Interferometer for observing the interference pattern of a surface under test utilizing an adjustable aperture stop

• US 5,561,525 A
• Filed: 12/20/1995
• Issued: 10/01/1996
• Est. Priority Date: 06/21/1993
• Status: Expired due to Fees

• Current Assignee: Nikon Corporation
• Original Assignee: Nikon Corporation
• Inventors: Toyonaga, Shuji, Gemma, Takashi
• Primary Examiner(s): Turner, Samuel A.
• Assistant Examiner(s): KIM, ROBERT H
• Application Number:
  US08/575,442
• Time in Patent Office: 286 Days
• Field of Search: 356/345, 356/351, 356/359, 356/360
• US Class Current: 356/512
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02057 by using common path configuration, i.e. reference and object path almost entirely overlapping

Abstract

An interferometer is used for observing the shape of a surface to be detected with the desired spatial resolution. A variable aperture stop is arranged at a Fourier transform image plane of the surface to be detected within an imaging optical system for forming an interference pattern of a reference light and a measuring light. The aperture diameter of the variable aperture stop is adjusted by a control in accordance with the desired spatial resolution.

Claims

1. An interferometer comprising:

a light source for producing a light beam of a predetermined wavelength;

a beam splitter for directing the light beam from said light source to a surface to be detected and a reference surface wherein reflected light from said surface to be detected and said reference surface are combined to form combined reflected light on the same optical axis;

an imaging optical system for forming an interference pattern of said combined reflected light;

a detecting optical system for observing said interference pattern formed by said imaging optical system;

a variable aperture stop arranged at a Fourier transform image plane of said surface to be detected within said imaging optical system; and

stop control means for varying an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect irregularities on said surface with a desired value of spatial resolution, wherein said stop control means adjusts an aperture diameter φ

of said variable aperture stop so as to satisfy the following equation

space="preserve" listing-type="equation">ν

.sub.m =|β

.sub.i | * φ

/2fλ

where ν

_m is said desired value of spatial resolution, β

_i is a magnification of said imaging optical system, f is the image-side focal length of said imaging optical system, and λ

is the wavelength of the light beam.

2. An interferometer according to claim 1, further comprising a beam expander for expanding a diameter of the light beam from said light source in accordance with an area of an observation region of said surface to be detected.

3. An interferometer comprising:

a light source for producing a light beam of a predetermined wavelength;

a beam splitter for directing the light beam from said light source to a surface to be detected and a reference surface wherein reflected light from said surface to be detected and said reference surface are combined to form combined reflected light on the same optical axis;

an imaging optical system for forming an interference pattern of said combined reflected light;

a detecting optical system for observing said interference pattern formed by said imaging optical system;

a variable aperture stop arranged at a Fourier transform image plane of said surface to be detected within said imaging optical system;

stop control means for varying an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts an aperture diameter φ

of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

=fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

4. An interferometer comprising:

a light source for producing a light beam of a predetermined wavelength;

a beam splitter for directing the light beam from said light source to a surface to be detected and a reference surface wherein reflected light from said surface to be detected and said reference surface are combined to form combined reflected light on the same optical axis;

an imaging optical system for forming an interference pattern of said combined reflected light;

a detecting optical system for observing said interference pattern formed by said imaging optical system;

a variable aperture stop arranged at a Fourier transform image plane of said surface to be detected within said imaging optical system;

stop control means for varying an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts an aperture diameter φ

of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

≦

fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

5. A Twyman-Green interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system wherein the light emitted from said light source is converted to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and reflecting said parallel light beam in a second direction perpendicular to said first direction thereby separating said parallel light beam into two light beams of a transmitted light and a reflected light of the directions perpendicular to each other;

a surface to be detected for substantially vertically receiving irradiation of said transmitted light to produce a reflection-diffracted light wherein said reflection-diffracted light is reflected by said beam splitter to travel in a third direction which is opposite to said second direction;

a reference surface arranged to substantially vertically receive irradiation of said reflected light to produce a reference reflected light wherein said reference reflected light is transmitted through said beam splitter, combined with said reflection-diffracted light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference reflected light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected; and

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect irregularities on said surface with a desired value of spatial resolution, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop so as to satisfy the following equation

space="preserve" listing-type="equation">ν

.sub.m =|β

.sub.i | * φ

/2fλ

where ν

_m is said desired value of spatial resolution, β

_i is a magnification of said imaging optical system, f is the image-side focal length of said imaging optical system, and λ

is the wavelength of the light beam.

6. An interferometer according to claim 5, further comprising a beam expander for expanding a diameter of said light from said light source in accordance with an area of an observation region of said surface to be detected.

7. A Twyman-Green interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system wherein the light emitted from said light source is converted to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and reflecting said parallel light beam in a second direction perpendicular to said first direction thereby separating said parallel light beam into two light beams of a transmitted light and a reflected light of the directions perpendicular to each other;

a surface to be detected for substantially vertically receiving irradiation of said transmitted light to produce a reflection-diffracted light wherein said reflection-diffracted light is reflected by said beam splitter to travel in a third direction which is opposite to said second direction;

a reference surface arranged to substantially vertically receive irradiation of said reflected light to produce a reference reflected light wherein said reference reflected light is transmitted through said beam splitter, combined with said reflection-diffracted light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference reflected light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

=fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

8. A Twyman-Green interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system wherein the light emitted from said light source is converted to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and reflecting said parallel light beam in a second direction perpendicular to said first direction thereby separating said parallel light beam into two light beams of a transmitted light and a reflected light of the directions perpendicular to each other;

a surface to be detected for substantially vertically receiving irradiation of said transmitted light to produce a reflection-diffracted light wherein said reflection-diffracted light is reflected by said beam splitter to travel in a third direction which is opposite to said second direction;

a reference surface arranged to substantially vertically receive irradiation of said reflected light to produce a reference reflected light wherein said reference reflected light is transmitted through said beam splitter, combined with said reflection-diffracted light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference reflected light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

≦

fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

9. A Fizeau interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system for converting the light emitted from said light source to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and for reflecting an incident light directed in a second direction opposite to said first direction in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of said parallel light beam transmitted through said beam splitter in said first direction is reflected and returned as a reference light in said second direction and the other part of said parallel light beam is transmitted as a measuring light in said first direction, said reference light being reflected by said beam splitter and directed in said third direction;

a surface to be detected arranged so as to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction, reflected by said beam splitter, combined with said reference light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect irregularities on said surface with a desired value of spatial resolution, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop so as to satisfy the following equation

space="preserve" listing-type="equation">ν

.sub.m =|β

.sub.i | * φ

/2fλ

where ν

_m is said desired value of spatial resolution, β

_i is a magnification of said imaging optical system, f is the image-side focal length of said imaging optical system, and λ

is the wavelength of the light beam.

10. An interferometer according to claim 9, further comprising a beam expander for expanding a diameter of the light beam from said light source in accordance with an area of an observation region of said surface to be detected.

11. A Fizeau interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system for converting the light emitted from said light source to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and for reflecting an incident light directed in a second direction opposite to said first direction in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of said parallel light beam transmitted through said beam splitter in said first direction is reflected and returned as a reference light in said second direction and the other part of said parallel light beam is transmitted as a measuring light in said first direction, said reference light being reflected by said beam splitter and directed in said third direction;

a surface to be detected arranged so as to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction, reflected by said beam splitter, combined with said reference light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

=fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

12. A Fizeau interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system for converting the light emitted from said light source to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and for reflecting an incident light directed in a second direction opposite to said first direction in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of said parallel light beam transmitted through said beam splitter in said first direction is reflected and returned as a reference light in said second direction and the other part of said parallel light beam is transmitted as a measuring light in said first direction, said reference light being reflected by said beam splitter and directed in said third direction;

a surface to be detected arranged so as to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction, reflected by said beam splitter, combined with said reference light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts an aperture diameter φ

of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

≦

fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

13. An interferometer comprising:

a laser light source for producing a laser light beam of a predetermined wavelength;

an optical system for converting said laser light beam to a substantially parallel light beam having a predetermined beam diameter and directed in a first direction;

a polarizing beam splitter arranged along an optical path of said parallel light beam directed in said first direction wherein first and second polarized light components are related so that vector vibration directions thereof are perpendicular to each other, only said first polarized light component is transmitted in said first direction and only said second polarized light component, directed in a second direction opposite to said first direction, is reflected in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of the first polarized light component of said parallel light beam transmitted through said polarizing beam splitter in said first direction is returned as a reference light in said second direction and the other part of the first polarized light component is transmitted as a measuring light in said first direction;

a surface to be detected arranged to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction;

a wave plate for converting said reference light and said reflection-diffracted light to second polarized components for introduction into said polarizing beam splitter, said second polarized components being reflected by said polarizing beam splitter, combined with each other and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction wherein an interference pattern of said second polarized components of said reflection-diffracted light and said reference light is formed on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop controller for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect irregularities on said surface with a desired value of spatial resolution, wherein said stop controller adjusts an aperture diameter φ

of said variable aperture stop so as to satisfy the following equation

space="preserve" listing-type="equation">ν

.sub.m =|β

.sub.i | * φ

/2fλ

where v_m is said desired value of spatial resolution, β

_i is a magnification of said imaging optical system, f is the image-focal length of said imaging optical system, and λ

is the wavelength of the light beam.

14. An interferometer according to claim 13, further comprising a beam expander arranged between said polarizing beam splitter and said surface to be detected for expanding a beam diameter of said measuring light in accordance with an area of an observation region of said surface to be detected.

15. An interferometer according to claim 14, wherein said wave plate is arranged between said semitransparent reference surface and said beam expander.

16. An interferometer according to claim 14, wherein said wave plate is arranged between said beam expander and said polarizing beam splitter.

17. An interferometer according to claim 14, wherein said wave plate is arranged within an optical system of said beam expander.

18. An interferometer according to claim 17, wherein said wave plate is arranged at a position closer to said surface to be detected than an inner focal position of said beam expander.

19. An interferometer comprising:

a laser light source for producing a laser light beam of a predetermined wavelength;

an optical system for converting said laser light beam to a substantially parallel light beam having a predetermined beam diameter and directed in a first direction;

a polarizing beam splitter arranged along an optical path of said parallel light beam directed in said first direction wherein first and second polarized light components are related so that vector vibration directions thereof are perpendicular to each other, only said first polarized light component is transmitted in said first direction and only said second polarized light component, directed in a second direction opposite to said first direction, is reflected in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of the first polarized light component of said parallel light beam transmitted through said polarizing beam splitter in said first direction is returned as a reference light in said second direction and the other part of the first polarized light component is transmitted as a measuring light in said first direction;

a surface to be detected arranged to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction;

a wave plate for converting said reference light and said reflection-diffracted light to second polarized components for introduction into said polarizing beam splitter, said second polarized components being reflected by said polarizing beam splitter, combined with each other and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction wherein an interference pattern of said second polarized components of said reflection-diffracted light and said reference light is formed on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop controller for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, controller adjusts an aperture diameter φ

of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

=fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

20. An interferometer comprising:

a laser light source for producing a laser light beam of a predetermined wavelength;

an optical system for converting said laser light beam to a substantially parallel light beam having a predetermined beam diameter and directed in a first direction;

a polarizing beam splitter arranged along an optical path of said parallel light beam directed in said first direction wherein first and second polarized light components are related so that vector vibration directions thereof are perpendicular to each other, only said first polarized light component is transmitted in said first direction and only said second polarized light component, directed in a second direction opposite to said first direction, is reflected in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of the first polarized light component of said parallel light beam transmitted through said polarizing beam splitter in said first direction is returned as a reference light in said second direction and the other part of the first polarized light component is transmitted as a measuring light in said first direction;

a surface to be detected arranged to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction;

a wave plate for converting said reference light and said reflection-diffracted light to second polarized components for introduction into said polarizing beam splitter, said second polarized components being reflected by said polarizing beam splitter, combined with each other and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction wherein an interference pattern of said second polarized components of said reflection-diffracted light and said reference light is formed on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop controller for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop controller adjusts an aperture diameter φ

of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

≦

fλ

|β

.sub.z |/p.sub.c

wherein β

_z is a magnification of said zoom lens system, f is the image-side focal length of the imaging optical system, λ

is the wavelength of the light beam, and p_c is the picture element pitch of the CCD camera.

21. An interferometer comprising:

a light source for producing a light beam of a predetermined wavelength;

a beam splitter for directing the light beam from said light source to a surface to be detected and a reference surface wherein reflected light from said surface to be detected and said reference surface are combined to form combined reflected light on the same optical axis;

an imaging optical system for forming an interference pattern of said combined reflected light;

a detecting optical system for observing said interference pattern formed by said imaging optical system;

a variable aperture stop arranged at a Fourier transform image plane of said surface to be detected within said imaging optical system;

stop control means for varying an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation

space="preserve" listing-type="equation">φ

.sub.x =fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y =fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD camera along a second direction which is perpendicular to said first direction.

22. An interferometer comprising:

a light source for producing a light beam of a predetermined wavelength;

a beam splitter for directing the light beam from said light source to a surface to be detected and a reference surface wherein reflected light from said surface to be detected and said reference surface are combined to form combined reflected light on the same optical axis;

an imaging optical system for forming an interference pattern of said combined reflected light;

a detecting optical system for observing said interference pattern formed by said imaging optical system;

a variable aperture stop arranged at a Fourier transform image plane of said surface to be detected within said imaging optical system;

stop control means for varying an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x ≦

fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y ≦

fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD along a second direction which is perpendicular to said first direction.

23. A Twyman-Green interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system wherein the light emitted from said light source is converted to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and reflecting said parallel light beam in a second direction perpendicular to said first direction thereby separating said parallel light beam into two light beams of a transmitted light and a reflected light of the directions perpendicular to each other;

a surface to be detected for substantially vertically receiving irradiation of said transmitted light to produce a reflection-diffracted light wherein said reflection-diffracted light is reflected by said beam splitter to travel in a third direction which is opposite to said second direction;

a reference surface arranged to substantially vertically receive irradiation of said reflected light to produce a reference reflected light wherein said reference reflected light is transmitted through said beam splitter, combined with said reflection-diffracted light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference reflected light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x =fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y =fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD camera along a second direction which is perpendicular to said first direction.

24. A Twyman-Green interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system wherein the light emitted from said light source is converted to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and reflecting said parallel light beam in a second direction perpendicular to said first direction thereby separating said parallel light beam into two light beams of a transmitted light and a reflected light of the directions perpendicular to each other;

a surface to be detected for substantially vertically receiving irradiation of said transmitted light to produce a reflection-diffracted light wherein said reflection-diffracted light is reflected by said beam splitter to travel in a third direction which is opposite to said second direction;

a reference surface arranged to substantially vertically receive irradiation of said reflected light to produce a reference reflected light wherein said reference reflected light is transmitted through said beam splitter, combined with said reflection-diffracted light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference reflected light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

said detecting optical system including a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x ≦

fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y ≦

fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD camera along a second direction which is perpendicular to said first direction.

25. A Fizeau interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system for converting the light emitted from said light source to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and for reflecting an incident light directed in a second direction opposite to said first direction in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of said parallel light beam transmitted through said beam splitter in said first direction is reflected and returned as a reference light in said second direction and the other part of said parallel light beam is transmitted as a measuring light in said first direction, said reference light being reflected by said beam splitter and directed in said third direction;

a surface to be detected arranged so as to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction, reflected by said beam splitter, combined with said reference light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x =fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y =fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD along a second direction which is perpendicular to said first direction.

26. A Fizeau interferometer comprising:

a light source for producing light of a predetermined wavelength;

an optical system for converting the light emitted from said light source to a substantially parallel light beam directed in a first direction;

a beam splitter for transmitting said parallel light beam in said first direction and for reflecting an incident light directed in a second direction opposite to said first direction in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of said parallel light beam transmitted through said beam splitter in said first direction is reflected and returned as a reference light in said second direction and the other part of said parallel light beam is transmitted as a measuring light in said first direction, said reference light being reflected by said beam splitter and directed in said third direction;

a surface to be detected arranged so as to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being trasnitted through said semitransparent reference surface in said second direction, reflected by said beam splitter, combined with said reference light and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction to form an interference pattern of said reflection-diffracted light and said reference light on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop control for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in accordance with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x ≦

fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y ≦

fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD camera along a second direction which is perpendicular to said first direction.

27. An interferometer comprising:

a laser light source for producing a laser light beam of a predetermined wavelength;

an optical system for converting said laser light beam to a substantially parallel light beam having a predetermined beam diameter and directed in a first direction;

a polarizing beam splitter arranged along an optical path of said parallel light beam directed in said first direction wherein first and second polarized light components are related so that vector vibration directions thereof are perpendicular to each other, only said first polarized light component is transmitted in said first direction and only said second polarized light component, directed in a second direction opposite to said first direction, is reflected in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of the first polarized light component of said parallel light beam transmitted through said polarizing beam splitter in said first direction is returned as a reference light in said second direction and the other part of the first polarized light component is transmitted as a measuring light in said first direction;

a surface to be detected arranged to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction;

a wave plate for converting said reference light and said reflection-diffracted light to second polarized components for introduction into said polarizing beam splitter, said second polarized components being reflected by said polarizing beam splitter, combined with each other and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction wherein an interference pattern of said second polarized components of said reflection-diffracted light and said reference light is formed on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop controller for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter 100_y of said variable aperture stop in conformity with a picture element pitch of the CCD camera so as to satisfy the following equation;

space="preserve" listing-type="equation">φ

.sub.x =fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y =fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD along a second direction which is perpendicular to said first direction.

28. An interferometer comprising:

a laser light source for producing a laser light beam of a predetermined wavelength;

an optical system for converting said laser light beam to a substantially parallel light beam having a predetermined beam diameter and directed in a first direction;

a polarizing beam splitter arranged along an optical path of said parallel light beam directed in said first direction wherein first and second polarized light components are related so that vector vibration directions thereof are perpendicular to each other, only said first polarized light component is transmitted in said first direction and only said second polarized light component, directed in a second direction opposite to said first direction, is reflected in a third direction perpendicular to said first direction;

a semitransparent reference surface wherein a part of the first polarized light component of said parallel light beam transmitted through said polarizing beam splitter in said first direction is returned as a reference light in said second direction and the other part of the first polarized light component is transmitted as a measuring light in said first direction;

a surface to be detected arranged to substantially vertically receive irradiation of said measuring light transmitted through said semitransparent reference surface in said first direction to produce a reflection-diffracted light, said reflection-diffracted light being transmitted through said semitransparent reference surface in said second direction;

a wave plate for converting said reference light and said reflection-diffracted light to second polarized components for introduction into said polarizing beam splitter, said second polarized components being reflected by said polarizing beam splitter, combined with each other and directed in said third direction;

an imaging optical system arranged in an optical path directed in said third direction wherein an interference pattern of said second polarized components of said reflection-diffracted light and said reference light is formed on an observation surface at a predetermined position;

a variable aperture stop arranged at a Fourier transform image plane in said imaging optical system with respect to said surface to be detected;

a stop controller for adjusting an aperture diameter of said variable aperture stop to adjust a spatial resolution of said interferometer in order to detect said surface with a desired value of spatial resolution;

a zoom lens system for varying a magnification of said interference pattern formed by said imaging optical system; and

a CCD camera for picking up said interference pattern varied in magnification by said zoom lens system, wherein said stop control means adjusts a first aperture diameter φ

_x and a second aperture diameter φ

_y of said variable aperture stop in accorandance with a picture element pitch of the CCD camera so as to satisfy the following equations;

space="preserve" listing-type="equation">φ

.sub.x ≦

fλ

|β

.sub.z |/P.sub.cx ; and

space="preserve" listing-type="equation">φ

.sub.y ≦

fλ

|β

.sub.z |/P.sub.cy ;

wherein β

_z is a magnification of said zoom lens system, f is an image-side focal length of the imaging optical system, λ

is the predetermined wavelength of the light beam, P_cx is a first picture element pitch of the CCD camera along a first direction, and P_cy is a second picture element pitch of the CCD along a second direction which is perpendicular to said first direction.