Phase Shift Interferometer and Shape Measurement Methods

Abstract

Phase-shift interferometer configured to measure a shape of a measurement object (402; 402a) by analyzing interference fringes based on a phase-shifting method, wherein the interference fringes are generated by a reference beam and a measuring beam or by a distortion in an optical reference path (401a) and an optical measuring path (401b), wherein the reference beam (401a) is a reflected beam from a reference surface (401) which serves as a reference for a measurement, wherein the measuring beam (401b) is obtained by reflection of a beam from a measuring object or item (402) or by propagation of a beam through the measuring object (402a), wherein the optical reference path (401a) does not contain the object being measured (402; 402a) arranged on the optical reference path (401a), wherein the optical measurement path (401b) includes the object being measured (402; 402a) arranged on the optical measurement path (401b), wherein the phase-shift interferometer is further configured to acquire images of the interference fringes using a plurality of cameras (604), and to independently perform a phase analysis of each of the interference fringes, and wherein the interference fringes are obtained by each of the plurality of cameras (604), and then to synthesize results of the phase analysis in order to calculate or determine at least part of the shape of the object being measured (402; 402a), and wherein images of the interference fringes with a phase difference relative to each other are provided, the majority of cameras (604) comprise two cameras (604a, 604b) which are provided with a phase difference of 90° relative to each other, and the two cameras (604a, 604b) are configured to capture the images of the interference fringes in order to measure or determine at least part of the shape or form of the object being measured (402; 402a), and wherein the results of the phase analysis are synthesized by calculating mean values ​​of the results of the phase analysis, wherein each of the mean values ​​is a mean value of results obtained from a set of corresponding pixels or picture points of the plurality of cameras (604).

Description

Technical field

[0001] The present invention relates to phase-shift interferometers, and in particular to a phase-shift interferometer that achieves reduced analysis errors due to shift errors and thus enables high-precision measurement using a reduced number of images (in a shorter time). The phase-shift interferometer is suitable for use in measuring the shapes of objects or items, such as optical flat surfaces, lenses, machined metal surfaces, and / or semiconductor surfaces, including wafers. Furthermore, the present invention relates to corresponding shape measurement methods. State of the art

[0002] An interferometer is an instrument for measuring the shape of an object with high precision by analyzing the phases of interference fringes generated by a reflected beam from the reference surface and a reflected beam from the object. Phase-shifting techniques are commonly used to analyze the phases of interference fringes (see JP H05-87541 A, JP 2000-329535 A, JP 4869656 B).

[0003] JP H05-87541 A describes the measurement of a two-dimensional physical quantity of a body to be measured using a rectilinearly polarized laser beam. This beam is converted into a parallel beam by a lens system and strikes an optical measuring system. The polarized light is separated into X-polarized and Y-polarized light by a polarizing semi-transparent mirror. The polarized light then passes through a transparent body to be measured and combined with the polarized light from the first mirror using a second polarizing semi-transparent mirror. After maintaining the polarization of its wave surface and splitting the wave surface into three amplitudes by an optical system, the beam passes through the polarizing plates, and the resulting interference fringes are photographed with telephoto cameras.The video signals from the telephoto cameras are converted into A / V signals in a signal processor, and the phase distribution is measured at a television rate by reading the phase data for a triple digital quantity. The phase difference is fed into a television monitor, and an image relative to the phase distribution is displayed as a light and shadow pattern.

[0004] JP 2000-329535 A describes a measuring device for phase-shifting interference fringes. A linearly polarized luminous flux, containing a sample light from a surface under test in an optical anti-interference state and a reference light, is split into three output luminous fluxes that have a phase difference of a specified amount with respect to the phase of the sample light and the reference light, while the optical anti-interference state is maintained. The sample light and the reference light, as output luminous fluxes, are optically made to interfere. In this case, an optical splitting system, which splits the sample light and the reference light into three output luminous fluxes with the specified phase difference, is equipped with a complex prism 9A onto which a linearly polarized luminous flux is incident, and which emits the three output luminous fluxes.The imaging and processing device is the simultaneous measuring instrument for the phase-shifting interference fringes, which includes a position reversal and conversion section.

[0005] JP 4 869 656 B2 describes a measurement in which the light emitted by a light source is converted into parallel light using a collimating lens. An (s)-polarized component of the parallel light passes through a polarizing plate and reaches an object W to be measured. A (p)-polarized component is reflected from the polarizing plate, which also serves as a reference surface. The synthesized light is reflected by a beam splitter, passes through a 1 / 4 wavelength plate, and is converted into light containing a left-handed circularly polarized component and a right-handed circularly polarized component. After conversion, the synthesized light is split into three beams using non-polarizing beam splitters and a mirror, and then polarized by plates with different phase shifts.

[0006] JP 4 799 766 B2 describes a phase-shift interference fringe simultaneous measuring instrument wherein the shape calculation is performed relative to an error at the shape calculation time which is generated by the difference in preload and amplitude between branch phase-shift interference fringes produced by splitting the original beam which includes reflected light from the reference surface and the test surface by re-accounting for the intensity ratio of the sample light at the preload and amplitude calculation time and at the shape measurement time, namely the reflection ratio.

[0007] US 2013 / 0 100 333 A1 describes a polarization imaging device comprising a laser light source and an image-taking element. The object light and reference light each contain a first polarization light component polarized in a first direction and a second polarization light component polarized in a second direction different from the first. The image-taking element simultaneously captures an image of an interference pattern comprising (i) a first interference figure, (ii) a second interference figure, (iii) a third interference figure, and (iv) a fourth interference figure.The polarization imaging device includes a reconstruction section that generates corresponding reconstructed images of the object from the first to fourth interference figures with respect to the first and second polarization light components, and a polarization light image calculation section that obtains polarization light images from the reconstructed images.

[0008] US 2002 / 0003628A1 describes a device for splitting, mapping, and measuring wavefronts with a reference wavefront and an object wavefront. A wavefront combination element receives an object wavefront from an object and a reference wavefront and combines them into a combined wavefront. A wavefront splitting element divides the combined wavefront into a plurality of sub-wavefronts such that each sub-wavefront is substantially adjacent to at least one other sub-wavefront. The wavefront splitting element can shift the relative phase between the reference wavefront and the object wavefront of the sub-wavefronts to obtain a corresponding plurality of phase-shifted sub-wavefronts.The wavefront-splitting element can then interfere with the reference and object wavefronts of the phase-shifted sub-wavefronts to obtain a corresponding number of phase-shifted interferograms. An imaging element receives and displays these phase-shifted interferograms. A computer connected to the imaging element measures various object parameters based on these phase-shifted interferograms. Examples of measurements include flow parameters such as the concentrations of selected gaseous species, temperature distributions, particle and droplet distributions, density, and so on. In addition to flow parameters, the displacement (e.g., vibration) and profile of an object can also be measured.

[0009] JP 2005 - 43 081 A describes an interferometer device that enables fringe scanning by physically moving a component irradiated by a light source and by changing the phase difference of two luminous fluxes that cause light wave interference. Summary of the invention: Technical problem

[0010] Fig. Figure 1 illustrates an example of an optical configuration of a Fizeau interferometer, which is a commonly used configuration in optics. Problems with the associated prior art are described here. The Fizeau interferometer is configured as follows. A light beam from a light source 1 is expanded to the size of the interferometer's field of view, for example, using a lens 2, an aperture 3, and a lens 5. A reference surface 6, which serves as a reference for a measurement, and a measurement object 7 are illuminated with the expanded light beam. The reference beam, which is a beam reflected from the reference surface 6, and the measurement beam, which is a beam reflected from the surface of the measurement object 7, are made to interfere with each other using a beam splitter 4. Phase shifting or...Interference fringing methods are performed as follows. A plurality of interference fringes are obtained by shifting the relative phase between the reference beam and the measurement beam. The interference fringes are then captured or recorded, for example, by a camera 8, and a numerical analysis is performed to calculate the phases of the interference fringes.

[0011] A typical method used for phase shifting is to move the reference surface 6 parallel to an optical axis A using, for example, a shifting platform 9. In this case, the shift is performed several times for one interference fringe cycle, and the reference surface 6 must be moved precisely by a distance in the range of a few tens to a few hundred nanometers. However, it is extremely difficult to perform a precise shift according to a calculated setting value in measurement environments where, for example, vibration or air disturbance / turbulence occurs, regardless of the fact that a shifting platform 9 is constructed to enable precise shifting with nanometer accuracy. As a result, a difference between the magnitude orthe size of a displacement predicted by a calculation and the actual extent of a displacement, and this results in the occurrence of errors in a calculation of the phase (referred to as phase analysis errors).

[0012] A typical method used to reduce phase analysis errors is to increase the number of phase shifts performed to increase the number of acquired or obtained images. These methods utilize the averaging effect by processing a larger number of images, or they predict a specific shift error and apply a corresponding shift to correct it. With these methods, as the analysis precision increases, the number of obtained images tends to rise.

[0013] This means that when performing a high-precision measurement using a state-of-the-art phase-shift interferometer, a displacement or shift stage 9 is necessary for shifting the reference surface 6 with high precision, which is costly. Another problem is that to reduce the phase analysis errors resulting from errors in the displacement of the displacement stage 9, the number of acquired images must be increased, and consequently, a longer measurement time is required.

[0014] Additionally, another problem is that if the measurement time is long, the gradual or increasing displacement of the position of the reference surface 6 is more likely to be affected by vibrations and / or temperature changes in the measurement environment, and therefore, depending on the user's operating environment, the effect of improving precision may not be sufficiently achieved.

[0015] JP H05-87541 A, JP 2000-329535 A, and JP 4869656 B disclose technologies similar to the invention for using a plurality of imaging devices that simultaneously image interference fringes exhibiting phases optically shifted relative to each other. However, these technologies have many limitations.

[0016] The invention was developed to solve the problems of the prior art. Accordingly, one objective or subject matter of the invention is to provide a phase-shift interferometer for shape measurement which is designed with less expensive components and achieves high-precision shape measurement in a shorter time.

[0017] This subject matter is solved according to the invention by the features of the independent claims. Particular embodiments of the invention are the subject of the dependent claims. Solution to the problem

[0018] According to one aspect, a phase-shift interferometer is provided to solve the problems mentioned above. The phase-shift interferometer is configured to measure the shape of a target object by analyzing interference fringes using a phase-shifting technique. The interference fringes are generated by a reference beam and a measurement beam, or by distortion in an optical reference path and an optical measurement path. The reference beam is a beam reflected from a reference surface, which serves as a reference for the measurement. The measurement beam is obtained either by reflection of a beam from the target object or by propagation of a beam through the target object.The optical reference path does not include the object being measured, but the optical measurement path does. The optical measurement path includes the object being measured, which is positioned along the optical measurement path. The phase-shift interferometer is further configured to capture images of the interference fringes using multiple cameras, to independently perform a phase analysis of each interference fringe (with the interference fringes being obtained by each of the multiple cameras), and then to synthesize the results of the phase analysis to calculate or determine at least part of the shape of the object being measured.

[0019] Furthermore, the images of the interference fringes are provided with a phase difference relative to each other. Most cameras consist of two cameras or comprise two cameras positioned with a phase difference of 90° relative to each other, and these two cameras are configured to capture the images of the interference fringes in order to measure or determine the shape or form of the object being measured.

[0020] Furthermore, the results of the phase analysis are synthesized by calculating mean values ​​of the phase analysis results, each of which is an average or mean value of results obtained from a set of corresponding pixels or image points of the majority of cameras.

[0021] According to a preferred embodiment, the phase-shifting method can be implemented in particular by extending or lengthening or shortening a length of the optical reference path or a length of the optical measurement path.

[0022] Furthermore, the length of the optical reference path and / or the length of the optical measurement path can be lengthened or shortened, in particular, by moving the reference surface and / or the object being measured and / or by lengthening or shortening an optical delay path.

[0023] According to a further aspect of the invention, a shape measurement method for measuring the shape or form of a measurement object or item is provided by analyzing interference fringes based on a phase-shifting method, in particular by using the interferometer according to the above aspect of a particular embodiment thereof, comprising the following steps: Generating interference fringes by a reference beam and a measurement beam or by distortion in an optical reference path and an optical measurement path, wherein the reference beam is a reflected beam from a reference surface which serves as a reference for a measurement, wherein the measurement beam is obtained by reflection of a beam from a measurement object or by propagation of a beam through the measurement object, wherein the optical reference path does not contain the measurement object arranged on the optical reference path, and wherein the optical measurement path contains the measurement object arranged on the optical measurement path. Capturing images of the interference fringes using a variety of cameras, Independently performing a phase analysis of each of the interference fringes, The interference fringes are obtained by each of the majority of cameras, and Synthesizing results of phase analysis to calculate or determine at least part of the form or shape of the object being measured.

[0024] Furthermore, images of the interference fringes with a phase difference relative to each other are provided, the majority of cameras comprise two cameras or are two cameras which are provided with a phase difference of 90° relative to each other, and the two cameras are configured to capture the images of the interference fringes in order to measure or determine at least part of the shape or form of the object being measured.

[0025] Furthermore, the results of the phase analysis are synthesized by calculating averages or mean values ​​of the results of the phase analysis, where each mean value is an average of results obtained from a set of corresponding pixels or image points of the majority of cameras.

[0026] According to a preferred embodiment, the phase-shifting method is implemented in particular by extending or lengthening or shortening a length of the optical reference path and / or a length of the optical measurement path.

[0027] Furthermore, the length of the optical reference path and / or the length of the optical measurement path is lengthened or shortened, in particular by moving the reference surface and / or the object being measured and / or by lengthening or shortening an optical delay path to implement a phase shift.

[0028] Advantageous Effects of the Invention: Prior art phase-shift interferometers require a precisely manufactured displacement platform for shifting the reference surface. In contrast, the invention allows phase analysis errors to be minimized, even if there is a shift or displacement error in the displacement platform. Accordingly, it is sufficient to use a displacement platform that is relatively low-precision and less expensive. While there is no need to perform additional measurements to reduce errors, a high level of measurement precision is achieved and maintained. Brief description of the drawings

[0029] These and other features, characteristics, and advantages of the present invention will become clearer upon reading the following detailed description of preferred embodiments and the accompanying drawings. It should be understood that, even though embodiments are described separately, individual features thereof can be combined to form additional embodiments. Fig. Figure 1 is a ray diagram illustrating a configuration example of the optics in a state-of-the-art Fizeau phase-shift interferometer. Fig. Figure 2 is a ray diagram illustrating a configuration according to a first embodiment of the invention. Fig. Figure 3 is a flowchart illustrating a measurement process according to the first embodiment. Fig. Figure 4 is a diagram illustrating an example of the results of an analysis according to the first embodiment. Fig. Figure 5 is a diagram illustrating a comparison between examples of results from an analysis performed on the outputs of two cameras according to the first embodiment. Fig. Figure 6 is a ray diagram illustrating a configuration according to a second embodiment of the invention. Fig. Figure 7 is a ray diagram illustrating a configuration according to a third embodiment of the invention. Fig. Figure 8 is a ray diagram illustrating a configuration according to a fourth embodiment of the invention. Fig. Figure 9 is a ray diagram illustrating a configuration according to a fifth embodiment of the invention. Description of embodiments

[0030] The following sections describe embodiments of the present invention in detail with reference to the drawings. It should be noted that the present invention is not intended to be limited to the content described in the following embodiments and examples. The illustrative or structural elements described in the following embodiments and examples include elements that can be readily conceived or developed by a person skilled in the art, elements that are essentially the same as those described, and elements that are considered equivalent within the scope of the invention. Furthermore, the structural elements disclosed in the following embodiments and examples can be suitably combined and / or appropriately selected and used.

[0031] Fig. Figure 2 illustrates a configuration or setup of a phase-shift interferometer according to a first particular embodiment of the invention. A light beam from a low-coherence light source 101 is split into two components, each having a polarization plane orthogonal to the other. The low-coherence light source 101 is a particular light source 100. In this embodiment, the light beam is split into a horizontally polarized light beam 11 and a vertically polarized light beam 12 by a polarization beam splitter (PBS) 201, which is in particular a component of an optical delay path 200. The vertically polarized reflected light beam is, or is, at least partially deflected, in particular by being reflected by a mirror 203, and then the horizontally polarized beam and the vertically polarized beam are combined by a PBS 202.The combined light beam is then expanded and collimated by beam expansion optics 300, which are in particular composed of or comprise a lens 301, a non-polarized beam splitter (NPBS) 302 and / or a collimating lens 303.

[0032] Subsequently, a reference surface 401 and a measurement object 402 of the phase difference generation unit 400 are irradiated with light beams to obtain a reference beam and a measurement beam, which are reflected beams from the reference surface 401 and the surface of the measurement object 402, respectively. The phase difference generation unit 400 is arranged on the same optical axis as the Fizeau phase-shift interferometers of the prior art. In the figure, reference numeral 403 denotes a displacement or displacement stage.

[0033] The light beams for analysis, which are formed or comprise the reference beam and the measuring beam, are extracted by the beam-expanding optics 300 through the NPBS 302 and guided through an imaging lens 500. The imaging lens 500 serves only to allow flexibility in the optical design, such as the magnification and focal point position for interference fringes in the phase-shift interferometer. Therefore, the presence or absence of the imaging lens 500 does not significantly affect the principle of the invention.

[0034] The light beams of the horizontally polarized beam and the vertically polarized beam are then passed through a λ / 4 plate 601 of a phase difference detection unit 600 to be converted into circularly polarized beams, which rotate in opposite directions relative to each other, either to the right or to the left. The light beams are then split for analysis by a beam splitter 602, and polarizers 603a and 603b, which can be rotated and adjusted relative to each split optical path, are positioned on the respective split optical paths. In this way, the phase difference of the reflected beam from the object 402 relative to the reference surface 401 is visualized as an interference fring.Images of the interference fringes, which have different phases, are captured by two cameras, 604a and 604b.

[0035] In this phase-shift interferometer, the difference L is measured. a the length of the optical path for a derivative and the difference L b The length of the optical path between the optical path of the reflected beam from the reference surface 401 and the optical path of the reflected beam from the surface of the object 402 is, in particular, substantially matched to each other. Furthermore, the light source 100, in particular the light source 101, has a low coherence, with a coherence length ΔL shorter than the difference L. athe length of the optical path. As a result, the interference fringes, which are made visible by the light beams passing through the polarizers 603a and 603b, which are arranged on the respective subdivided optical paths, are generated, in particular, only by the component of the vertically polarized beam, which is the reflected beam from the reference surface 401, and the component of the horizontally polarized beam, which is the reflected beam from the surface of the object 402. The interference fringes obtained by the two cameras 604a, 604b each have, or can have, a phase shift relative to the other according to the setting angles of the polarizers 603a and 603b, which are arranged in front of the cameras 604a and 604b.For example, the polarizer 603b can be positioned such that its transmission axis is rotated in a plane normal to the optical axis, in particular by about 45° relative to the transmission axis of the polarizer 603a. As a result, the phases of the interference fringes are shifted relative to each other, in particular by about 90°. The interference fringes are imaged by the cameras 604a and 604b and processed, for example, by a personal computer (PC) 700.

[0036] Next, illustrate Fig. 3. A measurement procedure or process is performed by the phase-shift interferometer of this embodiment. The displacement or displacement stage 403 is, in particular, slightly shifted mechanically in parallel to shift the phases of the interference fringes (step S1), and images of the interference fringes are obtained by the two cameras 604a and 604b in the phase-difference detection unit 600 (step S2). The two cameras 604a and 604b are optically phase-shifted relative to each other. Next, the phases of the interference fringes in the images obtained or captured by the two cameras 604a and 604b are analyzed, and a shape calculation is performed independently for each of them, in particular using the PC 700 (step S3). Subsequently, the shapes obtained by the cameras 604a and 604b are synthesized, for example, by addition, to determine the shape or...To determine the shape of the measuring object 402 (step S4).

[0037] Examples of the effects of this embodiment are provided below. In the four-phase step method, in which a cycle of the phase of an interference fring is divided into four segments, and the phase is shifted by 90° for each segment, the four images of the interference fringes are represented by the following equations. The wavelength of the light source 101 determines the extent of the shift. For example, if the wavelength is 633 nm, the extent of the shift per step is approximately 79 nm. {I1(x,y)=B(x,y)−A(x,y)cos[ϕ(x,y)]I2(x,y)=B(x,y)−A(x,y)cos[ϕ(x,y)+π2]I 3(x,y)=B(x,y)−A(x,y)cos[ϕ(x,y)+π]I4(x,y)=B(x,y)−A(x,y)cos[ϕ(x,y)+3π2]

[0038] By performing the following calculation based on the four equations, Φ can be determined, and by performing a phase unwrapping process, the shape or form of the measured item or object 402 can be determined. ϕ(x,y)=−tan−1[I2(x,y)−I4(x,y)I1(x,y)−I3(x,y)]

[0039] Here, in the process of shifting from I1 to I2, I3, and I4, if a shift is made different from a calculated setpoint or fixed value (here corresponding to a phase of 90°), the result of the calculation of equation (5) will be affected by the shift or displacement error. This is illustrated by the following example: Fig. 4 the result of a simulation of a case where a displacement error Er1 in I2 has occurred. In Fig. Figure 4 illustrates (a1) an analysis result Φ Analysis and (a2) illustrates a true or correct value in the simulation Φ True. (a3) ​​illustrates a phase analysis error Φ error , which represents the difference between the analysis result and the true value, Φ Analysis - Φ True As this is shown in Fig. 4 is illustrated, is Φ True A value based on the assumption that an ideal plane is inclined such that only one interference fring can be produced or generated. As illustrated in (a2), the phase analysis error cycle is specifically half the cycle of the phase of the interference fring (twice the frequency), with the magnitude increasing and decreasing within the cycle. The results are referred to as the results of a shape calculation by a phase-shift method from the interference fringes obtained by the single camera 604a.

[0040] Next, illustrate Fig. 5. The results of a shape calculation from four images of interference fringes obtained by the other camera 604b. The interference fringes exhibit a phase shifted by a fixed angle of 90° relative to the phase of the interference fringes obtained by camera 604a. The phase analysis results, which are presented in Fig. The four illustrations are, or will in turn be, in Fig. Figures (a1), (a2), and (a3) ​​illustrate this for comparison. Phase analysis errors have a frequency twice the frequency of the interference fringes' phase. Thus, by shifting the original phase of the interference fring obtained by camera 604b by 90° relative to the phase of the interference fring obtained by camera 604a, the phase analysis error is caused to be in exactly the inverted form. Accordingly, as illustrated in (c1), (c2), and (c3), phase analysis errors can be eliminated by calculating the mean of the results of a calculation for each set of pixels representing an identical position from the pixels of camera 604a and camera 604b.

[0041] Prior art phase-shift interferometers require a precisely manufactured displacement platform 9 for shifting the reference surface 6 to prevent the occurrence of errors corresponding to the phase of an interference fringe. Additionally, to reduce phase analysis errors due to displacement errors, a large number of images of interference fringes must be obtained, and consequently, a longer measurement period is required. As indicated by the simulation results, the technique of the invention minimizes phase analysis errors even if there is a displacement error in the displacement platform 403, and it is therefore sufficient to use a displacement platform 403 that is relatively low-precision and less expensive. In particular, high-precision measurements can be achieved with a smaller number of obtained or acquired images.

[0042] In this embodiment, a λ / 4 plate is not used in the phase difference generation unit 400, and therefore a measurement of spherical surfaces of lenses is also possible.

[0043] The components of the phase-shift interferometer are not limited to those of the first embodiment, which are described in Fig. 2 is illustrated. For example, as in a second special embodiment, which is shown in Fig. As illustrated in Figure 6, the optics are as follows. The light source 100 can, in particular, be a laser 102. A λ / 4 plate 404 can be arranged between the reference surface 401 and the surface of the object 402 to rotate the polarization plane of the reflected beam from the surface of the object 402 by 90°, in particular without using the optical delay path 200. This configuration is also achieved by a phase-shift interferometer, which produces a similar effect. The other components or elements and / or processes or actions are essentially the same or similar to those of the first particular embodiment, and thus such elements are designated with the same reference numerals and descriptions thereof are omitted.

[0044] In the second particular embodiment, since the laser 102 is a light source with high coherence, especially as the light source 100, the adjustment is easy or simple.

[0045] Furthermore, for example, as in a third special embodiment, which in Fig. Figure 7 illustrates the use of a Twyman-Green type phase-shift interferometer. In the phase-shift interferometer, a polarization beam splitter 405 is provided in the phase-difference generation unit 400 to divide the light beam into a reference beam and a measurement beam and to combine the beams. Figures 404a and 404b are shown in Figures 404a and 404b. Fig. The reference numeral 7 denotes a λ / 4 plate. The other components and / or processes are essentially the same or similar to those of the first embodiment, and such components are designated with the same reference numerals, and descriptions thereof are omitted.

[0046] In the first to third particular embodiments, the reference surface 401 must be displaced or relocated. Alternatively, the object being measured 402 can be displaced. Furthermore, in the first particular embodiment, the optical path length of the light beam l2 can be increased and / or decreased, in particular by displacing or relocating the mirror 203 in the optical delay path 200.

[0047] As in a fourth special embodiment, which in Fig. As illustrated in Figure 8, a Mach-Zehnder type interferometer can be used. The interferometer is often used to measure the shape of the wavefront of a light beam passing through a test object 402a, which is, in particular, a transparent object. The test object 402a is arranged beneath polarization beam splitters 405a and 405b and mirrors 406a and 406b, which are provided in the phase difference generation unit 400. In this case, the phase shift can be achieved, for example, by arranging the mirror 406a and the mirror 406b on a displacement platform 407 and moving the platform 407.

[0048] In the embodiment of Fig. In particular, no reference surface is provided, and one of the optical paths (the lower optical path in the figure) serves as an optical reference path 401a. This optical path does not contain the object 402a. That is, the distortion caused by the object 402a is measured, or can be measured, by measuring the relative distortion of the wavefront between the lower optical reference path 401a, in which no object is present, and the upper optical measurement path 401b, in which the object 402a is located, using interferometry.

[0049] In Fig. 8 are the optics for the case where ΔL ≤ L b In the case where ΔL < L b In particular, the optical delay path 200 can be added to the optics by using a light source with low coherence, in order to achieve the embodiment as shown in (b). Fig. 7 to be implemented. The measuring object 402a can be arranged between the polarization beam splitters 405a and 405b.

[0050] In the phase difference detection unit 600, the number of cameras in the imaging system is not limited to two. As in a fifth special embodiment, which is described in Fig. As illustrated in 9, three or more cameras can be used (in Fig. 9 three cameras 604a, 604b and 604c). This configuration also achieves the effect of reducing errors.

[0051] In the embodiment described above, the phase difference between the interference fringes obtained by the two cameras 604a and 604b is, in particular, 90°. However, if a phase analysis error occurs which has the same cycle as the phase of the interference fring (a fundamental wave), the magnitude of the phase difference to be provided between the two cameras 604a and 604b can be set, in particular, to 180° in order to eliminate the error. If a phase analysis error occurs which has a cycle of one-third of the cycle of the phase of the interference fring (a triple wave), a phase difference of 60° can, in particular, be provided.An effective way to provide the phase difference between cameras 604a and 604b is, for example, to adjust the angles of the transmission axes of polarizers 603a and 603b in such a way as to eliminate phase analysis errors that can be caused, for example, by the features of the displacement platform 403 for shifting the reference surface. The polarizers 603a and 603b are arranged (in particular, substantially directly) in front of cameras 604a and 604b. Even if the phase difference between the interference fringes obtained by cameras 604a and 604b is 0°, i.e., the phases of the two are identical, the present embodiment of the invention is effective because errors due to random noise, such as electrical noise, can be reduced.

[0052] Accordingly, a phase-shift interferometer and a corresponding shape measurement method are provided. The phase-shift interferometer uses fewer expensive components and / or achieves high-precision shape measurement in a shorter time.

[0053] The phase-shift interferometer is specifically configured and constructed to measure the shapes of measurement objects 402, 402a by obtaining multiple images of interference fringes while shifting the phases of the interference fringes. The interference fringes are provided with a phase difference of 90° relative to each other using polarization of light. Images of the interference fringes are captured by two cameras 604a, 604b, while, in accordance with a conventional phase-shifting method, a reference surface 401 or an optical reference path 401a is mechanically shifted to shift the phases. The phases of the interference fringes are calculated independently from the respective images obtained by the cameras 604a, 604b, and an average is obtained.The mean of the two phase calculation results is calculated. Thus, even if phase analysis errors occur for each of the cameras 604a, 604b due to errors in the relocation of the relocation stage 403, 407, the analysis errors can be eliminated by calculating the average of the two results, and a measurement of high precision is accordingly achieved or maintained. List of reference symbols 100 light sources 200 optical delay path 201, 202 polarized or polarization beam splitter (PBS) 300 beam expansion optics 302 Non-polarized beam splitter (NPBS) 400 Phase Difference Generation Unit 401 Reference surface 401a optical reference path 401b optical measurement path 402, 402a Object or item being measured 403, 407 Displacement or relocation platform 404a, 404b, 601 λ / 4 plate 500 imaging lens 600 Phase Difference Detection Unit 602 Beam splitter 603a, 603b, 603c Polarizer 604a, 604b, 604c camera 700 Personal Computers (PCs)

Claims

[1] Phase-shift interferometer configured to measure a shape of a measurement object (402; 402a) by analyzing interference fringes based on a phase-shifting method, wherein the interference fringes are generated by a reference beam and a measuring beam or by a distortion in an optical reference path (401a) and an optical measuring path (401b), wherein the reference beam (401a) is a reflected beam from a reference surface (401) which serves as a reference for a measurement, wherein the measuring beam (401b) is obtained by reflection of a beam from a measuring object or item (402) or by propagation of a beam through the measuring object (402a), wherein the optical reference path (401a) does not contain the object being measured (402; 402a) arranged on the optical reference path (401a), wherein the optical measurement path (401b) includes the object being measured (402; 402a) arranged on the optical measurement path (401b), wherein the phase-shift interferometer is further configured to acquire images of the interference fringes using a plurality of cameras (604), and to independently perform a phase analysis of each of the interference fringes, and wherein the interference fringes are obtained by each of the plurality of cameras (604), and then to synthesize results of the phase analysis in order to calculate or determine at least part of the shape of the object being measured (402; 402a), and wherein images of the interference fringes with a phase difference relative to each other are provided, the majority of cameras (604) comprise two cameras (604a, 604b) which are provided with a phase difference of 90° relative to each other, and the two cameras (604a, 604b) are configured to capture the images of the interference fringes in order to measure or determine at least part of the shape or form of the object being measured (402; 402a), and wherein the results of the phase analysis are synthesized by calculating mean values ​​of the results of the phase analysis, wherein each of the mean values ​​is a mean value of results obtained from a set of corresponding pixels or picture points of the plurality of cameras (604). [2] Phase-shift interferometer according to claim 1, wherein the phase-shift method is implemented by lengthening or shortening a length of the optical reference path (401a) and / or a length of the optical measurement path (401b). [3] Phase shift interferometer according to claim 2, wherein the length of the optical reference path (401a) and / or the length of the optical measurement path (401b) is lengthened or shortened by moving the reference surface (401) and / or the object being measured (402; 402a) and / or by lengthening or shortening an optical delay path for implementing a phase shift. [4] Form measurement method for measuring the shape of a measurement object (402; 402a) by analyzing interference fringes based on a phase-shifting method, comprising the following steps: Generating interference fringes by a reference beam and a measurement beam or by distortion in an optical reference path (401a) and an optical measurement path (401b), wherein the reference beam (401a) is a reflected beam from a reference surface (401) which serves as a reference for a measurement, wherein the measurement beam (401b) is obtained by reflection of a beam from a measurement object (402) or by propagation of a beam through the measurement object (402a), wherein the optical reference path (401a) does not contain the measurement object (402; 402a) arranged on the optical reference path (401a), and wherein the optical measurement path (401b) contains the measurement object (402; 402a) arranged on the optical measurement path (401b). Capturing images of the interference fringes using a variety of cameras (604), Independently performing a phase analysis of each of the interference fringes, The interference fringes are obtained by each of the plurality of cameras (604), and Synthesizing results of phase analysis to calculate or determine at least part of the form or shape of the object being measured (402; 402a), and wherein images of the interference fringes with a phase difference relative to each other are provided, the majority of cameras (604) comprise two cameras (604a, 604b) which are provided with a phase difference of 90° relative to each other, and the two cameras (604a, 604b) are configured to capture the images of the interference fringes in order to measure or determine at least part of the shape or form of the object being measured (402; 402a), and wherein the results of the phase analysis are synthesized by calculating mean values ​​of the results of the phase analysis, wherein each of the mean values ​​is a mean value of results obtained from a set of corresponding pixels or picture points of the plurality of cameras (604). [5] Shape measurement method according to claim 4, wherein the phase shift method is implemented by lengthening or shortening a length of the optical reference path (401a) and / or a length of the optical measurement path (401b). [6] Shape measurement method according to claim 5, wherein the length of the optical reference path (401a) and / or the length of the optical measurement path (401b) is lengthened or shortened by moving the reference surface (401) and / or the object being measured (402; 402a) and / or by lengthening or shortening an optical delay path to implement a phase shift.

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