JPH08288192A - Projection aligner - Google Patents

Abstract

PURPOSE: To compensate distortion without affecting other aberrations in a projection aligner for projecting a pattern on a reticle R to a light-sensitive substrate. CONSTITUTION: The pattern of a reticle R is projected on a wafer W in scanning exposure system via a projection optical system PL. An image distortion compensation body 31 consisting of an optical wedge 12 where an upper surface opposing the reticle R is vertical to a light axis IX and a lower surface is inclined and an optical wedge 13 where a lower surface is inclined vertically to the light axis IX and an upper surface is inclined in parallel to the lower surface of the optical wedge 12 is arranged between the reticle R and the projection optical system PL. By moving the optical wedges 12 and 13 in a direction opposite to the scanning direction being vertical to the light axis IX by an optical member control system 14, the light path length of illumination light IL in the image compensation body 31 is changed to continuously compensate distortion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection exposure apparatus used, for example, in manufacturing a semiconductor integrated circuit, a liquid crystal device or the like in a photolithography process, and more particularly to image forming characteristics such as distortion of a projection image by a projection optical system. The present invention relates to a projection exposure apparatus having a mechanism for correcting

[0002]

2. Description of the Related Art Conventionally, in this type of projection exposure apparatus, a fine pattern of a reticle (or a photomask, etc.) is projected with high resolution onto a wafer (or a glass plate, etc.) coated with a photoresist. In order to project a reticle pattern onto a pattern already formed on a wafer with high overlay accuracy, it is required to always maintain the imaging characteristics of the projection image by the projection optical system with high accuracy. In this case, the ambient pressure around the projection optical system, environmental changes such as temperature, shape change due to absorption of illumination light of the reticle or projection optical system, switching of the reticle illumination method, or when using a so-called phase shift mask, etc. The imaging characteristics may change gradually due to changes in the pattern on the reticle. Here, the switching of the reticle illumination method means switching from a normal illumination method to, for example, an annular illumination method or a modified light source method.

Therefore, conventionally, the amount of these environmental changes and the like are measured, the amount of change in the imaging characteristics is predicted from the measurement result, and the imaging characteristics are corrected so as to cancel the predicted amount of change. Was doing. Further, there are mainly two types of conventional correction targets of the image forming characteristics, that is, the defocus of the projected image and the projection magnification. In order to correct these, for example, regarding defocusing, a target value of the focus position is corrected in a mechanism (autofocus mechanism) that keeps the distance between the projection optical system and the wafer constant. Regarding the correction of the projection magnification, there has been proposed a method of sealing between the lenses inside the projection optical system to change the internal pressure, or a method of moving a part of the lenses of the projection optical system in the optical axis direction. There is.

With respect to this, in recent years, defocusing has occurred as the patterns of semiconductor integrated circuits have become finer and finer.
Not only the projection magnification but also the change of isotropic image distortion (so-called pincushion type or barrel type distortion) cannot be ignored.
As means for correcting the isotropic image distortion, a mechanism for moving the reticle in the optical axis direction of the projection optical system, a mechanism for moving some lenses of the projection optical system in the optical axis direction, an exposure light source (laser) There has been proposed a mechanism for changing the emission wavelength of a light source or the like, or a mechanism for sealing the lenses inside the projection optical system to change the internal pressure.

The conventional means for correcting isotropic image distortion as described above has the following disadvantages. First, isotropic image distortion is different from projection magnification and is a high-order aberration. Therefore, of the correction means, a mechanism for moving a part of the lens of the projection optical system in the optical axis direction and an exposure light source A mechanism that changes the emission wavelength,
Alternatively, if correction is performed using a mechanism that changes the pressure between predetermined lenses inside the projection optical system, other aberrations change and there is the inconvenience that only the isotropic image distortion cannot be corrected independently.
In this case, if the newly generated aberration is corrected by another mechanism, the entire correction mechanism becomes complicated. Further, even if an attempt is made to correct the isotropic image distortion by setting another aberration change within the allowable range, the amount that can be corrected becomes small, and a desired correction amount cannot be obtained. On the other hand, according to the method of moving the reticle in the optical axis direction, it is possible to correct only the isotropic image distortion without affecting other aberrations.

[0006]

As described above, in order to correct the isotropic image distortion, the method of moving the reticle in the optical axis direction can be said to be a simple method. However, recently, there has been an increasing demand for exposing a wider field area while maintaining the imaging characteristics, and in order to meet this demand, the reticle and the wafer are relatively scanned with respect to the projection optical system to perform scanning. Exposure type projection exposure apparatus (slit scan type or step-and-scan type projection exposure apparatus)
Is proposed. In this method, the maximum diameter of the effective exposure field of the projection optical system can be used by illuminating the reticle in a slit shape, and by scanning, the exposure field can be enlarged without being restricted by the optical system in the scanning direction. There are advantages. Further, since only a part of the projection optical system is used, there is an advantage that accuracy such as illuminance uniformity and distortion can be easily obtained. However, in this scanning exposure type projection exposure apparatus, since the reticle and the wafer have to be scanned in high precision in synchronization with each other, the reticle stage is required to have high rigidity.
In order to increase the rigidity of the stage on the reticle side, it is desirable that there is no mechanism for moving the reticle in the optical axis direction. Even in a batch exposure type device such as a stepper, it is desirable that the reticle side stage has high rigidity.

Further, since the reticle is directly driven, the driving error directly affects the image forming characteristic or the overlay accuracy. That is, when the reticle tilts from the plane perpendicular to the optical axis, the image surface tilts and the distortion changes.
In addition, when the reticle is laterally shifted, the positional relationship between the alignment sensor and the image is deviated, and an overlay error occurs. Alternatively, the reticle drive system requires very sophisticated control technology or position measurement technology in order to prevent these errors, resulting in an increase in manufacturing cost.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a projection exposure apparatus which can correct isotropic image distortion without adversely affecting other aberrations and without moving the reticle.

[0009]

A projection exposure apparatus according to the present invention has an illumination light (I) for exposure as shown in FIG.
L), in the projection exposure apparatus that projects the image of the transfer pattern formed on the mask (R) onto the photosensitive substrate (W) via the projection optical system (PL), the mask ( R) and its substrate (W), the light-transmissive substrates (12, 13) having variable thickness, and the optical path length of the illumination light by switching the thickness of the light-transmissive substrates. And an optical path length switching means (14) for changing the optical path length switching means (14), and the light transmitting substrate (12, 1) via the optical path length switching means (14).
The thickness of 3) is changed to adjust the image distortion of the projected image.

In this case, an example of the light-transmissive substrate is one or a plurality of optical wedges (12, 13) each having a continuously changing thickness, and in this case, the optical path length switching means is the same. It is preferably a moving means (14) for moving the optical wedges (12, 13) as a whole or relatively. Further, the projection exposure apparatus scans the substrate in a direction (-X direction) corresponding to the predetermined scanning direction in synchronization with scanning of the mask (R) in a predetermined scanning direction (X direction). Therefore, in the case of a scanning type exposure apparatus that sequentially exposes the pattern of the mask (R) onto the substrate (W), the optical path length switching means (14) has the optical wedge (1).
2, 13) is preferably moved along the predetermined scanning direction.

Another example of the light transmissive substrate is a plurality of light transmissive substrates (101 to 103) having different thicknesses, as shown in FIG. 10, and in this case, the optical path length switching means. Is an exchange means (14A) for exchanging the plurality of light transmissive substrates (101 to 103).

[0012]

According to the projection exposure apparatus of the present invention, the mask (R) and the projection optical system (PL) are changed by changing the thickness of the light transmissive substrates (12, 13) whose thickness can be changed. It is possible to correct the isotropic image distortion without changing the mask (R) and the projection optical system (PL) by changing the optical path length between the two. In addition, a light-transmissive substrate (12, 12 disposed between the mask (R) and the photosensitive substrate (W) without an optical member such as a lens interposed therebetween.
Since the image distortion is corrected only by 13), various aberrations caused by passing through an optical member such as a lens do not occur.
Therefore, isotropic image distortion (so-called pincushion type or barrel type distortion) can be changed independently of other aberrations by using the light transmissive substrates (12, 13).

Further, the light transmissive substrate has an optical wedge (12,
13) and the optical path length switching means is the moving means (14), the optical wedge (1) is moved by the moving means (14).
The optical path length can be continuously changed by moving (2, 13) in the direction in which the thickness continuously changes. Therefore, the image distortion can be continuously corrected. Further, if the change in the thickness is made gentle, the optical path length can be strictly controlled without strictly positioning the optical member.

Particularly, the upper surface of the light transmissive substrate (12) closest to the reticle (R) among the light transmissive substrates (12, 13) and the lower surface of the light transmissive substrate (13) closest to the substrate (W). If both are parallel planes, the light-transmissive substrate (12, 13)
The positioning may be three-dimensionally gradual, and may be gradual so as not to compare with the positioning accuracy when driving the mask (R) or the projection optical system (PL) itself.

When the projection exposure apparatus is a scanning type projection exposure apparatus and the optical wedges (12, 13) are moved along the scanning direction, a short slit-shaped illumination area is used in the scanning direction. The drive amount of the optical wedges (12, 13) is small. Further, in order to correct the isotropic image distortion, it is not necessary to physically drive the mask (R) or the projection optical system (PL) itself, so that a high rigidity is required especially for the entire apparatus. It is effective for a projection exposure apparatus.

Further, the light transmissive substrate comprises a plurality of light transmissive substrates (101 to 103), and the optical path length switching means (14).
Is the exchange means (14A), the correction is discontinuous, but the device configuration is simple. Also in this case, if the light transmissive substrates (101 to 103) to be replaced are parallel planes,
As described above, the positioning accuracy of the light transmissive substrates (101 to 103) may be rough. Therefore, the structure of the device can be further simplified.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the projection exposure apparatus according to the present invention will be described below with reference to FIGS. The present invention can be applied to both a batch exposure type (step and repeat system etc.) and a scanning exposure type (step and scan system etc.). Hereinafter, a case where the present invention is applied to a scanning exposure type in which the effects of the present invention can be more exerted will be described. However, the application to the collective exposure type is almost the same. In FIG. 1, the Z axis is taken parallel to the optical axis IX of the projection optical system PL, and the X axis is parallel to the paper surface of FIG. 1 in the plane perpendicular to the optical axis IX and is perpendicular to the paper surface of FIG. Take the Y axis.

FIG. 1 shows a schematic structure of the projection exposure apparatus of this example. In FIG. 1, the light source 1 is, for example, Kr.
An excimer laser light source such as an F excimer laser or an ArF excimer laser, a high wave generator such as a copper vapor laser or a YAG laser, or an ultrahigh pressure mercury lamp is used. When the light source 1 is an ultra-high pressure mercury lamp, the light source 1 emits illumination light IL composed of ultraviolet bright lines (g line, i line, etc.). The illumination light IL is incident on the illuminance homogenizing optical system 2 including a collimator lens, a fly-eye lens, etc., and after being converted into a luminous flux having a substantially uniform illuminance distribution, it is guided to a turret 3 for switching illumination conditions.

FIG. 2 is a front view of the turret 3 of FIG. 1. In this turret 3, diaphragms 41 to 44 are arranged at 90 ° intervals, and the turret 3 is rotated to form diaphragms 41 to 44. It is possible to change the light intensity distribution on the Fourier transform surface (pupil surface) of the projection optical system PL by switching between. This method is one of the techniques for improving the resolution of the projection optical system, and the optimum one is selected from these diaphragms 41 to 44 depending on the pattern to be exposed.

In the example of FIG. 2, the circular diaphragm 41 is a normal aperture diaphragm (σ diaphragm), and the ring-shaped diaphragm 42 is a diaphragm for performing ring-shaped illumination. Further, the small circular diaphragm 43 is for narrowing the angle of the light beam, and has a small coherence factor (σ value) in a normal illumination system (for example, σ
The value is about 0.1 to 0.4). A diaphragm 44 composed of four eccentric circular (or cross-shaped light-shielding portion) openings is a diaphragm for a plurality of inclined illuminations (deformed light sources) and is generally used for exposing a line and space pattern with high resolution. It is what is done. The turret 3 is used while being sequentially changed to the optimum one according to the pattern of the reticle R.

The illumination light IL which has passed through the predetermined stop of the turret 3 reaches the beam splitter 4 having photoelectric sensors 24 and 25 on both sides thereof. Beam splitter 4
Means that almost all the light fluxes are transmitted but a part of the light fluxes is reflected. The light flux coming from the turret 3 side and reflected by the beam splitter 4 enters the photoelectric sensor 24 and comes from the wafer W side. The light reflected by the beam splitter 4 enters the photoelectric sensor 25. The detection signals of the photoelectric sensors 24 and 25 are used to calculate the aberration change of the projection optical system PL as described later. The illumination light IL further passes through an illumination optical system 5 including a relay lens, a field stop, a condenser lens, etc., and is reflected by a dichroic mirror 6 to illuminate a reticle R on which a semiconductor circuit pattern or the like is drawn. The reticle R is vacuum-sucked on the reticle stage RST, and the reticle stage RST is a plane perpendicular to the optical axis (which coincides with the optical axis of the projection optical system PL) IX bent by the dichroic mirror 6 of the illumination optical system 5. The reticle R is positioned by finely moving two-dimensionally in the (XY plane).

Further, the reticle stage RST is provided with a reticle drive unit (not shown) composed of a linear motor or the like.
It is possible to move at a predetermined scanning speed in the X direction (scanning direction). Reticle stage RST has a moving stroke that allows the entire surface of reticle R to cross at least the optical axis IL of the illumination optical system. A movable mirror 8 that reflects the laser beam from the interferometer 9 is fixed to the end of the reticle stage RST.
The position of T in the scanning direction is set by the interferometer 9 to, for example, 0.0.
It is constantly detected with a resolution of about 1 μm. Position information of the reticle stage RST from the interferometer 9 is sent to the stage control system 30A, and the stage control system 30A drives the reticle stage RST via a reticle drive unit (not shown) based on the position information of the reticle stage RST. To do.
Since the initial position of the reticle stage RST is determined so that the reticle R is accurately positioned at a predetermined reference position by a reticle alignment system (not shown), the position of the movable mirror 8 can be simply measured by the interferometer 9. The position of the reticle R is measured with sufficiently high accuracy.

Now, the illumination light IL which has passed through the reticle R
Enters the image distortion correction body 31 formed of a pair of optical wedges 12 and 13 having the same shape. The optical wedges 12 and 13 are both formed to have the same size as the reticle R, and the illumination light IL transmitted through the reticle R can be entirely passed through the transmission region of the image distortion correction body 31. This image distortion correction body 3
The optical wedges 12 and 13 of No. 1 are driven in the scanning direction (X direction) to correct the distortion.
The image distortion correction body 31 will be described in detail later.

The illumination light IL transmitted through the image distortion correction body 31 is
Next, it is incident on the projection optical system PL which is telecentric on both sides,
The projection optical system PL forms a projection image obtained by reducing the circuit pattern of the reticle R with a reduction magnification β (for example, ⅕ or ¼) on the wafer W whose surface is coated with a photoresist (photosensitive material). . The optical axis I is provided near the pupil plane (Fourier transform plane for the reticle R) of the projection optical system PL.
An optical filter for blocking a light beam near X, that is, a central light blocking type pupil filter PF is detachably installed. The pupil filter PF improves resolution and depth of focus particularly when exposing a contact hole pattern. The main control system 30 controls the attachment / detachment of the pupil filter PF via the attachment / detachment device 30B. Further, the projection optical system PL of the present embodiment is equipped with mechanisms (15 to 22) for correcting the imaging characteristics, which will be described later.

FIG. 3 is a perspective view showing a scanning state of the reticle R and the wafer W shown in FIG. In FIG. 3, the projection optical system PL is expressed as if it is non-telecentric for convenience, but in reality the projection optical system PL is telecentric on both sides (or at least the wafer side). In the projection exposure apparatus of this embodiment, as shown in FIG. 3, a direction (Y) perpendicular to the scanning direction (X direction) of the reticle R is used.
The reticle R is illuminated in a rectangular (slit-shaped) illumination area IAR having a longitudinal direction (direction), and the reticle R is scanned at the speed V _R in the −X direction (or + X direction) during exposure. Illumination area IAR (center is almost coincident with optical axis IX)
The inner pattern is projected onto the wafer W via the projection optical system PL to form a slit-shaped projection area IA.

[0026] Since the wafer W is to the reticle R in inverted imaging relationship, the wafer W to the direction of the velocity V _R in the opposite + X direction (or -X direction), in synchronism with the reticle R, the speed V
Scanned in _W, the pattern of the entire surface on the reticle R in the shot area SA on the wafer W is sequentially exposed. The scanning speed ratio (V _W / V _R ) is exactly the same as the reduction ratio β of the projection optical system PL, and the pattern of the pattern area PA of the reticle R is accurate on the shot area SA on the wafer W. Is reduced and transferred to. The width of the illumination area IAR in the longitudinal direction is set to be wider than the pattern area PA on the reticle R and smaller than the maximum width of the light shielding area ST, and the entire area of the pattern area PA is illuminated by scanning the reticle R. It has become so.

Returning to the explanation of FIG. 1, the wafer W is vacuum-sucked on the wafer holder 7, and the wafer holder 7 is held on the wafer stage WST. The wafer holder 7 can be tilted in an arbitrary direction with respect to the best imaging plane of the projection optical system PL by a driving unit (not shown) and can be finely moved in the optical axis IX direction (Z direction). Further, the wafer holder 7 can also rotate about the optical axis IX. On the other hand, the wafer stage W
ST is a direction perpendicular to the scan direction (Y direction) so that the ST can be moved not only in the scan direction (X direction) described above but also to any shot area in a plurality of shot areas at any time.
And a step of repeating the operation of scanning and exposing each shot area on the wafer W and the operation of moving to the exposure start position of the next shot area.
And-scan operation is performed. A wafer stage drive unit (not shown) such as a motor moves the wafer stage WST to X and Y.
Drive in the direction. A movable mirror 10 that reflects the laser beam from the interferometer 11 is fixed to the end of the wafer stage WST, and the position of the wafer stage WST in the XY plane is constantly fixed by the interferometer 11 at a resolution of, for example, about 0.01 μm. It has been detected. The position information (or speed information) of wafer stage WST is sent to stage control system 30A, and stage control system 30A controls the wafer stage drive unit based on this position information (or speed information).

Further, in the apparatus shown in FIG. 1, an irradiation optical system 26 that supplies an image forming light beam for forming a pinhole image or a slit image toward the exposure surface of the wafer W in an oblique direction with respect to the optical axis IX. And a light receiving optical system 27 that receives the reflected light flux of the image-forming light flux on the exposed surface of the wafer W through the slit.
An oblique incidence type wafer position detection system (focus position detection system) consisting of and is fixed to a support portion (not shown) that supports the projection optical system PL. A more detailed structure of this wafer position detection system is disclosed in, for example, Japanese Patent Laid-Open No. 60-168112. The wafer position detection system detects the positional deviation of the exposure surface of the wafer in the Z direction from the best image plane of the projection optical system PL, and moves the wafer holder 7 in the Z direction so that the wafer W and the projection optical system PL maintain a predetermined distance. Used to drive to. Wafer position information from the wafer position detection system is sent to the stage control system 30A via the main control system 30. The stage control system 30A drives the wafer holder 7 in the Z direction based on this wafer position information.

In this embodiment, a parallel flat plate glass (plane, not shown) provided in advance inside the light receiving optical system 27 so that the best image forming surface (image forming surface) of the projection optical system PL becomes the zero point reference. (Parallel) angle is adjusted and the wafer position detection system is calibrated. Further, for example, a horizontal position detection system as disclosed in Japanese Patent Laid-Open No. 58-113706, which irradiates a parallel light flux on the surface to be detected and detects the lateral shift amount of the condensing point of the reflected light,
Alternatively, the wafer W is configured by configuring the wafer position detection system so as to detect the focal positions at arbitrary plural positions in the image field of the projection optical system PL (for example, projecting plural slit images in the image field). The inclination of the upper predetermined area with respect to the image plane may be detected. In this case, leveling is performed by adjusting the inclination angle of the wafer holder 7.

Further, a photoelectric sensor 28 is installed on the wafer stage WST, and an environment sensor 29 for measuring atmospheric pressure, temperature, humidity and the like is provided near the projection optical system PL, and each detection signal is detected. It is used to calculate the change in the imaging characteristics of the projection optical system PL. Details will be described later. Next, the image forming characteristic correction mechanism including the isotropic image distortion correction mechanism in the present embodiment will be described. As described above, the types of imaging characteristics to be corrected tend to increase as the exposure line width decreases. Therefore, also in this embodiment,
(A) In addition to isotropic image distortion (hereinafter also referred to as "distortion"), (b) projection magnification, (c) defocus,
And (d) an example of correcting four types of field curvature will be described.

First, in the distortion of (a), the image distortion correction body 31 arranged between the reticle R and the projection optical system PL is driven to change the optical path length between the reticle R and the projection optical system PL. By changing. In this embodiment, the projection optical system is an optical system in which the magnification component does not change with the change of the optical path length even if the mask side is telecentric or not telecentric.

The image distortion correction body 31 is composed of an optical wedge 12 close to the reticle R, and an optical wedge 13 arranged so as to overlap with the lower portion of the optical wedge 12. The optical wedges 12 and 13 are both made of wedge-shaped flat glass having substantially the same size with different thicknesses at both ends.
Both the upper surface of 2 and the lower surface of the optical wedge 13 are formed by planes perpendicular to the optical axis IX. Further, the lower surface of the optical wedge 12 and the upper surface of the optical wedge 13 are parallel to each other and are inclined with respect to a plane perpendicular to the optical axis IX as shown in FIG. By adopting such a configuration, there is an advantage that the distance between the image distortion correction body 31 and the reticle R and the projection optical system PL is not so strictly required. That is, the image distortion correction body 3
Since 1 is equivalent to a plane parallel plate, it has almost no effect on the projected image unless it is greatly inclined.

The optical wedges 12 and 13 are respectively fixed to drive guides (not shown), and can be driven in the scanning direction (X direction) shown by the arrow in FIG. Therefore, the upper surface of the optical wedge 12 and the lower surface of the optical wedge 13 are both on the optical axis IX.
It is driven in the X direction while being perpendicular to the above, and cannot be moved in any direction other than the X direction by the guide. The respective positions of the optical wedges 12 and 13 are measured by a position sensor such as a linear encoder or a potentiometer,
According to the target value set by the main control system 30, the optical member control system 14 is positioned by a drive mechanism including a motor or the like.

The size of the optical wedges 12 and 13, the interval between the optical wedges 12 and 13, and the optical wedge 1
The arrangement between the reticle R 2 and the reticle R and the projection optical system PL is not particularly limited, but the sizes of the optical wedges 12 and 13 are such that the optical wedges 12 and 13 do not bend and can be easily driven in terms of weight. It suffices if it is formed in a size of the order. However, the optical wedges 12 and 13 are preferably arranged near the reticle R in order to reduce the influence on aberrations other than distortion.

Next, the principle of controlling the isotropic image distortion by the image distortion corrector 31 will be described in detail with reference to FIGS. FIG. 4 is a diagram schematically illustrating the correction principle of isotropic image distortion in this example. FIG. 4A shows the optical wedge 12,
FIG. 4B shows a state in which the optical wedges 12 and 13 are displaced in the −X direction and the X direction, respectively. Further, FIG. 5 shows an example of isotropic image distortion in the present example, and FIG. 6 shows an example of an actual image distortion correction result.

If the projection optical system PL is telecentric on both sides, the principal rays traveling from the reticle R to the projection optical system PL should be all parallel to the optical axis IX, but in reality, aberrations that cannot be eliminated (pupil aberration, etc.). ), The chief ray between the reticle R and the projection optical system PL is slightly inclined at some image heights. Some projection optics are not bilateral telecentric in the strict sense (the aperture stop is at the image space focus), but are designed so that the magnification change components are well offset. However, the changes in distortion are the same, and the same effect can be obtained for such a projection optical system.

In these projection optical systems, normally, FIG.
As shown in FIG. 4B, the principal ray NL0 passing through the central portion and the periphery of the effective illumination area (the area conjugate with the effective exposure field of the projection optical system) of the pattern surface of the reticle R,
Since NL1 and NL2 are adjusted so as to be substantially parallel to the optical axis IX (see FIG. 3), the chief rays IL1 and IL2 in the middle of the left and right of the effective illumination area are inclined with respect to the normal optical axis IX. For this reason, the uppermost lens 15 of the projection optical system PL of the principal rays IL1 and IL2 that have passed through the optical wedges 12 and 13.
The positions of incidence on are shifted outward by Δa ₁ and Δa ₂ , respectively. In this case, the chief ray NL0 parallel to the optical axis IX,
NL1 and NL2 are also slightly shifted in the X direction. Specifically,
The inclination angles Δθ of the lower and upper surfaces of the optical wedges 12 and 13 with respect to the horizontal plane, and the distance from the lower surface of the optical wedge 12 to the upper surface of the optical wedge 13 (this is referred to as g (x)).
The principal rays NL0, NL1, and NL2 are laterally shifted in the X direction by the same amount from the initial state in accordance with the change amount of the shift amount, but when the change amount of the tilt angle Δθ and the interval g (x) is small, the shift amount is small. is there. However, depending on this shift amount,
For example, when the off-axis type alignment system is used, the so-called baseline, which is the offset amount between the detection center and the center of the exposure field, changes, and the alignment accuracy deteriorates. Therefore, the distance g (x) is obtained from the driving amount of the optical wedges 12 and 13 in the X direction, and the chief rays NL0, NL1, NL are calculated from the change amount of the distance g (x) and the inclination angle Δθ.
It is desirable to obtain the shift amount from the initial state of 2 and correct the baseline with this shift amount. Next, it is assumed that the position of the chief ray incident on the uppermost lens 15 of the projection optical system PL is approximately shifted in the X direction only at the intermediate image height.
Only the chief rays IL1 and IL2 will be described below.

As is clear from FIG. 3, the actual illumination area IAR is short in the scanning direction (X direction), so the amount of distortion and correction in the X direction within the illumination area IAR may be small. On the other hand, since the illumination area IAR is long in the non-scanning direction (Y direction), the isotropic distortion correction mechanism as in this example is particularly effective in correcting distortion in the non-scanning direction (Y direction). . In addition, the following explanation is X
Although it is performed in the direction, since the effect is isotropic, the distortion in the Y direction is the same.

In FIG. 4A, the optical wedges 12, 1
Assuming that the refractive index of 3 is n, and the distance between the optical wedges 12 and 13 is g ₀ , the optical wedge 1 starts from the upper surface of the optical wedge 12.
The optical path length to the lower surface of 3 is as follows. Initial value of optical path length = (L ₀ −g ₀ ) n + g ₀ Next, the optical wedges 12 and 13 are respectively moved to the left and right, and the points P and Q in FIG. 4A are respectively −x in the X direction.
And a state of being moved by x is shown in FIG. This Figure 4
In (b), the distance between the optical wedges 12 and 13 is g
Assuming that (x), the following relationship is established assuming that the inclination angle Δθ is small.

G (x) = g ₀ + 2xΔθ Therefore, the optical path length from the upper surface of the optical wedge 12 to the lower surface of the optical wedge 13 is as follows. Optical path length = (L ₀ −g (x)) n + g (x) = (L ₀ −g ₀ ) n + g ₀ −2x · Δθ (n−1)

Here, the refractive index n of the optical wedges 12 and 13
Is larger than 1 (for example, about 1.5), the principal rays IL1 and IL2 in the state of FIG.
The optical path length that passes through becomes shorter than in the state of (a). Therefore, the shift amounts Δa ₃ and Δa ₄ of the principal rays IL1 and IL2 incident on the uppermost lens 15 of the projection optical system PL are smaller than the shift amounts Δa ₁ and Δa ₂ of FIG. 4A, respectively. Since the above equation of the optical path length is the same in the XY plane, the shift amount of the light beam separated in the non-scanning direction (Y direction) with respect to the optical axis IX also changes.

Here, the distortion curves in the states of FIGS. 4A and 4B are referred to as curves a and curves b of FIG. 5, respectively. In FIG. 5, the vertical axis is a variable corresponding to the image height h in the Y direction, that is, the illumination light passage position on the reticle side (the distance from the optical axis in the Y direction), and the horizontal axis is the image height h. Shows the distortion ΔY in the Y direction. Since the distortion is isotropic, the distortion ΔX in the X direction at the position where the image height in the X direction is h is the same as in FIG. In such a case, the distortion curve should be in the middle of the curve a and the curve b in FIG. 5, that is, the positions of the optical wedges 12 and 13 should be almost the same as those in FIGS. 4 (a) and 4 (b). By setting to an intermediate state,
The distortion in the Y and X directions of the projected image is almost zero.
Can be However, since only the normal distortion does not change and the magnification component also changes, the distortion curve changes as shown by the curve c in FIG. 6 when adjusted to an intermediate state between the curve a and the curve b in FIG. That is,
A certain magnification error may remain. However, in the present embodiment, the magnification component and the distortion component are corrected by independent correction mechanisms, so that good correction can be performed.

Next, other correction items ((b) projection magnification,
The correction mechanism for (C) defocus and (D) field curvature will be briefly described. Projection magnification of (b) and (d)
For the correction of the field curvature of, the uppermost lens 15 and the next lens 16 among the lenses that form the projection optical system PL.
In this embodiment, a method of driving the optical axis in the optical axis direction is adopted.
In FIG. 1, the uppermost lens 15 is fixed to a holder 18, and the lens 16 is fixed to a holder 19. The holder 18 and the holder 19 are connected via a drive element 20 which is expandable and contractible. For example, a piezo element is used as the drive element 20, and about 2 to 4 drive elements 20 are arranged on the circumference. Further, the holder 19 is connected to the lens barrel main body of the projection optical system PL via a drive element 21. The drive controller 22 drives the drive elements 20 and 21 according to a command from the main control system 30. Usually, the drive element 20,
The expansion / contraction amount of 21 is feedback-controlled by a position sensor (not shown). Top lens 15 and next lens 16
The projection magnification and the curvature of field are changed by the movement in the optical axis direction. When it is desired to obtain desired projection magnification and field curvature characteristics, the driving amount of each of the uppermost lens 15 and the lens 16 is determined by solving a binary simultaneous equation relating to these characteristics. Further, regarding the correction of defocus in (c), the angle of the parallel plate glass in the light receiving optical system 27 of the wafer position detection system may be adjusted to position the wafer W at a desired position.

As described above, the distortion correction of (a) depends on the positions of the optical wedges 12 and 13, and the projection magnification of (b) and the field curvature of (d) are corrected by the uppermost lens 15,
The subsequent driving of the lens 16 and the correction of the defocus of (c) can be performed by the offset adjustment of the light receiving optical system 27. All can be corrected by correcting the defocus generated by correcting (a), (b), and (d) in the light receiving optical system 27 in (c). (B)
Various correction methods (d) have been devised in addition to this embodiment, and any method may be used, or may be omitted if not necessary. As another method, the projection optical system PL
There is a method of changing the air pressure inside the predetermined lens interval, or a method of changing the wavelength of the light source 1.

Next, how to determine the target value for the above-mentioned correction means, that is, the means for detecting the change amount of the correction target will be described. Since the detection means for each correction target are almost the same, the distortion of (a) will be described as an example. Distortion typically changes due to (e) changes in atmospheric pressure, (f) changes in illumination conditions, (g) absorption of illumination light in the projection optical system, and (h) absorption of illumination light in the reticle. In addition to this, when a plurality of exposure apparatuses are mixed and used, the distortion of the exposure apparatus to be used may be changed so as to match the distortion of the exposure apparatus used for exposing the front layer of the wafer.

First, with respect to the change in atmospheric pressure (e), the change in atmospheric pressure is measured by the environment sensor 29, and the measurement result is sent to the main control system 30. Usually, the change in atmospheric pressure and the change in distortion are in a proportional relationship, and therefore the amount of change in distortion can be calculated from the change in atmospheric pressure from the proportional constant obtained in advance by optical simulation, experiment, or the like. In addition to this, with respect to temperature, humidity, etc., the amount of change in distortion can be similarly calculated.

Next, regarding the change of the illumination condition of (f),
The projection optical system PL of the light flux from the reticle R depending on the illumination condition.
Since the optical path inside is different, distortion is generated by being affected by the aberration remaining in the projection optical system PL. On the other hand, since the illumination condition can be known by notifying the main control system 30 of the position of the turret 3, this can also be obtained from the amount of distortion change obtained in advance by experiments or the like. Other conditions for changing the optical path inside the projection optical system PL include the fineness of the pattern of the reticle R, the difference in the angle of the diffracted light depending on the presence or absence of a phase shifter, or the diaphragm (NA diaphragm) of the pupil plane of the projection optical system PL. )
Or there is a pupil filter PF on the pupil plane. Regarding these, similarly, the relationship with the distortion change may be obtained in advance. As another method,
A method of directly measuring the light intensity distribution on the pupil plane of the projection optical system PL is also conceivable. This is a method in which the relationship between the pupil surface light intensity distribution and the distortion change is obtained in advance and the distortion is obtained by comparing the obtained relation with the actual measurement value. As a method of measuring the light intensity distribution on the pupil plane, a method of inserting a sensor into the pupil plane, a method of opening and closing the diaphragm of the pupil plane while measuring the light quantity with the sensor on the image plane, and the like can be considered.

Next, when correcting the illumination light absorption of the projection optical system of (g), the photoelectric sensor 28 on the wafer stage WST determines the transmittance of the reticle R, and the photoelectric sensor 24 determines the light of the light source 1. By calculating the intensity, the amount of light energy incident on the projection optical system PL is calculated. Further, the light energy reflected from the wafer W and incident on the projection optical system PL again can be measured by the photoelectric sensor 25. Then, if the change characteristic between the incident light energy and the distortion is also obtained in advance by experiments and stored in the form of a differential equation or the like, the amount of distortion due to the absorption of the illumination light can be obtained by calculation.

Next, regarding the illumination light absorption of the reticle in (H), the transmittance of the reticle R, that is, the pattern density of the reticle R can be obtained by the photoelectric sensor 28 on the wafer stage WST, as in (G). , Photoelectric sensor 24
The intensity of light incident on the reticle R can be obtained. Reticle R
Since the illumination light absorption occurs in the pattern portion, not in the transmission portion, if the pattern density and the light absorption rate of the pattern are known, the amount of heat absorbed by the reticle R can be obtained. The light absorptance of the pattern is determined by the material of the pattern, so it may be input in advance. Further, similarly to (g), the change characteristic of the distortion with respect to the absorbed light energy may be obtained in advance by experiments or the like and stored in the form of a differential equation or the like. As described above, the distortion amount generated by (e), (f), (t), and (h) can be obtained. Therefore, the amount of distortion that must be corrected can be obtained by the sum of (e) to (h).

In the above method, the factors that change the distortion are measured and the amount of distortion change is calculated, but a method of directly measuring the distortion is also conceivable. This will be described with reference to FIG. Figure 7
7A shows a mark on the reticle R when measuring the image distortion, and in FIG. 7A, a mark MK for position measurement is provided outside the pattern area PA of the reticle R to directly measure the distortion. A plurality of marks are drawn, only the mark MK is illuminated in the illumination area IAR, and the position of the image of the mark is measured and obtained by the photoelectric sensor provided on wafer stage WST. As the photoelectric sensor on wafer stage WST, a two-dimensional or one-dimensional image pickup device such as CCD can be used. In this case, the image pickup device measures the position of the image of mark MK by image processing. Also known is a method of using a slit and a light receiving element for receiving the image of the mark MK through the slit as the photoelectric sensor, and obtaining the relative position between the slit position and the mark position from the signal of the light receiving element. Has been. Note that the mark MK in FIG.
What can be measured with is the distortion in the Y direction, but the distortion in the X direction is also the same. Since these methods cannot be frequently performed because the measurement takes time, it is more effective to use the method of the above calculation together with the method of correcting the calculation error.

FIG. 7B shows the mark MK of the reticle R.
Shows an example of the method without using. The mark MK of the reticle R has a positional error at the time of drawing, which differs for each reticle, so that accurate distortion measurement cannot be performed. Therefore, in the example of FIG. 7B, a mark plate MKP is provided near the reticle R, and a plurality of marks MKA are formed on the mark plate MKP. The distance between the plurality of marks MKA may be strictly measured in advance, and the mark plate MKP may be provided in a run-up area for scanning the reticle R at a constant speed during exposure, so that a new place is particularly required. do not do.

Since the amount of change in distortion is obtained as described above, the positions of the optical wedges 12 and 13 may be changed so as to cancel out the amount of change in distortion.
Although (a) distortion and (b) projection magnification are independently corrected in this example, when the projection optical system PL is not completely telecentric on the reticle R side, the optical wedge 1
The movement of 2 and 13 causes the magnification component and the distortion component to change simultaneously. Also, the lens 1 of the projection optical system PL
Although the magnification component and the distortion component also change at the same time by driving 5 and 16, if the component ratios are different, it is possible to independently correct the optimum driving amount by the simultaneous equations. Therefore, the method of this example can be applied to a projection optical system that is not both-side telecentric.

As described above, according to the projection exposure apparatus of this example, the optical wedges 12 and 13 of the image distortion correction body 31 whose thickness continuously changes in the optical axis IX direction are moved in the direction perpendicular to the optical axis IX direction. By doing so, the thickness of the image distortion correction body 31 through which the chief ray passes changes, and the optical path length between the reticle R and the projection optical system PL can be changed. Further, since the image distortion correction body 31 is arranged between the reticle R and the projection optical system PL, and no other optical member (lens) or the like is interposed, various aberrations due to these are not generated. Therefore, the aberration caused by the change in the optical path length due to the use of the image distortion correction body 31 is smaller than the aberration when changing the distance between other optical members. Thereby, isotropic image distortion (so-called pincushion type or barrel type distortion) can be changed independently of other aberrations. In this example, the optical wedges 12 and 13 are moved in the scanning direction (X direction), and the illumination area IAR of the reticle R is short in the scanning direction.
There is an advantage that 13 can be miniaturized. Further, although a lateral shift in the scanning direction of the image usually occurs due to a change in the thickness of the optical wedges 12 and 13 in the scanning direction, in the scanning type exposure apparatus like this example, a slit-shaped illumination area short in the scanning direction is formed. Since it is used, the influence of the change in the thickness of the optical wedges 12 and 13 in the scanning direction can be reduced.

Further, according to the method of this embodiment, it is not necessary to physically drive the reticle R and the projection optical system PL itself.
Even in the scanning type exposure apparatus as in this example, there is no inconvenience that the rigidity of the apparatus is lowered. In addition, the image distortion correction body 31
Uses optical wedges 12 and 13 whose thickness changes continuously, it is possible to continuously change the thickness of the portion through which the illumination light passes by moving in the scanning direction where the thickness changes. You can Therefore, the image distortion can be continuously corrected. Further, if the change in the thickness (inclination angle Δθ) is made gentle, the thickness of the image distortion correction body 31 can be strictly controlled without strictly positioning the optical wedges 12 and 13.
Particularly, as the image distortion correction body 31, two optical wedges 12 and 13 whose thickness continuously changes are used.
Since the upper surface of the optical wedge 13 facing the reticle R and the lower surface of the optical wedge 13 facing the projection optical system PL are both parallel planes, the positioning of the optical wedges 12 and 13 does not require three-dimensional accuracy. . Therefore, compared with the positioning accuracy when driving the reticle R or the projection optical system PL itself, the positioning accuracy can be far gentler, and the image distortion correction can be realized at low cost and without worrying about the influence on the optical performance.

The inclination angle Δθ of the optical wedges 12 and 13
Is large and the moving distance in the scanning direction is large, the thickness of the illumination light IL passing through changes greatly with the movement,
Spherical aberration occurs. Therefore, it is necessary to set the inclination angle Δθ and the moving distance of the optical wedges 12 and 13 within a range where spherical aberration does not occur. In this case, an optical wedge for spherical aberration correction may be provided between the projection optical system PL and the wafer W to correct spherical aberration.

In this example, the optical wedges 12 and 13 are moved in the direction perpendicular to the optical axis IX (X direction), but in this case, the gap between the optical wedges 12 and 13 changes. . When the inclination angle Δθ is large and the gap is largely changed, the lateral displacement of the image exceeds the allowable value and becomes large as described above. So, for example, optical wedge 1
It is desirable to provide a moving mechanism that moves the optical gaps 12 and 13 with a constant gap (that is, moves the optical wedges 12 and 13 relative to each other substantially along the slope). Or
As described above, the lateral shift amount of the image may be calculated, and the lateral shift amount may be added as an offset to the baseline in alignment.

Next, modified examples of the image distortion correction body used in the projection exposure apparatus of the present invention will be described with reference to FIGS. First, the first modification will be described with reference to FIG. In this example, only one of the optical members corresponding to the optical wedges 12 and 13 in FIG. 1 is driven. FIG. 8A shows the configuration of the image distortion correction body of this example. In FIG. 8A, the image distortion correction body 81 has an optical wedge 82 close to the reticle R and an optical wedge close to the projection optical system PL. The optical wedge 83 is composed of 83 and moves relative to the fixed optical wedge 82. In the case of this example, both optical wedges 82 and 83 are not of the same size, and in order to give the same change in optical path length to the principal ray symmetrically separated from the optical axis IX, as in the embodiment of FIG. The optical wedge 83 is formed large. Further, similarly to the embodiment of FIG. 1, the upper surface of the optical wedge 82 and the lower surface of the optical wedge 83 are formed by a plane perpendicular to the optical axis IX, and the optical wedge 83 is controlled by the main control system 30 to the optical member control system 14. By driving in the scanning direction (X direction) via the, the distance g ₁ between the optical wedges 82 and 83 is changed, and as a result, the optical path length is changed to correct the distortion. Other configurations are similar to those of the embodiment of FIG.

Although the upper surface of the optical wedge 82 is a plane perpendicular to the optical axis IX, it is anisotropic by polishing the upper surface into an irregular wavy shape as shown by the dotted line in FIG. 8A. It can be used as a correction member for irregular distortion. Further, as shown in FIG. 8B, the surface of one of the optical wedges facing the projection optical system may be formed by a lens having a curvature. In FIG. 8B, the image distortion correction body 84 is composed of an optical wedge 85 facing the reticle R and an optical member 86 facing the projection optical system. The optical member 86 has an upper surface having an inclined surface parallel to the lower surface of the optical wedge 85, but the lower surface facing the projection optical system PL is formed as a lens having a curvature.
In this case, it is necessary to make the optical path lengths of the chief rays passing through the image distortion correction body 84 the same, and the distances d ₁ and d ₂ between the upper surface of the optical wedge 85 and the left and right ends of the lower surface of the optical member 86. Are formed to be the same. In this example, the distortion is corrected by moving the optical wedge 85 relative to the fixed optical member 86 to change the gap g _{2 and} thus the optical path length.

The image distortion correction body shown in FIGS. 8A and 8B is an optical wedge 83, 8 driven relative to the embodiment of FIG.
Although the length and the driving amount of 5 are large, there is an advantage that only one driving unit is required and the positioning accuracy is half, and an optical wedge or an optical member that is not driven can be used for other purposes. Next, a second modification of the image distortion correction body will be described with reference to FIG. In this example, the image distortion correction body is configured by only one optical wedge.

FIG. 9 shows the structure of the image distortion correction body of this example.
In FIG. 9, the image distortion correction body 91 is in the scanning direction (X direction).
It consists of one long optical wedge. A surface of the image distortion correction body 91 facing the projection optical system PL is formed perpendicular to the optical axis IX, and a surface of the opposite side facing the reticle R is formed with an inclination angle. It is necessary that this inclination angle is such that the difference in thickness in the scanning direction within the illumination area IAR in FIG. 3 can be ignored and that the inclined surface does not adversely affect the imaging characteristics and the like. According to this method, the drive distance of the image distortion correction body 91 becomes long, but there is an advantage that only one drive unit is required.

Next, a third modification of the image distortion correction body will be described with reference to FIG. In this example, the optical wedge is not used as the image distortion correction body, and the distortion is corrected by using a plurality of parallel flat plates having different thicknesses. Figure 10
Shows a diagram for explaining the image distortion correction body of the present example. In FIG. 10, the image distortion correction body 104 is composed of three optical members 101 to 1 made of parallel plate glasses having different thicknesses.
It is composed of 03. These optical members 101 to 1
03 is replaced by the optical member control system 14A as necessary to change the optical path length to correct the distortion. According to the method of this example, continuous correction cannot be performed and the number of optical members increases, but since the optical members do not have to be processed into an inclined surface and the positioning accuracy can be rough, the total cost can be suppressed. There are advantages.

Although the above-described embodiment applies the present invention to a scanning exposure type projection exposure apparatus, the present invention corrects isotropic image distortion in a collective exposure type projection exposure apparatus such as a stepper. It can also be applied in cases. As described above, when the pair of optical wedges 12 and 13 of FIG.
It is desirable to set the relative movement direction of the optical wedges 12 and 13 along the short side direction of the rectangular pattern area on the reticle. This is because the optical wedges 12 and 13 can be downsized.

As described above, the present invention is not limited to the above-mentioned embodiments, and various configurations can be adopted without departing from the gist of the present invention.

[0064]

According to the projection exposure apparatus of the present invention, a light transmissive substrate having a variable thickness in the optical axis direction is arranged between the mask and the substrate, and an optical path length switching means for changing the thickness is provided. Therefore, the optical path length between the substrate and the mask can be changed without moving the mask. Therefore, there is an advantage that mainly isotropic image distortion can be corrected without lowering the rigidity of the stage that holds the mask. Furthermore, isotropic image distortion can be corrected with a simple device configuration without requiring highly accurate positioning as when moving the mask.

When the light transmissive substrate is composed of one or a plurality of optical wedges and the optical path length switching means is an optical wedge moving means, the image distortion can be continuously corrected by a simple moving operation. There are advantages. Further, when the projection exposure apparatus is a scanning projection exposure apparatus and the movement direction of the optical wedge is the scanning direction, it is not necessary to physically drive the mask itself in order to correct the isotropic image distortion. Therefore, it is particularly effective for a scanning type projection exposure apparatus which requires high rigidity as a whole. Further, since the width of the illumination region or the exposure region is narrow in the scanning direction, there is an advantage that the shape of the optical wedge can be downsized including the driving amount.

When the light transmissive substrate is composed of a plurality of light transmissive substrates and the optical path length switching means is the exchange means,
Although the correction is discontinuous, there is an advantage that the device configuration is simple.

[Brief description of drawings]

FIG. 1 is a partially cutaway schematic view showing an embodiment of a projection exposure apparatus according to the present invention.

FIG. 2 is an explanatory diagram of a turret 3 for switching illumination conditions of FIG.

3 is a perspective view showing a scanning state of a reticle R and a wafer W in the projection exposure apparatus of FIG.

FIG. 4 is a schematic explanatory diagram of a principle of correcting isotropic image distortion in the embodiment of FIG.

5 is a diagram showing a distortion state corresponding to each state of FIG. 4. FIG.

6 is a diagram showing a state of distortion (including a magnification error) when the intermediate state of FIG. 4 is set.

7A is a diagram showing marks on a reticle for measuring image distortion, and FIG. 7B is a diagram showing marks on a pattern plate different from the reticle for measuring image distortion.

FIG. 8 is a diagram showing a first modification of the image distortion correction body according to the embodiment.

FIG. 9 is a diagram showing a second modification of the image distortion correction body according to the embodiment.

FIG. 10 is a diagram showing a third modification of the image distortion correction body according to the embodiment.

[Explanation of symbols]

 R Reticle PL Projection optical system W Wafer IL Illumination light IX Optical axis 9 Laser interferometer (for reticle) 11 Laser interferometer (for wafer) 31, 81, 84, 91, 104 Image distortion corrector 12, 13, 82, 83 , 85, 86 Optical wedge 86, 101-103 Optical member 14 Optical member control system 30 Main control system 30A Stage control system 24, 25, 28 Photoelectric sensor 29 Environmental sensor

Claims (4)

[Claims] 1. A projection exposure apparatus for projecting an image of a transfer pattern formed on a mask onto a photosensitive substrate under exposure illumination light through a projection optical system, wherein the mask and the A light-transmissive substrate having a variable thickness disposed between the substrate and an optical path length switching means for changing the optical path length of the illumination light by switching the thickness of the light-transmissive substrate; A projection exposure apparatus, wherein the image distortion of a projected image is adjusted by changing the thickness of the light transmissive substrate via an optical path length switching means. 2. The projection exposure apparatus according to claim 1, wherein
The light-transmissive substrate is composed of one or a plurality of optical wedges each having a continuously changing thickness, and the optical path length switching means is a moving means for moving the optical wedges as a whole or relatively. And a projection exposure apparatus. 3. The projection exposure apparatus according to claim 2, wherein:
The projection exposure apparatus sequentially exposes the pattern of the mask onto the substrate by scanning the substrate in a direction corresponding to the predetermined scanning direction in synchronization with scanning the mask in the predetermined scanning direction. A projection exposure apparatus, which is a scanning exposure apparatus, wherein the optical path length switching means moves the optical wedge along the predetermined scanning direction. 4. The projection exposure apparatus according to claim 1, wherein
The projection exposure apparatus, wherein the light transmissive substrate is composed of a plurality of light transmissive substrates having different thicknesses, and the optical path length switching means is an exchange means for exchanging the plurality of light transmissive substrates.