JP2011127966A - Interference measurement apparatus and interference measurement method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To extend a dynamic range in the height direction without the use of a movement mechanism of a measurement object. <P>SOLUTION: The interference measurement apparatus includes: generation means (9, 20) for generating light flux having finite coherence lengths; a branch means (22) for branching the light flux into two light fluxes, irradiating a measurement target plane (25a) with one light flux, and irradiating a reference plane (24a) with the other light flux; integration means (22, 21) for integrating the measurement light flux through the measurement target plane and the reference light flux through the reference plane on the same optical path, and generating integrated light flux; a scan means (20) for scanning a coherence length of the light fluxes branched by the branch means; measurement means (27, 30) for obtaining relations information indicating a relationship between a scan position of the coherence length and an intensity of the integrated light flux; and a calculation means (30) for obtaining the height information of the measurement target plane as a boundary between a scan range in which the measurement light flux and the reference light flux are interfered with each other and a scan range in which the measurement light flux and the reference light flux are not interfered with each other based on the relation information. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an interference measuring apparatus and an interference measuring method for measuring the height and shape of a measurement target surface.

  Patent Document 1 discloses a surface shape measuring apparatus using the principle of a wavelength scanning interferometer. This device uses the fact that the change curve of the interference signal with respect to the wavelength varies depending on the height of the measurement target surface. Specifically, the interference fringe image is repeated while scanning the wavelength of the light with a variable wavelength light source. By obtaining the measurement, the change curve of the interference signal for each position on the measurement target surface is measured, and patterns that are close to the measured change curve are selected from the theoretical values (theoretical curves) of the change curves for various heights. By finding it by matching, the height of the position is obtained.

Japanese Patent No. 3861666

  However, if the height difference of the measurement target surface is larger than a predetermined value determined by the coherence distance of light, a significant interference signal cannot be obtained for some positions, so the surface shape must be measured at once. (See paragraph [0011] of Patent Document 1). Therefore, in this case, it is necessary to repeat the measurement by moving the measurement object in the optical axis direction, and it is necessary to allow the movement error of the measurement object to be superimposed on the shape measurement error.

  Therefore, the present invention provides an interference measuring apparatus and an interference measuring method capable of expanding a dynamic range in the height direction without using a moving mechanism of a measurement object.

  An aspect of the interference measuring apparatus illustrating the present invention includes a generating unit that generates a light beam having a finite coherence distance, the light beam is split into two light beams, and one of the two light beams is irradiated onto a measurement target surface. And a branching means for irradiating the other to the reference surface, an integrating means for integrating the measurement light flux passing through the measurement target surface and the reference light flux passing through the reference surface into the same optical path, and generating an integrated light flux, and the branching means Based on the relationship information, scanning means for scanning the coherence distance of the light beam branched by the above, a measurement means for acquiring relationship information indicating a relationship between the scanning position of the coherence distance and the intensity of the integrated light beam, And calculating means for acquiring, as the height information of the measurement target surface, a boundary between a scanning range in which the measurement light beam and the reference light beam interfere with each other and a scanning range in which the measurement light beam and the reference light beam do not interfere with each other.

  One aspect of the interference measurement method exemplifying the present invention is a generation procedure for generating a light beam having a finite coherence distance, the light beam is split into two light beams, and one of the two light beams is irradiated onto a measurement target surface. And a branching procedure for irradiating the other to the reference surface, an integration procedure for integrating a measurement light beam passing through the measurement target surface and a reference light beam passing through the reference surface into the same optical path to generate an integrated light beam, and the branching procedure On the basis of the scanning procedure for scanning the coherence distance of the light beam branched at, the measurement procedure for obtaining the relationship information indicating the relationship between the scanning position of the coherence distance and the intensity of the integrated light beam, And a calculation procedure for acquiring a boundary between a scanning range where the measurement light beam and the reference light beam interfere with each other and a boundary between the scanning range where the measurement light beam and the reference light beam do not interfere as the height information of the measurement target surface.

  ADVANTAGE OF THE INVENTION According to this invention, the interference measuring apparatus and interference measuring method which can expand the dynamic range of a height direction are realized, without using the moving mechanism of a measuring object.

1 is a configuration layout diagram of a surface shape measuring apparatus according to an embodiment. It is a figure explaining the spectrum of the light beam extracted by the wavelength selection element. 6 is a schematic diagram illustrating a change in coherence of a light beam extracted by a wavelength selection element 20. FIG. The relationship between the center wavelength λ and the coherence distance Z _C (Z _C = λ ² / | Δλ |) is shown as a graph. And the center wavelength lambda, is obtained by the relationship between the scanning pitch of the coherence length Z _C in the graph. It is a figure which shows the example of a tunable filter. 3 is a flowchart of measurement processing by a control unit 30. It is a figure which shows the shape of the measurement object surface 25a assumed by simulation. This is an image I ₁₅₁ acquired by the 151st sampling (when the center wavelength λ is set to λ ₁₅₁ = 0.55 μm and the coherence distance Z _C is set to Z _C151 = 3.025 μm). This is the image I ₆₅₁ acquired in the 651st sampling (when the center wavelength λ is set to λ ₆₅₁ = 1.05 μm and the coherence distance Z _C is set to Z _C651 = 11.025 μm). This is an image I ₁₁₅₁ acquired by the _1151st sampling (when the center wavelength λ is set to λ ₁₁₅₁ = 1.55 μm and the coherence distance Z _C is set to Z _C1151 = 24.025 μm). 3 is a flowchart of analysis processing by a control unit 30. It is a figure explaining step S23. It is a figure explaining step S26.

[First Embodiment]
Hereinafter, a surface shape measuring apparatus will be described as a first embodiment of the present invention.

  FIG. 1 is a configuration layout diagram of a surface shape measuring apparatus. As shown in FIG. 1, the surface shape measuring apparatus includes a white light source unit 9, a wavelength selection element 20, beam splitters 21 and 22, a correction plate 23, a reference mirror 24, an imaging optical system 26, an imaging element 27, a control unit 30, and the like. Is provided. Among these, the beam splitters 21 and 22, the imaging optical system 26, and the image pickup device 27 constitute an interferometer. Here, the interferometer type is assumed to be a Michelson type, but it can be changed to other types (Mach-Zehnder type, Mirau type, etc.).

  The white light source unit 9 includes a white light source 9A such as a halogen lamp and a beam expander 9B, and emits a white light beam composed of a parallel light beam having an appropriate diameter.

  The wavelength selection element 20 extracts only a component in a part of the wavelength band from the white light beam emitted from the white light source unit 9. The center wavelength of the wavelength band extracted by the wavelength selection element 20, the width of the wavelength band (wavelength width), and the intensity of each wavelength in the wavelength band can be controlled by the control unit 30, respectively.

  The light beam extracted by the wavelength selection element 20 passes through the beam splitter 21, and then reflects the separation surface 22a of the beam splitter 22 (reference light beam) and the light beam that passes through the separation surface 22a of the beam splitter 22 (measurement light beam). ) And branch.

  The reference light beam reflected by the separation surface 22a passes through the correction plate 23 and then enters the reference surface 24a of the reference mirror 24 from the front. The reference light beam is reflected by the reference surface 24 a to return the optical path, pass through the correction plate 23 again, and then return to the beam splitter 22.

  The measurement light beam transmitted through the separation surface 22a enters the measurement target surface 25a of the measurement target 25 from the front. The measurement light beam reflects the measurement target surface 25 a to return the optical path and return to the beam splitter 22.

  The reference light beam and the measurement light beam that have returned to the beam splitter 22 are integrated at the separation surface 22a, and the integrated light beam is directed toward the beam splitter 21. The integrated light beam reflects from the beam splitter 21 and enters the imaging surface 27 a of the imaging element 27 via the imaging optical system 26.

  The image sensor 27 generates an image showing the luminance distribution on the imaging surface 27a, that is, an image showing the integrated light flux intensity distribution. Note that the timing of image generation (imaging) by the imaging element 27 is controlled by the control unit 30.

Here, the complex amplitude of the wavefront of the reference beam reflected by the reference surface 24a is R _r · u (t), and the complex amplitude of the wavefront of the measurement beam reflected by the measurement target surface 25a is R _s · u (t−τ). Then, the complex amplitude of the wavefront (synthetic wavefront) of the integrated luminous flux is expressed as follows.

However, u (t) is the complex amplitude of those normalized wavefront (basic wave) of the light beam from the white light source 9A, reflectance R _r is the reference surface 24a, in reflectance R _s is the object surface 25a is there. τ represents the arrival time difference between the wave fronts caused by the optical path length difference L between the reference light beam and the measurement light beam, and is represented by τ = L / c (where c is the speed of light). .)

  At this time, the intensity I of the integrated light beam detected by the image sensor 27 is expressed as follows.

  In this equation, the third term represents the interference effect. However, the function Re [X] in the third term means the real part of the complex function of X.

Hereinafter, paying attention to the third term, this is _defined as I _int (τ). Since X of Re [X] in the third term is an autocorrelation function, I _int (τ) is expressed as follows using Wiener Hinchin's theorem.

That is, I _int (τ) corresponds to an inverse Fourier transform of the following energy spectrum.

  However, U (ω) is the Fourier transform of u (t) as follows.

  In this case, the following equation also holds from the relationship of inverse Fourier transform.

  Here, the wavelength selection element 20 of the present embodiment can set the spectrum of the extracted light beam to an arbitrary spectrum. Therefore, it is assumed that the wavelength selection element 20 sets A (ω) described above as follows.

In this case, I _int (τ) is expressed as follows.

  Therefore, the intensity I of the integrated light beam detected by the image sensor 27 is expressed as follows.

  The interference term in this case first becomes zero when | Δω | τ / 2 = π. In this case, if τ = 2π / | Δω | is defined as a coherent time, the following relationship holds.

  However, λ is the center wavelength of the light beam extracted by the wavelength selection element 20, and | Δλ | is the wavelength width of the light beam.

  Therefore, the coherence distance Zc of the light beam extracted by the wavelength selection element 20 is expressed as follows.

  In the present embodiment, as shown in FIG. 2, the center wavelength λ of the light beam extracted by the wavelength selection element 20 is within a predetermined wavelength range (here, 400 nm to 2000 nm) in minute units (here). The wavelength width | Δλ | of the luminous flux is fixed to a predetermined value (here, 100 nm) narrower than the wavelength range. Assume (see definition of A (ω) above). In addition, it is assumed that the intensity of each wavelength component included in the luminous flux is aligned with a predetermined value.

Thus, when the center wavelength λ of the light beam is gradually changed from 400 nm to 2000 nm, the coherence of the light beam gradually increases as shown schematically in FIG. 3, and the coherence distance Z _{C of the} light beam. (Z _C = λ ² / | Δλ |) is scanned from 1.6 μm to 40 μm. FIG. 4 is a graph showing the relationship between the center wavelength λ and the coherence distance Z _C (Z _C = λ ² / | Δλ |).

As described above, when the change pitch of the center wavelength λ is constant (here, 1 nm), the scanning pitch of the coherent distance Z _C (= λ ² / | Δλ |) becomes finer as the center wavelength λ is shorter. . FIG. 5 is a graph showing the relationship between the center wavelength λ and the scanning pitch.

  As the wavelength selection element 20 having the above functions, for example, a rio filter combining a liquid crystal tunable filter and a color conversion filter or a tunable filter as shown in FIG. 6 can be applied. Details of the tunable filter shown in FIG. 6 will be described later.

  The image sensor 27 has sensitivity to each light within the wavelength band (350 nm to 2050 nm in this case) of the apparatus, and other optical elements (beam expander 9B, beam splitters 21 and 22, correction) of the apparatus. As the glass material of the plate 23, the imaging optical system 26, and the like, those capable of guiding each light within the used wavelength band are selected. In order to further increase the interference intensity between the reference light beam and the measurement light beam, it is desirable that the reflectance of the reference mirror 24a be set as close as possible to the reflectance of the measurement object.

  Further, the optical characteristics (dispersion, refractive index, etc.) of the correction plate 23 arranged in the single optical path of the reference beam are such that the deformation given from the correction plate 23 to the wavefront of the reference beam is changed from the beam splitter 22 to the wavefront of the measurement beam. It is preset to be the same as the given deformation. Thereby, the condition of the single optical path of the reference light beam and the condition of the single optical path of the measurement light beam are made common.

  In the vicinity of the measurement object 25, the reference surface 25b optically conjugate with the imaging surface 27a satisfies the following condition. That is, the distance from the reference surface 25b to the separation surface 22a of the beam splitter 22 is the same as the distance from the reference surface 24a to the separation surface 22a of the beam splitter 22.

  Each position of the measurement target surface 25a exists above or below the reference surface 25b. Here, it is assumed that each position of the measurement target surface 25a exists above the reference surface 25b (on the side close to the beam splitter 22).

  Here, attention is paid to a certain position on the measurement target surface 25a (hereinafter, this position is referred to as a “target position”). The integrated light beam incident on the pixel corresponding to the target position (target pixel) on the imaging surface 27a is composed of the reference light beam and the measurement light beam as described above. The optical path length difference Z between these light beams is based on the reference surface 25b. A value (Z = 2Z ′) corresponding to the height Z ′ of the target position is taken.

If the scanning position of the coherent distance Z _C is near the lower end value (1.6 μm) of the scanning range described above, and the optical path length difference Z is longer than the coherent distance Z _C (that is, Z ′> Z by the time) _{the C /} 2 holds, since their reference beam and the measurement light beam do not interfere with each other, the contrast of the intensity of the coherence length Z _C signal pixel of interest is generated during the scanning of zero.

On the other hand, when the scanning position of the coherent distance Z _C is near the upper end value (40 μm) of the scanning range described above, and the optical path length difference Z is shorter than the coherent distance Z _C (that is, Z ′ <Z _C / by the time 2 holds), since their reference beam and measurement beam interfere with each other, the contrast of the intensity of the coherence length Z _C signal pixel of interest is generated during the scanning of no zero.

Accordingly, the present apparatus is adapted to scan toward the lower end value of the scan range described above the coherence length Z _C from (1.6 [mu] m) to the upper value (40 [mu] m), the intensity of the signal generated by the target pixel in the scanning If the contrast is monitored and a scanning position (boundary position Z _b ) corresponding to the boundary between the scanning range in which the contrast changes and the scanning range in which the contrast does not change is found, the height Z ′ of the target position is determined as Z ′ = Z _b / It can be known by the equation of 2. This is true for each position of the measurement target surface 25a.

Incidentally, it expressed scanning range of the coherence length _{Z C} a (1.6μm~40μm) in units of height Z ', in the range of 0.8Myuemu～20myuemu. Therefore, the measurement range in the height direction of this apparatus is a range of 0.8 μm to 20 μm.

  Therefore, at the start of the measurement process described later, the position of the measurement object 25 in the optical axis direction is adjusted so that the height Z ′ of each position of the measurement object surface 25a falls within the range of 0.8 μm to 20 μm. .

  The control unit 30 of this apparatus has a function as a control apparatus and a function as an arithmetic unit.

  The control unit 30 as a control device obtains an image group including height information of each position of the measurement target surface 25a by driving and controlling the light source unit 9, the wavelength selection element 20, and the imaging element 27 (measurement processing). ). In addition, the control unit 30 as a calculation device calculates the height distribution of the measurement target surface 25a by performing calculation on the image group (analysis process).

  Next, the measurement process by the control unit 30 will be described. FIG. 7 is a flowchart of measurement processing by the control unit 30. Hereinafter, each step will be described in order.

  Step S10: The control unit 30 sets the sampling number i to an initial value (1).

  Step S11: The control unit 30 turns on the light source unit 9 and the wavelength selection element 20.

Step S12: The control unit 30 instructs the wavelength selection element 20 to set the center wavelength λ of the light beam to be extracted to the i-th wavelength λ _i . The i-th wavelength λ _i (nm) is represented by λ _i = 400 + (i−1). Thus, the scanning position of the coherence length Z _C is set to i-th scan position Z _Ci. Incidentally, i-th scan position _Z Ci (nm) _is represented by ^{_{Z Ci = λ i 2/100}} .

Step S13: The control unit 30 drives the image sensor 17 and samples an image for one frame. Hereinafter, an image sampled by the i-th sampling is referred to as “image I _i ”.

  Step S14: The control unit 30 determines whether or not the sampling number i has reached the final value (1600 in this case). If the final value has not been reached, the process proceeds to step S15. The process proceeds to S16.

Step S15: The control unit 30 increments the sampling number i and then returns to step S12. Accordingly, the control unit 30 repeats the acquisition of the image _{I i} 1600 times while scanning the coherent length _{Z C.}

  Step S16: The control unit 30 turns off the light source unit 9 and the wavelength selection element 20.

Step S17: The control unit 30 stores the series of images I ₁ ,..., I ₁₆₀₀ acquired in the above steps in the storage memory in the control unit 30 and ends the flow.

Next, a part of the images I ₁ ,..., I ₁₆₀₀ obtained by the measurement process will be described. The images shown in FIGS. 8 to 11 are obtained by simulation.

  In the simulation, the shape of the measurement target surface 25a is set to Z ′ = A · | cos (X) · cos (Y) | as shown in FIG. In FIG. 8, regions having a common height are represented by the same density.

  In the simulation, A for determining the peak value of the measurement target surface 25a was set to 10 μm. In the simulation, it is assumed that the reflectance of the measurement target surface 25a is the same as the reflectance of the reference surface 24a.

FIG. 9 is an image I ₁₅₁ acquired by the 151st sampling. In the 151st sampling, the center wavelength λ was set to λ ₁₅₁ = 0.55 μm, and the coherence distance Z _C was set to Z _C151 = 3.025 μm.

As is clear from FIG. 9, in the image I ₁₅₁ , the interference effect is high in a region lower than the height corresponding to the coherent distance Z _C151 (Z ′ = Z _C151 /2=1.5125 μm), and the average when there is no interference An output having a large difference from the intensity (intensity 2 in the figure) is shown, but in an area higher than that, it is close to the average intensity when there is no interference.

FIG. 10 is an image I ₆₅₁ acquired in the 651st sampling. In the 651st sampling, the center wavelength λ was set to λ ₆₅₁ = 1.05 μm, and the coherence distance Z _C was set to Z _C651 = 11.025 μm.

As is clear from FIG. 10, in the image I ₆₅₁ , the interference effect appears in a region lower than the height corresponding to the coherent distance Z _C651 (Z ′ = Z _C651 /2=5.5125 μm), and there is no interference. Although the difference from the average intensity (intensity 2 in the figure) is large, the intensity is close to the average intensity in a region higher than that, and the interference effect does not appear.

FIG. 11 is an image I ₁₁₅₁ acquired by the 1151st sampling. In the _1151st sampling, the center wavelength λ was set to λ ₁₁₅₁ = 1.55 μm, and the coherence distance Z _C was set to Z _C1151 = 24.025 μm.

As apparent from FIG. 11, in the image I ₁₁₅₁ , the intensity is an average value (ie, all regions) lower than the height corresponding to the coherent distance Z _C1151 (Z ′ = Z _C1151 /2=12.0125 μm). In the figure, the difference from intensity 2) is large, and an interference effect appears.

Next, analysis processing by the control unit 30 will be described. FIG. 12 is a flowchart of analysis processing by the control unit 30. Hereinafter, each step will be described in order.
It is assumed that the measurement process has been executed at the start of the analysis process, and a series of images I ₁ ,..., I ₁₆₀₀ are stored in the storage memory in the control unit 30.

Step S21: The control unit 30 reads out a series of images I ₁ ,..., I ₁₆₀₀ stored in the storage memory onto the work memory of the control unit 30, and sets the pixel number j to the initial value (1). Set.

  Step S22: The control unit 30 sets the sampling number i to an initial value (1).

Step S23: The control unit 30 calculates the contrast value C _ij relating to the pixel number j and the sampling number i by the following procedures (a) to (d).

(A) The control unit 30 refers to a series of signals S _1j ,..., S _1600j relating to the jth pixel in the series of images I ₁ ,..., I ₁₆₀₀ , and as shown schematically in FIG. The signals S _1j ,..., S _1600j are mapped onto the “coordinate space indicating the relationship between the scanning position of the coherent distance Z _C and the signal intensity”. Arrangement pitch across the scanning direction of the mapped signals to the coordinate space is irregular intervals (This is because the scanning pitches of the coherence length Z _C was heterogeneous.). Since FIG. 13 is a schematic diagram, the number of data and the data interval may be different from actual ones.

(B) The control unit 30 assumes, in the coordinate space, a reference range A _i centered on the scanning position Z _Ci corresponding to the sampling number i and having a predetermined size spread in the scanning direction.

(C) The control unit 30 refers to the intensities of a plurality of signals belonging to the reference range A _i among the series of signals S _1j ,..., S _1600j . Note that since the arrangement pitch of the series of signals S _1j ,..., S _1600j is not uniform, the number of signals belonging to the reference range A _i may vary depending on the sampling number i.

(D) the control unit 30 finds the maximum value Imax _ij and the minimum value Imin _ij from among the intensities of a plurality of signals referred to in steps (c), the maximum value Imax _ij and the minimum value Imin _{ij, C ij} = The contrast value C _ij is obtained by fitting to the formula (Imax _ij −Imin _ij ) / (Imax _ij + Imin _ij ).

  Step S24: The control unit 30 determines whether or not the sampling number i has reached the final value (1600 in this case). If the final value has not been reached, the process proceeds to step S25. The process proceeds to S26.

Step S25: The control unit 30 increments the sampling number i and then returns to step S23. Therefore, the control unit 30 sequentially acquires the contrast values C _ij (i = 1 to 1600) regarding all the sampling numbers i of the j-th pixel. FIG. 14 schematically shows examples of acquired contrast values C _{j1 to} C _j1600 . Since FIG. 14 is a schematic diagram, the number of data and the data interval may be different from actual ones.

Step S26: Based on the series of contrast values C _{1j to} C _1600j for the jth pixel, the control unit 30 creates a contrast value change curve in the scanning direction as shown by the curve in FIG. This change curve can be obtained by _performing an interpolation process on a series of contrast values C _{1j to} C _1600j . Alternatively, the change curve can be obtained by fitting a function to a series of contrast values C _{1j to} C _1600j . Then, the control unit 30 finds the sampling number i corresponding to the boundary between the scanning range where the contrast fluctuates greatly and the scanning range where the contrast hardly fluctuates (scanning range where the contrast attenuates) from the created change curve. Is converted into a coherent distance to obtain the boundary position Z _bj . The boundary position Z _bj shows an optical path length difference corresponding to the j-th pixel on the object surface 25a.

Step S27: The control unit 30 applies the boundary position Z _bj related to the j th pixel to the expression Z _j ′ = Z _bj / 2 to thereby obtain the height of the position corresponding to the j th pixel on the measurement target surface 25a. Z _j ′ is calculated.

  Step S28: The control unit 30 determines whether or not the pixel number j has reached the final value (here, 512 × 512 = 262144). If not, the process proceeds to step S29. If so, the process proceeds to step S30.

Step S29: The control unit 30 increments the pixel number j and returns to step S22. Therefore, the control unit 30 sequentially acquires the heights Z _j ′ (j = 1 to 262144) for all the pixel numbers j.

Step S30: The control unit 30 acquires the height distribution data of the measurement target surface 25a by arranging the heights Z _j ′ (j = 1 to 262144) acquired in the above steps in the order of the pixel number j. Then, the control unit 30 visualizes the height distribution data on a monitor (not shown), stores it in a storage memory as necessary, and ends the flow.

Above, the control unit 30 of the apparatus, as well as measuring the relationship between the intensity of the integrated light beam and the scanning position of the coherence length Z _C (FIG. 13), based on the relationship (FIG. 13), the reference beam and the measuring beam There the interfering scanning range, the measurement light beam and said reference light beam is calculated boundary position Z _b of the scanning range that does not interfere (Fig. 14), converted it to the height Z _{j '.}

Therefore, a height direction of the measurement range of the device is not determined by the length itself of the coherence length Z _C, determined by the width of the scanning range of the coherence length Z _C. Specifically, the width of the measurement range in the height direction is 1/2 times larger than the scanning range of the coherence length Z _C.

Accordingly, the present apparatus can be extended only in the measurement range in the height direction to extend the scanning range of the coherence length Z _C. Indeed, in this apparatus, by setting the scanning range of the coherence length _{Z C} to 1.6Myuemu～40myuemu, set the measurement range in the height direction 0.8Myuemu～40myuemu. As a result, the dynamic range in the height direction was 20−0.8 = 19.2 μm.

Incidentally, the control unit 30 of the device has been changed only the wavelength λ of the light beam to scan the coherence length Z _C, the wavelength width | [Delta] [lambda] | only may be changed, also the wavelength λ and the wavelength width Both | Δλ | may be changed.

For example, the control unit 30 of the present apparatus may gradually change only the wavelength λ from 400 nm to 2000 nm and start to gradually narrow the wavelength width | Δλ | toward zero when it reaches 2000 nm. In this way, the scanning coherence length Z _C by two parameters, it is possible to widen the scanning range of the coherence length Z _C without increasing the operating wavelength range of the device.

Further, the control unit 30 of the apparatus, by changing only the wavelength λ of the light beam to scan the coherence length Z _C, and since the uniform the changes pitch, the scanning pitch of the coherence length Z _C is uneven became a coherence length Z _C of the scan pitch is uniform and so as the wavelength λ and the wavelength width | may set a combination of the change pattern | [Delta] [lambda].

[Example of tunable filter]
Hereinafter, the tunable filter shown in FIG. 6 will be described in detail. In FIG. 6, reference numeral 20 denotes a tunable filter.

  Light incident from the light source unit 9 enters the first optical system 1 and is converted into a light beam parallel to the optical axis. This light is perpendicularly incident on the first Wollaston prism 2, and the traveling directions of P-polarized light and S-polarized light are separated. Since the first Wollaston prism 2 is used as the polarization separation element, a high extinction ratio can be obtained as compared with a thin film type polarization beam splitter or a polarization filter. In FIG. 6, the traveling direction of light is indicated by an arrow, and P-polarized light is indicated by an arrow and S-polarized light is indicated by a circle (showing a vibration direction) with reference to the Wollaston prism input / output surface.

  The P-polarized light and the S-polarized light emitted from the first Wollaston prism 2 become parallel lights, which respectively enter the second optical system 3, and the principal ray is converted into convergent light parallel to the optical axis. The light emitted from the second optical system 3 enters the pretilt correction wavelength plate 4 where the pretilt phase of the reflective liquid crystal element array device 6 described later is half compensated. Such a method of using the pretilt correction wavelength plate 4 is well known when the reflective liquid crystal element array device 6 is used.

  The second optical system 3 is an exit side telecentric optical system, and the principal ray of the light emitted from the second optical system 3 enters the pretilt correction wavelength plate 4 perpendicularly. Thereby, the retardation amount by the pretilt correction wavelength plate 4 is prevented from fluctuating depending on the light beam passing angle. In order to obtain such an optical system, the polarization separation point by the first Wollaston prism 2 and the front focal point of the second optical system 3 are substantially matched.

  The light emitted from the pretilt correction wavelength plate 4 converges to the slit 51 of the wavelength dispersion spectrometer 5. The slit 51 is disposed at the rear focal position of the second optical system 3. The slit 51 is thin in the spectral direction of the grating 53 of the wavelength dispersive spectrometer 5 and is elongated in a direction perpendicular thereto. The light transmitted through the slit 51 forms an image of the slit 51 on the grating 53 of the wavelength dispersive spectrometer 5 by the spectrometer collimator 52. That is, the slit 51 and the grating 53 are arranged at the front focal point and the rear focal point position of the spectrometer collimator 52, respectively.

  The light separated by the grating 53 is converted into two spectrum images of the P-polarized light and the S-polarized light by the spectroscope camera optical system 54, respectively, and the first liquid crystal element array of the reflective liquid crystal element array device 6. 61, an image is formed on the second liquid crystal element array 62. That is, the grating 53, the first liquid crystal element array 61, and the second liquid crystal element array 62 are arranged at the front focal position and the rear focal position of the spectroscope camera optical system 54, respectively. As a result, an S-polarized spectrum image is formed on the first liquid crystal element array 61 in the array direction, and a P-polarized spectrum image is formed on the second liquid crystal element array 62 in the array direction. The Since the first liquid crystal element array 61 and the second liquid crystal element array 62 operate independently of each other, it is also possible to compensate for the presence of polarization characteristics in the optical system.

  Further, as can be seen from this optical system, the spectroscope camera optical system 54 is an exit side telecentric, and the principal rays are perpendicular to the first liquid crystal element array 61 and the second liquid crystal element array 62, respectively. Incident. Further, since there is no change in the incident angle according to the wavelength, the retardation given by the liquid crystal element is prevented from changing depending on the light beam passage angle, and is accurate.

  The reflective liquid crystal element array driver 13 adjusts the voltage applied to each unit liquid crystal element (each of which is arranged as an array) of the first liquid crystal element array 61 and the second liquid crystal element array 62. To adjust the retardation of each unit liquid crystal element. As a result, the polarization state (generally, elliptically polarized light) of the light beam reflected from the first liquid crystal element array 61 and the second liquid crystal element array 62 and returning along the incident path is reversed. It can be changed for each unit liquid crystal element, that is, for each wavelength of incident light.

  The light modulated and reflected by the first liquid crystal element array 61 and the second liquid crystal element array 62 is collected on the grating 53 by the spectroscope camera optical system 54, where the light dispersed for each wavelength is reflected. It is collected into two light beams (P-polarized light and S-polarized light). The collected light beam is collected on the slit 51 by the spectroscope collimator 52, passes through the slit 51, and passes through the pretilt correction wavelength plate 4, where the P-polarized light and the S-polarized light are converted into the first Wollaston prism 2. It is focused on.

  The P-polarized light and the S-polarized light focused on the first Wollaston prism 2 are emitted as three light beams. That is, the P-polarized light and the S-polarized light that have not been subjected to retardation by the first liquid crystal element array 61 and the second liquid crystal element array 62 are combined into one light beam by the first Wollaston prism 2 and returned to the incident light. Go. The component of the P-polarized light that has undergone retardation and is converted to S-polarized light, and the component of the S-polarized light that has undergone retardation and has been converted to P-polarized light is further bent in the optical path by the first Wollaston prism 2. As shown in FIG. Then, after being condensed by the first optical system 1, it is converted into parallel light by the third optical system 7 and enters the second Wollaston prism 8. The second Wollaston prism 8 collectively outputs the P-polarized light and the S-polarized light as one light. In the example shown in FIG. 6, the first optical system 1 and the third optical system 7 constitute an afocal optical system.

  Further, the polarization separation point by the first Wollaston prism 2, the wavelength resolving point in the grating 53 used in the wavelength dispersion spectroscope 5, and the polarization combining point by the second Wollaston prism 8 are substantially conjugate. Has been. As a result, the cross-sectional area of the light beam passing through the grating 53, the first Wollaston prism 2 and the second Wollaston prism 8 can be minimized and the size of the expensive element can be reduced.

  In the optical system as described above, the spectrum of the light emitted from the second Wollaston prism 8 changes depending on the retardation received by the first liquid crystal element array 61 and the second liquid crystal element array 62. Therefore, the reflective liquid crystal element The spectrum distribution can be changed by the output of the array driver 13, and the filter functions as a tunable filter.

DESCRIPTION OF SYMBOLS 9 ... White light source part, 20 ... Wavelength selection element, 21, 22 ... Beam splitter, 23 ... Correction plate, 24 ... Reference mirror, 26 ... Imaging optical system, 27 ... Imaging element, 30 ... Control unit

Claims (7)

Generating means for generating a light beam having a finite coherence distance;
Branching means for branching the luminous flux into two luminous fluxes, irradiating one of the two luminous fluxes to the surface to be measured and irradiating the other to the reference surface;
An integration means for generating an integrated light beam by integrating the measurement light beam passing through the measurement target surface and the reference light beam passing through the reference surface into the same optical path;
Scanning means for scanning the coherence distance of the light beam branched by the branching means;
Measuring means for acquiring relationship information indicating a relationship between the scanning position of the coherent distance and the intensity of the integrated luminous flux;
Based on the relationship information, a boundary between a scanning range in which the measurement light beam and the reference light beam interfere and a scanning range in which the measurement light beam and the reference light beam do not interfere is acquired as height information of the measurement target surface. Computing means;
An interference measurement apparatus comprising: The interference measurement apparatus according to claim 1,
The scanning means includes
In order to scan the coherence distance of the light beam, at least one of the center wavelength and the wavelength width of the light beam is changed. In the interference measuring device according to any one of claims 1 and 2,
The computing means is
A scanning range in which the contrast in the scanning direction of the intensity is greater than or equal to a threshold value is regarded as a scanning range in which the measurement light beam and the reference light beam interfere with each other, and scanning in which the contrast in the scanning direction of the intensity is less than the threshold value The height measurement information is obtained by regarding the range as a scanning range in which the measurement light beam and the reference light beam do not interfere with each other. In the interference measuring device according to any one of claims 1 to 3,
The measuring means includes
Obtaining the relation information for each position of the measurement target surface;
The computing means is
The interference measurement apparatus, wherein the height information is acquired for each position of the measurement target surface. The interference measurement apparatus according to claim 4,
The computing means is
A scanning range in which the contrast in the spatial direction of the intensity is equal to or greater than a threshold value is regarded as a scanning range in which the measurement light beam and the reference light beam interfere with each other, and scanning in which the contrast in the spatial direction of the intensity is less than the threshold value. The height measurement information is obtained by regarding the range as a scanning range in which the measurement light beam and the reference light beam do not interfere with each other. In the interference measuring apparatus according to any one of claims 1 to 5,
The reference plane in which the optical path length difference from the reference light beam is zero among the measurement light beams is located on a surface that is out of the measurement target surface,
An interference measuring device, wherein the intensity detection surface by the measuring means is located on a surface conjugate with the reference surface. A generation procedure for generating a light beam having a finite coherence distance;
A branching procedure for branching the light flux into two light fluxes, irradiating one of the two light fluxes to the surface to be measured and irradiating the other to the reference surface;
An integration procedure for generating an integrated light beam by integrating the measurement light beam passing through the measurement target surface and the reference light beam passing through the reference surface into the same optical path;
A scanning procedure for scanning the coherence distance of the light beam branched in the branching procedure;
A measurement procedure for obtaining relationship information indicating a relationship between the scanning position of the coherent distance and the intensity of the integrated luminous flux;
Based on the relationship information, a boundary between a scanning range in which the measurement light beam and the reference light beam interfere with each other and a scanning range in which the measurement light beam and the reference light beam do not interfere is acquired as height information of the measurement target surface. Calculation procedure and
An interference measurement method comprising: