JPH11271630A - Differential interference microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized differential interference microscope of small power consumption at a low cost. SOLUTION: Light from a light source is divided into two lights of different polarization components by a Nomarski prism 6 and then irradiates an object 8 to be tested, and reflected light from the object 8 to be tested is synthesized in the Nomarski prism 6 again and made incident on a polarizing filter 13. The polarizing filter 13 selects the two polarization components from incident light and sends them to a two-dimensional image sensor 14. The two-dimensional image sensor 14 receives one of the polarization components selected in the polarizing filter 13 by one of a pair of adjacent picture elements and receives the other selected polarization component by the other one of the pair of the picture elements. An image processor 15 forms a differential interference image of the object 8 to be tested based on image signals outputted from the pair of the picture elements.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential interference microscope, and more particularly to a differential interference microscope suitable for observing an IC pattern or a metal surface.

[0002]

2. Description of the Related Art Differential interference microscopes are widely used as means for observing microstructures and microsteps on IC patterns and metal surfaces. The applicant of the present application has already applied for a differential interference microscope in which the contrast of a differential interference image of a test object can be changed by changing the analyzer angle of an analyzer in a differential differential interference microscope. It is disclosed in Japanese Patent Application Laid-Open No. 9-15503. Another type of differential interference microscope (or step measuring device) filed by the applicant of the present application is disclosed in
145329.

First, the differential interference microscope disclosed in Japanese Patent Application Laid-Open No. 9-15503 will be described below. FIG.
FIG. 503 is a schematic configuration diagram of a differential interference microscope disclosed in 503. In this differential interference microscope, a laser beam having a polarization direction parallel to the paper is emitted from a laser light source 51, and the emitted laser beam is collimated by a collimating lens 52.
The light becomes parallel light through the mirror and enters the half mirror 53.
The laser beam reflected downward by the half mirror 53 is spatially deflected by the two-dimensional scanning device 54 and then enters the Nomarski prism 55. The Nomarski prism 55, which is a type of birefringent prism, has an optical axis that intersects at 45 degrees with the polarization direction of the incident laser beam, and splits the incident laser beam into two laser beams having different oscillation directions. . Nomarski prism 55
The laser beams split into two by the laser beam are condensed via an objective lens 56, and form two laser spots on a test object 57 arranged on a stage 58.
These two laser spots are scanned by a two-dimensional scanning device 54.
Scans the object 57 two-dimensionally.

[0004] The two laser beams reflected by the test object 57 pass through the objective lens 56 again, and are combined via a Nomarski prism 55. The Nomarski prism 55, for two laser beams,
The insertion position with respect to the optical axis is defined so as to add a phase difference of an integral multiple of π in the round trip. Therefore, the test object 5
If 7 is a mirror surface, the phase difference between the two reflected laser beams via the Nomarski prism 55 will be an integral multiple of π. In other words, two reflected laser beams from the test object 57 corresponding to the two laser spots are combined via the Nomarski prism 55 into a linearly polarized laser beam having a polarization direction parallel to the plane of the drawing.

The combined laser beam combined by the Nomarski prism 55 becomes a parallel beam via a two-dimensional scanning device 54 and enters a half mirror 53.
The combined laser beam transmitted through the half mirror 53 enters the quarter-wave plate 60 via the mirror 59. 1/4
The wave plate 60 is １ ／ when the test object 57 is a mirror surface.
It is positioned at an azimuth angle π / 4 with respect to the direction of linear polarization of the synthetic linearly polarized laser beam incident on the wave plate 60. Therefore, when the test object 57 is a mirror surface, 1 /
The laser beam passing through the four-wavelength plate 60 becomes circularly polarized light and enters the half-wavelength plate 61. 1/2 wave plate 61
Is installed so that it can rotate about the optical axis.

[0006] The laser beam having passed through the half-wave plate 61 is split by a polarization beam splitter 62 into transmitted light and reflected light. Since the half-wave plate 61 is an optical rotator having a variable polarization rotation angle, the rotatable half-wave plate 61 and the fixed polarization beam splitter 62 constitute a polarization beam splitter with a variable analyzer angle. Have been.
The light transmitted through the polarization beam splitter 62 is
3 and is photoelectrically converted into an electric signal.
On the other hand, the light reflected by the polarization beam splitter 62 is detected by the photodetector 64 and photoelectrically converted.

The electric signals photoelectrically converted by the light detector 63 and the light detector 64 are supplied to a differential circuit 65, respectively. The differential circuit 65 obtains a difference signal based on the signal from the photodetector 63 and the signal from the photodetector 64, and supplies the obtained difference signal to the synchronizer 66. The synchronization device 66 synchronizes the scanning position information of the laser spot according to the operation of the two-dimensional scanning device 54 with the difference signal from the operation circuit 65 and supplies the information to the image display device 67.

The image display device 67 forms a differential interference image based on the scanning position information of the laser spot and the difference signal, and displays the formed differential interference image. Image display device 6
The contrast of the differential interference image displayed by 7 changes depending on the polarization rotation angle of the half-wave plate 61 and, consequently, the analyzer angle of the polarization beam splitter 62. Therefore, by appropriately rotating the half-wave plate 61 about the optical axis, the contrast of the obtained differential interference image can be freely adjusted from the maximum to the minimum.

On the other hand, the differential interference microscope of JP-A-9-145329 uses a light source having a finite size as the light source 51 of the differential interference microscope shown in FIG. Are imaging differential differential microscopes in which a lens and a two-dimensional image sensor are arranged, respectively, and a two-dimensional scanning device 54 is removed. This differential interference microscope,
Similar to the one shown in FIG. 5, it has a function of adjusting the contrast of the differential interference image. However, since a two-dimensional image sensor is used, there is no need for a movable part such as a two-dimensional scanning device.
A high-speed differential interference image can be formed.

[0010]

However, as described above, when two two-dimensional image sensors are used as the photodetectors of the differential interference microscope, pixels are accurately matched between the two two-dimensional image sensors. You have to
Alignment of both was difficult. In addition, there is another problem that the use of two two-dimensional image sensors increases the size of the apparatus and increases the cost. In addition, there is a problem that power consumption is further increased, which causes an increase in heat generation of the apparatus, which also affects the accuracy of a differential interference microscope, which is an optical precision instrument.

[0011]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has high image quality without requiring a photodetector such as a polarizing beam splitter or two two-dimensional image sensors. Capable of forming a differential interference image,
It is an object of the present invention to provide a low-cost differential interference microscope that is relatively small and consumes less power.

In order to solve the above-mentioned problems, a differential interference microscope according to the present invention divides light from a light source into two lights having different polarization components, emits the light to a test object, and emits the light from the test object. A birefringent prism that combines two lights, and a first polarized component and a second polarized component that intersects the first polarized component are selected from the two lights combined by the birefringent prism. A polarizing plate, comprising a plurality of pixels, receives the first polarized component selected by the polarizing plate in one of the pixels adjacent to each other, the second polarized component selected by the polarizing plate A sensor that receives light at the other of the adjacent pixels, and an image forming apparatus that forms a differential interference image of the test object based on the respective image signals output from the adjacent pixels. Since this differential interference microscope does not require two sensors to form a differential interference image, there is no need for alignment between the sensors, and the apparatus can be provided at a small size and at low cost.

It is preferable that the first polarization component and the second polarization component are polarized in directions orthogonal to each other. Further, the differential interference microscope according to the present invention further includes a contrast changer arranged between the birefringent prism and the polarizing plate, for changing a contrast of the differential interference image of the test object. Is also good.

Further, in the image forming method according to the present invention, the step of dividing light from a light source into two lights having different polarization components and emitting the light to a test object, and synthesizing the two lights from the test object Selecting a first polarization component and a second polarization component that intersects the first polarization component from the two combined lights; and selecting the first polarization component selected. Receiving one of a pair of pixels in a sensor having a plurality of pixels, and receiving the selected second polarization component at the other of the pair of pixels,
Forming a differential interference image of the test object based on signals output from each of the pair of pixels. It is preferable that the first polarization component and the second polarization component are polarized in directions orthogonal to each other.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a differential interference microscope according to the present invention will be described below with reference to the drawings.
The following embodiment shows an example of the present invention, and the present invention is not limited to this.

FIG. 1 is a diagram schematically showing a configuration of an embodiment of a differential interference microscope according to the present invention. As shown in this figure, the differential interference microscope of the present embodiment is provided with a light source 1 such as a tungsten lamp, and the light emitted from the light source 1 passes through a collimating lens 2 and becomes parallel light. The wavelength is selected by passing through the interference filter 3. In the present embodiment, a filter that selects a wavelength of about 550 nm is used as the interference filter 3.

The light transmitted through the interference filter 3 has its oscillation direction limited by the polarizing plate 4, becomes linearly polarized light whose polarization direction is, for example, parallel to the paper surface, and then enters the half mirror 5. The light reflected by the half mirror 5 in the direction of the test object (downward in the figure) enters a Nomarski prism 6 which is a birefringent prism. The Nomarski prism 6 has an optical axis that crosses the polarization direction of the incident light at 45 degrees, and divides the incident light into two lights having different vibration directions (in this case, the vibration planes are orthogonal).
Note that a Wollaston prism or the like may be used instead of the Nomarski prism 6.

The light split into two by the Nomarski prism 6 is condensed through an objective lens 7 to form two illumination lights on a test object 8 arranged on a stage 9. In this way, the test object 8 is illuminated by two illumination lights whose vibration planes are orthogonal to each other and whose centers are slightly apart. The test object 8 has a low step on its surface.

The two illumination lights illuminating the test object 8 are reflected by the test object 8, pass through the objective lens 7 again, enter the Nomarski prism 6, and are synthesized by the Nomarski prism 6. The insertion position of the Nomarski prism 6 with respect to the optical axis is defined so as to give a phase difference of an integral multiple of π between two illumination lights in a round trip. Therefore, when the test object 8 is a flat surface such as a mirror surface with no change in reflectance, the phase difference between the two reflected lights passing through the Nomarski prism 6 is an integral multiple of π. Therefore, the combined light emitted from the Nomarski prism 6 becomes light linearly polarized having a polarization direction parallel to the plane of the drawing.

The combined light that has exited the Nomarski prism 6 enters the half mirror 5 again, and the combined light that has passed through the half mirror 5 enters the quarter-wave plate 10. 1/4 wavelength plate 1
0 is a quarter-wave plate 10 when the object 8 is a mirror surface.
Azimuth angle π / 4 with respect to the direction of linear polarization of the combined light
It is positioned with. Therefore, when the test object 8 is a mirror surface, the combined light that has passed through the quarter-wave plate 10 becomes circularly polarized light, and then enters the half-wave plate 11. 1/2
The wave plate 11 is installed so as to be rotatable around the optical axis. The light transmitted through the half-wave plate 11 is condensed by the lens 12 and transmitted through the polarization filter 13 to form an image on a two-dimensional image sensor 14 having a plurality of pixels.

FIG. 2 shows the configuration of a part of the polarizing filter 13. Each square in this figure represents the elements a to p (also referred to as a pixel) of the polarizing filter 13, and the line in the square represents the polarization direction of each element.

Each of the elements a to p of the polarizing filter 13 is constituted by a polarizing plate arranged so as to correspond one-to-one with a pixel of the two-dimensional image sensor 14 arranged behind. It is configured such that the polarization directions are orthogonal between adjacent elements. Accordingly, the polarizing filter 13 selectively transmits only two polarized components of the light that has exited the lens 12 and makes the two-dimensional image sensor 14 enter the two-dimensional image sensor 14. Receives light having a polarization component different from that of the other pixel.

The light focused on the two-dimensional image sensor 14 is photoelectrically converted for each pixel and sent to the image processing device 15. The image processing device 15 treats the signal received from the two-dimensional image sensor 14 as a set of two pixels adjacent to each other in the horizontal direction of the two-dimensional image sensor 14, and obtains a difference between outputs of the two adjacent pixels for each set. Thus, a two-dimensional image (differential differential interference image) of the test object 8 is formed. That is, the elements of a certain row of the polarizing filter 13 (for example, a, e,
Each output of the pixel of the two-dimensional image sensor 14 behind the i, m) is combined with each output of the pixel behind the element (b, f, j, n) on the right side of the polarization filter 13 (polarization filter). If we make correspondence with the element on the left of 13,
a and b, e and f, i and j, and m and n), and a two-dimensional image is formed from the output difference between the combined pixels. Therefore, the number of horizontal pixels of the two-dimensional image formed by the image processing device 15 is half of the number of horizontal pixels of the two-dimensional image sensor 14.

The two-dimensional image formed by the image processing device 15 is displayed on an image output device 16 such as a CRT or a liquid crystal display. The image displayed on the image output device 16 is
Even when a two-dimensional image sensor such as this embodiment is used, an image substantially equivalent to an image obtained based on the intensity difference between transmitted light and reflected light of the polarizing beam splitter as described in the related art. Becomes

The above-mentioned half-wave plate 11 and polarizing filter 13 constitute an analyzer whose analyzer angle is variable. This analyzer is composed of two types of analyzers having different analyzer angles by 90 degrees for each adjacent pixel in the two-dimensional image sensor 14. Therefore,
By rotating the half-wave plate 11 constituting the analyzer around the optical axis, the contrast of the differential interference image of the test object 8 formed by the image display device 15 can be adjusted. The contrast of the differential interference image is adjusted by using the polarizing filter 13 without using the half-wave plate 11.
And the two-dimensional image sensor 14 may be simultaneously rotated about the optical axis. As described above, the half-wave plate 11 for adjusting the contrast of the differential interference image and the like constitute a contrast adjuster (or a contrast changer).

The two-dimensional image sensor 14 includes, for example, C
An image sensor such as a CD is used. The pixel interval of the two-dimensional image sensor 14 is preferably １ ／ of the minimum resolution of the objective lens 7 in the horizontal direction, and is 以下 or less in the vertical direction.

In the present embodiment, the image processing device 15 uses the two-dimensional image sensor 1 to obtain a differential differential interference image.
Although the difference between the signals output from two pixels adjacent in the horizontal direction has been determined, the differential differential interference image may be formed by calculating the output difference between two pixels adjacent in the vertical direction. A set of pixels (2 × 2) may be used to determine the sum of the outputs of the pixels that have received light in the same polarization direction, and then determine the difference between them to form a differential differential interference image. Further, the shape of the element of the polarization filter 13 and the pixel of the two-dimensional image sensor 14 corresponding to the element may be a rectangle extending in the horizontal direction or the vertical direction, and a pair may be formed into a square. In this case, since the pair of pixels of the two-dimensional image sensor corresponding to the pixels of the differential differential interference image formed by the image processing device 15 have a square shape, the efficiency of the differential interference image is very high. A good pixel arrangement is obtained.

In this embodiment, the insertion position of the Nomarski prism 6 with respect to the optical axis such that the illumination light becomes circularly polarized just before the quarter-wave plate 10 when a mirror surface is observed as the test object 8. When the optical system is adjusted by appropriately defining the above, the quarter-wave plate 10 can be omitted.

In the above embodiment, the image processing device 15
Has formed a differential differential interference image indicating a change in the phase or reflectance of the test object 8 based on the difference between the output signals of a pair of pixels of the two-dimensional image sensor 14. Alternatively, a device that forms an image based on the sum of output signals of a pair of pixels of the two-dimensional image sensor 14 may be used. In this case, the formed image is a bright-field image representing a difference in reflectance of the test object 8. Further, the image processing apparatus 1
5 may have both a function of forming a differential differential interference image based on a difference between output signals of a pair of pixels and a function of forming a bright-field image based on the sum thereof. A switching device for switching between the two functions may be added.

The reflection type differential interference microscope according to the present invention has been described above. Next, an embodiment (second embodiment) in which the present invention is applied to a transmission type differential interference microscope will be described.

FIG. 3 is a view schematically showing an embodiment of a transmission type differential interference microscope according to the present invention. In the following description, the same elements as those of the reflection type differential interference microscope of FIG. 1 are denoted by the same reference numerals, and a specific description thereof will be omitted.

As shown in FIG. 3, in the transmission type differential interference microscope of the present embodiment, the light emitted from the light source 1 passes through the collimating lens 2 to become parallel light, and then passes through the interference filter 3. Wavelength is selected. The light transmitted through the interference filter 3 becomes linearly polarized light with its vibration direction being limited by the polarizing plate 4, and enters the Nomarski prism 6.
In this embodiment, the half mirror 5 shown in FIG. 1 is not used.

The light split into two by the Nomarski prism 6 is condensed via the objective lens 7 and illuminates the test object 8 arranged on the transparent stage 19 as two illumination lights. In this way, the test object 8 is illuminated by two illumination lights whose vibration planes are orthogonal to each other and whose centers are slightly apart.

The two illuminating lights irradiating the test object 8 pass through the test object 8, pass through the objective lens 7 ', enter the Nomarski prism 6', and are synthesized by the Nomarski prism 6 '. The combined light that has exited the Nomarski prism 6 ′ enters the quarter-wave plate 10. The combined light that has become circularly polarized light via the quarter-wave plate 10 is then turned into a half light.
The light enters the wave plate 11. The half-wave plate 11 is installed so that it can rotate about the optical axis. The light transmitted through the half-wave plate 11 is collected by the lens 12,
2D image sensor 1 transmitted through polarizing filter 13
4.

The light focused on the two-dimensional image sensor 14 is photoelectrically converted for each pixel and sent to the image processing device 15. The image processing device 15 treats the signal received from the two-dimensional image sensor 14 as two sets of two pixels adjacent to each other in the horizontal direction of the two-dimensional image sensor 14 and obtains a difference between outputs of the two adjacent pixels for each set. Thus, a two-dimensional image (differential differential interference image) of the test object 8 is formed. The formed two-dimensional image is displayed on the image output device 16. Note that the modifications and additional functions of the differential interference microscope of the first embodiment can be applied to this embodiment.

Next, a third embodiment according to the present invention will be described. In the following description, components having the same functions as those of the above-described prior art are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 4 is a schematic configuration diagram of a differential interference microscope according to a third embodiment of the present invention. In this differential interference microscope, a laser beam emitted from a laser light source 51 becomes parallel light via a collimator lens 52 and enters a half mirror 53. The laser beam reflected downward by the half mirror 53 is spatially deflected by the two-dimensional scanning device 54 and then enters the Nomarski prism 55. The laser beams split into two by the Nomarski prism 55 are condensed via an objective lens 56 to form two laser spots on a test object 57 arranged on a stage 58. These two laser spots scan the test object 57 two-dimensionally by the two-dimensional deflection action of the two-dimensional scanning device 54.

The two laser beams reflected by the test object 57 pass through the objective lens 56 again, and are combined via the Nomarski prism 55. The combined laser beam combined by the Nomarski prism 55 becomes a parallel beam via the two-dimensional scanning device 54 and enters the half mirror 53. The combined laser beam transmitted through the half mirror 53 is
The light enters the wavelength plate 60 and then enters the 波長 wavelength plate 61. The laser beam passing through the half-wave plate 61 passes through the polarization filter 13 and forms an image on a two-dimensional image sensor 14 having a plurality of pixels. Polarizing filter 13
Is such that only two polarized components of the light that has exited the half-wave plate 11 are selectively transmitted and made incident on the two-dimensional image sensor 14, and an arbitrary pair of adjacent pixels of the two-dimensional image sensor 14 , Respectively, receives light having a polarization component different from that of the other pixel.

The light focused on the two-dimensional image sensor 14 is photoelectrically converted for each pixel and sent to the image processing device 26. The image processing device 26 treats the signal received from the two-dimensional image sensor 14 as a set of two adjacent pixels of the two-dimensional image sensor 14 and obtains a difference between outputs of the two adjacent pixels. Then, a differential interference image is formed based on the scanning position information of the laser spot obtained from the two-dimensional scanning device 54 and the obtained difference, and displayed on the image output device 67.

The structures of the polarizing filter 13 and the two-dimensional image sensor 14 are the same as those described in the first embodiment. However, since the laser spot is operated by the two-dimensional scanning device 54, the object Pixel regions corresponding to all 57 regions are not required. Therefore, the image display device 26 only needs to process signals from pixels on which the laser beam is incident. Also, the laser spot is scanned by the two-dimensional scanning device so that
In the case where the incident area of the laser beam incident on the two-dimensional image sensor 14 is constant or limited to a certain range while irradiating all the areas 7, the size of the pixel area of the two-dimensional image sensor 14 is It is sufficient if the size is large enough to cover the fixed area or the limited range.

The angle of the mirror 59 is adjusted according to the irradiation position of the laser spot scanned by the two-dimensional scanning device 54, so that the incident position of the laser beam incident on the two-dimensional image sensor 14 is the same as the irradiation position of the laser spot. You may make it correspond one-to-one. In this case, the pixel of the two-dimensional image sensor 14 on which the laser beam is incident has a one-to-one correspondence with the position on the test object 57 irradiated with the laser spot, so that the image processing device 26 forms a differential interference image. In doing so, it is not necessary to synchronize the output signal of the pixel with the scanning position of the two-dimensional scanning device 54.
The position of the laser beam incident on the two-dimensional image sensor 14 is adjusted not by adjusting the mirror 59 but by adjusting the position of the mirror 59.
A laser beam deflector may be provided in front of the two-dimensional image sensor 14, and the deflector may adjust the incident position of the laser beam on the two-dimensional image sensor 14 so as to correspond to the scanning position of the laser spot. .

As in the first embodiment, the image processing device 26 may form a bright-field image using the sum of pixel outputs, or may switch between forming a bright-field image and a differential interference image. It may be. In addition, modifications and additional functions of the differential interference microscope of the first embodiment can be applied to this embodiment. Note that the above-mentioned differential interference microscope has great power in inspecting defects such as IC patterns and inspecting foreign substances on masks and wafers used in semiconductor manufacturing equipment.

[0043]

According to the present invention, a high-quality differential interference microscope capable of adjusting the contrast with one two-dimensional image sensor without using a photodetector such as a polarizing beam splitter or two two-dimensional image sensors is provided. Therefore, the burden on the alignment of the constituent members can be reduced, and a small-sized, low-power, low-cost differential interference microscope can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of an embodiment of a differential interference microscope according to the present invention.

FIG. 2 is a diagram schematically showing a partial configuration of a polarizing filter used in a differential interference microscope according to the present invention.

FIG. 3 is a diagram showing a schematic configuration of an embodiment of a transmission type differential interference microscope according to the present invention.

FIG. 4 is a diagram illustrating a schematic configuration of an embodiment of a differential interference microscope using a light beam according to the present invention.

FIG. 5 is a diagram showing a schematic configuration of a conventional differential interference microscope.

[Explanation of symbols]

 Reference Signs List 1 light source 2, 52 collimating lens 3 interference filter 4 polarizing plate 5, 53 half mirror 6, 6 ', 55 Nomarski prism 7, 7', 56 objective lens 8, 57 test object 9, 19, 58 stage 10, 60 1 / 4 wavelength plate 11, 61 1/2 wavelength plate 12 lens 13 polarizing filter 14 two-dimensional image sensor 15, 26 image processing device 16 image output device 21, 51 laser light source 24, 54 two-dimensional scanning device 25, 59 mirror 62 polarization Beam splitter 63, 64 Photodetector 65 Operating circuit 66 Synchronizer 67 Image display

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI G02B 21/36 G02B 21/36

Claims (5)

[Claims] 1. A birefringent prism that splits light from a light source into two lights having different polarization components and emits the light to a test object, and combines the two lights from the test object, and the birefringent prism. A polarizing plate that selects a first polarization component and a second polarization component that intersects with the first polarization component from the light combined in step (a). Receiving the first polarization component on one of the pixels adjacent to each other;
A sensor that receives the second polarization component selected by the polarizing plate at the other of the adjacent pixels; and a differential of the test object based on respective image signals output from the adjacent pixels. A differential interference microscope including an image forming apparatus that forms an interference image. 2. The method according to claim 1, wherein the first polarization component and the second polarization component are polarized in directions orthogonal to each other.
Differential interference microscope described in 1. 3. The apparatus according to claim 1, further comprising a contrast changer arranged between the birefringent prism and the polarizing plate, for changing a contrast of the differential interference image of the test object. Differential interference microscope described in 1. 4. a step of dividing light from a light source into two lights having different polarization components and emitting the light to a test object; and combining the two lights from the test object. Selecting a first polarization component and a second polarization component that intersects the first polarization component from the two lights; and using the selected first polarization component as a sensor having a plurality of pixels. Receiving light at one of a pair of pixels in the other, and receiving the selected second polarization component at the other of the pair of pixels; based on a signal output from each of the pair of pixels, Forming a differential interference image of the test object. 5. The apparatus according to claim 4, wherein the first polarization component and the second polarization component are polarized in directions orthogonal to each other.
2. The image forming method according to 1.,