JP2023071068A - Optical interference device - Google Patents

Abstract

To enlarge a measurement range in a depth direction, using Fabry-Perot etalon, and identify the number of times of repeated reflection by a single camera.SOLUTION: An optical interference device 1A is provided that has: a camera 100A; a BS 110; a BS 120; a lens 170; a FP 150; a grating 160; generation means 182; and identification means 184A. The BS 110, BS 120, lens 170, and grating 160 consist of a Mach-Zehnder optical system, and the FP 150 is provided in an optical path ranging from the BS 110 to the grating 160. The camera 100A is configured to receive first return light R1 from a measurement object OB, and second return light R2 from the grating 160. The generation means 182 is configured to generate a two-dimensional image representing an interference fringe generating by the first return light R1 and second return light R2. The identification means 184A is configured to identify the number of times of repeated reflection by the FP 150 in the second return light R2.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to optical interference devices.

As an example of a measurement technique for measuring the unevenness of the surface of an object to be measured, there is a technique using a Mach-Zehnder optical system (see, for example, Patent Document 1). In this type of measurement technology, light emitted from a light source is separated into measurement light that irradiates an object to be measured and reference light. The surface shape of the measurement object is measured by measuring the interference fringes of light).

JP 2016-099239 A

In the surface measurement technique using the Mach-Zehnder optical system, it is conceivable to arrange a Fabry-Perot etalon in the optical path of the reference light in order to expand the measurement range in the depth direction such as the normal direction of the surface. A Fabry-Perot etalon is an optical element in which two reflecting planes are arranged to face each other. In the Fabry-Perot etalon, light interference occurs due to repeated reflections between reflecting surfaces arranged facing each other, and only light with a wavelength that satisfies a predetermined interference condition is transmitted. By placing the Fabry-Perot etalon in the optical path of the reference light, the reference light whose phase is delayed by the number of repeated reflections is generated for each repeated reflection. When the thickness (optical length) of the Fabry-Perot etalon is less than the basic measurement range of the grating, the basic The measurement range is continuously repeated, and the depth measurement range is expanded by simultaneously measuring multiple interference fringes with the camera.

When a Fabry-Perot etalon is placed in the reference light path, it is possible to determine how many times each fringe in the interference fringes formed by repeated reflections in the Fabry-Perot etalon is the interference fringe due to repeated reflection. necessary. This is because the number of repeated reflections (order) corresponds to the depth of the depressions on the surface of the object to be measured. In principle, it is also possible to generate interference fringe images for each wavelength using a plurality of cameras, and to analyze the plurality of images and calculate the repetition order of the reference light caused by the interference signal. However, in a mode in which a plurality of cameras are used to generate an image of interference fringes for each wavelength, the signal intensity per camera decreases and the S/N deteriorates. S/N means the ratio of signal to noise. Therefore, it would be desirable to be able to identify the number of repeated reflections with a single camera, but there has been no technology that enables such a thing.

The present invention expands the depth measurement range using a Fabry-Perot etalon in surface measurement technology using a Mach-Zehnder optical system, and provides a technology that enables specifying the number of repeated reflections with a single camera. intended to provide

An optical interference device according to a first aspect of the present invention includes a light receiving means, first and second beam splitters, a lens, a Fabry-Perot etalon, a grating, a generating means, and an identifying means. Light emitted from the light source is incident on the first beam splitter. The lens converges the measurement light, which is the first light split by the first beam splitter, onto the measurement object. The reference light, which is the second light different from the first light and split by the first beam splitter, enters the Fabry-Perot etalon. Reference light emitted from the Fabry-Perot etalon is guided to the grating. A second beam splitter is provided between the first beam splitter and the lens. The second beam splitter guides the first light to the lens and guides the first return light from the object to be measured to the light receiving means. Also, the second beam splitter guides the second return light from the grating to the light receiving means. The generator generates a two-dimensional image representing interference fringes generated by the first return light and the second return light received by the light receiver. The identifying means identifies the number of repeated reflections by the Fabry-Perot etalon of the second return light that causes the interference fringes, based on the thickness of the interference fringes represented in the two-dimensional image generated by the generating means.

An optical interference device according to a second aspect of the present invention includes a light receiving means, first and second beam splitters, a lens, a Fabry-Perot etalon, a grating, a generating means, and an identifying means. Prepare. The light receiving means separates the received light into two or more predetermined wavelength bands. Light emitted from the light source is incident on the first beam splitter. The lens converges the measurement light, which is the first light split by the first beam splitter, onto the measurement object. The reference light, which is the second light different from the first light and split by the first beam splitter, enters the Fabry-Perot etalon. Reference light emitted from the Fabry-Perot etalon is guided to the grating. A second beam splitter is provided between the first beam splitter and the lens. The second beam splitter guides the first light to the lens and guides the first return light from the object to be measured to the light receiving means. Also, the second beam splitter guides the second return light from the grating to the light receiving means. The generator generates a two-dimensional image representing interference fringes generated by the first return light and the second return light received by the light receiver. If two or more interference fringe sets exist in the depth direction of the object to be measured in the two-dimensional image generated by the generating means, the specifying means determines the interference fringe set based on the interval between the interference fringes that form the interference fringe set. Determine the number of repeated reflections by the Fabry-Perot etalon in the second return light that causes the set.

In the optical interference device of the second aspect, the light receiving means may include a filter that transmits or reflects light in two or more predetermined wavelength bands, and a monochrome camera that is arranged after the filter.

Moreover, in the optical interference device of the second aspect, the light receiving means may include a color camera that separates the received light into at least three wavelength bands.

Further, in the optical interference device of each aspect described above, the beam shape of the measurement light immediately before entering the object to be measured may be linear.

It is a figure which shows the structural example of 1 A of optical interference apparatuses by 1st Embodiment of this invention. FIG. 10 is a diagram showing an example of output timing of reference light L2 from the FP 150 when dispersion is not imparted; 4 is a diagram showing an example of output timing of reference light L2 from the FP 150 in this embodiment. FIG. FIG. 3 is a diagram showing an example of an actual surface state of an object to be measured OB; FIG. 10 is a diagram showing an example of a determination result based on interference fringes when dispersion is not given; It is a figure which shows an example of the interference fringe imaged by camera 100A in this embodiment. FIG. 5 is a diagram showing a configuration example of an optical interference device 1B according to a second embodiment of the present invention; It is a figure which shows an example of the interference fringe imaged by camera 100A. It is a figure which shows the structural example of 1 C of optical interference apparatuses by 3rd Embodiment of this invention. FIG. 4 is a diagram showing an example of wavelength sensitivity of each pixel of R, G, and B in the camera 100C; It is a figure which shows an example of the interference fringes imaged by the camera 100C.

Various technically preferable limitations are attached to each embodiment described below. However, embodiments of the present invention are not limited to the forms described below.

A. 1st embodiment
FIG. 1 is a diagram showing a configuration example of an optical interference device 1A according to the first embodiment of the present invention. The optical interference device 1A is a device for measuring unevenness on the surface of the object to be measured OB. In FIG. 1, a light source SC and a measurement object OB are illustrated in addition to the optical interference device 1A. As shown in FIG. 1, the optical interference device 1A includes a camera 100A, a beam splitter (abbreviated as BS in FIG. 1, hereinafter the same in this specification) 110, BS120 and BS130, a mirror 140, and a Fabry It includes a Perot etalon (abbreviated as FP in FIG. 1, hereinafter the same in this specification) 150 , a grating 160 , a processor 180 A, a relay lens 190 , a relay lens 200 and a lens 210 . In this embodiment, the axis along the normal line of the surface of the measurement object OB whose unevenness is to be measured by the optical interference device 1A is referred to as the Z axis. One of the two axes orthogonal to the Z-axis is called the X-axis and the other is called the Y-axis.

In optical interference apparatus 1A, BS 110, BS 120 and BS 130, mirror 140, and grating 160 constitute a Mach-Zehnder optical system. Each of BS 110, 120 and BS 130 is a polarizing beam splitter. The reason why BS110, BS120 and BS130 are polarizing beam splitters is that it is easy to adjust the intensity of transmitted light and reflected light at each.

As shown in FIG. 1, the light L emitted from the light source SC is incident on the BS 110 . In a preferred embodiment, the light source SC is a coherent, broadband, high-intensity pulsed light source generated by imparting a nonlinear effect to light emitted from an ultrashort pulse laser. The wavelength range preferably covers red, green and blue light.
The BS 110 splits the incident light L into measurement light L1 and reference light L2. BS 110 is an example of a first beam splitter in this disclosure. The measurement light L1 is an example of first light in the present disclosure. The reference light L2 is an example of second light in the present disclosure, that is, second light different from the first light.

The reference light L2 is incident on the FP 150 after being reflected by the mirror 140 . The reference light L2 that has entered the FP150 is emitted from the FP150 after being reflected multiple times within the FP150. In this embodiment, the thickness of the FP 150 (the optical length of one round trip of the reference light L2 within the FP 150) is equal to or less than the basic measurement range of the depth of the depression along the Z-axis on the surface of the measurement object OB. The reference light L2 emitted from the FP 150 is guided by the BS 130 and enters the grating (diffraction grating) 160 . In this embodiment, the FP 150 is provided with dispersion so that the output timing differs for each color of the reference light L2 emitted from the FP 150, that is, so that the phase delay differs for each wavelength.

FIG. 2 is a diagram showing an example of the output timing of the reference light L2 that is output after N round trips in the FP 150 (N=1 to 4 in FIG. 2) when dispersion is not imparted. Time T in FIG. 2 is the time required for the reference light L2 to make one round trip inside the FP 150 . In FIG. 2, the hatching with a downward slope represents the output spectrum of blue light, the hatching with horizontal lines represents the output spectrum of green light, the hatching with vertical lines represents the output spectrum of yellow light, and the hatching with an upward slope. represents the output spectrum of red light (the same applies to FIG. 3 described later). As shown in FIG. 2, when no dispersion is imparted, the blue, green, yellow, and red lights included in the reference light L2 are output at the same timing.

FIG. 3 is a diagram showing an example of output timing of the reference light L2 emitted from the FP 150 in this embodiment. In this embodiment, since dispersion is imparted to the FP 150, a phase delay occurs for each wavelength in the reference light L2 for each round trip. As is clear from FIG. 3, if the number of round trips is the same, the output timing of yellow, green, and blue light is delayed in the order of red light. The width Δ of the delay from the light output to the blue light output increases.

A plurality of sawtooth-shaped grooves are provided on the incident surface of the reference light L2 in the grating 160 . FIG. 1 shows one of these grooves. In this embodiment, the depth d of this groove is the basic measurement range of the depth of the depression along the Z-axis on the surface of the object to be measured OB. Grating 160 reflects incident reference light L2. Reflected light R2 of reference light L2 from grating 160 is guided by BS 130, relay lens 190 and relay lens 200 and enters BS 120. FIG. Reflected light R2 is an example of second returned light in the present disclosure. Below, the reflected light R2 is also referred to as the second return light R2.

As shown in FIG. 1, BS 120 is provided between BS 110 and lens 170 . The BS 120 guides the measurement light L1 to the lens 170 and guides the reflected light R1 of the measurement light L1 from the measurement object OB to the camera 100A. The reflected light R1 of the measurement light L1 from the measurement object OB is an example of the first return light in the present disclosure. Below, the reflected light R1 is also referred to as the first return light R1. BS 120 also guides reflected light R2 to camera 100A. BS 120 is an example of a second beam splitter in this disclosure.

The lens 170 is a cylindrical lens that linearly collects the incident measurement light L1 along the X axis. In this embodiment, the linear measurement light L1 having a length of about 20 mm along the X-axis and a width of about 10 microns is used to measure the unevenness of the surface of the object to be measured OB. Since the unevenness of the surface of the measurement object OB is measured using the linear measurement light L1 along the X-axis, scanning in the X-axis direction is unnecessary in this embodiment, and only scanning in the Y-axis direction is performed. should be performed. The linear measurement light L1 emitted from the lens 170 is condensed by the lens 210 and irradiated onto the surface of the object to be measured OB. Reflected light R1 of measurement light L1 from measurement object OB is guided by lens 210, lens 170, and BS 120 and enters camera 100A.

Camera 100A is a monochrome camera. The camera 100A outputs an image signal GM representing the received light of the first return light and the second return light, that is, an image representing a monochrome image of interference fringes generated by the first return light R1 and the second return light R2. It outputs the signal GM to the processor 180A. Camera 100A is an example of a light receiving means in the present disclosure.

The processing device 180A includes a processor such as a CPU (Central Processing Unit), that is, a computer. The processing unit 180A may include one computer or may include multiple computers. The processing device 180A functions as the generating means 182 and the specifying means 184A by operating according to a program stored in a storage device (not shown in FIG. 1). That is, the generation means 182 and the identification means 184A in FIG. 1 are software modules realized by operating a computer according to a program. Note that the processing device 180A and the storage device may be part of a personal computer.

The generator 182 generates a two-dimensional image, that is, a monochrome two-dimensional image representing interference fringes generated by the first return light R1 and the second return light R2 based on the image signal GM. In the following, each bright portion of the interference fringes produced by the first return light R1 and the second return light R2 is referred to as a bright line, and each dark portion is referred to as a dark line. Interference fringes alternate between bright and dark lines.

Based on the thickness of the interference fringes represented by the two-dimensional image generated by the generating unit 182, that is, the thickness of the bright line or the dark line, the identifying means 184A determines The number of repeated reflections by the FP 150 (that is, the number of round trips of the reference light L2 within the FP 150) is specified. The reason why the number of repeated reflections by the FP 150 can be specified based on the thickness of the interference fringes is as follows.

In the reference light L2, the phase of each wavelength (color of light) differs for each number of repeated reflections. Color bleeding occurs according to the As described above, the camera 100A that captures the interference fringes in this embodiment is a monochrome camera. Therefore, the color blur corresponding to the number of repeated reflections appears as a difference in the thickness of the interference fringes in the image captured by the camera 100A. This is the reason why the number of repeated reflections by the FP 150 can be specified based on the thickness of the interference fringes.

Here, it is assumed that the measurement range in the Z-axis direction is expanded to the 10th order by providing the FP 150, but the FP 150 is not provided with dispersion. In this case, as shown in FIG. 2, the output timing of each color of the reference light L2 is the same for each number of times of reflection, and the above-described color bleeding does not occur.
FIG. 4 shows an example of the actual surface state of the measurement object OB. In this example, recesses RE (...P1-P2-P3-P4-P5-P6...) are formed on the surface of the measurement object OB. That is, the wall surface of the recess RE is inclined with respect to the depth direction (the Z-axis direction, in other words, the direction parallel to the direction in which the measurement light is incident on the measurement object OB) between P2 and P3. Between P5, it is parallel to the depth direction (in other words, the hole is cut vertically). FIG. 5 conceptually shows the interference fringes obtained when the measurement object OB is measured without imparting dispersion to the FP 150 . In FIG. 5, the interference fringes corresponding to the area between P4 and P5 appear in multiple overlaps corresponding to the extension of the measurement range. As a result, the depth of the recess RE between P4 and P5 cannot be correctly determined from the interference fringes.

FIG. 6 schematically shows interference fringes obtained when the measurement object OB shown in FIG. 4 is measured by imparting dispersion to the FP 150 . As shown in the figure, the number of repeated reflections appears as a difference in the thickness of the interference fringes. That is, the interference fringes are thicker (wider; in other words, the lines are blurred) in the area on the image corresponding to the area between P2 and P6. More specifically, the interference fringes gradually thicken in the area on the image corresponding to P2-P3, and the area on the image corresponding to P1-P2, P3-P4, and P5-P6 (Fig. ), the thickness of the interference fringes is constant.
Based on the thickness of this interference fringe (in other words, the degree of blurring of the line), the number of repeated reflections is specified, and the shape of the hole (for example, in this example, information including the depth of the recess RE and the distance in the Z-axis direction) ) can be correctly determined. As described above, according to the optical interference apparatus 1A of the present embodiment, in the surface measurement technique using the Mach-Zehnder optical system, the depth measurement range is expanded by using the Fabry-Perot etalon, and the depth measurement range is repeatedly reflected by one camera. It becomes possible to specify the number of times of

B. Second Embodiment FIG. 7 is a diagram showing a configuration example of an optical interference device 1B according to a second embodiment of the present invention. In FIG. 7, the same components as in FIG. 1 are labeled with the same reference numerals. As is clear from comparing FIG. 7 and FIG. 1, the configuration of the optical interference device 1B differs from the configuration of the optical interference device 1A in the following two points of difference.

The first difference is that there is a filter 220 between camera 100A and BS120. The filter 220 is a color filter that transmits light in two separate wavelength bands (a wavelength band of 500 nm±10 nm and a wavelength band of 700 nm±10 nm in this embodiment). In this embodiment, the filter 220 and the camera 100A form a light receiving means for separating the received light into two or more predetermined wavelength bands. Instead of the filter 220, a color filter that reflects light in two or more predetermined wavelength bands is used, and by guiding the light reflected by the color filter to the camera 100A, the received light is divided into two or more predetermined wavelength bands. A separate light receiving means may be formed.

A second difference is that a processing device 180B is provided instead of the processing device 180A. The processing device 180B is the same as the processing device 180A in that it functions as the generation means 182, but differs from the processing device 180A in that it functions as the identification means 184B instead of the identification means 184A. The identifying means 184B is common to the identifying means 184A in that it identifies the number of repeated reflections by the FP 150 based on the interference fringes of the first return light R1 and the second return light R2. When two or more interference fringe sets exist in the depth direction of the measurement object OB in the two-dimensional image generated by the generating means 182, the identifying means 184B performs the It differs from the identifying means 184A in that it identifies the number of repeated reflections. When two or more sets of interference fringes exist in the Z-axis direction, the reason why the number of repeated reflections can be specified based on the interval between the interference fringes that make up the set of interference fringes is as follows.

Also in the present embodiment, since the FP 150 is provided with dispersion, a phase delay for each wavelength occurs in the reference light L2 of each number of reflections, as shown in FIG. Since the reference light L2 has a different phase for each wavelength depending on the number of repeated reflections, the interference fringes generated by the interference between the first return light R1 and the second return light R2 have colors corresponding to the number of repeated reflections. Bleeding occurs. In this embodiment, since the filter 220 is arranged in front of the camera 100A, the interference fringes of the two wavelength bands that pass through the filter 220 in the color blur are formed as a set of two as shown in FIG. (In the figure, F1R and F1L form a pair of interference fringes, and F2R and F2L form a pair of interference fringes). In other words, there are two Z-axis values corresponding to a given X-axis position. The fringe intervals of the above two wavelength bands in the interference fringes forming one set by the two lines differ according to the number of repeated reflections. This is the reason why, when there are two or more sets of interference fringes in the Z-axis direction, the number of repeated reflections can be specified based on the interval between the interference fringes forming the set of interference fringes.

As described above, in the surface measurement technique using the Mach-Zehnder optical system, the optical interference apparatus 1B of the present embodiment also expands the depth measurement range using the Fabry-Perot etalon, and repeats the measurement using a single camera. It becomes possible to specify the number of reflections.

C. Third Embodiment FIG. 9 is a diagram showing a configuration example of an optical interference device 1C according to a third embodiment of the present invention. In FIG. 9, the same components as in FIG. 7 are labeled with the same reference numerals. As is clear from a comparison of FIGS. 9 and 7, the configuration of the optical interference device 1C differs from the configuration of the optical interference device 1B in the following two points of difference. A first difference is that the optical interference device 1C does not have the filter 220 . A second difference is that a camera 100C is provided instead of the camera 100A. The camera 100C is different from the camera 100A in that it is a color camera that outputs image signals GC of respective colors of red, green, and blue, that is, a color camera that separates received light into at least three wavelength bands. The camera 100C, like the filter 220 and the camera 100A in the second embodiment, serves as light receiving means for separating received light into two or more predetermined wavelength bands.

In this embodiment, the identifying means 184B identifies the number of repeated reflections based on the difference in the interval between interference fringes in two predetermined colors among red, green, and blue. For example, if the camera 100C is a Bayer pattern camera whose wavelength sensitivity of each pixel of R (red), G (green), and B (blue) is shown in FIG. In the case of a prism camera in which the wavelength sensitivity of each pixel of is shown in FIG. 10B, the wavelength near the sensitivity boundary of each color is used. The number of repeated reflections may be specified based on the difference in spacing.
FIG. 11 is a diagram showing an example of interference fringes captured by the camera 100C. In the example shown in FIG. 11, the dashed-dotted line represents the interference fringes of red light, the solid line represents the interference fringes of green light, and the dotted line represents the interference fringes of blue light. As shown in FIG. 11, in the interference fringes captured by the camera 100C, the positions of the red and blue interference fringes with respect to the green light interference fringes are different from each other, and the blue light interference fringes and the red light interference fringes are at different positions. The distance from the interference fringes differs depending on the number of repeated reflections.

As described above, in the surface measurement technique using the Mach-Zehnder optical system, the optical interference apparatus 1C of the present embodiment also expands the depth measurement range using the Fabry-Perot etalon and repeats the measurement using a single camera. It becomes possible to specify the number of reflections. Although the number of repeated reflections is specified based on the distance between the interference fringes of two predetermined colors, the distance in the Z-axis direction is measured using R, G , and B.

D. Modification Each embodiment described above may be modified as follows.
(1) In each of the above embodiments, the Fabry-Perot etalon 150 may be replaced with a plurality of Fabry-Perot etalons arranged in series (for example, two Fabry-Perot etalons arranged in series). Due to the limited machining precision of the reflective surface of the Fabry-Perot etalon, the spectral envelope of the output light does not have a very clean waveform. can mitigate the impact of

(2) Although the generating means 182 and the identifying means 184A in the first embodiment are software modules, either one or both of the generating means 182 and the identifying means 184A may be hardware modules such as ASIC. Even if one or both of the generating means 182 and the identifying means 184A are hardware modules, the same effects as in the first embodiment can be obtained. The identifying means 184B in the second embodiment may also be a hardware module.

(3) In each of the above embodiments, the beam shape of the measurement light L1 immediately before entering the measurement object OB was linear. However, if scanning in the Y-axis direction is also performed in addition to the X-axis direction, the beam shape of the measurement light L1 does not have to be linear.

1A, 1B, 1C... optical interference device 100A... camera 110, 120, 130... beam splitter 140... mirror 150... Fabry-Perot etalon 160... grating 170... lens 180A, 180B... processor 182... Generating means 184A, 184B... Identifying means, 190, 200,... Relay lens, 210... Lens, 220... Filter.

Claims (5)

a light receiving means;
a first beam splitter into which light emitted from a light source is incident;
a lens for condensing the measurement light, which is the first light split by the first beam splitter, onto the measurement object;
a Fabry-Perot etalon on which the reference light, which is the second light different from the first light and split by the first beam splitter, is incident;
a grating through which the reference light emitted from the Fabry-Perot etalon is guided;
provided between the first beam splitter and the lens, guides the first light to the lens and guides the first return light from the object to be measured to the light receiving means; a second beam splitter that guides the second return light of to the light receiving means;
generating means for generating a two-dimensional image representing interference fringes generated by the first return light and the second return light received by the light receiving means;
an optical interference apparatus comprising: specifying means for specifying, based on the thickness of the interference fringes represented in the two-dimensional image, the number of repeated reflections by the Fabry-Perot etalon of the second return light that causes the interference fringes. . a light receiving means for separating the received light into two or more predetermined wavelength bands;
a first beam splitter into which light emitted from a light source is incident;
a lens for condensing the measurement light, which is the first light split by the first beam splitter, onto the measurement object;
a Fabry-Perot etalon on which the reference light, which is the second light different from the first light and split by the first beam splitter, is incident;
a grating through which the reference light emitted from the Fabry-Perot etalon is guided;
provided between the first beam splitter and the lens, guides the first light to the lens and guides the first return light from the object to be measured to the light receiving means; a second beam splitter that guides the second return light of to the light receiving means;
generating means for generating a two-dimensional image representing interference fringes generated by the first return light and the second return light received by the light receiving means; when there is an interference fringe set, the number of repeated reflections by the Fabry-Perot etalon in the second return light that causes the interference fringe set is specified based on the interval of the interference fringes that make up the interference fringe set. and an optical interferometer. 3. The optical interference device according to claim 2, wherein said light receiving means includes a filter that transmits or reflects light in two or more predetermined wavelength bands, and a monochrome camera arranged after said filter. 3. The optical interference device according to claim 2, wherein said light receiving means includes a color camera that separates said received light into at least three wavelength bands. a beam shape of the measurement light immediately before entering the measurement object is linear;
An optical interference device according to any one of claims 1 to 4.

Images