JP2005308876A - Optical member, optical system and exposure device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce stress double refraction quantity and the anisotropy of stress double refraction in an optical system having crystalline optical members. <P>SOLUTION: A plurality of optical members each of which consists of single crystal whose crystal structure belongs to a point group O, Oh or Td and has axially symmetrical stress distribution having a prescribed crystal axis direction as a symmetrical axis are arranged so as to be relatively rotated around the symmetrical axis by prescribed angles so that the symmetrical axes of the stress distribution coincide with each other. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical system with reduced stress birefringence and an exposure apparatus equipped with the optical system, and more particularly to an optical system suitable for an exposure apparatus used when manufacturing a microdevice such as a semiconductor element in a photolithography process. Is.

In recent years, in the field of precision processing and measurement using light in semiconductor manufacturing, the wavelength of a light source has been shortened in accordance with improvement in required processing / measurement accuracy. Accordingly, usable optical members are limited to materials that absorb less light up to shorter wavelengths. Optical members that can be used for the ArF excimer laser (wavelength: 193 nm), which is currently the mainstream light source, are synthetic quartz glass or fluoride single crystals such as calcium fluoride, and a short wavelength F ₂ laser (wavelength: 157 nm) as the light source. In this case, virtually only a fluoride single crystal can be used as the optical member.

  An equiaxed crystal is preferably used for the fluoride single crystal as the optical member. This is because the equiaxed crystal generally has no birefringence. However, it is known that even if it is an equiaxed crystal, birefringence occurs when the wavelength of light is close to the absorption edge wavelength of the crystal, or when stress is applied to the crystal to cause distortion. Hereinafter, the birefringence generated in the former case is referred to as intrinsic birefringence (also referred to as intrinsic birefringence, intrinsic birefringence or intrinsic birefringence), and the birefringence generated in the latter case is referred to as stress birefringence. Unless otherwise specified in the future, the crystal refers to an equiaxed crystal.

  Intrinsic birefringence is a reflection of the periodic structure, which is an essential property of the crystal, and therefore occurs even in an ideal crystal and cannot be eliminated by means of crystal manufacturing. Therefore, in an optical system using a crystalline optical member in which intrinsic birefringence is recognized, a plurality of optical members oriented in a predetermined crystal orientation with respect to a light beam are combined to apparently cancel the birefringence of the entire optical system. . This method is generally called clocking.

Burnett et al. As a whole optical system by placing a pair of calcium fluoride (CaF ₂ ) single crystal lenses having the <111> direction as the optical axis in a crystallographically rotated 60 degrees around the optical axis. It was shown that birefringence is improved (see Non-Patent Document 1). Calcium fluoride crystals have an intrinsic birefringence distribution that is three-fold symmetric around the optical axis with respect to the light beam in the <111> direction, but according to the method of Burnett et al., The three-fold symmetric component is canceled and the intrinsic birefringence is A concentric distribution, that is, an axially symmetric distribution, improves the optical design problem to some extent. However, since the axially symmetric component still remains, as a means for reducing the axially symmetric component of this intrinsic birefringence, in addition to a pair of calcium fluoride single crystal lenses having the <111> direction as the optical axis, the <100> direction is There has been proposed an optical system in which a pair of calcium fluoride single crystal lenses which are optical axes and are rotated by 45 degrees around the optical axis are added (Patent Document 1).

Unless otherwise specified, in the following description, axial symmetry refers to concentric ∞ rotation symmetry around the symmetry axis.
JP 2003-50349 A JHBurnett, ZHLevine, ELShirley and JHBruing, "Symmetry of spatial-dispersion-induced birefringence and its implications for CaF2 ultraviolet optics", Journal of Microlithography, Microfabrication, and Microsystems, Vol.1 No.3, p.213-224, October 2002

Among the birefringence exhibited by the crystalline optical member, the intrinsic birefringence has been studied in terms of optical design such as clocking, while the stress birefringence has not been particularly studied so far. Recently, however, the existence of stress birefringence has been highlighted, and has become a serious problem that affects the feasibility of light exposure in the vacuum ultraviolet wavelength region such as ArF and F ₂ . This is because, when an optical member such as a lens has birefringence, optical performance such as imaging performance is deteriorated.

At present, stress birefringence at a level that poses a problem when an optical system of an exposure apparatus is assumed is observed in the <100> direction of a calcium fluoride single crystal.
As described above, in the clocking for canceling the intrinsic birefringence, in addition to the single crystal pair having the <111> direction as the optical axis, a single crystal pair having the <100> direction as the optical axis may be used. It is effective (patent document 1). However, in the calcium fluoride single crystal having the <100> direction as the optical axis, a relatively large stress birefringence of about several nanometers to several tens of nanometers / cm is observed by retardation. Further, although the birefringence amount is smaller than that in the <100> direction, it is known that there is a non-negligible amount of stress birefringence in the <111> direction. In other words, even if the intrinsic birefringence can be completely canceled by clocking, the influence of stress birefringence cannot be eliminated. Also, around the <111> direction, the phenomenon that the amount of stress birefringence differs greatly depending on the angle between the light beam and the <111> axis, that is, the incident angle dependency of the stress birefringence is known to be large. It is a design problem.

Intrinsic birefringence is derived from the intrinsic symmetry of the crystal and is a characteristic of the crystal itself regardless of external factors. Therefore, measures such as clocking against intrinsic birefringence have the same symmetry. As long as the crystals are provided, the same effect can always be obtained regardless of the production method and use conditions. On the other hand, stress birefringence is a phenomenon caused by stress acting on the crystal, and stress depends on external factors associated with individual crystals such as crystal manufacturing conditions and usage conditions. Is difficult to generalize. Moreover, since stress birefringence shows a complicated behavior depending on both the symmetry of the crystal and the stress, the anisotropy is higher than the intrinsic birefringence and it is considered difficult to eliminate it.
In order to reduce the effect of stress birefringence, there are attempts to reduce the absolute value of the stress itself by measures such as improving the crystal annealing method, but it is difficult to create a completely stress-free state at present. No effective solution has been found.
An object of the present invention is to reduce the amount of stress birefringence and the anisotropy of stress birefringence in an optical member and an optical system.

The present invention provides an optical member and an optical system having improved anisotropy of stress birefringence.
The optical member according to claim 1 provided by the present invention has an axially symmetric stress distribution having a crystal structure of a single crystal belonging to any of the point group O, Oh, or Td, and having the <111> axis as an axis of symmetry. It is characterized by having.

Stress birefringence is particularly problematic for short wavelength light. Therefore, if it is applied to a calcium fluoride single crystal that is an optical member that is transparent to light having a short wavelength, a great effect can be obtained. Accordingly, an optical member according to a second aspect of the present invention is the optical member according to the first aspect, wherein the single crystal is a calcium fluoride (CaF ₂ ) single crystal.

An optical system according to a third aspect has at least one optical member according to the first or second aspect.
The present invention also includes a plurality of optical members having a crystal structure of a single crystal belonging to any of the point group O, Oh, or Td, and having a stress distribution that is axially symmetric with respect to a predetermined crystal orientation. The following optical system is provided.

  An optical system according to claim 4 is the optical system according to claim 3, wherein the optical system has two optical members, the symmetry axes of the two optical members are aligned, and one optical member Is arranged by being rotated 60 degrees around the axis of symmetry with respect to the other optical member.

  The optical system according to claim 5 includes two optical elements each having an axially symmetric stress distribution with a <100> axis as an axis of symmetry, the crystal structure being a single crystal belonging to any of point group O, Oh, or Td. The members are arranged such that the symmetry axes coincide with each other and one optical member is rotated 45 degrees around the symmetry axis with respect to the other optical member.

  The optical system according to claim 12 is composed of a single crystal having a crystal structure belonging to any of the point group O, Oh, or Td, and has four axisymmetric stress distributions with the <110> axis as an axis of symmetry. The members are aligned with the symmetry axis, and three of the four optical members are 45 degrees, 90 degrees, 135 around the symmetry axis with respect to the other optical member, respectively. It is characterized in that it is arranged rotated at a degree.

In the above optical system, when the stress distributions of the two or four optical members are equal to each other, the anisotropy of the stress distribution is most reduced. Therefore, the present invention provides the following optical system.
An optical system according to a sixth aspect is the optical system according to the fourth or fifth aspect, wherein the stress distributions of the two optical members are equal to each other.

An optical system according to a thirteenth aspect is the optical system according to the twelfth aspect, wherein stress distributions of the four optical members are equal to each other.
The present invention is also an optical system in which a plurality of optical members made of single crystals whose crystal structure belongs to any one of the point groups O, Oh, or Td are combined, and each of the optical members has a predetermined birefringence amount and a phase advance. The following optical system is provided which has an axial distribution and is arranged in a predetermined direction. Here, the predetermined birefringence amount and the distribution in the fast axis direction are distributions specifically observed in the optical member having the axially symmetric stress distribution.

  The optical system according to claim 7 is made of a single crystal having a crystal structure belonging to any of the point group O, Oh, or Td, and has a birefringence amount and a fast axis direction of a light beam incident in a <100> direction of [100]. The two optical members exhibiting a four-fold rotational symmetry in the plane are aligned with the four-fold rotational symmetry axis, and one optical member is placed around the symmetry axis with respect to the other optical member. It is characterized by being rotated 45 degrees.

  The optical system according to claim 9 is made of a single crystal having a crystal structure belonging to any of the point group O, Oh, or Td, and the birefringence amount and the fast axis direction with respect to the light incident in the <111> direction are [111]. The two optical members exhibiting an axially symmetric distribution in the plane are arranged by aligning the symmetric axes and rotating one optical member 60 degrees around the symmetric axis with respect to the other optical member. It is characterized by that.

  The anisotropy of stress birefringence is most reduced in the above optical system when the birefringence amount and the distribution in the fast axis direction of the two optical members are equal to each other. Therefore, the present invention provides the following optical system.

  An optical system according to claim 8 is the optical system according to claim 7, wherein the birefringence amount and the distribution in the fast axis direction in the [100] plane of the two optical members are equal to each other. Features.

  An optical system according to claim 10 is the optical system according to claim 9, wherein the birefringence amount and the distribution in the fast axis direction in the [111] plane of the two optical members are equal to each other. Features.

  When the two or four optical members have the same stress distribution or birefringence distribution, the anisotropy of the stress birefringence of the optical system is most reduced by arranging the optical members adjacent to each other. Therefore, the present invention provides the following optical system.

An optical system according to an eleventh aspect is the optical system according to the sixth, eighth, or tenth aspect, wherein the two optical members are arranged adjacent to each other.
An optical system according to a fourteenth aspect is the optical system according to the thirteenth aspect, wherein the four optical members are arranged adjacent to each other.

  When the optical system having the above configuration is applied to the optical system of the exposure apparatus, there is an effect that the imaging performance is improved by improving the birefringence. In particular, birefringence becomes a problem in an optical system using ultraviolet light as a light source and an exposure apparatus equipped with the optical system. In the optical system, a calcium fluoride single crystal is used as an optical member. Therefore, the present invention provides the following optical system and exposure apparatus.

An optical system according to claim 15 is the optical system according to any one of claims 5 to 14, wherein the single crystal is a calcium fluoride (CaF ₂ ) single crystal. To do.
An exposure apparatus according to a sixteenth aspect includes the optical member according to the first or second aspect.

An exposure apparatus according to a seventeenth aspect includes the optical system according to any one of the third to fifteenth aspects.
In the present invention, the stress distributions being equal to each other means that the components of the stress tensor at a distance r from the symmetry axis of the stress distribution are equal to each other for each optical member. Refers to positioning the axes on the same straight line. The birefringence amount is an amount represented by the retardation amount in the fast axis direction and the slow axis direction.

  According to the present invention, the amount of stress birefringence and the anisotropy of stress birefringence in an optical member made of a single crystal whose crystal structure belongs to any one of point groups O, Oh, or Td, and an optical system having the optical member. Can be reduced.

  First, an analysis result performed by the present inventor on the behavior of stress birefringence when a crystal belonging to the point group O, Oh, or Td has an axially symmetric stress distribution is shown. Reduction of stress birefringence according to the present invention is shown. Explain the effect. Note that the point groups O, Oh, and Td are displayed using the Shane Fleece notation.

  The relationship between stress and birefringence is generally expressed by equation (1).

Here, π _ijmn is a photoelastic constant tensor, and T _mn is a stress tensor. The left side is a tensor representing the difference between the negative square of the refractive index and the case where no stress is applied (subscript 0). In the following description, the tensor represented by the equation (1) is referred to as a stress birefringence tensor. To do.

The feature of the present invention is that the stress of the optical member is axisymmetric. Therefore, the stress tensor T ′ _mn is defined on cylindrical coordinates where the symmetric axis direction of stress is z, the radial direction perpendicular to z is r, and the azimuth direction of r is φ. Considering an axially symmetric, ie, concentric, stress distribution that is uniform in the z direction, the stress tensor is a function T ′ _mn (r) of only r, and is expressed by the following equation.

Here, if r = constant, that is, a point on the circumference that is equidistant from the z-axis (stress symmetry axis) is to be analyzed, the components of the stress tensor T _mn ′ (r) are constants, and the stress tensor is T _mn 'Become.

In the following description, the second-order tensor A _ij and the fourth-order tensor X _ijmn are respectively expressed in the form of column vectors and matrices as follows.

Since the tensor π _{ijmn of the} photoelastic constant is generally displayed with reference to the crystal axis (orthogonal coordinates in the equiaxed crystal system to which the crystal point group according to the present invention belongs), it is defined by cylindrical coordinates for convenience of calculation. The stress tensor T _mn ′ is once converted into a tensor T _mn on orthogonal coordinates with [100] [010] [001] of the crystal as three axes. At this time, if the symmetry axis of stress, that is, the z-axis is made coincident with the [001] axis of the crystal and φ = 0 is taken as the [100] axis, the stress tensor T _mn is expressed by equation (6).

Further, π _ijmn of the crystal belonging to the point group O, Oh, or Td is as shown in the equation (7). (Terms that are equivalent due to crystal symmetry are represented by a single symbol.)

  Therefore, the stress birefringence when the stress of the axis acts on the crystal belonging to the point group O, Oh, or Td is expressed by the product of both tensors as shown in the equation (8).

The tensor in equation (8) corresponds to the orthogonal coordinates. If this is converted again into cylindrical coordinates, the stress birefringence tensor on the cylindrical coordinates _uses the component of the tensor π _ijmn of the photoelastic constant of the crystal and the component of the stress tensor T _mn ′ defined by the cylindrical coordinates. Finally, it is expressed as in equation (9).

  The stress birefringence tensor of the formula (9) is a distance from the symmetry axis of stress for an optical member having an axisymmetric stress distribution made of a crystal belonging to the point group O, Oh, or Td and having the symmetry axis in the <100> direction. r represents stress birefringence at a point at an azimuth angle φ.

  Subsequently, the present inventor solved the eigenvalue equation for the stress birefringence tensor of Equation (9), and derived the amount of stress birefringence and the fast axis direction. Although the eigenvalue and the eigenvector are derived from the eigenvalue equation, the amount of stress birefringence is the difference between the two eigenvalues, and the eigenvector corresponding to the larger eigenvalue represents the fast axis direction. By performing this calculation, the amount of stress birefringence for a light ray incident in the <100> direction parallel to the symmetry axis of the stress distribution is

The direction of the fast axis is the angle between the r vector and the fast axis as Ψ,

It was derived that.
FIG. 1 (a) shows the amount of stress birefringence and the fast axis direction at a point where r = a constant distance from the stress symmetry axis on the calcium fluoride single crystal [100] plane. It is the result calculated by the formula. The plane of the paper is the [100] plane, the center of the figure is the axis of symmetry of the stress, and r = diameter of the circle written on a constant circumference indicates the relative magnitude of the stress birefringence at that point, A straight line represents the direction of the fast axis.

  As is clear from FIG. 1A, neither the stress birefringence amount nor the fast axis direction is axially symmetric, and an anisotropy of four-fold rotational symmetry is recognized. The amount of stress birefringence has a four-fold rotational symmetric distribution around the symmetry axis of stress, which is maximum in the <100> direction and minimum in the <110> direction. The fast axis direction has a four-fold rotationally symmetric distribution in which the <100> direction is the circumferential direction and the <110> direction is the radial direction (when (Tr-Tφ)> 0). Between the <100> direction and the <110> direction, the fast axis direction rotates continuously, and the fast axis rotates once counterclockwise while the observation position rotates once clockwise. Indicates. The fast axis direction when (Tr-Tφ) <0 is a direction orthogonal to the fast axis direction when (Tr-Tφ)> 0.

  From the above analysis results, the crystalline optical member belonging to any of the point groups O, Oh, and Td has a stress birefringence amount and a phase advance even if the stress distribution is an axially symmetric distribution with the <100> direction as the symmetric axis. It became clear that the axial direction was not axisymmetric. Therefore, the present inventor has started development of a method for reducing stress birefringence by combining a plurality of optical members based on the knowledge obtained in the above analysis process. As a result, it has been found that an optical system in which the anisotropy of stress birefringence is canceled can be realized by disposing an optical member having a predetermined stress distribution in a predetermined crystal orientation. It has come.

Hereinafter, embodiments of the present invention will be described.
[First embodiment]
In the first embodiment of the present invention, two optical elements each having an axially symmetric stress distribution with a <100> axis as an axis of symmetry, the crystal structure of which is a single crystal belonging to either the point group O, Oh, or Td. The present invention relates to an optical system in which a member is arranged such that the symmetry axis coincides and one optical member is rotated 45 degrees around the symmetry axis with respect to the other optical member.

  Equation (9) is a stress birefringence tensor for one optical member having an axially symmetric stress distribution with the <100> direction as the symmetry axis. FIG. 1A shows the stress birefringence and the fast axis direction. It was a distribution. Here, the stress birefringence tensor in which φ is replaced with φ + 45 degrees in the equation (9) is calculated. This corresponds to the case where the same optical member as the analysis target of the equation (9) is rotated 45 degrees around the stress symmetry axis. The <100> axis (stress symmetry axis in this case) of the crystal belonging to the point group O, Oh, or Td is a four-fold rotational symmetry axis with respect to the symmetry of the crystal structure. It doesn't matter. The result is equation (12).

  Next, the arithmetic average of the equations (9) and (12) is obtained. The result is equation (13). Equation (13) shows that two optical members having the same axially symmetric stress distribution and whose stress symmetry axis is in the <100> direction are arranged so that the stress symmetry axes coincide with each other and around the stress symmetry axis. This corresponds to the stress birefringence tensor of the entire optical system arranged with a relative rotation of 45 degrees.

  FIG. 8 is a schematic view showing the optical system of the present embodiment with the <100> direction perpendicular to the stress symmetry axis as the reference for the rotation angle. The optical members 21 and 22 constituting the optical system of FIG. 8 have the same axially symmetric stress distribution and are rotated by 45 degrees around the symmetric axis of the stress distribution.

  The feature of the stress birefringence tensor of the equation (13) is that the tensor component does not include φ. The fact that the stress birefringence tensor does not contain φ means that the eigenvalue and eigenvector of the tensor are independent of both the amount of stress birefringence and the fast axis direction, and the anisotropy around the symmetry axis of the stress is completely Means a concentric distribution that has been canceled by That is, the two optical members of the present embodiment, which are composed of a single crystal whose crystal structure belongs to any of the point group O, Oh, or Td, and have an axially symmetric stress distribution with the <100> axis as the axis of symmetry, An optical system in which the symmetry axes are aligned and one optical member is rotated by 45 degrees around the symmetry axis with respect to the other optical member cancels the anisotropy of stress birefringence as a whole. Thus, it has an axially symmetric birefringence distribution around the stress symmetry axis.

  FIG. 1B illustrates the amount of stress birefringence and the fast axis direction of the optical system according to the present invention obtained from the equation (13) in the same manner as in FIG. 1A. Compared with FIG. 1A showing the stress birefringence distribution of a single optical member, the optical system of this embodiment eliminates the anisotropy around the symmetry axis of the stress, and the absolute value of the stress birefringence is also It is clear that it has been reduced.

  The above analysis results are for the case where the stress distributions of the two optical members constituting this embodiment are completely equal. When optical members having different stress distributions are combined, the φ dependence of the stress tensor is not completely canceled, but it goes without saying that a reduction effect is observed in terms of stress birefringence and anisotropy in the fast axis direction.

In the present embodiment, the two optical members arranged to rotate by a predetermined angle are desirably arranged adjacent to each other in the optical system, and most preferably arranged in close contact using an optical contact or the like. . This is because the smaller the distance between the two optical members, the smaller the difference in the distance r from the central axis of the stress at which one light beam passes through each optical member, thereby canceling the effect of stress birefringence. This is because becomes larger.
[Second Embodiment]
The second embodiment of the present invention relates to an optical member having an axially symmetric stress distribution with a <111> axis as a symmetric axis, the crystal structure being made of a single crystal belonging to any of point group O, Oh, or Td. It is.

  The present inventor conducted an analysis similar to that in the <100> direction for an optical member having an axially symmetric stress distribution with the <111> direction as the symmetry axis. Expression (14) is a stress birefringence tensor of an optical member having an axially symmetric stress distribution with the <111> direction as an axis of symmetry.

When the eigenvalue equation is solved for equation (14), the amount of stress birefringence for light traveling in the <111> direction parallel to the symmetry axis of the stress distribution becomes equation (15), and the fast axis direction becomes equation (16). .

Neither φ (15) nor (16) includes φ. That is, light traveling in the <111> direction parallel to the symmetry axis of the stress distribution of an optical member belonging to any of the point groups O, Oh, Td and having an axially symmetrical stress distribution having the symmetry axis in the <111> direction. It has been clarified that the stress birefringence amount and the fast axis direction show an axially symmetric distribution around the stress symmetry axis.

This property found by the present inventor is a property peculiar to an optical member having a symmetry axis of stress in the <111> direction, and is different from that of an optical member having a symmetry axis of stress in the <100> direction. There is a feature that an axially symmetric stress birefringence distribution and a fast axis distribution can be realized without combining these optical members.
[Third embodiment]
In the third embodiment of the present invention, four optical elements each having an axially symmetric stress distribution with a <110> axis as a symmetric axis, the crystal structure being a single crystal belonging to any of the point group O, Oh, or Td. The members are aligned with the symmetry axis, and three of the four optical members are 45 degrees, 90 degrees, 135 around the symmetry axis with respect to the other optical member, respectively. The present invention relates to an optical system that is rotated at a degree.

Hereinafter, the analysis result about the optical member which has the symmetrical axis of stress in <110> direction which this inventor performed is demonstrated.
The equation (17) is a single crystal having a crystal structure of a single crystal belonging to any of the point group O, Oh, or Td, and having an axially symmetrical stress distribution with the <110> axis as the axis of symmetry. Refractive tensor.

Since the stress birefringence tensor in this case includes φ in the component, it is considered that the stress birefringence exhibits anisotropy around the stress symmetry axis. Therefore, as a result of various analyzes on the combination of the optical members, the present inventor has come up with a combination method that can cancel the anisotropy of stress birefringence and obtain an axially symmetric stress birefringence distribution.

  Expressions (18) to (20) are stress birefringence tensors when the optical member represented by Expression (17) is rotated by 45 degrees, 90 degrees, and 135 degrees around the stress symmetry axis, respectively. Equation (21) is an arithmetic average of Equations (17) to (20).

  The formula (21) is expressed as follows: four optical members having an axially symmetric stress distribution with a <110> axis as a symmetric axis, the crystal structure being made of a single crystal belonging to any of the point group O, Oh, or Td. The symmetry axes are matched, and three of the four optical members are rotated 45, 90, and 135 degrees around the symmetry axis with respect to the other optical member, respectively. This corresponds to a tensor representing the stress birefringence of the optical system as a whole.

FIG. 12 is a diagram schematically showing the optical system of the present embodiment with the <100> direction as a reference for the rotation angle. The optical members 61 to 64 are arranged so that the symmetry axis of stress is on the same straight line, the optical member 62 is 45 degrees, the optical member 63 is 90 degrees, and the optical member 64 is 135 degrees with respect to the optical member 61. Is arranged around the axis of symmetry.
Since the tensor component of equation (21) does not include φ, it is clear that the amount of stress birefringence and the fast axis direction obtained by solving equation (21) as an eigenvalue equation do not depend on φ. Therefore, the optical system has axially symmetric stress birefringence in which the anisotropy of stress birefringence is canceled as a whole optical system.

  As is apparent from the above analysis process, the effect of reducing the stress birefringence in the present embodiment is greatest when the stress distributions in the four optical members constituting the optical system are equal to each other. When optical members having different stress distributions are combined, the φ dependence of the stress tensor is not completely canceled, but it goes without saying that a reduction effect is observed in terms of stress birefringence and anisotropy in the fast axis direction.

In the present embodiment, the four optical members that are rotated by a predetermined angle are desirably disposed adjacent to each other in the optical system, and most preferably disposed in close contact using an optical contact or the like. . This is because the smaller the distance between the four optical members, the smaller the difference in the distance r from the central axis of the stress at which one light beam passes through each optical member, thereby canceling the effect of stress birefringence. Because it becomes higher.
[Fourth Embodiment]
By the way, the analysis performed by the present inventor so far has been the case where the incident optical axis is parallel to the symmetry axis of stress. However, in the case of a calcium fluoride single crystal that is actually used as an optical member, stress birefringence has an incident angle dependency, and particularly for light incident around the <111> direction, stress birefringence is incident. It is known that the angle dependency is large. Therefore, the present inventor has analyzed an optical system in which a plurality of members having a symmetry axis of stress in the <111> direction are combined for the purpose of reducing the incident angle dependency of stress birefringence. The results will be described below.

  The inventor converted the stress birefringence tensor of the optical member into cylindrical coordinates having the incident direction of the light beam as the z axis, solved the eigenvalue equation, and calculated the stress birefringence for the light beam incident in an arbitrary direction. In the analysis, as shown in FIG. 2, the tilt angle of incident light from the <111> direction parallel to the symmetry axis of the stress distribution was θ, and the tilt direction was χ.

Since the stress birefringence tensor after coordinate conversion is an enormous expression, the explanation is omitted, and the incident angle (θ) dependence of the finally obtained stress birefringence is shown in FIG. FIG. 3A shows the amount of stress birefringence of a calcium fluoride single crystal having an axisymmetric stress distribution in the <111> direction at a point in the φ = π / 6, that is, [01-1] direction, χ = 0. This is a result calculated by the present inventor for light having a wavelength of 157 nm incident at 1. Stress birefringence are designated with a value normalized by _{T r (Pa) × (-n} o 3/2) × 10 -12. FIG. 3A clearly shows that the amount of stress birefringence increases rapidly as the incident direction of the light beam tilts from the <111> direction parallel to the symmetry axis of the stress distribution. This is consistent with the trend observed with actual optical members.

  Here, in the stress birefringence tensor of the equation (14), the tensor replaced with φ → φ + π / 3 is calculated. The result is Equation (22).

Taking the geometric mean of the equations (14) and (22), the stress birefringence tensor of the equation (23) is obtained.

  Equation (23) is composed of crystals belonging to any one of the point groups O, Oh, and Td, has an axially symmetric stress distribution with the <111> direction as the symmetric axis, and two optical elements having the same stress distribution. Corresponding to the stress birefringence tensor of the optical system according to the present invention, in which the members are arranged so that the symmetry axes coincide with each other and one optical member is rotated 60 degrees around the symmetry axis with respect to the other optical member. To do.

  FIG. 11 is a schematic diagram of the optical system according to the present embodiment, displaying the <110> direction as a reference for the rotation angle. The first optical member 51 and the second optical member 52 are arranged so that the symmetry axis of stress is on the same straight line, and are rotated by 60 degrees around the symmetry axis.

  FIG. 3 (b) assumes that the stress birefringence amount with respect to the light incident from an arbitrary direction using the stress birefringence tensor of the equation (23) is the case where light having a wavelength of 157 nm is incident on calcium fluoride. It is the result of calculation. Comparing FIG. 3A and FIG. 3B, the amount of stress birefringence when the incident light is tilted from the <111> direction parallel to the axis of symmetry of the stress distribution, that is, θ = 0, is almost the same. A large difference occurs in the amount of stress birefringence with respect to incident light deviating from this direction (θ ≠ 0), and by combining two optical members at a predetermined angle, the optical member is used alone. It is clear that the incident angle dependence of the stress birefringence is remarkably improved.

The above effect is maximized when the stress distributions of the two optical members are equal, including the absolute value, and the effect decreases if there is a difference between the stress distributions of the two optical members.
Further, in the present embodiment, the two optical members arranged to rotate by a predetermined angle are preferably arranged adjacent to each other in the optical system, and are arranged in close contact using an optical contact or the like. Most preferred. This is because the smaller the distance between the two optical members, the smaller the difference in the distance r from the central axis of the stress at which one light beam passes through each optical member, thereby canceling the effect of stress birefringence. Because it becomes higher.
[Fifth Embodiment]
In the present embodiment, the crystal structure is made of a single crystal belonging to any of the point group O, Oh, or Td, and the birefringence amount and the fast axis direction with respect to the light incident in the <100> direction are 4 in the [100] plane. Two optical members exhibiting a rotationally symmetric distribution are aligned with the four-fold rotationally symmetric symmetry axis, and one optical member is rotated 45 degrees around the symmetric axis with respect to the other optical member. The present invention relates to the arranged optical system.

  The optical member and the optical system described in the first to fourth embodiments are derived from the result of theoretical analysis of an optical member having an axially symmetric stress distribution in a predetermined crystal orientation. In order to realize such an optical member or optical system, it is necessary to actually measure and evaluate the crystal orientation and stress distribution of the optical member constituting the optical system.

  As a method for measuring the crystal orientation of the optical member, an X-ray structure analysis method is generally used. For example, the crystal orientation can be easily determined by analyzing a diffraction spot measured by the Laue method.

  On the other hand, the stress distribution measurement of the optical member is not as easy as the crystal orientation measurement, and in order to confirm that the stress distribution of the actually manufactured optical member is axisymmetric, a simple stress distribution measurement method is required. Therefore, the present inventor has found a means for easily determining that the stress distribution of the optical member is axially symmetric based on the knowledge obtained through the stress birefringence analysis so far.

According to the analysis result of the present inventor, in an optical member made of a crystal belonging to any one of the point groups O, Oh, and Td and having an axisymmetric stress distribution with the <100> axis as an axis of symmetry, the stress distribution is symmetric. FIG. 1A shows the stress birefringence amount and the relative distribution in the fast axis direction for light incident in the <100> direction parallel to the axis, regardless of the values of T _r , Tφ, and T _z . It is shown. Therefore, the amount of stress birefringence and the fast axis direction with respect to light incident in the <100> direction of the crystal optical member belonging to any of the point groups O, Oh, and Td are measured, and the distribution is shown in FIG. If it is four-fold rotational symmetry as shown in (a), the optical member will have an axisymmetric stress distribution with the <100> axis as the axis of symmetry. As described in the first embodiment, the optical birefringence is improved by arranging an optical member having an axially symmetric stress distribution in the <100> direction in a predetermined direction. It is obvious that the system can obtain the same effect as the optical system of the first embodiment.
[Sixth Embodiment]
In the sixth embodiment, the crystal structure is made of a single crystal belonging to any one of the point group O, Oh, or Td, and the birefringence amount and the fast axis direction with respect to the light incident in the <111> direction are in the [111] plane. The optical system in which two optical members exhibiting an axially symmetric distribution are arranged so that the symmetry axes coincide with each other and one optical member is rotated 60 degrees around the symmetry axis with respect to the other optical member. Is.

According to the inventor's analysis of an optical member made of a crystal belonging to any one of the point groups O, Oh, and Td and having an axisymmetric stress distribution with the <111> axis as a symmetry axis, the stress distribution is symmetric. The amount of stress birefringence and the relative distribution in the fast axis direction with respect to light incident in the <111> direction parallel to the axis is an axisymmetric distribution regardless of the values of T _r , Tφ, and T _z ( (15) and (16)). Therefore, the amount of stress birefringence and the fast axis direction with respect to light incident in the <111> direction of the crystal optical member belonging to any of the point groups O, Oh, and Td are measured, and the distribution is axially symmetric in the [111] plane. The optical member has an axisymmetric stress distribution with the <111> axis as the symmetry axis. As described in the fourth embodiment, if the optical member having such an axially symmetric stress distribution is combined in a predetermined direction, the incident angle dependency of the stress birefringence is reduced. Therefore, the optical system of the present embodiment can also reduce the incident angle dependency of stress birefringence as in the fourth embodiment.

  In Example 1, an optical member made of a calcium fluoride single crystal having an axially symmetric stress distribution in which the symmetry axis is the <100> direction was manufactured. FIG. 4 is a schematic view of a heat treatment apparatus for producing a calcium fluoride single crystal having an axisymmetric stress distribution.

  The heat treatment apparatus is sealed and evacuated after housing the first container 1 made of carbon containing the calcium fluoride single crystal to be treated and the first container 1 containing the calcium fluoride single crystal inside. A stainless steel second container 2 that can be hermetically sealed and a heater 3 disposed outside the second container 2 are provided. A vacuum exhaust device (not shown) is connected to the exhaust port 4 of the second container 2.

  In the heat treatment apparatus, the first container 1 and the second container 2 and the heater 3 are configured so that the internal temperature distribution of the calcium fluoride single crystal is axisymmetric during the heat treatment. Specifically, the heater 3 is arranged on the circumference of a circle centering on the center position of the horizontal section of the first container 1 and the second container 2, and the calcium fluoride single crystal to be treated is at its center. Is accommodated in the center of the first container 1, the second container 2, and the heater 3.

  As the heater 3, a linear heating element wound around a cylindrical support member at equal intervals or a cylindrical heating element can be used. As a specific example, a nichrome wire heater is used, and the heater is arranged at an equal distance from the second container so as to spirally surround the outer periphery of the second container so that the heaters are spaced apart from each other. The heater is used so that the longitudinal direction of the heating element coincides with the longitudinal direction of the second container, and is arranged at regular intervals on the entire circumference of the second container, and the heater is shaped along the outer circumference of the second container. Specially manufactured and processed can be listed as heating elements.

Moreover, as a material of the heater, silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), molybdenum disilicide (MoSi ₂ ), carbon, or the like can be used.
Next, an actual heat treatment process will be described.

  In this example, an ingot of a high-purity calcium fluoride single crystal manufactured by a conventional vertical Bridgman method is prepared, and a cylindrical optical member having a diameter of 240 mm and a thickness of 50 mm, having an upper surface and a lower surface of [100]. 10 were cut out. As shown in FIG. 5, the optical member 10 cut out from the ingot is sandwiched between disc-like carbon materials 7 having a diameter of 300 mm and a thickness of 10 mm, and the periphery is covered with a felt-like carbon material 8. Ten central axes (<100> axes) were stored in the first container 1 in a state in which the central axes coincided with the central axis of the heat treatment apparatus. The disk-shaped carbon material 7 has a diameter of 300 mm and a thickness of 10 mm. After sealing the heat treatment apparatus, it was evacuated to a predetermined degree of vacuum and heat treatment was performed according to the schedule shown in FIG.

  About the optical member after heat processing, the birefringence with respect to the light of wavelength 633nm which injects into the axial direction (<100> direction) of a cylindrical member was measured. FIG. 7 shows the measurement results of the birefringence amount and the fast axis in the [100] plane perpendicular to the axial direction of the cylindrical member. The paper plane is the [100] plane, the vertical and horizontal directions are <110>, 45 degrees. The direction is <100>. The amount of birefringence at each measurement point in the [100] plane is indicated by the size of the circle, and the fast axis direction at that point is indicated by a straight line in the circle.

From FIG. 7, the following features can be read about the optical member manufactured in this example. First, the distribution of birefringence exhibits a four-fold rotational symmetry with the <100> direction as a maximum and the <110> direction as a minimum, and its axis of symmetry substantially coincides with the geometric center of the optical member. Secondly, the direction of the fast axis is the <100> direction perpendicular to the diameter, and the <110> direction is the four-fold rotational symmetry in the diameter direction, and the axis of symmetry is substantially coincident with the geometric center of the optical member. . From the above measurement results, it is confirmed that the optical member manufactured in this example has an axially symmetric stress distribution with the <100> direction as the symmetry axis, and the symmetry axis substantially coincides with the symmetry axis of the geometric shape. It has been found. Further, from the relationship between the crystal orientation and the fast axis direction, it was found that the optical member manufactured in this example satisfies T _r −Tφ> 0.

The effects of the disk-like carbon material 7 and the felt-like carbon material 8 in this embodiment can be interpreted as follows.
Both the disk-like carbon material 7 and the felt-like carbon material 8 act as heat conductive members. By these heat conductive members, the temperature distribution in the circumferential direction of the optical member 10 to be heat-treated is made uniform, the residual stress formation process proceeds in a symmetric manner in the circumferential direction, and the optical member processed into a cylindrical shape has an axis. It is considered that a symmetric stress distribution is realized. Further, by covering the side surface of the optical member 10 with a heat conductive member, cooling from the side surface of the cylinder is promoted, and tensile stress is formed between the center of the cylinder and the tensile stress in the radial direction is dominant, that is, T It is considered that _r− Tφ> 0 is realized.

  Through the above steps, an optical member made of a calcium fluoride single crystal and having an axially symmetric stress distribution with the <100> direction as the symmetric axis has been manufactured. An optical system with reduced anisotropy is realized.

  FIG. 8 shows an example of such an optical system, in which the <100> direction of the optical member is the optical axis, and the symmetrical axis of stress of each optical member is aligned with the optical axis. The optical members 21 and 22 constituting the optical system of FIG. 8 both have the same axially symmetric stress distribution and are rotated by 45 degrees around the symmetric axis of the stress distribution. Anisotropy is canceled out and it becomes axisymmetric. Further, since the optical axis coincides with the symmetry axis of the birefringence distribution, the optical design of the higher-order optical system including the optical system of FIG. 8 as a constituent element is easy.

  In this example, an optical member having a symmetry axis of stress distribution in the <100> direction is manufactured. However, an optical member having a symmetry axis of stress in another crystal orientation, that is, the <111> direction or the <110> direction is manufactured. In this case, the heat treatment may be performed with the central axis of the optical member at the time of the heat treatment being the <111> axis or the <110> axis.

  In Example 2, an optical member made of a calcium fluoride single crystal having an axially symmetrical stress distribution in which the axis of symmetry is the <100> direction was manufactured. The same heat treatment apparatus as that of Example 1 was used for producing a calcium fluoride single crystal having an axisymmetric stress distribution.

  First, a cylindrical optical member 11 having a diameter of 270 mm and a thickness of 55 mm was cut out from an ingot of calcium fluoride single crystal with the upper surface and the lower surface being a [100] surface. This ingot is a calcium fluoride single crystal produced by a conventional vertical Bridgman method and has not undergone heat treatment. As shown in FIG. 9, the optical member 11 cut out from the ingot is sandwiched between carbon plates 9 having a diameter of 200 mm and a thickness of 10 mm, and the central axis (<100> axis) of the optical member 11 is a heat treatment apparatus. Was stored in the first container 1 in a state of being aligned with the central axis. At this time, the first container 1 and the carbon plate 9 were in thermal contact with the felt-like carbon 12.

The heat treatment was performed according to the schedule shown in FIG.
About the optical member after heat processing, the birefringence with respect to the light of wavelength 633nm which injects into the axial direction (<100> direction) of a cylindrical member was measured. FIG. 10 shows the measurement results of the birefringence amount and the fast axis in the [100] plane perpendicular to the axial direction of the cylindrical member. The paper plane is the [100] plane, the vertical and horizontal directions are <100>, 45 degrees. The direction is <110>. The amount of birefringence at each measurement point in the [100] plane is indicated by the size of the circle, and the fast axis direction at that point is indicated by a straight line in the circle.

From FIG. 10, the following features can be read about the optical member manufactured in this example. First, the distribution of birefringence exhibits a four-fold rotational symmetry with the <100> direction as a maximum and the <110> direction as a minimum, and its axis of symmetry substantially coincides with the geometric center of the optical member. Secondly, the direction of the fast axis shows four-fold rotational symmetry in the diametrical direction in the <100> direction and the direction perpendicular to the diameter in the <110> direction, and the symmetric axis substantially coincides with the geometric center of the optical member. . From the above measurement results, it is confirmed that the optical member manufactured in this example has an axially symmetric stress distribution with the <100> direction as the symmetry axis, and the symmetry axis substantially coincides with the symmetry axis of the geometric shape. It has been found. Further, from the relationship between the crystal orientation and the fast axis direction, it was found that the optical member manufactured in this example satisfies T _r −Tφ <0.

In this embodiment, since not arranged a felt-like carbon on a side surface of a cylindrical optical member 11 unlike the first embodiment, tensile stress in the circumferential direction by the cylindrical side surface cooling is delayed from the center portion is formed, T _r It is considered that −Tφ <0 is realized.

  Through the above-described steps, an optical member made of a calcium fluoride single crystal and having an axially symmetric stress distribution with the <100> direction as the symmetry axis is manufactured, and this is rotated by a predetermined angle in the same manner as in Example 1. If arranged, an optical system with reduced stress birefringence is realized.

  FIG. 13 is a configuration example of an exposure apparatus according to the present invention. In Embodiment 3, the present invention is applied to an exposure apparatus provided with a refractive projection exposure apparatus. The exposure apparatus of this embodiment includes an illumination device 30 for illuminating a reticle (mask) 31 disposed on the first surface.

  The illumination device 30 is, for example, an ArF excimer laser light source that supplies light having a wavelength of 193 nm, an optical integrator that forms a secondary light source of a predetermined shape (circular, ring-shaped, bipolar, quadrupolar, etc.) by laser light from the light source. The illumination field stop for defining the illumination area on the reticle 31 is provided, and the illumination area on the reticle 31 is illuminated with a substantially uniform illuminance distribution.

  Here, the illumination optical path in the illumination device 30 is preferably purged with an inert gas. In this embodiment, the illumination optical path is purged with a high-purity nitrogen gas. The reticle 31 is placed on a reticle stage 32, and the reticle 31 and the reticle stage 32 are isolated from the external atmosphere by a casing 33. The internal space of the casing 33 is also preferably purged with an inert gas, and in this embodiment, it is purged with high-purity nitrogen gas.

  The light from the reticle 31 illuminated by the illumination device 30 is a projection optical system having a plurality of lens elements 34 to 39 arranged along the optical axis AX and an aperture stop 40 for controlling a coherence factor (σ value). A pattern image of the reticle 31 is formed in a photosensitive area on the wafer 42 by being guided to a wafer 42 as a photosensitive substrate through 41. The projection optical path in the projection optical system 41 is preferably purged with an inert gas. In this embodiment, the projection optical path is purged with high-purity nitrogen gas.

  The wafer 42 is placed on the wafer stage 43 so that the surface thereof is positioned on the second surface as the image plane of the projection optical system 41. The wafer 42 and the wafer stage 43 are separated from the external atmosphere by the casing 44. Isolated. The internal space of the casing 44 is also preferably purged with an inert gas. In this embodiment, the casing 44 is purged with high-purity nitrogen gas. Then, exposure on the wafer 42 is performed by illuminating the reticle 31 while moving the reticle stage 32 and the wafer stage 43 relative to the projection optical system 41 at a speed ratio corresponding to the magnification of the projection optical system 41. The pattern on the reticle 31 is transferred into the region.

  In this embodiment, adjacent lens elements 37 and 38 in the refraction-type projection optical system 41 are made of a calcium fluoride single crystal, and each has the same axially symmetric stress distribution manufactured in the first embodiment. The optical member is processed and polished into a lens shape, and the antireflection treatment is applied to the surface. That is, the lens element 37 has an axially symmetric stress distribution with the <100> direction as the symmetric axis, and is arranged so that the symmetric axis of the stress coincides with the optical axis. The lens element 38 has the same stress distribution as the lens element 37, and is also arranged so that the symmetry axis of the stress coincides with the optical axis, and the crystallography around the optical axis with respect to the lens element 37. Specifically, they are arranged by rotating 45 degrees. In the projection optical system of the present embodiment, the lens elements 37 and 38 are arranged in a predetermined direction provided by the present invention, and the lens elements 37 and 38 are arranged adjacent to each other in the projection optical system 41. Therefore, the anisotropy of stress birefringence is sufficiently reduced.

In this embodiment, a calcium fluoride single crystal is used as the optical member. However, the present invention is not limited to this, and any crystalline optical member belonging to any of the point groups O, Oh, Td can be used. Barium fluoride (BaF ₂ ), strontium titanate (SrTiO ₃ ), and the like can also be used in the same manner as calcium fluoride.

  In the exposure measure of the present embodiment, a reticle (mask) is illuminated by an illuminating device, and a transfer pattern formed on the mask is exposed to a photosensitive substrate using a projection optical system, whereby a semiconductor element, an imaging element, Micro devices such as liquid crystal display elements can be manufactured.

It is a figure showing distribution of stress birefringence. It is a figure showing the coordinate system used for the analysis. It is a figure which shows the light beam incident angle dependence of the amount of stress birefringence. It is the schematic of a heat processing apparatus. It is a figure which shows the accommodation arrangement | positioning of the optical member at the time of heat processing. 2 is a temperature schedule for heat treatment in Example 1. FIG. It is a stress birefringence measurement result of the optical member manufactured in Example 1. It is the schematic of the optical system concerning this invention. It is a figure which shows the accommodation arrangement | positioning of the optical member at the time of heat processing. It is a stress birefringence measurement result of the optical member manufactured in Example 2. It is the schematic of the optical system concerning this invention. It is the schematic of the optical system concerning this invention. It is a structural example of the exposure apparatus concerning this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... 1st container, 2 ... 2nd container, 3 ... Heater, 4 ... Exhaust port, 7 ... Disc shaped carbon material, 8 ... Felt-like carbon material, 9 ... Disc shaped carbon material 10 ... Optical member, 11 ... Optical member, 12 ... felt-like carbon material, 21 ... optical member, 22 ... optical member, 30 ... illumination device, 31 ... reticle, 34-39 ... lens element, 41 ... projection optical system, 42 ... wafer, 51-52 ... Optical member, 61-64: Optical member

Claims (17)

An optical member comprising a single crystal whose crystal structure belongs to any of point group O, Oh, or Td, and having an axially symmetric stress distribution with the <111> axis as the axis of symmetry. The optical member according to claim 1, wherein the single crystal is a calcium fluoride (CaF ₂ ) single crystal. An optical system having at least one optical member according to claim 1. An optical system having two optical members, wherein the symmetry axes of the two optical members are matched, and one optical member is rotated 60 degrees around the symmetry axis with respect to the other optical member. The optical system according to claim 3, which is arranged as described above. Two optical members each having an axially symmetrical stress distribution with a <100> axis as a symmetry axis are made to coincide with the symmetry axis, and the crystal structure is made of a single crystal belonging to either the point group O, Oh, or Td. An optical system in which one optical member is rotated 45 degrees around the axis of symmetry with respect to the other optical member. The optical system according to claim 4, wherein stress distributions of the two optical members are equal to each other. The crystal structure is a single crystal belonging to either point group O, Oh, or Td, and the birefringence amount and the fast axis direction with respect to the light incident in the <100> direction are four-fold rotationally symmetric in the [100] plane. An optical system in which two optical members indicating the same are aligned with the four-fold rotational symmetry axis, and one optical member is rotated 45 degrees around the symmetry axis with respect to the other optical member. The optical system according to claim 7, wherein the birefringence amount and the distribution in the fast axis direction in the [100] plane of the two optical members are equal to each other. The crystal structure is composed of a single crystal belonging to any of the point group O, Oh, or Td, and the birefringence amount and the fast axis direction with respect to the light incident in the <111> direction show an axisymmetric distribution in the [111] plane. An optical system in which two optical members are arranged such that the symmetry axes coincide with each other and one optical member is rotated 60 degrees around the symmetry axis with respect to the other optical member. The optical system according to claim 9, wherein the birefringence amount and the distribution in the fast axis direction in the [111] plane of the two optical members are equal to each other. The optical system according to claim 6, wherein the two optical members are arranged adjacent to each other. Four optical members comprising a single crystal having a crystal structure belonging to any of point group O, Oh, or Td, and having an axially symmetric stress distribution with the <110> axis as the axis of symmetry, are made to coincide with the axis of symmetry, An optical system in which three of the four optical members are rotated 45, 90, and 135 degrees around the symmetry axis with respect to the other optical member, respectively. The optical system according to claim 12, wherein stress distributions of the four optical members are equal to each other. The optical system according to claim 13, wherein the four optical members are arranged adjacent to each other. The optical system according to claim 5, wherein the single crystal is a calcium fluoride (CaF ₂ ) single crystal. An exposure apparatus comprising the optical member according to claim 1. An exposure apparatus comprising the optical system according to any one of claims 3 to 15.