JP4419291B2 - Inspection method of fluoride single crystal - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to light having a wavelength of 200 nm or less, such as ArF excimer laser, F₂Regarding light-transmitting optical members such as lenses, prisms, and plates used in optical systems such as lasers, solid-state lasers, and the like, for example, projection exposure apparatuses, CVD apparatuses, and laser processing apparatuses, particularly from fluoride single crystals It relates to an optical member.
[0002]
The present invention also relates to a method for inspecting a fluoride single crystal used for such an optical member, and further to a projection exposure apparatus.
[0003]
[Prior art]
Semiconductor devices such as LSIs have been highly integrated and exposure using light is performed. In the photolithography process, a method of transferring a pattern drawn on a mask onto a wafer with a lens using a projection exposure apparatus is mainly performed. In general, the resolution of a transfer pattern is improved in proportion to the numerical aperture of a lens and the inverse of the wavelength of light to be exposed. However, in order to increase the numerical aperture of the lens, a large-diameter lens is required, and there is a limit to manufacturing. Therefore, in order to improve the resolution of photolithography, it is effective to shorten the wavelength of the light source.
[0004]
For this reason, an exposure apparatus using g-line (436 nm) or i-line (365 nm) as a light source has been used so far, but now, an exposure apparatus using a KrF excimer laser (248 nm) as a light source is widely used. Furthermore, an exposure apparatus equipped with an ArF excimer laser (193.4 nm), which is vacuum ultraviolet light, has begun to be put into practical use. Recently, F has a shorter wavelength.₂An exposure apparatus using a laser (157.6 nm) as a light source is also expected.
[0005]
In an exposure apparatus using g-line or i-line as a light source, optical glass has been used as a lens material. However, for an ultraviolet light source of 300 nm or less, optical glass can no longer be used as a lens material from the viewpoint of light transmittance. Synthetic quartz glass and fluoride crystals can be used for light in this region. For example, in a KrF excimer laser exposure apparatus, synthetic quartz glass is used because of the advantage that a large aperture is easily obtained.
[0006]
It is considered that synthetic quartz glass and fluoride crystals can be used for light in a wavelength range of 200 nm or less (vacuum ultraviolet wavelength range). In particular, a fluoride crystal such as a calcium fluoride crystal excellent in light transmittance has attracted attention.
[0007]
In the region of 160 nm or less, even synthetic quartz glass cannot be used because of its lack of light transmittance, and only fluoride crystals such as calcium fluoride crystals can be used. For example, F₂Calcium fluoride crystals and barium fluoride crystals are expected for an exposure apparatus using a laser as a light source. In particular, calcium fluoride crystals are considered to be practical.
[0008]
Optical materials used in an exposure apparatus are required to have a quality that is excellent in light transmission, small in birefringence and large in diameter. The same applies to fluoride crystals.
[0009]
First, regarding the demand for excellent light transmission, it is generally said that the smaller the transmission loss due to scattering and absorption caused by the inside of the optical material, the better.
[0010]
The transmission loss due to absorption is said to change the refractive index distribution of a lens or the like or cause deformation due to the change of light energy to thermal energy due to electron transition, and is said to have a great influence directly on the imaging characteristics.
[0011]
On the other hand, it is said that transmission loss due to scattering does not have a large direct effect on the imaging characteristics themselves. That is, if the optical material has very strong scattering, the weak stray light generated by the scattering can reduce the contrast of the image. However, in the case of scattering, it is said that the energy of incident light does not change to thermal energy within the material, so that it does not directly affect the imaging characteristics.
[0012]
Optical glass and plastics are known to have scattering such as Rayleigh scattering, but even this scattering is very small compared to absorption, so most transmission loss is considered to be absorption loss. . Furthermore, it is said that such Rayleigh scattering does not exist essentially in a fluoride crystal which is a crystal.
[0013]
Thus, conventionally, it has been considered that the transmission loss in a fluoride crystal is only an absorption loss and does not include a scattering loss.
[0014]
Thus, conventionally, it has been considered that the transmission loss in a fluoride crystal is only an absorption loss that directly affects the imaging characteristics, and does not include scattering loss.
[0015]
Although the details of the fluoride crystal having excellent light transmittance will be described later, it is possible to perform a selection operation based on an inspection based on the transmittance in a part of the manufacturing process. The transmittance is a value obtained by measuring with a spectrophotometer or the like, and is a quantitative expression of light transmittance.
[0016]
Next, regarding the requirement that the birefringence is small and the aperture is large, it is generally necessary to be a single crystal to satisfy this requirement. In other words, simply having a large aperture is not sufficient. An aggregate of single crystals is called polycrystalline. If the fluoride crystal is not a single crystal but a polycrystal, strain remains at the interface between the adjacent single crystals and exhibits a large birefringence. In addition, when a lens or the like is configured such that the refractive index distribution near the interface becomes discontinuous, the imaging performance becomes insufficient. Therefore, a fluoride single crystal is used as an optical material used in the exposure apparatus, not a polycrystalline fluoride crystal.
[0017]
Fluoride single crystals are usually produced by the Bridgman method. For example, a method for producing a calcium fluoride single crystal, which is one of fluoride single crystals, is described in the “Bridgeman Stock Burger Method” in the “Crystal Growth Handbook (September 1, 1995, First Edition, 1st Edition, P583)”. It is introduced as (B-S method). The outline of the contents disclosed there is as follows.
[0018]
The Bridgman method is a method of crystallizing the melt in the crucible when the crucible descends through a temperature gradient set in the furnace. A crystal growth furnace comprising a crucible, a heater, a heat insulating material, a bell jar, an exhaust mechanism, and a descending mechanism is used. Further, it has been shown that a calcium fluoride single crystal can be grown not only by the Bridgeman method but also by a pulling method or a zone melt method.
[0019]
The mass produced in this way is called an ingot. A part of the ingot is cut out as a cylindrical member or cut out as a circular member at an appropriate interval. Such a member is polished or coated to form an optical member such as a lens. Furthermore, according to the optical design, a lens group or the like is formed to form an optical system.
[0020]
Hereinafter, a manufacturing method of a calcium fluoride single crystal will be described as a fluoride single crystal used as a material for an optical member of an exposure apparatus. The process for producing this calcium fluoride single crystal mainly comprises the following three processes (1) to (3). The single crystallization by the Bridgman method described above corresponds to the following step (2).
[0021]
Step {circle around (1)}: A step of deoxygenating the raw material to obtain a pretreated product,
Step (2): A step of growing a crystal to obtain a single crystal ingot,
Step (3): Step of obtaining a heat-treated product by performing a heat treatment to reduce birefringence
[0022]
First, the step (1) will be described.
[0023]
When a calcium fluoride single crystal used in the ultraviolet or vacuum ultraviolet region is produced by the Bridgman method, natural calcium fluoride is not used as a raw material, and a raw material of artificially synthesized high-purity powder is used. Even if such a raw material is melted and crystallized only, it shows a tendency to become cloudy and devitrified. Therefore, a reagent called a scavenger is added and heated to prevent white turbidity. An additive substance that chemically reacts with and removes impurities contained therein is generally called a scavenger.
[0024]
A typical scavenger for producing a calcium fluoride single crystal includes lead fluoride. Lead fluoride reacts with oxygen, which is an impurity contained in the calcium fluoride single crystal, to become lead oxide. Since the produced lead oxide volatilizes at a high temperature, oxygen can be removed from the calcium fluoride single crystal. Since devitrification due to white turbidity is considered to be caused by oxygen contained in the calcium fluoride single crystal, devitrification can be prevented by removing this oxygen.
[0025]
The procedure for reacting the raw material with the scavenger is explained. The powder raw material mixed with the scavenger is filled in a clean container such as graphite, and the temperature is raised to the melting point of calcium fluoride by sufficient evacuation to deoxygenate. Advance the reaction. After that, the temperature is lowered to room temperature and used as a pre-processed product.
[0026]
Next, step (2) will be described.
[0027]
It is widely known that crystal growth methods can generally be broadly divided into solidification of a melt, precipitation from a solution, precipitation from a gas, and growth of solid particles. Of these, a method often used is a method in which the pretreated product obtained in the step (1) is once melted and solidified from the melt to grow crystals. One of the methods for promoting crystal growth by solidifying from a melt is the Bridgman method.
[0028]
As described above, the Bridgman method is a solution from the upper part to the lower part in the temperature distribution consisting of the high temperature part (upper part) controlled to a temperature higher than the melting point and the low temperature part (lower part) controlled to a temperature lower than the melting point. In this method, the (container) is pulled down for crystallization. Hereinafter, a procedure for growing a calcium fluoride single crystal by the Bridgman method will be described.
[0029]
The pretreated product obtained in the step (1) is filled in a clean container such as graphite, and is placed at a predetermined position of the Bridgman apparatus that can be evacuated. Under sufficient evacuation, the temperature of the pretreatment product is raised and melted by a heating means such as electric heating. After reaching the melting point, crystallization (reduction) is started after a few hours have elapsed after the temperature has stabilized. The pulling is performed at a speed of about 0.1 mm to 5 mm per hour. When pulling progresses and all the melt is crystallized, it is slowly cooled to room temperature and taken out as an ingot.
[0030]
A part of the ingot obtained by the step (2) is cut, and the inside is observed with the naked eye using a condensing lamp with strong illuminance. As a result of this observation, there may be a minute particle (what looks like a minute particle) that scatters light inside. This fine grain is considered to be a crystal defect or the like. The cause of the formation of fine grains is unknown, but it seems to be due to fluctuations in conditions during crystal growth, such as temperature fluctuations.
[0031]
Although it is not such fine particles, there are bubbles and foreign substances of optical glass as similar to such fine particles. A method for measuring bubbles and foreign matters in optical glass is in the Japan Optical Glass Industry Association Standard (JOGIS), and all of them classify by measuring the diameter and number of bubbles and foreign matters present in optical glass. In actual use, it is unclear how bubbles and foreign substances in the optical glass affect.
[0032]
The same applies to the calcium fluoride single crystal, and it is not known what optical effect the fine particles have. If the macroscopic observation as described above is performed and there is not a very large amount of fine particles, the ingot is cut into an appropriate shape, and further subjected to a molding process such as a rounding process to form a member, and the process proceeds to step (3).
[0033]
Next, the step (3) will be described.
[0034]
The member for heat treatment (the member cut out from the ingot after the step (2) and molded) is processed in advance to the same extent as the target optical member in both shape and size. For example, when the objective is an optical lens, it has a thin cylindrical shape, and its diameter and thickness are often determined according to the optical lens. It is said that the atmosphere during heat treatment should avoid oxygen and moisture. Moreover, it is better for the atmosphere at the time of heat processing not to react with calcium fluoride at high temperature. For this reason, the heat treatment is performed in a vacuum atmosphere, an inert gas atmosphere, or a fluorine atmosphere. The temperature during the heat treatment is about 1000 ° C. First, the temperature of the member is gradually raised from room temperature, and is held for about one day at a constant temperature of about 1000 ° C., for example. Thereafter, the temperature is gradually lowered and the mixture is gradually cooled to room temperature.
[0035]
Since it is necessary that there is no temperature unevenness during the heat treatment, the member is accommodated in a container made of graphite having good thermal conductivity. A heating means is provided around the container. When the temperature is lowered to room temperature, the member is taken out. The member (heat-treated product) that has been heat-treated in this manner is sufficiently gradually strained and has reduced birefringence.
[0036]
By the steps {circle around (1)} to {circle around (3)} described above, the calcium fluoride single crystal used as the material of the optical member of the exposure apparatus is completed. However, the calcium fluoride single crystal produced in this way does not always have the desired characteristics.
[0037]
Therefore, conventionally, a calcium fluoride single crystal has been inspected by the following method.
[0038]
That is, as described above, the member is obtained from the ingot obtained in the step (2), but a small molded product is manufactured separately from the member from the same ingot. From this molded product, a sample called a test piece is manufactured as an object to be measured when measuring the transmittance. That is, apart from the columnar member for optical material, a test piece is manufactured from the same ingot. The test piece is usually molded and processed into an appropriate size and shape so that it can be installed in a sample chamber of a measuring device such as a spectrophotometer. In addition, two parallel faces of the test piece facing each other are polished in a mirror shape.
[0039]
By measuring the transmittance of such a test piece, the light transmittance of the manufactured calcium fluoride single crystal member can be quantitatively evaluated. Therefore, the member is inspected by determining whether or not the light transmittance satisfies a predetermined level. That is, when the predetermined level is satisfied, it is evaluated that the member obtained from the same ingot as the test piece is suitable as a material for the optical member of the exposure apparatus. On the other hand, when the predetermined level is not satisfied, the test is performed. A member obtained from the same ingot as the piece is evaluated as inappropriate and is not used as a material for an optical member of the exposure apparatus.
[0040]
The specific level of light transmission was as follows. In other words, it is said that the light transmittance should be as high as possible. Therefore, whether the internal transmittance per cm of the optical path of the calcium fluoride single crystal member is 99.8% or more at the wavelength of the light to be used. No is used as a reference for determining whether or not the optical member of the exposure apparatus can be used as a material. For example, when an attempt is made to use a calcium fluoride single crystal as an optical member such as a lens constituting an exposure apparatus using an ArF excimer laser as a light source, the internal wavelength at 193.4 nm is achieved despite such a short wavelength. Those having a transmittance of 99.8% / cm or more were selected and used.
[0041]
Here, the internal transmittance will be described. Internal transmittance refers to the transmittance of the material itself, which does not include reflection on the surface, etc. When there is no transmission loss of the material, it is indicated as 100%, and when the material does not transmit light at all Is 0% as a percentage. The internal transmittance of 99.8% / cm is the same as the transmission loss of 0.2% per 1 cm of the optical path. However, whether the transmission loss is a transmission loss due to absorption or a transmission loss due to scattering cannot be determined from the transmittance measurement result.
[0042]
The internal transmittance can be obtained by calculating from the transmittance including reflection on the surface. The transmittance T obtained by measuring from the spectrophotometer includes reflection on the surface. Since the transmittance T and the internal transmittance τ have the following relationship, the conversion from the transmittance T to the internal transmittance τ is easy. However, R in Formula 1 is a reflectance, and the refractive index n of the atmosphere₀And refractive index n of the object to be measured_SIs determined by the following equation (2). When the internal transmittance is 100% (τ = 1), the transmittance T including reflection is determined by the reflectance R, and the theoretical transmittance T₀Sometimes called. In that case, theoretical transmittance T₀Is represented by Equation 3 below.
[0043]
[Expression 1]
T = {(1-R)²τ} / (1-R²τ²)
[0044]
[Expression 2]
R = {(n₀-N_S) / (N₀+ N_S)}²
[0045]
[Equation 3]
T₀= {(1-R)²} / (1-R²)
[0046]
Here, the measurement error regarding the internal transmittance is estimated. Usually, the reflectance R is about 0.04 and the internal transmittance τ is about 1, so that R is sufficiently smaller than 1 and τ. The degree of measurement error that occurs in the actual measurement of the transmittance T including reflection may be regarded as being directly propagated to the error of the internal transmittance τ.
[0047]
Needless to say, in order to conduct a precise inspection such as the internal transmittance of 99.8% / cm, it is necessary to study in the order of 0.1%. Similarly, it is shown that it is necessary to accurately measure the transmittance with an order of 0.1%.
[0048]
When such a measurement is to be performed with a spectrophotometer, fixed-point measurement is performed in which the transmittance is measured by fixing the wavelength to a target wavelength, instead of measuring the spectral transmittance by wavelength scanning. .
[0049]
The conventional inspection method of the calcium fluoride single crystal described above is shown as a flowchart in FIG. The conventional inspection method will be briefly described with reference to FIG. 4. First, the reflection and transmission of the test piece at the target wavelength in the exposure apparatus is measured with a spectrophotometer, and the measured reflection and transmission is measured. The internal transmittance A of the test piece is measured by converting the rate according to the formulas 1 and 2 (step S1). Next, it is determined whether or not the internal transmittance A is 99.8% / cm or more (step S2). If it is 99.8% / cm or more, it is obtained from the same ingot as the test piece. The member is evaluated as being suitable as a material for the optical member of the exposure apparatus (step S3), and if it is less than 99.8% / cm, the member obtained from the same ingot as the test piece is the exposure apparatus. It is evaluated that it is not suitable as a material for the optical member (step S4), and the inspection is terminated.
[0050]
If the result of the inspection is "appropriate", the member collected from the original ingot is a material suitable for the optical member that constitutes the ultraviolet light exposure device, and is subjected to polishing or coating to produce an optical member such as a lens. To do. In the case of “inappropriate”, the original ingot is not used as an optical member of the ultraviolet light exposure apparatus but is disposed as a defective product.
[0051]
For a calcium fluoride single crystal intended for an exposure apparatus using an ArF excimer laser as a light source, the internal transmittance at a wavelength of 193.4 nm is about 99.4 to 100.0% / cm. All of these are not used for the optical members of the exposure apparatus, and only some of them are used for the optical members.
[0052]
[Problems to be solved by the invention]
As described above, in the conventional inspection method, by setting a very high level regarding the light transmittance such that the internal transmittance is 99.8% / cm or more, a desired exposure apparatus using vacuum ultraviolet light is desired. Optical characteristics were ensured.
[0053]
However, since the high level was set in this way, the ratio of the manufactured calcium fluoride single crystals that did not satisfy the level and were discarded as defective products was considerably high, and the yield was low. For this reason, there have been problems such as disturbing the production plan of the exposure apparatus due to the insufficiency of the planned quantity, and increasing the capital investment in the apparatus for manufacturing the calcium fluoride single crystal.
[0054]
Therefore, it is conceivable to set the inspection standard of the calcium fluoride single crystal to be lower than the internal transmittance of 99.8% / cm. However, if the inspection standard is unnecessarily lowered, even if the calcium fluoride single crystal passes the inspection, desired optical characteristics cannot be obtained in an exposure apparatus using an optical member made of the calcium fluoride single crystal. It may lead to a situation.
[0055]
In the above description, the case where the material is a calcium fluoride single crystal has been described as an example, and the case where an optical member using the material is used in an exposure apparatus has been described as an example. In the description, various devices using a light source having a wavelength of 200 nm or less as a light source other than an exposure device when the material is a fluoride single crystal other than the calcium fluoride single crystal or an optical member using the fluoride single crystal is used. The same applies to the case where it is used.
[0056]
The present invention has been made in view of the above-described circumstances, can improve the yield of fluoride single crystals, and obtain desired optical characteristics in an apparatus using an optical member using fluoride single crystals. It is an object of the present invention to provide a method for inspecting a fluoride single crystal that does not cause a situation where it cannot be performed.
[0057]
In addition, the present invention provides an optical member using a fluoride single crystal that is inherently sufficient although it is a fluoride single crystal that has been disposed of as a defective product according to a conventional inspection method. It is another object of the present invention to provide a projection exposure apparatus.
[0058]
[Means for Solving the Problems]
In order to solve the above problems, a fluoride single crystal inspection method according to a first aspect of the present invention is a fluoride single crystal inspection method used for an optical member that transmits light having a specific wavelength of 200 nm or less. , When the first wavelength α (nm) and the second wavelength β (nm) are in the relationship of α ≦ 200 ≦ β and (β−α) ≧ 15, the first single crystal of the fluoride single crystal From the internal transmittance A (% / cm) at the wavelength α and the internal transmittance B (% / cm) at the second wavelength β of the fluoride single crystal, C = A + (100−B) Including the step of determining whether the value C (% / cm) obtained by the equation is 99.8% / cm or more, and the value C is 99.8% / cm or more as a necessary condition It is evaluated that the fluoride single crystal is suitable as a material for the optical member.
[0059]
The fluoride single crystal includes lithium fluoride single crystal (LiF), magnesium fluoride single crystal (MgF).₂), Calcium fluoride single crystal (CaF)₂), Strontium fluoride single crystal (SrF)₂), Barium fluoride single crystal (BaF)₂)including.
[0060]
Said 1st wavelength (alpha) may be made to correspond with the said specific wavelength, and does not necessarily need to correspond.
[0061]
Whether the value C (% / cm) is 99.8% / cm or more is determined directly by determining whether the value C obtained in advance is 99.8% / cm or more. Or may be performed by determining whether the determination is equivalent to the determination, for example, whether B−A ≦ 0.2% / cm. In the latter case, it is not always necessary to obtain the value C.
[0062]
The first wavelength α may be 140 nm or more. The second wavelength β may be 200 nm or more. The second wavelength β may be 400 nm or less.
[0063]
The above point is the same also about each aspect of this invention mentioned later.
[0064]
The method for inspecting a fluoride single crystal according to the second aspect of the present invention includes the step of determining whether or not the internal transmittance A is 99.6% / cm or more in the first aspect, On the condition that the internal transmittance A is 99.6% / cm or more, it is evaluated that the fluoride single crystal is suitable as a material for the optical member.
[0065]
In the present invention, 99.6% / cm may be replaced with 99.7% / cm in the second embodiment.
[0066]
In the method for inspecting a fluoride single crystal according to the third aspect of the present invention, the first wavelength α is 193.4 nm or 157.6 nm.
[0067]
When a fluoride single crystal is used in an apparatus using an ArF excimer laser as a light source, the first wavelength α may be set to 193.4 nm in accordance with the wavelength of the light source light as in the third aspect. preferable. In this case, the second wavelength β can be set to, for example, an i-line wavelength of 365.1 nm. F₂When a fluoride single crystal is used in an apparatus using a laser as a light source, it is preferable to set the first wavelength α to 157.6 nm in accordance with the wavelength of the light source light as in the third aspect. In this case, the second wavelength β can be set to 248.3 nm, for example. These points are the same also about the 8th aspect mentioned later.
[0068]
The inspection method for a fluoride single crystal according to the fourth aspect of the present invention is the method according to any one of the first to third aspects, wherein the fluoride single crystal is a calcium fluoride single crystal.
[0069]
The optical member according to the fifth aspect of the present invention is an optical member that is made of a fluoride single crystal and transmits light having a specific wavelength of 200 nm or less, and has a first wavelength α (nm) and a second wavelength β (nm ) Is in a relationship of α ≦ 200 ≦ β and (β−α) ≧ 15, the internal transmittance A (% / cm) of the fluoride single crystal at the first wavelength α is 99.99. Less than 8% / cm, and C = A + (100−B) from the internal transmittance A and the internal transmittance B (% / cm) at the second wavelength β of the fluoride single crystal. The value C (% / cm) obtained by the above is 99.8% / cm or more. In the present invention, in the fifth aspect, the internal transmittance A may be less than 99.7% / cm.
[0070]
The optical member according to a sixth aspect of the present invention is the optical member according to the fifth aspect, wherein the internal transmittance A is 99.6% / cm or more.
[0071]
The optical member according to a seventh aspect of the present invention is the optical member according to the fifth or sixth aspect, wherein the fluoride single crystal contains fine particles having a particle size of 10 μm or more and 500 μm or less.
[0072]
The optical member according to an eighth aspect of the present invention is the optical member according to any one of the fifth to seventh aspects, wherein the first wavelength α is 193.4 nm or 157.6 nm.
[0073]
An optical member according to a ninth aspect of the present invention is the optical member according to any one of the fifth to eighth aspects, wherein the fluoride single crystal is a calcium fluoride single crystal.
[0074]
The optical member according to a tenth aspect of the present invention is the optical member according to the ninth aspect, wherein barium contained in the calcium fluoride single crystal is less than 0.02 ppm, cerium is less than 0.01 ppm, iron is less than 0.1 ppm, Manganese is less than 0.02 ppm, lead is less than 0.1 ppm, and yttrium is less than 0.01 ppm.
[0075]
A projection exposure apparatus according to an eleventh aspect of the present invention is the projection exposure apparatus for guiding light of a predetermined wavelength from a light source onto a mask on which a pattern is formed, and projecting and exposing the pattern on the mask onto a substrate. An optical system including the optical member according to any one of the fifth to tenth aspects is provided.
[0076]
DETAILED DESCRIPTION OF THE INVENTION
[Principle of the present invention]
[0077]
Prior to the description of the embodiments of the present invention, the principle of the present invention will be described.
[0078]
Hereinafter, a calcium fluoride single crystal will be described as an example of the fluoride single crystal, but the same applies to other fluoride single crystals. An exposure apparatus will be described as an example of an apparatus that uses an optical member such as a lens using a fluoride single crystal, but the same applies to other apparatuses.
[0079]
Conventionally, precise measurement of the transmittance of a calcium fluoride single crystal has been carried out at the wavelength of the light source used in the exposure apparatus, that is, the target wavelength, but in the present invention, the conventional single wavelength is used. In addition to precise transmittance measurement, precise transmittance is measured at another wavelength longer than this.
[0080]
The specific wavelength of the intended vacuum ultraviolet light (for example, the wavelength of ArF excimer laser or F₂The wavelength of the laser) is the first wavelength α (nm), and the specific wavelength of ultraviolet light longer than this is β (nm). The selection of the first wavelength α depends on what light source is used in the exposure apparatus. However, the first wavelength α does not necessarily need to match the wavelength of the light source of the exposure apparatus. In the present invention, the first wavelength α is selected to be 200 nm or less because it is intended for an apparatus that uses light in the vacuum ultraviolet wavelength region, which is a wavelength region of 200 nm or less, as a light source. The second wavelength β needs to be selected depending on the properties of the calcium fluoride single crystal and the performance of the transmittance measuring device, but will be described in detail later. good.
[0081]
Here, the transmittance measuring device will be described. One type of transmittance measuring device is a spectrophotometer. Such an apparatus includes a vacuum ultraviolet spectrophotometer, an ultraviolet spectrophotometer, a visible spectrophotometer, an infrared spectrophotometer, and the like depending on a wavelength range to be measured and a measurement atmosphere corresponding thereto. In the present invention, vacuum ultraviolet spectrophotometer and ultraviolet spectrophotometer are targeted from the wavelength range.
[0082]
With Cary5 (trade name) manufactured by Varian, which is one of the ultraviolet spectrophotometers, the transmittance can be measured in a range of at least about 185 nm to 400 nm. In addition, in a CAMS (trade name) manufactured by Acton, which is one of vacuum ultraviolet spectrophotometers, transmittance can be measured in a range of at least about 140 nm to 200 nm.
[0083]
The first wavelength α to be used in the exposure apparatus may be set to 140 to 200 nm as belonging to the vacuum ultraviolet region where the transmittance can be measured with such an apparatus. In addition, for the second wavelength β longer than the first wavelength α, a maximum of about 400 nm may be considered. Of course, if an appropriate spectrophotometer is used, the second wavelength β may be set to a wavelength longer than 400 nm.
[0084]
However, if the first wavelength α and the second wavelength β are too close, it is meaningless to measure at another wavelength, so at least the half width of the light to be measured (usually about 1 to 5 nm). It is better that the distance is about 3 times or more, and therefore about 15 nm or more as a guide. The reason for the existence of the transmittance measurement by the second wavelength β and the effect thereof will be described later from the viewpoint of the properties of the calcium fluoride single crystal.
[0085]
For the above reason, the first wavelength α (nm) and the longer second wavelength β (nm) are set to satisfy α ≦ 200 and β ≧ α + 15. Further, although not particularly limited, 140 ≦ α and β ≦ 400 may be satisfied.
[0086]
When measuring the transmittance at the first wavelength α and the second wavelength β, it is not particularly necessary to use the same measuring device, but if there is no problem in performance such as measurement accuracy, the same device is used. Measurement may be performed.
[0087]
When the target wavelength is the ArF excimer laser wavelength 193.4 nm in vacuum ultraviolet light, the first wavelength α is set to 193.4 nm, for example, and the second wavelength β is selected to 250 nm, 350 nm, etc. .
[0088]
The target wavelength is F of vacuum ultraviolet light.₂When the laser wavelength is 157.6 nm, the first wavelength α is set to 157.6 nm, for example, and the second wavelength β is selected to be 200 nm, 250 nm, 350 nm, or the like.
[0089]
Here, the properties of the calcium fluoride single crystal will be described. As described above, conventionally, it has been considered that the transmission loss in a fluoride crystal is only an absorption loss and does not include a scattering loss. However, as will be described in detail below, as a result of the inventor's research, transmission loss in fluoride crystals is considered to include not only absorption loss but also scattering loss (particularly, scattering loss due to the fine particles). Turned out to be.
[0090]
The inventor actually produced a calcium fluoride single crystal by the above-described three steps (1) to (3). At this time, as described later, from the pretreated product obtained after the step (1), the ingot obtained after the step (2), and the heat-treated product obtained after the step (3), respectively. A test piece for measuring transmittance was manufactured.
[0091]
That is, a treatment for preventing white turbidity was performed by adding a scavenger to the powder raw material and heating. Lead fluoride was used as the type of scavenger. The powder raw material mixed with the scavenger was filled into a clean container such as graphite, and the temperature was raised to about the melting point of calcium fluoride under sufficient vacuum evacuation to perform the deoxygenation reaction. Thereafter, the temperature was lowered to room temperature to prepare a pretreated product. The pretreated product was filled in a clean container such as graphite and installed at a predetermined position of the Bridgman apparatus which can be evacuated. Under sufficient evacuation, the temperature of the pretreated product was increased and melted by a heating means such as electric heating. After reaching the melting point, crystallization was started after several hours had passed after waiting for the temperature to stabilize rather than immediately starting crystallization by pulling down. Pulling down was performed at a speed of 2 mm per hour. When the pulling progressed and all the melt was crystallized, it was gradually cooled to room temperature and taken out as an ingot. The member cut out from the ingot was accommodated in a graphite container having good thermal conductivity. A heating means was provided around the container, and after heat treatment, the temperature was lowered to room temperature and the member was taken out.
[0092]
The inventor manufactured a test piece for measuring transmittance from each of the pretreated product, the ingot, and the heat-treated product. The parallel two surfaces facing each other of the test piece were polished to a mirror surface with high accuracy, the parallelism was about 10 seconds or less, and the surface roughness was about 0.25 nm or less in RMS display. It is desirable to clean the surface of the test piece by performing wet cleaning or the like. Such a clean surface has very good wettability with pure water and a contact angle of about 2 ° or less.
[0093]
A clear absorption band centered at a wavelength of 204 nm was observed in the pretreated product. This is thought to be because a trace amount of the lead component added as a scavenger remains. The internal transmittance of the ArF excimer laser at a wavelength of 193.4 nm is often 98.0% / cm or less, and a large influence due to absorption is observed.
[0094]
In contrast, ingots and heat-treated products did not have clear absorption bands. The internal transmittance of the ingot at a wavelength of 193.4 nm is about 99.4 to 100.0% / cm. Despite the fact that a clear absorption band was seen in the pre-treated product, the ingot was not found because the lead component, which is an impurity, was prevented from being incorporated by the segregation effect during crystal growth. It is thought that. It is considered that not only lead but also other impurities are prevented from being taken in by the same effect. The ingot thus obtained becomes high-purity calcium fluoride, the impurity content is reduced, and it is no longer difficult to quantify impurities by an analytical method such as emission analysis or mass spectrometry. In the heat treatment step (step {circle around (3)}), since no lead component or other impurities are taken in, it is considered that a clear absorption band was not seen not only in the ingot but also in the heat treatment product.
[0095]
From the above, it was considered that the cause of absorption in the calcium fluoride single crystal was due to the trace impurities contained.
[0096]
The internal transmittance at a wavelength of 193.4 nm is not only relatively good as 99.8% / cm or more, but also has a narrow range of 99.4 to 100.0% / cm. It was thought that this was due to the fact that such impurities remained in a trace amount.
[0097]
Therefore, an attempt was made to reduce the impurities that were presumed to remain even though the amount was extremely small by producing a calcium fluoride single crystal of higher purity as follows.
[0098]
Regarding the purity of high-purity raw materials for artificial synthesis, for example, in vacuum ultraviolet applications, barium is less than 0.02 ppm, cerium is less than 0.01 ppm, iron is less than 0.1 ppm, manganese is less than 0.02 ppm, lead is 0.1 ppm. The powder raw material of calcium fluoride single crystal, which is guaranteed high quality such as less than 0.01 ppm, is used. Even with such a high-purity raw material, when only the raw material is melted and crystallized, it becomes cloudy and devitrified. Therefore, a treatment for preventing white turbidity was performed by adding a scavenger and heating. As the type of scavenger, copper fluoride, silver fluoride, zinc fluoride and the like were used in addition to lead fluoride. The powder raw material mixed with the scavenger is filled into a clean container such as graphite, and the temperature is raised to about the melting point of calcium fluoride under sufficient vacuum evacuation to proceed with the deoxygenation reaction. Thereafter, the temperature was lowered to room temperature to prepare a pretreated product. The pretreated product was filled in a clean container such as graphite and installed at a predetermined position of the Bridgman apparatus which can be evacuated. Under sufficient evacuation, the temperature of the pretreated product was increased and melted by a heating means such as electric heating. After reaching the melting point, crystallization was started after several hours had passed after waiting for the temperature to stabilize rather than immediately starting crystallization by pulling down. Pulling down was performed at a speed of 1 mm per hour. When the pulling progressed and all the melt was crystallized, it was gradually cooled to room temperature and taken out as an ingot.
[0099]
The inventor manufactured a test piece for transmittance measurement from each of the pretreated product, the ingot, and the heat-treated product, similarly to the test piece described above. The internal transmittance of these test pieces was 99.5-100.0% / cm, and the range was narrowed. In terms of transmission loss, 0.0 to 0.5% / cm is indicated. Thus, the internal transmittance is in the direction of improvement, but there are still those with low internal transmittance.
[0100]
Therefore, the present inventor has inferred that such transmission loss may be due not only to absorption due to impurities but also to the fine particles described above. Even for fine particles, it is estimated that there is a relatively small contribution to the small transmission loss that should be considered at a level of 0.0 to 0.5% / cm, that is, 0.1% / cm. It was.
[0101]
In that case, it is thought that the influence by the fine particles is scattering and not absorption, so even if it is a calcium fluoride single crystal that seemed to have low internal transmittance at first glance, it is actually an optical member. I thought it could be used. If it is used for an optical member, an important point is an allowable amount of the fine particles, and an inspection method for measuring it. The above-mentioned measurement of the Japan Optical Glass Industry Association standard is insufficient because it only counts the diameter and number of bubbles and foreign matters, and does not refer to optical effects when bubbles or foreign matters exist. On the other hand, the present inventor has focused on the light transmission itself of the calcium fluoride single crystal in which fine particles are present.
[0102]
In the high-purity calcium fluoride single crystal produced as described above, the transmission loss due to absorption is so small that it is not detected in the wavelength region of 200 nm or more. Therefore, if a transmission loss was detected in a wavelength region of 200 nm or more, it was estimated that it was not due to absorption but due to scattering by fine particles. In order to examine this estimation, a plurality of ingots were made, and the transmittance of a test piece made from each ingot was measured.
[0103]
Speaking of measurement reproducibility, it is necessary to be ± 0.05% or better than ± 0.05% in order to investigate in the order of 0.1%. The reproducibility of measurement should be evaluated by performing statistical processing. For example, the standard deviation σ is obtained by measuring the transmittance in a state where a sample is not installed in the transmittance measuring device (blank) many times. Alternatively, the standard deviation σ may be obtained by actually measuring the transmittance of the sample many times.
[0104]
The wavelength 193.4 nm of the ArF excimer laser was taken as the first wavelength α, and the internal transmittance at this wavelength was A (% / cm). To measure this wavelength, an ultraviolet spectrophotometer was used because of the excellent reproducibility of transmittance measurement.
[0105]
The second wavelength was 365.1 nm of i-line by Hg, and the internal transmittance at this wavelength was B (% / cm). An ultraviolet spectrophotometer was also used for measurement at this wavelength, but the second wavelength β was set at a wavelength of 365.1 nm because the reproducibility of transmittance measurement in this vicinity was very good.
[0106]
When A = 100.0% / cm, B = 100.0% / cm. When A = 99.9% / cm, B = 99.9 to 100.0% / cm. When A = 99.8% / cm, B = 99.8 to 100.0% / cm. Similarly, when A = 99.7% / cm, B = 99.7 to 99.9% / cm, when A = 99.6% / cm, B = 99.6 to 99.8% / cm, A = When 99.5% / cm, the result of B = 99.5-99.7% / cm was obtained.
[0107]
However, the extreme wavelength dependence is not seen in the transmission loss as in Rayleigh scattering well known for optical glass. If Rayleigh scattering is performed when the size of the scattering source and the wavelength of the light are approximately the same, it is inversely proportional to the fourth power of the wavelength. Although more transmission loss is exhibited at a short wavelength, such extreme wavelength dependence is not observed in the case of calcium fluoride. This can be considered as a phenomenon in which the microparticles shield the light because the size of the microparticles is sufficiently larger than the wavelength of the light.
[0108]
Actually, the wavelength of the light is 140 nm to 400 nm, whereas the diameter of the fine particles observed with a microscope is about 10 μm to 500 μm, and is about 50 to 2500 times the wavelength. The influence of such fine particles should no longer depend on the wavelength and should be regarded as constant.
[0109]
Therefore, it was considered that the transmission loss (100-B) at the second wavelength β was caused by such fine particles, and this also had the same effect at the first wavelength α. That is, the transmission loss due to the fine particles at the first wavelength α is also (100−B).
[0110]
Accordingly, the transmission loss (100-A) at the first wavelength α is equal to the sum of the transmission loss (100-B) due to the fine particles and the transmission loss not due to the fine particles, and thus the transmission loss not due to the fine particles. Is represented as (100-C) for the sake of convenience, the following relationship is established. When Equation 4 is transformed into Equation 5 below, C can be indicated by A and B that can be actually measured.
[0111]
[Expression 4]
100-A = (100-B) + (100-C)
[0112]
[Equation 5]
C = A + (100-B)
[0113]
The transmittance C (% / cm) calculated by Equation 5 is a net transmittance excluding the influence of fine particles. Therefore, if the calcium fluoride single crystal has a transmittance C of 99.8% / cm or more, even if the calcium fluoride single crystal has an internal transmittance A of less than 99.8% / cm, the internal transmission Similar to a calcium fluoride single crystal having a rate A of 99.8% / cm or more, it can be used for an optical member that transmits light of a specific wavelength of 200 nm or less, and imaging of an exposure apparatus using the optical member. The characteristics are not inferior. The reason why the net transmittance C should be a problem by excluding the transmission loss due to the fine particles as described above is that the transmission loss due to the fine particles is not an absorption loss but a scattering loss, as described above. This is because there is almost no change to thermal energy, and the imaging characteristics and the like are not greatly affected.
[0114]
However, if the transmittance A (% / cm) itself at the first wavelength α (nm) used in the exposure apparatus is too low, the illuminance of the entire lens group may be reduced. Although depending on the optical path length of the entire lens, for example, when about 10 lenses based on a calcium fluoride single crystal are used, it is desirable that at least the internal transmittance A is 99.6% / cm or more.
[0115]
[First Embodiment]
[0116]
A method for inspecting a fluoride single crystal according to the first embodiment of the present invention in accordance with the above-described principle will be described with reference to FIG. FIG. 1 is a flowchart showing this inspection method.
[0117]
In this inspection method, first, as described above, the first wavelength α (nm) is determined so as to satisfy α ≦ 200 according to the specific wavelength of the originally intended vacuum ultraviolet light, and then the second wavelength β (Nm) is determined such that (β−α) ≧ 15 (steps S11 and S12).
[0118]
Next, a fluoride single crystal test piece to be inspected is prepared as in the conventional inspection method described above. Using this test piece, the reflection-containing transmittance at the first wavelength α of the fluoride single crystal is measured with a spectrophotometer or the like, and the measured reflection-containing transmittance is converted according to the above formulas 1 and 2. Thus, the internal transmittance A at the first wavelength α of the fluoride single crystal is measured (step S13).
[0119]
Next, using the test piece, the reflection-containing transmittance of the fluoride single crystal at the second wavelength β is measured with a spectrophotometer or the like, and the measured reflection-containing transmittance is measured according to the equations 1 and 2. By converting, the internal transmittance B at the second wavelength β of the fluoride single crystal is measured (step S14).
[0120]
Thereafter, a value C (% / cm) is calculated from the internal transmittances A and B obtained in steps S13 and S14 according to the equation 5 (step S15).
[0121]
Next, it is determined whether or not the value C calculated in step S15 is 99.8% / cm or more (step S16), and if it is 99.8% / cm or more, the same ingot as the test piece. It is evaluated that the member of the fluoride single crystal obtained from the above is suitable as a material for an optical member such as a lens of the exposure apparatus (step S17). If it is less than 99.8% / cm, it is the same as the test piece. It is evaluated that the fluoride single crystal member obtained from the ingot is not suitable as a material for an optical member such as a lens of the exposure apparatus or the like (step S18), and the inspection is terminated.
[0122]
According to the present embodiment, “appropriate” is evaluated when the value C is 99.8% or more, and therefore, “appropriate” is evaluated when the internal transmittance A is 99.8% or more. Compared with the conventional inspection method, the inspection standard is relaxed. Therefore, according to the present embodiment, the yield of the fluoride single crystal is improved. As a result, problems such as disturbing the production plan of the exposure apparatus due to the shortage of the planned quantity and causing an increase in capital investment in the apparatus for producing the calcium fluoride single crystal are solved.
[0123]
Further, according to the present embodiment, since the above-described principle is followed, similar to the conventional inspection method, desired imaging characteristics and the like in an exposure apparatus or the like using an optical member using a fluoride single crystal. This does not cause a situation in which the optical characteristics cannot be obtained.
[0124]
[Second Embodiment]
[0125]
FIG. 2 is a flowchart showing a method for inspecting a fluoride single crystal according to the second embodiment of the present invention. In FIG. 2, steps that are the same as or correspond to those in FIG. 1 are given the same reference numerals, and redundant descriptions thereof are omitted.
[0126]
This embodiment is different from the first embodiment in that it is determined whether or not the internal transmittance A at the first wavelength α of the fluoride single crystal to be inspected is 99.6% or more. If step S20 is added and the internal transmittance A is less than 99.6%, it is evaluated as unsuitable in step S18, and the condition that the internal transmittance A is 99.6% or more is also evaluated as “appropriate”. It is only a point.
[0127]
As already described, if the internal transmittance A at the first wavelength α is too low, the decrease in illuminance becomes large, which is not preferable. On the other hand, according to the present embodiment, the internal transmittance A of 99.6% or more is also evaluated as “appropriate” as a necessary condition.
[0128]
[Third Embodiment]
[0129]
The optical member according to the third embodiment of the present invention has an internal transmittance at the first wavelength α among the fluoride single crystals evaluated to be suitable by the inspection method according to the first or second embodiment. It is made of a fluoride single crystal having A of less than 99.8% (that is, a fluoride single crystal that is regarded as a defective product according to a conventional inspection method).
[0130]
Such a fluoride single crystal has been found to have sufficient characteristics for the first time based on the principle described above. Conventionally, such a fluoride single crystal has been disposed of as a defective product before being formed into an optical member such as a lens, and there was no optical member made of such a fluoride single crystal. .
[0131]
In the optical member according to the third embodiment, the fluoride single crystal may contain fine particles having a particle size of 10 μm or more and 500 μm or less. This is because the diameter of fine particles observed with a microscope is about 10 μm to about 500 μm, and in the present invention, as described with respect to the principle, the net transmittance excluding the influence of fine particles is a problem. .
[0132]
In the optical member according to the third embodiment, the fluoride single crystal is a calcium fluoride single crystal, barium contained in the calcium fluoride single crystal is less than 0.02 ppm, cerium is less than 0.01 ppm, Iron may be less than 0.1 ppm, manganese may be less than 0.02 ppm, lead may be less than 0.1 ppm, and yttrium may be less than 0.01 ppm. Thus, it is preferable that the impurity concentration is small because the absorption loss in the calcium fluoride single crystal is considered to be due to a trace amount of impurities contained in the calcium fluoride single crystal, as described above with respect to the principle.
[0133]
[Fourth Embodiment]
[0134]
FIG. 3 is a view showing the basic structure of a projection exposure apparatus according to the fourth embodiment of the present invention.
[0135]
The exposure apparatus according to the present embodiment is prepared as a wafer stage 3 on which a substrate 8 (the whole is simply referred to as “substrate W”) coated with a photosensitive agent 7 placed on the surface 3a can be placed as exposure light. Illumination optical system 1 for transferring a mask pattern (reticle R) prepared on the substrate W by irradiating vacuum ultraviolet light of a different wavelength, a light source 100 for supplying exposure light to the illumination optical system 1, and a substrate A first surface P1 (object plane) on which a mask R for projecting an image of the pattern of the mask R on W is placed and a second surface (image plane) matched with the surface of the substrate W are placed. Projection optical system 5 described above. The illumination optical system 1 also includes an alignment optical system 110 for adjusting the relative position between the mask R and the wafer W, and the mask R can move in parallel to the surface of the wafer stage 3. It is arranged on the reticle stage 2. The reticle exchange system 200 exchanges and transports a reticle (mask R) set on the reticle stage 2. The reticle exchange system 200 includes a stage driver for moving the reticle stage 2 parallel to the surface 3 a of the wafer stage 3. The projection optical system 5 has an alignment optical system applied to a scan type exposure apparatus.
[0136]
The exposure apparatus according to the present embodiment uses the optical member (for example, an optical lens) according to the third embodiment. Specifically, the present exposure apparatus shown in FIG. 3 is according to the third embodiment as one or both of the optical lens 9 of the illumination optical system 1 and the optical lens 10 of the projection optical system 5. An optical lens is used. In FIG. 3, 300 is a stage control system for controlling the wafer stage 3, and 400 is a main control unit for controlling the entire apparatus.
[0137]
In this exposure apparatus, since the optical member according to the third embodiment is used, since the yield of the optical member is good, the cost of the entire apparatus can be reduced, and sufficient imaging characteristics and the like can be achieved. Can be secured.
[0138]
【Example】
[Example 1]
[0139]
Hereinafter, a calcium fluoride single crystal is manufactured for use in an optical member used in an exposure apparatus using an ArF excimer laser as a light source, and the calcium fluoride single crystal is inspected by the inspection method according to the present invention. An example of “appropriate” will be described.
[0140]
Regarding the purity of high-purity raw materials for artificial synthesis, barium is less than 0.02 ppm, cerium is less than 0.01 ppm, iron is less than 0.1 ppm, manganese is less than 0.02 ppm, lead is less than 0.1 ppm, and yttrium is less than 0.01 ppm. A powdered raw material of calcium fluoride single crystal was used.
[0141]
As a scavenger, 1 mol% of silver fluoride was added to the powder raw material and mixed well with stirring. The mixed powder raw material was filled in a clean container made of graphite. The deoxygenation reaction was advanced by evacuation and raising the temperature to a melting point of calcium fluoride of 1400 ° C. under a degree of vacuum better than 0.01 Pa. Thereafter, the temperature was lowered to room temperature and taken out as a pretreatment product.
[0142]
The pretreated product was filled in a clean graphite crucible. This was installed at a predetermined position of the Bridgeman apparatus. When the degree of vacuum reached 0.01 Pa by sufficient evacuation, the temperature of the pretreated product was increased and melted by energization heating. When the temperature at the bottom of the crucible reached the melting point, crystallization was started by pulling down after 8 hours had passed. Pulling down was performed at a speed of 1 mm per hour. When the pulling progressed and all the melt was crystallized, it was gradually cooled to room temperature and taken out as an ingot.
[0143]
A test piece for measuring transmittance was produced from this ingot. Two parallel surfaces of the test piece facing each other were polished into a mirror surface with high accuracy. The parallelism was 5 to 10 seconds, and the surface roughness was 0.10 to 0.15 nm in RMS display. Moreover, the surface was made into the clean state by wet-cleaning just before the transmittance | permeability measurement.
[0144]
The wavelength 193.4 nm of the ArF excimer laser was taken as the first wavelength α, and the internal transmittance at this wavelength α was A (% / cm). A = 99.7% / cm. In order to measure the transmittance at this wavelength α, an ultraviolet spectrophotometer Cary 5 was used because of excellent reproducibility of transmittance measurement. The reproducibility of the transmittance measurement of the blank was as good as 3σ = 0.03%.
[0145]
The second wavelength β is a wavelength of 365.1 nm, and the internal transmittance at this wavelength β is B (% / cm). B = 99.9% / cm. The ultraviolet spectrophotometer Cary 5 was also used to measure the transmittance at this wavelength β, but the reproducibility of the blank transmittance measurement at a wavelength of 365.1 nm, which was the second wavelength β, was extremely 3σ = 0.01%. It was good.
[0146]
The transmittance C (% / cm) calculated according to the equation 5 by A = 99.7% / cm and B = 99.9% / cm was C = 99.8% / cm. Since C is 99.8% / cm or more and A is 99.6% / cm or more, it is evaluated as “suitable” by any of the inspection method shown in FIG. 1 and the inspection method shown in FIG. It was done.
[0147]
Then, the member cut out from this ingot was accommodated in the container made from graphite with favorable heat conductivity. A heating means was provided around the container, and after heat treatment, the temperature was lowered to room temperature and the member was taken out. An optical lens as an optical member was produced using the heat treated member of the calcium fluoride single crystal, and this lens was used in the illumination optical system 1 and the projection optical system 5 of the exposure apparatus using an ArF excimer laser as a light source. However, there was no particular problem because the desired imaging characteristics could not be obtained.
[0148]
Since the calcium fluoride single crystal produced in this example has an A of less than 99.8% / cm, it is evaluated as a defective product according to the conventional inspection method described above.
[0149]
[Example 2]
[0150]
A calcium fluoride single crystal is manufactured for use in an optical member used in an exposure apparatus using an ArF excimer laser as a light source, and the calcium fluoride single crystal is inspected by the inspection method according to the present invention. The example which was was demonstrated.
[0151]
Also in this example, a calcium fluoride single crystal was manufactured separately from Example 1, using the same material as in Example 1 and setting the same manufacturing conditions. Similarly to Example 1, a test piece for measuring transmittance similar to the test piece of Example 1 was manufactured from an ingot.
[0152]
Using this test piece, as in Example 1, the first wavelength α takes the wavelength 193.4 nm of the ArF excimer laser, the second wavelength β takes the wavelength 365.1 nm, and the first wavelength α The internal transmittance A (% / cm) at the wavelength α and the internal transmittance B (% / cm) at the second wavelength β were measured using the same spectrophotometer as in Example 1.
[0153]
As in Example 1, A = 99.7% / cm, but unlike Example 1, B = 100.0% / cm. Thus, the transmittance C (% / cm) calculated according to Equation 5 was C = 99.7% / cm. Since C is less than 99.8% / cm, it was evaluated as “unsuitable” by both the inspection method shown in FIG. 1 and the inspection method shown in FIG.
[0154]
Therefore, the calcium fluoride single crystal manufactured in Example 2 is not adopted as an optical member used in an exposure apparatus using an ArF excimer laser as a light source.
[0155]
However, the following tests were performed. That is, the member cut out from the ingot produced in Example 2 was accommodated in a graphite container having good thermal conductivity. A heating means was provided around the container, and after heat treatment, the temperature was lowered to room temperature and the member was taken out. Using this heat treated member of calcium fluoride single crystal to produce an optical lens as an optical member, when this lens is used in an exposure apparatus using an ArF excimer laser as a light source, an uncorrectable aberration occurs. The image could not be formed.
[0156]
[Example 3]
[0157]
F₂A calcium fluoride single crystal is manufactured for use in an optical member used in an exposure apparatus using a laser as a light source, and the calcium fluoride single crystal is inspected by the inspection method according to the present invention, and the inspection result is “appropriate”. An example will be described.
[0158]
Also in this example, a calcium fluoride single crystal was manufactured separately from Example 1, using the same material as in Example 1 and setting the same manufacturing conditions. Similarly to Example 1, a test piece for measuring transmittance similar to the test piece of Example 1 was manufactured from an ingot.
[0159]
The first wavelength α is F₂The laser wavelength was 157.6 nm, and the internal transmittance at this wavelength was A (% / cm). A = 99.7% / cm. In order to measure the transmittance at this wavelength α, a vacuum ultraviolet spectrophotometer CAMS was used because of the excellent reproducibility of transmittance measurement. The reproducibility of the transmittance measurement of the blank was as good as 3σ = 0.05%.
[0160]
The second wavelength was 248.3 nm, and the internal transmittance at this wavelength was B (% / cm). B = 99.9% / cm. Although the ultraviolet spectrophotometer Cary 5 was used for the measurement of the transmittance at this wavelength β, the reproducibility of the blank transmittance measurement at the wavelength 248.3 nm with the second wavelength β was as good as 3σ = 0.03%. there were.
[0161]
When A = 99.7% / cm and B = 99.9% / cm, the transmittance C (% / cm) calculated according to Equation 5 was C = 99.8% / cm. Since C is 99.8% / cm or more and A is 99.6% / cm or more, it is evaluated as “suitable” by any of the inspection method shown in FIG. 1 and the inspection method shown in FIG. It was done.
[0162]
Then, the member cut out from this ingot was accommodated in the container made from graphite with favorable heat conductivity. A heating means was provided around the container, and after heat treatment, the temperature was lowered to room temperature and the member was taken out. Using this heat-treated member of calcium fluoride single crystal, an optical lens as an optical member is manufactured.₂When used in the illumination optical system 1 and the projection optical system 5 of an exposure apparatus using a laser as a light source, there was no problem that desired imaging characteristics could not be obtained, and there was no particular problem.
[0163]
Since the calcium fluoride single crystal produced in this example has an A of less than 99.8% / cm, it is evaluated as a defective product according to the conventional inspection method described above.
[0164]
[Example 4]
[0165]
A calcium fluoride single crystal is manufactured for use in an optical member used in an exposure apparatus using an ArF excimer laser as a light source, and this calcium fluoride single crystal is produced by the inspection method shown in FIG. 1 and the inspection method shown in FIG. An example in which inspection is performed and the inspection results are divided into “appropriate” and “inappropriate” will be described.
[0166]
Also in this example, a calcium fluoride single crystal was manufactured separately from Example 1, using the same material as in Example 1 and setting the same manufacturing conditions. Similarly to Example 1, a test piece for measuring transmittance similar to the test piece of Example 1 was manufactured from an ingot.
[0167]
Using this test piece, as in Example 1, the first wavelength α takes the wavelength 193.4 nm of the ArF excimer laser, the second wavelength β takes the wavelength 365.1 nm, and the first wavelength α The internal transmittance A (% / cm) at the wavelength α and the internal transmittance B (% / cm) at the second wavelength β were measured using the same spectrophotometer as in Example 1.
[0168]
Unlike Example 1, A = 99.7% / cm and B = 99.6% / cm. Thus, the transmittance C (% / cm) calculated according to Equation 5 was C = 99.9% / cm. Since C is 99.8% / cm or more and A is less than 99.6% / cm, according to the inspection method shown in FIG. 1, it is “suitable”, and according to the inspection method shown in FIG. It was evaluated.
[0169]
The following tests were conducted. That is, the member cut out from the ingot manufactured in Example 4 was accommodated in a graphite container having good thermal conductivity. A heating means was provided around the container, and after heat treatment, the temperature was lowered to room temperature and the member was taken out. Using this heat treated member of calcium fluoride single crystal to produce an optical lens as an optical member and using this lens in an exposure apparatus using an ArF excimer laser as a light source, there is a problem in terms of imaging characteristics. The illuminance was slightly decreased. As described above, although the illuminance is slightly reduced, the exposure apparatus is not necessarily a serious problem.
[0170]
As mentioned above, although each embodiment and each Example of this invention were demonstrated, this invention is not limited to these.
[0171]
【The invention's effect】
As described above, according to the present invention, the yield of a fluoride single crystal can be improved, and desired optical characteristics cannot be obtained in an apparatus using an optical member using a fluoride single crystal. It is possible to provide a method for inspecting a fluoride single crystal that does not cause a situation.
[0172]
In addition, according to the present invention, a fluoride single crystal that originally has sufficient characteristics while being a fluoride single crystal that was disposed of as a defective product according to a conventional inspection method was used. An optical member and a projection exposure apparatus can be provided.
[Brief description of the drawings]
FIG. 1 is a flowchart showing an inspection method according to a first embodiment of the present invention.
FIG. 2 is a flowchart showing an inspection method according to a second embodiment of the present invention.
FIG. 3 is a view showing a basic structure of a projection exposure apparatus according to a fourth embodiment of the present invention.
FIG. 4 is a flowchart showing a conventional inspection method.
[Explanation of symbols]
1 Illumination optics
2 Reticle stage
3 Wafer stage
5 Projection optical system
100 light sources

Claims (4)

A method for inspecting a fluoride single crystal used in an optical member that transmits light having a specific wavelength of 200 nm or less,
When the first wavelength α (nm) and the second wavelength β (nm) are in the relationship of α ≦ 200 ≦ β and (β−α) ≧ 15, the first single crystal of the fluoride single crystal From the internal transmittance A (% / cm) at the wavelength α and the internal transmittance B (% / cm) at the second wavelength β of the fluoride single crystal, the formula C = A + (100−B) Determining whether the value C (% / cm) obtained by the above is 99.8% / cm or more,
A method for inspecting a fluoride single crystal, wherein the value C is evaluated to be suitable as a material for the optical member on the condition that the value C is 99.8% / cm or more.   Including the step of determining whether or not the internal transmittance A is 99.6% / cm or more, and the fluoride single substance is used as a necessary condition that the internal transmittance A is 99.6% / cm or more. The method for inspecting a fluoride single crystal according to claim 1, wherein the crystal is evaluated as suitable as a material for the optical member.   3. The method for inspecting a fluoride single crystal according to claim 1, wherein the first wavelength α is 193.4 nm or 157.6 nm.   4. The method for inspecting a fluoride single crystal according to claim 1, wherein the fluoride single crystal is a calcium fluoride single crystal.

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