JP2006315915A - Optical component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical component wherein the presence of impurities does not adversely affect its imaging performance by regulating the local distribution of impurities. <P>SOLUTION: In the first face and the second face of a crystalline optical member of an optical device, a C<SB>1</SB>/C<SB>2</SB>value (wherein, C<SB>1</SB>and C<SB>2</SB>are respectively the maximum and minimum concentration of alkaline earth metal impurities at least at three points such as center, periphery and middle) is lower than a specified value. The C<SB>1</SB>/C<SB>2</SB>value is 1.05 or less. The crystalline optical component is a fluoride, in particular calcium fluoride. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical member used in an exposure apparatus (semiconductor exposure apparatus) for manufacturing a semiconductor integrated circuit.

  Conventionally, an optical member such as a lens is used in a telescope, a camera, a semiconductor exposure apparatus, or the like. In particular, high-quality optical materials are desired for optical members of semiconductor exposure apparatuses. In recent years, with the high integration of semiconductor integrated circuits, the demand for ultra fine pattern formation is increasing, and a reduction projection type exposure apparatus (stepper) for transferring a fine pattern onto a wafer is widely used. In order to achieve high integration, it is necessary to increase the resolution of the stepper. In order to increase the resolution, it is necessary to use short-wavelength light and increase the numerical aperture of the projection lens, that is, to increase the diameter (for example, 300 mm or more).

  In addition, from the Kr-F excimer laser light (wavelength 248 nm), the use of Ar-F excimer laser light (wavelength 193 nm) and F2 laser light (wavelength 157 nm) is expected to be promising. A short wavelength laser after the Kr-F excimer laser beam has low transmittance, and it is impossible to use conventional optical glass.

  For this reason, a fluoride crystal having a high transmittance of extreme ultraviolet light is used for the optical member of the excimer laser exposure apparatus. These materials are crystalline, whereas conventional optical glass is amorphous.

  One of the requirements for these materials is that the optical characteristics are uniform throughout the optical member. One of the obstacles to optical uniformity is the problem of strain in crystals. Strain in the crystal causes refractive index non-uniformity. It is known that the non-uniformity of the refractive index becomes an aberration and affects the image performance of the semiconductor exposure apparatus. On the other hand, another possible cause is impurities in the crystal.

  Here, the production method of the fluoride crystal will be described. The fluoride crystal is usually produced in a crystal production apparatus by the Bridgman method (see Patent Document 1 below). In this apparatus, a crucible loaded with raw materials is placed in the furnace body, and an annular heating element is vertically arranged around the furnace. It controls the temperature inside the body. In the process of melting the raw material in the crucible at the upper part of the furnace body using this temperature distribution and then pulling down the crucible to the lower part of the furnace body at a predetermined rate, the seed crystal or the growth occurs when passing through the temperature range near the melting point A single crystal is grown by sequential precipitation from the melt while preserving the crystal orientation on the surface of the single crystal.

  As described above, the presence of impurities in the crystal hinders the optical uniformity, so it is desirable that there is no impurity. Therefore, in the process of melting and purifying the raw material in advance, a metal fluoride is added as a scavenger in order to remove oxides generated by the reaction of the raw material with moisture and the impurities in the raw material.

However, all the impurities in the raw material cannot be removed by the scavenger alone. In particular, when the fluoride crystal is calcium fluoride (CaF ₂ , fluorite), magnesium (Mg), strontium (Sr), and barium (Ba), which are alkaline earth metals of the same family as calcium, can be separated and purified in the raw material. It is difficult, and removal is difficult even by crystal growth.

There is an example in which the optical characteristics are improved by defining the allowable amount of these impurities (see Patent Document 2 below). In this example, when fluorite is irradiated with ArF excimer laser light, high energy photons such as γ rays, or particles, a color center is generated and the transmittance is prevented from decreasing. The total concentration of the alkaline earth metal impurities is 1E18 atom / cm ³ or less, more preferably, the concentration of Sr among the alkaline earth metal impurities is 1E18 atom / cm ³ or less.
JP 2005-89204 A JP-A-10-203899

  In the above-mentioned Patent Document 2, the concentration of alkaline earth metal is specified to obtain the required imaging performance. In the present invention, more importantly, how these impurities are distributed in the crystal. I found out. The presence of these localized distributions of impurities results in loss of uniformity in optical characteristics and affects the imaging performance of the semiconductor exposure apparatus. There is a need to.

  Therefore, the present invention is to provide an optical member that does not affect the imaging performance deterioration by defining the local distribution of impurities.

In order to solve the above-described problems, the present invention provides a crystalline optical member of an optical device, and the alkaline earth metal impurity concentration at least at the center, the outer peripheral portion, and the middle of the first and second surfaces. When the maximum and minimum values of C ₁ and C ₂ are C ₁ / C ₂ , the value of C ₁ / C ₂ is not more than a specified value.

  According to the present invention, an optical member having good imaging performance can be efficiently manufactured, and the image performance of an excimer laser exposure apparatus can be improved.

  Hereinafter, embodiments of the present invention will be described in detail.

  In the pretreatment step, metal fluoride is added to the raw material calcium fluoride as a scavenger, and the impurities in the raw material are removed as much as possible by melting and solidifying. Using this pretreated raw material, crystal growth is performed by a crucible pulling down method (Bridgeman method) or the like. At this time, precipitation of impurities occurs on the surface of the single crystal, but the phenomenon in which impurities are eliminated on the liquid side of the solid-liquid interface is generally called segregation. The intracrystalline distribution of alkaline earth metal impurities is mainly determined by the segregation effect. That is, by flattening the solid-liquid interface shape as much as possible, the in-plane distribution can be suppressed, and at the same time, thermal stress can be reduced, leading to higher quality of crystals.

  In the present invention, crystal growth is performed by reducing the thermal stress by reducing the temperature gradient in the crucible pulling direction of the outer wall of the crucible to 10 ° C./cm or less. For the quantification of impurities, the distribution of the amount of impurities in the surface is measured using a laser ablation type ICP-MS (inductively coupled plasma ion source mass spectrometer).

  FIG. 1 is a schematic diagram showing impurity analysis measurement points (●) in the optical member surface of a fluorite crystal.

In first side and a second side of the optical member, C _{1 /} C ₂ when at least around, the maximum value and the minimum value of the alkaline earth metal impurity concentration in the outer peripheral portion and the three-point of the intermediate was C _1, C ₂ Is less than or equal to the specified value. Further, when at least the above three points are taken on a plurality of radial lines from the center, and the maximum value and the minimum value of the alkaline earth metal impurity concentration at all points are C ₁ and C ₂ , the value of C ₁ / C ₂ is Below the specified value. The value of C ₁ / C ₂ is 1.05 or less.

In addition to calcium fluoride (CaF ₂ ), fluoride crystals such as lithium fluoride (LiF), magnesium fluoride (MgF ₂ ), and barium fluoride (BaF ₂ ) have transmittance in the ultraviolet region. And is used as an optical material. The present invention can be carried out on these fluoride crystals.

[Example]
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these Examples.

  Using commercially available high-purity calcium fluoride as a raw material, 0.1 mol% of zinc fluoride, which is a scavenger for preventing oxidation, was added and stirred, and then charged in a pretreatment crucible. After vacuum drying at 200 ° C. for 5 hours, the temperature was raised to the melting point of calcium fluoride for melting to remove impurities, and then the temperature was returned to room temperature to solidify the calcium fluoride to obtain a crystal growth raw material.

Crystal growth was performed by a pulling-down method (Bridgeman method) using a <111> seed crystal. 0.01 mol% of zinc fluoride was added as a scavenger to the raw material for crystal growth, and the crucible for crystal growth was filled. The temperature was raised to a vacuum of 8 × 10 ⁻² Pa and a temperature of 1420 ° C., and the degree of vacuum was 2.6 × 10 ⁻⁴ Pa and the temperature was 1420 ° C. for 20 hours in order to dissolve and degas the CaF ₂ in the crucible.

  Next, while controlling the output of the heater, the temperature gradient in the crucible pulling direction of the outer wall of the crucible is controlled to 5 ° C./cm, and the crucible is lowered at a rate of 0.5 mm / h to grow a good quality calcium fluoride single crystal. It was. Since the pulling rate at this time preferably corresponds to the crystal growth rate, it goes without saying that it is necessary to consider the size and shape of the crystal to be manufactured. In general, as the crystal size increases, the pulling speed needs to be reduced.

  Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace for heat treatment, and zinc fluoride with an addition amount of 0.01 mol% with respect to the calcium fluoride single crystal were added. The furnace was evacuated and the temperature of the crucible was increased from room temperature to 1100 ° C. at a rate of 100 ° C./h, and then maintained at about 1100 ° C. for about 50 hours. Then, the temperature was lowered to 5 ° C./h and cooled to room temperature. At this time, the cooling rate needs to be reduced as the crystal size increases. In other words, it is difficult to make the birefringence very small unless slow.

  The crystal thus obtained is processed into a disk shape, and when it is converted into an optical element, the upper bottom surface intersecting with the optical axis is measured by ICP-MS at 8 points at intervals of 20 mm from the center and at intervals of 60 ° in the circumferential direction. Concentration analysis was performed. The laser was measured at a wavelength of 213 nm, the laser beam diameter was 20 μm, and the irradiation pulse was 15 Hz. As a result, the most contained alkaline earth metal impurity was 20 ppm in Sr. The largest concentration ratio was also Sr, and the ratio of the maximum value and the minimum value of Sr concentration on the top surface was 1.04, and 1.03 on the bottom surface.

When the wavefront aberration of this crystal was measured and the RMS (root mean square) after correcting the power component of the refractive index difference was calculated, the value was 2.45 × 10 ⁻⁸ and the image of the excimer laser exposure apparatus. The performance could be improved.

[Comparative example]
Crystal growth was performed by a pulling-down method (Bridgeman method) using a <111> seed crystal. 0.01 mol% of zinc fluoride was added as a scavenger to the raw material for crystal growth, and the crucible for crystal growth was filled. The temperature was raised to a vacuum degree of 8 × 10 ⁻² Pa and a temperature of 1420 ° C., and the degree of vacuum was 2.6 × 10 ⁻⁴ Pa and the temperature was 1420 ° C. for 20 hours in order to degas and degas the CaF ₂ in the crucible.

  Next, while controlling the output of the heater, the temperature gradient in the crucible pulling direction of the outer wall of the crucible was controlled to 15 ° C./cm, and the crucible was lowered at a rate of 0.5 mm / h to grow a calcium fluoride single crystal. Since the pulling rate at this time preferably corresponds to the crystal growth rate, it goes without saying that it is necessary to consider the size and shape of the crystal to be manufactured. In general, as the crystal size increases, the pulling speed needs to be reduced.

  Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace for heat treatment, and zinc fluoride with an addition amount of 0.01 mol% with respect to the calcium fluoride single crystal were added. The furnace was evacuated and the temperature of the crucible was increased from room temperature to 1100 ° C. at a rate of 100 ° C./h, and then maintained at about 1100 ° C. for about 50 hours. Then, the temperature was lowered to 5 ° C./h and cooled to room temperature. At this time, the cooling rate needs to be reduced according to the increase in the size of the crystal. In other words, it is difficult to make the birefringence very small unless slow.

  The crystal thus obtained is processed into a disk shape, and when it is converted into an optical element, the upper bottom surface that intersects the optical axis is measured by ICP-MS at 8 points at intervals of 20 mm from the center and every 60 ° in the circumferential direction Concentration analysis was performed. The laser was measured at a wavelength of 213 nm, the laser beam diameter was 20 μm, and the irradiation pulse was 15 Hz. As a result, the most contained alkaline earth metal impurity was 50 ppm in Sr. The largest concentration ratio was Sr, and the ratio of the Sr concentration to the maximum value and the minimum value on the top surface was 1.10, and 1.07 on the bottom surface.

When the wavefront aberration of this crystal was measured and the RMS after correcting the power component of the refractive index difference was calculated, the value was 2.01 × 10 ⁻⁷ and the image performance of the excimer laser exposure apparatus could not be improved. It was.

Schematic diagram showing impurity analysis measurement points (●) in the optical member surface of fluorite crystal

Claims (8)

A crystalline optical member of an optical device, wherein a maximum value and a minimum value of an alkaline earth metal impurity concentration at least at the center, the outer peripheral portion, and the middle of the first surface and the second surface are defined as C ₁ , C an optical member, characterized in that ₂ to the value of C _{1 /} C ₂ when is less than a specified value. When at least the center, the outer periphery and the middle three points are taken on a plurality of radial lines from the center, and the maximum and minimum values of the alkaline earth metal impurity concentration at all points are C ₁ and C ₂ , C ₁ The optical member according to claim 1, wherein the value of / C ₂ is a specified value or less. The optical member according to claim _1, wherein the value of at least C ₁ / C ₂ is 1.05 or less.   The optical member according to claim 1, wherein the crystalline optical member is a fluoride.   The optical member according to claim 4, wherein the fluoride is calcium fluoride, lithium fluoride, magnesium fluoride, or barium fluoride. The method for producing an optical member according to claim 4, wherein a scavenger is added using fluoride as a raw material, and the pretreatment is performed by melting and solidifying.
The raw material obtained by the above step has a step of growing a fluoride single crystal by the Bridgman method,
A method for producing an optical member, characterized in that a temperature gradient in a crucible pulling direction of a crucible outer wall is set to 10 ° C./cm or less.   An optical device using the optical member according to claim 1.   A semiconductor exposure apparatus using the optical member according to claim 1 and using an excimer laser as a light source.

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