JP2006124242A - Fluoride single crystal, calcium fluoride single crystal and its sorting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a calcium fluoride single crystal having total characteristics more suitable as an optical element material used in a short-wavelength light source; and to provide a method for sorting the same. <P>SOLUTION: The method for sorting the calcium fluoride single crystal comprises irradiating a calcium fluoride single crystal with a gamma ray of a dose of 1×10<SP>5</SP>rad/h for 1 h, then measuring the transmissivity T<SB>140</SB>at 140 nm wavelength and the transmissivity T<SB>190</SB>at 190 nm wavelength, and selecting the single crystal having T<SB>140</SB>and T<SB>190</SB>values of both ≥80% and a transmissivity ratio T<SB>140</SB>/T<SB>190</SB>of ≥0.9. Thereby, a calcium fluoride single crystal maintaining high transmissivity both at 140 nm and 190 nm wavelengths, almost free from lowering of the transmissivity in a wavelength range of 190 to 140 nm, and having total characteristics more suitable as an optical element material used in a short wavelength light source can be sorted. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fluoride single crystal and a calcium fluoride single crystal used for an optical element and the like, and a selection method thereof.

In recent years, it has been required to increase the exposure resolution due to the demand for miniaturization of optical elements, and as one means, the wavelength of the exposure light source has been shortened. Specifically, the wavelength is shortened from a conventional KrF laser (248 nm) to an ArF laser (193 nm) or an F ₂ laser (157 nm). As an optical element capable of dealing with such a short wavelength, a fluoride single crystal such as calcium fluoride, which has better transmittance in the vacuum ultraviolet region than conventional synthetic quartz (SiO ₂ ), has attracted attention.

As a technique relating to the characteristics of calcium fluoride single crystals and inspection methods that can cope with shorter wavelengths, there is a technique described in Patent Document 1, for example. According to the literature 1, gamma ray irradiation is proposed as a relatively inexpensive means, and the transmittance per 30 mm thickness at a wavelength of 140 nm after irradiation with a gamma ray with a dose of 1 × 10 ⁶ R / h for one hour is proposed. Decrease of 12% or less (preferably 7% or less) before irradiation, or decrease in transmittance per 30 mm thickness at 190 nm wavelength after irradiation for 1 hour with a dose of 1 × 10 ⁶ R / h gamma rays It is said that a calcium fluoride single crystal whose amount is 9% or less (preferably 7% or less) with respect to that before irradiation is suitable. (Paragraph number 0007, etc. in the same document)

JP 2003-206197 A

  However, the transmittance of a fluoride single crystal generally decreases sharply as it goes into the short wavelength region, but it is difficult to analogize other crystal characteristics simply by measuring the transmittance at a single wavelength. Therefore, it was insufficient to judge the overall characteristics of the crystal including the optical characteristics.

  The present invention has been made in view of the above problems, and provides a fluoride single crystal and a calcium fluoride single crystal having comprehensive characteristics more suitable as an optical element material used for a short wavelength light source, and a selection method thereof. With the goal.

In the fluoride single crystal according to the present invention, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm after irradiation with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour are both 80% or more. , T ₁₄₀ / T ₁₉₀ is 0.9 or more.

The calcium fluoride single crystal according to the present invention has a transmittance T _{140 at a} wavelength of ₁₄₀ nm and a transmittance T _{190 at a} wavelength of 190 nm of 80% or more after irradiation with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour. And _T140 / _T190 is 0.9 or more, It is characterized by the above-mentioned.

In addition, the method for selecting a fluoride single crystal according to the present invention is that the fluoride single crystal is irradiated with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour, and then the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance at a wavelength of 190 nm. T ₁₉₀ is measured, and T ₁₄₀ and T ₁₉₀ are both 80% or more, and T ₁₄₀ / T ₁₉₀ is 0.9 or more.

Further, the calcium fluoride single crystal selection method according to the present invention is such that the calcium fluoride single crystal is irradiated with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour, and then has a transmittance T _{140 at a} wavelength of ₁₄₀ nm and a wavelength of 190 nm. The transmittance T ₁₉₀ is measured, and T ₁₄₀ and T ₁₉₀ are both 80% or more and T ₁₄₀ / T ₁₉₀ is 0.9 or more.

According to the fluoride single crystal of the present invention, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm after irradiation with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour are both 80% or more. In addition, since T ₁₄₀ / T ₁₉₀ is 0.9 or more, the transmittance at both 190 nm and 140 nm is kept high, and the decrease in the transmittance at 190 nm to 140 nm is small. It can be said that it has comprehensive characteristics more suitable as an element material.

In addition, according to the calcium fluoride single crystal according to the present invention, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm after irradiation with a gamma ray with a dose of 1 × 10 ⁵ rad / h for 1 hour are both 80. Since the T ₁₄₀ / T ₁₉₀ is not less than 0.9 percent and the T ₁₄₀ / T ₁₉₀ is not less than 0.9, the transmittance at both 190 nm and 140 nm is maintained high, and the decrease in the transmittance at 190 nm to 140 nm is small, and the short wavelength light source It can be said that it has a more suitable overall characteristic as an optical element material used in the above.

Further, according to the method for selecting a fluoride single crystal according to the present invention, the fluoride single crystal is irradiated with a gamma ray having a dose of 1 × 10 ⁵ rad / h for 1 hour, and then the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the wavelength at a wavelength of 190 nm. The transmittance T ₁₉₀ is measured, and T ₁₄₀ and T ₁₉₀ are both 80% or more, and T ₁₄₀ / T ₁₉₀ is 0.9 or more. Therefore, the transmittance at both 190 nm and 140 nm is increased. It is possible to select a fluoride single crystal that is maintained and has a low decrease in transmittance at 190 nm to 140 nm, and has comprehensive characteristics more suitable as an optical element material used for a short wavelength light source.

Moreover, according to the selection method of the calcium fluoride single crystal according to the present invention, the calcium fluoride single crystal is irradiated with gamma rays having a dose of 1 × 10 ⁵ rad / h for 1 hour, and then the transmittance T ₁₄₀ and the wavelength at a wavelength of ₁₄₀ nm are used. The transmittance T ₁₉₀ at ₁₉₀ nm is measured, and the transmittance at both 190 nm and 140 nm is selected in order to sort out those having both T ₁₄₀ and T ₁₉₀ of 80% or more and T ₁₄₀ / T ₁₉₀ of 0.9 or more. Can be selected, and a single crystal of calcium fluoride having a comprehensive characteristic more suitable as an optical element material used for a short wavelength light source can be selected.

A method for selecting a calcium fluoride single crystal according to the present embodiment will be described with reference to the flowchart shown in FIG. First, the calcium fluoride single crystal to be selected is irradiated with gamma rays with a dose of 1 × 10 ⁵ rad / h for 1 hour (S1). This is known to have a correlation between the F ₂ laser durability deterioration amount and the gamma ray durability deterioration amount with respect to the calcium fluoride single crystal as shown in FIG. This is because gamma irradiation is more advantageous than laser irradiation in terms of accuracy and measurement cost.

After gamma ray irradiation for 1 hour, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm are measured (S2). The 140 nm and 190 nm are known to be peaks obtained in common with the F ₂ laser and the gamma ray, as shown in paragraph No. 0019 of Patent Document 1 and the like. It is selected as the evaluation wavelength. In addition, a known spectrophotometer for measuring the vacuum ultraviolet wavelength region is used for measuring the transmittance. _Then, there is _{T 140} and _{T 190} are both 80% or _{more, T} 140 _{/ T 190} which is the ratio of the _{T 140} and _{T 190} are sorted those less than 0.9 (S3).

FIG. 2 is a graph showing the results of the investigation of the relationship between wavelength and transmittance by the present inventors for three types of calcium fluoride single crystals A, B, and C after irradiation with gamma rays. In the figure, A shows a calcium fluoride single crystal grown without adding a scavenger effective for the purification of the single crystal to the raw material, that is, a calcium fluoride single crystal not subjected to impurity exclusion treatment. Is. According to the figure, the transmittance of the single crystal A rapidly decreases near 200 nm, and both _T140 and _T190 do not satisfy the condition of 80% or more. Therefore, the single crystal A can be excluded in the calcium fluoride single crystal selection method according to the present embodiment.

B in the figure shows a calcium fluoride single crystal that was grown by adding a scavenger and contained impurities inherent in the raw material. According to the figure, the transmittance of the single crystal B rapidly decreases near 145 nm, and the condition that T ₁₉₀ is 80% or more is satisfied, but the condition that T ₁₄₀ is 80% or more is not satisfied. Therefore, this single crystal B can be excluded in the calcium fluoride single crystal selection method according to the present embodiment.

Further, the transmittance of the calcium fluoride single crystal shown in FIG. 9 of Patent Document 1 is drooping in the vicinity of 160 nm, and the condition that T ₁₉₀ is 80% or more is satisfied, but T ₁₄₀ is 80% or more. The condition that T ₁₄₀ / T ₁₉₀ is 0.9 or more is not satisfied. Therefore, in the calcium fluoride single crystal selection method according to the present embodiment, the single crystal of Patent Document 1 can be eliminated.

On the other hand, C in FIG. 2 shows a calcium fluoride single crystal grown by adding a suitable scavenger using a raw material with few intrinsic impurities. According to the figure, the transmittance of the single crystal C maintains a high transmittance up to about 125 nm, both T ₁₄₀ and T ₁₉₀ are 80% or more, and T is the ratio of T ₁₄₀ and T _{190. The} condition that ₁₄₀ / T ₁₉₀ is 0.9 or more is satisfied. Therefore, in the method for selecting a calcium fluoride single crystal according to the present embodiment, the single crystal C can be selected as one having few defects due to impurities and high overall characteristics.

  As described above, according to the calcium fluoride single crystal sorting method according to the present embodiment, it is possible to sort as having an overall characteristic more suitable as an optical element material used in a short wavelength light source with few defects due to impurities. it can. In the above embodiment, the calcium fluoride single crystal has been described as an example, but the present invention can be applied to other fluoride single crystals.

It is a flowchart explaining the selection method of the calcium fluoride single crystal which concerns on this embodiment. It is a graph which shows the relationship between a wavelength and the transmittance | permeability about the calcium fluoride single crystal after gamma ray irradiation.

Claims (4)

The transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm after irradiation with gamma rays with a dose of 1 × 10 ⁵ rad / h for 1 hour are both 80% or more, and T ₁₄₀ / T ₁₉₀ is 0.9 A fluoride single crystal characterized by the above. The transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T _{190 at a} wavelength of 190 nm after irradiation with gamma rays with a dose of 1 × 10 ⁵ rad / h for 1 hour are both 80% or more, and T ₁₄₀ / T ₁₉₀ is 0.9 A calcium fluoride single crystal characterized by the above. After irradiating a fluoride single crystal with a dose of 1 × 10 ⁵ rad / h of gamma rays for 1 hour, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T ₁₉₀ at a wavelength of 190 nm are measured, and both T ₁₄₀ and T ₁₉₀ are 80%. A method for screening a fluoride single crystal, characterized in that a screen having T ₁₄₀ / T ₁₉₀ of 0.9 or more is selected. After irradiating the calcium fluoride single crystal with gamma rays with a dose of 1 × 10 ⁵ rad / h for 1 hour, the transmittance T _{140 at a} wavelength of ₁₄₀ nm and the transmittance T ₁₉₀ at a wavelength of 190 nm are measured, and both T ₁₄₀ and T ₁₉₀ are 80 A method for selecting a calcium fluoride single crystal, characterized in that those having a percentage of T ₁₄₀ / T ₁₉₀ of 0.9 or more are selected.

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