JP2004145309A - Method for manufacturing optical member formed of fluoride crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical member formed of fluoride single crystal minimizing the influence of birefringence, its manufacturing method, and an exposure device equipped with the optical member. <P>SOLUTION: The manufacturing method for the optical member formed of fluoride crystal includes the processes of: growing a fluoride crystal ingot; cutting a columnar raw material having desired crystal plane azimuths on two parallel planes out of the ingot; determining the crystal azimuth of the flank of the columnar raw material; measuring the birefringence of the azimuth-determined flank in a specified crystal axis direction; and determining the fluoride crystal according to a measurement result of the birefringence. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a camera, a microscope, an optical device such as a telescope, and a manufacturing method for manufacturing an optical member constituting an optical system in an optical lithography apparatus such as a stepper, and an optical material obtained by the manufacturing method. The present invention relates to an optical member made of a fluoride crystal used as an optical member for photolithography of 250 nm or less, and a method for manufacturing the same.

In recent years, lithography technology for drawing integrated circuit patterns on wafers has been rapidly developing. The demand for higher integration of integrated circuits is only increasing, and to achieve this, it is necessary to increase the resolution of the projection optical system of the projection exposure apparatus. The resolution of the projection lens is governed by the wavelength of the light used and the NA (numerical aperture) of the projection lens. To increase the resolution, the wavelength of the light used is made shorter and the NA of the projection optical system is made larger (larger). Diameter).

First, we will discuss how to shorten the wavelength of light. The wavelength of the light source used in the projection exposure apparatus has been shortened to g-line (wavelength 436 nm), i-line (wavelength 365 nm), KrF excimer laser light (wavelength 248 nm), and further ArF excimer laser light (wavelength 193 nm). If an F2 laser beam (wavelength: 157 nm) having a shorter wavelength is to be used in the future, the use of general optical glass as a lens material for an imaging optical system such as a projection optical system causes a decrease in transmittance. It is no longer possible because of problems.

For this reason, it is generally considered that a fluoride crystal such as calcium fluoride (fluorite) is used as an optical member in the optical system of the F ₂ laser stepper.

Next, the enlargement of the diameter will be described. In order to satisfy the optical performance as an optical member used in the optical system of the KrF, ArF excimer laser stepper, or F ₂ laser stepper, it is said that the crystalline material is preferably a single crystal.

(4) Recently, with the improvement in the performance of projection exposure apparatuses, large-diameter calcium fluoride single crystals having a diameter of about 100 mm to 350 mm have recently been required. Such a calcium fluoride (fluorite) single crystal has a lower refractive index and a smaller dispersion (wavelength dependence of the refractive index) as compared with general optical glass, and therefore when used together with an optical member made of another material. This is very effective in that chromatic aberration can be corrected. Further, compared to other crystal materials (such as barium fluoride), it is easily available on the market, and large-diameter single crystals having a diameter of φ100 mm or more are also available.

カ ル シ ウ ム Calcium fluoride single crystals having these advantages have been used as lens materials for cameras, microscopes, and telescopes in addition to optical materials for steppers. Recently, single-crystals of barium fluoride and strontium fluoride other than calcium fluoride single crystal also belong to the same equiaxed crystal system and have similar properties. As attention has been drawn.

As the fluoride single crystal, a single crystal growing method by a melt method such as the Bridgman method (Stockberger method or pull-down method) or the Tamman method is known.

(4) An example of a method for producing a calcium fluoride single crystal by the Bridgman method is described below.

In the case of a fluorite single crystal used in the ultraviolet or vacuum ultraviolet region, it is common to use a high-purity raw material produced by chemical synthesis without using natural fluorite as a raw material.

The raw material can be used as it is as a powder, but in this case, a semi-molten product or a crushed product thereof is usually used because the volume of the material when melted is drastically reduced. First, a crucible filled with the raw materials is placed in a growing apparatus, and the inside of the growing apparatus is kept in a vacuum atmosphere of 10 ⁻³ to 10 ⁻⁴ Pa. Next, the temperature in the growing device is raised to the melting point of fluorite or higher (1370 ° C. to 1450 ° C.) to melt the raw material. At this time, control by constant power output or high-precision PID control is performed in order to suppress temporal fluctuation of the temperature in the growing apparatus.

(4) In the crystal growth (crystal growth) stage, the crucible is pulled down at a speed of about 0.1 to 5 mm / h to gradually crystallize from the lower part of the crucible. When the crystal is crystallized to the uppermost part of the melt, the crystal growth is completed, and simple slow cooling is performed avoiding rapid cooling so that the grown crystal (ingot) does not crack. When the temperature in the growing device has dropped to about room temperature, the device is opened to the atmosphere and the ingot is taken out.

育成 In this crystal growth, a graphite crucible is usually used. The shape of the tip is a pencil shape with a conical tip, and the other portions are cylindrical. Crystals are grown from a pencil-shaped tip located at the lower end of the crucible to obtain a crystallized ingot. Although a seed crystal is inserted into the tip portion to control the crystal plane orientation of the ingot to some extent, when the diameter of the ingot exceeds φ100 mm, the orientation control becomes extremely difficult.

Fluoride crystals manufactured by the Bridgman method are basically considered to have no advantage in the growth orientation, and the horizontal plane of the ingot becomes a random plane every time the crystal grows.

Since a large residual stress exists in the removed ingot after the crystal growth, a simple heat treatment is performed while keeping the shape of the ingot.

カ ル シ ウ ム The calcium fluoride single crystal ingot thus obtained is cut into a suitable size for each target product. Here, when the crystal plane orientation is not taken into account, the ingot is cut horizontally (round cut) in order to efficiently cut out a larger material for producing an optical element (such as a lens) from the ingot. Thereafter, the cut-out material is subjected to a heat treatment to obtain desired optical performance (homogeneity of refractive index and birefringence).

As an example of a method for producing a calcium fluoride single crystal using the Bridgman method and heat treatment as described above, a fluorite single crystal is grown by the Bridgman method, and then subjected to a heat treatment so that the refractive index difference is 5 × 10 5 ^A method for obtaining an optical material of ^-6 or less (see Patent Document 1 below) is known.

By the way, since the optical performance of the fluoride single crystal in the direction perpendicular to the {111} crystal plane is higher than that of the other crystal planes, the {111} crystal plane of the fluoride single crystal ingot was measured, and the {111} crystal plane was measured.熱処理 A heat treatment is performed after cutting out a material for producing an optical element so that the planes become two parallel planes.

Alternatively, after subjecting a fluoride single crystal ingot obtained by crystal growth to heat treatment for obtaining desired optical performance (homogeneity of refractive index and birefringence), {111} crystal planes are parallel two planes. There is a method of cutting out a material for producing an optical element so as to obtain a fluoride single crystal having good optical performance.

On the other hand, since the intrinsic birefringence of the fluoride crystal increases as the measurement wavelength becomes shorter, the projection lens of 193 nm or less has a {100} plane or a {100} plane in addition to the {111} plane fluoride crystal. It has been studied to use a fluoride crystal having two parallel 110 ° planes. Even for a fluoride crystal having a {100} plane or a {110} plane as two parallel planes, a cutting step and a heat treatment step similar to those of the {111} plane are performed.

Also, in order to control and reduce the influence of the intrinsic birefringence of the fluoride crystal, at least two crystal plane orientations have been measured by, for example, the Laue method.

In the text, birefringence is a phenomenon in which the refractive index differs depending on the polarization direction of light (= electromagnetic wave). The polarization direction with the minimum refractive index is the “fast axis” and the polarization direction with the maximum refractive index is Each is called the "slow axis." Birefringence is usually represented by the optical path difference (called retardation) between the polarization of the fast axis and the polarization of the slow axis when passing through the unit length of the substance, and the unit is nm / cm. In addition, birefringence may be caused not only by intrinsic birefringence inherent to a substance or a crystal structure but also by distortion (strain) caused by thermal stress or the like. Such birefringence is simply referred to as distortion. Sometimes called.

Intrinsic birefringence has a material-specific value irrespective of the crystal manufacturing method and heat treatment conditions. Therefore, without measuring the amount of birefringence or the direction of the fast axis, the amount of birefringence can be controlled by measuring only the crystal plane orientation, and the effects of combining multiple optical members can be canceled out. It is possible.

On the other hand, with respect to the occurrence of birefringence due to thermal stress, the value of the birefringence of the fluoride crystal after the heat treatment is reduced in the optical axis direction by performing a heat treatment as in the case of multi-component optical glass and quartz glass. Was measured to be about 1 to 2 nm / cm (measurement wavelength: 633 nm), and it was thought that the stress due to thermal stress was reduced to a level that would not hinder free optical design. For example, there has been known a method for producing a fluorite single crystal having a large diameter and a small birefringence that can be used for photolithography with a wavelength of 250 nm or less (see Patent Document 2 below). According to this method, the birefringence in the side direction perpendicular to the optical axis of the fluorite single crystal after the heat treatment was reported to be substantially the same at a rotation angle of 360 °. The birefringence in the lateral direction (at any angle) was measured without determining
JP-A-8-5801 JP-A-11-240796

The present inventors measured the amount of birefringence in a variety of plane orientations of a calcium fluoride single crystal, and found that stress-induced birefringence that cannot be suppressed by heat treatment remains in the calcium fluoride single crystal. all right. It has been found that such birefringence is at a level that adversely affects the optical design. Furthermore, it has been found that this type of birefringence cannot cancel the effect even when a plurality of optical members are combined. That is, it is necessary to constitute the optical system with an optical member of this type having a birefringence of a predetermined level or less.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an optical member formed of a single crystal of fluoride and minimizing the influence of birefringence, and a method of manufacturing the same. Still another object of the present invention is to provide an exposure apparatus provided with the optical member of the present invention.

That is, the present invention provides a growing step of growing a fluoride crystal ingot, a cutting step of cutting out a cylindrical material having a desired crystal plane orientation as two parallel surfaces from the ingot, and determining a crystal orientation of a side surface of the cylindrical material. Orientation determination step, a birefringence measurement step of measuring birefringence in a predetermined crystal axis direction of the side surface obtained in the orientation determination step, and a determination step of determining a fluoride crystal based on the measurement result of the birefringence. And a method for producing an optical member comprising a fluoride crystal having the following.

The present inventors measured the amount of birefringence in a variety of plane orientations of a calcium fluoride single crystal, and found that birefringence due to thermal stress may not be sufficiently suppressed by the conventional method. . Then, when the butterfly was further continued, it was found that the amount of birefringence of the side surface perpendicular to the optical axis of the calcium fluoride single crystal generated due to thermal stress fluctuates greatly depending on the crystal direction. Therefore, the present inventors previously determined a specific crystal plane orientation in which birefringence due to thermal stress increases, and by managing the amount of birefringence in that orientation, an optical member having good optical characteristics can be obtained. Succeeded in sorting. For example, when the two parallel planes are {111} planes, it is only necessary to determine whether the birefringence in the specific crystal direction <110> on the side surface is equal to or less than a specified value. In this case, it may be determined whether the birefringence in the specific crystal direction <100> direction or <110> direction of the side surface is equal to or less than a specified value. Then, only a member having a specified value or less may be used as a material constituting an optical system such as a lens.

In particular, recently, since an optical lens having a large NA is used to improve the resolution, light passing through the lens has a large component oblique to the optical axis. This oblique component is affected by birefringence not only in the optical axis direction but also in the direction perpendicular to the optical axis, that is, in the side surface direction of the columnar crystal (in-plane direction perpendicular to the optical axis). Therefore, according to the present invention, by inspecting the amount of birefringence in the side direction perpendicular to the optical axis, it can be determined whether or not the optical member is suitable for an optical system used in an exposure apparatus or the like.

に お い て In the orientation determining step of the method of the present invention, the crystal orientation of the side surface can be obtained by measuring the birefringence of the side surface at a plurality of angles. In the determination step, it can be determined whether or not the maximum value of the birefringence in the specific crystal axis direction on the side surface is 10 nm / cm or less at a measurement wavelength of 633 nm. In the projection optical system used for the exposure apparatus, good imaging performance can be obtained by suppressing the maximum value of birefringence in the direction of the specific crystal axis on the side surface to 10 nm / cm or less at a measurement wavelength of 633 nm. When the maximum value of the amount of birefringence in the specific crystal axis direction on the side surface is 10 nm / cm or less at a measurement wavelength of 633 nm, the material can be formed into a predetermined optical member shape. Otherwise, the columnar material may be discarded as inappropriate as an optical member.

According to the present invention, there is provided a fluoride crystal optical member manufactured by the manufacturing method of the present invention. In this optical member, the maximum value of the amount of birefringence in the direction of the specific crystal axis of the side surface measured with a columnar material shape having a specific crystal plane orientation as two parallel surfaces is 10 nm / cm or less at a measurement wavelength of 633 nm. The fluoride crystal may be a calcium fluoride single crystal.

According to the present invention, there is provided an exposure apparatus including an optical member manufactured by the manufacturing method of the present invention. The exposure device may include an excimer laser or F ₂ laser as a light source.

According to the present invention, it is possible to provide an optical member in which the crystal plane orientation other than the optical axis is controlled, and it is possible to secure desired optical performance. Excimer laser with an optical member obtained in this way, by constituting the projection optical system of a projection exposure apparatus that uses ultraviolet, etc. F ₂ laser as a light source, it is possible to provide a high resolution exposure apparatus.

Mode of discretion for carrying out the invention

FIG. 1 shows an example of a flow of a method for manufacturing an optical member of the present invention. The method for manufacturing an optical member according to the present invention basically includes a step of growing a crystal, a step of cutting out a columnar material, and measuring the crystal orientation and birefringence of the side surface of the material. And a sorting step of judging the quality or usage of the device.

The crystal growth step 601 specifically includes a raw material refining step, a pretreatment step, a crystal growth step, an annealing step in a crystal growth furnace, and the like.

(4) A step 602 of measuring a crystal plane orientation in an ingot shape may be performed before the step of cutting out a columnar material.

(4) The columnar material cutting step 604 specifically includes an inspection step for foreign matter inside, a cutting step, a rounding step, a surface grinding step, a chamfering step, and the like. When cutting is performed, a cutting orientation determining step 603 for determining a cutting plane and a cutting position by using a Laue method or a cleavage plane described later is performed.

After the cutting step, a step of measuring the crystal orientation and birefringence of the side surface of the material is performed as necessary.

{Circle around (5)} The annealing process (heat treatment process) 605 is performed on the cut material. Specifically, the annealing step includes a heating step of gradually increasing the temperature in the annealing furnace, a holding step of holding the material at a constant temperature, a cooling step of gradually lowering the temperature, and an annealing step. A cooling step of turning off the heater of the furnace and allowing the furnace to cool.

After the annealing step, a step 606 of measuring the birefringence of the material after annealing is performed. Specifically, the in-plane birefringence, the distribution of the direction of the fast axis, the birefringence in the lateral direction, the direction of the fast axis, and the like are measured. Based on the measurement result, the material is sorted. If the desired amount of birefringence has been obtained, it is processed as it is into a lens shape (step 607), and an optical system is formed through an assembly step 608. If the desired amount of birefringence is not obtained, for example, a process is performed again through an annealing step so that the desired amount of birefringence is obtained.

The manufacturing method of the present invention measures, in particular, the crystal orientation in the side direction of a cylindrical material having two parallel surfaces cut out by a predetermined crystal plane orientation, the amount of birefringence in the orientation, the direction of the fast axis, and the like. It is characterized by the following.

結晶 The crystal plane orientation of the material does not change due to the heat treatment, but the birefringence changes the birefringence amount and the direction of the fast axis before and after the heat treatment. Therefore, in the present invention, the crystal plane orientation is measured in the state of the ingot or at least one stage after or after the heat treatment, and the birefringence (amount, fast axis) is measured at least after the heat treatment. Is required.

It is preferable to measure the crystal plane orientation and the birefringence in each direction before the heat treatment, and appropriately determine the heat treatment conditions based on this information in order to optimize the birefringence. When the crystal plane orientation in the lateral direction is measured before the heat treatment, the orientation can be marked on the material, and the birefringence in the orientation of the marking can be measured after the heat treatment.

Furthermore, as described later, the present inventors have confirmed that the birefringence in the side direction has a periodicity in the circumferential direction in relation to the crystal plane orientation. Therefore, it is also possible to estimate the crystal plane orientation by measuring the birefringence in the side direction from a plurality of angles without measuring the crystal plane orientation in the side direction. That is, the crystal orientation of the side surface can be determined by measuring the birefringence amount at a plurality of angles in the side surface direction.

Hereinafter, each step will be described in detail.
<Positive crystal growth>
In the crystal growing step, a method similar to the conventional method for producing a fluoride crystal is used. Hereinafter, a method for growing a calcium fluoride single crystal by the Bridgman method (stock burger method or reduction method) will be described.

FIG. 2 is a conceptual diagram showing a method for growing a calcium fluoride single crystal.

The raw materials use high purity raw materials made by chemical synthesis. First, a semi-molten product of a raw material was produced by the following procedure. The graphite crucible filled with the powder raw material is stacked and placed in the pretreatment device, and the inside of the pretreatment device is maintained in a vacuum atmosphere of 10 ^-3 to 10 ^-4 Pa. Next, the temperature in the pretreatment device is raised to the melting point of calcium fluoride or higher (1370 ° C. to 1450 ° C.), and the raw material is melted. At this time, it is preferable to perform PID control in order to suppress temporal fluctuation of the temperature inside the apparatus. A fluorinating agent such as lead fluoride was added to this powder raw material. The semi-molten product thus obtained is transferred to a crystal growing furnace and heated again to the melting temperature, and then, at the crystal growth stage, the crucible is lowered at a rate of about 0.1 to 5 mm / h, whereby the crucible is reduced. Gradually crystallizes from the bottom of When the crystal is crystallized to the uppermost part of the melt, the crystal growth is completed, and simple slow cooling is performed avoiding rapid cooling so that the grown crystal (ingot) does not crack. When the temperature in the growing device has dropped to about room temperature, the device is opened to the atmosphere and the ingot is taken out.

In this crystal growth, a pencil-shaped ingot having a conical tip is manufactured using a crucible 702 made of graphite (carbon) having a diameter of 300 mm in a growth furnace 701 having heat insulation and airtightness. At this time, single crystals can be formed by growing crystals from the tip of the conical portion located at the lower end of the crucible. The crucible 702 is movable in the vertical direction via a support 703. In the growth furnace 701, a high-temperature side heater 704 is arranged at an upper part and a low-temperature side heater 705 is arranged at a lower part along the inner peripheral surface of the furnace 701, so that the lower part has a lower temperature than the upper part in the furnace. Can be heated. Further, an exhaust line 706 for reducing the pressure inside the furnace is provided below the growing furnace 701. First, a seed crystal was placed at the tip of the crucible. This seed crystal also has the purpose of controlling the crystal growth orientation, but in actuality, the horizontal plane of the ingot is a random plane for each crystal growth, and thus the plane orientation cannot be estimated at this stage.

In the development step, first, placing the crucible 702 a raw material for fluoride single crystal is filled, as shown in FIG. 2A in the upper portion of the growth furnace 701, 10 ^-3 to exhaust to the growth furnace from the exhaust line A vacuum atmosphere of 10 ^-4 Pa is maintained. Next, the inside of the growth furnace is raised to a temperature higher than the melting point of fluoride (1370 ° C. to 1450 ° C. in the case of calcium fluoride) by the heaters 704 and 705 to melt the raw material.

Next, as shown in FIG. 2B, the crucible 702 is pulled down at a predetermined speed (0.1 to 5 mm / hour) through the support rod 703, and the melt 707 is gradually crystallized from the lower side of the crucible. To grow a fluoride single crystal 708.

Next, as shown in FIG. 2C, when the melt 707 has crystallized to the top of the crucible, the crystal growth is completed. In order to prevent the grown crystal 709 (ingot) from cracking and reduce residual stress, the temperature is slowly lowered to about room temperature in a furnace, and the ingot is taken out (A). <Plane orientation measurement process>
When cutting a columnar (cylindrical) material from an ingot, the crystal orientation of at least two parallel surfaces is determined. Further, at this stage, two or more crystal orientations may be measured using the Laue method or the like. In addition, the present invention is not limited to measuring the crystal orientation of the entire ingot. The test piece may be cut out from the top portion or the cone portion of the ingot and the crystal orientation may be measured to determine the crystal orientation of the entire ingot. Further, the crystal orientation may be measured in a state of a base material obtained by cutting or processing the ingot to some extent.

For the measurement of the plane orientation, for example, the Laue method of measuring the crystal plane orientation by irradiating the test object with X-rays is used.

The Perau method has an advantage that various crystal plane orientations such as {111} and {110} can be easily measured and managed.

If the Laue method is based on the side reflection method, there is an advantage that measurement can be performed even on a large-diameter test object without damaging the test object.

When determining the crystal orientation, it is preferable to keep the deviation angle from the desired orientation within 3 °. The method for measuring the crystal orientation suitable for the present invention will be described in detail below.

Evaluation methods for crystal orientation include X-ray methods, mechanical methods, and optical methods.

The method of determining the orientation of a crystal by X-rays is the Laue method of irradiating X-rays while the crystal is stationary, the rotation method of irradiating X-rays while rotating or oscillating the crystal, the Weissenberg method of changing them. And the precession method.

Next, the mechanical method is described.

Generally, when a crystal is plastically deformed by an appropriate means, various surface patterns characterized by crystal orientation appear on the surface. For example, an impression image (or a hitting image) having a shape specific to a crystal plane, a slip band along a specific crystal plane, a twin crystal, a cleavage, and the like. Among them, twins include annealing twins and growth twins in addition to twinning deformation caused by plastic deformation, and these still form a surface pattern.

Specifically, a method using an indentation image, a method using a slip ellipse, a slip line, a twin, a method using an intersection angle between other surface patterns, a method using a cleavage plane, a slip, a twin, And cleavage analysis.

光学 Moreover, the optical method includes an angle measurement method, an eclipse method, an optical image method, a polarization method, and the like.

Ｘ Among these measurement methods, the method using X-rays is suitable for use in the present invention because of high measurement accuracy and high speed. Hereinafter, a method using X-rays will be described.

In the case of using an X-ray diffractometer, a rear reflection Laue camera is attached to the X-ray tube on the side opposite to the diffractometer. The distance between the sample surface and the film is set to several tens mm. The X-ray tube is a Mo target, and images are taken at a tube voltage of 40 kV, a tube current of 50 mA, and an exposure time of 60 sec. The azimuth analysis is performed manually from the obtained polaroid photograph of the Laue pattern, or the photograph is taken into a computer by a scanner and calculated.

The Laue method is one of the X-ray diffraction methods in which white X-rays are applied to a fixed single crystal. Since the Bragg angle θ is fixed to all surfaces of the crystal, each surface is An X-ray having a wavelength λ that satisfies the Bragg condition λ = 2d · sin θ with respect to the plane distance d and the Bragg angle θ is selected and diffracted. The Laue method includes three methods, a transmission method, a back reflection method, and a side reflection method, by changing the relative positional relationship of an X-ray source, a crystal, a film, or a CCD camera. In transmission, a film or CCD camera is placed behind the crystal to record the forward diffracted beam. In the back reflection method, the film is placed between the crystal and the X-ray source, and the incident beam is recorded as a diffracted beam backward through a hole in the film. In the lateral reflection method, the X-ray source is set so that it is incident on the crystal at a certain incident angle ω, and the film or CCD camera is positioned at ψ rotation with respect to the incident beam to record a diffracted beam in any lateral direction. Is placed. The diffracted beam forms Laue spots on the film or phosphor plate in either way. In any method, the position of the Laue spot is determined by the relative relationship of the crystal direction to the incident beam, and this is applied to determine the crystal direction.

The Perau method can easily measure various crystal plane orientations such as {111} and {110}, and is suitable for the present invention together with measurement accuracy and speed.

ミ ラ ー The Miller index is used as a method of indicating the crystal orientation. The Miller index is the reciprocal of the ratio of the distance from the origin of the unit cell of the crystal to the point where the plane intersects the crystal axis with respect to the unit length of that axis, and in the case of a cubic system such as calcium fluoride, Assuming that the unit length is a, the Miller index is (hkl) when a plane intersects the axis at points a / h, a / k, and a / l from the origin. In the cubic system, the direction [hkl] is always perpendicular to the plane (hkl) of the same index, and the symmetrical direction is represented by one index and is indicated by <>, and the symmetric equivalent lattice The surface is also represented by a single index and is indicated by ｛｝. For example, the cube diagonal lines [111], [1-11], [-1-11], [-111], etc. are all represented by <111>, and the cube surfaces (100), (010), (- 100), (0-10), (001), and (00-1) are represented by {100}.

A device for measuring the crystal plane orientation by the Laue method comprises an X-ray source, a sample stage and a CCD camera (FIG. 3). X-rays are incident on the sample and the resulting diffraction beam is analyzed as a Laue pattern.

In the present invention, it is proposed to preferably use the following method for measuring a large sample such as a φ300 × t60 mm block.

First, the measurement sample 800 is placed flat on a stage, and an X-ray source 820 and a film or an optical system of a CCD camera 830 are installed thereunder. By placing the measurement sample 800 flat on the stage 810, it is possible to cope with a large sample, and it is also possible to perform detailed measurement such as mapping, and it is only necessary to select spots and determine whether or not the simulation is correct. Efficient measurement has become possible. Further, damage to the sample by X-rays is weakened by the side reflection method.

(4) The crystal plane orientation is measured by the Laue method. The measurement of the crystal plane orientation is synonymous with the measurement of the crystal axis, that is, measuring the {111} plane orientation is equivalent to measuring the <111> axis.

Using the measurement method described above, manage the plane orientation by the procedure of cutting the top and cone of the ingot → measuring the orientation → estimating the orientation of the ingot from the top and cone data → cutting out the member → measuring the effective outside diameter of the member (in some cases mapping) I do.

手 順 The procedure for determining the plane orientation of the fluoride single crystal obtained by the Bridgman method is specifically described below.

(1) For a fluoride single crystal ingot, first, a side portion facing the front in the furnace is scraped and smoothed with a wire brush, and one straight line is drawn with a glass pencil to be a position reference line (B).

Subsequently, the conical shape at the tip (referred to as a cone portion) and the opposing portion (referred to as a top portion) are cut to a thickness of 30 mm to obtain a test piece for plane orientation measurement (C). The plane orientation of the main body is estimated by measuring the plane orientation of the two test pieces. The positional relationship between the two test pieces and the ingot body is confirmed by the first position reference line. The plane orientation of the test piece was measured by the Laue method.

In addition, it is possible to measure the plane orientation of the entire ingot as it is by the Laue method, but it is difficult to handle because the ingot weighs several tens of kg, and calcium fluoride has a large expansion coefficient. It is necessary to cut out the test piece (φ30 ~ 60 × t20mm) for the management of the transmittance and the excimer laser resistance from the top and the cone part that the mechanical strength is not great and the ingot is also at risk of breakage. Since the cutting step is indispensable, it is more advantageous to perform measurement using a test piece.

It should be noted that both calcium fluoride (or barium fluoride) single crystals have cleavage properties on the {111} crystal plane. Therefore, when a thermal stress or the like is applied to the ingot, the ingot is broken (cleaved) by the {111} crystal plane. Also, in the case of an ingot that has not been cleaved, it is cleaved by tapping the end portion lightly with something like a chisel. By cutting the ingot on the basis of the cleaved surface (cleaved surface) so as to be parallel to the cleaved surface, a material for producing an optical element can be collected. The obtained material has two parallel {111} crystal planes. As described above, there is a method based on the cleavage plane, but according to the Laue method, each plane orientation can be instantaneously measured without destruction.

(4) An automatic measuring apparatus based on the Laue method will be described below. The automatic measuring device includes an X-ray source, a sample stage, and a CCD camera.

The sample is placed flat on a stage, and an X-ray source and an optical system of a film or a CCD camera are set under the stage, so that a large sample can be handled. The top and cone test pieces are installed so that the reference line is located in front. Since the cone part is conical, it is measured with the flat part facing downward. For this reason, the measured value of the plane orientation is turned over and collated with another part. The X-ray tube uses a W target with a maximum output of 2 kW, for example, a tube voltage of 50 kV and a tube current of 40 mA. X-rays generated by the X-ray tube are substantially collimated by a double pinhole collimator having a diameter of about 1 mm, focused to a beam diameter of about 2 mm, and then incident on a sample. X-ray irradiation time is about 1 minute. The diffracted beam is projected on a fluorescent plate, imaged by a CCD camera, and taken into a computer as a Laue pattern. The CCD is cooled to −50 ° C. by the Peltier device, and the S / N ratio is improved. The captured Laue pattern is analyzed on an orientation analysis screen. The Laue pattern is composed of a plurality of dot sequences, and one dot sequence represents diffraction spots from the same zone axis. Indexing is automatically performed by designating four points from the spots (intersections of the point sequence) belonging to a plurality of crystal zone axes from among them, and the simulation pattern is superimposed on the Laue pattern when a match is made. The measurer determines the degree of coincidence between the two patterns. When the index is determined, a stereo projection, a stereo triangle, and the plane azimuth angle of each plane are output as the azimuth analysis result. For the plane azimuth angle, an angle between the z-axis and <111> is α, and <111> is projected on the measurement surface in a coordinate system in which the depth of the sample stage is the X-axis direction and the vertically downward direction of the sample stage is the z-axis direction. + X on X axis of line
The angle formed counterclockwise from the direction is represented as β. FIG. 4 is a conceptual diagram of α and β in a cylindrical member.

On the other hand, the remaining ingot body obtained by cutting the cone and the top portion is rounded, and the cylindrical surface portion is made into a surface equivalent to sanding (E). It is also possible to observe the inside by grinding the side surface with a width of several cm (D). In addition to observation from the surface on the sand surface, observation of the inside in a dark room by applying a matching oil of refractive index, observation of stress concentration at the interface by the crossed Nicol optical system, etc., sub grain boundary (sub bun), Check the state of the polycrystal and the position of its interface. Further, the state of bubbles and foreign substances is also checked at the same time. If the entire ingot has grown into a single crystal, measuring the plane orientation of one of the cone and the top can estimate the plane orientation of the entire ingot. It is desirable to measure the plane orientation and confirm that the plane orientations do not conflict with each other. Furthermore, ingots are often polycrystalline or have sub-vans. In such a case, it is necessary to measure the crystal orientation for each single crystal portion in the ingot.
<Determination of cutting direction and cutting process>
The cutting direction of the ingot is determined based on α and β obtained by the above test pieces. When using the azimuth angle of the top part, the side face in the (90 ° -β) direction when the ingot is viewed from the top direction and the counterclockwise direction from the reference line direction is centered around the surface normal of the cut surface of the top part Is the seat surface, and the direction of α in the clockwise direction is the cutting direction with the cut surface of the top portion as the reference surface. When the azimuth angle of the cone is used, the side face in the (90 ° -β) direction when the ingot is viewed from the cone direction and the counterclockwise direction from the reference line direction is centered around the surface normal of the cut surface of the cone part Is the seating surface (processing reference surface), and the direction of α is the clockwise direction with the cutting surface of the top portion as the reference surface (F). Further, while avoiding the position of bubbles and foreign substances inside the ingot, the cutting position is determined to a desired part size with a processing allowance of the annealing step, a thickness and a diameter of +5 to 10 mm (G). For cutting, first make a ground surface (seat surface) parallel to the axis of the ingot in the direction of the bearing surface of the side surface of the ingot determined from the plane azimuth, and set the ingot with the bearing surface facing down on the stage of the cutting machine, and set the top surface The ingot is rotated α and cut as a reference plane (H). The disc on the ellipse obtained by cutting is coated with a matching oil of refractive index to observe the inside in a dark room, observe the stress concentration at the interface by the cross Nicol optical system, etc., sub grain boundary (sub bun), poly crystal Check the state of the interface and the position of the interface, and avoid the position of bubbles and foreign matter inside the ingot, and adjust the part sampling position to the desired part size with the machining allowance, thickness and diameter of +5 to 10 mm for the annealing process. Then, cutting (rough cutting) (I) and rounding (J) are performed. Further, rough grinding and chamfering for inspecting the plane orientation are performed (K). At this time, by maintaining a marking that makes the relationship with the plane orientation obtained from the first reference line clear, the plane orientation of the final member can also be managed.

Thus, by providing the plane orientation determining step before the cutting-out step, the deviation angle of the manufactured optical member from the desired orientation can be made within 3 °. Although this angle can be used up to 4 ° and up to about 6 ° in the most deviated state, it is preferably 3 ° or less, particularly preferably 2 ° or less.

The Laue method used in the present invention can be used not only for such management of the plane orientation but also for detection and management of sub-boundary and twin. Slightly sub-boundary is not easy to detect visually, and it is usually necessary for a skilled person to detect light by obliquely shining light on the ground surface. If such a mapping measurement is performed, if the sub-boundary exists, a deviation of the plane orientation of about several degrees exists, and therefore, it can be easily detected.
<Heat treatment process>
For the two columnar materials of φ260 × t50 and φ200 × t60 obtained in this way, the crystal plane (axis) orientation in the lateral direction is measured, and the amount of birefringence and the direction of the fast axis in that direction are measured. After the measurement, heat treatment (annealing) is performed for quality improvement. Measuring the birefringence of light passing through a direction perpendicular to the normal direction of the two parallel planes, that is, the direction of the side surface of the member, is hereinafter referred to as measurement of the side direction.
<Plane orientation / birefringence measurement process>
For the measurement of the lateral direction, a method similar to the above-described surface orientation measurement by the Laue method with an ingot is used.

The plane orientation of the two parallel planes is determined as a {111} plane or a {100} plane, and the measurement in the optical axis direction is unique, but the measurement in the side direction is directed toward the center of the member. There is an optionality of 180 °. When the two parallel planes are {111} planes, the optical axis direction is the <111> axis, but in the side direction orthogonal thereto, for example, the <110> axis and the <211> axis exist. When the two parallel planes are {100} planes, the direction of the optical axis is the <100> axis, and the side directions orthogonal thereto are, for example, the <100> axis and the <110> axis. As a result of detailed measurement of the lateral direction by the present inventors, it has a certain periodicity with respect to the rotation direction of 180 °, and when the two parallel planes are {111} planes, the direction of the <110> axis And when the two parallel planes are {100} planes, the birefringence in the lateral direction has a maximum (maximum) value in both the case of the <110> axis direction and the case of the <100> axis direction. I understand.

Therefore, the relationship between the crystal plane orientation in the side direction and the birefringence is measured by measuring the side direction.

In this measurement, for example, a crystal plane orientation in a specific side direction is measured by the Laue method, a birefringence amount in the direction is measured, and thereafter, a birefringence amount in a side direction having a predetermined angle from the direction is sequentially measured. I do. Specifically, when the two parallel planes are {111} planes, the birefringence in the direction of the <110> axis of the side surface is measured. In this case, the <110> axis exists in the rotation direction of 120 ° in the side direction. Preferably, the amount of birefringence is measured at an angular interval of, for example, 30 ° in the rotational direction, preferably about 10 °, with respect to the specific <110> axis direction.

However, without using the Laue method to measure the crystal plane orientation in the lateral direction, it is also possible to estimate the crystal plane orientation in the lateral direction by using the periodicity described above. That is, the birefringence amount is measured in a rotation direction of 180 ° in, for example, 10 ° steps with reference to an arbitrary position in the side direction. The direction of the maximum value thus obtained is the <110> axis when the parallel two planes are {111} planes, and the <110> or <100> axis direction when the parallel two planes are {100} planes. It becomes.

As described above, the crystal plane direction and the amount of birefringence in the lateral direction before the heat treatment, and the direction of the fast axis are measured.

At this time, it is preferable to perform marking in order to maintain the crystal orientation of the material during annealing. For marking, a soft pencil or red oil-based ink that does not damage the calcium fluoride surface and does not cause impurity contamination is used. Since the red oil-based ink turns black after annealing, it is possible to identify before and after annealing.

Further, the mapping measurement of the plane orientation in the plane of the two parallel surfaces of the material is performed. In the plane orientation measurement accompanied by X-ray irradiation such as the Laue method, the calcium fluoride material is damaged and a color center is generated. Is suitable for mapping measurement. After approaching the lens shape, it is only possible to measure outside the effective diameter of the optical design, that is, within a few mm range around the lens.

In the present invention, it is preferable to precisely control the angle shift of the crystal plane. When measuring the angle shift, an apparatus for measuring the angle shift between the sample surface and the crystal plane from Laue spots obtained by side reflection using a side reflection Laue method was used. The Laue method generally employs a back reflection method or a transmission method. In this case, it is preferable to control the transmittance after X-ray irradiation so as to minimize damage to the sample.
<Heat treatment process>
Hereinafter, the material whose crystal plane orientation and birefringence have been measured is subjected to heat treatment to improve optical performance such as birefringence.

(4) The columnar material is placed in a container of the heat treatment apparatus so that the plane faces up and down, and is heated by a heater to perform heat treatment (annealing, heat treatment temperature: 1080 ° C.) (L).

The heat treatment device is a vacuum device and has a structure to prevent the incorporation of oxygen, which causes calcium fluoride to be smudged. The outer structure is made of stainless steel, and a graphite heater and a graphite container are installed inside. In order to completely exclude oxygen inside and coat the metal exposed on the inner surface of the furnace with fluoride, at the time of heat treatment, about 100 g of ammonium acid fluoride and calcium fluoride are enclosed simultaneously with the calcium fluoride member inside the furnace. I do. In this state, the inside of the furnace is evacuated by a vacuum pump, and then the temperature is increased. Before and after the furnace temperature exceeds 500 ° C., vaporization of ammonium acid fluoride starts, and the furnace pressure turns to a weak positive pressure. The temperature is increased while maintaining the pressure, and the temperature is maintained at 1080 ° C. and the temperature is gradually cooled so as to maintain the weak positive pressure (2 to 8 kPa).

By performing annealing as described above, the amount of birefringence due to thermal stress in the specific crystal plane orientation of the calcium fluoride member can be reduced.

In particular, when maintaining at a predetermined temperature, a heat treatment is performed so that the temperature distribution inside the bulk (member) at the time of cooling and cooling is 0.5 ° C. or less in any case, so that the predetermined crystal plane on the side surface is obtained. It is possible to reduce the amount of birefringence in the direction.

It is preferable that the heat treatment apparatus has a heat insulating material or a heater arranged so as to cover the entire surface of the object to be treated (calcium fluoride material). In order to improve the soaking state in the apparatus, it is preferable that the apparatus has a sufficiently large capacity with respect to the member, for example, a device having a volume of 10 times or more the member.

In addition, in order to further increase the temperature uniformity, it is preferable to rotate the object in the heat treatment apparatus.

Alternatively, a heat treatment in which a heater is arranged so as to provide a heat distribution that eliminates the circumferential distribution of the birefringence in accordance with the crystal plane orientation of the side surface of the cylindrical material and the measurement result of the birefringence in each orientation. By using the apparatus, it is also possible to reduce the amount of birefringence in the side direction of the material to be processed.

(4) The side surfaces of the heat-treated cylindrical material were window-processed, and the upper and lower surfaces were ground by 2.5 mm (M). Then, the birefringence of the side surface and the homogeneity of the refractive index are confirmed (N). Thereafter, rounding is performed (O).

After annealing, polishing (temporary luster) and chamfering (P), the value of the birefringence of light traveling in the direction of the normal to the two parallel planes was measured by using an automatic birefringence measuring device manufactured by Oak Works or Uniopt Co., Ltd. The point is automatically measured (measurement wavelength is 633 nm). This measurement is called measurement in the optical axis direction. The birefringence of light passing through a direction orthogonal to the normal direction of the two parallel planes, that is, the direction of the side surface of the member is also measured. If the outer circumference is a circle, an auxiliary tool that allows light to travel straight is required, but automatic measurement is possible by devising the holding of the member. This measurement is called a lateral measurement.

According to the various heat treatment methods described above, in the present invention, the absolute value of the birefringence amount on the side surface of the material can be reduced regardless of the crystal orientation. As described above, by reducing the amount of birefringence in the side direction, it becomes easy to control the intrinsic birefringence due to the crystal plane orientation. That is, the birefringence due to the thermal stress of the present invention is measured and managed at a wavelength of 633 nm, but it is necessary to accurately control the influence of the intrinsic birefringence which increases at a wavelength (for example, 193 nm) actually used as an optical member. It is very effective to first minimize the amount of birefringence due to thermal stress at 633 nm.
<Example of projection exposure apparatus>
Next, an example of a projection exposure apparatus equipped with an optical member made of a fluoride crystal obtained according to the present invention will be described.

The projection exposure apparatus shown in FIG. 5 includes an F2 laser (wavelength: 157 nm) as a light source 11 for supplying illumination light in an ultraviolet region. The light emitted from the light source 11 uniformly illuminates the mask 13 on which a predetermined pattern is formed, via the illumination optical system 12.

In the optical path from the light source 11 to the illumination optical system 12, one or a plurality of bending mirrors for changing the optical path as necessary are arranged. The illumination optical system 12 includes, for example, a fly-eye lens, an internal reflection type integrator, and the like, and projects a field stop for defining a surface light source having a predetermined size and shape on the mask 13. And an optical system such as a field stop imaging optical system. Further, an optical path between the light source 11 and the illumination optical system 12 is sealed by a casing (not shown), and the light path from the light source 11 to an optical member disposed on the side of the illumination optical system 12 close to the mask 13 is provided. The space is replaced with an inert gas (nitrogen, helium, or the like) having a low absorption rate of exposure light.

The mask 13 is held on the mask stage 15 in parallel with the XY plane via the mask holder 14. A pattern to be transferred is formed on the mask 13, and a slit-shaped pattern region having a long side along the Y-axis direction and a short side along the X-axis direction in the entire pattern region is illuminated. .

The mask stage 15 is two-dimensionally movable along a mask plane (XY plane), and its position coordinates are measured and controlled by an interferometer 17 using a mask moving mirror 16. .

As described above, the mask 13, the mask holder 14, and the mask stage 15 disposed between the illumination optical system 12 and the projection optical system 18 are housed in a casing (not shown), and an inert gas ( Nitrogen, helium, etc.).

Light from the pattern formed on the mask 13 forms a mask pattern image on the wafer 19 as a photosensitive substrate via the catadioptric projection optical system 18. The wafer 19 is held on a wafer stage 21 via a wafer holder 20 in parallel to the XY plane. Then, on the wafer 19, a slit-like shape having a long side along the Y-axis direction and a short side along the X-axis direction is provided so as to optically correspond to the slit-like illumination region on the mask 13. A pattern image is formed in the exposure area.

The wafer stage 21 is two-dimensionally movable along a wafer plane (XY plane), and its position coordinates are measured and controlled by an interferometer 23 using a wafer moving mirror 22. .

The wafer 19, the wafer holder 20, and the wafer stage 21 are housed in a casing (not shown), and the inside of the casing is replaced with an inert gas (nitrogen, helium, or the like).

As described above, in the projection exposure apparatus shown in FIG. 5, an atmosphere in which absorption of exposure light is suppressed is formed over the entire optical path from the light source 11 to the wafer 19. Further, as described above, the shapes of the illumination region (viewing region) of the mask 23 formed by the projection optical system 18 and the projection region (exposure region) on the wafer 19 have a slit shape having a short side along the X-axis direction. It is. Therefore, while controlling the position of the mask 13 and the wafer 19 using the drive system and the interferometers 17 and 23, the mask stage 15 is moved along the short side direction (X-axis direction) of the slit-shaped illumination area and exposure area. , The wafer stage 21 or the mask 13 and the wafer 19 are synchronously moved so that the wafer 19 has a width equal to the long side of the exposure area on the wafer 19 and corresponds to the scanning amount (moving amount) of the wafer 19. A region having a length is scanned and exposed.

{Circle around (2)} As the optical members (lenses, prisms, etc.) constituting the illumination optical system 12 and the projection optical system 18, it is useful to use the optical member of the present invention in which two or more crystal plane orientations are controlled.

The projection exposure apparatus shown in FIG. 5 is an example, and the optical member manufactured by the present invention may be applied to various projection exposure apparatuses disclosed in, for example, US Pat. No. 6,341,007B1. <Example of projection optical system>
FIG. 6 is a schematic configuration diagram showing an example of a projection optical system used in the projection exposure apparatus of the present invention.

In FIG. 6, a projection optical system uses a catadioptric first imaging optical system G1 for forming an intermediate image of a pattern on a reticle R as a projection original plate and an intermediate image formed by the first imaging optical system G1 as a work. And a refraction-type second imaging optical system G2 for re-imaging on the wafer W. On the optical axis AX1, a reflection surface S1 for deflecting the optical path from the reticle R toward the first imaging optical system G1 by 90 °, and a reflection surface S1 from the first imaging optical system G1 to the second imaging optical system G2. An optical path bending member including an optical path bending reflecting mirror 31 having a reflecting surface S2 for deflecting the optical path toward 90 ° is disposed.

The first imaging optical system G1 has a plurality of lens components and a concave reflecting mirror arranged along the optical path AX1, and forms an intermediate image with approximately equal or slightly reduced magnification.

The second imaging optical system G2 has a plurality of lens components arranged along an optical axis AX2 orthogonal to the optical axis AX1, and a variable aperture stop AS for controlling a coherence factor. Based on the light from the image, a secondary image is formed with a predetermined reduction magnification.

Here, the optical axis AX0 in FIG. 6 is the optical axis between the reticle R and the reflecting mirror 31, which is orthogonal to the optical axis AX1 of the first imaging optical system G1, and the optical axis AX0 and the optical axis AX2 are May be co-linear.

FIG. 6 shows the projection optical system including the first imaging optical system G1 and the second imaging optical system G2 each including a plurality of lens components, but is arranged along the optical axes AX1 and AX2. The lens component may be singular or plural.

{Furthermore, the angle between the optical axis AX0 and the optical axis AX1 is not necessarily 90 °, and may be, for example, an angle obtained by rotating the concave reflecting mirror CM counterclockwise. At this time, it is preferable to set the bending angle of the optical axis on the reflection surface S2 so that the reticle R and the wafer W are parallel.

In the present invention, as shown in FIG. 7, a projection optical system including two reflecting mirrors 31 and 32 can be used.

{Further, in the present invention, a projection optical system having the configuration shown in FIG. 8 can be used.

8, the projection optical system includes a catadioptric first imaging optical system G1 that forms an intermediate image of a pattern on a reticle R as a projection original plate. A first optical path bending reflecting mirror 31 is arranged near the first intermediate image formed by the first imaging optical system G1. A light beam from the first intermediate image is deflected toward the second imaging optical system. The second imaging optical system G2 has a concave reflecting mirror CM and at least one negative lens 33, and based on a light beam from the first intermediate image, a second intermediate image of substantially the same size as the first intermediate image. (A secondary image of the pattern, which is an image of the first intermediate image) is formed near the first intermediate image.

In the vicinity of the formation position of the second intermediate image formed by the second imaging optical system G2, a second optical path bending reflecting mirror 32 is disposed, and the second optical path bending reflecting mirror 32 causes the second intermediate image to be formed. The incoming light beam or the light beam from the second intermediate image is deflected toward the third imaging optical system G3. The reflecting surface of the first optical path bending reflecting mirror 31 and the reflecting surface of the second optical path bending mirror 32 are arranged so as not to spatially overlap with each other.

The third imaging optical system G3 converts a reduced image of the pattern of the reticle R (an image of the second intermediate image, which is the final image of the catadioptric optical system) based on the light beam from the second imaging optical system. It is formed on a wafer W as a work (photosensitive substrate) arranged on two surfaces.

The projection optical system shown in FIGS. 6 to 8 is suitably used, for example, when the exposure light source is an F2 laser. On the other hand, for example, when the exposure light source is an ArF excimer laser, a projection optical system having a lens configuration shown in FIG. 9, for example, is preferably used.

In FIG. 9, a first lens group G1 having a positive power, a second lens group G2 having a positive power, and a third lens group G3 having a negative power are formed in order from the reticle R side as the first object. The object side (reticle R side) and the image side (wafer W side) are almost telecentric and have a reduction magnification. Also, the N.D. A. Is 0.6, the projection magnification is 1/4, and the diameter of the exposure area on the image side is 30.6.

When the projection optical system has the lens configuration shown in FIG. 9, usually, the material of the square lens is appropriately selected to correct chromatic aberration. For example, quartz glass as a material of fourteen lenses L11 to L114 forming the first lens group G1, quartz glass as a material of four lenses L21 to L24 forming the second lens group G2, and a third lens group G3. Correction of chromatic aberration is preferably performed by using a calcium fluoride crystal as a material of L31, L33, L35, L37, L38, and L310 among the eleven lenses L31 to L311 and quartz glass as the other five materials. Can do it.

Ingots were manufactured using the Bridgman method. The raw materials used were high-purity raw materials made by chemical synthesis. The graphite crucible filled with the powder raw material was stacked and placed in the growing apparatus, and the inside of the growing apparatus was kept in a vacuum atmosphere of 10 ^-3 to 10 ^-4 Pa. Next, the temperature in the growing device was raised to the melting point of the fluorite or higher, and the raw materials were melted and then cooled to room temperature. At this time, PID control was performed in order to suppress temporal fluctuation of the temperature in the growing apparatus. Lead fluoride was added to this powder raw material as a fluorinating agent. The semi-molten product thus obtained is transferred to a crystal growing furnace and heated again to a melting temperature, and then, at the crystal growth stage, the crucible is lowered at a speed of about 0.1 to 5 mm / h, whereby the crucible is reduced. And gradually crystallized from the bottom. When the crystal is crystallized to the uppermost part of the melt, the crystal growth is completed, and simple slow cooling is performed avoiding rapid cooling so that the grown crystal (ingot) does not crack. When the temperature in the growing apparatus dropped to about room temperature, the apparatus was opened to the atmosphere and an ingot of φ290 × t300 mm was taken out.

In this crystal growth, a pencil-shaped ingot having a conical tip was manufactured using a graphite crucible having a diameter of 300 mm. At this time, a seed crystal was put into the tip of the conical portion located at the lower end of the crucible, and the crystal was grown while controlling the plane orientation of the crystal growth to obtain a single crystal. Since the removed ingot had a very large residual stress, the temperature was slowly lowered to room temperature in the furnace.

The conical shape (referred to as a cone) at the tip of the ingot of the fluorite single crystal thus obtained and the opposing portion (referred to as a top) are cut into a thickness of 30 mm (diameter 290 mm), A test piece for azimuth measurement. The plane orientation of the main body was estimated by measuring the plane orientation of the two test pieces. The plane orientation of the test piece was measured by the Laue method.

The plane orientations of the two cylindrical materials of φ260 × t50 mm and φ200 × t60 mm of {111} planes were cut out and the birefringence was measured. The object to be measured was measured in 18 directions at intervals of 10 ° with reference to the <110> direction in accordance with the distribution of birefringence in the {111} plane and the crystal plane direction of the side surface obtained in advance by the Laue method.

From the {measurement results}, the birefringence in the optical axis direction on the {111} crystal plane was 10 nm / cm or more before the heat treatment. The birefringence in the side direction showed a maximum value of 12 nm / cm in the <110> direction. Therefore, a heat treatment was performed on this material.

(4) A cylindrical material is placed on a heat treatment apparatus so that the plane is up and down, and is heated by a heater to perform heat treatment (heating to 1080 ° C., holding, and annealing) (L). At this time, the temperature schedule was adjusted so that the temperature distribution inside the member was within 0.5 ° C. in any of holding, cooling, and cooling.

(4) The heat treatment device is a vacuum device, which has a structure to prevent the intrusion of oxygen which causes calcium fluoride slag, an external structure made of stainless steel, and a graphite heater and a graphite container installed inside.

The graphite container used was sufficiently large (about 10 times or more the volume of the material) with respect to the material. Approximately 100 g of ammonium acid fluoride was sealed inside the graphite container together with the calcium fluoride member. After the inside of the furnace was evacuated with a vacuum pump, the temperature was raised. While controlling the pressure so as to keep the furnace temperature at a weak positive pressure (2 to 8 kPa), the temperature was raised, the temperature was maintained at 1080 ° C., and the cooling was performed slowly.

By the heat treatment as described above, the birefringence in the optical axis direction on the {111} crystal plane of the calcium fluoride member after the heat treatment became 0.8 nm / cm. The birefringence in the side direction showed a maximum value of 2.5 nm / cm in the <110> direction. The maximum value of the birefringence satisfies the requirement as a lens optical member of a projection optical system used for an exposure apparatus. Therefore, a lens optical member was formed using this material.

カ ル シ ウ ム A calcium fluoride single crystal ingot was grown in the same manner as in Example 1. From this ingot, a plurality of cylindrical members were cut out such that the {100} plane became the upper and lower two planes. The birefringence in the optical axis direction on the {100} crystal face of the cut-out member was 20 nm / cm or more before heat treatment. The birefringence in the side direction of this member showed a maximum value of 18 nm / cm in the <110> direction. Therefore, heat treatment was performed in the same manner as in Example 1. By this heat treatment, the birefringence in the optical axis direction on the {100} crystal plane of the calcium fluoride member became 2.8 nm / cm. The birefringence in the side direction showed a maximum value of 5.9 nm / cm in the <110> direction. The maximum value of the birefringence satisfies the requirement as a lens optical member of a projection optical system used for an exposure apparatus. Therefore, a lens optical member was formed using this material.

In the same manner as in Example 1, a calcium fluoride single crystal ingot was grown. From this ingot, a plurality of cylindrical members were cut out such that the {100} plane became the upper and lower two planes. The cut-out member had a birefringence in the optical axis direction of the {100} crystal plane of 20 nm / cm or more (before heat treatment). The birefringence in the side direction showed a maximum value of 18 nm / cm in the <110> direction. Therefore, this member was heat-treated.

設置 The cylindrical material is placed in the heat treatment apparatus so that the plane is up and down, and is heated by a heater to perform heat treatment (heating to 1080 ° C., holding, then annealing). The heat treatment device is a vacuum device, which has a structure that prevents the intrusion of oxygen that causes calcium fluoride lagging, a piping structure that can introduce a gas-based fluorinating agent, and a material that is resistant to corrosion. have. Specifically, the external structure was made of stainless steel, and a graphite heater and a graphite container were installed inside. Calcium fluoride was put in the graphite container, and the graphite container had a mechanism for rotating with respect to the center of the member, and was rotated in the range of 1 to 5 rpm.

The graphite container used was sufficiently large (about 10 times or more the volume of the material) with respect to the material. Approximately 100 g of ammonium acid fluoride was sealed inside the graphite container together with the calcium fluoride member. After the inside of the furnace was evacuated with a vacuum pump, the temperature was raised. While controlling the pressure so as to keep the furnace temperature at a weak positive pressure (2 to 8 kPa), the temperature was raised, the temperature was maintained at 1080 ° C., and the cooling was performed slowly. The heat treatment conditions were the same as in Example 1. By this heat treatment, the birefringence in the optical axis direction on the {100} crystal plane of the calcium fluoride member became 2.2 nm / cm. The birefringence in the side direction showed a maximum value of 3.4 nm / cm in the <100> direction.

カ ル シ ウ ム A calcium fluoride single crystal ingot was grown in the same manner as in Example 1. From this ingot, a plurality of cylindrical members were cut out such that the {111} plane was two upper and lower planes. The birefringence in the {111} crystal plane direction of the cut member was 10 nm / cm or more (before heat treatment). The birefringence in the side direction showed a maximum value of 11 nm / cm in the <110> direction. Therefore, heat treatment was performed at a higher cooling rate than the condition of Example 1. By this annealing, the birefringence in the {111} crystal plane direction of the calcium fluoride member after the heat treatment became 1.5 nm / cm. The birefringence in the side direction was 5.9 nm / cm when measured in an arbitrary direction. However, the measurement in the <110> direction showed a maximum value of 12 nm / cm. Therefore, this maximum birefringence is inappropriate (nonstandard) as an optical member of a lens for a projection optical system of an exposure apparatus, and therefore this member was not used as a material for forming a lens. Further, from the results of this example, it was found that the material could not be sufficiently evaluated even if the birefringence in the side direction was measured in an arbitrary direction (angle).

カ ル シ ウ ム A calcium fluoride single crystal ingot was grown in the same manner as in Example 1. From this ingot, a plurality of cylindrical members were cut out such that the {100} plane became the upper and lower two planes. The birefringence of the cut member in the {100} crystal plane direction was 20 nm / cm or more (before heat treatment). The birefringence in the side direction showed a maximum value of 18 nm / cm in the <110> direction. Therefore, heat treatment was performed at a higher cooling rate than the condition of Example 2. By this heat treatment, the birefringence of the calcium fluoride member in the {100} crystal plane direction was 3.8 nm / cm. The birefringence in the side direction was 7.5 nm / cm when measured in an arbitrary direction. However, when measured in the <110> direction, a maximum value of 15 nm / cm was shown. Therefore, since this maximum birefringence value is inappropriate (nonstandard) for an optical member of a lens for a projection optical system of an exposure apparatus, this member was not used as a material for forming a lens. Further, similarly to Example 4, it was found that the material could not be sufficiently evaluated even if measurement was performed in an arbitrary direction.

A conceptual diagram showing an example of a method for manufacturing an optical member according to the present invention. Conceptual diagram showing the method of growing calcium fluoride single crystal Diagram showing crystal plane orientation measurement device by Laue method (side reflection method) It is a conceptual diagram of (alpha) and (beta) in a cylindrical member. Schematic diagram showing an example of a projection exposure apparatus Schematic showing an example of a projection optical system Schematic diagram showing another example of the projection optical system Schematic diagram showing another example of the projection optical system Schematic diagram showing another example of the projection optical system

Claims (11)

A growing step of growing a fluoride crystal ingot,
From the ingot, a cutting step of cutting out a cylindrical material having a desired crystal plane orientation as two parallel surfaces,
An orientation determining step of determining the crystal orientation of the side surface of the cylindrical material,
Birefringence measurement step of measuring the birefringence in the predetermined crystal axis direction of the side surface, determined in the orientation determination step,
A determination step of determining a fluoride crystal based on the measurement result of the birefringence,
The manufacturing method of the optical member which consists of a fluoride crystal which has this. 2. The optical member comprising a fluoride crystal according to claim 1, wherein the crystal orientation of the side surface of the columnar material is determined by measuring the birefringence of the side surface at a plurality of angles. Production method. 2. The fluoride crystal according to claim 1, wherein in the determination step, it is determined whether or not the maximum value of the birefringence in the specific crystal axis direction on the side surface is 10 nm / cm or less at a measurement wavelength of 633 nm. A method for manufacturing an optical member. When the maximum value of the birefringence in the specific crystal axis direction of the side surface is 10 nm / cm or less at a measurement wavelength of 633 nm, the method includes forming the columnar material into a predetermined optical member shape. A method for producing an optical member comprising the fluoride crystal according to claim 3. 4. The method of claim 3, wherein the two parallel surfaces are {111} surfaces, and the specific crystal direction of the side surface is <110>. 5. 4. The method of claim 3, wherein the two parallel surfaces are {100} surfaces, and the specific crystal direction of the side surface is a <100> or <110> direction. 5. . An optical member made of a fluoride crystal manufactured by the manufacturing method according to claim 1,
The maximum value of the amount of birefringence in the specific crystal axis direction of the side surface measured with the columnar material shape having the specific crystal plane orientation as two parallel surfaces is 10 nm / cm or less at a measurement wavelength of 633 nm. An optical member made of a fluoride crystal. The optical member made of a fluoride crystal according to claim 7, wherein the {specific crystal plane direction is a {111} plane, and a specific crystal axis direction of a side surface is <110>. The optical member according to claim 7, wherein the specific crystal plane orientation is a {100} plane, and a specific crystal axis direction of a side surface is a <100> or <110> direction. 8. The optical member according to claim 7, wherein the fluoride crystal is a calcium fluoride single crystal. An exposure apparatus including an optical system comprising the optical member according to claim 7.

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