JP2009031043A - Transmittance measuring method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device capable of measuring highly accurately a lens having an uneven various curvature, in measurement of a transmittance. <P>SOLUTION: In a means for measuring each photodetection intensity when a test lens is interposed in an optical path through which measuring light reaches a photodetection means, to thereby allow the measuring light to pass, and when the test lens is not interposed, and calculating the transmittance of the test lens by comparison thereof, the shape, the material, and the position in the optical path of the test lens, and a measurement error portion caused by convergence or divergence of a light flux by a measurement wavelength are measured beforehand in both cases where the test lens is interposed in the optical path and where the test lens is not interposed, to thereby correct a measured value. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical lens measurement method and apparatus, and more particularly to transmittance evaluation effective for measurement evaluation of light in the ultraviolet region, and in particular, to improve measurement accuracy in the vacuum ultraviolet region, which is difficult to measure in the past and has low accuracy. The present invention relates to a measurement method and apparatus.

  Conventionally, the optical lens has been measured for spectral energy distribution, spectral transmittance, spectral reflectance, and the like, and these measurements have been mainly performed in the visible region.

  FIG. 6 shows an example of a conventional spectral transmittance measuring apparatus. In a conventional spectroscopic measurement device, light emitted from a light source is converted into monochromatic light by a spectroscope, and the monochromatic light is divided into reference light and sample light by a sector mirror or a half mirror. The reference light is directly guided to the light receiving sensor by the reflecting mirror, while the sample light is guided to the light receiving sensor by the reflecting mirror via the lens to be tested, directly or via an integrating sphere. The light energy is measured by comparing the luminous flux of each light. This type of spectroscopic measurement apparatus is disclosed in Patent Document 1, for example.

As the light receiving elements used in the spectroscopic measurement apparatus, silicon photodiodes, photomultiplier tubes, CCDs, and the like have been properly used depending on the application, wavelength used, required accuracy, and the like.
JP 2000-321126 A

  By the way, in contrast to the conventional spectroscopic measurement apparatus mainly performed in the visible region, apparatuses using light in the ultraviolet region are being used in various directions. In particular, light sources such as steppers used in semiconductor manufacturing include mercury lamp g-line (λ = 4358 Å) to i-line (λ = 3650 Å), gas laser KrF (λ = 2486 、) laser, and vacuum ultraviolet region ArF ( λ = 1934Å) Transition to laser.

  The major problem here is the optical system used there. It is necessary to evaluate and develop the characteristics of the lens and the antireflection film formed on the lens surface, and as a tool for evaluating and developing these, the characteristics of glass materials and optical films are accurately measured in the vacuum ultraviolet wavelength region.・ A spectral transmittance measuring device that can be evaluated is required. However, the spectral transmittance measuring device used for the measurement evaluation so far does not support the measurement of lenses having various concave and convex curvatures in the vacuum ultraviolet region, and thus it has not been possible to expect accurate measurement evaluation.

The reason is as follows according to the knowledge of the present inventor.
In order to achieve accurate measurement, light detection is performed when the measurement light is passed through the optical path where the measurement light reaches the light detection means, and when the measurement light is not interposed. Intensities must be measured and compared accurately. However, it is rare that the light receiving characteristics depending on the location in the light receiving unit are completely uniform. In the case of a silicon photodiode, there is a non-uniformity in sensitivity depending on the location due to deterioration due to the manufacturing process or use time in the light receiving surface where photoelectric conversion is performed. Further, in the case of an integrating sphere, there is a non-uniformity of reflectance depending on the location due to non-uniform coating and deterioration of the integrating sphere inner wall paint. Therefore, there is a problem that accurate comparison cannot be made unless light is received at the same portion in the light receiving surface at the time of 100% measurement when the test lens is not in the optical path and at the time of measurement when the test lens is put in the optical path. .

  In particular, in the vacuum ultraviolet region, there is a problem that light is strongly absorbed by various substances, and the influence on sensitivity unevenness due to contamination of the light receiving surface and the like is significant. Further, when measuring a spherical surface having various curvatures, the measurement light irradiated to the test lens by the convergence and divergence of the light beam is refracted by the shape and thickness of the test lens. For this reason, unlike the light beam in the case where the test lens is not interposed in the optical path, the sensitivity uniformity of the light receiving unit is not perfect, which causes a measurement error.

  Here, the case where a spherical surface having various curvatures is measured will be schematically described with reference to FIG. In this case, the measurement light applied to the test lens by the convergence and divergence of the light beam is refracted by the shape and thickness of the test lens, and changes to the light beam when the test lens is not interposed in the optical path. FIG. 7A shows a case where the test lens is not interposed in the optical path, and shows the vicinity of the light receiving unit using the photomultiplier tube 23 as the detector via the integrating sphere 24. From the left side of the drawing, the monochromatic measurement incident light 33 is a convergent light having a certain NA, and enters the integrating sphere with the vicinity of the integrating sphere opening as a focal position. Next, FIG. 7B shows a case where a lens 34 having a negative power is interposed in the measurement optical path as a test lens. It can be seen that the light is refracted by the lens 34, the focal position is changed to the back side, and the luminous flux is changed as compared with the case where the test lens is not interposed in the optical path in FIG. Next, FIG.7 (c) is a case where the lens 35 which has positive power as a to-be-tested lens is interposed in a measurement optical path. It can be seen that the light is refracted by the lens 35, the focal position is changed to the near side, and the luminous flux is diverged as compared with the case where the test lens is not interposed in the optical path in FIG.

  In this way, when measuring a spherical surface having various curvatures, the measurement light irradiated to the test lens due to the convergence and divergence of the light beam is refracted by the shape and thickness of the test lens, and the test lens is in the optical path. It is different from the luminous flux when no intervening. For this reason, there is a problem that measurement errors occur due to sensitivity variations in the light receiving section.

Further, when an integrating sphere is used for the light receiving portion, an opening for receiving light by the detector and for receiving light by the detector is necessary. Further, the way of influence varies depending on the convergence and divergence of the incident measurement light beam, depending on the size and number of the openings, the ratio to the inner diameter of the integrating sphere itself, the position of the light receiving portion of the detector, and the like. Another problem is that the amount of error varies depending on the magnitude and positive / negative of the power determined by the curvature and refractive index of the lens to be examined. Also, depending on the measuring machine, there is a problem that the measurement light flux, the unevenness of the light receiving part, etc. are different, and the amount of error is also different for each measuring machine.
An object of the present invention is to solve the problems in the above-described conventional example.

  In the transmittance measuring method of the present invention, the light detection intensity when the measurement light is allowed to pass through the optical path through which the measurement light reaches the light detection means and when the measurement light is not interposed are measured. A transmittance measurement method for measuring and measuring the transmittance of a test lens by comparing the measurement, and measuring a measurement error of the transmittance due to convergence or divergence of a light beam by a test lens interposed in an optical path A measurement error measurement step, and a correction step of correcting the measured transmittance value of the lens to be measured in accordance with the measurement error.

  Further, the transmittance measuring apparatus of the present invention is capable of detecting light when the measurement light is allowed to pass through the optical path through which the measurement light reaches the light detection means and when the measurement lens is not interposed. A transmittance measuring device that measures intensity and measures the transmittance of a test lens by comparing the intensity, and measures the measurement error of the transmittance caused by convergence or divergence of a light beam by a test lens interposed in an optical path And a correction means for correcting the measured transmittance value of the lens to be measured in accordance with the measurement error.

  According to the present invention, the measurement error amount resulting from the convergence or divergence of the light beam due to the sensitivity unevenness of the light receiving unit and the shape, material, position in the optical path, measurement wavelength, etc. of the lens to be measured is measured in advance. By correcting the measurement error, it is possible to reduce the measurement error. As a result, lenses with various curvatures can be measured with high accuracy even in the spectral transmittance measurement in the vacuum ultraviolet region, and the lens and antireflection film formed on the lens surface can be evaluated with high accuracy. It has a special effect.

  The present invention measures the light detection intensity when the measurement light is allowed to pass through the optical path where the measurement light reaches the light detection means and when the measurement light is not interposed, and contrasts the measurement light. This relates to a transmittance measuring method and apparatus for calculating the transmittance of a test lens. In a preferred embodiment of the present invention, the shape or material of the test lens, the position in the optical path, and the convergence or divergence of the light beam depending on the measurement wavelength when the test lens is interposed in the optical path or not. The resulting measurement error is measured in advance, and the measured value is corrected. In the present embodiment, the measurement error measurement process for measuring the measurement error is performed in advance, but in the present invention, the measurement error measurement process and the transmittance measurement process of the test lens are either May be first.

  In measuring the measurement error, a plurality of types of lenses having different shapes and made of a material cut out from the same material having little or no absorption and having uniform optical characteristics are used. For each of these lenses, the transmittance is measured, and the relationship between the measurement condition (parameter) and the transmittance measurement value error is approximated from the measurement result. Further, the transmittance measurement value of the test lens is corrected by a transmittance measurement value error (measurement error) according to the conditions for measuring. The conditions for measurement are the lens power, shape, material, position in the optical path, measurement wavelength, etc. of the lens to be measured.

  In addition, the transmittance measuring apparatus according to the present embodiment includes a plurality of types of lenses having different shapes and made of a material cut out from the same material that has little or no absorption and has uniform optical characteristics. Moreover, the control apparatus which performs the following processes is provided. The control device corrects the lens power, the position of the test lens, the process of finding the relationship between the measurement wavelength and the transmittance measurement value error, the process of deriving the correction formula, the process of measuring the test lens, and the power of the test test lens. The measured values are output individually or continuously through the steps to be performed.

  Hereinafter, with reference to FIG. 1 and FIG. 2, Example 1 of this invention is demonstrated in detail. It should be noted that the drawings only schematically show the shape, size, and arrangement relationship of each component to the extent that the invention can be understood, and therefore the present invention is not limited to the illustrated examples.

  In FIG. 2, the spectroscope used for spectroscopy in the present embodiment is a so-called Zellnitana type monochromator comprising a reflection type plane diffraction grating 14 and two off-axis paraboloidal mirrors 13 and 15 as light dispersion elements. It is said. As the light source 11, a deuterium lamp that emits light having a continuous wavelength from the vacuum ultraviolet region to the visible region is used. A photomultiplier tube 23 is used via an integrating sphere 24 for a light receiving sensor for reference light and measurement light. In this example, the inverse dispersion, which is the wavelength difference per unit length on the exit slit 16 surface, was set to 2 nm / mm from the relationship between the light quantity and the wavelength resolution.

  The configuration of the spectrometer of FIG. 2 will be described in more detail along the optical path of light emitted from the light source. The spectroscope unit includes a first off-axis parabolic mirror 13, a reflective planar diffraction grating 14, a second off-axis parabolic mirror 15, and an exit slit 16. The light emitted from the light source 11 using the deuterium lamp is changed by 90 ° by the first plane mirror 12, becomes parallel light by the first off-axis parabolic mirror 13, and enters the diffraction grating 14. The light split by the diffraction grating 14 is condensed again by the second off-axis parabolic mirror 15 and forms an image on the exit slit 16 surface so that only a specific wavelength passes. The light that has passed through the exit slit 16 becomes parallel light by the third off-axis parabolic mirror 17 and is time-divided into reference light and measurement light by rotating a semicircular plane mirror (sector mirror) 18. That is, when the sector mirror 18 is on the optical path, the direction is changed by 90 ° and is incident on the fourth off-axis paraboloidal mirror 19 to be condensed and incident on the integrating sphere 24. On the other hand, when the mirror 18 is not on the optical path, it enters the fifth off-axis parabolic mirror 21 as it is, and is condensed and enters the integrating sphere 24.

  If the measurement area is visible light, barium sulfate or the like is generally applied to the inner surface of the integrating sphere 24. In this embodiment, a phosphor having a characteristic of emitting light at a vacuum ultraviolet wavelength is applied. A photomultiplier tube (photomultiplier) 23 is used as a detector, and is attached to the opening on the front side of the integrating sphere 24 in the drawing. 2 is the same as the conventional example shown in FIG. 6 and FIG. The vacuum ultraviolet light incident on the integrating sphere 24 irradiates the phosphor coated on the inner wall of the integrating sphere, and the emitted fluorescent light diffuses and reflects the surface of the phosphor coating on the inner surface of the integrating sphere while photomultiplier serving as a detector. The tube 23 is reached for measurement.

  Next, a method for correcting the measured value will be described. In this embodiment, a correction value at a wavelength of 193.4 nm for synthetic quartz glass as a lens material is obtained. First, six types of lenses having different shapes are prepared from materials cut out from the same material that has little or no absorption and has uniform optical characteristics. The transmittance of each lens is based on the value obtained by subtracting the reflection loss R% obtained from the refractive index of the material from 100 when the absorption is negligible, and the value obtained by further subtracting the absorption when the absorption is known. Let it be the transmittance To. Alternatively, each lens may be polished and measured before spherical polishing, and the value may be used as the reference transmittance To.

Each lens has a diameter of φ30 and a center thickness of 10 mm, and one surface is polished to a flat surface, and the other surface is a concave surface having a radius of curvature of 28, 56, 112, and also convex of 112, 56, 28. A spherical surface. As a result, the lens powers were set to -0.02, -0.01, -0.005, +0.005, +0.01, and +0.02, respectively. The lens power is the reciprocal of the focal length and is expressed by the following equation.
P = 1 / f = (n−1) (1 / R1-1 / R2) + t · (n−1) ² / (n · R1 · R2)
In the above equation, P is power, f is the focal length, n is the refractive index of the lens material, R1 is the radius of curvature of one surface (first surface), R2 is the radius of curvature of the other surface (second surface), t Is the center thickness.

  Next, the transmittance is measured in the same manner as measuring the normal test object for each of the six types of lenses. At that time, transmittance data was obtained by changing the distance between the light receiving unit and the lens to 27 mm, 32 mm, 42 mm, and 52 mm. The result is shown in the graph of FIG. When the horizontal axis represents the lens power and the vertical axis represents the transmittance, it can be seen that the measured values differ depending on the distance between the lens and the light receiving unit and the lens power, as shown in the graph.

Next, the function is obtained by approximating the six measured values for each measurement distance by polynomial approximation using the least square method or the like. In the present embodiment, a sixth-order approximation and an expression of the following form are obtained.
Ta = aP ⁶ + bP ⁵ + cP ⁴ + dP ³ + eP ² + fP + g
In the above equation, Ta is an approximate value, P is power, a to f are coefficients, and g is a constant (intercept).

Finally, the correction equation for obtaining the correction value Tc from the measurement value Tm measured with the same measuring machine, the same distance, and the same wavelength as the above-mentioned approximate equation for the lens to be tested is as follows.
Tc = To / Ta · Tm = To / (ap ⁶ + bp ⁵ + cp ⁴ + dp ³ + ep ² + fp + g) · Tm

  By correcting the measured value by the above method, the sensitivity variation of the light receiving part and the measurement error due to the shape or material of the lens to be tested, the position in the optical path, and the convergence or divergence of the light flux depending on the measurement wavelength can be corrected. Measurement error can be reduced. Actually, when the correction of the present embodiment was performed, a lens with a large power, which had a particularly large error, was measured with a transmittance of 5% or more lower than that of the reference plane substrate. Thus, the absolute value accuracy was greatly improved. As a result, even in the measurement of spectral transmittance in the vacuum ultraviolet region, lenses having various curvatures can be measured with high accuracy, and the lens and the antireflection film formed on the lens surface can be evaluated with high accuracy. Became.

  Hereinafter, the second embodiment of the present invention will be described in detail with reference to FIG. 3, FIG. 4, and FIG. It should be noted that the drawings only schematically show the shape, size, and arrangement relationship of each component to the extent that the invention can be understood, and therefore the present invention is not limited to the illustrated examples.

  Since the main configuration of the spectrometer (see FIG. 4) in the present embodiment is the same as that of the first embodiment, only the differences will be described. The transmissivity measuring apparatus in FIG. 4 includes an error amount measuring lens for obtaining a correction formula in a measuring machine, and is configured to be automatically set at a test lens set position in a measurement optical path. Here, six error amount measurement lenses are attached to the holder 25, and when measuring the error amount, each lens is alternatively (one by one) set in the measurement optical path under the control of the control device. Is done.

Also in the present embodiment, a correction value at a wavelength of 193.4 nm is obtained for synthetic quartz glass as a lens material. The error amount measurement lens is a material cut out from the same material that has little or no absorption and has uniform optical properties. Prepare six types of lenses with different shapes and measure the transmittance by sequentially interposing them in the optical path. It is set in a movable holder so that it can be done. When the transmittance of each lens is so small that the absorption is negligible, a value obtained by subtracting the reflection loss R% obtained from the refractive index of the material from 100, and a value obtained by further subtracting when the absorbance is known, are referred to as the reference transmittance To. To do. Alternatively, each lens may be subjected to surface polishing before spherical polishing, and the reference transmittance To may be actually measured. Each lens has a diameter of φ30, a center thickness of 10 mm, one surface is polished to a flat surface, and the other surface is a concave surface with a radius of curvature of 28, 56, 112, and a convex spherical surface with 112, 56, 28 as well. It was. The lens powers were set to -0.02, -0.01, -0.005, +0.005, +0.01, and +0.02, respectively. The lens power is the reciprocal of the focal length and is expressed by the following equation.
P = 1 / f = (n−1) (1 / R1-1 / R2) + t · (n−1) ² / (n · R1 · R2)
Here, P is power, f is the focal length, n is the refractive index of the lens material, R1 is the radius of curvature of one surface (first surface), R2 is the radius of curvature of the other surface (second surface), and t is Center thickness.

FIG. 5 is a flowchart showing the operation of the control device (not shown in FIG. 4).
Next, a method for deriving a measurement value corrected by the control device in succession or individually in accordance with the flow procedure shown in FIG. 5 will be described.
First, the first step “step of finding the relationship between error amount and changing parameter” will be described. The changing parameter is only the power in this embodiment, but may be the distance between the integrating sphere and the test lens (position in the optical path of the test lens), the wavelength of the measurement light, or the like. Alternatively, the shape and material of the lens to be examined may be used. As described above, the transmittance is measured by driving the movable holder with the lenses having different powers and sequentially interposing them in the optical path. At that time, the distance between the light receiving portion and the lens was all set to 33 mm.

Next, the second step “step for deriving a correction formula” will be described. The graph of FIG. 3 shows the transmittance results obtained in the first step. When the horizontal axis represents the lens power and the vertical axis represents the transmittance, it can be seen that the measured values differ depending on the lens power, as shown in the graph. From this result, a function is obtained by polynomial approximation of the six measured values by the least square method or the like by a calculation execution program or the like. Also in this embodiment, a sixth-order approximation and the following form are obtained.
Ta = aP ⁶ + bP ⁵ + cP ⁴ + dP ³ + eP ² + fP + g
Here, Ta is an approximate value, P is power, a to f are coefficients, and g is a constant (intercept).
The correction formula for obtaining the correction value Tc is as follows.
Tc = To / Ta · Tm = To / (ap ⁶ + bp ⁵ + cp ⁴ + dp ³ + ep ² + fp + g) · Tm

  Next, the third step “step of measuring a sample” will be described. The error amount measuring lens in the first step is retracted from the optical path by driving the movable holder, and this time, a test lens with a known power p is set at a position 33 mm from the light receiving portion. The test lens is set in a movable holder (not shown) different from the error amount measurement lens, and is configured to be moved by a command from a control device such as a PC. The transmittance is measured, and the measured value Tm is stored in the control PC.

  Finally, the fourth process “correcting process” will be described. In this step, the measurement value Tm obtained in the third step is substituted into the correction equation obtained in the second step to obtain a correction value Tc, and the result is stored in a control device such as a PC. Can be displayed on a display or the like.

  In this embodiment, six types of lenses having different powers are used. However, the power can be varied by changing the distance between the lenses by combining two or more convex and concave lenses. You may make it acquire the relationship of quantity.

  By correcting the measured value by the above method, the sensitivity variation of the light receiving part and the measurement error due to the shape or material of the lens to be tested, the position in the optical path, and the convergence or divergence of the light flux depending on the measurement wavelength can be corrected. Measurement error can be reduced. Actually, when the correction of the present embodiment was performed, a lens with a large power, which had a particularly large error, was measured with a transmittance of 5% or more lower than that of the reference plane substrate. Thus, the absolute value accuracy was greatly improved. As a result, even in the measurement of spectral transmittance in the vacuum ultraviolet region, lenses having various curvatures can be measured with high accuracy, and the lens and the antireflection film formed on the lens surface can be evaluated with high accuracy. It became.

It is a graph for calculating | requiring the correction value which concerns on Example 1 of this invention. It is a schematic sectional drawing which shows the structure of the measuring device as one side of Example 1 of this invention. It is a graph for calculating | requiring the correction value which concerns on Example 2 of this invention. It is a schematic sectional drawing which shows the structure of the measuring apparatus as one side of Example 2 of this invention. It is a flow for demonstrating the process performed in the apparatus of FIG. It is a schematic sectional drawing which shows the structure of the conventional measuring apparatus. Schematic sectional view for explaining a problem to be solved by the invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Light source 12, 13, 15, 17, 19, 21 Mirror 14 Diffraction grating 16 Slit 18 Sector mirror 23 Photomultiplier tube 24 Integrating sphere 25 Holder 26 Test lens

Claims (7)

In the optical path where the measurement light reaches the light detection means, the light detection intensity is measured when the measurement light is passed with the test lens interposed and when the test light is not interposed. A transmittance measuring method for measuring the transmittance of
A measurement error measuring step for measuring a measurement error of the transmittance due to convergence or divergence of a light beam by a lens to be measured interposed in the optical path;
And a correction step of correcting the measured transmittance value of the lens to be measured in accordance with the measurement error.   In the measurement error measurement step, a plurality of types of error amount measurement lenses having different shapes and made from a material cut out from the same material having uniform optical characteristics are used to measure the respective transmittances, and to measure these transmittances. The measurement method according to claim 1, wherein a correction of the transmittance measurement value is performed by approximating a relationship between the power of the test lens and the transmittance measurement value error from the value.   The error measurement lens is used to further approximate a relationship between a position or a measurement wavelength in the optical path of the lens to be examined and a transmittance measurement value error, and correct the transmittance measurement value. Item 3. The measuring method according to Item 2.   The step of finding the relationship between the lens power of the test lens and the measurement error, the step of deriving a correction formula from the found relationship, the step of measuring the transmittance of the test lens, and the measured transmittance 4. The measurement method according to claim 1, wherein the measurement value corrected through the step of correcting by the power of the lens is individually or continuously corrected. In the optical path where the measurement light reaches the light detection means, the light detection intensity is measured when the measurement light is passed with the test lens interposed and when the test light is not interposed. A transmittance measuring device for measuring the transmittance of
A measurement error measuring means for measuring the measurement error of the transmittance due to the convergence or divergence of the light beam by the lens to be measured interposed in the optical path;
A transmittance measuring apparatus comprising: correction means for correcting the measured transmittance value of the lens to be measured according to the measurement error. The measurement error measuring means includes a plurality of types of error amount measuring lenses having different shapes made of a material cut out from the same material having uniform optical characteristics, and these lenses are alternatively interposed in the optical path. Means for measuring the transmittance of the lens and approximating the relationship between the measured lens power and the transmittance measurement value error from the measured values,
The measurement apparatus according to claim 5, wherein the correction unit corrects the transmittance measurement value based on a relationship between a power of the lens to be measured and a transmittance measurement value error.   7. The measurement error measurement means further uses the error amount measurement lens to further approximate a relationship between a position or a measurement wavelength of the lens to be measured in the optical path and a transmittance measurement value error. The measuring device described in 1.

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