JP3266691B2 - Inspection method for optical materials for excimer laser - Google Patents

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 such as a lens and a prism for an excimer laser as an ultraviolet laser.

[0002]

2. Description of the Related Art In recent years, a super LSI using an excimer laser has been developed.
With the development of manufacturing processes, CVD processes using excimer lasers, and the like, the demand for optical materials for excimer lasers has been particularly increasing.

An excimer laser is a high-power pulse laser that mainly oscillates in the ultraviolet region. XeF (wavelength 350 nm, photon energy 3.5 eV) and 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 of 7.9 eV). Of these, KrF and ArF excimer lasers have attracted attention in relation to lithography and photo-CVD. In particular, KrF excimer lasers are being developed mainly in the lithography process for manufacturing semiconductors.

[0004] Excimer lasers such as ArF and KrF have a higher energy density and a much higher power than ultraviolet light sources such as conventional mercury lamps and deuterium lamps, and thus are more likely to damage quartz glass. In the case of synthetic quartz glass as an optical system material such as a photomask substrate, the emission band at 650 nm is 260 nm due to sputtering or plasma etching in the manufacturing process.
m, an absorption band may appear and the transmission performance in the ultraviolet region may decrease, and even more, when excimer laser light is used as an exposure light source in a lithography process for semiconductor manufacturing instead of a conventional mercury lamp. This decrease in transmittance is very large.

[0005] Such a substrate material which deteriorates in transmission process due to deterioration in the photomask manufacturing process must be inspected and removed in advance, but there is no method of knowing it in advance, and sputtering or plasma etching is required. After that, it is necessary to measure the fluorescence characteristics and absorption characteristics of the substrates one by one, which is troublesome and costly, so it is not realistic and inspects the deterioration of the absorption characteristics of synthetic quartz glass by a simple method. There was a demand for the development of means to do this.

[0006] Therefore, the applicant of the present invention, as a method of examining the deterioration of the absorption characteristics of synthetic quartz glass, from the viewpoint that materials can be preliminarily selected if an accelerated test can be performed by any method, synthesizes excimer laser light. When illuminated on quartz glass, the synthetic quartz glass used as the photomask substrate undergoes a change in absorption characteristics similar to that caused by alteration due to sputtering or plasma etching in the manufacturing process, and emits red light of about 650 nm. It was found that the emission occurred, and an inspection method for selecting the presence or absence of red light emission upon irradiation with an excimer laser was found (JP-A-1-189654).

[0007] That is, depending on the presence or absence of red light emission,
260 nm generated in the reticle fabrication process for pattern transfer
A method of determining the presence or absence of an absorption band having a peak at the time of the generation of this absorption band is based on the stoichiometric requirement of the supply amount of hydrogen in the flame during 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, thereby lowering the transmittance of the excimer laser itself. As described above, in a reduction projector (stepper) for lithography of an ultra LSI, there is a problem that even if there is no problem in use for a short period of time, the quartz glass is deteriorated after a long use and an absorption band is generated. Further, the presence or absence of the generation of the absorption band of 220 nm can be determined in a relatively short time at the time of ArF excimer laser irradiation.
Irradiation with an rF excimer laser is difficult to generate, and becomes noticeable only with irradiation of about 10 ⁶ shots. Therefore, it takes a long time to select materials, and the efficiency is low.

As described above, the absorption band generated during KrF excimer laser irradiation becomes very noticeable only after a very weak excimer laser irradiation, and it takes several days to several weeks for inspection. It was difficult to conduct such a program, and it was required to provide an accelerated test to solve such a problem.

[0010]

Means for Solving the Problems Accordingly, 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 an E ′ center composed of a ≡Si structure, and also causes emission at 280 nm. Therefore, whether or not such an absorption band is generated by a KrF excimer laser When X-rays are used to determine whether an absorption band is generated in a shorter time than with a KrF excimer laser, the presence or absence of an absorption band generated by an excimer laser having a photon energy of 5 eV or more can be determined. With this knowledge, the present invention has been completed.

[Action]

As a sorting method, for example, a sample piece obtained by polishing a parallel plane having a thickness of about 5 to 30 mm to a mirror surface is cut out from a raw material block of an optical material from which an optical material is cut out.
The spectral transmittance of a portion to be irradiated with the excimer laser beam and the X-ray is measured by a spectrophotometer, and then the excimer laser beam is irradiated under predetermined conditions (energy density, repetition frequency, number of shots). . Immediately after the irradiation, the spectral transmittance of the irradiated part was measured.
The difference in extinction coefficient at 20 nm is determined and used as an index of the degree of absorption, and X-rays are irradiated under predetermined conditions. Similarly, the difference in extinction coefficient at 220 nm is determined and the degree of absorption is used as an index. At this time, KrF, ArF, F
₍₂₎ Calibration curves are determined for samples having different degrees of absorption during excimer laser and X-ray irradiation, and inspection is performed based on the calibration curves.

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 excimer laser but also by irradiation with γ-rays, X-rays, neutron rays, or the like. Analysis of the electron spin resonance (ESR) spectrum confirmed that this absorption band was due to a ≡Si structure called an E ′ center.
Spin concentration obtained from signal intensity of SR and 220n
The absorption band intensity of m is proportional.

Various precursors for the E ′ center upon irradiation with an excimer laser have been considered. One is the irradiation of excimer laser with {Si-O-Si}

Embedded image Or

Embedded image Generation by the mechanism of (1) is conceivable. here

Embedded image Is a planar structure bonded to three oxygen molecules.

In addition, {Si-H H-O-Si}
Structure is conceivable.

Embedded image

In either mechanism, an E ′ center is generated by absorption of a photon. The energy gap of the band of quartz glass 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. Since the photon energy is 5 eV or more, two-photon absorption is considered. Therefore, it is considered that the generation efficiency is high because the photon energy is high in the order of F ₂ excimer laser, ArF excimer laser, and KrF excimer laser.

X-rays, on the other hand, have a higher photon energy, but ArF and KrF
The correlation is obtained with the absorption band intensity at the time of excimer laser irradiation. However, if the irradiation is performed for a long time, the network structure of the glass starts to be broken, and there is no correspondence with the induced absorption at the time of excimer laser irradiation.

As synthetic quartz glass, silicon tetrachloride which is directly deposited and vitrified by hydrolysis in an acid / hydrogen flame is excellent in excimer laser resistance such as transmission characteristics in the wavelength region of an excimer laser. As an optical material for use. The hydrolyzate in an oxyhydrogen flame produced in excess of the stoichiometric amount of hydrogen produced an absorption band at 260 nm and associated with 65%.
It is effective for preventing emission of red light of 0 nm, and is preferable as an optical material for excimer laser.

Various types of X-ray sources can be considered.
For example, an X-ray diffractometer using a W or Cu bulb or an X-ray fluorescence analyzer using a Rh bulb can be used as it is. When irradiating X-rays using these devices, the X-ray irradiation conditions vary greatly depending on the method of holding the sample and the control conditions (voltage and current) of the X-ray tube. 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 under which a material that absorbs when irradiated with an excimer laser causes sufficient absorption for evaluation. For example, when a W target tube is used in an X-ray diffractometer, it is about 10 to 30 minutes under the conditions of 50 kV and 30 mA. Similarly, a Cu tube is 50 kV and 30 to 120 mA at 30 mA using the X-ray diffractometer. It can be used with irradiation for about a minute. In addition, it is also possible to manufacture and use an irradiation device using an appropriate tube.

[0019]

【Example】

Example 1 As a sample, silicon tetrachloride was produced by direct deposition vitrification by hydrolysis in an acid / hydrogen flame, and the OH concentration was A: 450 ppm, B: 650 ppm,
C: 850 ppm, D: 1120 ppm, E: 1300
5 kinds including those of ppm, 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 a Rh target were applied at 50 mA and 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. Table 1 shows the results. Each sample is 3
Four pieces were cut out into a shape of 0 mm × 10 mm × 10 mm, and two surfaces were mirror-polished so as to have a thickness of 10 mm.

The numerical values shown in Table 1 are obtained by multiplying the absorption coefficient by 1000.

[Table 1]

The following equations (4) to (7) are derived from the results of Table 1 obtained by measuring the absorption coefficients αArF, αKrF, and αX induced by ArF excimer laser, KrF excimer laser, and X-ray irradiation. , ΑKrF, α
ArF and αX were found to be in 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 degree of absorption obtained with an irradiation time of 000 seconds (about 2.8 hours) can be predicted with 10 to 15 minutes of X-ray irradiation. By irradiating X-rays in this way, it is 220 nm in a short time.
The presence or absence of the generation of an absorption band can be determined.
By using the line, it is also possible to predict the degree of absorption upon irradiation with an ArF excimer laser.

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 Five pieces of quartz glass of different lots were selected from quartz glass synthesized according to Example 1, and 30 pieces were selected according to Example 1.
Three pieces each having a shape of mm × 10 mm × 10 mm were cut out, and two surfaces were mirror-polished so as to have a thickness of 10 mm. An X-ray (50 m) was applied to one of the samples according to Example 1.
A, 50 kV) for 15 minutes, and the absorption coefficient αX induced by the irradiation was measured. Based on the result (4),
The predicted value of the absorption coefficient at 220 nm when the KrF and ArF excimer lasers were irradiated in accordance with Example 1 was calculated by the equation (5). After that, the other sample
After irradiation with an rF or ArF excimer laser according to Example 1, the absorption coefficient at 220 nm was measured. Table 2 shows the results. From the results in Table 2, it can be seen that the predicted value and the measured value are in good agreement, which indicates that the inspection method according to the present invention is effective. Table 2
Numerical values shown in Table 1 are obtained by multiplying the absorption coefficient by 1000.

[Table 2]

[0024]

[Effect] By measuring the absorption coefficient induced by irradiating the synthetic quartz glass with X-rays, the absorption coefficient is determined when the excimer laser beam is transmitted for a long time based on the correlation between the absorption coefficient and the previously determined correlation. Can be sorted out. Also, by using X-rays, KrF
22 times faster than excimer laser irradiation
The presence / absence of generation of an absorption band of 0 nm can be determined, and under certain conditions, the degree of absorption generated when an excimer laser having a photon energy of 5 eV or more is irradiated can be predicted.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-189654 (JP, A) JP-A-55-89707 (JP, A) JP-A-62-232347 (JP, A) JP-A-2- 132313 (JP, A) Japanese Utility Model 63-60052 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 23/06 G01M 11/00

Claims (5)

(57) [Claims] An absorption coefficient induced by irradiating a synthetic quartz glass with X-rays is measured, and an absorption coefficient induced by irradiating an excimer laser is determined based on a correlation between the absorption coefficient and a previously determined correlation. A method for inspecting synthetic quartz glass that selects and does not cause absorption by excimer laser irradiation. 2. A method for inspecting a synthetic quartz glass according to claim 1, wherein the excimer laser has a photon energy of 5 eV or more and an absorption around 220 nm. 3. A method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is a KrF excimer laser. 4. The method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is an ArF excimer laser. 5. The method for inspecting synthetic quartz glass according to claim 1, wherein the excimer laser is an F ₂ excimer laser.