JP6666792B2 - Optical image measurement device, optical image measurement method - Google Patents

Description

  The present invention relates to an optical image measurement device that observes a measurement target using light interference.

  Optical coherence tomography (OCT: Optical Coherence Tomography) is a technique for acquiring a tomographic image of an object to be measured using light interference, and has been put into practical use in the field of fundus examination since 1996. Applications in various fields such as oncology, oncology, food industry and regenerative medicine are being studied.

  Patent Document 1 listed below describes a technique relating to OCT. As described in the same document, in OCT, light from a light source is split into two parts, a signal light for irradiating a measurement target and a reference light reflected by a reference light mirror without irradiating the measurement target. A measurement signal is obtained by multiplexing and interfering the signal light reflected from the device with the reference light.

  OCT is largely divided into a time domain OCT and a Fourier domain OCT by a scanning method of a measurement position in the optical axis direction (hereinafter, referred to as z-scan). In the time domain OCT, a z-scan is performed by using a low coherence light source as a light source and scanning a reference light mirror during measurement. As a result, only the component having the same optical path length as the reference light included in the signal light interferes, and the desired signal is demodulated by performing envelope detection on the obtained interference signal. Fourier domain OCT is further divided into wavelength scanning OCT and spectral domain OCT. In the wavelength scanning OCT, a wavelength sweep light source capable of scanning the wavelength of the emitted light is used, and the wavelength is scanned at the time of measurement to perform z-scanning. A desired signal is obtained by Fourier-transforming the spectrum. In the spectral domain OCT, using a broadband light source as a light source, dispersing generated interference light with a spectroscope, and detecting interference light intensity (interference spectrum) for each wavelength component corresponds to the z-scan. A desired signal is obtained by Fourier-transforming the obtained interference spectrum.

US2014 / 0204388

  In the conventional OCT apparatus described above, the depth resolution is determined by the wavelength bandwidth or the wavelength sweep width of light. For this reason, a light source having a wide wavelength band such as a super luminescence diode (SLD) or a wavelength sweep light source is used. These light sources are more expensive than ordinary laser light sources that generate narrow band light. In addition, since the wavelength band of light to be used is wide, an optical element corresponding to broadband light is required, and chromatic dispersion compensation is also essential. For these reasons, it has been difficult to reduce the cost of the conventional OCT apparatus.

  Therefore, the present inventors have invented an optical measurement device described in Patent Document 1. This optical measurement device uses a high NA (Numerical Aperture) objective lens to converge and irradiate laser light (signal light) onto a measurement target and scans the objective lens to scan the condensing position and perform measurement. Obtain a target tomographic image. This optical measurement device enables three-dimensional measurement using the principle that reflected light components other than the focal point of the objective lens included in the signal light do not seem to interfere with the reference light because the wavefront curvature does not match. Therefore, the principle is fundamentally different from that of a conventional OCT apparatus using an SLD or a wavelength-swept light source. In this configuration, an expensive light source is not required, so that an inexpensive optical image measurement device can be provided. On the other hand, the measurement time tends to be long because it takes time to scan the light condensing position.

  The present invention has been made in view of the above problems, and has as its object to provide an optical image measurement device that can acquire images at a plurality of depth positions in a short measurement time without using a broadband light source. I do.

  In the present invention, the defocus amount of the reference light is changed in a plurality of stages while the position where the signal light irradiates the measurement target moves a distance corresponding to one pixel of the measurement image. This makes it possible to acquire tomographic images at a plurality of depth positions in a single measurement, so that the measurement time per tomographic image can be reduced.

  As an example, when the number of pixels in the scanning direction of the acquired image is N, the modulation frequency of the defocus amount of the reference light is set to be N times or more the scanning frequency of the condensing position of the signal light. This makes it possible to obtain a signal sufficient to generate an image of N pixels corresponding to each measurement depth position.

  As an example, as a means for modulating the defocus amount of the reference light, the relative position between the reflection surface of the stepped mirror and the condensing position of the reference light is displaced in a direction parallel to the reflection surface of the stepped mirror. Element to be used. This makes it possible to modulate the defocus amount of the reference light without using an element that drives the objective lens and the step-type mirror in the optical axis direction at high speed. Therefore, the measurement time can be reduced with an inexpensive configuration.

  As an example, an optical deflecting element is used as an element for displacing the relative position between the reflection surface of the staircase mirror and the condensing position of the reference light in a direction parallel to the reflection surface of the staircase mirror. . In this configuration, since the relative position between the objective lens and the staircase-type mirror is fixed, robustness against unintended changes in the defocus amount due to disturbance or the like is high, and the defocus amount of the reference light is accurately determined. Can be modulated.

  As an example, as an element for displacing the relative position between the reflection surface of the stepped mirror and the condensing position of the reference light in a direction parallel to the reflection surface of the stepped mirror, driving for displacing the position of the objective lens An element was used. This allows the reference light to always enter the objective lens perpendicularly, so that it is possible to suppress the occurrence of aberrations other than the defocus aberration that occurs when light obliquely enters the lens.

  As an example, the position of the step mirror is displaced as an element for displacing the relative position between the reflection surface of the step mirror and the condensing position of the reference light in a direction parallel to the reflection surface of the step mirror. A driving element was used. This allows the reference light to always enter the objective lens perpendicularly, so that it is possible to suppress the occurrence of aberrations other than the defocus aberration that occurs when light obliquely enters the lens. Further, since the relative position between the reference light and the objective lens is fixed, the fluctuation of the power of the reference light passing through the opening of the objective lens can be suppressed.

  As an example, the area of the reflection surface of the staircase mirror is made larger than the spot area of the reference light on each reflection surface. Thereby, most of the reference light power can be reflected, so that the SN ratio of the signal can be improved.

As an example, the number of pixels in the first direction of the image is N, the number of unit structures of the staircase mirror is M, the frequency at which the scanner scans the irradiation position of the signal light in the first direction is f _sig , the reference light. When a frequency at which the focusing position displacer changes the defocus amount from the start stage to the end stage is f _ref, it is determined that Mf _ref ≧ Nf _sig is satisfied. Accordingly, an element having a small driving frequency can be applied as a means for displacing the reference light condensing position, so that the measurement time can be reduced with an inexpensive configuration.

  As an example, as a means for modulating the defocus amount of the reference light, an element for displacing the relative position between the reflection surface of the mirror and the condensing position of the reference light in a direction perpendicular to the reflection surface is used. Thus, the defocus amount of the reference light can be modulated with a simple and inexpensive configuration.

  As an example, a detector for detecting the defocus amount is provided. Thereby, the depth measurement position corresponding to the detection signal can be calculated with high accuracy.

  According to the present invention, it is possible to provide an optical image measurement device that acquires a tomographic image of a measurement target in a short time without using a broadband light source such as an SLD or a wavelength swept light source. Problems, configurations, and effects other than those described above will be apparent from the following description of the embodiments.

1 is a schematic diagram illustrating a configuration example of an optical image measurement device according to a first embodiment. It is a side view which shows the structure of the step type mirror 112. 5 is a flowchart illustrating a procedure in which the optical image measurement device according to the first embodiment measures a measurement target. It is a schematic diagram explaining the state of the reference light and the measurement depth position corresponding to it. FIG. 3 is a diagram illustrating a temporal change of a condensing position of a signal light and a temporal change of a defocus amount of a reference light. This is an example of a method of manufacturing the step mirror 112. FIG. 9 is a schematic diagram when a reflective film 603 is formed by a sputtering method and an evaporation method. This is an example in which a metal mask is combined as a mask at the time of etching. This is an example of a method of transferring a step element formed in advance as a matrix. It is an example of a transfer method using a film. It is an example of stereolithography. It is an example of injection molding. FIG. 9 is a schematic diagram illustrating a configuration example of an optical image measurement device according to a second embodiment. 4 shows a configuration example of a four-division photodiode 1305. FIG. 13 is a diagram illustrating a configuration example of a defocus modulation unit in an optical image measurement device according to a third embodiment. FIG. 14 is a diagram illustrating a configuration example of a defocus modulation unit in an optical image measurement device according to a fourth embodiment. FIG. 15 is a diagram illustrating a configuration example of a defocus modulation unit in an optical image measurement device according to a fifth embodiment. FIG. 14 is a schematic diagram illustrating a configuration example of an optical image measurement device according to a sixth embodiment.

<First Embodiment: Configuration of Optical System>
FIG. 1 is a schematic diagram illustrating a configuration example of the optical image measurement device according to the first embodiment of the present invention. One laser beam emitted from the light source 101 is converted into a parallel beam by a collimating lens 102, the polarization is rotated by a λ / 2 plate 103 whose optical axis can be adjusted, and then a beam formed by a polarization beam splitter 104. The splitter splits the signal light and the reference light into two. The signal light passes through the λ / 4 plate 105 whose optical axis direction is set to about 22.5 degrees with respect to the horizontal direction, converts the polarization state from s-polarized light to circularly polarized light, and then has a numerical aperture of NA _sig . The sample 108 is condensed and irradiated by the objective lens 106. The position of the objective lens 106 is linearly driven in the y-direction while being repeatedly driven in the x-axis direction, for example, in the form of a sine wave by the objective lens actuator 107, whereby the condensing position (measurement position) of the signal light is on the xy plane. Two-dimensional scanning is performed. The signal light reflected or scattered from the sample 108 passes through the objective lens 106 again, and the polarization state is changed from circularly polarized light to p-polarized light by the λ / 4 plate 105, and is incident on the polarization beam splitter 104. The sample 108 is made of a material that transmits light from the light source 101 to some extent, and may be any material that allows noninvasive observation of the internal structure. Examples include multilayer semiconductor structures, foods, plants, cultured cells, human tissues, and the like.

The reference light is converted from p-polarized light to circularly polarized light by the λ / 4 plate 109, and then is composed of a MEMS (Micro Electro Mechanical Systems) mirror 110, an objective lens 111 having a numerical aperture of NA _ref , and a step mirror 112. The light enters the focus modulator 113. The reference light that has entered the defocus modulator 113 is reflected by the MEMS mirror 110, and is condensed and irradiated on the step mirror 112 by the objective lens 111. The defocus modulation unit 113 shifts the condensing position of the reference light on the step mirror 112 in the horizontal direction of the reflection surface of the step mirror 112 by changing the inclination of the MEMS mirror 110. Thereby, the height of the reflection surface at the light condensing position changes, and the defocus amount of the reference light is modulated. The reference light reflected by the step mirror 112 passes through the objective lens 111 again, and is reflected by the MEMS mirror 110. Then, the polarization state is changed from circularly polarized light to s-polarized light, and then enters the polarization beam splitter 104.

  The signal light and the reference light are multiplexed by the polarization beam splitter 104 to generate a combined light. The combined light is guided to an interference optical system 121 including a half beam splitter 114, a λ / 2 plate 115, a λ / 4 plate 116, condenser lenses 117 and 118, and Wollaston prisms 119 and 120. The combined light that has entered the interference optical system 121 is split into two by the half beam splitter 114 into transmitted light and reflected light. The transmitted light passes through a λ / 2 plate 115 whose optical axis is set to about 22.5 degrees with respect to the horizontal direction, is then condensed by a condenser lens 117, and is polarized and separated by a Wollaston prism 119. A first interference light and a second interference light having a phase relationship different from each other by 180 degrees are generated. The first interference light and the second interference light are detected by a current differential type photodetector 122, and a differential output signal 124 proportional to the difference between their intensities is output.

  The reflected light passes through a λ / 4 plate 116 whose optical axis is set to about 45 degrees with respect to the horizontal direction, is then collected by a condenser lens 118, and is polarized and separated by a Wollaston prism 120, so that the reflected lights are separated from each other. A third interference light and a fourth interference light having a phase relationship different by about 180 degrees are generated. The third interference light has a phase difference of about 90 degrees from the first interference light. The third interference light and the fourth interference light are detected by the current differential type photodetector 123, and a differential output signal 125 proportional to the difference between their intensities is output. The differential output signals 124 and 125 generated in this way are input to the image generation unit 126. The image generation unit 126 generates an image based on these signals, and the image display unit 127 displays this.

FIG. 2 is a side view showing the structure of the step mirror 112. The staircase mirror 112 has a structure in which a plurality of unit structures are periodically arranged. The unit structure has a plurality of reflection surfaces arranged in steps and having different heights. Unit structure illustrated in FIG. 2, the width different heights by p _z has five reflecting surfaces of the p _x. In Figure _2, the unit structure in the _{X ref} is lined M pieces.

<Embodiment 1: Function of optical system>
Hereinafter, the operation principle and effects of the first embodiment will be described using mathematical expressions. The Jones vector of the combined light at the time of entering the interference optical system 121 is represented by the following equation 1. E _sig represents the complex electric field amplitude of the signal light, and E _ref represents the complex electric field amplitude of the reference light.

The complex electric field amplitude of the signal light and the reference light at a position separated by a distance r from the center of the optical axis reflects the position in the optical axis direction where the signal light is condensed from z ₀ and the position z and is combined with the reference light. The electric field amplitude and the phase of the signal light component are _denoted by A _sig (z) and θ _sig (z), respectively, and the defocus amount of the reference light by the objective lens 111 (the condensing position of the reference light in the z direction and the staircase mirror 112 Assuming that the difference between the z positions of the reflecting surfaces) is Δz _ref and the effective diameters of the objective lens 106 and the objective lens 111 are a, the following expressions 2 and 3 can be obtained.

W _sig (z−z ₀ ) r ² and W _ref Δz _ref r ² represent defocus aberrations of the signal light and the reference light reflected from the position z, respectively. The defocus aberration coefficients W _sig and W _ref are represented by the following equations 4 and 5, respectively.

  The Jones vector of the combined light transmitted through the half beam splitter 114 and further transmitted through the λ / 2 plate 115 is expressed by the following equation (6).

The combined light represented by Expression 6 is polarization-separated into a p-polarized component and an s-polarized component by the Wollaston prism 119, and then differentially detected by the current differential type photodetector 122. The photodetector 122 generates a differential output signal 124 represented by the following equation (7). r represents a coordinate vector of a light beam cross section. D represents a detection area. ∫ _D dr means an integral operation in the entire area of the light flux. The conversion efficiency of the detector was set to 1 for simplicity.

  The Jones vector of the combined light reflected by the half beam splitter 114 and further transmitted through the λ / 4 plate 116 is represented by the following Expression 8.

  The combined light represented by Expression 8 is polarization-separated into a p-polarized component and an s-polarized component by the Wollaston prism 120, and then differentially detected by the current differential photodetector 123. The photodetector 123 generates a differential output signal 125 represented by the following equation (9).

  By substituting Equations 2 and 3 into Equations 7 and 9, the following Equations 10 and 11 are obtained.

  F (z) and Δθ (z) are expressed by the following equations 12 and 13, respectively.

  Using approximation equation 14 that holds for a sufficiently large constant k, equations 10 and 11 are simplified to equations 15 and 16, respectively.

The image generation unit 126 obtains the sum of squares of the signal I and the signal Q, and thereby obtains luminance data S (z ₀ , Δz _{ref) of the} image corresponding to the condensing position z ₀ of the signal light and the defocus amount Δz _ref of the reference light. ).

Although the signal light represented by Expression 2 includes reflected light components from all depths (in the direction of the optical axis), the signal light and the reference light interfere with each other to specify Only the reflected light component from the depth position can be selectively detected. Further, the expression 17 changes the defocus amount of the reference light by displacing the focus position of the reference light by Δz _{ref in the} optical axis direction, thereby changing the measurement position from the focus position z ₀ of the signal light to (NA _ref / NA _sig ) ² means that it can be shifted by Δz _ref .

  Therefore, the present invention scans the condensing position of the signal light in a plane perpendicular to the optical axis (in the xy plane) while rapidly changing the measurement position depth by modulating the defocus amount of the reference light at high speed. By doing so, signals from a plurality of depth positions are simultaneously acquired while scanning the condensing position of the signal light on the xy plane by one pixel. Thereby, the measurement time per one image can be reduced. In the method for modulating the defocus amount of the reference light in the first embodiment, since the relative position between the objective lens 111 and the step mirror 112 is fixed, an unintended change in the defocus amount due to disturbance or the like. And the defocus amount of the reference light can be modulated with high accuracy.

<Embodiment 1: Measurement procedure>
FIG. 3 is a flowchart illustrating a procedure in which the optical image measurement device according to the first embodiment measures a measurement target. Hereinafter, each step of FIG. 3 will be described.

(FIG. 3: Step S300)
When the user selects the measurement start, the optical image measurement device starts the present flowchart (S300). The optical image measurement device modulates the defocus amount of the reference light by driving the MEMS mirror 110 (S301). A specific mode of modulating the defocus amount of the reference light will be described again with reference to FIG.

(FIG. 3: Steps S302 to S303)
The optical image measurement device starts scanning of the condensing position of the signal light by driving the objective lens actuator 107 (S302). The image generation unit 126 obtains measurement data by performing AD conversion on the differential output signals 124 and 125 while the light-converging position of the signal light is being scanned (S303).

(FIG. 3: Steps S304 to S307)
When the optical image measurement device scans the condensing position of the signal light over the entire measurement target area, the scanning of the signal light ends (S304). The image generation unit 126 sorts the measurement data for each measurement depth position (S305), and generates an image of a measurement target for each measurement depth position based on the sorted measurement data (S306). The image display unit 127 displays the image generated by the image generation unit 126 (S307). As will be described later, while scanning the condensing position of the signal light for one pixel, the defocus amount of the reference light changes in multiple steps, so that images at a plurality of depth positions can be obtained for each pixel position. .

<Embodiment 1: Control method>
Hereinafter, a control method for scanning the condensing position of the signal light and a control method for modulating the defocus amount of the reference light will be described. Here, as an example, a case where five xy-plane tomographic images are acquired by one measurement will be described.

FIG. 4 is a schematic diagram illustrating the state of the reference light and the corresponding measurement depth position. As shown in FIG. 4A, the condensing position of the reference light on the step mirror 112 moves from 1 to 5 due to a change in the angle of the MEMS mirror 110. Accordingly, the defocus amount is applied to the reference light is changed in five stages from -2p _z to 2p _z. Measurement depth position at this time is, as shown in FIG. 4 (b), corresponding to the defocus amount of the reference light _{(NA ref /} NA ^sig) changes in five stages in 2 _{p z} interval.

FIG. 5 is a diagram for explaining the time change of the condensing position of the signal light and the time change of the defocus amount of the reference light. As shown in FIG. 5 (a), the condensing position of the signal light is moved by a distance corresponding to N pixels in the xy tomographic image acquired during a time 1 / 2f _sig. Therefore, as shown in FIG. 5B, the condensing position of the signal light moves on average by a distance corresponding to one pixel during a time 1/2 Nf _sig . As shown in FIG. 5C, the defocus amount of the reference light is modulated in five stages during the time (1 / 2Nf _sig ) during which the light condensing position of the signal light moves by one pixel. By the above process is repeated N ² times during the measurement, it is possible to obtain the five xy tomographic image in the xy plane scanning of one degree of condensing position of the signal light.

Hereinafter, an example of the control method will be specifically described using expressions. The condensing position (x, y, z) of the signal light is scanned by the objective lens actuator 107 according to the following equation (19). X _sig and Y _sig are the measurement area widths in the xy directions, respectively. f _sig is the drive frequency of the objective lens actuator 107. N is the number of pixels of the tomographic image to be obtained (assuming that the number of pixels in the x direction and the number of pixels in the y direction are equal).

By the control according to Expression 19, the focus position of the signal light is moved at a constant speed of Y _sig in the y direction during the time (N / 2f _sig ), and only N / 2 times with the amplitude X _sig in the x direction. Go back and forth. On the other hand, the condensing position of the reference light on the step mirror 112 is repeatedly scanned in the x direction by the MEMS mirror 110, thereby modulating the defocus amount of the reference light. The x coordinate x _ref of the condensing position of the reference light is given by Expression 20 below, where the driving frequency of the angle of the MEMS mirror 110 is f _ref .

The focal length of the objective lens 111 and F, when the driving amplitude range of the angle of the MEMS mirror 110 and ± φ _{ref, X ref} can be expressed approximately by the following equation 21.

As shown in FIG. 2, when the focus position of the reference light is displaced in the x direction, the height of the reflection surface changes, so that the defocus amount of the reference light changes. Assuming that the focal position of the objective lens 111 is set to the third reflecting surface from the bottom among the five reflecting surfaces in the unit structure, the defocus amount Δz _ref of the reference light at this time is Takes the five values shown in Expression 22 below. Δz _ref <0 when the focal point deviates deeper than the reflection surface.

While the MEMS mirror 110 performs one reciprocal scan, the condensing position of the reference light passes through the unit structure of the staircase-type mirror 112 by only 2M in a reciprocating manner. Then, the defocus modulation frequency f defocus of the reference light by the MEMS mirror 110 is _expressed by the following equation (23).

To obtain an image of N × N pixels, between one line of scanning of the signal light converging position (1 _{/ 2f sig),} the defocus state of the reference light to 2p _z from (-2p _z It is necessary to change N times or more (assuming the change in five steps is one). That is, the defocus modulation frequency f _defocus of the reference light (scanning frequency of the condensing position of the signal light at this time to be two lines scanned by reciprocating becomes 2f _sig) scanning frequency of the condensing position of the signal light Must be N times or more. When this condition is expressed by an equation, the following equation 24 or 25 is obtained.

In the first embodiment, if the driving frequencies of the objective lens actuator 107 and the MEMS mirror 110 are f _sig = 100 Hz, f _ref = 4000 Hz, and the number of pixels of the image is N = 400, then M ≧ 10.

Next, we describe the pitch _{p x} and z direction of the pitch _{p z} in the x direction in the unit structure of the stepped mirror 112. The pitch p _{z in the} z direction is given by the following equation 26 when the measurement depth interval is Δz _sig .

The measurement depth interval can be set arbitrarily according to the purpose, but can be, for example, about the depth resolution of the optical image measurement device. This is because even if the measurement depth interval is set to be equal to or less than the depth resolution of the optical image measurement device, the optical image measurement device cannot resolve the measurement depth interval. The depth resolution Δz of the optical image measurement device according to the present invention is given by the following Expression 27 using the numerical aperture NA _sig of the objective lens 106 and the wavelength λ of the light source 101. α is a constant satisfying sinc ² (α) = １ ／ and is approximately 1.39.

Using equation 27, pitch p _z in the z direction when the measured depth interval are equal to the depth resolution is given by the following equation 28. If λ = 780 nm and NA _ref = 0.1 are used in the first embodiment, p _z = 69 μm.

Regarding the pitch p _{x in the} x direction, in order for most of the reference light power to be reflected, the width of the reflection surface must be larger than the spot diameter at the time of maximum defocusing of the reference light (Δz _ref = ± 2p _z ). Must. Therefore, it is necessary to satisfy the following Expression 29.

ω _ref (z) is a spot radius when the reference light is defocused by z. Here, for the sake of simplicity, it is assumed that the reference light behaves as a Gaussian beam in the vicinity of the condensing position, and the following expression 30 is approximately used as ω _ref (z).

  By substituting Equations 28 and 30 into Equation 29, the following Equation 31 is obtained.

There are M unit structure in the width X _ref, since each unit structure has five reflecting surfaces, the pitch p _x in the x-direction is given by the following equation 32.

In the first embodiment, if M = 10, F = 20 mm, and φ _ref = 12 degrees are used, p _x ≒ 84 μm, which satisfies the condition of Expression 30. The structure of the step-type mirror described here is merely an example, and any structure may be used as long as the defocus amount of the reference light can be modulated, and other structures and dimensions may be considered. Can be

<First Embodiment: Theoretical Limit of Multiplicity>
Up to this point, the case where five xy plane tomographic images are acquired by one measurement (the multiplicity of the reference light is 5) has been described as an example, but the multiplicity can be further increased. In the following, the theoretical upper limit of the multiplicity is determined. If multiplicity is set to an odd number, and be expressed as 2m + 1 with any natural number m, the pitch p _x in the x direction can be expressed by the following equation 33.

  By solving Equation 33 for M and substituting it into Equation 25, the following Equation 34 is obtained.

  The condition corresponding to Expression 29 when the multiplicity is 2m + 1 is Expression 35 below.

  From the conditions of Expressions 35 and 34, the following Expression 36 is obtained as a conditional expression for the multiplicity.

When the maximum m satisfying Equation 36 was m _0, 2m 0 ₊₁ is the theoretical upper limit value of the multiplicity. In order to obtain a more explicit condition for the multiplicity, the following equation 37 is obtained by substituting equation 30 into equation 36 and rearranging the equation.

As can be seen from Expression 37, the number N of pixels of the acquired image, the driving frequency f _{sig of} the condensing position of the signal light, the driving frequency f _ref and the driving amplitude X _{ref of} the condensing position of the reference light, the NA _{ref of the} objective lens 111, And the wavelength λ, the theoretical upper limit of the multiplicity is determined. In order to increase the degree of multiplicity, it is necessary to select a defocus modulation means for the reference light having a large X _ref × f _ref × NA _ref . Generally, when a resonance-type scanning element (resonance-type galvanometer mirror or MEMS mirror) is used, X _ref × f _ref can be increased. It is preferable that the NA _ref is simply larger as the objective lens 111 from the viewpoint of the multiplicity. However, when light is obliquely incident on the lens as in the first embodiment, the larger the NA _ref , the more the obliqueness. Since the aberration caused by the incident light is apt to occur, if the NA _ref is too large, it causes the signal deterioration.

Substituting the values of the above parameters (N = 400, f _sig = 100 Hz, f _ref = 4000 Hz, X _ref = 4189 μm, NA _ref = 0.1, λ = 0.78 μm) in the first embodiment into equation 37, The maximum m that satisfies 37 is m ₀ = 4. That is, the maximum multiplicity 2m ₀ +1 in the first embodiment is 9.

  Based on the above description, the staircase mirror 112 can be designed, for example, by the following procedure. However, this procedure is merely an example of the design procedure, and various other procedures can be considered.

(Procedure 1) A means for modulating the defocus amount of the reference light is determined so that X _ref × f _ref × NA _ref is increased while considering signal degradation factors such as aberration.
(Procedure 2) The multiplicity is determined (m is determined) within a range satisfying Expression 36 (or Expression 37).
(Procedure 3) The number M of unit structures is determined so as to satisfy Equation 25.
The (Step 4) 28 and Equation 33 to determine the pitch _{p z} and x direction of the pitch _{p x} in the z direction.

<Embodiment 1: Manufacturing method of step-type mirror 112>
FIG. 6 shows an example of a method of manufacturing the step mirror 112. Here, three steps are formed as an example.

  A resist mask 602 is formed on a quartz substrate 601 (FIG. 6A). Thereafter, a first step is formed by etching a region other than the mask (FIG. 6B). As a mask resist, a negative resist, ZPN1150 manufactured by ZEON was used, and as a developer, NMD-3 manufactured by Tokyo Ohka was used. For exposure to the resist, a mask aligner (PLA-501F manufactured by Canon) was used. The quartz substrate 601 was etched using an HF aqueous solution, and after the etching, the resist was removed by ultrasonic cleaning in an acetone solution (FIG. 6C).

  As the resist, a positive type may be used in combination with the reticle design at the time of contact exposure, or laser exposure for direct exposure or electron beam exposure may be used. The etching may be dry etching by RIE (Reactive Ion Etching) other than wet etching.

  A second mask including the first step is formed by aligning the position on the quartz substrate 601 where the first step has been formed (FIG. 6D). Thereafter, regions other than the mask are similarly etched to form a second step (FIG. 6E). A mask including the first step and the second step is formed in accordance with the first step and the second step (FIG. 6F). Thereafter, regions other than the mask are similarly etched to form third steps (FIG. 6G). After forming a desired step, an Al film having a thickness of 150 nm is formed as a reflective film 603 by sputtering (FIG. 6H).

  The reflective film 603 may be made of Ag, Co, Ti, Ni, W, Pt, Au, or an alloy containing these, in addition to Al. A material having sufficient reflected light at the wavelength to be used may be used. As a film formation method, an evaporation method, a CVD method, or the like may be used in addition to the sputtering. It is desirable that the film surface be smooth so that the required reflectance is obtained.

  FIG. 7 is a schematic diagram when the reflective film 603 is formed by a sputtering method and an evaporation method. FIG. 7A shows an example formed by a sputtering method, and FIG. 7B shows an example formed by a vapor deposition method. In a sputtering method generally suitable for mass production, a target material adheres from various directions, so that a film is formed evenly on a step boundary. In the vapor deposition method, the film is formed on a flat surface because of its high directivity, but is hardly formed on a step boundary. In order to reduce unnecessary reflected light from the step boundary surface, it can be said that the vapor deposition method is suitable. The substrate may be an opaque substrate such as a Si substrate in addition to a quartz substrate. This is because the reflection film 603 is formed after the processing, so that the reflectance is not affected by the substrate. Any substrate can be used as long as it can be etched.

  FIG. 8 shows an example in which a metal mask is combined as a mask at the time of etching. The first step is formed using the metal mask 804 and the resist mask 802. The mask for forming the first step formed first is the narrowest. In this example, a metal mask 804 is provided in order to prevent processing defects due to resist collapse easily occurring during wet etching. As a result, desired processing could be realized even when the resist width was narrow. Since the mask width becomes wider after the second step, stable etching can be performed without the metal mask 804. Al was used as the material of the metal mask 804, and processing was performed with an aqueous sodium hydroxide solution. However, W and Cr did not pose any problem. In the case of W, treatment can be performed using a mixed solution of ammonia and hydrogen peroxide water, and in the case of Cr, treatment can be performed using MPM-E350 manufactured by DNP Fine Chemicals. The material of the metal mask 804 may be selected in combination with an etching solution.

  As another example, a thick reflective material may be formed on a substrate, and the reflective material may be directly processed using a mask as described above.

  FIG. 9 shows an example of a method of transferring a step element formed in advance as a matrix. A photocurable resin (DVD003N manufactured by Nippon Kayaku) 903 is sandwiched between the transparent substrate 901 and the step element 902 formed as described above, and pressure is applied so as to be uniform. Thereafter, the resin is cured by irradiating ultraviolet rays from the transparent substrate 901 side. After that, the step element 902 is peeled off to form a reflection film. When a non-transparent substrate is used, a thermosetting resin may be used. Thus, the transfer method can be said to be excellent in productivity because a plurality of elements can be manufactured by a simple method by manufacturing one matrix. In this method, since the laser beam is incident from the transparent substrate 901 side, the step can be increased by designing the refractive index difference between the substrate and the resin. Therefore, the effect of increasing the processing margin can be obtained.

  FIG. 10 shows an example of a transfer method using a film. After sandwiching the film 1003 between the substrate 1001 and the matrix 1002 and bringing the film 1003 into close contact with a pressure roller 1004, the film 1003 is cured and the matrix 1002 is peeled off. Thus, the transferred step element is completed. If a plurality of matrixes 1002 are arranged side by side on a plane, a plurality of step elements can be produced at one time. Thereafter, a reflective film is formed and the mold is removed. The film 1003 may be either photo-curable or thermo-curable, and may be provided with a necessary curing mechanism.

  FIG. 11 shows an example of the optical shaping method. A desired shape is formed on the molding stage 1103 while irradiating the photocurable resin 1101 with the laser beam 1102 to cure the resin. The photo-curable resin 1101 is cured at a portion where the light intensity of the laser focusing portion is high. The laser beam 1102 is scanned in the X and Y directions, and the modeling stage 1103 moves in the Z direction. First, a first step 1104 is formed, and then the Z-axis is moved to form a second step 1105 and then a third step 1106. The step accuracy and the flatness of the mirror surface are determined according to the size of the condensed spot and the pitch in the XY and Z directions. Therefore, if the pitch for forming one step is made finer according to the step condition and the processing is performed in a plurality of times, the performance is further improved. Uncured resin adhering to the surroundings when taken out is washed with alcohol. Thereafter, a reflective film is formed to produce a step mirror element integrated with a resin material.

  FIG. 12 shows an example of injection molding. The resin 1201 melted by heat is poured into the mold 1202, and cured by applying pressure. Although precision and cost are required to form the mold 1202, a large number of copies can be made by manufacturing one mold 1202, which is a method suitable for mass production. Thereafter, a reflective film is formed to produce a step mirror element integrated with a resin material.

<Embodiment 2>
FIG. 13 is a schematic diagram illustrating a configuration example of the optical image measurement device according to the second embodiment of the present invention. The same components as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The second embodiment is different from the first embodiment in that the configuration of the defocus modulation unit 113 is different from that of the first embodiment, and that a defocus amount detection unit 1306 is further provided. Other configurations, operations, and functions are the same as those in the first embodiment, and a description thereof will not be repeated.

  The defocus modulator 113 includes an objective lens actuator 1301, an objective lens 111, and a step mirror 112. In Embodiment 2, the objective lens 111 is driven by the objective lens actuator 1301 in a direction perpendicular to the optical axis, thereby displacing the focusing position of the reference light on the staircase mirror 112 and reducing the defocus amount of the reference light. Modulate. Unlike the first embodiment, since the reference light can always be perpendicularly incident on the objective lens 111, it is possible to suppress the occurrence of aberrations other than the defocus aberration that occurs when light is obliquely incident on the lens. Can be. As a result, the measurement time per image can be reduced without deteriorating the image quality.

  The defocus amount detection unit 1306 includes a beam splitter 1302, a condenser lens 1303, a cylindrical lens 1304, and a four-division photodiode 1305. A part of the reference light reflected from the step mirror 112 is reflected by the beam splitter 1302, then collected by the condenser lens 1303 and the cylindrical lens 1304, and detected by the four-division photodiode 1305.

FIG. 14 shows a configuration example of a four-division photodiode 1305. FIG. 14A shows an example of the structure of the light receiving unit of the four-division photodiode 1305 and an example of the spot shape of the reference light in the light receiving unit. Due to the effect of astigmatism caused by the cylindrical lens 1304, the spot shape in the light receiving section depends on the magnitude and sign of the defocus aberration given to the reference light by the defocus modulation section 113 as shown in the figure. different. So the four light receiving portions A in the present embodiment 2, B, C, respectively output from _{D I A, I B, I} C, as _{I D,} was that the focus error signal FE is defined by formula 38 .

FIG. 14B shows the relationship between the defocus aberration Δz _ref given to the reference light by the defocus modulator 113 and the focus error signal FE. Since the value of Δz _ref and the value of FE have a one-to-one correspondence within the range of the fixed defocus amount Δz _ref, the defocus amount Δz _ref can be calculated from the value of FE. Thereby, the depth measurement position corresponding to the signals I and Q detected at any time can be calculated with high accuracy.

<Embodiment 3>
FIG. 15 is a diagram illustrating a configuration example of the defocus modulation unit 113 in the optical image measurement device according to the third embodiment of the present invention. The configuration other than the defocus modulation unit 113 is the same as in the first and second embodiments, and a description thereof will be omitted. As shown in FIG. 15A, the defocus modulator 113 according to the third embodiment includes an objective lens 111, a disc-shaped step-shaped mirror 1501, and a motor 1502.

  FIG. 15B is a schematic diagram showing a structural example of a disk-shaped step-shaped mirror 1501. The disk-shaped step mirror 1501 has a structure in which a plurality of types of reflection surfaces (five types of 1 to 5) having different heights are periodically arranged on the circumference. In the third embodiment, the reference light is defocused by focusing and irradiating the reference light onto the disc-shaped step mirror 1501 by the objective lens 111 and rotating the disc-shaped step mirror 1501 by the motor 1502. The aberration is to be modulated. In this configuration, since the relative position between the reference light and the objective lens 111 is fixed, it is possible to suppress the intensity fluctuation of the signals I and Q due to the fluctuation of the reference light power passing through the opening of the objective lens 111. it can.

<Embodiment 4>
FIG. 16 is a diagram illustrating a configuration example of the defocus modulation unit 113 in the optical image measurement device according to the fourth embodiment of the present invention. The configuration other than the defocus modulation unit 113 is the same as in the first and second embodiments, and a description thereof will be omitted. As shown in FIG. 16A, the defocus modulator 113 according to the fourth embodiment includes an objective lens 111, an objective lens actuator 1601, and a mirror 1602.

  FIG. 16B shows a relative position change between the reflection surface of the mirror 1602 and the condensing position of the reference light. In the fourth embodiment, the objective lens 111 is driven in the optical axis direction using the objective lens actuator 1601, and the relative position between the reflection surface of the mirror 1602 and the condensing position of the reference light is changed in the optical axis direction. The defocus aberration of the reference light is modulated. In this configuration, the defocus aberration of the reference light can be modulated with a simple and inexpensive configuration without using the step mirror 112. By configuring the objective lens 111 as a variable focus lens instead of the objective lens actuator 1601, the same effect can be exhibited.

<Embodiment 5>
FIG. 17 is a diagram illustrating a configuration example of the defocus modulation unit 113 in the optical image measurement device according to the fifth embodiment of the present invention. The configuration other than the defocus modulation unit 113 is the same as in the first and second embodiments, and a description thereof will be omitted. As shown in FIG. 17A, the defocus modulator 113 according to the fifth embodiment includes an objective lens 111, a mirror 1701, and a piezo element 1702.

  FIG. 17B shows a relative position change between the reflection surface of the mirror 1701 and the condensing position of the reference light. In the fifth embodiment, the mirror 1701 is driven in the optical axis direction using the piezo element 1702, and the relative position between the reflection surface of the mirror 1701 and the condensing position of the reference light is changed in the optical axis direction. In addition, the defocus aberration of the reference light is modulated. As the mirror 1701, a mirror having a small size and a small mass is used as long as it has a size of the reflection surface about the spot size of the reference light. This makes it possible to modulate the defocus amount faster than in the case where the objective lens 111 is driven as in the fourth embodiment.

<Embodiment 6>
FIG. 18 is a schematic diagram illustrating a configuration example of an optical image measurement device according to Embodiment 6 of the present invention. The defocus modulator 113 according to the sixth embodiment includes a staircase mirror actuator 1801 instead of the objective lens actuator 1301. Other configurations are the same as those of the second embodiment.

  In the sixth embodiment, the staircase mirror actuator 1801 drives the staircase mirror 112 in a direction perpendicular to the optical axis, thereby displacing the focusing position of the reference light on the staircase mirror 112 and defocusing the reference light. Modulate the amount. Thereby, the same effect as in the second embodiment can be exhibited. Further, since the relative position between the reference light and the objective lens 111 is fixed, the fluctuation of the power of the reference light passing through the opening of the objective lens 111 can be suppressed.

<Regarding Modification of the Present Invention>
The present invention is not limited to the embodiments described above, and includes various modifications. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described above. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of one embodiment can be added to the configuration of another embodiment. Also, for a part of the configuration of each embodiment, it is possible to add, delete, or replace another configuration.

101: light source 102: collimating lens 103: λ / 2 plate 104: polarizing beam splitter 105: λ / 4 plate 106: objective lens 107: objective lens actuator 108: sample 109: λ / 4 plate 110: MEMS mirror 111: objective lens 112: Step mirror 113: Defocus modulator 114: Half beam splitter 115: λ / 2 plate 116: λ / 4 plate 117: Condensing lens 118: Condensing lens 119: Wollaston prism 120: Wollaston prism 121: Interference optical system 122: photodetector 123: photodetector 126: image generation unit 127: image display unit

Claims (15)

An optical image measurement device that acquires an image of a measurement target,
A light source for emitting laser light,
An optical branching unit that branches the laser light emitted from the light source into signal light and reference light,
A first objective lens for condensing and irradiating the signal light onto the measurement target,
A scanner that moves a position where the first objective lens irradiates the signal light in a plane orthogonal to an optical axis of the signal light;
A defocus modulator that changes a defocus amount of the reference light,
An interference optical system that generates interference light by multiplexing the signal light reflected or scattered by the measurement target and the reference light in which the defocus amount is modulated,
A photodetector for detecting the interference light,
An image generation unit that generates the image using the interference light detected by the photodetector,
With
The defocus modulation unit changes the defocus amount of the reference light in a plurality of stages while a position where the signal light irradiates the measurement target moves a distance corresponding to one pixel of the image. Optical image measurement device. The defocus modulation unit includes a mirror having a plurality of reflection surfaces whose positions in the height direction are different from each other,
The defocus modulation unit, by reflecting the reference light using the mirror, imparts a defocus aberration corresponding to the position of the reflection surface to the reference light,
The optical image according to claim 1, wherein the image generation unit generates images of the measurement target at a plurality of positions along an optical axis direction of the signal light for each of the plurality of levels of defocus amounts. Measuring device. The image generation unit generates the image having N pixels in a first direction in the plane,
The defocus modulation unit changes the defocus amount in a plurality of stages from a start stage to an end stage,
The defocus modulation unit changes the defocus amount from the start stage to the end stage at a frequency that is at least N times a frequency at which the scanner scans the irradiation position of the signal light in the first direction. The optical image measurement device according to claim 1, wherein: The defocus modulator,
A mirror having a plurality of reflection surfaces whose positions in the height direction are different from each other,
A second objective lens for condensing the reference light on the mirror,
A reference light condensing position displacer for displacing the condensing position of the reference light by the second objective lens in a direction parallel to the reflection surface;
The optical image measurement device according to claim 1, further comprising:   The reference light condensing position displacer is configured using an optical deflecting element that displaces a condensing position of the reference light by changing an incident angle of the reference light with respect to the second objective lens. The optical image measurement device according to claim 4, wherein The said reference light condensing position displacer is comprised using the drive element which displaces the condensing position of the said reference light by displacing the position of the said 2nd objective lens. Optical image measurement device. The said reference light condensing position displacer is comprised using the drive element which displaces the condensing position of the said reference light with respect to the said mirror by displacing the position of the said mirror. Optical image measurement device. The optical image measurement device according to claim 4, wherein an area of each of the reflection surfaces of the mirror is larger than a spot area of the reference light on each of the reflection surfaces. The image generation unit generates the image having N pixels in a first direction in the plane,
The mirror has a structure in which a plurality of unit structures including at least two or more of the reflection surfaces are periodically arranged,
The reference light condensing position displacement unit changes the defocus amount in a plurality of stages from a start stage to an end stage,
The number of the unit structures of the mirror is M, the frequency at which the scanner scans the irradiation position of the signal light in the first direction is f _sig , and the reference light condensing position displacement unit is the defocus amount. 5. The light according to claim 4, wherein when the frequency changed from the start stage to the end stage is f _ref , the reference light condensing position displacement unit satisfies M × f _ref ≧ N × f _sig. Image measurement device. The image generation unit generates the image having N pixels in a first direction in the plane,
The mirror has a structure in which a plurality of unit structures including at least two or more of the reflection surfaces are periodically arranged,
The reference light condensing position displacement unit changes the defocus amount in a plurality of stages from a start stage to an end stage,
The number of the reflection surfaces of the unit structure is M, the number of the unit structures of the mirror is M, the frequency at which the scanner scans the irradiation position of the signal light in the first direction is f _sig , and the reference light is When a frequency at which the light condensing position displacer changes the defocus amount from the start stage to the end stage is f _ref , M × f _ref ≧ N × f _sig is satisfied;
The number of the reflection surfaces of the unit structure is further within a range satisfying that the area of each of the reflection surfaces of the mirror is larger than the spot area of the reference light on each of the reflection surfaces. The optical image measurement device according to claim 4, wherein The defocus modulator,
mirror,
A second objective lens for condensing the reference light on the mirror,
A reference light condensing position displacer for displacing the condensing position of the reference light by the second objective lens in a direction normal to a reflection surface of the mirror;
The optical image measurement device according to claim 1, further comprising: The defocus modulator,
mirror,
A second objective lens for condensing the reference light on the mirror,
A reference light condensing position displacer that displaces the condensing position of the reference light by the second objective lens relative to the reflecting surface of the mirror in a direction normal to the reflecting surface of the mirror;
With
The reference light condensing position displacement instrument, an optical image measuring apparatus according to claim 1, characterized by being constituted by using a driving device for displacing the position of the mirror. The optical image measurement device according to claim 11, wherein the reference light condensing position displacer is configured using a drive element that displaces the position of the second objective lens. The optical image measurement device according to claim 1, wherein the optical image measurement device further includes a defocus amount detector that detects a defocus amount of the reference light. An optical image measurement method for acquiring an image of a measurement target,
Emitting a laser beam,
Branching the emitted laser light into signal light and reference light,
Converging and irradiating the signal light onto the measurement object,
Moving the position to irradiate the signal light in a plane orthogonal to the optical axis of the signal light,
A defocus modulation step of changing a defocus amount of the reference light,
Generating interference light by multiplexing the signal light reflected or scattered by the measurement target and the reference light in which the defocus amount is modulated,
Detecting the interference light,
Generating the image using the detected interference light,
Has,
In the defocus modulation step, the defocus amount of the reference light is changed in a plurality of steps while a position where the signal light irradiates the measurement target moves a distance corresponding to one pixel of the image. Optical image measurement method.

Images