JPH06273355A - Inspection method for optical material for excimer laser - Google Patents

Abstract

PURPOSE:To provide a method for selecting such a material as causing no absorption, when excimer laser light is transmitted for a long time by irradiating a synthetic silica glass with X-rays, easily in a short time. CONSTITUTION:Silicon tetrachloride is subjected to hydrosis in hydrogen flame and deposited directly to produce a synthetic silica glass sample which is then irradiated with excimer laser of KrF, ArF, F2, etc., or X-rays. In this regard, a linear relationship exists between absorption coefficients alpha derived through irradiation. Extent of absorption when the sample is subjected to 10<6> shots (about 2.8 hours) of 100HZ KrF excimer laser, for example, can be predicted by irradiating the sample with Rh target X-rays of 50mA, 5OkV for 10-15 min in the vacuum of about 10<-2>Torr. When X-rays are used, generation of 220nm absorption band can be decided in a short time and the extent of absorption can be predicted when the sample is irradiated with excimer laser having photon energy of 5eV or above. Consequently, a material causing no absorption even after transmission of excimer laser for a long time can be selected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for inspecting synthetic quartz glass used as an optical material for lenses and prisms for excimer lasers, which are ultraviolet lasers.

[0002]

2. Description of the Related Art Recently, a VLSI using an excimer laser
With the development of manufacturing processes and CVD processes using excimer lasers, the demand for optical materials for excimer lasers has become particularly strong.

The excimer laser is a high-power pulse laser that oscillates mainly in the ultraviolet region, and mainly consists of XeF (wavelength 350 nm, photon energy 3.5 eV), XeCl (wavelength 308 nm, photon energy 4. 0 eV), KrF (wavelength 248 nm,
Photon energy 5.0 eV, ArF (wavelength 193n)
m, photon energy 6.4 eV), F ₂ (wavelength 157 n
m, photon energy 7.9 eV). Of these, KrF and ArF excimer lasers are drawing attention in relation to lithography and photo-CVD, and in particular, in the lithography process for semiconductor manufacturing, KrF excimer lasers are being developed.

Excimer lasers such as ArF and KrF have high energy density and much higher power than ultraviolet light sources such as conventional mercury lamps and deuterium lamps, and are therefore likely to damage quartz glass. Further, in synthetic quartz glass as an optical system material such as a photomask substrate, the emission band at 650 nm is 260 n due to sputtering or plasma etching in the manufacturing process.
In some cases, an absorption band may appear in m and the transmission performance in the ultraviolet region may deteriorate, and when an excimer laser beam is used as an exposure light source in a semiconductor manufacturing lithography process instead of a conventional mercury lamp. However, this decrease in transmittance is extremely large.

Substrate materials that are deteriorated in the photomask manufacturing process and deteriorate in transmission performance must be inspected and eliminated in advance, but there is no way to know them in advance, and there is no way to know them in advance. Since it is necessary to measure the fluorescence characteristics and absorption characteristics of the substrates one by one after each step, it is time consuming and costly, so it is not realistic, and the deterioration of the absorption characteristics of synthetic quartz glass is inspected by a simple method. There was a demand for the development of means for doing so.

Therefore, the applicant of the present invention synthesized excimer laser light as a method for inspecting alteration of absorption characteristics of synthetic quartz glass from the viewpoint that materials can be selected in advance if an accelerated test can be carried out by some method. When the quartz glass is irradiated, the synthetic quartz glass used as the photomask substrate undergoes a change in absorption characteristics similar to that caused by spattering, plasma etching, etc. in the manufacturing process, and red fluorescence of about 650 nm is emitted. It was found that the light emitted was emitted, and an inspection method for selecting according to the presence or absence of red light emission during irradiation of an excimer laser was found (Japanese Patent Laid-Open No. 1-189654).

That is, depending on the presence or absence of red light emission, the VLSI
260 nm generated in the reticle manufacturing process for pattern transfer
It is a method to determine the presence or absence of an absorption band with a peak at the time of generation of this absorption band, because the amount of hydrogen supplied in the flame is more than the stoichiometrically required amount during the hydrolysis of silicon tetrachloride in an oxyhydrogen flame. Can also be prevented by making it excessive.

[0008]

However, if the irradiation of the excimer laser is repeated for a long time, an absorption band having a peak at 220 nm is generated, so that the transmittance of the excimer laser itself is lowered. As described above, in a reduction projector (stepper) for VLSI lithography, there is a problem that silica glass deteriorates and an absorption band is generated when used for a long time even if there is no problem in a short-term use. Whether or not the 220 nm absorption band is generated can be determined in a relatively short time when ArF excimer laser irradiation is performed.
It was difficult to generate during irradiation with rF excimer laser, and became conspicuous only after irradiation with about 10 ⁶ shots. Therefore, it took a long time to select the material and the efficiency was poor.

As described above, the absorption band generated during irradiation of the KrF excimer laser is very weak and becomes conspicuous only after irradiation of the excimer laser for a very long time. Therefore, it takes a few days to a few weeks to carry out the inspection, and therefore it is a reality. Was difficult to implement and was required to provide accelerated testing to solve these problems.

[0010]

Therefore, the present inventors have
As a result of intensive studies to solve the above problems, 22
The absorption band having a peak at 0 nm is caused by a defect structure called E ′ center composed of ≡Si · structure and also causes emission of light at 280 nm. Therefore, whether or not such an absorption band is generated by the KrF excimer laser. In order to determine whether or not the X-ray is used, it is possible to determine whether or not the absorption band is generated in a shorter time than the KrF excimer laser, and it is also possible to determine whether or not the absorption band is generated by the excimer laser having a photon energy of 5 eV or more. The present invention has been completed based on the knowledge that

[Action]

As a selection method, a sample piece obtained by polishing a parallel plane having a thickness of, for example, about 5 to 30 mm into a mirror surface is cut out from a lump of the optical material from which the optical material is cut out.
This sample piece is measured with a spectrophotometer for the spectral transmittance of the portion where the excimer laser beam and X-rays are irradiated, and then the excimer laser beam is irradiated under predetermined conditions (energy density, repetition frequency, number of shots). . The spectral transmittance of the irradiated part was measured immediately after the end of irradiation, and 2
The difference in absorption coefficient at 20 nm is obtained and used as an index of the degree of absorption, and X-ray irradiation is performed under a predetermined condition, and the difference in absorption coefficient at 220 nm is similarly obtained, and the degree of absorption is used as an index. At this time, KrF, ArF, F
₍₂₎ Examine the calibration curve for samples with different generation of absorption during excimer laser and X-ray irradiation, and perform the inspection based on the calibration curve.

When the excimer laser is irradiated for a long time,
An absorption band having a peak around 220 nm is generated. The absorption band having a peak at 220 nm is generated not only by the excimer laser but also by irradiation with γ rays, X rays, neutron rays, or the like. From the analysis of electron spin resonance (ESR) spectrum, it was confirmed that this absorption band is due to the ≡Si · structure called E ′ center.
220 n of spin concentration obtained from SR signal intensity
The absorption band intensity of m is proportional.

Various precursors of the E'center when irradiated with an excimer laser are considered. One is ≡Si-O-Si≡ by irradiation with an excimer laser.

[Chemical 1] Or

[Chemical 2] It is considered that the generation is caused by the mechanism of. here

[Chemical 3] Is a planar structure in which three oxygen molecules are bonded.

In addition to this, ≡Si—H H—O—Si≡
The structure is conceivable.

[Chemical 4]

In either mechanism, the E'center is generated by the absorption of photons. The energy gap of the quartz glass band is about 9 eV. The photon energy of the KrF excimer laser is 5.0 eV, the photon energy of the ArF excimer laser is 6.4 eV, and the photon energy of the F ₂ excimer laser is 7.9 eV. Therefore, in any case, defects are generated. The photon energy is 5 eV or more, and two-photon absorption is considered. Therefore, since the photon energy is higher in the order of F ₂ excimer laser, ArF excimer laser, and KrF excimer laser, it is considered that the generation efficiency is good.

On the other hand, X-rays have higher photon energy, but ArF and KrF can be obtained by irradiating them for an appropriate time.
The correlation with the absorption band intensity during excimer laser irradiation is obtained. However, when irradiation is carried out for a long time, cleavage of the network structure of the glass begins, and there is no correspondence with the induced absorption during irradiation of the excimer laser.

As the synthetic quartz glass, one obtained by directly depositing vitrified silicon tetrachloride by hydrolysis in an acid / hydrogen flame is excellent in excimer laser resistance such as transmission characteristics in the wavelength region of the excimer laser. Preferred as an optical material for use. In addition, the hydrolyzate produced in the oxyhydrogen flame produced in excess of stoichiometric amount of hydrogen produced an absorption band of 260 nm, and the resulting 65
It is effective for preventing red emission of 0 nm and is preferable as an optical material for excimer laser.

Various types of X-ray sources are conceivable,
For example, an X-ray diffractometer using a W or Cu tube or a fluorescent X-ray analyzer using a Rh tube can be used as it is. When irradiating X-rays using these devices, the X-ray irradiation conditions greatly differ depending on the sample holding method and the control conditions (voltage, current) of the X-ray tube, so the control conditions should be kept constant. Of course, the shape of the sample must be constant and an appropriate sample fixing jig must be used. In addition, it is preferable to empirically select a condition in which sufficient absorption for evaluation occurs with respect to a material that absorbs when an excimer laser is irradiated. For example, when a W target tube is used in the X-ray diffractometer, it is used for about 10 to 30 minutes under the conditions of 50 kV and 30 mA. It can be used with irradiation for about a minute. In addition, an irradiation device can also be manufactured and used by using an appropriate tube.

[0019]

【Example】

Example 1 A sample was produced by subjecting silicon tetrachloride to direct deposition vitrification by hydrolysis in an acid / hydrogen flame, and OH concentrations were A: 450 ppm and B: 650 ppm, respectively.
C: 850 ppm, D: 1120 ppm, E: 1300
5 kinds including ppm are selected, and KrF excimer laser (200 mJ / cm ² , 100 Hz, 10 ⁶ shots), ArF excimer laser (100 mJ / cm ² ,
50 Hz, 10 ⁴ shots, and X-rays (using a fluorescent X-ray device, X-rays of Rh target at 50 mA, 50 kV)
Under a vacuum of about 10 ^-2 torr for 10 minutes and 15 minutes), and the absorption coefficient α induced by the irradiation was measured. The results are shown in Table 1. Each sample has 3
Four pieces each having a shape of 0 mm x 10 mm x 10 mm were cut out, and two surfaces were mirror-polished to have a thickness of 10 mm.

[0020]

[Table 1]

Further, the following formulas (4) to (7) are derived from the results of Table 1 in which the absorption coefficients αArF, αKrF and αX induced by ArF excimer laser, KrF excimer laser and X-ray irradiation are measured. , ΑKrF, α
It was found that ArF and αX have a linear relationship. In this example, the KrF excimer laser is 1 Hz at 100 Hz.
Since it is 0 ⁶ shot, 10 ⁶ / 100Hz = 10
The extent of absorption obtained with an irradiation time of 000 seconds (about 2.8 hours) can be predicted by X-ray irradiation for 10 to 15 minutes. By irradiating X-rays in this way, 220 nm can be obtained in a short time.
It is possible to determine whether or not the absorption band of
It is also possible to predict the degree of absorption upon irradiation with ArF excimer laser by using the line.

○ In the case of irradiation for 15 minutes αKrF = 0.14 (αX−0.045 cm ⁻¹ ) (4) αArF = 0.16 (αX−0.045 cm ⁻¹ ) (5) ○ In the case of irradiation for 10 minutes αKrF = 0.16 (αX-0.031 cm ^-1 ) (6) αArF = 0.19 (αX-0.031 cm ^-1 ) (7)

Example 2 From the quartz glass synthesized according to Example 1, five different lots were selected, and 30 according to Example 1 respectively.
Three pieces each having a shape of mm × 10 mm × 10 mm were cut out, and two surfaces thereof were mirror-polished to have a thickness of 10 mm. X-ray (50 m
A, 50 kV) was irradiated for 15 minutes, and the absorption coefficient αX induced by irradiation was measured. Based on the result, (4)
From the formula (5), the value of the absorption coefficient at 220 nm when the KrF and ArF excimer laser was irradiated according to Example 1 was predicted. Then, KrF,
The ArF excimer laser was irradiated according to Example 1 and then the absorption coefficient at 220 nm was measured. The results are shown in Table 2. From the results in Table 2, it was found that the predicted value and the actually measured value were in good agreement, which indicates that the inspection method according to the present invention is effective.

[Table 2]

[0024]

[Effect] By irradiating synthetic quartz glass with X-rays, it is possible to select a material that does not absorb when excimer laser light is transmitted for a long time. In addition, by using X-rays, it is possible to determine whether or not an absorption band of 220 nm is generated in a shorter time than in the case of irradiating with a KrF excimer laser, and under certain conditions, the excimer with a photon energy of 5 eV or more is used. The extent of absorption that occurs when irradiated with a laser can be predicted.

Claims (6)

[Claims] 1. A method for inspecting synthetic quartz glass, which comprises selecting a material that does not absorb when irradiated with an excimer laser having a photon energy of 5 eV or more by irradiating the synthetic quartz glass with X-rays. 2. A method for inspecting synthetic quartz glass, which comprises irradiating synthetic quartz glass with X-rays to select a material that does not cause absorption around 220 nm when an excimer laser having a photon energy of 5 eV or more is emitted. 3. Synthetic quartz glass is irradiated with X-rays to produce 220n produced by E ′ center when irradiated with an excimer laser having a photon energy of 5 eV or more.
A method for inspecting synthetic quartz glass that selects materials that do not cause absorption near m. 4. The method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is a KrF excimer laser. 5. The method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is an ArF excimer laser. 6. The method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is an F ₂ excimer laser.