Optical interference optical system and optical interference measurement system

Abstract

To eliminate unwanted light arising from measurement light being mixed into a reference optical system in an optical interference measurement system using a Mach-Zehnder optical system.SOLUTION: An optical interference measurement system 1 for measuring irregularity of a surface of a measurement object OB has: a light source SC; an optical interference optical system 10; a grating 20; a camera 30; and a processing device 40. In the optical interference optical system 10, a special filter 180 is provided between a relay lens 150 and a relay lens 160 that is aimed at preventing stray light serving as third light other than measurement light L1 from being guided to the grating 20.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical interference measurement device. [Background technology]

[0002] One example of a measurement technology for measuring the surface shape, such as the unevenness of the surface of a measurement object, is an optical interference measurement system that uses a Mach-Zehnder optical system (see, for example, Patent Document 1). In this type of optical interference measurement system, light emitted from a light source is separated into measurement light and reference light that are irradiated onto the measurement object, and the measurement light and reference light reflected by the measurement object are combined to form a combined light (interference light), thereby measuring the interference fringes of the combined light, thereby measuring the surface shape of the measurement object. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-099239 Summary of the Invention [Problem to be solved by the invention]

[0004] An optical interferometry system using a Mach-Zehnder optical system may include a first beam splitter for splitting light emitted from a light source into measurement light and reference light, and a second beam splitter for directing the measurement light to a measurement object and directing the reflected light of the measurement light from the measurement object to a camera or the like. This type of optical interferometry system may also include a reference optical system for directing the reference light to a grating, which may include a mirror and a third beam splitter. In an optical interferometry system with second and third beam splitters, the diffracted or reflected light of the reference light by the grating is directed by the second and third beam splitters to a camera for measuring interference fringes.

[0005] In an optical interferometry system using a Mach-Zehnder optical system, two beams of light are required to measure the surface shape of a measurement target: measurement light and reference light. However, in an optical interferometry system with the aforementioned first, second, and third beam splitters, the measurement light may enter the reference optical system, generating unwanted light that interferes with the measurement. Specifically, when the measurement light split by the first beam splitter is guided to a grating by the second and third beam splitters, and the diffracted or reflected light of the measurement light by the grating (hereinafter referred to as stray light) is guided to a camera by the third and second beam splitters, the stray light becomes unwanted light.

[0006] An object of the present invention is to provide a technique for removing unnecessary light that occurs when measurement light enters a reference optical system in an optical interference measurement system using a Mach-Zehnder optical system. [Means for solving the problem]

[0007] In one aspect, the optical interference optical system according to the present invention includes first and second beam splitters, a lens, a reference optical system, and a spatial filter. Light emitted from a light source is incident on the first beam splitter. The first beam splitter splits the incident light into measurement light, which is the first light, and a second light other than the first light. The lens focuses the first light onto the measurement object. The reference optical system guides the second light to a grating. The second beam splitter is disposed between the first beam splitter and the lens. The second beam splitter guides the first light to the lens, guides first return light from the measurement object to a light receiving means, and guides second return light from the grating to the light receiving means. The spatial filter prevents stray light, which is the third light other than the first light split by the second beam splitter, from being guided to the grating.

[0008] In a more preferred embodiment of the optical interference optical system, a first lens and a second lens are provided between the grating and the light receiving means, and the spatial filter may be positioned between the first lens and the second lens at a position that is in an optically Fourier transform relationship with the grating, or alternatively, a first lens and a second lens are provided between the grating and the light receiving means, and the grating may be positioned between the first lens and the second lens at a position that is optically conjugate with the light receiving means.

[0009] In a further preferred embodiment of the optical interference optical system, in each of the above embodiments, the spatial filter may be a liquid crystal element capable of selectively setting the light beam to prevent stray light, which is a third light other than the first light split by the second beam splitter, from being guided to the grating, and the spatial filter may be an obstruction target having a circular opening.

[0010] In a further preferred embodiment of the optical interference optical system in each of the above embodiments, the first beam splitter and the second beam splitter may be polarizing beam splitters.

[0011] In a more preferred embodiment of the light interference optical system, the beam shape of the measurement light incident on the object to be measured may be linear.

[0012] In another aspect, the present invention provides an optical interference measurement system having a light source, a light receiving means, a grating, a first beam splitter onto which light emitted from the light source is incident, a lens that focuses measurement light, which is a first light split from the first beam splitter, onto a measurement object, a reference optical system that guides reference light, which is a second light split from the first beam splitter and is other than the first light, to the grating, a second beam splitter that is provided between the first beam splitter and the lens and that guides the first light to the lens, guides first return light from the measurement object to the light receiving means, and guides second return light from the grating to the light receiving means, a spatial filter that prevents stray light, which is a third light other than the first light split by the second beam splitter, from being guided to the grating, and means for measuring the measurement object by interference caused by the first return light and the second return light received by the light receiving means. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing an example of the configuration of an optical interference measurement system 1 according to an embodiment of the present invention. [Figure 2] 10 is a diagram for explaining the size of the condensed spot of the relay lens 150. FIG. [Figure 3] 10 is a diagram for explaining how light is blocked by the spatial filter 180, focusing on the spectrum. [Figure 4] 10 is a diagram for explaining how light is blocked by the spatial filter 180, focusing on image formation. DETAILED DESCRIPTION OF THE INVENTION

[0014] Although various technically preferable limitations are applied to each of the embodiments described below, the embodiments of the present invention are not limited to the following embodiments.

[0015] A. Embodiment FIG. 1 is a diagram showing an example of the configuration of an optical interferometry system 1 according to an embodiment of the present invention. The optical interferometry system 1 is a system for measuring the unevenness of the surface of a measurement object OB, i.e., the surface shape of the measurement object OB. In addition to the optical interferometry system 1, FIG. 1 also shows the measurement object OB. As shown in FIG. 1, the optical interferometry system 1 includes a light source SC, an optical interference optical system 10, a grating (diffraction grating) 20, a camera 30, and a processing device 40. In this embodiment, the axis normal to the surface of the measurement object OB, whose surface shape is to be measured by the optical interferometry system 1, is referred to as the Z-axis. One of the two axes perpendicular to the Z-axis is referred to as the X-axis, and the other is referred to as the Y-axis.

[0016] The optical interference optical system 10 is a Mach-Zehnder optical system and includes a beam splitter (abbreviated as BS in FIG. 1 , and the same applies hereinafter in this specification) 110, BSs 120 and 130, a mirror 140, relay lenses 150, 160 and 170, a spatial filter 180, and a lens 190. Each of the BSs 110, 120 and 130 is a polarizing beam splitter. The reason for using polarizing beam splitters as the BSs 110, 120 and 130 is that it is easy to adjust the intensity of transmitted light and reflected light at each.

[0017] As shown in Fig. 1, light L emitted from a light source SC is incident on the BS 110. In a preferred embodiment, the light source SC is a coherent, broadband, and high-intensity pulsed light source produced by applying a nonlinear effect to the light emitted from an ultrashort pulse laser. Its wavelength range preferably covers red, green, and blue light. The BS110 splits the incident light L into measurement light L1, which is light transmitted through the BS110, and reference light L2, which is light reflected by the BS110. The BS110 is an example of a first beam splitter in the present invention. In this embodiment, the transmitted light through the BS110 is the measurement light L1, but the reflected light of the light L by the BS110 may be the measurement light L1 and the transmitted light may be the reference light L2. The measurement light L1 is an example of the first light in the present invention. The reference light L2 is an example of the second light in the present invention, i.e., a second light different from the first light.

[0018] The reference light L2 is reflected by the mirror 140 and then incident on the BS 130. The reference light L2 is guided by the BS 130 and then incident on the grating 20. In other words, the mirror 140 and the BS 130 are an example of a reference optical system that guides the reference light L2 to the grating 20.

[0019] The grating 20 has a plurality of sawtooth grooves on its incident surface for the reference light L2. FIG. 1 shows one of these grooves. The grating 20 diffracts the incident reference light L2. The diffracted light R2 of the reference light L2 by the grating 20 is an example of the second returned light in the present invention. In this embodiment, the diffracted light R2 is the second returned light, but the reflected light of the reference light L2 by the grating 20 may also be the second returned light. Hereinafter, the diffracted light R2 is also referred to as the second returned light R2.

[0020] The second return light R2 is guided by the BS 130, the relay lens 150, and the relay lens 160, and then enters the BS 120. In this embodiment, a spatial filter 180 is disposed at the focal position between the relay lens 150 and the relay lens 160. Details of the spatial filter 180 will be made clear later. The relay lens 150 is an example of the first lens in the present invention, and the relay lens 160 is an example of the second lens in the present invention.

[0021] As shown in FIG. 1, the BS 120 is provided between the BS 110 and the lens 190. The BS 120 guides the measurement light L1 to the lens 190 and also guides the reflected light R1 of the measurement light L1 by the measurement object OB to the camera 100. The reflected light R1 of the measurement light L1 by the measurement object OB is an example of the first returned light in the present invention. Hereinafter, the reflected light R1 will also be referred to as the first returned light R1. The BS 120 also guides the second returned light R2 to the camera 100. The BS 120 is an example of the second beam splitter in the present invention.

[0022] The lens 190 is a cylindrical lens that focuses the incident measurement light L1 into a line along the X-axis. In this embodiment, the surface shape of the measurement object OB is measured using a linear measurement light L1 that is 20 mm long and 10 microns wide along the X-axis. Because the surface shape of the measurement object OB is measured using the linear measurement light L1 along the X-axis, in this embodiment, scanning with the measurement light L1 in the X-axis direction is not necessary; only scanning with the measurement light L1 in the Y-axis direction is required. The linear measurement light L1 emitted from the lens 190 is focused by the relay lens 170 and then irradiated onto the surface of the measurement object OB. Reflected light R1 of the measurement light L1 by the measurement object OB, i.e., first return light R1, is guided by the relay lens 170, the lens 190, and the BS 120 and enters the camera 30.

[0023] The camera 30 outputs an image signal GC representing the received light, i.e., an image of the interference fringes generated by the first return light R1 and the second return light R2, to the processing device 40. The camera 30 is an example of the light receiving means of the present invention.

[0024] The processing device 40 includes a processor, i.e., a computer, such as a CPU (Central Processing Unit). The processing device 40 may include one computer or multiple computers. The processing device 40 operates in accordance with a program stored in a storage device (not shown in FIG. 1) to measure the unevenness of the surface of the measurement object OB based on the image signal GC provided by the camera 30. In other words, the processing device 40 functions as a means for measuring the measurement object OB by the interference caused by the first return light R1 and the second return light R2. The processing device 40 and the storage device may be part of a personal computer.

[0025] As described above, the spatial filter 180 is disposed at the focal position between the relay lens 150 and the relay lens 160. More specifically, the spatial filter 180 is disposed at a position between the relay lens 150 and the relay lens 160, and at a position that is optically conjugate with the camera 30. The spatial filter 180 is also disposed at a position between the relay lens 150 and the relay lens 160, and at a position that is optically in a Fourier transform relationship with the grating 20.

[0026] The spatial filter 180 is a spatial notch filter that blocks the third return light R3 (stray light) traveling from the relay lens 160 to the relay lens 150 and blocks only the center of the optical axis of the light R2 traveling from the relay lens 150 to the relay lens 160 in a circular or strip-like manner. The blocking may be achieved by reflection, absorption, or scattering. The spatial filter 180 prevents light other than the reference light L2 from being guided to the grating 20. Specifically, the spatial filter 180 may be formed of a liquid crystal element that can selectively set the wavelength and transmittance of light to be transmitted. Alternatively, the spatial filter 180 may be a so-called obstruction target with an annular opening that allows light rays reaching the spatial filter 180 to pass through the opening and blocks light rays that reach areas other than the opening.

[0027] The spatial filter 180 is not limited to the above-described configuration and may be configured, for example, by a pinhole and a lens. Furthermore, the spatial filter 180 does not need to separate a light beam into a completely transmitting portion and a completely blocking portion (change the shape of the light beam). In short, the spatial filter 180 may be any optical element that does not completely block the necessary reference light to be guided to the camera 30 (allows at least a portion to pass through), while changing the intensity distribution of the light beam guided to the spatial filter 180 so that unnecessary light (stray light) is not guided to the grating 20. In the present invention, the amount of diffracted light R2 guided to the camera is reduced compared to when the spatial filter 180 is not provided, but the accuracy (S / N) of the interference signal obtained by the camera 30 is improved by removing the stray light. Furthermore, since the spatial filter 180 can be understood to have a function of adjusting the amount of reference light, the arrangement, size, and other characteristics of the spatial filter 180 may be designed taking into account the amount of reference light required, as necessary.

[0028] The size of the spatial filter 180 may be from equal to to twice the size of the focused spot size d of the relay lens 150 (i.e., 100% to 200% of the size of the focused spot size d). When the spatial filter 180 is an obstruction target having an annular opening, the size of the spatial filter 180 refers to the diameter of the opening. When the spatial filter 180 is a rectangular liquid crystal element that blocks only the center of the optical axis of the second return light R2 in a strip shape, the size of the spatial filter 180 refers to the length of one side of the rectangle. When the wavelength of the light focused by the relay lens 150 is λ, the focused spot size d of the relay lens 150 is expressed by the following equation 1, where D is the beam diameter of the second return light R2 incident on the relay lens 150 as shown in FIG. 2 and f is the focal length of the relay lens 150. Note that in equation 1, π is the ratio of the circumference of a circle to its diameter.

number

[0029] 3 is a diagram conceptually illustrating the state of light blocking by spatial filter 180, focusing on spectroscopy. In optical interferometry system 1, second return light R2 is spatially separated into wavelengths at the focal position of relay lens 150 by grating 20 and relay lens 150. In the example shown in FIG. 3, blue light contained in second return light R2 is indicated by a dashed line, green light by a solid line, and red light by a dashed-dotted line. Note that in FIG. 3, to avoid cluttering the drawing, only the grating 20, BS 130, relay lens 150, spatial filter 180, relay lens 160, BS 120, and camera 30 are shown among the components of optical interferometry system 1 (the same applies to FIG. 4).

[0030] As shown in FIG. 3, of the second return light R2 traveling from the relay lens 150 toward the relay lens 160, the green light is blocked by the spatial filter 180, but the blue and red light are not blocked by the spatial filter 180 and reach the camera 30. That is, in this embodiment, the surface of the measurement object OB is measured using the blue and red light contained in the second return light R2. Also, in FIG. 3, the reflected light R3 of the measurement light L1 by the BS 120 is shown by a dotted line. The reflected light R3 is stray light unnecessary for measuring the unevenness of the surface of the measurement object OB, and is an example of the third light in the present invention. As shown in FIG. 3, all wavelengths of the reflected light R3 are collected at the focus of the relay lens 160 and blocked by the spatial filter 180. Therefore, in this embodiment, the reflected light R3 is prevented from being reflected by the grating 20 and returning, and unnecessary light toward the camera 30 is removed.

[0031] FIG. 4 is a diagram conceptually illustrating the manner in which light is blocked by spatial filter 180, focusing on image formation. Reference symbols PSC1, PSC2, and PSC3 in FIG. 4 represent point light sources when the grooves provided on the surface of grating 20 are considered to be an array of point light sources. In FIG. 4, reflected light R3 is indicated by a dotted line, as in FIG. 3. In the example shown in FIG. 4, the light emitted in the optical axis direction of second return light R2 from each of point light sources PSC1, PSC2, and PSC3 is blocked by spatial filter 180, but the other light is not blocked by spatial filter 180 and reaches camera 30. Also, reflected light R3 is blocked by spatial filter 180, as in FIG. 3.

[0032] As described above, according to this embodiment, unnecessary light resulting from the measurement light being mixed into the reference optical system is removed in the optical interference measurement system 1 using the Mach-Zehnder optical system. As a result, it becomes possible to measure the surface shape of the measurement object by effectively utilizing the dynamic range of the camera 30.

[0033] B. Deformation The above-described embodiments may be modified as follows. (1) In the above embodiment, the optical interference measurement system 1 is described as including the light source SC, the optical interference optical system 10, the grating 20, the camera 30, and the processing device 40. However, the optical interference optical system 10, i.e., the optical interference optical system including the BSs 110, 120, and 130, the mirror 140, the relay lenses 150, 160, and 170, and the spatial filter 180, may be implemented as a single unit.

[0034] (2) In the above embodiment, the beam shape of the measurement light L1 immediately before it is incident on the measurement object OB is linear. However, if scanning is performed in the Y-axis direction in addition to the X-axis direction, the beam shape of the measurement light L1 does not have to be linear. [Explanation of symbols]

[0035] 1...optical interference measurement system, SC...light source, 10...optical interference optical system, 110, 120, 130...beam splitter, 140...mirror, 150, 160, 170...relay lens, 180...spatial filter, 20...grating, 30...camera, 40...processing device.

Claims

1. a first beam splitter onto which the light emitted from the light source is incident; a lens that focuses the measurement light, which is the first light split from the first beam splitter, onto a measurement object; a reference optical system that guides a reference light, which is a second light other than the first light and is split from the first beam splitter, to a grating; a second beam splitter provided between the first beam splitter and the lens, which guides the first light to the lens, guides first return light from the object to a light receiving means, and guides second return light from the grating to the light receiving means; a spatial filter that blocks the center of the optical axis of the second return light in a circular or stripe shape, thereby preventing stray light, which is a third light other than the first light split by the second beam splitter, from being guided to the grating; An optical interference optical system having:

2. a first lens and a second lens are provided between the grating and the light receiving means; the spatial filter is between the first lens and the second lens, The grating is disposed at a position that is in an optical Fourier transform relationship with the grating. The optical interference system according to claim 1 .

3. a first lens and a second lens are provided between the grating and the light receiving means; the spatial filter is between the first lens and the second lens, The light receiving means is disposed at a position that is optically conjugate with the light receiving means. The optical interference system according to claim 1 .

4. The spatial filter is a liquid crystal element that can be selectively configured to block stray light from being directed onto the grating. The optical interference system according to any one of claims 1 to 3.

5. The spatial filter is an obstruction target having an annular opening. The optical interference system according to any one of claims 1 to 3.

6. The first beam splitter and the second beam splitter are polarizing beam splitters. The optical interference system according to any one of claims 1 to 5.

7. The beam shape of the measurement light incident on the measurement object is linear. The optical interference system according to any one of claims 1 to 6.

8. A light source and Light receiving means; Grating and a first beam splitter onto which the light emitted from the light source is incident; a lens that focuses the measurement light, which is the first light split from the first beam splitter, onto a measurement object; a reference optical system that guides a reference light, which is a second light other than the first light and is split from the first beam splitter, to the grating; a second beam splitter provided between the first beam splitter and the lens, which guides the first light to the lens, guides first return light from the object to the light receiving means, and guides second return light from the grating to the light receiving means; a spatial filter that blocks the center of the optical axis of the second return light in a circular or stripe shape, thereby preventing stray light, which is a third light other than the first light split by the second beam splitter, from being guided to the grating; a means for measuring the object to be measured by interference caused by the first return light and the second return light received by the light receiving means; An optical interferometry system having:

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