photodetecting area of said one-dimensional photodetecting means projected onto the plane surface of said patterned object through said optical system; and

non-additively processing said first detection signal and said second detection signal to cancel a component of said first detection signal and said second detection signal due to light scattered from said raised pattern and detect said foreign substance.

2. A method according to claim 1, wherein said oblique illumination and said vertical illumination have mutually different polarizations.

3. A method according to claim 1, wherein the step of non-additively processing said first detection signal and said second detection signal includes the steps of obtaining a ratio of said first detection signal and said second detection signal, comparing the ratio with a predetermined reference value, and detecting the presence of the foreign substance depending upon whether or not the ratio exceeds the predetermined reference value.

4. A method according to claim 1, wherein said oblique and vertical illuminations have mutually different wavelengths, and the separated scattered lights are separated from each other according to their wavelengths and are independently detected by said photodetecting means.

5. A method according to claim 1, wherein said oblique and vertical illuminations are applied in a time-sharing manner, and the separated scattered lights are detected by said photodetecting means in a time-sharing manner in synchronism with the time-sharing of said oblique and vertical illuminations such that the first and second detection signals are obtained independently.

6. A method of detecting a foreign substance on an unpatterned object having an unpatterned mirror-finished surface by illuminating said unpatterned object and detecting via an optical system light reflected by said unpatterned object to detect said foreign substance on said unpatterned mirror-finished surface of said unpatterned object, comprising the steps of:

applying oblique illumination onto an oblong area of the unpatterned mirror-finished surface of said unpatterned object in an oblique direction relative to an optical axis of said optical system, and applying vertical illumination vertically onto the oblong area of the unpatterned mirror-finished surface of said unpatterned object along the optical axis of said optical system;

receiving, via said optical system, light from said oblique illumination reflected and scattered from the oblong area, and light from said vertical illumination reflected and scattered from the oblong area, separating the received scattered lights from the received reflected lights, and detecting the separated scattered lights with one-dimensional photodetecting means to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination, wherein the oblong area is large enough to contain an image of an entire photodetecting area of said one-dimensional photodetecting means projected onto the unpatterned mirror-finished surface of said unpatterned object through said optical system; and

additively processing said first detection signal and said second detection signal to detect said foreign substance.

7. A method of detecting a foreign substance on an object by illuminating said object and detecting via an optical system light reflected by said object to detect said foreign substance, comprising the steps of:

applying oblique illumination onto said object in an oblique direction relative to an optical axis of said optical system and vertical illumination vertically onto said object along the optical axis of said optical system;

condensing via said optical system lights scattered by said object from said oblique and vertical illuminations, and independently detecting the condensed scattered lights with photodetecting means to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination;

when the object has a plane surface and a raised pattern formed on said plane surface, detecting the foreign substance on the basis of a difference between the first and second detection signals; and

when the object has a mirror-finished surface, detecting the foreign substance by adding together the first and second detection signals to obtain a sum and comparing the sum with a predetermined reference value.

8. An apparatus for detecting a foreign substance on a patterned object having a plane surface and a raised pattern formed on said plane surface by illuminating said patterned object and detecting via an optical system light reflected by said patterned object to detect said foreign substance in distinction from said raised pattern comprising:

an optical system;

oblique illumination means for applying oblique illumination onto an oblong area of the plane surface of said patterned object having said raised pattern formed thereon in an oblique direction relative to an optical axis of said optical system;

vertical illumination means for applying vertical illumination vertically onto the oblong area of the plane surface of said patterned object along the optical axis of said optical system;

means for receiving, via said optical system, light from said oblique illumination reflected and scattered from the oblong area and light from said vertical illumination reflected and scattered from the oblong area;

means for separating the received scattered lights from the received reflected lights by shutting off the received reflected lights;

one-dimensional photodetecting means for independently detecting the separated scattered lights to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination, wherein the oblong area is large enough to contain an image of an entire photodetecting area of said one-dimensional photodetecting means projected onto the plane surface of said patterned object through said optical system; and

signal processing means for non-additively processing said first detection signal and said second detection signal to cancel a component of said first detection signal and said second detection signal due to light scattered from said raised pattern and detect said foreign substance.

9. An apparatus according to claim 8, wherein said oblique illumination and said vertical illumination have mutually different polarizations, wherein said one-dimensional photodetecting means includes a polarization beam splitter for separating the scattered light from the oblique illumination and the scattered light from the vertical illumination from each other according to their mutually different polarizations, and wherein said one-dimensional photodetecting means independently detects the scattered lights separated from each other according to their mutually different polarizations.

10. An apparatus according to claim 8, wherein said separating means includes shutoff means for preventing zeroth-order diffracted light in light reflected from the oblong area from being incident to the one-dimensional photodetecting means.

11. An apparatus according to claim 8, wherein said one-dimensional photodetecting means includes one-dimensional solid pickup means in which plural pixels are arranged linearly, wherein said vertical illumination means includes a cylindrical lens for applying a linear light beam onto the oblong area along the optical axis of said optical system as said vertical illumination, and wherein the oblong area is large enough to contain an image of an entire photodetecting area of said one-dimensional solid pickup means projected onto the plane surface of said patterned object through said optical system.

12. An apparatus according to claim 8, wherein said oblique and vertical illuminations have mutually different wavelengths, wherein said one-dimensional photodetecting means includes means for separating the scattered light from said vertical illumination and the scattered light from said oblique illumination from each other according to their mutually different wavelengths, and a first one-dimensional photodetecting element and a second one-dimensional photodetecting element for independently detecting the scattered lights separated from each other according to their mutually different wavelengths.

13. An apparatus according to claim 12, wherein the signal processing means includes a division comparator for obtaining a ratio of the first detection signal and the second detection signal to cancel the component of the first detection signal and the second detection signal due to the light scattered from the raised pattern, and a binarizing circuit for converting the ratio to a binary signal with respect to a predetermined threshold value to detect the foreign substance depending on whether or not the ratio exceeds the predetermined threshold value.

14. An apparatus according to claim 12, wherein the signal processing means includes a subtracter for obtaining a difference between the first detection signal and the second detection signal to cancel the component of the first detection signal and the second detection signal due to the light scattered from the raised pattern, and a binarizing circuit for converting the difference to a binary signal with respect to a predetermined threshold value to detect the foreign substance depending on whether or not the difference exceeds the predetermined threshold value.

15. An apparatus according to claim 12, wherein the means for separating the scattered lights from each other according to their mutually different wavelengths includes a color-separation element for separating the scattered lights from each other according to their mutually different wavelengths.

16. An apparatus according to claim 15, wherein the color-seperation element is selected from the group consisting of a color-separation prism, a combination of a semitransparent mirror and a color filter, a combination of a dichroic mirror and a color filter, and a dichroic mirror prism.

17. An apparatus according to claim 8, wherein said oblique and vertical illumination means are operated in a time-sharing manner, wherein said one-dimensional photodetecting means detects the first and second detection signals in a time-sharing manner corresponding to the oblique and vertical illuminations, respectively, and wherein said signal processing means includes means for storing and holding one of the first detection signal and the second detection signal while the other one of the first detection signal and the second detection signal is detected such that the first detection signal and the second detection signal are available at the same time.

18. An apparatus according to claim 17, wherein the signal processing means includes a division comparator for obtaining a ratio of the first detection signal and the second detection signal to cancel the component of the first detection signal and the second detection signal due to the light scattered from the raised pattern, and a binarizing circuit for converting the ratio to a binary signal with respect to a predetermined threshold value to detect the foreign substance depending on whether or not the ratio exceeds the predetermined threshold value.

19. An apparatus according to claim 17, wherein the signal processing means includes a subtracter for obtaining a difference between the first detection signal and the second detection signal to cancel the component of the first detection signal and the second detection signal due to the light scattered from the raised pattern, and a binarizing circuit for converting the difference to a binary signal with respect to a predetermined threshold value to detect the foreign substance depending on whether or not the difference exceeds the predetermined threshold value.

20. An apparatus for optically detecting a foreign substance on an object, comprising:

an objective lens;

oblique illumination means for applying oblique illumination onto said object in an oblique direction relative to an optical axis of said objective system;

vertical illumination means for applying vertical illumination vertically onto said object via said objective lens along the optical axis of said objective lens;

an optical system for condensing light scattered by said object;

signal detecting means responsive to the condensed scattered light from said optical system for independently detecting lights scattered by said object from said oblique and vertical illuminations to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination;

first signal processing means for detecting the foreign substance on the basis of a difference obtained by comparing the first and second detection signals with each other;

second signal processing means for detecting the foreign substance on the basis of a sum obtained by adding together the first and second detection signals; and

means for selecting the first signal processing means when detecting a foreign substance on an object having a plane surface and a raised pattern formed on said plane surface, and for selecting the second signal processing means when detecting a foreign substance on an object having a mirror-finished surface.

21. An apparatus for detecting a foreign substance on an unpatterned object having an unpatterned mirror-finished surface by illuminating said unpatterned object and detecting via an optical system light reflected by said unpatterned object to detect said foreign substance on said unpatterned mirror-finished surface of said unpatterned object comprising:

an optical system;

oblique illumination means for applying oblique illumination onto an oblong area of the unpatterned mirror-finished surface of said unpatterned object in an oblique direction relative to an optical axis of said optical system;

vertical illumination means for applying vertical illumination vertically onto the oblong area of the unpatterned mirror-finished surface of said unpatterned object along the optical axis of said optical system;

means for receiving, via said optical system, light from said oblique illumination reflected and scattered from the oblong area and light from said vertical illumination reflected and scattered from the oblong area;

means for separating the received scattered lights from the received reflected lights by shutting off the received reflected lights;

one-dimensional photodetecting means for detecting the separated scattered lights to obtain a first detection signal based on the scattered light from the oblique illumination and a second detection signal based on the scattered light from the vertical illumination, wherein the oblong area is large enough to contain an image of an entire photodetecting area of said one-dimensional photodetecting means projected onto the plane surface of said patterned object through said optical system; and

signal processing means for additively processing the first detection signal and the second detection signal to detect the foreign substance.