JPH1019727A - Method for estimating durability to irradiation by excimer laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To estimate deterioration of an optical material because of the irradiation of excimer laser by using an estimation formula for each of an absorbance, the amount of a change of a refractive index and a surface change. SOLUTION: An estimation formula for an absorbance is obtained from experiments using a KrF excimer laser as a light source and quartz glass as an optical material. The absorbance (/cm) is expressed by K1×E<a> ×p<b> ×H<c> wherein E is an energy density, (p) is an integral count of projected pulses, H is a concentration of dissolved H2 and K1, (a), (b), (c) are constants. An estimation formula for a change of a refractive index (Δn) is (Δn)=K2×E<d> ×p<e> wherein Δn is the amount of the change of a refractive index of 632.8nm, E is the energy density, (p) is the integral count of projected pulses and K2, (d), (e) are constants. Moreover, an estimation formula for the amount of a surface change (μm) holds (μm)=K3×Δn wherein Δn is the change amount of the refractive index of 632.8nm and K3 is a constant. With the use of the estimation formulae, deterioration of an optical part used in an excimer laser optical system can be estimated simply. Since the estimation formulae hold only in a linear region of the change, if the change is clearly not linear, curve fitting is performed to obtain a dependent formula.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical material,
The present invention relates to a method for predicting deterioration of an optical system lens such as an rF (248 nm), ArF (193 nm) excimer laser lithography projection lens / illumination system lens, and an excimer laser processing machine, and a method for estimating the number of durable pulses. Excimer lasers used in excimer laser optical systems, such as synthetic quartz glass and optical crystal materials
The characteristic feature is that the transmittance change, the refractive index change, and the surface change due to the deterioration due to the irradiation can be easily and accurately predicted using an empirical formula or in consideration of a safety coefficient, that is, an error component. .

[0002]

2. Description of the Related Art Conventionally, in an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit onto a wafer such as silicon, an exposure apparatus called a stepper is used. The wavelength of the light source of this stepper has been reduced from the g-line to the i-line with the recent high integration of the LSI, and the light source of the stepper has become K with the further high integration of the LSI.
The trend has shifted to rF and ArF excimer lasers. For the illumination system or the projection lens of such an excimer laser stepper, general optical glass can no longer be used, but is limited to optical materials such as quartz glass and fluorite.

[0003] Such an excimer laser stepper quartz glass used for the illumination system or a projection lens of, even in fluorite, the internal transmittance is required 0.998Cm ^-1 or 0.999Cm ^-1 or more. Therefore,
Development is underway to increase the transmittance of the optical material in the ultraviolet light region. Furthermore, the so-called solarization and compaction of the aging of the optical material due to irradiation caused by the short-wavelength property and the flash property of the excimer laser become a serious problem. The progress of such optical deterioration affects the imaging performance.

[0004]

First, it is necessary to precisely and experimentally examine what causes deterioration of quartz glass as an excimer laser optical material and other optical materials. Heretofore, although some data of literatures or the like examined fragmentally or qualitatively have been studied, no prediction formula having strict quantitative properties has been shown.

By the way, an excimer laser is used as a light source.
In order to predict the life of an optical system of a product such as an excimer laser stepper and a processing machine, a strict prediction formula by excimer laser irradiation of a lens material is required. However, since there was no reliable prediction formula, the useful life of the lens, the cumulative number of useful pulses, the allowable energy density of irradiation, and the like were not clearly shown.

In particular, in order to examine the deterioration of a quartz glass member used by irradiating with an energy of 10 mJ / cm ² · pulse or less, it is necessary to irradiate with a real energy density and to obtain a transmittance, a refractive index, a surface change and the like. It is most desirable to confirm the change in physical properties of the material. However, in the region where the irradiation energy-density is low, the change in each physical property per irradiation pulse is very small.
To confirm the rate of change, the number of irradiation pulses must be increased. However, there are practically limited human, economic, and time constraints, and it is difficult to test a single sample for several years. In addition, it is necessary to guarantee the performance of expensive quartz glass optical lenses and the like for more than ten years.

[0007]

Means for Solving the Problems Accordingly, the present inventors have:
For many years, we have been studying the transmittance, refractive index, and surface change of ultraviolet optical materials such as quartz glass. In order to achieve the object, excimer irradiation is performed under various conditions, and experimental data such as transmittance, refractive index, and surface change measured at that time are obtained.

Next, it is necessary to calculate the relational expression of the change in the absorption coefficient, the change in the refractive index, and the change in the surface change from the obtained experimental data by using a statistical method and a theoretical method. . This patent provides a prediction formula and a prediction method for deterioration of an excimer laser optical material, that is, a change in an absorption coefficient, a change in a refractive index, and a change in a surface change.

[0009]

The following is an example of a method of calculating a prediction formula for deterioration of an optical material due to excimer laser irradiation, that is, a change in an absorption coefficient, a change in a refractive index, and a change in a surface change. The following description relates to an experiment and a method of calculating a prediction formula, in which a light source is a KrF excimer laser and quartz glass is used as an optical material.

FIG. 1 shows an optical system and a measuring system for an excimer laser irradiation experiment. The change in the absorption coefficient will be described. First, the irradiation energy-density dependence was investigated. The experiment was performed using the same sample while keeping other conditions constant. FIG. 2 shows the irradiation energy-density dependence. Number of accumulated pulses 3E6
Irradiation energy density during pulse (mJ / cm ² per
pulse) 50, 100, 200, 400, 800
Asked for dependencies.

The dependence calculated by the least squares method was 248.3 nm absorption coefficient (/ cm) = K × E ^1.75 . Energy-density dependence is 1.75 ± 0.2
(3σ). Generally, the causes of the absorption band generation induced by the KrF excimer laser are the E ′ center (215 nm band) generated in the two-photon absorption process and oxygen-related defects (2
60 nm band). At 248.3 nm, it seems to be slightly deviated from the energy square law due to the composite peak.

Next, the dependence on the number of accumulated pulses was examined. The results are shown in FIG. Irradiation energy density (mJ / cm ² pe
r pulse) 50, 100, 200, 400, 80
At 0, the dependence was determined by the least squares method. In the drawing, 5E6 or the like means 5 × 10 ⁶ . Or later,
In the figure, such an abbreviation is used for display. 248.3 nm absorption coefficient (/ cm) = K × P ^{0.998 The} accumulated pulse number dependence was 0.998 ± 0.1 (3σ).

[0013] This dependence is approximately equal to the absorption coefficient (/ cm).
It holds up to 0.2. Further, the dissolved H ₂ concentration (mole.
/ Cm ³ ). Irradiation energy-density 40
The relationship between the concentration of dissolved H ₂ after irradiation with 0 (mJ / cm ² per pulse) and the cumulative pulse number of 3E6 pulses and the 248.3 nm absorption coefficient (/ cm) was examined. FIG. 4 shows the results.
From this, the dependency equation was obtained by the least squares method. 248.3 nm absorption coefficient (/ cm) = K × H ^−1.03627 Correlation coefficient r = 0.92 where H: dissolved H ₂ concentration (mole./cm ³ ) As shown by correlation coefficient r = 0.92 , Kr of quartz glass
The main factors that determine F excimer laser resistance-absorption generation-are:
It is considered to be the concentration of dissolved H ₂ molecules.

From the above, an equation for predicting the absorption coefficient was calculated from three equations of energy-density dependence, accumulated pulse number dependence, and dissolved H ₂ concentration dependence.

[0015]

(Equation 1)

The constants are k1: 38396.2, a: 1.
75 ± 0.2 (3σ) b: 0.998 ± 0.1 (3σ), c: -1.0362
7 ± 0.1 (3σ). The change in the refractive index will be described. Here, the change in the refractive index indicates a change in the refractive index at 632.8 nm (He-Ne laser wavelength). The measurement is a value measured by a fringe scanning interferometer using an oil-on-plate method. The symbol is △ n
I will show you. The refractive index is increased by excimer laser irradiation of quartz glass.

First, the irradiation energy-density dependence was investigated. The experiment was performed using the same sample while keeping other conditions constant. FIG. 5 shows the irradiation energy-density dependence. Irradiation energy density (mJ / c) when the number of accumulated pulses is 3E6
m ² per pulse) 50, 100, 200, 40
Dependencies were determined at 0,800. The dependency calculated by the least square method was Δn (632.8 nm) = K × E ^0.965 .

The energy-density dependence is 0.965 ±
0.1 (3σ). This indicates that KrF excimer laser irradiation-induced change in refractive index—compaction—is not a two-photon process. Next, the dependence on the number of accumulated pulses was examined. FIG. 6 shows the results. Irradiation energy density (mJ /
cm ² per pulse) 50, 100, 200, 4
At 00 and 800, the dependence was determined by the least squares method. Δn (632.8 nm) = K × P ^{0.49 The} accumulated pulse number dependence was 0.49 ± 0.1 (3σ).

This dependency is satisfied when the Δn (632.8 nm) change is about 〜1 × 10 ⁴ . From the above, the energy
A prediction formula for a change in the refractive index was calculated from two equations, ie, density dependence and accumulated pulse number dependence.

[0020]

(Equation 2)

Each constant is k2 = 6.1 × 10 ^-12 and d =
0.965 ± 0.1 (3σ) e = 0.49 ± 0.1 (3σ). 248.3 nm
In order to obtain Δn, it can be calculated by a calculation in consideration of chromatic dispersion. The same applies to other wavelengths. Excimer laser irradiation-induced increase in the refractive index of quartz glass is considered to be a phenomenon called compaction. The above equation shows that the increase in the refractive index has a dependency on the dose amount of about 0.5 power when the excitation light source is KrF excimer. This is considered to mean that the defect is essentially generated by SiO ₂ .

In the case of the KrF excimer laser, the dependence on the concentration of dissolved hydrogen molecules in quartz glass is small.
In the F excimer laser, it is necessary to consider this and include a prediction formula. A description is given of the surface change. Here, the term “surface change” means a change in the shape of the surface of the optical component at the portion irradiated with the excimer laser.

Similar to the change in the refractive index, it is considered to be shrinkage of the SiO ₂ structure due to compaction—densification—a volume shrinkage phenomenon due to densification. That is, the surface changes in the concave direction. The correlation between the excimer laser irradiation-induced increase in the refractive index and the laser incident surface was examined. FIG. 7 shows the result. Also,
The equation obtained by the correlation coefficient r = 0.913 and the least square method is shown below.

[0024]

(Equation 3)

The constant is k3 = 4703. As described above, equations (1), (2), and (3) are obtained, and
The life expectancy of a product can be predicted by comparing the operating conditions-irradiation energy-density, number of pulses-and required specifications of a product using an excimer laser as a light source, that is, absorption, refractive index change, and surface change. .

Supplementally, the prediction equation described here holds only in the linear region of change. In the case of precision optical products, the specifications are strict, that is, the amount of change in physical properties is very small, and therefore, in most cases, it can be predicted by a method such as Expressions (1), (2) and (3).
If the behavior of the change is clearly not a straight line, it is possible to obtain a dependency equation by performing an optimum curve fitting.

Further, the absorption and the like can be predicted by calculating the irradiation energy and the pulse number dependence of the saturation amount. By the way, in order to consider the safety coefficient, it is necessary to obtain a measurement error, a curve fitting error, and the like, calculate (sum of squares) ^1/2 of these, and substitute the calculated sum into a prediction equation.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A high-purity quartz glass ingot, which is an optical material, uses high-purity silicon tetrachloride as a raw material, mixes and burns oxygen gas and hydrogen gas with a burner made of quartz glass, and discharges the raw material gas from the center. It was diluted with a carrier gas (usually oxygen gas or hydrogen gas) and jetted, deposited on a target, melted and synthesized. Thereby, the diameter of 180
mm and a length of 550 mm were obtained.

[0029]

Example 1 An optical lens component for a KrF excimer laser stepper was cut out from the quartz glass ingot, and a sample for partially measuring physical properties was also prepared. The sample was subjected to an irradiation experiment for evaluating excimer laser resistance obtained by the present invention, and (Equation 1), (Equation 2), and (Equation 3) were obtained.

The H ₂ concentration of this optical quartz glass member is 5
× 10 ¹⁷ mole. / Cm. The specifications required for this lens part, which is a part of the KrF excimer laser stepper optical system, are as follows: absorption coefficient: 1% / cm or less, refractive index change Δn (632.8 nm): 1 × 10 ⁻⁵ or less, Surface change (one side): not more than 0.04 μm.

It is considered that a precise optical lens has an effect on optical performance unless the change in physical properties is at least within the above range. The condition for using this lens is 5 mJ / cm
² per pulse, repetition frequency 500 Hz, the number of irradiation pulses per day is 1 × 10 ⁷ pulses. Then, the predicted value of the quartz glass optical component 5 years later calculated by the above equation was compared with the actual amount of change in physical properties.

The results are shown in Table 1. The prediction error was within 10% for both absorption, Δn and concave.

[0033]

[Table 1]

Further, in order to obtain the service life, the number of pulses may be calculated by substituting the specifications and use conditions into (Equation 1), (Equation 2) and (Equation 3). First, when the absorption is calculated, P = EXP ((LN
(0.01 / (38396 × E ^1.75 ×
H ^-1.036271 ))) / 0.998) Next, the refractive index is shown. P = EXP ((LN (△ n / (6.1 × 10 ⁻¹² × E
^0.965 )) / 0.49)) Next, the surface change will be described. P = EXP (LN ((concave / 4703) / (6.1 × 10
⁻¹² × E ^0.965 )) / 0.49) By substituting the respective specification values and the energy used and calculating the number of serviceable pulses, the absorption was found to be 3.59 × 10 ¹⁰ , Δn
Was 2.03 × 10 ¹¹ pulses, and 1.46 × 10 ¹¹ pulses for concave (μm).

When this is converted into a service life, absorption is 9.8 years, Δn is 55.4 years, and concave is 39.9 years.
Year. Since the service life of the lens depends on the shortest item of each specification, it can be predicted to be 9.8 years.

[0036]

Embodiment 2 An optical lens part for a KrF excimer laser stepper was cut out from the quartz glass ingot, and a part of the sample was also prepared. The sample was subjected to an irradiation experiment for evaluating excimer laser resistance obtained by the present invention, and (Equation 1), (Equation 2), and (Equation 3) were obtained.

The specifications required for this lens part, which is a part of the KrF excimer laser stepper optical system, are as follows: an absorption coefficient: 0.2% / cm or less, and a refractive index change Δn (632.
8 nm): 2 × 10 ⁻⁶ or less, surface change (one side): 0.01
μm or less. The condition for using this lens is 1 m
J / cm ² per pulse, repetition frequency 500
Hz, the number of irradiation pulses per day is 1 × 10 ⁷ pulses.

Further, a safety factor and a measurement error were considered.
To find the useful life, (Equation 1), (Equation 2), (Equation 2)
Substitute each specification, usage condition and each measurement error into 3)
The number of resources may be calculated. Here, the measurement errors are all 1σ.
When there are multiple error factors (square sum) ^0.5Measure
What is necessary is just to make it a fixed error.

The error factors to be considered will be described. First, regarding absorption, the transmittance measurement accuracy was 0.01%, the effect of absorption relaxation was 0.002%, and the irradiation energy during the irradiation experiment was low.
Density error ± 10%, influence of irradiation beam profile ±
When 10% is converted into absorption, it is 0.02% each.
When these (sum of squares) ^0.5 are calculated, AE = ± at 1σ
0.03%. The measurement accuracy of the dissolved H ₂ concentration is 1
HE = ± 2.5 × 10 ¹⁷ mole. / Cm.

The relaxation model of excimer laser irradiation-induced absorption of quartz glass is given by -B × (t ＾ 0.5) B is about 0.4
t: It is considered that it can be defined by the relaxation time. next,
The measurement accuracy of Δn is ΔnE = ± 5 × 10 ⁻⁷ at 1σ. In addition, the measurement accuracy of the surface shape concave is 1σ and the concave E = ± 0.
002 μm. (Equation 1), (Equation 2), and (Equation 3) were modified using the obtained error to obtain an equation for obtaining the number of usable pulses.

First, when the absorption is calculated, P = EXP ((LN ((0.002- (AE / 10
0)) / (38396 × E 1.7 5 × (H-H
E) ^-1.036271 ))) / 0.998) Next, the refractive index is shown. P = EXP ((LN ((△ n- △ nE) / (6.1 × 1
0 ⁻¹² × E ^0.965 )) / 0.49)) Next, the surface change will be described. P = EXP (LN (((concave-concave E) / 4703) /
(6.1 × 10 ⁻¹² × E ^0.965 )) / 0.49) By substituting the respective specification values and energy used and calculating the number of durable pulses, 8.1 × 10 ¹⁰ pulses for absorption were obtained.
The Δn was 1.0 × 10 ¹¹ pulses, and the concave (μm) was 1.3 × 10 ¹¹ pulses.

When this is converted into the service life, the absorption is 22.2 years, Δn is 27.5 years, and concave is 35.
5 years. The service life of the lens depends on the shortest item of each specification, and can be predicted to be 22.2 years.

[0043]

By using the method and formula for predicting the durability of the excimer laser of the present invention, the absorption, refractive index change, and surface change of the optical components used in the excimer laser optical system such as quartz glass. The amount can be easily predicted. The method of the present invention can be applied to prediction of a change in physical properties such as a distortion amount (birefringence amount).

In addition, the life expectancy of the excimer laser optical system,
The service life can also be calculated. Further, the same life expectancy can be predicted for an optical thin film.

[Brief description of the drawings]

FIG. 1 is a schematic view of an excimer irradiation experiment apparatus.

FIG. 2 is a diagram showing the energy density dependence of the absorption at 248.3 nm induced by KrF excimer laser irradiation of quartz glass.

FIG. 3 is a graph showing the cumulative pulse number dependence of the absorption at 248.3 nm induced by KrF excimer laser irradiation of quartz glass.

FIG. 4 is a graph showing the dependence of the absorption at 248.3 nm induced by KrF excimer laser irradiation of quartz glass under the same conditions on the concentration of hydrogen molecules dissolved in quartz glass.

FIG. 5 is a diagram showing the energy dependence of a refractive index change induced by KrF excimer laser irradiation of quartz glass.

FIG. 6 is a graph showing the dependence of a change in refractive index induced by irradiation of KrF excimer laser on quartz glass with the number of accumulated pulses.

FIG. 7 is a diagram showing a refractive index change amount and a surface change amount of quartz glass induced by KrF excimer laser irradiation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Excimer laser 2 Beam shaping and homogenizer optical system 3 Irradiation sample 4 Laser beam 5 Energy monitor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical indication G01N 33/38 G01N 33/38

Claims (6)

[Claims] (1) 0.01 to 100 mJ / cm ² · pulse
e in a test method for predicting the deterioration due to light irradiation of the quartz glass optical member used by irradiating light of a specific wavelength,
A method for testing a quartz glass optical member, comprising the following steps. Step 1: The correlation between the hydrogen concentration in the quartz glass optical member and the change in transmittance, and the correlation between the energy density of the irradiated light and the change in transmittance are obtained. For obtaining a relational expression between the hydrogen concentration of the silicon and the energy density of the irradiated light. Step 2: 10 to 10,000 mJ /
Step 3: Measuring the transmittance change upon irradiation with light of cm ² · pulse Step 3: Substitute the transmittance change amount, hydrogen concentration, and energy density obtained in Step 2 into the relational expression obtained in Step 1 to obtain 0 Step of finding a change in transmittance at a specific wavelength of 0.1 to 100 mJ / cm ² · pulse 2. The quartz glass optical member according to claim 1, wherein said quartz glass optical member has a test method of 0.01 to 100 mJ / cm ^2.
A quartz glass optical member, wherein a change in transmittance at a specific wavelength of pulse is 1% or less. (3) 0.01 to 100 mJ / cm ² · pulse
e in a test method for predicting the deterioration due to light irradiation of the quartz glass optical member used by irradiating light of a specific wavelength,
A method for testing a quartz glass optical member, comprising the following steps. Step 1: The correlation between the hydrogen concentration in the quartz glass optical member and the change in transmittance, and the correlation between the energy density of the irradiated light and the change in the refractive index are obtained. For obtaining a relational expression between the hydrogen concentration of the silicon and the energy density of the irradiated light. Step 2: 10 to 10,000 mJ /
Step 3: Measuring the change in refractive index when irradiating light of cm ² · pulse. Step 3: Substituting the amount of change in refractive index, hydrogen concentration and energy density obtained in step 2 into the relational expression obtained in step 1 to obtain 0 Step of finding refractive index change at a specific wavelength of 0.1 to 100 mJ / cm ² · pulse 4. A quartz glass optical member according to claim 1, which is obtained by the test method of 0.01 to 100 mJ / cm ^2.
A change in the refractive index at a specific wavelength of pulse is 1 × 10
A quartz glass optical member, characterized by having a diameter of ^-5 or less. (5) 0.01 to 100 mJ / cm ² · pulses
e in a test method for predicting the deterioration due to light irradiation of the quartz glass optical member used by irradiating light of a specific wavelength,
A method for testing a quartz glass optical member, comprising the following steps. Step 1: The correlation between the hydrogen concentration in the quartz glass optical member and the change in transmittance, and the correlation between the energy density of irradiation light and the surface change are determined. From these correlations, the surface change amount and the hydrogen in the quartz glass optical member are determined. Step of obtaining a relational expression between the concentration and the energy density of the irradiated light Step 2: Apply 10 to 10,000 mJ / to the quartz glass optical member.
Step 3: Measuring the surface change upon irradiation with light of cm ² · pulse Step 3: Substituting the surface change amount, hydrogen concentration, and energy density obtained in Step 2 into the relational expressions obtained in Step 1,
Step of finding a surface change at a specific wavelength of 0.01 to 100 mJ / cm ² · pulse 6. A method for testing a quartz glass optical member according to claim 1, wherein the measured value is 0.01 to 100 mJ / cm ^2.
The surface change at a specific wavelength of pulse is 0.04 μm
A quartz glass optical member characterized by the following.