JP5228828B2 - Low coherence interferometer, low coherence interferometer, and low coherence interferometry method - Google Patents

Description

  The present invention relates to a low coherence interferometer using a broadband light source such as a white interference microscope, a low coherence interferometer, and a low coherence interference measurement method.

  When measuring the shape of a measurement target surface with a large difference in height, such as a measurement target surface including a step structure, or when measuring the distance of an object, the width of the distance range to be measured (required dynamic range) A low coherence interferometer using a light source with a short coherent length (broadband light source) is effective (see Patent Document 1).

  In a low coherence interferometer, a peak appears in the envelope of the interference signal only when the optical path length difference between the measurement light from the measurement object and the reference light from the reference object becomes zero. The width of the range can be measured (ie the dynamic range is wide). In the present specification, various distances are represented by optical path length differences between the measurement light from the measurement object and the reference light from the reference object.

Incidentally, if the phase information of the interference signal is detected with a low-coherence interferometer, it is possible to measure a minute distance with an accuracy close to that of a high-coherence interferometer using a laser light source (the dynamic range is less than the light source wavelength). Yes (see Patent Document 2 etc.).
JP 2004-28647 A Japanese Patent No. 2679876

  However, low-coherence interferometers need to scan the optical path length difference while repeatedly acquiring interference signals in order to search for the envelope peak. The width of the scan range is at least as large as the required dynamic range. Is done. For this reason, the low coherence interferometer requires a large stroke moving mechanism and tends to increase the measurement time.

  Therefore, the present invention provides a low-coherence interferometer having a configuration suitable for reducing the width of the scan range of the optical path length difference and improving the efficiency of measurement, and also provides a low-efficiency measurement with high efficiency. It is an object of the present invention to provide a coherence interference measurement apparatus and a low coherence interference measurement method.

An aspect of the low coherence interferometer illustrating the present invention is a light source unit that generates a broadband modulated light having an optical frequency width corresponding to a low coherence interferometry and having a spectrum that is intensity-modulated at the optical frequency. And branching the broadband modulated light generated by the light source unit to both the measurement object and the reference object, and causing interference between the measurement light from the measurement object and the reference light from the reference object An optical system for generating a signal.

Also, one aspect of the low coherence interference apparatus exemplifying the present invention is one aspect of a low coherence interferometer, and a moving mechanism that relatively moves the measurement object and the reference object in the optical axis direction of the optical system. And a control device that captures an interference signal generated by the optical system at a predetermined time interval with the relative movement, and positional information of the measurement surface on the measurement object based on the interference signal captured by the control device And an arithmetic unit that calculates the peak value of the envelope of the interference signal taken in by the control device, and performs offset correction according to the type on the position information.

In addition, the low coherence interference measurement method exemplifying the present invention generates wideband modulated light having an optical frequency width corresponding to the low coherence interferometry and having a spectrum whose intensity is modulated at the optical frequency. The generated broadband modulated light is branched and guided to both the measurement object and the reference object, and the measurement light from the measurement object and the reference light from the reference object are interfered to generate an interference signal. And moving the measurement object and the reference object relative to each other in the optical axis direction, and capturing an interference signal generated by the interference at a predetermined time interval with the relative movement. Calculating the position information of the measurement surface of the measurement object based on the captured interference signal, and the calculating includes the type of the peak of the envelope of the captured interference signal Discriminated, it comprises applying an offset correction according to the type to the position information.

  According to the present invention, a low coherence interferometer having a configuration suitable for reducing the width of the scanning range of the optical path length difference and improving the efficiency of measurement, and the low coherence interference measurement capable of performing the measurement with high efficiency. An apparatus and a low coherence interferometry method are realized.

[First Embodiment]
Hereinafter, a first embodiment of the present invention will be described. The present embodiment is an embodiment of an interference measurement apparatus for acquiring three-dimensional information such as a surface shape.

  First, the configuration of the interference measurement apparatus will be described. FIG. 1 is a schematic configuration diagram of an interference measurement apparatus. As shown in FIG. 1, the interference measuring apparatus includes a broadband light source 1, a modulation filter 21, polarizing plates 2 and 16, a polarization beam splitter (PBS) 9, quarter-wave plates 10 and 13, objective optical systems 11 and 14, A measuring object 12, a reference object 15, an imaging optical system 17, a two-dimensional image detector 18, a moving mechanism 22 such as a piezo element, a control device 23, a computer 24, and the like are arranged. The computer 24 is connected to a monitor, an input device, etc. (not shown).

  The measurement object 12 is, for example, an optical component, a metal component, a plastic component, or the like, and is supported by a stage (not shown). Here, the height difference of the measurement target surface 12a of the measurement target 12 is 400 μm as the optical path length difference. Therefore, the dynamic range required in the height direction in this embodiment is 400 μm. The reference of the shape of the measurement target surface 12a is the reference surface 15a of the reference object 15, and the shape thereof is a plane.

  The light emitted from the broadband light source 1 enters the PBS 9 through the modulation filter 21 and the polarizing plate 2.

  Of the light beam incident on the PBS 9, the component that becomes S-polarized light with respect to the PBS 9 is reflected by the PBS 9 and reaches the measurement target surface 12 a via the quarter-wave plate 10 and the objective optical system 11. The light (measurement light) reflected by the measurement target surface 12 a returns to the objective optical system 11, passes through the quarter-wave plate 10 and the PBS 9, passes through the polarizing plate 16 and the imaging optical system 17, and is a two-dimensional image detector. 18 is reached. On the two-dimensional image detector 18, an image of the measurement target surface 12 a is formed by the objective optical system 11 and the imaging optical system 17.

  On the other hand, the component that becomes P-polarized light with respect to the PBS 9 out of the light beam incident on the PBS 9 passes through the PBS 9 and reaches the reference surface 15 a through the quarter-wave plate 13 and the objective optical system 14. The light (reference light) reflected by the reference surface 15 a returns to the objective optical system 14, passes through the quarter-wave plate 13, reflects the PBS 9, passes through the polarizing plate 16 and the imaging optical system 17, and is a two-dimensional image. The detector 18 is reached. Therefore, an interference image due to the measurement light and the reference light is formed on the two-dimensional image detector 18.

  Here, the control device 23 drives the moving mechanism 22 to move the reference object 15 in the optical axis direction, and scans the optical path length difference between the measurement light and the reference light. As a result, the phase of the interference image formed on the two-dimensional image detector 18 changes.

  The control device 23 drives the two-dimensional image detector 18 at a constant frame rate while scanning the optical path length difference at a predetermined pattern (for example, at a constant speed), and acquires interference images for a plurality of frames. Are sent to the computer 24 in the order of acquisition.

  The computer 24 sequentially accumulates interference images sent from the control device 23 in a memory (RAM or the like). Furthermore, the computer 24 analyzes the interference signal (scan position-pixel value change curve) of each pixel indicated by a plurality of frames of interference images stored in the memory, and obtains height information of each position of the measurement target surface 12a. Then, the shape of the measurement target surface 12a is calculated.

  Next, features of the configuration of the interference measuring apparatus will be described.

  The broadband light source 1 is, for example, a super luminescence diode (SLD), and its optical frequency width is a width suitable for measuring the shape of the measurement target surface 12a by the low coherence interference method. Here, the emission spectrum of the broadband light source 1 is a Gaussian type with a center wavelength of 680 nm as shown in FIG. 2, and its full width at half maximum is about 15.4 nm.

  The modulation filter 21 is an interference filter having a characteristic that intensity-modulates broadband light emitted from the broadband light source 1 in the optical frequency direction (wavelength direction). The intensity modulation amount by the modulation filter 21 is a periodic function of the optical frequency as shown in FIG. 3, and is a sine function here. In this case, the emission spectrum of the light source unit composed of the broadband light source 1 and the modulation filter 21 is as shown in FIG.

  Next, the behavior of the interference signal of each pixel (when the scan range is sufficiently wide) will be described.

  First, when there is no modulation filter 21, a peak appears in the envelope of the interference signal only when the optical path length difference is zero, as shown in FIG. However, since the modulation filter 21 is arranged in this embodiment, as shown in FIG. 6, not only a peak (main peak A) appears in the envelope of the interference signal when the optical path length difference is zero, but also the optical path. A peak (sub peak B) also appears when the length difference is a predetermined value.

  The intensity of the sub peak B is lower than the intensity of the main peak A, and the interval between the main peak A and the sub peak B is uniquely determined by the period of intensity modulation by the modulation filter 21 and is 200 μm in this embodiment.

  Next, the scanning range of the optical path length difference by the control device 23 will be described.

  FIG. 7 is a schematic diagram for explaining the scanning range of the optical path length difference. Here, each scan position will be described based on the height position of the measurement object 12.

  As described above, the required dynamic range in the height direction of the present embodiment is the distance (400 μm) from the base surface s1 to the top surface s2 of the measurement target surface 12a.

  In the conventional example, the width of the scan range a1 is set to a level (400 μm + 2α) obtained by adding a margin to the required dynamic range. For example, the scan start position in the conventional example is set to a position s0 that is slightly lower than the base position s1, and the scan stop position in the conventional example is set to a position s3 that is slightly higher than the top position s2 (approximately The base position s1 and the approximate top position s2 are identified based on the design data of the measurement object 12 input in advance by the measurer.

  However, the width of the scan range a2 of the present embodiment is set to a width (200 μm + 2α) shorter by 200 μm as the optical path length difference than that of the conventional example. The difference (200 μm) from the conventional example coincides with the interval between the main peak A and the sub peak B shown in FIG. For example, the scan start position of this embodiment is set to a position s0 that is slightly lower than the base position s1, and the scan stop position of this embodiment is set to a position s4 that is 200 μm lower than the scan stop position s3 of the conventional example. (Note that the approximate base position s1 and the approximate top position s2 are identified based on the design data of the measurement object 12 previously input by the measurer).

  Therefore, if the frame rate and the scan pattern are made common between this embodiment and the conventional example, the number of frames of the interference image to be acquired by the control device 23 of this embodiment is 200 μm than that of the conventional example. Only reduced.

  Next, analysis processing by the computer 24 of this embodiment will be described.

  FIG. 8 is a flowchart showing a procedure of analysis processing by the computer 24 of the present embodiment. Hereinafter, each step will be described in order.

  Step S1: The computer 24 sets the pixel number of the pixel of interest to an initial value.

  Step S2: The computer 24 reads the interference signal of the current target pixel from the memory. Incidentally, when the location corresponding to the pixel of interest on the measurement target surface 12a is a location 50 μm higher than the scan start position, an interference signal as shown in FIG. 9 is read out. The envelope of this interference signal peaks when the scan position is 50 μm. Further, when the position corresponding to the target pixel on the measurement target surface 12a is a position higher by 250 μm than the scan start position, an interference signal as shown in FIG. 11 is read out. The envelope of this interference signal also peaks when the scan position is 50 μm. However, since this peak is not a main peak but a sub peak, its peak intensity is lower than that of FIG.

  Step S3: The computer 24 performs a Fourier transform on the read interference signal, and obtains a primary Fourier spectrum (including a phase component and an intensity component) of the interference signal. The primary Fourier spectrum of the interference signal shown in FIG. 9 is as shown in FIG. 10, and the primary Fourier spectrum of the interference signal shown in FIG. 11 is as shown in FIG.

  Subsequently, the computer 24 extracts phase slope information when the intensity component takes a peak from the obtained first-order Fourier spectrum, and based on the phase slope information, scans when the interference signal takes a peak. The position is calculated, and the scan position is written into the memory as the height data of the pixel of interest. Therefore, the height data is “50 μm” when the interference signal transmitted from the pixel of interest is either of FIG. 9 and FIG.

  Step S4: The computer 24 compares the maximum value of the signal intensity in the vicinity of the above-described scan position among the interference signals of the pixel of interest with the threshold value, and if it is greater than the threshold value, the peak described above is the main peak A. Accordingly, the process proceeds to step S6, and if it is smaller than the threshold value, the above-described peak is regarded as the sub peak B, and the process proceeds to step S5. Note that the threshold used in this step is set in advance to an intermediate value (or the vicinity thereof) between the intensity of the main peak A and the intensity of the sub peak B shown in FIG. Therefore, the peak of the interference signal shown in FIG. 9 is regarded as the main peak A, and the peak of the interference signal shown in FIG. 11 is regarded as the sub-peak.

  Step S5: The computer 24 performs offset correction by adding a predetermined offset amount to the height data of the pixel of interest. This offset amount is the interval (200 μm here) between the main peak A and the sub-peak B shown in FIG. Therefore, when the interference signal transmitted from the pixel of interest is as shown in FIG. 11, the height data is offset-corrected from “50 μm” to “250 μm” in this step.

  Step S6: The computer 24 determines whether or not steps S2, S3, and S4 have been executed for all the pixels. If the computer 24 has been executed, the process proceeds to step S8. The process proceeds to step S7.

  Step S7: The computer 24 increments the pixel number of the pixel of interest by 1, and then returns to step S2.

  Step S8: The computer 24 obtains the shape of the measurement target surface 12a based on the height data of all the pixels stored in the memory, outputs it to a monitor (not shown), etc., and ends the flow.

  As described above, the interference measuring apparatus according to the present embodiment not only generates a peak (main peak A) in the interference signal when the optical path length difference is zero, but also the optical path length difference. A peak (sub peak B) is also generated in the interference signal at a predetermined value (see FIG. 6), and the relatively high portion of the measurement target surface 12a is measured by the sub peak B instead of the main peak A.

  Therefore, in this embodiment, the measurement time and the analysis time can be shortened by narrowing the scan range by the interval between the main peak A and the sub peak B.

  Further, according to the modulation filter 21 of the present embodiment, the intensity of the sub-peak B is lower than the intensity of the main peak A. Therefore, the peak expressed by the computer 24 in the interference signal of each pixel is the difference between the main peak A and the sub-peak B. It can be determined automatically.

  Furthermore, since the computer 24 of this embodiment appropriately offset-corrects the pixel height data according to the determination result, the shape of the measurement target surface 12a is correctly measured.

[Supplement to the first embodiment]
In the first embodiment, the above-described threshold value set to an intermediate value (or the vicinity thereof) between the intensity of the main peak A and the intensity of the sub-peak B can be derived from actual measurement data acquired prior to the above-described measurement. The actual measurement data is an interference signal acquired from the representative pixel in a state where the width of the scan range is set sufficiently wide.

  Further, the offset amount set to the same value as the interval between the main peak A and the sub peak B can be derived from the design value of the modulation filter 21 (specifically, the period of intensity modulation). It may be derived from the above-described actual measurement data assuming a manufacturing error of the filter 21 for use.

  In addition, the interference measurement apparatus may automatically perform a part or all of the above-described threshold value calculation and the above-described offset amount calculation.

  In addition, when the reflectance of the measurement target surface 12a is non-uniform, the above-described threshold value may differ depending on the pixel. In this case, the reflectance distribution (design data) of the measurement target surface 12a is determined. The measurer inputs in advance to the computer 24, and the computer 24 may normalize the threshold value of each pixel according to the distribution.

  Further, a Fabry-Perot etalon can be used for the modulation filter 21 of the present embodiment. The Fabry-Perot etalon is obtained by applying a coating having an appropriate reflectance on both surfaces of a spacer such as a glass substrate. A design example of the modulation filter 21 using a Fabry-Perot etalon is as shown in FIG. In the example shown in FIG. 13, since the intensity modulation amount of the modulation filter 21 is slightly deviated from the sine function of the optical frequency, the emission spectrum of the light source unit composed of the broadband light source 1 and the modulation filter 21 is as shown in FIG. Become.

  In this case, the number of sub-peaks appearing in the envelope of the interference signal is not one but a plurality as shown in FIG. However, since the width of the scan range of this embodiment is set to (200 μm + 2α) as described above, there is no possibility that the second and subsequent sub-peaks (such as sub-peak C) will affect the measurement.

[Second Embodiment]
Hereinafter, a second embodiment of the present invention will be described. Since this embodiment is a modification of the first embodiment, only differences from the first embodiment will be described.

  In the present embodiment, the emission spectrum of the light source unit composed of the broadband light source 1 and the modulation filter 21 is set as shown in FIG. The behavior of the interference signal in this case is as shown in FIG.

  In the present embodiment, the required dynamic range in the height direction is set to 600 μm, which is wider than that in the first embodiment (400 μm). However, the width of the scan range in the present embodiment is set to the same (200 μm + 2α) as in the first embodiment.

  FIG. 16 is a flowchart showing the procedure of analysis processing by the computer 24 of this embodiment. In FIG. 16, steps different from those in FIG. 8 are indicated by bold lines.

  Step S1: The computer 24 sets the pixel number of the pixel of interest to an initial value.

  Step S2: The computer 24 reads the interference signal of the current target pixel from the memory.

  Step S3: The computer 24 performs a Fourier transform on the read interference signal, and obtains a primary Fourier spectrum (including a phase component and an intensity component) of the interference signal. Subsequently, the computer 24 extracts phase slope information when the intensity component takes a peak from the obtained first-order Fourier spectrum, and based on the phase slope information, scans when the interference signal takes a peak. The position is calculated, and the scan position is written into the memory as the height data of the pixel of interest.

  Step S4-1: The computer 24 compares the maximum value of the signal intensity in the vicinity of the above-described scan position among the interference signals of the pixel of interest with the first threshold value. If it is considered that the peak is the main peak A, the process proceeds to step S6. If the peak is smaller than the first threshold value, the peak described above is regarded as the sub peak B or the sub peak C, and the process proceeds to step S4-2. Note that the first threshold value used in this step is set in advance to an intermediate value (or its vicinity) between the intensity of the main peak A and the intensity of the first sub-peak B shown in FIG.

  Step S4-2: The computer 24 compares the above-mentioned maximum value with the second threshold value, and if it is larger than the second threshold value, the computer 24 regards the above-mentioned peak as the first sub-peak B and proceeds to Step S5-1. If the value is smaller than the second threshold value, the peak described above is regarded as the second sub-peak C, and the process proceeds to step S5-2. Note that the second threshold value used in this step is set in advance to an intermediate value (or the vicinity thereof) between the intensity of the first sub-peak B and the intensity of the second sub-peak C shown in FIG.

  Step S5-1: The computer 24 performs offset correction by adding the first offset amount to the height data of the target pixel. This first offset amount is the interval (200 μm here) between the main peak A and the first sub-peak B shown in FIG.

  Step S5-2: The computer 24 performs offset correction by adding the second offset amount to the height data of the target pixel. The second offset amount is an interval (here, 400 μm) between the main peak A and the second sub-peak C shown in FIG.

  Step S6: The computer 24 determines whether or not Steps S2, S3, and S4-1 have been executed for all the pixels. If the computer 24 has been executed, the process proceeds to Step S8, and has not been executed. To step S7.

  Step S7: The computer 24 increments the pixel number of the pixel of interest by 1, and then returns to step S2.

  Step S8: The computer 24 obtains the shape of the measurement target surface 12a based on the height data of all the pixels stored in the memory, outputs it to a monitor (not shown), etc., and ends the flow.

  As described above, since the interference measuring apparatus of the present embodiment generates a plurality of sub-peaks (first sub-peak B, second sub-peak C) in the interference signal (see FIG. 15), the height direction without expanding the scan range. Can expand the dynamic range.

[Third Embodiment]
The third embodiment of the present invention will be described below. Since the present embodiment is a modification of the second embodiment, only differences from the second embodiment will be described.

  In the present embodiment, the emission spectrum of the light source unit composed of the broadband light source 1 and the modulation filter 21 is set as shown in FIG. Incidentally, the emission spectrum of the light source section is as shown in FIG. 17 when the intensity modulation amount by the modulation filter 21 is a pulse function of the optical frequency. The behavior of the interference signal in this case is as shown in FIG. 18, and there is almost no difference in the intensity of the main peak A, sub-peaks B, C,. In this case, since the type of peak cannot be determined based on the peak intensity, it is necessary to perform the determination based on the design data of the measurement object 12. The design data of the measurement object 12 is previously input by the measurer into the computer 24.

  FIG. 19 is a flowchart showing a procedure of analysis processing by the computer 24 of the present embodiment. In FIG. 19, steps different from those in FIG. 16 are indicated by bold lines.

  Step S0: The computer 24 and the design data of the measurement object 12 inputted in advance by the measurer and the actual measurement data obtained prior to measurement (obtained from the representative pixel in a state where the scan range is set sufficiently wide) Based on the interference signal), the imaging surface of the two-dimensional image detector 18 is divided into a first region, a second region, and a third region which will be described below.

  The first region is a region where pixels corresponding to the lowest portion of the measurement target surface 12a are present, and the peak that appears in the interference signal of these pixels is the main peak A.

  The second region is a region where pixels corresponding to the next highest portion of the measurement target surface 12a are present, and the peak appearing in the interference signal of these pixels is the first sub-peak B.

  The third region is a region where pixels corresponding to the highest portion of the measurement target surface 12a are present, and the peak appearing in the interference signal of these pixels is the second sub-peak C.

  Step S1: The computer 24 sets the pixel number of the pixel of interest to an initial value.

  Step S2: The computer 24 reads the interference signal of the current target pixel from the memory.

  Step S3: The computer 24 performs a Fourier transform on the read interference signal, and obtains a primary Fourier spectrum (including a phase component and an intensity component) of the interference signal. Subsequently, the computer 24 extracts phase slope information when the intensity component takes a peak from the obtained first-order Fourier spectrum, and based on the phase slope information, scans when the interference signal takes a peak. The position is calculated, and the scan position is written into the memory as the height data of the pixel of interest.

  Step S4′-1: The computer 24 determines whether or not the pixel of interest belongs to the first region. If the target pixel belongs to the first region, the computer 24 regards the above-described peak as the main peak A and proceeds to Step S6. If the peak does not belong to the first region, the peak described above is regarded as the sub peak B or the sub peak C, and the process proceeds to step S4′-2.

  Step S4'-2: The computer 24 determines whether or not the pixel of interest belongs to the second area. If the target pixel belongs to the second area, the computer 24 regards the above-described peak as the sub peak B and proceeds to step S5-1. If the transition does not belong to the second region, the peak described above is regarded as the sub-peak C, and the process proceeds to step S5-2.

  Step S5-1: The computer 24 performs offset correction by adding the first offset amount to the height data of the target pixel. This first offset amount is the interval (200 μm here) between the main peak A and the first sub-peak B shown in FIG.

  Step S5-2: The computer 24 performs offset correction by adding the second offset amount to the height data of the target pixel. This second offset amount is an interval (here, 400 μm) between the main peak A and the second sub-peak C shown in FIG.

  Step S6: The computer 24 determines whether or not Steps S2, S3, and S4 have been executed for all the pixels. If they have been executed, the process proceeds to Step S8. The process proceeds to step S7.

  Step S7: The computer 24 increments the pixel number of the pixel of interest by 1, and then returns to step S2.

  Step S8: The computer 24 obtains the shape of the measurement target surface 12a based on the height data of all the pixels stored in the memory, outputs it to a monitor (not shown), etc., and ends the flow.

  As described above, although the interference measurement apparatus according to the present embodiment does not generate a difference in the plurality of peak intensities, the type of peak is automatically determined based on the design data of the measurement object 12, and thus the same effect as that of the second embodiment is obtained. be able to.

[Supplement of each embodiment]
In step S3 of each embodiment described above, the method described in Japanese Patent No. 2679876 is employed as a method for calculating height data, but other methods may be employed. Incidentally, when the method of calculating the height data without using the information of the phase slope is adopted, the optical frequency width of the broadband light source 1 may be set wider than that shown in FIG. In that case, a halogen lamp or the like can be used as the broadband light source 1.

  In each of the above-described embodiments, a wavelength invariant light source is used as the broadband light source 1, but a wavelength scanning narrow band light source (wavelength scanning laser light source) may be used. The wavelength scanning laser light source is, for example, a semiconductor laser light source using a Littman type external resonator.

  By changing the emission wavelength and emission intensity of this wavelength scanning laser light source at high speed and setting the integrated emission spectrum of the wavelength scanning laser light source within the charge accumulation period for one frame to be the same as that shown in FIG. An effect equivalent to any of the above-described embodiments can be obtained.

  In each of the above-described embodiments, the combination of the broadband light source 1 and the modulation filter 21 is used as the light source unit. However, if there is a light source capable of spontaneously emitting intensity-modulated light, that is used as the light source unit. May be used.

  For example, a wavelength scanning laser light source may be used as the light source unit, and both the function of the broadband light source 1 and the function of the modulation filter 21 may be assigned to the wavelength scanning laser light source. That is, the integrated emission spectrum of the wavelength scanning laser light source within the charge accumulation period for one frame may be set to the same as that shown in any of FIGS. Also in this case, an effect equivalent to that of any of the embodiments described above can be obtained.

  Further, when the function of the modulation filter 21 is assigned to the wavelength scanning laser light source, the period of intensity modulation over the optical frequency direction can be switched and the interval between peaks can be switched. For example, the required dynamic range in the height direction If the peak is wide, the interval between peaks can be set wide, and when the required dynamic range in the height direction is narrow, the interval between peaks can be set narrowly (however, the interval between peaks changes) In this case, the offset amount described above is also changed.)

  Further, when the wavelength scanning laser light source has the function of the modulation filter 21, the period of intensity modulation over the optical frequency direction can be shifted and the position of the sub peak can be shifted in the optical frequency direction. It is also possible to scan the optical path length difference without using it.

  In each of the above-described embodiments, an example of a white interference microscope that measures a surface shape has been described. However, the present invention can also be applied to an interferometer device that measures an interval and a distance between objects.

[Principle of each embodiment]
Hereinafter, the outline of the principle of each embodiment will be described using mathematical expressions.

  In general, an interference signal obtained by the low coherence interferometry is expressed as shown in Expression (1).

In equation (1), k is the wave number of light, S (k) is the spectral intensity of the broadband light source, M (k) is the intensity modulation amount in the optical frequency direction of the broadband light source, z ₀ is the optical path length difference at the start of scanning, z is the scanning amount of the optical path length difference.

  In the conventional example, since the modulation filter 21 is not disposed, M (k) = 1. Therefore, the interference signal of the conventional example is expressed by Expression (2).

In equation (2), k _c is the center wave number. G (z) is a function representing an envelope, and has a peak at z = 0. As is apparent from the equation (2), the envelope has a peak when zz ₀ = 0, that is, when the optical path length difference between the measurement light and the reference light is zero.

  On the other hand, in the first embodiment, since the intensity modulation amount is a sine function of the optical frequency, it is expressed by Expression (3).

  In Equation (3), δ is the period of intensity modulation. Therefore, the interference signal of the first embodiment is expressed by Expression (4).

  From equation (4), it can be seen that sub-peaks appear in addition to the main peak. It can also be seen that the interval between the main peak and the sub-peak is given by 2π / δ.

  In the third embodiment, since the intensity modulation amount is a pulse function (comb type) at equal intervals of the optical frequency, the interference signal is expressed by Equation (5), and a large number of peaks appear at intervals of 2π / δ. You can see it.

  The interference signal behaves as described above because the envelope function and the spectral intensity distribution of the light source section are in a Fourier transform relationship. In the second embodiment or the third embodiment, the high-order sub-peak appears because the intensity modulation amount with respect to the optical frequency deviates from the sine function, and the harmonic component is superimposed on the spectral intensity distribution of the light source unit. Because it was.

[Supplement to the aspect described above]
In one aspect of the low coherence interferometer described above, the intensity modulation amount of the broadband modulated light may be a periodic function of optical frequency.

  The intensity modulation amount of the broadband modulated light may be a sine function of optical frequency.

  The intensity modulation amount of the broadband modulated light may be a periodic pulse function of optical frequency.

  The light source unit includes a broadband light source that generates broadband light having an optical frequency width for measuring the distance range to be measured by low coherence interferometry, and broadband light generated by the broadband light source. An interference filter that generates the broadband modulated light by performing intensity modulation in the optical frequency direction may be provided.

  The interference filter may be a Fabry-Perot etalon type interference filter.

  The broadband light source may be a wavelength scanning light source that generates light equivalent to the broadband light by time-modulating the emission wavelength of the wavelength scanning light source during one generation period of the interference signal. Good.

  The light source unit may include a wavelength scanning light source that generates light equivalent to the broadband modulated light by temporally modulating the emission wavelength of the wavelength scanning light source during one generation period of the interference signal. Good.

It is a schematic block diagram of the interference measuring apparatus of 1st Embodiment. It is an emission spectrum of the broadband light source 1 of the first embodiment. It is the wavelength dependence characteristic of the transmittance | permeability of the filter 21 for modulation of 1st Embodiment. It is an emission spectrum of the light source part of 1st Embodiment. It is the optical path length difference dependence characteristic of the conventional interference signal. It is the optical path length difference dependence characteristic of the interference signal of 1st Embodiment. It is a schematic diagram explaining the scanning range of 1st Embodiment. It is a flowchart which shows the procedure of the analysis process of 1st Embodiment. It is the interference signal which the certain pixel of 1st Embodiment output. 10 is a first order Fourier spectrum of the interference signal shown in FIG. 9. It is the interference signal which the other pixel of 1st Embodiment output. 12 is a first-order Fourier spectrum of the interference signal shown in FIG. It is the wavelength dependence characteristic (design example) of the transmittance of the modulation filter 21. It is the emission spectrum (design example) of a light source part. It is an optical path length difference dependence characteristic (design example) of an interference signal. It is a flowchart which shows the procedure of the analysis process of 2nd Embodiment. It is an emission spectrum of the light source part of 3rd Embodiment. It is an optical path length difference dependence characteristic of the interference signal of 3rd Embodiment. It is a flowchart which shows the procedure of the analysis process of 3rd Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Broadband light source, 21 ... Modulation filter, 2, 16 ... Polarizing plate, 9 ... Polarization beam split (PBS), 10, 13 ... 1/4 wavelength plate, 11, 14 ... Objective optical system, 12 ... Measurement object, 15 ... Reference object, 17 ... Imaging optical system, 18 ... 2D image detector, 22 ... Movement mechanism, 23. ..Control devices, 24 ... computers

Claims (14)

A light source unit for generating broadband modulated light having an optical frequency width corresponding to low coherence interferometry and having a spectrum whose intensity is modulated at the optical frequency ;
The broadband modulated light generated by the light source unit is branched and guided to both the measurement object and the reference object, and the interference light is generated by interfering the measurement light from the measurement object and the reference light from the reference object. An optical system to generate,
Low coherence interferometer with The low coherence interferometer according to claim 1,
The intensity modulation characteristic is expressed by a periodic function of optical frequency.
Low coherence interferometer. The low coherence interferometer according to claim 1 or 2,
The characteristic of the intensity modulation is expressed by a sine function of optical frequency.
Low coherence interferometer. The low coherence interferometer according to claim 1 or 2,
The intensity modulation characteristic is expressed by a periodic pulse function of optical frequency.
Low coherence interferometer. In the low coherence interferometer according to any one of claims 1 to 4,
The light source unit is
A broadband light source for generating broadband light having an optical frequency width corresponding to low coherence interferometry ;
An interference filter that generates the broadband modulated light by intensity-modulating the spectrum of the broadband light generated by the broadband light source at the optical frequency ;
Low coherence interferometer with The low coherence interferometer according to claim 5,
The interference filter is
A low-coherence interferometer, a Fabry-Perot etalon-type interference filter. The low coherence interferometer according to claim 5 or 6,
The broadband light source is:
Low coherence interferometer is out morphism wavelength scanning light source for modulating the wavelength time. In the low coherence interferometer according to any one of claims 1 to 4,
The light source unit is
Low coherence interferometer including a wavelength scanning light source for modulating the output morphism wavelength time. The low coherence interferometer according to any one of claims 1 to 8,
A moving mechanism for relatively moving the measurement object and the reference object in the optical axis direction of the optical system;
A control device that captures interference signals generated by the optical system at a predetermined time interval with the relative movement;
An arithmetic device that calculates position information of a measurement surface of the measurement object based on an interference signal captured by the control device , and
The arithmetic unit is:
A low-coherence interference device that discriminates the type of an envelope peak of an interference signal captured by the control device and applies offset correction according to the type to the position information. Generating broadband modulated light having an optical frequency width according to low coherence interferometry and having a spectrum intensity-modulated at the optical frequency;
The generated broadband modulated light is branched and guided to both the measurement object and the reference object, and an interference signal is generated by causing the measurement light from the measurement object and the reference light from the reference object to interfere with each other. When,
Relatively moving the measurement object and the reference object in the optical axis direction;
Capturing the interference signal generated by the interference with the relative movement at a predetermined time interval;
Calculating position information of a measurement surface of the measurement object based on the captured interference signal, and
Said calculating is
  Determining the type of the peak of the envelope of the captured interference signal, and applying offset correction according to the type to the position information,
  Low coherence interference measurement method.   The low coherence interferometer according to claim 9,
  The peak type includes the intensity of the peak
  Low coherence interferometer.   The low coherence interference apparatus according to claim 9 or 11,
  The position information of the measurement surface includes the shape of the measurement surface.
  Low coherence interferometer.   The low coherence interferometry method according to claim 10,
  The peak type includes the intensity of the peak
  Low coherence interference measurement method.   The low coherence interference measurement method according to claim 10 or 13,
  The position information of the measurement surface includes the shape of the measurement surface.
  Low coherence interference measurement method.

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