JP2010025689A - Curvature radius measuring method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve inspection efficiency and measurement accuracy in a curvature radius measuring method and apparatus. <P>SOLUTION: The curvature radius measuring apparatus 50 includes: a Fizeau surface 4a; a linear motion stage 16 which relatively moves a surface under measurement 5a in the optical axis C direction between two adjustment target positions, namely the confocal state and the cat's-eye state, and in the vicinity thereof; a piezoelectric element 9; a laser length measuring instrument 12 for measuring a relatively moved position; a wavefront measuring section which relatively moves the surface under measurement 5a to at least two positions in the optical axis C direction, measures a wavefront on the basis of the image information of interference fringes, and stores the wavefront together with information on each relatively moved position corresponding to the wavefront; a displacement evaluated value calculation section for calculating a displacement evaluated value by analyzing the wavefront; and an adjustment target position calculation section for calculating the estimated value of each adjustment target position on the basis of the relatively moved position and the displacement evaluated value. The curvature radius of the surface under measurement 5a is determined from a difference between these estimated values of the adjustment target positions. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method and apparatus for measuring a radius of curvature.

2. Description of the Related Art Conventionally, a curvature radius measuring method and apparatus using an interferometer are known in order to measure the radius of curvature of a measurement target surface having a curved shape, for example, a lens surface or a mirror surface.
When irradiating measurement light from a reference lens placed on the interferometer toward the surface to be measured, and observing interference fringes between the reflected light on the surface to be measured and the reference light reflected by the reference lens, from the surface to be measured The interference fringes due to the interference between the reflected light and the reference light are clearly observed because the converging position of the measurement light is close to the position of the center of curvature of the measurement surface and the measurement target. This is a case in the vicinity of the position of the cat's eye state that coincides with the top of the surface. In the confocal state and the cat's eye state, no interference fringes are generated due to misalignment, so the number of interference fringes is minimized. Therefore, while measuring the interference fringe, move the measurement light condensing position relative to the surface to be measured, and measure the relative movement distance between the confocal state and the cat's eye state to obtain the radius of curvature of the surface to be measured. A method of measuring a radius of curvature is known.
In this measurement method, the measurement accuracy of the radius of curvature depends on the accuracy of the position adjustment to the confocal state and the cat's eye state performed by the inspector looking at the interference fringe image. It is likely to occur.

For this reason, a technique for obtaining a radius of curvature using an interference fringe image acquired at a position slightly deviated from the confocal state and the cat's eye state has been proposed.
For example, Patent Document 1 is provided with a means for changing the relative distance between the lens (reference lens) and the surface to be measured, a distance detection means for detecting the relative distance, and a CCD camera that captures interference fringes as an image. This is due to the coincidence error between the shape virtually generated based on the output from the lens and the condensing point obtained from the interference fringes generated when the condensing point by the lens is substantially coincident with the vertex of the surface to be measured. Virtually generated due to the coincidence error between the condensing point and the center of curvature obtained from the interference fringes generated when the converging point generated by the lens substantially coincides with the center of curvature of the surface to be measured. A curvature radius measuring method and apparatus for obtaining a curvature radius based on the generated shape are described.
Here, the “virtually generated shape” is obtained by converting the reflection height of the irradiated light on the surface to be measured using a proportional position obtained from the pixel position on the CCD, the design value of the curvature radius of the lens, and the F number. Looking for. Then, an aberration function analysis is performed on the interference fringe images in a substantially confocal state and a substantially cat's eye state, and defocus coefficients are obtained respectively. From these values, the coincidence error between the focal point of the lens and the apex of the surface to be measured and the coincidence error between the focal point of the lens and the center of curvature of the surface to be measured are obtained, and these are obtained from the amount of change in the distance between the lens and the surface to be measured. The radius of curvature is obtained by subtracting the coincidence error of the lens and removing the influence of the coincidence error between the focal point of the lens and the vertex of the surface to be measured or the center of curvature of the surface to be measured.
JP 2002-228426 A

However, the conventional curvature radius measuring method and apparatus as described above have the following problems.
The technique described in Patent Document 1 analyzes the wavefront of the interference fringe image observed at each position shifted in the optical axis direction from the confocal state or the cat's eye state, thereby removing the signal from the confocal state or the cat's eye state. A positional deviation amount is calculated, and the curvature radius of the surface to be measured is obtained using the positional deviation amount.
For this reason, it is necessary to obtain in advance various coefficients necessary for the calculation from the design values such as the radius of curvature and F number of the surface to be measured, and the optical characteristics of the imaging lens that acquires the surface to be measured and the interference fringes. It was. Since these various factors must be changed when the inspection object is changed, there is a problem that the inspection efficiency is lowered.
In addition, since the design value of the surface to be measured is assumed in order to convert the distance on the image sensor into the light beam height, there is a problem that the manufacturing error of the surface to be measured is reflected in the measurement error.

  The present invention has been made in view of the above problems, and an object thereof is to provide a curvature radius measuring method and apparatus capable of improving inspection efficiency and measurement accuracy.

In order to solve the above-described problem, in the invention described in claim 1, an interferometer divides one light beam into measurement light and reference light that interferes with the measurement light, and the divided measurement light is divided. Irradiating with a convergent position converged toward the measurement surface made of a curved surface, moving the measurement surface relative to the measurement light condensing position, and reflected light from the measurement surface and the reference A curvature radius measurement method for obtaining a curvature radius of the surface to be measured from image information of interference fringes formed by light and information on a relative movement distance of the surface to be measured, wherein a condensing position of the measurement light is determined The relative position in the optical axis direction of the surface to be measured as a reference is referred to as the relative movement position of the surface to be measured, and the confocal state in which the collection position of the measurement light coincides with the center of curvature of the surface to be measured; A cat-aper where the condensing position of the measurement light coincides with the top of the surface to be measured When the relative movement positions of the measured surface in the state are referred to as adjustment target positions, respectively, at least two positions in the vicinity of the adjustment target position in the optical axis direction for each adjustment target position in the confocal state and the cat's eye state Then, the wavefront is measured based on the image information of the interference fringes, and the wavefront measuring step for acquiring the information on the relative movement position of the measured surface corresponding to the wavefront, and the wavefront measured in the wavefront measuring step And calculating a displacement evaluation value corresponding to the amount of displacement of the relative movement position of the measured surface corresponding to the wavefront from the adjustment target position, and the wavefront measurement step. Based on the acquired information on the relative movement position of the surface to be measured corresponding to the wavefront and the positional deviation evaluation value calculated in the positional deviation evaluation value calculating step, An adjustment target position calculation step for calculating an estimated value of the adjustment target position, and an estimated value of the adjustment target position in the confocal state calculated in the adjustment target position calculation step, and an adjustment target in the cat's eye state The radius of curvature is obtained from the difference from the estimated position value.
According to the present invention, the wavefront is measured based on the image information of the interference fringes at at least two positions in the vicinity of one of the adjustment target positions in the confocal state and the cat's eye state, and corresponds to the measured wavefront. Information on the relative movement position of the surface to be measured is acquired.
Next, in the positional deviation evaluation value calculation step, the wavefront measured in the wavefront measurement step is analyzed, and the positional deviation corresponding to the deviation amount from the adjustment target position of the relative movement position of the measured surface corresponding to this wavefront. An evaluation value is calculated.
Next, in the adjustment position target calculation step, the information on the relative movement position of the measurement target surface corresponding to the wavefront acquired in the wavefront measurement step, and the positional deviation evaluation value calculated in the positional deviation evaluation value calculation step, An estimated value of the adjustment target position, that is, a relative movement position of the surface to be measured from which a positional deviation evaluation value corresponding to a deviation amount 0 from the adjustment target position is obtained is calculated.
Then, the above steps are repeated for the other adjustment target position in the confocal state and the cat's eye state. Accordingly, the radius of curvature can be obtained from the difference between the estimated value of the adjustment target position in the confocal state and the estimated value of the adjustment target position in the cat's eye state calculated in the adjustment target position calculation step.
In this way, by analyzing the wavefront based on the images of the plurality of interference fringes at at least two positions in the optical axis direction in the vicinity of the adjustment target position, at least two position displacement evaluation values are calculated. Since the estimated value of the adjustment target position is calculated from the position deviation evaluation value of the position and the curvature radius is obtained, the relative displacement position of the surface to be measured is not moved to the adjustment target position, and the displacement amount itself can be obtained. It is possible to measure the radius of curvature.

According to a second aspect of the present invention, in the curvature radius measuring method according to the first aspect, in the positional deviation evaluation value calculating step, the wavefront is approximated by a quadratic expression represented by the following expression, The approximation coefficient of the quadratic term in the equation is used as the positional deviation evaluation value.
Z (X, Y) = C ₀ + C ₁ · X + C ₂ · Y + C ₃ · (X ² + Y ² ) (1)
Here, (X, Y, Z) is the coordinate value of the wavefront in the XYZ rectangular coordinate system in which the optical axis direction is the Z-axis direction, C ₀ is a constant term, C ₁ and C ₂ are primary terms, and C ₃ Is an approximation coefficient of a quadratic term.
According to the present invention, to evaluate the positional deviation amount from the relationship between the power components of the wavefront represented by the approximation coefficient C ₃ of the second-order terms of the positional deviation amount and the formula (1) from the measurement reference position in the optical axis direction Can do.

In the invention according to claim 3, in the curvature radius measuring method according to claim 1 or 2, in the wavefront measuring step, the relative movement position of the surface to be measured is moved in one direction in the optical axis direction,
During this movement, n interference fringe images that can be regarded as having a phase difference of ± Δφ · K, where Δφ is a constant phase increment (where K is 0 ≦ K, where N is an integer equal to or greater than 1) ≦ N integer, n is a plurality of interference fringe images from which at least two sets of n = 2N + 1) can be selected are acquired as time-series images, and the measured surface at the time of each acquisition of the time-series images A step of acquiring information on a relative movement position, and after the step, the at least two sets of the n interference fringe images are selected from the time-series images, and the selected n interference fringe images are selected. Based on the image information, the wavefront is measured at at least two positions near the adjustment target position.
According to the present invention, the relative movement position of the surface to be measured is moved in one direction in the optical axis direction, and during this movement, at least two sets of n interference fringe images for measuring the wavefront can be selected. A plurality of interference fringe images can be acquired as time-series images, and information on the relative movement position of the measurement target surface of each time-series image can be acquired. For this reason, image information of all interference fringes necessary for wavefront measurement at at least two positions in the vicinity of the adjustment target position and the relative movement position of the surface to be measured corresponding to the wavefront are simultaneously acquired by one relative movement. After that, since the positional deviation evaluation value calculation step and the adjustment target position calculation step can be performed, the mechanical operation can be reduced and the curvature radius can be measured quickly.

According to a fourth aspect of the present invention, in the method of measuring a radius of curvature according to the third aspect, in the wavefront measuring step, the relative measurement of the surface to be measured is performed at a moving pitch corresponding to a phase increment smaller than the constant phase increment Δφ. It is assumed that the time-series image is acquired by moving the movement position.
According to the present invention, the relative movement position of the surface to be measured is moved with the movement pitch corresponding to the phase increment smaller than the constant phase increment Δφ, and therefore, the positions for measuring the wavefronts at at least two positions are measured with a fine pitch. Therefore, the estimation accuracy of the estimated value of the adjustment target position in the adjustment target position calculation step can be improved.

According to a fifth aspect of the present invention, in the curvature radius measuring method according to the first or second aspect, in the wavefront measuring step, a relative movement position of the surface to be measured is moved in the optical axis direction, and a constant phase increment is performed. Is an interference fringe image that can be regarded as having a phase difference of ± Δφ · K (where K is an integer of 0 ≦ K ≦ N, where N is an integer of 1 or more, and n is n = 2N + 1), and information on the relative movement position of the measured surface when an interference fringe image corresponding to at least K = 0 is obtained, and based on image information of the n interference fringe images The method includes a step of measuring one wavefront, and the step is repeated at at least two positions near the adjustment target position.
According to the present invention, the wavefront can be accurately measured based on the image information of n interference fringe images.

According to a sixth aspect of the present invention, in the curvature radius measuring method according to any one of the first to fourth aspects, in the adjustment target position calculating step, information on a relative movement position of the measured surface corresponding to the wavefront and Then, the estimated value of the adjustment target position is calculated by performing regression analysis on the positional deviation evaluation value.
According to the present invention, since the estimated value of the adjustment target position is calculated by performing regression analysis on the positional deviation evaluation value, the estimated value can be calculated with high accuracy.

According to a seventh aspect of the present invention, in the method of measuring a radius of curvature according to the sixth aspect, in the adjustment target position calculating step, after the regression equation is obtained by the regression analysis, the regression of the data used for the regression analysis is obtained. When data with a large divergence with respect to the equation is generated, the data with the large divergence is excluded and a regression analysis is performed again.
According to the present invention, when data having a large deviation from the regression equation is generated for some reason, this data is excluded and the regression analysis is performed again, so that the influence of the estimation error due to the abnormal measurement value is reduced. Can do.

In the invention according to claim 8, one light beam is divided into measurement light and reference light that interferes with the measurement light by an interferometer, and the divided measurement light is directed to a measurement surface that is a curved surface. Then, the light is converged on the condensing position and irradiated, and the surface to be measured is moved relative to the light condensing position of the measurement light, and interference fringes formed by the reflected light and the reference light on the surface to be measured A curvature radius measuring device for obtaining a radius of curvature of the surface to be measured from image information and information on a relative movement distance of the surface to be measured, the light of the surface to be measured based on a condensing position of the measurement light The relative position in the axial direction is referred to as the relative movement position of the surface to be measured, the confocal state in which the light collection position of the measurement light coincides with the center of curvature of the surface to be measured, and the light collection position of the measurement light is the The measured surface in a cat's eye state that coincides with the top of the measured surface When each of the relative movement positions is referred to as an adjustment target position, the relative movement position of the surface to be measured is relative to at least two positions in the vicinity of the adjustment target position in the confocal state and the cat's eye state in the optical axis direction. A relative movement mechanism for moving; a relative movement position measuring unit for measuring a relative movement position of the surface to be measured; and a relative movement position of the surface to be measured by the relative movement mechanism in the optical axis direction in the vicinity of the adjustment target position. A wavefront measuring unit that relatively moves to at least two positions, measures a wavefront based on image information of the interference fringes, and stores the wavefront together with information on a relative moving position of the measured surface corresponding to the wavefront, and the wavefront measuring unit The positional deviation corresponding to the amount of deviation from the adjustment target position of the relative movement position of the measured surface corresponding to the wavefront The positional deviation evaluation value calculation unit for calculating the value, the information on the relative movement position of the measured surface corresponding to the wavefront acquired by the wavefront measurement unit, and the positional deviation evaluation value calculation unit An adjustment target position calculation unit that calculates an estimated value of the adjustment target position based on a positional deviation evaluation value, the estimated value of the adjustment target position in the confocal state calculated by the adjustment target position calculation unit, and The curvature radius is obtained from the difference from the estimated value of the adjustment target position in the cat's eye state.
According to this invention, since it is a curvature radius measuring apparatus which can be used for the curvature radius measuring method of Claim 1, it has the same effect as the invention of Claim 1.

  According to the method and apparatus for measuring the radius of curvature of the present invention, the radius of curvature can be measured without moving and adjusting the relative movement position of the inspection surface to the adjustment target position, and without determining the displacement amount itself. There is an effect that efficiency and measurement accuracy can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram showing a schematic configuration and a confocal arrangement of a curvature radius measuring apparatus according to an embodiment of the present invention. FIG. 2 is a functional block diagram showing a functional configuration of the control means of the curvature radius measuring apparatus according to the embodiment of the present invention. FIG. 3 is a schematic configuration diagram showing an arrangement in a cat's eye state of the curvature radius measuring device according to the embodiment of the present invention.

A curvature radius measuring device according to an embodiment of the present invention will be described.
As shown in FIG. 1, the schematic configuration of the radius-of-curvature measuring apparatus 50 of this embodiment is as follows: a laser light source 1, a collimating lens 2, a beam splitter 3, a Fizeau lens 4, a linear motion stage 16 (relative movement mechanism), and a perforated stage. 8, a measurement object holding unit 11, a piezo element 9 (relative movement mechanism), a piezo element controller 10, a laser length measuring device 12 (relative movement position measurement unit), a condenser lens 6, a CCD 7, and a measurement control unit 13. The measurement control unit 13 displays, for example, an operation unit 15 including a keyboard and a mouse, an image captured by the CCD 7, a shape measurement result, and the like in order to perform operation input and setting information necessary for shape measurement. Therefore, the display unit 14 including a monitor or the like is electrically connected.
Here, the laser light source 1, the collimating lens 2, the beam splitter 3, the Fizeau lens 4, and the condenser lens 6 constitute an interferometer 51 having a Fizeau-type optical system.
The measured surface 5a may be a convex spherical surface, but in the following, an example in the case of a concave spherical surface of the measured object 5 made of a plano-concave lens will be described. A person skilled in the art can easily understand the arrangement and configuration of the interferometer when the surface to be measured 5a is a convex spherical surface.

The laser light source 1 is a light source that generates coherent light for forming interference fringes. In this embodiment, as an example, a light source that generates laser light having a wavelength λ = 632.8 (nm) as divergent light is employed. ing.
The divergent light generated by the laser light source 1 is converted into parallel light 30 a by the collimator lens 2 and is incident on the beam splitter 3.
The beam splitter 3 reflects and guides the parallel light 30a onto the optical axis C of the Fizeau lens 4, and also splits the light to be measured, which will be described later from the Fizeau lens 4 side, and the measured surface reflected light 30c and the reference surface reflected light 30d. It is an element.

The Fizeau lens 4 reflects a part of the parallel light 30a incident on the optical axis C by the Fizeau surface 4a to form reference surface reflected light 30d (reference light), and the parallel light incident on the optical axis C. It is a lens that transmits the other part of the light 30a as the transmitted light 30b and collects the transmitted light 30b, and has a function of dividing the parallel light 30a (one light beam).
The shape of the Fizeau surface 4a is a spherical surface finished with high accuracy. Therefore, the Fizeau surface 4a constitutes a reference surface for converting the wavefront of the measurement surface reflected light 30c reflected by the measurement surface 5a to form an interference fringe with the reference surface reflected light 30d.
The optical axis C of the Fizeau lens 4 constitutes the optical axis of the interferometer 51.

The linear motion stage 16 is a relative movement mechanism that is fixed to the apparatus main body of the curvature radius measuring device 50 and holds the perforated stage 8 in a direction along the optical axis C so as to be linearly movable. A configuration including a linear guide that guides the movement along the axis and driving means such as a ball screw or a linear motor can be employed.
The linear motion stage 16 is electrically connected to the measurement control unit 13, and the movement amount is controlled by the measurement control unit 13.
The movable range of the linear motion stage 16 is set to a range larger than the radius of curvature of the surface to be measured 5a.

The perforated stage 8 is a plate-like member having a through-hole 8a having a size capable of transmitting a laser beam 12a for length measurement, which will be described later, in the center, and a direction along the optical axis C by the linear motion stage 16. Is supported so as to be movable.
On the plane 8 b formed on the perforated stage 8 and orthogonal to the optical axis C, a device holding part 11 is attached via a piezo element 9.

The DUT holding unit 11 holds the DUT 5 on the back side of the measurement surface 5a.
The object-to-be-measured holding unit 11 positions and holds the object to be measured 5 in a direction along the optical axis C and in a direction perpendicular to the optical axis C. Is provided. As a result, the optical axis of the surface to be measured 5 a can be held in a state where it is aligned with the optical axis C of the interferometer 51.
Further, on the back side of the holding position of the DUT 5 of the DUT holding part 11, there is provided a length measuring light reflecting part 11a that reflects the light that passes through the through hole 8a and enters parallel to the optical axis C. It has been.

The piezo element 9 is provided so as to be extendable in the direction along the optical axis C between the flat surface 8 b of the perforated stage 8 and the measured object holding part 11, and the measured object holding part with respect to the perforated stage 8. 11 is a relative movement mechanism that minutely moves 11 in the direction along the optical axis C.
Thereby, the distance between the measured surface 5a held on the measured object holding portion 11 and the Fizeau surface 4a of the Fizeau lens 4 can be changed by a minute amount sufficiently smaller than λ / 8. It is like that.
The amount of movement of the piezo element 9 is controlled by changing the applied voltage by the piezo element controller 10 electrically connected to the piezo element 9.

The piezo element controller 10 controls the voltage applied to the piezo element 9 based on a control signal from the measurement control unit 13 and controls the amount of expansion / contraction of the piezo element 9, whereby the surface to be measured 5 a with respect to the Fizeau surface 4 a is controlled. The relative position in the direction along the optical axis C is controlled.
As shown in FIG. 2, the schematic configuration of the piezo element controller 10 includes a calculation unit 35, a timer 36, a D / A conversion unit 37, and an amplifier unit 38.
The calculation unit 35 generates applied voltage data that linearly increases the applied voltage from 0 V to the command voltage within the rise time in accordance with the rise time and command voltage control signal sent from the measurement control unit 13, and performs measurement. The applied voltage data is sequentially supplied to the D / A conversion unit 37 in accordance with a movement start signal from the control unit 13.
The timer 36 supplies a reference clock for sending applied voltage data from the computing unit 35 at appropriate intervals.
The D / A converter 37 converts the applied voltage data supplied by the calculator 35 into a voltage signal. This voltage signal is amplified by the amplifier unit 38 and output to the piezo element 9 as an applied voltage for driving the piezo element 9.
Thus, according to the piezo element controller 10 of the present embodiment, an applied voltage that continuously drives the piezo element 9 in a certain direction can be supplied in accordance with the movement start signal from the measurement control unit 13. It has become.

  The linear movement stage 16 and the piezo element 9 move the relative movement position of the measured surface 5a to the vicinity of the adjustment target position in the confocal state and the cat's eye state, and the relative movement position of the measured surface 5a. The relative movement mechanism is configured to relatively move in the optical axis direction in the vicinity of the respective adjustment target positions in the confocal state and the cat's eye state.

The laser length measuring device 12 irradiates the length measuring light reflecting portion 11a of the measured object holding portion 11 with the laser light 12a traveling in the direction along the optical axis C, and the length measuring light reflecting portion from an appropriate reference position. The distance in the optical axis C direction of 11a is measured, and the length measurement result is sent to the measurement control unit 13 as a coordinate z. The coordinate z represents a relative movement position of the measurement surface 5a in the direction along the optical axis C because it has a fixed positional relationship with the measurement surface 5a held by the measurement object holding unit 11.
The laser length measuring device 12 constitutes a relative movement position measuring unit that measures the relative movement position of the surface to be measured 5a.
As the apparatus configuration of the laser length measuring device 12, an appropriate configuration can be adopted as long as the length can be measured with an accuracy of a fraction of λ / 8 or less, for example, interference of the laser beam 12a. A configuration that measures the length by counting the fringes can be suitably employed.

Transmitted light 30b transmitted through the Fizeau lens 4 is condensed to the condensing position Q _R.
Condensing position Q _R, in this embodiment, since the light incident on the Fizeau lens 4 is collimated light 30a, coincides with the focal position of the Fizeau lens 4.
In the following, the positional relationship between the Fizeau surface 4a and the measurement surface 5a, referred to state condensing position Q _R coincides with the center of curvature Q _L of the surface to be measured 5a (see FIG. 1) and confocal state, condenser position Q _R is referred to as a cat's eye state a state that matches the surface apex Q _P (see FIG. 3) of the measurement surface 5a.
In addition, the relative movement position of the measurement target surface 5a in the confocal state and the relative movement position of the measurement target surface 5a in the cat's eye state are referred to as the adjustment target position in the confocal state and the cat's eye state, or simply as the adjustment target position, respectively.

For the positional relationship confocal state, as shown in FIG. 1, diverges from the condensing position Q _R, transmitted light 30b incident on the measurement surface 5a is reflected as a measurement surface reflected light 30c. At this time, the light beam of the transmitted light 30b enters along the normal line of the measured surface 5a, and the measured surface reflected light 30c travels backward along the same optical path as the transmitted light 30b and reenters the Fizeau lens 4. And transmitted toward the beam splitter 3. The measurement surface reflected light 30c and the reference surface reflected light 30d reflected by the Fizeau surface 4a interfere with each other and form interference fringes because the parallel light 30a is a light beam divided by the Fizeau surface 4a. .
The measured surface reflected light 30c is reflected by the measured surface 5a, thereby being measured light having a wavefront corresponding to the shape of the measured surface 5a. On the other hand, the reference surface reflected light 30d is reflected by the Fizeau surface 4a, thereby becoming reference light having a wavefront corresponding to the shape of the Fizeau surface 4a. Further, the reflected light 30c to be measured is reflected by the measured surface 5a and travels backward in substantially the same optical path, so that the optical path between the Fizeau surface 4a and the measured surface 5a with respect to the reference surface reflected light 30d. The optical path difference is twice the length.
Since the Fizeau surface 4a is manufactured with sufficiently high accuracy compared to the measured surface 5a, the interference fringe image represents the aberration of the measured surface 5a. Similarly, in the cat's eye state, an interference fringe image representing the internal aberration of the interferometer itself is obtained.

The condenser lens 6 is an optical element that projects interference fringes due to the measurement surface reflected light 30 c and the reference surface reflected light 30 d onto the imaging surface 7 a of the CCD 7.
The CCD 7 is an image sensor that photoelectrically converts an interference fringe image projected on the imaging surface 7a at a predetermined video rate. As the video rate, an appropriate value can be adopted as necessary. For example, a video rate of 30 fps can be preferably adopted.
The CCD 7 is electrically connected to the measurement control unit 13, the imaging operation is controlled by the measurement control unit 13, and the image signal captured by the CCD 7 is sent to the measurement control unit 13.

  As shown in FIG. 2, the functional block configuration of the measurement control unit 13 includes a signal conversion unit 20, a wavefront measurement unit 21, a positional deviation evaluation value calculation unit 22, a storage unit 23, an adjustment target position calculation unit 24, and a radius of curvature calculation unit. 25 and a display control unit 27.

  The signal conversion unit 20 captures the image signal sent from the CCD 7 for each image frame at the timing instructed by the wavefront measurement unit 21, converts it into luminance data, and displays the wavefront measurement unit 21 and display as two-dimensional image data. This is sent to the control unit 27.

The wavefront measuring unit 21 relatively moves the relative movement position of the surface to be measured 5a to at least two positions in the optical axis direction near the adjustment target position by the linear motion stage 16 and the piezo element 9 to obtain image information of interference fringes. The wavefront is measured based on this, and is stored in the storage unit 23 together with information on the relative movement position of the measured surface 5a corresponding to this wavefront.
Therefore, when the wavefront measuring unit 21 of the present embodiment receives an operation input for starting measurement from the operation unit 15, the linear motion stage 16 and the piezoelectric element controller are based on measurement conditions such as a preset moving distance and moving speed. 10, a control signal is sent to control the amount of movement of the measured object holding unit 11, the measured object holding unit 11 is positioned at the measurement start position z ₁ , the position of the linear motion stage 16 is fixed, and the piezo element Control is performed to continuously change the amount of expansion and contraction 9 in the direction along the optical axis C. The coordinate z can be acquired from the laser length measuring device 12 at each moving position.
Here, the measurement start position z ₁ is the position coordinates z _0cf , z of the measured object holding part 11 corresponding to the design confocal state and cat's eye state determined from the design value of the radius of curvature of the measured surface 5a. _In this vicinity, the measurement surface 5a is moved in the direction along the optical axis C in advance to obtain an interference fringe image, and the moving range δ has a contrast that allows the interference fringe image to be analyzed. be prepared with the position previously, within this movement range [delta], may be employed spaced appropriately positioned from the center of the [delta] as a measurement start position z _1.
In the present embodiment, as an example, in the range of [delta], the position where the measurement surface 5a moves away from the most Fizeau surface 4a and the measurement start position z _1, 1, of [delta] from the illustrated lower side of the measurement object holder 11 It is assumed that the measurement is performed while the surface to be measured 5a is brought closer to the Fizeau surface 4a within the range.

Such measurement start position z _1, if the measurement surface 5a having the same radius of curvature, it is not necessary to re-set each time to replace the measured surface 5a, after once set, employing the same configuration can do.

The wavefront measuring unit 21 sends an interference fringe image capturing start signal to the signal converting unit 20, and when the image converting is started by the signal converting unit 20, each of the interference fringe images transmitted from the signal converting unit 20. Image data (hereinafter referred to as a time-series image) is stored in the storage unit 23 in association with information on the capturing order thereof (hereinafter referred to as time-series position information).
For example, the i-th time-series image acquired from the signal converter 20 on the time-series is F _i (where i = 1,..., M), and the coordinates acquired from the laser length measuring device 12 are z _i . At this time, i, F _i , and z _i are stored in the storage unit 23.
Here, the time-series image F _i is a candidate for the wavefront measurement image F _j for measuring the wavefront, and has a constant phase with respect to the interference fringe image selected as the wavefront measurement image F _j. When the increment is Δφ, n interference fringe images that can be regarded as having a phase difference of ± Δφ · K (where K is an integer of 0 ≦ K ≦ N, where N is an integer of 1 or more, n Constitutes a candidate image as a candidate of n = 2N + 1).
In this embodiment, the condition of the movement operation set by the wavefront measuring unit 21 in the piezo element controller 10 is a command to the piezo element 9 to move by δ in the direction along the optical axis C within a certain rise time T. Set the condition that the voltage is changed at a constant speed.
Since the wavefront measuring unit 21 acquires m (where m is an integer of 4 or more) candidate images during the rise time T, the sampling period ΔT is ΔT = T / (m−1). . The minimum value of the sampling period ΔT is the reciprocal of the video rate of the CCD 7.

The wavefront measuring unit 21 can perform wavefront measurement based on a known measurement method known as a fringe scan method, a phase shift method, or the like as a wavefront measurement method.
That is, in this embodiment, the wavefront measuring unit 21, the time-series images F _j for measuring a wavefront from among the series images F _i when acquired selected as the wavefront measurement image, the image F for wavefront measurement _{When j is} a constant phase increment with respect to _j , n interference fringe images that can be regarded as having a phase difference of ± Δφ · K (where K is 0, where N is an integer equal to or greater than 1) ≦ K ≦ N, n is a wavefront measurement using n = 2N + 1). Hereinafter, referred to as the n pieces of the interference fringe image and the wavefront measurement image group G _j.

In the present embodiment, an example in which Δφ = π / 2 and n = 5 will be described. This Δφ = π / 2 corresponds to λ / 8 as the relative movement distance of the measured surface 5a. Further, when the number m of series images F _i is adapted to set so as to measure the wavefront at least two positions in the optical axis direction, a phase increment between n sheets of interference fringe image in the image group G _j for wavefront measurement Select so that the error with respect to Δφ is sufficiently small. In the present embodiment, a case where m = 40 will be described as an example.
Examples of wavefront measurement calculation methods include P. Hariharan, BF Oreb, T. Eiju, "Applied Optics", (USA), 1987, 26, pp. 2504-2506, the so-called 5-bucket method is employed.
The 5-bucket method will be briefly described. For example, it is _assumed that the wavefront measurement image group G _j is G _j = {F _j2− , F _j1− , F _j , F _{j1 +} , F _{j2 +} }. Here, the subscript jk ± (k = 1, 2) represents a subscript of the time-series image F _i that can be regarded as the phase difference with respect to F _j being ± kΔφ (composite same order).
Each of the interference fringe images _{F j2-, F j1-, F j} , F j1 +, F j2 + constant position each position intensity luminance in (image on the coordinates (x, y) represents) data _g k (x, y) (where k = −2, −1, 0, 1, 2), the phase θ _j (x, y) at the coordinates (x, y) can be calculated from the following equation. However, in formula (2), for simplicity, θ _j (x, y) and g _k (x, y) are represented by θ and g _k , respectively.

Therefore, the wavefront h _j (x, y) calculated from the wavefront measurement image group G _j can be expressed by the following equation using the phase θ _j (x, y) expressed by the equation (2). it can. In the following, when there is no possibility of misunderstanding, (x, y) may be omitted and simply referred to as wavefront h _j and phase θ _j .

h _j (x, y) = λ · θ _j (x, y) / 4π (3)

The storage unit 23 stores time-series position information i, a time-series image F _i, and coordinates z _i sent from the wavefront measurement unit 21.

Position shift evaluation value calculation unit 22 analyzes the wavefront h _j measured for each image group G _j wavefront measured by the wavefront measurement section 21, the relative movement position of the measuring surface 5a that corresponds to the wavefront hj, adjusting and it calculates the positional deviation evaluation value P _j corresponding to the amount of deviation from the target position.
The positional deviation evaluation value P _j of this embodiment employs an approximation coefficient C ₃ that is a power component when the wavefront h _j obtained from the expression (3) is approximated by the following expression (1). The power component is 0 in the confocal state and the cat's eye state.

Z (X, Y) = C ₀ + C ₁ · X + C ₂ · Y + C ₃ · (X ² + Y ² ) (1)
Here, (X, Y, Z) is the coordinate value of the wavefront in the XYZ rectangular coordinate system in which the optical axis direction is the Z-axis direction, C ₀ is a constant term, C ₁ and C ₂ are primary terms, and C ₃ Is an approximation coefficient of a quadratic term.

As the positional deviation evaluation value P _j , other evaluation values may be used as long as the corresponding value when the positional deviation amount from the confocal state or the cat's eye state becomes 0 is known.
Expression (1) is an approximate expression of a curved surface by a quadratic expression, and does not assume the optical characteristics of the surface to be measured 5a, for example, the radius of curvature and the F number. Therefore, even if the shape of the measured surface 5a changes, the positional deviation evaluation value _Pj can be calculated in exactly the same manner. As a result, even if there is a variation in the optical characteristics due to the manufacturing error of the measured surface 5a, the positional deviation evaluation value is not affected.

The adjustment target position calculation unit 24 calculates the confocal state and the cats based on the coordinate z _i acquired by the wavefront measurement unit 21 and the position shift evaluation value P _j at the time of acquisition calculated by the position shift evaluation value calculation unit 22. The estimated values of the adjustment target positions in the eye state are calculated as z _cf and z _ce , respectively.
In this embodiment, the estimated value z _cf (z _ce ) of the adjustment target position is obtained by regression analysis of the data of (P _j , z _j ) to calculate a regression equation z = f (P), and z _cf = f (0) (z _ce = f (0)) is adopted.

The curvature radius calculation unit 25 calculates the curvature radius R from the following expression from the adjustment target position estimated values z _cf and z _ce calculated by the adjustment target position calculation unit 24, and sends the calculation result to the display control unit 27. Is.

R = | z _cf −z _ce | (4)

  The display control unit 27 converts the image data sent from the signal conversion unit 20 into, for example, an NTSC signal and sends it to the display unit 14 to display an interference fringe image on the display unit 14 or to calculate an adjustment target position. Image information and character information of measurement results such as a graph of an approximate curve calculated by the unit 24 and a value of the curvature radius calculated from the curvature radius calculation unit 25 are displayed on the display unit 14.

  The apparatus configuration of the measurement control unit 13 may be realized by using the dedicated hardware for each function described above, but in the present embodiment, the apparatus is configured by a computer including a CPU, a memory, an input / output interface, an external storage device, and the like. These functions are realized by executing appropriate control programs by this computer.

Next, the operation of the curvature radius measuring device 50 will be described focusing on the curvature radius measuring method of the present embodiment.
FIG. 4 is a flowchart showing a measurement flow of the curvature radius measurement method according to the embodiment of the present invention. FIG. 5 is a flowchart for explaining the operation of the wavefront measuring step of the curvature radius measuring method according to the embodiment of the present invention. FIG. 6 is a schematic diagram of a time-series image acquired in the wavefront measurement step of the curvature radius measurement method according to the embodiment of the present invention. FIGS. 7A and 7B are diagrams for explaining an example of the wavefront measurement image group selected in the positional deviation calculation step of the curvature radius measurement method according to the embodiment of the present invention and a selection method of another example, respectively. It is a typical graph. The horizontal axis indicates time-series position information i, and the vertical axis indicates the amount of phase change. FIG. 8 is a graph showing a regression analysis result in the positional deviation calculation step of the curvature radius measurement method according to the embodiment of the present invention. The horizontal axis represents the positional deviation evaluation value P, and the vertical axis represents the coordinate z representing the relative movement position of the surface to be measured.

In the Fizeau type optical system used for the curvature radius measuring apparatus 50, in the confocal state shown in FIG. 1, when the laser light source 1 is oscillated, divergent light of wavelength λ is generated, and collimated lens 2 forms parallel light 30a. Reflected by the beam splitter 3 and incident on the optical axis of the Fizeau lens 4.
The parallel light 30a is divided by the Fizeau surface 4a, and a part thereof is reflected by the Fizeau surface 4a toward the beam splitter 3 and proceeds as reference surface reflected light 30d. Other light is transmitted through the transmission light 30b, is focused by the lens action of the Fizeau lens 4 from being condensed on the condensing position Q _R, it is guided to the measuring surface 5a, a normal of the measurement surface 5a By being incident in the direction, it is reflected as the measured surface reflected light 30c. The measured surface reflected light 30c travels backward along the same optical path as the transmitted light 30b, passes through the Fizeau lens 4, and is emitted to the beam splitter 3 side. At that time, an interference fringe image corresponding to the optical path difference between the measured surface reflected light 30c and the reference surface reflected light 30d is formed on the Fizeau surface 4a.
This interference fringe image is projected onto the imaging surface 7 a of the CCD 7 by the condenser lens 6. Then, the interference fringe image is photoelectrically converted by the CCD 7, sent as an image signal to the measurement control unit 13, and displayed on the display unit 14 via the display control unit 27.

Also, the cat's eye state shown in FIG. 3, the transmitted light 30b incident on the Fizeau lens 4 in the same manner is focused by the lens action of the Fizeau lens 4, it is focused on the surface apex Q _P of the measuring surface 5a . Then, the measured surface reflected light 30c reflected by the measured surface 5a passes through the Fizeau lens 4 and is emitted as parallel light to the beam splitter 3 side. At that time, an interference fringe image corresponding to the optical path difference between the measured surface reflected light 30c and the reference surface reflected light 30d is formed on the Fizeau surface 4a.
This interference fringe image is displayed on the display unit 14 in the same manner as in the confocal state.
When the relative movement position of the surface to be measured 5a is deviated in the optical axis C direction from the confocal state or the cat's eye state, a different optical path difference is generated depending on the position deviation amount or the light beam height. An interference fringe image having an increased number of interference fringes compared to the eye state is formed.

In the radius of curvature measurement method of this embodiment, the surface to be measured 5a is relatively moved to the vicinity of the respective adjustment target positions in the confocal state and the cat's eye state, and the wavefront measurement step, the positional deviation evaluation value calculation step, and the adjustment target position This is a method of calculating the curvature radius R of the surface 5a to be measured from the difference between the estimated values z _cf and z _ce of the adjustment target position calculated in each adjustment target position calculation step in order. Hereinafter, description will be given with reference to the flowchart shown in FIG.

First, in step S1, the surface to be measured 5a is relatively moved so that the relative movement position z of the surface to be measured 5a is close to the adjustment target position in the confocal state.
In other words, the measurer sets the length of the piezo element 9 to the shortest initial value through the operation unit 15 and adjusts the amount of movement of the linear motion stage 16 to bring the measured object holding unit 11 into the measurement start position z _1. Move to.
Accordingly, the center of curvature Q _L of the measuring surface 5a is arranged near the condensing position Q _R.

In the case where the measurement starting position z ₁ is not known in advance, the first in this step, measuring person, it drives the linear movement stage 16 while watching the interference fringe image shown in the display unit 14, the δ actually measuring the extent performs an operation for obtaining the measurement start position z ₁ and.

In steps S2 to S4, a wavefront measurement process at the adjustment target position in the confocal state is performed. This step, at least two positions near adjusting a target position in the optical axis direction, to measure the wavefront h _j based on the image information of the interference fringe, the information of the relative movement position of the measuring surface 5a that corresponds to the wavefront h _j a step of acquiring the coordinate z _j is.
In step S2, during the rise time T, it is moved continuously toward the only condensing position Q _R side measurement object holder 11 from the measurement starting position z ₁ distance δ by using a piezoelectric element 9, the sampling period For each ΔT, m interference fringe images are sequentially acquired, and time-series images F ₁ , F ₂ ,..., F _m are stored in the storage unit 23 together with time-series position information i representing the acquisition order. 6).

The operation of this step will be described with reference to FIG. In the present embodiment, a case where δ = 1.3λ / 2 and T = 1.3 (s) will be described as an example.
The sampling period ΔT is set to ΔT = 1/30 (s) in accordance with the video rate 30 fps of the CCD 7 of the present embodiment. That is, in this example, each frame image during the time T is captured and a time-series image with m = 40 is acquired.

First, as shown in FIG. 5, in step S20, a command voltage for extending the piezo element 9 by δ at the rising time T is sent from the movement control unit 22 to the calculation unit 35 of the piezo element controller 10. A movement start signal 100 is sent out. Then, on the measurement control unit 13 side, the process proceeds to step S21.
On the piezo element controller 10 side, when information on the rise time T and the command voltage is received from the movement control unit 22, step S23 is executed.
In step S23, the calculation unit 35 performs setting so that applied voltage data proportional to time is output from 0 V to the command voltage within the rising time T. When the movement start signal is received, the timer 36 is initialized, and step S24 is executed.
In step S <b> 24, the applied voltage data is converted into a voltage signal by the D / A converter 37 according to the value of the timer 36, amplified appropriately by the amplifier 38, and applied to the piezo element 9.
In step S25, it is determined whether or not the value of the timer 36 has exceeded the rise time T. If the value has not exceeded the rise time T, the process proceeds to step S24.
When the rise time T is exceeded, the movement control unit 22 is notified of the movement end by sending the movement end signal 101, the voltage output is stopped, and the position of the piezo element 9 is returned to the initial state.

In step S21, the wavefront measuring unit 21 acquires the interference fringe image from the CCD 7 in synchronization with the sampling period ΔT, acquires the coordinates z _i from the laser length measuring device 12, and the value of the counter i representing the image acquisition order. At the same time, it is stored in the storage unit 23.
In step S22, the presence / absence of the movement end signal 101 from the piezo element controller 10 is determined. If the movement end signal 101 is not received, the process returns to step S21.
If the movement end signal 101 has been received, the operation in step S2 in FIG. 4 is terminated, and the process proceeds to step S3.
In this way, by repeating step S21, the measured object holding unit 11 is moved by the piezo element 9 and the measured surface 5a is moved relative to the Fizeau surface 4a. Along with the information of the counter i and the coordinates z _i , m time-series images F _i are sequentially stored in the storage unit 23.

Next, in step S3, first, a wavefront measurement image _Fj is selected from the _m time-series images F ₁ , F ₂ ,..., Fm acquired in step S2, and this wavefront measurement image F _{j is} further selected. Interference fringe images that can be regarded as having a phase difference of −π, −π / 2, + π / 2, + π with respect to the phase difference are selected, and a wavefront measurement image group G _j = {F _{j2 −} , F _{j1. -, F j, F j1 +} , selects the _{F j2 +}.}
Here, the subscript jk ± (where k = 1, 2) is acquired using the relationship that a change in λ / 8 at the coordinate z becomes a phase change of π / 2. For example, z (j, k) = z _j ± k · (λ / 8) is calculated, z (j, k) is compared with each coordinate z _i stored in the storage unit 23, and | z _i − z (j, k) | is set to jk ± as a minimum value of i that is equal to or smaller than an allowable value.
If a value of _{i for which} | z _i −z (j, k) | is less than or equal to an allowable value is not found, it is determined that the wavefront measurement image group G _j cannot be selected for that j.
For example, when the phase change curve representing the relationship between the position information i on the time series and the phase change amount is represented by the phase change curve 41 shown in FIGS. 7A and 7B, the wavefront measurement image is expressed as F. ₁₆ , the coordinate z ₁₆ is compared with each coordinate z _i to obtain j 2 = 1, j 1 = 10, j 1 + = 21, j 2 + = 26 (see FIG. 7A), and G ₁₆ = {F ₁ , F ₁₀ , F ₁₆ , F ₂₁ , F ₂₆ }.
Further, when the wavefront measurement image is F ₂₉ , the coordinates z ₂₉ are compared with each z _i to obtain j 2 = 20, j 1 = 25, j 1 + = 34, j 2 + = 38 (FIG. 7 (b )), G ₂₉ = {F ₂₀ , F ₂₅ , F ₂₉ , F ₃₄ , F ₃₈ }.
As can be seen from FIG. 7 (a), (b) , in the present embodiment, the image group _{G j} for wavefront measurement _can elect 14 sets up G 16 _{~G 29.}

The selection method of the wavefront measurement image group G _j described above is an example. For example, when the subscript j2- is changed from 1 to m, z _j2- and approximately π / 2, π, 3π / F _j1- , F _j , F _{j1 +} , F _{j2 +} corresponding to the subscripts j1-, j, j1 +, j2 + are selected from z _i that can be regarded as having a phase difference of 2, _2π . A method such as

In the example of this embodiment, [pi / 2, that is, the moving distance of lambda / 8 are divided 9-4, the measurement position accuracy of the individual time-series images F _i is, lambda / 8 1 / 18-1 / 8, that is, about 4.4 nm to 9.9 nm, but this is only an example, and it may be possible to use a CCD with a higher video rate or increase the rise time T of driving the piezo element 9. Te, by reducing the sampling period [Delta] T, it is possible to further reduce the variation of the phase increment in the image group G _j for wavefront measurement.

Next, in step S4, obtaining the wavefront h _j measured from the wavefront measurement image group G _j that is selected in step S3.
The wavefront measurement portion _{21, G j = {F j2-} , F j1-, F j, F j1 +, F j2 +} using to calculate the wavefront _{h j} by 5 bucket method. That is, for each position (x, y), the formula (2), (3) was used to calculate the wavefront _{h j,} and the wavefront _{h j,} in association with each coordinate _{z j} corresponding to the wavefront _{h j} Store in the storage unit 23.
This is the end of the wavefront measurement process.

In the next steps S5 and S6, a positional deviation evaluation value calculation step at the adjustment target position in the confocal state is performed. This step analyzes the wavefront h _j measured by the wavefront measuring step, the wavefront h _j of the relative movement position of the measuring surface 5a corresponding to the position deviation corresponding to a deviation amount from the adjustment target location estimate P _This is a step of calculating _j .
First, in step S5, the wavefronts _{h j} approximated by applying the above equation (1), approximation coefficients _C 0j corresponding to _j, C _1j, C _2j, seek _{C 3j.}
Next, in step S6, the calculated position shift evaluation value P _j from approximation coefficients calculated in step S5, the storage unit 23 in association with the wavefront h _j and coordinates z _j. In the present embodiment, each approximate coefficient C _3j of the quadratic term is set as the positional deviation evaluation value P _j as in the following equation (5).

P _j = C _3j (5)
Approximation coefficients C _3j is a coefficient of the second order term of the above formula (1), which is the power component of the wavefront is calculated from the wavefront measurement image group G _j.
This completes the positional deviation evaluation value calculation step.

Next, in steps S7 and S8, an adjustment target position calculation step is performed. This step includes a coordinate z _j which is information of a relative movement position of the measuring surface 5a that corresponds to the wavefront h _j, the positional shift evaluation value P _j corresponding to the wavefront h _j calculated by the positional shift evaluation value calculation step Thus, an estimated value of the adjustment target position is calculated.
First, in step S7, the adjustment target position calculation unit 24 performs regression analysis on the data group (P _j , z _j ) composed of the coordinates z _j and the positional deviation evaluation value P _j stored in the storage unit 23 and performs regression. Find the formula.
In the present embodiment, the data group (P _j , z _j ) is well approximated by a regression line. Therefore, a regression equation such as the following equation (6) is obtained with the regression coefficients α and β.
z = f (P) = β · P + α (6)

The adjustment target position calculation unit 24 sends an image obtained by plotting the obtained regression line and the data group (P _j , z _j ) on a graph to the display control unit 27, as shown in FIG. It is displayed on the display unit 14. The regression line is displayed as a straight line 42, for example. In FIG. 8, for the sake of simplicity, the subscript j of (P _j , z _j ) is changed to j = 1,...
Thus, the measurer can determine whether the straight line 40 is a regression line that approximates the data group (P _j , z _j ) well.
For example, in the example shown in FIG. 8, it can be seen that the point (P ₁₀ , z ₁₀ ) is significantly deviated from the distribution of other measured values. As a result, the straight line 42 is not a good approximate straight line for most data.
In this embodiment, in such a case, the measurer removes an abnormal value such as a point (P ₁₀ , z ₁₀ ) from the data group through the operation unit 15 and performs an operation for regression analysis again. You can input. A straight line 43 shown in FIG. 8 shows a regression line obtained by excluding the point (P ₁₀ , z ₁₀ ).

Then, calculated in step S8, in the above formula based on the final regression analysis results again regression analysis as necessary (6), as P = 0, the estimated value z _cf the adjustment target position of the confocal condition Then, z _cf = f (0) is sent to the curvature radius calculation unit 25.
Thus, the adjustment target position calculation process is completed.

Next, in steps S9 to S16, the estimated value z _ce of the adjustment target position in the cat's eye state is obtained by performing the steps performed to obtain the estimated value z _cf of the adjustment target position in the confocal state in steps S1 to S8. Is what we do. Therefore, the points different from the steps S1 to S8 will be mainly described.

Step S9 is a step for relatively moving the measurement surface 5a such that the relative movement position z of the measuring surface 5a is near the position of the adjustment target position of the cat's eye state, the measurement start position z ₁ is only different, The same as step S1 above. The measurement start position z ₁ may be determined by actually measuring a range in which sufficient contrast of the interference fringes can be obtained in the same manner as in step S _{1. In} the present embodiment, the measurement start position z ₁ is set at the measurement start position z ₁ in step S _1. A value obtained by adding the design radius of curvature of 5a is used. Also, δ can adopt the same value as in step S1.
Thus, a surface apex Q _R of the measuring surface 5a is arranged near the condensing position Q _R.

Next, steps S10 to S12 are wavefront measurement steps, which are the same steps as steps S2 to S4.
The next steps S13 and S14 are a positional deviation evaluation value calculation process, which is the same process as steps S5 and S6.
However, acquired in step S10, S12, S14, time-series images _{F i,} and the wavefront _{h j,} coordinate _{z j} corresponding to the wavefront _{h j,} the data such as position shift evaluation value _{P j,} the steps S2, S4 , Stored in the storage unit 23 separately from each data acquired in S6.

Next steps S15 and S16 are adjustment target position calculation steps, which are the same steps as steps S7 and S8.
Thereby, the regression equation of the above equation (6) is obtained using the coordinates z _j and the positional deviation evaluation value P _j corresponding to the wavefront h _j acquired in steps S10 and S14, and the cat's eye state is obtained from this regression equation. of the estimated value _{z ce} of the adjustment target position _is calculated as z ce = f (0), and sends to the radius of curvature calculating section 25.

Next, in step S17, the curvature radius calculation unit 25 calculates the curvature radius R of the measured surface 5a from the above equation (4) using the estimated values z _cf and z _ce calculated in steps S8 and S16. . Then, information on the calculated curvature radius R is sent to the display control unit 27 and displayed on the display unit 14.
Thus, the curvature radius measurement method using the curvature radius measurement device 50 of the present embodiment is completed.

Thus, according to the curvature radius measuring apparatus 50 of the present embodiment, the relative movement position of the surface to be measured 5a is relatively moved to the vicinity of the adjustment target position in the confocal state and the cat's eye state, and the surface to be measured 5a is moved. While moving δ in the direction along the optical axis C, m time-series images F _i are acquired, coordinates z _i at the time of acquisition are acquired, and the estimated value z of the adjustment target position is obtained from the plurality of data. _cf and z _ce are calculated, and the radius of curvature R can be calculated from the above equation (4).
Since the estimated values z _cf and z _ce are estimated as values that minimize the error by regression analysis from a plurality of data groups (P _j , z _j ), it is possible to improve the measurement accuracy of the radius of curvature.
In addition, it is not necessary to perform precise position adjustment as in the case of relative movement of the relative movement position of the measurement target surface 5a toward the adjustment target position while viewing the interference fringe image. As a result, the position adjustment can be greatly reduced. Therefore, inspection efficiency can be improved.
Further, as in the case of obtaining a defocus coefficient or the like from the interference fringe image at a position shifted from the adjustment target position to obtain the absolute value of the positional deviation amount, information on the optical characteristics of the measurement target surface 5a is used, or interference fringes are used. Since it is not necessary in the present embodiment to convert the distance on the image into an actual distance, for example, when the optical characteristic value of the measured surface 5a is different from the design value due to a manufacturing error or the like, or the distance on the interference fringe image The influence of the error included in the converted value of can be eliminated, and highly accurate measurement can be performed. For the same reason, even when switching to the measurement of the measuring surface 5a of different shapes, only by changing the measurement start position z ₁ in the mobile process, since the conditions of processing do not need to be changed, labor for changing setup Inspection efficiency can be improved.

Next, a curvature radius measuring device according to a first modification of the present embodiment will be described.
FIG. 9 is a schematic configuration diagram showing a schematic configuration of a curvature radius measuring apparatus according to a first modification of the embodiment of the present invention. FIG. 10 is a functional block diagram showing the functional configuration of the control means of the curvature radius measuring apparatus according to the first modification of the present invention.

As shown in FIGS. 9 and 10, the curvature radius measuring device 50 </ b> A of this modification example is replaced with the piezoelectric element 9 </ b> A, the piezoelectric element controller 10, and the measurement control unit 13 of the curvature radius measuring device 50 of the above embodiment. (Relative movement mechanism), a piezo element controller 10A, and a measurement control unit 13A, the perforated stage 8 is deleted, and a strain gauge 9a (relative movement position measurement unit) and a support plate 17 are added.
The measurement control unit 13A is obtained by replacing the wavefront measurement unit 21 of the measurement control unit 13 of the above embodiment with a wavefront measurement unit 21A.
Hereinafter, a description will be given focusing on differences from the above embodiment.

The support plate 17 is fixed to the apparatus main body of the curvature radius measuring device 50 (not shown) between the DUT 5 and the Fizeau lens 4, and is formed on a flat support surface 17 b orthogonal to the optical axis C on the Fizeau lens 4 side. The annular flat plate member is formed and provided with a through-hole 17a having a size that allows the transmitted light 30b and the measured surface reflected light 30c to pass through the optical axis C.
The piezo element 9A has a ring shape surrounding the through-hole 17a, one end is fixed on the support surface 17b of the support plate 17, and can be expanded and contracted in the direction along the optical axis C. At the other end of the piezo element 9A, a holding portion for holding the Fizeau lens 4 is provided so that the Fizeau lens 4 can be moved along the optical axis C.
The material and configuration of the piezo element 9A can be the same material and configuration as the piezo element 9 of the above embodiment. As a result, the Fizeau lens 4 can be moved in a direction along the optical axis C with a minute movement sufficiently smaller than λ / 8.
The piezoelectric element 9A is provided with a strain gauge 9a for measuring strain in the direction along the optical axis C.
As shown in FIG. 10, the piezo element 9A is electrically connected to the amplifier section 38 of the piezo element controller 10A, and the strain gauge 9a is electrically connected to the movement amount measuring section 39 of the piezo element controller 10A. .

The piezo element controller 10A includes a calculation unit 35A instead of the calculation unit 35 of the piezo element controller 10 of the above-described embodiment, and includes a movement amount measurement unit 39.
The movement amount measuring unit 39 detects the resistance change of the strain gauge 9a, calculates the strain change, calculates the expansion / contraction amount of the piezo element 9, and sends it to the calculation unit 35A.
The calculation unit 35A is electrically connected to the wavefront measurement unit 21A and the movement amount measurement unit 39. In addition to the same function as the calculation unit 35 of the above embodiment, the calculation unit 35A includes the piezoelectric element 9 measured by the movement amount measurement unit 39. information deformation amount, for example, pre-distortion and in such previously obtained calibration curve showing the relationship between the moving amount, in terms of coordinates z _R of the movement position in the direction along the optical axis C of the Fizeau surface 4a, measured has coordinates z _R that is to be sent to the wavefront measurement portion 21.

As described above, in the present modification, the Fizeau lens 4 is provided so as to be movable minutely, and therefore, the measured object holding portion 11 is held movably only by the linear motion stage 16 as shown in FIG. For this reason, the coordinate z of the length measuring light reflecting portion 11a measured by the laser length measuring device 12 shows only the moving position of the object 5 to be measured, and represents the relative moving position of the surface to be measured 5a in the above embodiment. It is not a thing. Therefore, hereinafter, the coordinate z by the laser length measuring device 12 will be referred to as coordinate z _L again.

The wavefront measuring unit 21A is different from the wavefront measuring unit 21 of the above-described embodiment only in the calculation method of the relative movement position of the measured surface 5a.
The wavefront measuring unit 21A, the following equation and coordinates z _L obtained from the laser length measuring machine 12, using the coordinate z _R obtained from the piezoelectric element controller 10A, a coordinate z that represents the relative movement position of the measuring surface 5a Calculate as follows.
z = z _L −z _R (10)
Here, the positive directions of the coordinates z _L and z _R are assumed to be matched.

The operation of the radius-of-curvature measuring apparatus 50A having the above-described configuration is the same as that of the above-described embodiment except that the coordinate z of the above-described embodiment is different from the above equation (10) only by the wavefront measuring unit 21A. The radius of curvature of the surface to be measured 5a can be measured.
That is, the radius-of-curvature measuring apparatus 50A moves the measured surface 5a and the Fizeau surface 4a separately in the direction along the optical axis C by the linear motion stage 16 and the piezo element 9A, and changes the distance between them. This is an example of relative movement.

Next, a second modification of the present embodiment will be described.
In the description of the above embodiment, the approximation coefficient _C3j , which is a power component when the wavefront is approximated using the equation (1), is used as the positional deviation evaluation value _Pj as in the above equation (5). The evaluation of the positional deviation amount is not limited to this.
For example, using the technique disclosed in Japanese Patent No. 2951366 (see the equations (1) and (2) of the same document), the wavefront is approximated by the following equation (11). As in 12), (2C ₃ -6C ₈ ) that is an approximation coefficient representing the defocus amount may be adopted as the positional deviation evaluation value P _j .

However, (ρ, θ) is polar coordinates when X = ρ cos θ and Y = ρ sin θ.
Further, the subscript j in the expression (12) has the same meaning as in the above expression (5).

Next, a third modification of the present embodiment, which is a modification of the wavefront measurement process, will be described.
In the description of the above embodiment, the time series image F _i and the coordinate z _i are acquired by one relative movement in one direction in the vicinity of the adjustment target position in the optical axis direction, and 2 from the acquired time series image F _i. The wavefront h _{j at} the M position above the position and the corresponding z coordinate z _j were measured.
In this modified example, a step corresponding to the phase increment Δφ is disclosed in each of two or more M positions in the vicinity of the adjustment target position in the optical axis direction, as disclosed in, for example, Japanese Patent Laid-Open No. 2000-275007. Jo to by changing the relative movement position, the measured wavefront h _j acquires the n pieces of interference fringe images whose phase is shifted, by measuring the z-coordinate z _j corresponding to the wavefront h _j, 2 position The wavefront h _{j at} the M position and the corresponding z coordinate z _j are measured.
The z coordinate z _j is preferably obtained by actually measuring the z coordinate of the position corresponding to the wavefront h _j (for example, if the 5-bucket method is used, the instantaneous z coordinate at the time of acquiring the third image is actually measured). but if the apparatus constitutively difficult to actually measure the z-coordinate at the time or closing from the phase shift, those z-coordinate in terms by a distance corresponding to a phase difference with respect to the wavefront h _j, acquires the z-coordinate z _j May be.

Next, a fourth modification of the present embodiment, which is a modification of the wavefront measurement process, will be described.
In this modification, a carrier method (spatial carrier method) classified into a spatial phase shift method is used for wavefront measurement, not the fringe scan method. In this carrier method, a reference lens Fizeau lens 4 is adjusted by a tilt stage (not shown) so that interference fringes are tilted with respect to the optical axis so that, for example, a predetermined number of 20 to 30 interference fringes appear. This is a technique for acquiring the wavefront and measuring the wavefront from one interference fringe image by performing Fourier transform. The carrier method is disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-86057.
According to this modification, even when measuring wavefronts at two or more M positions, it is only necessary to acquire M interference fringe images, so that the wavefront can be measured quickly.

  In the above description, an example in which a Fizeau interferometer is used as an interferometer has been described. However, as long as a phase-shifted interference fringe image can be acquired, the type of interferometer is not limited to these interferometers. For example, a Twiman Green interferometer, a Mach-Zehnder interferometer, or the like can be preferably used.

  In the above description, the wavefront is calculated using the so-called 5-bucket method in which n = 5. However, the 7-bucket method and the 9-bucket method in which n = 7, 9, and 13 are used. The 13 bucket method may be adopted. If n is 3 or more, all known calculation methods used in the fringe scan method and the phase shift method can be similarly employed.

  In the above description, a regression line is used as a regression equation for regression analysis. However, depending on the characteristic of the misalignment evaluation value, an appropriate regression equation that matches the characteristic can be used. For example, a high-order polynomial such as a quadratic expression or a cubic expression, or a function other than a polynomial may be used. That is, the positional deviation evaluation value may not be linearly related to the positional deviation amount.

In the above description, in the vicinity of the position adjustment target position, 14 wavefronts _hj are obtained from 14 sets of wavefront measurement image groups _Gj , and 14 sets of data groups ( _Pj , z) corresponding thereto are obtained. the _j) by regression analysis, to estimate the position adjustment target position, this is an example, and the wavefront h _j, according to the measurement accuracy of the coordinate z _j, the number may be fewer or more.
In addition, the estimated value of the position adjustment target position may be estimated without necessarily performing regression analysis. For example, if the wavefront h _j at two positions in the vicinity of the position adjustment target position in the optical axis direction is obtained, a straight line is determined from two sets of (P _j , z _j ). Therefore, as an intersection of this straight line and z = 0 Then, an estimated value of the position adjustment target position is obtained. Therefore, the wavefront may be measured at at least two positions in the vicinity of the position adjustment target position in the optical axis direction.

In the above description, the time series image F _j is described as an example in which the phase difference between each other is acquired as m images whose phase difference is smaller than Δφ. However, the time series image F _j has at least two wavefronts. It is only necessary to obtain n interference fringe images that can be regarded as having a phase difference of ± Δφ · K with respect to the measurement image. For example, the image is sampled at a sampling period corresponding to the phase increment Δφ ( It may be n + 1) or more interference fringe images.

  In the above description, in order to efficiently perform wavefront measurement, the relative movement of the surface to be measured in the wavefront measurement process has been described as an example in which the relative movement is continuously performed in one direction. Can be moved intermittently at an appropriate movement pitch.

  In the description of the first modification, the example in which the strain gauge 9a is used as the relative movement position measurement unit for measuring the relative movement position of the Fizeau surface 4a has been described. The present invention is not limited to this as long as the axial movement amount can be detected. For example, a capacitance sensor, a glass scale, a laser length measuring device, or the like may be employed.

  In the above description, the piezoelectric element 9, 9A, which is a piezoelectric element, is described as an example of a relative movement mechanism that performs minute movement. However, the mechanism is not limited to a mechanism that includes only a piezoelectric element. A mechanism using a piezoelectric element in part may be used, or a minute driving mechanism other than the piezoelectric element may be adopted.

  In the above description of the embodiment and the third modification, an example in which an interference fringe image having a phase difference is acquired by moving the relative movement position of the measurement target surface 5a has been described. The wavefront may be measured by the n-bucket method by optically shifting the phase by π / 2 and acquiring n interference fringe images at the same time.

It is a schematic block diagram which shows schematic structure of the curvature-radius measuring apparatus which concerns on embodiment of this invention, and arrangement | positioning of a confocal state. It is a functional block diagram which shows the function structure of the control means of the curvature radius measuring apparatus which concerns on embodiment of this invention. It is a schematic block diagram which shows arrangement | positioning of the cat's eye state of the curvature-radius measuring apparatus which concerns on embodiment of this invention. It is a flowchart which shows the measurement flow of the curvature radius measuring method which concerns on embodiment of this invention. It is a flowchart explaining operation | movement of the wave front measurement process of the curvature-radius measuring method which concerns on embodiment of this invention. It is a schematic diagram of the time series image acquired at the wavefront measurement process of the curvature radius measuring method according to the embodiment of the present invention. It is a typical graph for demonstrating the selection method of an example of the image group for wavefront measurement selected at the position shift calculation process of the curvature-radius measuring method concerning the embodiment of the present invention, and other examples. It is a graph which shows the regression analysis result in the position shift calculation process of the curvature radius measuring method which concerns on embodiment of this invention. It is a schematic block diagram which shows schematic structure of the curvature-radius measuring apparatus which concerns on the 1st modification of embodiment of this invention. It is a functional block diagram which shows the function structure of the control means of the curvature-radius measuring apparatus which concerns on the 1st modification of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Collimate lens 3 Beam splitter 4 Fizeau lens 4a Fizeau surface 5a Surface to be measured 6 Condensing lens 7 CCD
7a Imaging surface 9, 9A Piezo element (relative movement mechanism)
9a Strain gauge (relative movement position measurement unit)
10, 10A Piezo element controller 11 DUT holding unit 12 Laser length measuring device (relative movement position measuring unit)
13, 13A Measurement control unit 16 Linear motion stage (relative movement mechanism)
21, 21A Wavefront measurement unit 22 Position deviation evaluation value calculation unit 23 Storage unit 24 Adjustment target position calculation unit 25 Radius of curvature calculation unit 30a Parallel light 30b Transmitted light 30c Surface to be measured reflected light (measurement light)
30d Reference surface reflected light (reference light)
50, 50A Curvature radius measuring device 51 Interferometer F _i , F _j Time series image G _j Wavefront measurement image group P _j Position shift evaluation value Q _L Center of curvature Q _P plane top Q _R Condensing position z, z _i coordinates ( Relative movement position of measured surface)
z ₁ Measurement start position

Claims (8)

An interferometer divides one light beam into measurement light and reference light that interferes with the measurement light, and irradiates the divided measurement light toward a measurement surface that is a curved surface by converging at a condensing position. And moving the surface to be measured relative to the focusing position of the measurement light, image information of interference fringes formed by the reflected light on the surface to be measured and the reference light, and the surface of the surface to be measured A curvature radius measurement method for obtaining a curvature radius of the surface to be measured from information on a relative movement distance,
The relative position in the optical axis direction of the measurement surface relative to the measurement light condensing position is referred to as the relative movement position of the measurement surface, and the condensing position of the measurement light is the center position of curvature of the measurement surface. And the relative movement position of the surface to be measured in the cat's eye state in which the focusing position of the measurement light coincides with the top of the surface to be measured is referred to as an adjustment target position, respectively.
For each adjustment target position of the confocal state and the cat's eye state,
A wavefront for measuring a wavefront based on image information of the interference fringes at at least two positions in the vicinity of the adjustment target position in the optical axis direction, and acquiring information on a relative movement position of the measured surface corresponding to the wavefront Measuring process;
A positional deviation evaluation that analyzes the wavefront measured in the wavefront measuring step and calculates a positional deviation evaluation value corresponding to a deviation amount from the adjustment target position of the relative movement position of the measured surface corresponding to the wavefront. A value calculation process;
Based on the information on the relative movement position of the surface to be measured corresponding to the wavefront acquired in the wavefront measurement step and the positional deviation evaluation value calculated in the positional deviation evaluation value calculation step, the adjustment target position An adjustment target position calculation step for calculating an estimated value,
The curvature radius is obtained from the difference between the estimated value of the adjustment target position in the confocal state and the estimated value of the adjustment target position in the cat's eye state calculated in the adjustment target position calculation step, respectively. Curvature radius measurement method. In the positional deviation evaluation value calculation step,
The curvature radius measurement according to claim 1, wherein the wavefront is approximated by a quadratic expression expressed by the following equation, and an approximation coefficient of a quadratic term of the quadratic equation is used as the displacement evaluation value. Method.
Z (X, Y) = C ₀ + C ₁ · X + C ₂ · Y + C ₃ · (X ² + Y ² ) (1)
Here, (X, Y, Z) is the coordinate value of the wavefront in the XYZ rectangular coordinate system in which the optical axis direction is the Z-axis direction, C ₀ is a constant term, C ₁ and C ₂ are primary terms, and C ₃ Is an approximation coefficient of a quadratic term. In the wavefront measuring step,
Moving the relative movement position of the surface to be measured in one direction of the optical axis;
During this movement, n interference fringe images that can be regarded as having a phase difference of ± Δφ · K, where Δφ is a constant phase increment (where K is 0 ≦ K, where N is an integer equal to or greater than 1) ≦ N integer, n is a plurality of interference fringe images from which at least two sets of n = 2N + 1) can be selected are acquired as time-series images, and the measured surface at the time of each acquisition of the time-series images Comprising a step of acquiring information of a relative movement position;
After the step, the at least two sets of the n interference fringe images are selected from the time-series images, and based on the image information of the selected n interference fringe images, the vicinity of the adjustment target position is selected. 3. The method of measuring a radius of curvature according to claim 1, wherein wavefronts at at least two positions are measured. In the wavefront measuring step,
The time-series image is acquired by moving a relative movement position of the measurement surface with a movement pitch corresponding to a phase increment smaller than the constant phase increment Δφ. Curvature radius measurement method. In the wavefront measuring step,
The relative movement position of the surface to be measured is moved in the optical axis direction, and a constant phase increment is Δφ, and n interference fringe images (where K is a phase difference) can be regarded as having a phase difference of ± Δφ · K. , N is an integer of 1 or more, and 0 ≦ K ≦ N, n is n = 2N + 1), and at least the interference fringe image corresponding to K = 0 is acquired. Obtaining information of relative movement positions, and measuring one wavefront based on image information of the n interference fringe images,
3. The method of measuring a radius of curvature according to claim 1, wherein the step is repeated at at least two positions near the adjustment target position. In the adjustment target position calculation step,
The estimated value of the adjustment target position is calculated by performing regression analysis of information on the relative movement position of the measurement target surface corresponding to the wavefront and the positional deviation evaluation value. The curvature radius measuring method according to any one of the above. In the adjustment target position calculation step,
After obtaining a regression equation by the regression analysis, when data having a large divergence from the regression equation of the data used for the regression analysis occurs, the data having the large divergence is excluded, and the regression analysis is performed again. The method of measuring a radius of curvature according to claim 6. An interferometer divides one light beam into measurement light and reference light that interferes with the measurement light, and irradiates the divided measurement light toward a measurement surface that is a curved surface by converging at a condensing position. And moving the surface to be measured relative to the focusing position of the measurement light, image information of interference fringes formed by the reflected light on the surface to be measured and the reference light, and the surface of the surface to be measured A radius-of-curvature measuring device for obtaining a radius of curvature of the surface to be measured from information on a relative movement distance,
The relative position in the optical axis direction of the measurement surface relative to the measurement light condensing position is referred to as the relative movement position of the measurement surface, and the condensing position of the measurement light is the center position of curvature of the measurement surface. And the relative movement position of the surface to be measured in the cat's eye state in which the focusing position of the measurement light coincides with the top of the surface to be measured is referred to as an adjustment target position, respectively.
A relative movement mechanism that relatively moves the relative movement position of the measurement target surface to at least two positions in the vicinity of each of the adjustment target positions in the optical axis direction in the confocal state and the cat's eye state;
A relative movement position measuring unit for measuring a relative movement position of the surface to be measured;
The relative movement mechanism relatively moves the relative movement position of the surface to be measured to at least two positions in the optical axis direction in the vicinity of the adjustment target position, and measures the wavefront based on the image information of the interference fringes. A wavefront measuring unit that stores the information together with information on the relative movement position of the surface to be measured corresponding to
A positional deviation evaluation that analyzes the wavefront measured by the wavefront measuring unit and calculates a positional deviation evaluation value corresponding to a deviation amount from the adjustment target position of the relative movement position of the measurement target surface corresponding to the wavefront. A value calculator,
Based on the information on the relative movement position of the surface to be measured corresponding to the wavefront acquired by the wavefront measuring unit and the positional deviation evaluation value calculated by the positional deviation evaluation value calculating unit, An adjustment target position calculation unit for calculating an estimated value,
The curvature radius is obtained from the difference between the estimated value of the adjustment target position in the confocal state calculated by the adjustment target position calculation unit and the estimated value of the adjustment target position in the cat's eye state. measuring device.

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