JPH11211613A - Method and apparatus for measuring inner loss coefficient - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the inner loss coefficient of an optical material in the wavelength region of 200 nm or less. SOLUTION: Divergence angle of the measuring light from a light source 2 is set at 10 milli radian or less at a position transmitting an object M by varying the width of slits 4, 6 or the diameter of a pinhole 9, or shifting a collimator lens 10, or the like, in the direction of the optical axis. Furthermore, pressure in a vacuum chamber 20 is set at 10×10<-2> Torr or below.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring an internal loss coefficient of an optical material which transmits ultraviolet light in a wavelength region of 200 nm or less, and a measuring apparatus suitable for carrying out the method.

[0002]

2. Description of the Related Art In recent years, various optical systems using an excimer laser as a light source, such as an excimer laser lithography apparatus, an excimer laser CVD apparatus, and an excimer laser processing apparatus, or an ultraviolet pulse laser beam performing a pulse oscillation operation in the same manner as an excimer laser Various industrial optical devices having an optical system using a light source have been studied. These devices are
In particular, ArF excimer laser lithography apparatus, ArF
ArF having a wavelength of 193 nm incorporated in an excimer laser CVD apparatus, an ArF excimer laser processing apparatus, and the like.
Various optical systems using an excimer laser as a light source, or lens members such as an illumination optical system or an imaging optical system using a UV light having a wavelength of 200 nm or less, vacuum ultraviolet light or a laser having the same wavelength region as a light source, a fiber, a window member, It is useful for quartz glass and various optical crystal materials used as optical elements such as mirrors, etalons, and prisms.

[0003] The above-described optical elements constituting the optical system installed in these apparatuses have a high initial transmittance (high transmittance itself) to ultraviolet light of the light source and a high ultraviolet resistance (transmittance). Is small). For this reason, materials having a high transmittance for ultraviolet rays, such as quartz glass and calcium fluoride crystals, are used as materials for the optical element. However, even if an optical element material having a high transmittance is used, However, in an optical system made by combining a large number of optical elements, there is a problem how to increase the transmittance of the entire optical system. Therefore, it is necessary to accurately evaluate how much the material of such an optical element has transmittance and how much light is absorbed. That is, it is necessary to accurately measure the internal loss coefficient of the optical material.

Conventionally, the internal loss coefficient of an optical material with respect to ultraviolet light such as excimer laser light is measured by using a spectrophotometer, and the intensity I of transmitted light after passing through the object to be measured and a reference before passing through the object to be measured. The light intensity I ₀ was obtained, and the transmittance T = I / I ₀ was calculated.

[0005]

However, when the above-described transmittance measurement (that is, internal loss coefficient measurement) is performed using a commercially available spectrophotometer, the following problems have occurred. And because of these problems, 200
It has been difficult to measure the internal loss coefficient of the optical material with high accuracy in the wavelength region of nm or less.

First, sufficient collimation of the measuring light is not performed at the position where the measuring light passes through the measuring object. Therefore, the measurement light (transmitted light) transmitted through the measurement target is refracted and the optical path changes, and the image of the measurement target detected on the light receiving surface is not of an accurate size. Since the light receiving surface of the detector (for example, a photomultiplier tube) has spatial sensitivity unevenness, the output value from the detector changes when the size of the image of the measurement target changes as in
That is the problem. For this reason, there is a possibility that the internal transmittance of the measurement target may be calculated to be larger than the actual value (in some cases, the internal transmittance may exceed 100%).

Second, as the wavelength of the measurement area becomes shorter, the intensity of the light source of the spectrophotometer generally decreases, and it becomes difficult to measure the transmittance with high accuracy. This is especially true when the internal transmittance of the measurement target is very high (the internal absorption is weak), and it is very difficult to measure the internal transmittance with high accuracy by the conventional method.

Third, in the wavelength region of 200 nm or less, the influence of absorption of oxygen molecules in the atmosphere (normally, air) on the optical path of the measurement light cannot be ignored. That is, in the wavelength region of 200 nm or less, the absorption coefficient of oxygen molecules is on the order of 0.001 cm ^−1, and when the internal loss coefficient β of the measurement object is very large (for example, β> 0.1
cm ⁻¹ ), the absorption of oxygen molecules does not matter, but if the internal loss coefficient β of the measurement object is very small (eg, β <0.01 cm ⁻¹ ), the absorption of oxygen molecules Has a great influence on the measurement of the internal loss factor.

This will be described specifically. When measuring the transmittance of an optical material using a spectrophotometer, it is necessary to determine the transmitted light intensity I and the reference light intensity I ₀ , but usually,
The transmitted light intensity I is detected in a state where the measurement target is inserted on the measurement optical path, while the reference light intensity I ₀ is detected in a blank state where the measurement target is not inserted on the measurement optical path. However, since the measurement target has the thickness L, the measurement optical path length is relatively longer by the length L when measuring the reference light intensity than when measuring the transmitted light intensity. Therefore, the required reference light intensity I ₀ is larger than the actual value by the amount absorbed by the oxygen molecules in the atmosphere having the optical path length L. Therefore, the reference light intensity I
₀ is measured lower than the actual value, and there is a possibility that the transmittance of the measurement object may be measured higher than the actual value.

The present invention has been made in view of such a problem, and a method and a method for measuring an internal loss coefficient capable of measuring an internal loss coefficient of an optical material with high accuracy in a wavelength region of 200 nm or less. It is intended to provide a device.

[0011]

In order to achieve such an object, a measuring method according to the present invention irradiates a measuring object with a wavelength of 200 nm or less to a measuring object and extracts the measuring light from the measuring light before passing through the measuring object. Measuring the intensity of the reference light and the intensity of the transmitted light extracted from the measurement light after transmission through the measurement target to determine the transmittance of the measurement target. The divergence angle of the measurement light at the position where the light passes through the target is made small (preferably, 10 milliradians or less), and the atmosphere around the optical path of the measurement light is evacuated. In the present invention, the vacuum is
For example, the pressure of a closed atmosphere is preferably 1 × 10
^-2 Torr or less. The divergence angle is
The divergence angle of one side measured from the optical axis of the measurement light.

As described above, by adjusting the divergence angle of the measurement light when transmitted through the measurement object within a minute angle range and making it substantially parallel light, a change in the measurement optical path due to refraction in the measurement object is reduced. Therefore, it is possible to reduce the influence of a measurement error caused by uneven sensitivity of the light receiving surface of the detector. In addition, by setting the atmosphere around the measurement optical path to a vacuum, the influence of absorption by oxygen molecules in the difference between the optical path length of the transmitted light and the optical path length of the reference light due to the thickness of the measurement object can be reduced. When the transmittance of each of the plurality of test pieces made of the same optical material and having different thicknesses is measured using this method, the transmittance is 200 n.
In the wavelength region of m or less, the internal loss coefficient of the optical material can be measured with high accuracy.

[0013] Further, in order to preferably carry out such a method, the measuring apparatus according to the present invention comprises a light source for irradiating a measuring object with a wavelength of 200 nm or less to the measuring object, and a measuring light at a position transmitting the measuring object. Divergence angle adjusting means capable of adjusting the divergence angle of the light, and light for detecting the intensity of the reference light extracted from the measurement light before passing through the measurement target and the intensity of the transmitted light extracted from the measurement light after passing through the measurement target The apparatus includes an intensity detecting means, and a vacuum chamber capable of sealing the atmosphere around the optical path of the measurement light and setting the pressure of the sealed atmosphere to at least 1 × 10 ^-2 Torr or less.

The above-described method and apparatus for measuring the intensity of the reference light and the intensity of the transmitted light separate the measurement light before transmission through the measurement target into two, and allow one of the separated measurement lights to pass through the measurement target. In addition to the method and apparatus for transmitting the measurement object and simultaneously measuring these intensities as transmitted light as the reference light, the measurement light measured without inserting the measurement object on the measurement optical path is referred to as the reference light. A method and an apparatus for separately measuring the intensities of measurement light measured with the measurement target inserted on the same measurement light path as the transmitted light are included.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of an internal loss coefficient measuring device 1 of the present invention. The measurement light emitted from the ArF excimer laser (wavelength 193.4 nm) light source 2 is condensed by the first condenser lens 3 and then incident on the diffraction grating 5 via the spectroscope entrance slit 4. The measuring light reflected and diffracted by the diffraction grating 5 is applied to a second condenser lens 7 via a spectroscope exit slit 6, where it is condensed again, and the brightness is adjusted by a stop 8.
The measurement light that has passed through the stop 8 is shaped into a predetermined shape in the pinhole 9, converted into a parallel light beam by the collimator lens 10, and emitted to the first half mirror 11.

The measuring light reflected by the first half mirror 11 is irradiated to the measuring object M via an optical chopper 13 for measuring light. The transmitted light that has passed through the measurement target M is the mirror 15, the second half mirror 16, and the condenser lens 1.
The light is radiated to the detector 18 through 7. On the other hand, the reference light transmitted through the first half mirror 11 is applied to the second half mirror 16 via the first mirror 12 and the optical chopper 14 for reference light, and the reference light reflected by the second half mirror 16 is collected. The light is emitted to the detector 18 via the optical lens 17. The transmitted light and the reference light received by the detector 18 are separated by the processing device 19 to obtain the transmitted light intensity I and the reference light intensity I ₀ . All devices except the detector 18 and the processing device 19 are hermetically provided in a vacuum chamber 20. The vacuum chamber 20 can set the atmosphere around the measurement light to a pressure close to vacuum.

The divergence angle of the measurement light at the position transmitting the object to be measured M changes the slit width of the spectroscope entrance slit 4 and the spectrometer exit slit 6 and the hole diameter of the pinhole 9, and the second collection point. Optical lens 7, aperture 8, pinhole 9
It can be adjusted by moving the collimator lens 10 along the optical axis.

A procedure for obtaining the internal loss coefficient of the optical material using the measuring device 1 as described above will be described. First, a plurality of test pieces are prepared using an optical material whose internal loss coefficient is to be measured. Then, the transmittance T of each test piece is obtained by using the measuring device 1, where the transmittance T is determined by the detector 18.
And T = I / I ₀ using the intensity I _{0 of the} reference light and the intensity I of the transmitted light obtained from the processing device 19.
Here, the transmittance T is the internal loss coefficient β and the thickness L to be measured.
And Equation (1).

[0019]

T = I / I ₀ = exp (−βL) (1)

The internal loss coefficient β has a relation of β = α + τ with the internal coefficient α and the internal scattering coefficient τ. As shown in Expression (1), the internal loss coefficient β of the measurement target M can be obtained by calculating the transmittance T by obtaining the transmitted light intensity I and the reference light intensity I ₀ by the measuring device 1.

Here, when the measuring object M is an optical material, that is, a solid material, the refractive index of the measuring object M is different from the refractive index of the atmosphere in which the measuring object is placed. Equation (1) of the transmittance T can be rewritten as Equation (2) due to the reflection caused by

[0022]

## EQU2 ## T = I / I ₀ ^{= {(1-R) 2} exp (-βL)} / {1-R 2 exp (-2βL)} ··· (2)

Here, the measurement object M is processed into the shape of a parallel plate having a thickness of L, and its two planes are optically polished. It is arranged perpendicular to the axis. Therefore, in equation (2), multiple reflection between the two polished surfaces is also considered. Further, R in the formula (2) is a reflectance per polished surface of the measuring object M, and is calculated using only the refractive index n ₁ of the atmosphere around the optical path of the measuring light and the refractive index n ₂ of the measuring object M. , Can be represented by the following equation.

[0024]

R = {(n ₁ −n ₂ ) / (n ₁ + n ₂ )} ² (3)

In equation (2), the transmittance T when the internal loss coefficient β of the measuring object M is assumed to be zero is called the theoretical transmittance, which is the refractive index n _{1 of the} atmosphere and the refractive index of the measuring object M. If the rate n ₂ is known, it can be obtained by calculation using equations (2) and (3). At this time, the theoretical transmittance T ₀ is expressed by equation (4).

[0026]

T ₀ = (1−R) ² / (1−R ² ) (4)

Assuming that 2 · β · L≪1 holds in equation (2), equation (5) is obtained by dividing equation (2) by equation (4).

[0028]

T / T ₀ = exp (−βL) (5)

From equation (5), the known theoretical transmittance T
_From the value of ₀ and the transmittance T obtained by the measuring device 1, L (thickness of the measuring object M) and β · L (internal loss coefficient β
And the product of the test piece thickness L). Then, by performing the same measurement for all the test pieces, plotting their relationship and examining the slope,
The value of the internal loss coefficient β can be obtained.

The measurement of the internal loss coefficient β according to the above procedure was carried out by changing the conditions of the divergence angle of the measuring light at the position transmitting the measuring object M and the pressure in the vacuum chamber 20. As a result, when the divergence angle of the measurement light at the position transmitting the measurement target M is set to 10 milliradians (0.57 degrees) or less, the change in the measurement optical path due to refraction in the measurement target M can be reduced, and the detector It has been found that the occurrence of measurement error due to uneven sensitivity of the light receiving surface of No. 18 can be prevented. It should be noted that the method of eliminating the influence of the uneven sensitivity of the detector 18 has an advantage that the light amount of the measurement light does not decrease compared to the method of placing an integrating sphere in front of the detector 18.

The atmosphere around the optical path of the measuring light is 1 × 1
When the pressure is 0 ^-2 Torr (1.33 Pa) or less and the partial pressure of oxygen is a vacuum of 2 × 10 ^-3 Torr (0.27 Pa) or less, the thickness of the object to be measured has conventionally been a problem. It has been found that the influence of absorption by oxygen molecules in the difference between the optical path length of the transmitted light and the optical path length of the reference light can be reduced. Then, the divergence angle of the measurement light at the position transmitting the measurement object M is set to 10 milliradians (0.57 degrees).
And the atmosphere around the optical path of the measurement light is 1 × 10
^-2 Torr (1.33 Pa) or less, when the oxygen partial pressure is set to 2 × 10 ⁻³ Torr (0.27 Pa), 2
Weak internal loss coefficient β in the wavelength region of 00 nm or less
(Β <0.01 cm ⁻¹ or less) could be accurately measured.

In the measuring apparatus 1 described above, the reference light intensity and the transmitted light intensity are detected by the same detector 18, but the reference light intensity and the transmitted light intensity are detected by different detectors. It may be configured. Further, the measuring method of the reference light intensity and the transmitted light intensity in the measuring device 1 is to separate the measurement light before transmission through the measurement target into two, and to use one of the separated measurement light as the reference light without transmitting the measurement target, The other method is based on a method of simultaneously measuring these intensities as transmitted light by transmitting an object to be measured. Instead of such an apparatus and method, measurement is performed without inserting the object to be measured on a measurement optical path. It is also possible to use an apparatus and a method for separately measuring the intensities of the measurement light as reference light and the measurement light measured in a state where the measurement target is inserted on the same measurement optical path as the transmitted light.

[0033]

EXAMPLES Specific examples will be described below. For the test piece of this example, a synthetic quartz glass synthesized by a direct method was used. Mg and Ca of alkaline earth metals and Sc, Ti, V, Cr, Mn, Fe, Co, Ni and C of transition metals
Each of the impurity element concentrations of u, Zn, and Al is 2
It was 0 ppb or less. Further, the Cl concentration is 30 ppm,
The Na concentration was below the lower limit of detection (1 ppb), and the K concentration was below the lower limit of detection (50 ppb). The OH group concentration was 1000 ppm. The quantification of Na, K, and Cl was performed by activation analysis using thermal neutron irradiation. The quantification of alkaline earth metals, transition metals and Al elements was performed by inductively coupled plasma emission spectroscopy.
The OH group concentration was measured by infrared absorption spectroscopy (measuring the absorption amount of 1.38 μm by the OH group).

The above-mentioned synthetic quartz glass is processed to have a diameter of φ60.
2,5,10,20,30,40,50mm thickness in mm
7 types of test pieces having the following shapes were produced. On these test pieces, two polished surfaces facing each other were parallelized within 10 seconds, the flatness of each side was within 3 Newton rings,
Precision polishing was performed so that the surface roughness of each side was rms = 10 Å or less, and the test pieces were finally polished so that the thickness of each test piece was within a tolerance of ± 0.1 mm. Further, a finish polishing process using high-purity SiO ₂ powder was performed so that an abrasive agent causing surface absorption did not remain on the surface.

Using the measuring apparatus 1, the transmittance was measured for each of the above seven types of test pieces as a measurement object M, and the equation (7) was obtained.
Was used to determine the relationship between the product β · L value of the internal loss coefficient β and the test piece thickness L and the test piece thickness L. Here, the divergence angle of the measurement light at the position transmitting the measurement object M was 8.7 mrad (0.5 degrees). Also, the vacuum chamber 20
The internal pressure was 1 × 10 ⁻³ Torr (about 0.13 Pa). Note that this pressure is 2 × 10 ⁻⁴ in terms of oxygen partial pressure.
Torr (about 0.03 Pa). FIG. 2 shows the result of plotting the β · L value with respect to the thickness L to be measured. From the slope of the obtained straight line, the internal loss coefficient β of the optical material in this example was determined to be 0.0024 cm ⁻¹ .

[0036]

Comparative Example Next, a comparative example with respect to the above embodiment will be described. In Comparative Example 1, the same apparatus configuration and the same divergence angle as those of the Example were used, and the inside of the vacuum chamber 20 was not purged with nitrogen but purged with nitrogen. The result is shown in FIG. In Comparative Example 1, the internal loss coefficient β was determined to be 0.0022 cm ⁻¹ . In Comparative Example 2, the same apparatus configuration and the same divergence angle as those of the Example were used, and the inside of the vacuum chamber 20 was opened to the atmosphere instead of vacuum. The result is shown in FIG. In Comparative Example 2, the internal loss coefficient β was determined to be 0.0009 cm ⁻¹ . In Comparative Example 3, the divergence angle of the measurement light at the position transmitting the measurement target M was set to 17
The pressure was set to 4 milliradians (10 degrees), and the inside of the vacuum chamber 20 was purged with nitrogen instead of vacuum. The measurement result at this time is shown in FIG. As shown in FIG. 2, in Comparative Example 3, linearity of the β · L value with respect to the thickness L to be measured was not obtained, and the internal loss coefficient β could not be calculated.

[0037]

As described above, according to the method and the apparatus for measuring the internal loss coefficient according to the present invention, the divergence angle of the measuring light at the position transmitting the object to be measured is made small, so that the measuring light can be substantially Since the change in the measurement optical path due to refraction at the measurement target as parallel light can be reduced, the influence of measurement error due to uneven sensitivity of the light receiving surface of the detector can be reduced. In addition, by making the atmosphere around the measurement optical path a vacuum, the influence of absorption by oxygen molecules in the difference between the optical path length of the transmitted light and the optical path length of the reference light due to the thickness of the measurement object can be reduced. . As a result, 20
In the wavelength region of 0 nm or less, the internal loss coefficient of the optical material can be measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an apparatus for measuring an internal loss coefficient according to the present invention.

FIG. 2 is a diagram showing β · L values with respect to a test piece thickness L in Examples and Comparative Examples.

[Explanation of symbols]

 2 Light source 5 Diffraction grating 8 Aperture 9 Pinhole 10 Collimator lens 18 Detector 20 Vacuum chamber M Measurement target

Claims (5)

[Claims] 1. A measurement light having a wavelength of 200 nm or less is irradiated on a measurement target, and the intensity of reference light extracted from the measurement light before transmission through the measurement target and the measurement light after transmission through the measurement target are measured. In the method for measuring an internal loss coefficient of an optical material having a step of determining the transmittance of the measurement target by measuring the intensity of the extracted transmitted light, the divergence angle of the measurement light at a position transmitting the measurement target is minute. A method of measuring an internal loss coefficient, wherein an atmosphere around an optical path of the measurement light is a vacuum. 2. A plurality of test pieces made of the same optical material and having different thicknesses are used as the measurement object, and the transmittance of each of the plurality of test pieces is determined to determine the internal loss coefficient of the optical material. The method for measuring an internal loss coefficient according to claim 1, wherein the value is obtained. 3. The method for measuring an internal loss coefficient according to claim 1, wherein the divergence angle of the measurement light at a position transmitting the measurement target is 10 milliradians or less. 4. The method for measuring an internal loss coefficient according to claim 1, wherein a pressure of an atmosphere around an optical path of the measurement light is 1 × 10 ⁻² Torr or less. . 5. A light source for irradiating a measurement light having a wavelength of 200 nm or less to a measurement target, a divergence angle adjusting means capable of adjusting a divergence angle of the measurement light at a position transmitting the measurement target, and transmitting the measurement target. Light intensity detecting means for detecting the intensity of the reference light extracted from the measurement light before the measurement and the intensity of the transmitted light extracted from the measurement light after passing through the measurement object, and the atmosphere around the optical path of the measurement light. The atmosphere is sealed and the pressure of the sealed atmosphere is at least 1 × 10 ⁻² Torr.
An internal loss coefficient measuring device, comprising: a vacuum chamber that can be set to r or less.