JP5181403B2 - Interferometer - Google Patents

Description

  The present invention relates to an optical apparatus for measuring physical characteristics such as the surface shape of an object using the principle of light wave interference.

  Interferometric measurement technology is indispensable for precise measurement of the surface shape and spectral characteristics of an object, the thickness of a transparent body, and the movement distance. There are numerous types of interferometers such as Twiman-Green Interferometer, Mach-Zehnder Interferometer, and Fizeau Interferometer. When considering the measurement, it is necessary to be an interferometer capable of acquiring a two-dimensional image by an imaging optical system having a certain NA (Numerical Aperture).

  As an interferometer used in such a case, there are a Michelson interferometer, a Miro interferometer, and a linic interferometer. As shown in FIG. 5, the Michelson interferometer divides the optical path between the objective lens 401 and the object 108 by a semi-transparent mirror 501 inclined at 45 °, and is reflected by the reference mirror 502 and the object 108. Interfere with light. Although it is an easy to handle and highly versatile interferometer, when the objective lens 401 has a high magnification and high NA, it becomes difficult to obtain a sufficient working distance (hereinafter referred to as OWD) of the objective lens 401. , It becomes difficult to insert the semi-transparent mirror 501 inclined at 45 °. Therefore, it is used for observation / measurement of low magnification and low NA.

  The Miro interferometer is an interferometer for observation / measurement with high magnification and high NA. As shown in FIG. 6, the illumination light is branched to the object side and the objective lens 401 side by the semi-transparent mirror 104 arranged perpendicular to the optical axis, and the light collected by providing the reference mirror 502 on the front surface of the objective lens 401 is collected. In this structure, the light is further reflected by the semi-transparent mirror 104, returned to the objective lens 401, reflected by the object 108, and interfered with the light transmitted through the semi-transparent mirror 104. In this structure, since the semi-transparent mirror 104 does not need to be 45 °, it can be realized even if the OWD is small. However, this structure requires the objective lens 401 to have a large NA and a small field of view because the reference mirror 502 having a size larger than the field of view of the objective lens 401 is provided in front of the objective lens 401. become. In the case of the objective lens 401 having a wide field of view and a small NA as shown in FIG. 5, this is generally the case with the objective lens 401 with a low magnification. End up.

  As shown in FIG. 7, the linic interferometer has a structure in which an optical path is branched into two by a 45 ° semi-transparent mirror 501 before illumination light enters the objective lens 401, and the objective lens 401 is arranged in both of the two optical paths. In the case of such a structure, since it is not necessary to insert anything between the objective lens 401 and the object 108, it can be realized even with the high magnification / high NA objective lens 401 having a remarkably small OWD. However, it is economically undesirable to use the two most expensive objective lenses 401 in the optical system, and the use of two objective lenses 401 having complicated aberration characteristics is the same in terms of specifications. Even if one is used, it is not preferable in view of the fact that the aberration situation does not become exactly the same due to processing assembly errors.

The advantages and disadvantages of the conventional interferometer are briefly described above, but these are described in detail in Non-Patent Document 1.
Daniel Malacara, "Optical Shop Testing 2nd. Edition" John Wiley & Sons, Inc., pp. 700-703, 1992

The NA of the objective lens 401 is the most important value that governs the performance of the optical system. The larger the NA, the higher the lateral resolution. In addition, when the reflection angle of the object reflected light is large due to the surface inclination of the object 108 or the like, there is a possibility of deviating from the optical axis. High observation / measurement becomes impossible.

  In addition, the wider the field of view, the more convenient for observation and measurement. In particular, considering the application to in-line measurement, which has become widespread recently, the demand for high speed is very strong, and it is required to have as wide a field of view as possible directly related to the measurement speed.

That is, when observing and measuring the object 108, it can be said that a high NA and a wide field of view (low magnification) are required. The higher the NA and the wider field of view, the more difficult it is to manufacture the objective lens 401. Since the aberration situation becomes very severe, the number of lenses increases, and assembly adjustment also requires accuracy. If the NA is wide and the field of view is large, the lens aperture itself needs to be increased. Further, if it is necessary to take a longer OWD, the difficulty increases both in terms of aberration and lens aperture. If the OWD cannot be taken long, the Michelson interferometer cannot be used. Further, such a linic interferometer using two large objective lenses 401 having a high manufacturing difficulty is economically undesirable. Of course, the Millo interferometer is not possible because it cannot be used at low magnifications.

  The problem to be solved by the present invention is to realize an economically excellent interferometer that can be realized even with the objective lens 401 having a high NA and a wide field of view, or in other cases.

In order to solve the above-mentioned problem, the polarization component in a direction perpendicular to or substantially perpendicular to the incident direction of incident light, transmits all or most of the polarization component in a specific direction, and is orthogonal to the polarization direction. Polarized optical element 102 having a function of reflecting all or most of the light, first quarter-wave plate 103 for converting linearly polarized light transmitted through the polarized optical element 102 into circularly polarized light, and light that has been circularly polarized light An interferometer is constituted by a semi-transparent mirror 104 that reflects a part of the light and transmits a part thereof, and a second quarter-wave plate 105 that converts the light transmitted through the semi-transparent mirror into linearly polarized light again.

  The polarizing optical element 102 can be realized by a wire grid polarizer or a photonic crystal.

The interferometer includes an imaging optical system that generates an optical image of the object 108, a photoelectric conversion element that photoelectrically converts the optical image and outputs an electrical signal, and part or all of the imaging optical system. An illumination system that illuminates the object 108 through the objective lens 401 is attached.

  And a moving mechanism for moving the polarizing optical element 102 in the optical axis direction and a moving mechanism for moving the object in the optical axis direction. An interferometer system incorporating a function of performing an arithmetic process for obtaining physical characteristics such as a surface shape of an object by analyzing the stored signal by digitizing and storing an electrical signal obtained from the photoelectric conversion element.

  By configuring as described above, even if an objective lens 401 having a high NA and a wide field of view is used, an economically excellent interferometer can be realized.

  In the following, embodiments that are considered to be the best for concrete implementation of the present invention will be described.

  First, a first example of an embodiment embodying the present invention will be described with reference to FIG.

Basic components are a polarizing optical element 102, a first quarter-wave plate 103, a semi-transparent mirror 104, and a second quarter-wave plate 105 in order from the incident direction of the illumination light 101. A polarization optical element 102 as a key part is a polarization beam splitter, and is used by being inserted perpendicularly or substantially perpendicular to the optical axis. It has a function of transmitting a specific linearly polarized light component and reflecting a linearly polarized light component orthogonal to the component. As shown in FIG. 2, such an element is a so-called wire grid polarizer in which thin wires (wires) of a conductor such as aluminum are arranged on a glass substrate at a pitch at a level below the wavelength of light, or a fine below the wavelength level. This is realized by a so-called photonic crystal that manipulates the characteristics of light by the structure. Hereinafter, an example using a wire grid polarizer will be described.

  The incident illumination light 101 is linearly polarized light in a direction orthogonal to the wire of the polarizing optical element 102 and passes through the polarizing optical element 102. The transmitted illumination light 101 is incident on a first quarter-wave plate 103 installed so that the optical axis is 45 degrees with respect to the polarization direction of the illumination light 101, and becomes two components in a direction perpendicular to the optical axis. Circular polarization is obtained by giving a phase difference of ¼ wavelength.

A quarter-wave plate having such a function is manufactured mainly using the birefringence of quartz. Quartz has birefringence characteristics in which the refractive index differs between the optical axis and the axis orthogonal thereto. By controlling the thickness of the crystal, it is possible to give a desired amount of phase difference between the components in both axial directions of the transmitted light. Since the thickness that gives a quarter wavelength is usually very thin, tens of microns, it is difficult to process. Use two quartz plates with a thickness that can be processed, and bond them so that their optical axes are orthogonal to each other. It is possible to give the necessary phase difference by canceling the influence of refraction and making one to be thicker than the other. In addition to such a so-called zero-order wave plate, a multi-order wave plate that gives a phase difference of an integral multiple of the illumination wavelength + 1/4 wavelength using a single crystal may be used. A refractive material may be used. It is also possible to manufacture a quarter wave plate with a fine periodic structure such as a wire grid polarizer or a photonic crystal. In any case, any element that converts incident linearly polarized light into circularly polarized light is within the scope of the present invention.

The illumination light 101 that has become circularly polarized light enters the semi-transparent mirror 104 and is branched into transmitted light and reflected light. Various methods for realizing the semi-transparent mirror 104 are conceivable. However, it is preferable to manufacture the semi-transparent mirror 104 by a technique in which the intensity is branched while maintaining the polarization characteristics of incident light as much as possible. For example, it can be realized by depositing a chromium film on a glass substrate. Hereinafter, the transmitted light is referred to as object light 106, and the reflected light is referred to as reference light 107. The reference beam 107 reflected by the semi-transparent mirror 104 is incident on the first quarter-wave plate 103 again, and a quarter-wave phase difference is given between the optical axis and the orthogonal component, and a half-wavelength is also obtained. The phase difference becomes linearly polarized light orthogonal to the polarization direction at the time of incidence and reaches the polarizing optical element 102.

The direction of polarization orthogonal to the incident time is the direction in which the electric field oscillates parallel to the wire and is reflected by the polarizing optical element 102, and this reflecting surface acts as a reference mirror. The reflected reference beam 107 passes through the first quarter-wave plate 103 three times. The reference beam 107 that has been circularly polarized again by the first quarter-wave plate 103 is reflected by the semi-transparent mirror 104, passes through the first quarter-wave plate 103 four times, and becomes linearly polarized light in the original polarization direction. It passes through the optical element 102 and returns to the incident direction of the illumination light 101. On the other hand, the object beam 106 transmitted through the semi-transparent mirror 104 is converted into linearly polarized light in the polarization direction orthogonal to the incident light by the second quarter wavelength plate 105 to illuminate the object 108. The reflected object beam 106 is sequentially transmitted through the second quarter-wave plate 105, the semi-transparent mirror 104, and the first quarter-wave plate 103, has the same polarization direction as that at the time of incidence, passes through the polarization optical element 102, and is illuminated. 101 returns to the incident direction. From the above, it can be seen that both the reference beam 107 and the object beam 106 pass through the polarization optical element 102 and are superposed to function as an interferometer. The polarization optical element 102 is an interferometer that functions as a reference mirror.

  For ease of explanation, the polarizing optical element 102, the first quarter-wave plate 103, the semi-transparent mirror 104, and the second quarter-wave plate 105 have been described as separate parts. However, as shown in FIG. Can be manufactured. If a semi-permeable film is deposited on the back surface of the first quarter-wave plate 103 or the front surface of the second quarter-wave plate 105 and the first quarter-wave plate 103 and the second quarter-wave plate 105 are bonded together, The three parts can be one part. The polarizing optical element 102 can also be bonded to the first quarter-wave plate 103, but it is better to have a space in consideration of matching with the object side optical path. Even so, it is possible to manufacture all parts as a single unit using the frame 301. The main feature is that it can be made very compact and can be used easily.

If a light source having a very long coherent length such as a laser is used as the illumination light source, the optical path length between the polarizing optical element 102 and the semi-transparent mirror 104 as a reference mirror, and the optical path length between the semi-transparent mirror 104 and the object 108 are necessarily used. Even if they do not match, it functions as an interferometer, but the superiority of this interferometer over other interferometers is not clear. In other words, this interferometer has the great advantage of simplicity and compactness that an interferometer can be configured simply by inserting a flat part into the illumination optical path as described above, but such an interferometer is a Fizeau interferometer. Since the Fizeau interferometer is simpler in structure, there is no significant advantage of this interferometer.

  When this interferometer is configured as an interferometer having an equal optical path length or an approximately equal optical path length, the superiority becomes clear. The Fizeau interferometer cannot be configured as an equal optical path length interferometer. This interferometer can use a light source with much lower coherence while having the same advantages (simpleness, compactness, common optical path property) as the Fizeau interferometer. When the coherent length is short, there is an advantage that interference due to back surface reflection can be prevented, for example, in the case of measuring the surface accuracy of a flat substrate. If the optical path length is completely equal, it can be used as a white interferometer.

  When combined with an imaging optical system, the advantages of this interferometer become clearer. As a second example of the embodiment of the present invention, a case where it is combined with an imaging optical system will be described with reference to FIG.

The imaging optical system is a telecentric imaging system including an objective lens 401 and an imaging lens 402 that is connected to the objective lens 401 at infinity. A semi-transparent mirror 403 is provided between the objective lens 401 and the imaging lens 402 so that illumination light can be introduced. The illumination optical system 404 is a Koehler illumination system and is arranged to form an image of the light source 405 at the pupil position of the objective lens 401. An interference filter 406 is inserted in the illumination optical path to narrow the band, and the time coherency is increased to facilitate interference. In addition, the illumination light is linearly polarized by the polarization filter 407. A two-dimensional image sensor 408 is disposed at the focal position of the imaging lens, and a video signal is output from a camera including the two-dimensional image sensor 408. The video output signal is digitized and stored and processed in the arithmetic processing unit 409.

  The interferometer is disposed between the objective lens 401 and the object 108 of the imaging optical system, the optical path length between the focal position (object plane) of the objective lens 401 and the semi-transparent mirror 104, and the reference mirror of the interferometer. The interferometer is adjusted so that the optical path length between the polarizing optical element 102 and the semi-transparent mirror 104 is the same. That is, the arrangement of the objective lens 401, the semi-transparent mirror 104, the reference mirror 502 (FIG. 6), and the object 108 on the optical axis is exactly the same as that of the Miro interferometer.

Since the optical arrangement is the same as that of the Miro interferometer, it basically functions as an interferometer with the same light beam movement as that of the Miro interferometer. The major difference is the reference mirror 502 (FIG. 6). The reference mirror 502 (FIG. 6) of the Miro interferometer is an obstacle that blocks the illumination beam / imaging beam, but in this interferometer, the polarizing optical element 102, which is the reference mirror, blocks the illumination beam / image beam. Absent.

Since the reference mirror 502 (FIG. 6) of the Miraud interferometer needs to have a size larger than the field of view, in the case of the objective lens 401 having a low NA and a low magnification (wide field of view), the light flux near the optical axis is the reference mirror 502. Since it is completely blocked by (FIG. 6), no data can be obtained. Even if the NA is large and the light beam in the vicinity of the optical axis is not completely blocked as shown in FIG. 6, the peripheral light beam becomes a very deviated imaging light beam, so that significant coma aberration occurs. In any case, the Millo interferometer is in no way preferred for the imaging performance of the objective lens 401 and cannot be used at least in a wide field objective lens.

On the other hand, in the present interferometer, as long as the illumination light is linearly polarized light, the illumination light beam and the imaging light beam are not blocked. On the other hand, the space required for the interference optical system portion (the portion where the light beam is split into two beams and then overlapped again) is not much different from that of the Milo interferometer and is very compact, so it is not necessary to take a large OWD like a Michelson interferometer.

  Increasing the OWD greatly increases the difficulty of manufacturing the objective lens 401, and also increases the size of the objective lens 401, making it impossible to manufacture the objective lens 401 with a wide field of view and high NA. It is also possible to become. Although not impossible, it is not preferable in terms of cost and size.

With this interferometer, it is possible to realize an interferometer that can be used regardless of the magnification and NA of the objective lens 401 at a compact and low cost.

If the object 108 is mounted on the Z table 410 and can move in the optical axis direction, the phase can be shifted. Therefore, the interference image obtained by shifting the phase by shifting the Z table 410 by a predetermined amount is used as the two-dimensional image sensor 408. The surface shape of the object 108 can be measured by converting it into an electronic image, storing it in the arithmetic processing unit 409, and analyzing it.

Further, if the transmission band width by the interference filter 406 is increased to obtain low coherence light, the 0th order interference fringe that is an interference fringe that appears when the optical path difference between the two optical paths of the interferometer becomes exactly zero can be specified. It can be used as a so-called white interferometer.

That is, by moving the Z table 410 and acquiring interference images at intervals of several tens to several hundreds of nanometers in the optical axis direction, and specifying the position where the 0th-order interference fringe appears for each pixel, Surface shape can be measured.

  The above is a typical embodiment of the present invention, but the present invention is not necessarily limited to the above embodiment. For example, the imaging optical system is described as a telecentric optical system, and the illumination system is a Koehler illumination system. These are preferable for precision measurement, but are not essential for the present invention. There may be a so-called finite correction system objective lens 401 that does not have a lens corresponding to the imaging lens 402 and generates an image only by the objective lens 401, or may be a critical illumination system. The light source 405 is not necessarily a white light source, and may be a laser or an LED.

Also, the Z table 410 for phase shift or white interference measurement does not necessarily need to move the object, and the objective lens 401 and the interference optical system part may be moved together in the Z direction. A phase shift can also be realized by moving only the polarizing optical element 102 in the Z direction. If the polarizing optical element 102 is set slightly tilted from the state perpendicular to the optical axis of the objective lens 401, an interference fringe having a high spatial frequency can be obtained and analyzed by a well-known Fourier transform method.

Further, the imaging optical system is not limited to the general optical system as described above, but may be a special optical system such as a confocal optical system. Any optical system that can finally obtain an optical image may be used.

  In the above description, a two-dimensional image is targeted, but there is no change even if it is a one-dimensional image. The two-dimensional image sensor 408 only becomes a one-dimensional image sensor. Further, even point measurement like an optical stylus does not change technically.

  According to the present invention, it is possible to realize an interferometer that is economically and excellent in size regardless of the NA and magnification of the objective lens. There is a great demand in industrial fields where precise measurement is required.

It is the figure which showed the 1st Example of this invention. It is a figure for demonstrating a wire grid polarizer. It is a figure for demonstrating the compact state of a 1st Example. It is the figure which showed the 2nd Example of this invention which has an imaging optical system. It is a figure for demonstrating a Michelson interferometer. It is a figure for demonstrating a Miro interferometer. It is a figure for demonstrating a linlic interferometer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Illumination light 102 ... Polarizing optical element 103 ... 1st 1/4 wavelength plate 104 ... Semi-transparent mirror 105 ... 2nd 1/4 wavelength plate 106 ... Object light 107 ... Reference light 108 ... Object 301 ... Frame 401 ... Objective lens 402 DESCRIPTION OF SYMBOLS ... Imaging lens 403 ... Semi-transparent mirror 404 ... Illumination optical system 405 ... Light source 406 ... Interference filter 407 ... Polarization filter 408 ... Two-dimensional image sensor 409 ... Arithmetic processor 410 ... Z table 501 ... Semi-transparent mirror 502 ... Reference mirror

Claims (5)

  A function that is arranged perpendicular or nearly perpendicular to the incident direction of the incident light, transmits all or most of the polarization component in a specific direction, and reflects all or most of the polarization component in a direction orthogonal to the polarization direction A polarizing optical element, a first quarter-wave plate that converts linearly polarized light transmitted through the polarizing optical element into circularly polarized light, and a part of the circularly polarized light is reflected and partially transmitted. An interferometer comprising: a semi-transparent mirror, and a second quarter-wave plate that converts light transmitted through the semi-transparent mirror into linearly polarized light again.   The interferometer according to claim 1, wherein the polarizing optical element is a wire grid polarizer.   The interferometer according to claim 1, wherein the polarizing optical element is a photonic crystal.   An object is illuminated through an imaging optical system that generates an optical image of the object, a photoelectric conversion element that photoelectrically converts the optical image and outputs an electrical signal, and an objective lens that is a part or all of the imaging optical system. The interferometer according to claim 1, further comprising an illumination system.   The photoelectric conversion element has a moving mechanism for moving the polarization optical element in the optical axis direction and a moving mechanism for moving the object in the optical axis direction, and controls the moving mechanism and the photoelectric conversion element. The interferometer according to claim 4, wherein the interferometer has a function of digitizing and storing the electrical signal obtained from the above, and performing arithmetic processing for obtaining the physical characteristics such as the surface shape of the object by analyzing the stored signal.

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