Foreign particle inspection apparatus and method with front and back illumination

Claims

What is claimed is:

1. A method of inspecting a reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate, and a pattern of a transparent or translucent film formed on the front surface of the substrate, said method comprising:

obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system;

concentrating scattered light scattered by the front surface of the substrate and surfaces of the patterns formed on the front surface of the substrate;

obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate by a back illuminating system;

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate;

intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and scattered by the patterns with spatial filters disposed on Fourier transform planes;

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on detectors, respectively; and

comparing a detection signal of the detector representing the scattered light and a detection signal of the detector representing the transmitted-and-diffracted light to see if there are any foreign particles adhering to the substrate.

2. A method of inspecting a reticle according to claim 1, wherein the reticle is provided with a phase shifter.

3. A method of inspecting a reticle according to claim 1, wherein the scattered light and the transmitted-and-diffracted light are concentrated by a focusing optical system disposed on a side of the front surface of the reticle with its optical axis extended substantially perpendicularly to the front surface of the substrate.

4. A method of inspecting a reticle according to claim 1, wherein a position on the front surface of the reticle is illuminated from a plurality of directions with the front illuminating light beam, and a position on the back surface of the reticle is illuminated from a plurality of directions with the back illuminating light beam.

5. A method of inspecting a reticle according to claim 1, wherein either the front illuminating light beam or the back illuminating light beam is used at a time.

6. A method of inspecting a reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate, and a pattern of a transparent or translucent film formed on the front surface of the substrate, said method comprising:

obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system;

concentrating scattered light scattered by the front surface of the substrate and surfaces of the patterns formed on the front surface of the substrate;

obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate by a back illuminating system;

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate;

separating the concentrated scattered light and the concentrated transmitted-and-diffracted light:

intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes;

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively; and

comparing detection signals provided by the first detector and the second detector to detect a defect including a foreign particle.

7. A method of inspecting a reticle according to claim 6, wherein the reticle is provided with a phase shifter.

8. A method of inspecting a reticle according to claim 6, wherein the scattered light and the transmitted-and-diffracted light are concentrated by a focusing optical system disposed on the side of the front surface of the reticle with its optical axis extended substantially perpendicularly to the front surface of the substrate.

9. A method of inspecting a reticle according to claim 6, wherein a position on the front surface of the reticle is illuminated from a plurality of directions with the front illuminating light beam, and a position on the back surface of the reticle is illuminated from a plurality of directions with the back illuminating light beam.

10. A method of inspecting a reticle according to claim 6, wherein the wavelength of the front illuminating light beam is in a range of 600 to 800 nm, and the wavelength of the back illuminating light beam is in a range of 450 to 550 nm.

11. A method of inspecting a reticle according to claim 10, wherein the wavelength of the front illuminating light beam is about 780 nm and the wavelength of the back illuminating light beam is one of about 488 nm and about 515 nm.

12. A projection exposure method comprising:

a reticle inspecting process for inspecting a phase shift reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate, and contamination preventing devices each comprising a frame and a transparent thin film held on the frame, and put on the front surface and a back surface of the substrate, respectively, comprising:

obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system,

concentrating scattered light scattered by the front surface of the substrate and surfaces of the patterns formed on the front surface of the substrate,

obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through the back surface of the substrate by a back illuminating system,

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate,

intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes,

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively, and

comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles adhering to the substrate;

a conveying process for conveying the phase shift reticle provided with the contamination preventing devices, inspected by the reticle inspecting process and proved to be free from foreign particles to a projection exposure apparatus and locating the phase shift reticle at a predetermined exposure position; and

an exposure process for projecting exposure light through the phase shift reticle located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the circuit pattern of the transparent or translucent film to prevent interference, to print an image of the pattern of the opaque film of the phase shift reticle on the wafer

13. A projection exposure method comprising:

a reticle inspecting process for inspecting a phase shift reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate, comprising:

obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system,

concentrating scattered light scattered by the front surface of the substrate and surfaces of the patterns formed on the front surface of the substrate,

obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through the back surface of the substrate by a back illuminating system,

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate,

separating the concentrated scattered light and the concentrated transmitted-and-diffracted light,

intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, respectively,

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively, and

comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles adhering to the substrate;

a conveying process for conveying the phase shift reticle inspected by the reticle inspecting process and proved to be free from foreign particles to a projection exposure apparatus and locating the phase shift reticle at a predetermined exposure position; and

an exposure process for projecting exposure light through the phase shift reticle located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the circuit pattern of the transparent or translucent film to prevent interference, to print an image of the pattern of the opaque film of the phase shift reticle on the wafer.

14. A projection exposure method comprising:

a first reticle inspecting process for inspecting a phase shift reticle comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate, comprising:

obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system,

concentrating scattered light scattered by the front surface of the substrate and surfaces of the patterns formed on the front surface of the substrate,

obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through the back surface of the substrate by a back illuminating system,

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate,

separating the concentrated scattered light and the concentrated transmitted-and-diffracted light,

intercepting the scattered light scattered by the patterns and transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, respectively,

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively, and

comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles adhering to the substrate;

a contamination preventing device attaching process for attaching contamination preventing devices each comprising a frame and a transparent thin film held on the frame to the front surface and the back surface, respectively, of the substrate of the phase shift reticle inspected by the first reticle inspecting process and proved to be free from foreign particles;

a second reticle inspecting process comprising:

obliquely projecting a front illuminating light beam through the transparent thin film of the contamination preventing device on the front surface of the substrate by the front illuminating system,

concentrating scattered light scattered by the front surface of the substrate and the surfaces of the patterns formed on the front surface of the substrate, and traveling through the transparent thin film of the contamination preventing device,

obliquely projecting a back illuminating light beam through the transparent thin film of the contamination preventing device on the back surface of the substrate by the back illuminating system,

concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns formed on the front surface of the substrate, and traveling through the transparent thin film of the contamination preventing device,

separating the concentrated scattered light and the concentrated transmitted-and-diffracted light,

intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with the spatial filters disposed on Fourier transform planes, respectively,

focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on the first detector and the second detector, respectively, and

comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles adhering to the substrate;

a conveying process for conveying the phase shift reticle inspected by the second reticle inspecting process and proved to be free from foreign particles to a projection exposure apparatus and locating the phase shift reticle at a predetermined exposure position;

an exposure process for projecting exposure light through the phase shift reticle located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the circuit pattern of the transparent or translucent film to prevent interference, to print an image of the pattern of the opaque film of the phase shift on the wafer.

15. A reticle inspecting apparatus for inspecting a reticle comprising a transparent or translucent substrate, a pattern of an opaque film formed on a front surface of the substrate, and a pattern of a transparent or translucent film formed on the front surface of the substrate, said reticle inspecting apparatus comprising:

a front illuminating system for obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate;

a back illuminating system for obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate;

a focusing optical system for concentrating and focusing scattered light, which is part of the front illuminating light beam scattered by the front surface of the substrate and the surfaces of the patterns, and transmitted-and-diffracted light, which is part of the back illuminating light beam transmitted and diffracted by the substrate and the patterns;

spatial filters disposed on Fourier transform planes to intercept the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns, respectively;

detectors for receiving the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters, and for providing detection signals representing the received scattered light and the received transmitted-and-diffracted light, respectively; and

a comparing means for comparing a detection signal provided by the detector and representing the received scattered light, and a detection signal provided by the detector and representing the received transmitted-and-diffracted light to determine if there are any foreign particles on the reticle.

16. A reticle inspecting apparatus according to claim 15, wherein the reticle is a phase shift reticle provided with a phase shifter.

17. A reticle inspecting apparatus according to claim 15, wherein the focusing optical system is disposed on the side of the front surface of the reticle with its optical axis extended perpendicularly to the front surface of the substrate.

18. A reticle inspecting apparatus according to claim 15, wherein the front illuminating system is capable of illuminating a position on the surface of the substrate from a plurality of directions with the front illuminating light beam and the back illuminating system is capable of illuminating one position on the back surface of the substrate from a plurality of directions with the back illuminating light beam.

19. A reticle inspecting apparatus according to claim 15, wherein the front illuminating system has two front illuminating units, the back illuminating system has two back illuminating units, and a switching means selects either of the two front illuminating units and either of the back illuminating units for enabling inspection.

20. A reticle inspecting apparatus for inspecting a reticle comprising a transparent or translucent substrate, a pattern of an opaque film formed on a front surface of the substrate, and a pattern of a transparent or translucent film formed on the front surface of the substrate, said reticle inspecting apparatus comprising:

a front illuminating system for obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate;

a back illuminating system for obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate;

a focusing optical system for concentrating and focusing scattered light, which is part of the front illuminating light beam scattered by the surface of the substrate and surfaces of the patterns, and transmitted-and-diffracted light, which is part of the back illuminating light beam transmitted and diffracted by the substrate and the patterns;

a separating optical system for separating the scattered light and the transmitted-and-diffracted light concentrated by the focusing optical system;

a first spatial filter and a second spatial filter disposed on Fourier transform planes to intercept the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns, respectively;

a first detector and a second detector for receiving the scattered light and the transmitted-and-diffracted light transmitted by the first and second spatial filter and focused by the focusing optical system and for converting the respective intensities of the received scattered light and the received transmitted-and-diffracted light into corresponding detection signals, respectively; and

a comparing means for comparing a detection signal provided by the first detector and representing the received scattered light, and a detection signal provided by the second detector and representing the received transmitted-and-diffracted light to determine if there are any foreign particles on the reticle.

21. A reticle inspecting apparatus according to claim 20, wherein the reticle is a phase shift reticle provided with phase shifter.

22. A reticle inspecting apparatus according to claim 20, wherein the focusing optical system is disposed on the side of the front surface of the reticle with its optical axis extended perpendicularly to the front surface of the substrate.

23. A reticle inspecting apparatus according to claim 20, wherein the front illuminating system is capable of illuminating a position on the front surface of the substrate from a plurality of directions, and the back illuminating system is capable of illuminating a position on the back surface of the substrate from a plurality of directions.

24. A reticle inspecting apparatus according to claim 20, wherein the wavelength of the front illuminating light beam is in a range of 600 to 800 nm and that of the back illuminating light beam is in a range of 450 to 550 nm.

25. A reticle inspecting apparatus according to claim 24, wherein the wavelength of the front illuminating light beam is about 780 nm and the wavelength of the back illuminating light beam is one of about 488 nm and about 515 nm.

26. A projection exposure apparatus comprising:

a reticle inspecting means for inspecting a phase shift reticle provided with a phase shifter and comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate, a pattern of a transparent or translucent film formed on the front surface of the substrate, and contamination preventing devices each comprising a frame and a transparent thin film held on the frame, and attached to the front surface and back surface of the substrate, respectively, by obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle through the transparent thin film on the front surface of the substrate by a front illuminating system, concentrating scattered light scattered by the front surface of the substrate and the surfaces of the patterns and traveling through the transparent thin film, obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through the transparent thin film on the back surface of the substrate, concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns and traveling through the transparent thin film, separating the concentrated scattered light and the concentrated transmitted-and-diffracted light, intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, respectively, focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively, and comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles on the reticle;

a conveying means for conveying the phase shift reticle provided with the contamination preventing devices, inspected by the reticle inspecting means and proved to be free from foreign particles to and locating the same at a predetermined exposure position; and

a projection exposure means for projecting exposure light through the phase shift reticle provided with the contamination preventing devices and located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the circuit pattern of the transparent or translucent film to prevent interference, to print an image of the pattern of the opaque film of the phase shift reticle on the wafer.

27. A projection exposure apparatus comprising:

a reticle inspecting means for inspecting a phase shift reticle provided with a phase shifter and comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate by obliquely projecting a front illuminating light beam having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle on the front surface of the substrate by a front illuminating system, concentrating scattered light scattered by the front surface of the substrate and the surfaces of the patterns, obliquely projecting a back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate by a back illuminating system, concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns, separating the concentrated scattered light and the concentrated transmitted-and-diffracted light, intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, respectively, focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, and comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles on the reticle;

a conveying means for conveying the phase shift reticle inspected by the reticle inspecting means and proved to be free from foreign particles to and locating the same at a predetermined exposure position; and

a projection exposure means for projecting exposure light through the phase shift reticle located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the circuit pattern of the transparent or translucent film to prevent interference, to print an image of the pattern of the opaque film on the wafer.

28. A projection exposure apparatus comprising:

a reticle inspecting means for inspecting a phase shift reticle provided with a phase shifter and comprising a transparent or translucent substrate, a circuit pattern of an opaque film formed on a front surface of the substrate and a pattern of a transparent or translucent film formed on the front surface of the substrate, before and after attaching contamination preventing devices to the reticle, by obliquely projecting a front illuminating light beam on having a wavelength about 1.6 times of a smallest size of a foreign particle to be inspected on the reticle the front surface of the substrate, concentrating scattered light scattered by the surface of the substrate and surfaces of the patterns, obliquely projecting back illuminating light beam having at least one wavelength about 1.0 times the smallest size of the foreign particle to be inspected on the reticle through a back surface of the substrate by a back illuminating system, concentrating transmitted-and-diffracted light transmitted and diffracted by the substrate and the patterns, separating the concentrated scattered light and the concentrated transmitted-and-diffracted light, intercepting the scattered light scattered by the patterns and the transmitted-and-diffracted light transmitted and diffracted by the patterns with spatial filters disposed on Fourier transform planes, respectively, focusing the scattered light and the transmitted-and-diffracted light transmitted by the spatial filters on a first detector and a second detector, respectively, and comparing a detection signal provided by the first detector and a detection signal provided by the second detector to see if there are any foreign particles on the reticle;

a conveying means for conveying the phase shift reticle provided with contamination preventing devices, inspected by the reticle inspecting means and proved to be free from foreign particles to and locating the same at a predetermined exposure position; and

a projection exposure means for projecting exposure light through the phase shift reticle provided with contamination preventing devices and located at the exposure position toward a wafer and focusing the exposure light transmitted by the phase shift reticle on the wafer by a focusing optical system, which changes a phase of the light transmitted by the pattern of the transparent or translucent film to prevent interference, to print an image of the circuit pattern of the opaque film of the reticle on the wafer.

BACKGROUND OF THE INVENTION

The present invention relates to a method of inspecting a reticle or a photomask (hereinafter referred to inclusively as "reticle") provided with a circuit pattern and a phase shifter formed of a light-transmissive film for defects, such as foreign particles adhering to the reticle, and, more particularly, to a method of inspecting a reticle provided with phase shifter for defects, such as foreign particles, having sizes on the submicron order before printing the reticle on a wafer, and a reticle inspecting apparatus for carrying out the same reticle inspecting method.

When fabricating LSI chips or printed wiring boards, a reticle having a circuit pattern is inspected for defects before printing the reticle on wafers by a photographic process. If the reticle has minute foreign particles having sizes on the submicron order thereon, the reticle cannot be correctly printed on wafers and, consequently, LSI chips fabricated by using such wafers become defective. Problems attributable to the minute foreign particles adhering to the reticle have become more and more significant with the recent progressive increase in the degree of integration of LSIs and the existence of even foreign particles having sizes on the submicron order on the reticle is not permissible.

The inspection of the reticle for foreign particles before printing the reticle on a wafer is indispensable to preventing defective reticle printing, and various techniques for inspecting the reticle for foreign particles have been proposed. A prevalent method of inspecting a reticle for foreign particles, which is employed widely because of its capability of quick and highly sensitive inspection, irradiates the reticle obliquely with a light beam having a high directivity, such as a laser beam, and detects scattered light scattered by foreign particles. However, since the light beam is diffracted at edges of the pattern of the reticle, the diffracted light and the scattered light scattered by foreign particles must be discriminated from each other. Various technical means for discriminating between the diffracted light and the scattered light have been proposed.

A first previously proposed technical means is an inspecting apparatus disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 54-101390. This inspecting apparatus comprises a laser that emits a linearly polarized laser beam, an irradiating means for irradiating a circuit pattern obliquely with the linearly polarized laser beam so that the linearly polarized laser beam fall on the circuit pattern at a given incidence angle, and an oblique focusing optical system including a polarizing plate and lenses. When the circuit pattern is irradiated with the linearly polarized laser beam, the diffracted light diffracted by the circuit pattern and the scattered light scattered by foreign particles differ from each other in the plane of polarization, i.e., the plane of vibration, therefore, only the light scattered by the foreign particles can be detected.

A second previously proposed technical means is an inspecting apparatus disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 59-65428, 1-117024 or 1-153943. This inspecting apparatus comprises a scanning means for scanning an object with a laser beam obliquely projected on the object, a first lens disposed above the object to condense scattered laser light so that the point of irradiation of the laser beam coincides substantially with the condensing point, a filter plate disposed on the Fourier transform image plane of the first lens to filter regular diffracted light diffracted by the circuit pattern of the object, a second lens for the inverse Fourier transform of the scattered light scattered by the foreign particles and transmitted through the screen plate, a slit plate disposed at the focal point of the second lens to screen scattered light from portions of the object other than a portion corresponding to the point of irradiation with the laser beam, and a light receiving device for receiving the scattered light scattered by the foreign particles and passed through the slit of the slit plate. This inspecting apparatus is based on a fact that generally the elements of a circuit pattern are extended in a single direction or in several directions, and filters the diffracted light diffracted by the elements of the circuit pattern extending in a specified direction by a spatial filter disposed on the Fourier transform image plane to detect only the scattered light scattered by the foreign particles.

A third previously proposed technical means is an arrangement disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 58-62543. This arrangement is based on a fact that diffracted light diffracted by the edges of a circuit pattern is directional light while scattered light scattered by foreign particles is not directional and identifies foreign particles on the basis of the logical product of the outputs of a plurality of obliquely arranged detectors.

A fourth previously proposed technical means is an arrangement disclosed in, for example, Japanese Patent Laid-open (Kokai) No. 60-154634 or 60-154635. This arrangement is based on a fact that diffracted light diffracted by the edges of a circuit pattern converges only along a specific direction while scattered light scattered by foreign particles is scattered in all directions and identifies foreign particles from the outputs of a plurality of detectors.

Apparatuses and methods relating to the inspection of objects for minute foreign particles, such as a schlieren method, a phase-contrast microscope and a technique relating to a diffraction image having a finite size are disclosed in, for example, Hiroshi Kubota, "Oyou Kogaku", Iwanami Zensho, pp. 129-136.

When an array type detector, such as a one-dimensional solid-state imaging device provided with an array of solid-state image sensors, an output signal representing a foreign particle is distributed to a plurality of pixels if the foreign particle corresponds to a plurality of elements of the detector and, consequently, the output of the detector is reduced and therefore there is the possibility that the detector fails in detecting the foreign particle. An invention made to obviate such a possibility, disclosed in Japanese Patent Laid-open (Kokai) No. 61-104242 disposes an array type detector at an angle to the direction of scanning operation of an inspection table. Other inventions made for the same purpose, disclosed in Japanese Patent Laid-open (Kokai) Nos. 61-104244 and 61-104659 employ an array type detector having a unique shape and provided with elements arranged in a unique arrangement.

Irregular illumination and the variation of illumination affects adversely to the repeatability and accuracy of detection. An invention disclose in Japanese Patent Laid-open (Kokai) No. 60-038827 calibrates the intensity of scattered light automatically by using a standard sample having a known characteristics.

Japanese Patent laid-open (Kokai) No. 56-132549 discloses an invention for obviating misidentifying a large amount of scattered light scattered by a comparatively large foreign particle as scattered light scattered by a plurality of comparatively small foreign particles.

As mentioned above, failure in finding foreign particles which affect adversely to the quality of LSI chips has become a significant problem with the reduction of the size of foreign particles to be detected. The first previously proposed technical means, for example, the invention disclosed in Japanese Patent Laid-open (Kokai) No. 54-101390, is unable to detect minute foreign particles because the difference between the plane of polarization of the scattered light scattered by minute foreign particles and the plane of polarization of the diffracted light diffracted by the edges of the circuit pattern is small.

The second previously proposed technical means, for example, the inventions disclosed in Japanese Patent Laid-open (Kokai) Nos. 59-65428, 1-117024 and 1-153943, detect only the scattered light scattered by foreign particles by separating the scattered light scattered by foreign particles from the diffracted light diffracted by the circuit pattern with the filter plate and the slit plate. Although these inventions uses a detecting mechanism for detecting foreign particles by a simple binary method having a simple configuration to their advantage, the diffracted light diffracted by the intersection of the elements of the circuit pattern does not travel unidirectionally like the diffracted light diffracted by the straight edge of the circuit pattern, and hence the spatial filter is unable to filter the diffracted light diffracted by the intersection of the elements of the circuit pattern perfectly. Furthermore, since the diffracted light diffracted by a minute circuit pattern on the micron order for a LSI having a very high degree of integration is analogous in behavior to the scattered light scattered by foreign particles and hence it is practically difficult to discriminate between the circuit pattern and foreign particles by a simple binary method.

The apparatus proposed as the third previously proposed technical means in, for example, Japanese Patent Laid-open (Kokai) No. 58-62543) and the those proposed as the fourth previously proposed technical means in, for example, Japanese Patent Laid-open (Kokai) Nos. 60-154634 and 60-154635 have difficulty in employing an optical system having a sufficiently high condensing ability because of their configurations and hence it is practically difficult for these apparatuses to detect faint scattered light scattered by foreign particles.

The apparatuses disclosed as the fifth previously proposed technical means in, for example, Japanese Patent Laid-open (Kokai) Nos. 61-104242 and 61-104244 need a special detector and a special optical system, which are costly.

The apparatus disclosed as the sixth previously proposed technical means in, for example, Japanese Patent Laid-open (Kokai) No. 60-038827 has drawbacks in application to an array type detector suitable for quick detection and in structural accuracy for detecting minute foreign particles.

The apparatus disclosed as the seventh previously proposed technical means in, for example, Japanese Patent Laid-open (Kokai) No. 56-132544 detects only a single point on a large foreign particle and hence the apparatus is unable to recognize the shape of an elongate foreign particle accurately.

A reticle recently developed to improve resolution in transferring a circuit pattern formed on the reticle is provided with a transparent or translucent thin film, which is called a phase shift film or a phase shifter, having a thickness equal to odd times half the wavelength of the light used for exposure and formed so as to cover spaces between the elements of the circuit pattern. Although this thin film is transparent or translucent, the thickness of this thin film is several times the thickness on the order of 0.1 .mu.m of the circuit pattern. Consequently, the intensity of the diffracted light diffracted by the edges of the thin film is several to several tens times the intensity of the diffracted light diffracted by the edges of the circuit pattern, which reducing the foreign particle detecting sensitivity significantly.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method of inspecting a reticle, such as a reticle fabricated by forming a circuit pattern and a phase shift film for improving transfer resolution on a transparent or translucent substrate for defects, such as minute foreign particles having sizes on the submicron order adhering to the circuit pattern, capable of separating the defects from the circuit pattern and of stably detecting the defects, and an apparatus for carrying out the method.

Another object of the present invention is to provide a projection exposure method capable of eliminating defects, such as minute foreign particles, from a mask, such as a reticle provided with a phase-shift film (hereinafter referred to as "phase shift reticle") and of projecting a circuit pattern formed on the mask on a substrate, such as a wafer, by using an image-reducing projection exposure apparatus for exposure, and an apparatus for carrying out the projection exposure method.

The present invention has been made by introducing improvements into the invention proposed in U.S. patent application Ser. No. 08/192,036 related with U.S. patent application Ser. No. 07/902,819 or the invention proposed in Korean Pat. Application No. 3209/1994 related with Korean Pat. Application No. 11092/1992.

With the foregoing object in view, the present invention provides a reticle inspecting apparatus for inspecting a reticle fabricated by forming a circuit pattern on a substrate, for defects, such as foreign particles adhering to the reticle, comprising: a stage unit for supporting the substrate, capable of being optionally moved in directions along an X-axis, a Y-axis and a Z-axis; an illuminating system having a first illuminating system and a second illuminating system individually provided with light sources that emit light beams of approximately 780 nm in wavelength, disposed opposite to each other on the side of the front surface of the substrate provided with the circuit pattern so as to illuminate the substrate with light beams that travel obliquely to the surface of the substrate, either the first or the second illuminating system that can project the light beam on the surface of the substrate so that the light beam is not intercepted by a pellicle holding frame being used; an illuminating system disposed on the opposite side of the reticle with respect to the former illuminating system, having a third illuminating system corresponding to the first illuminating system and a fourth illuminating system corresponding to the second illuminat