JP2013257302A - Heterodyne interference device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To set a signal frequency of an image to not more than approximately 100 Hz so as to detect a two-dimensional interference image with a normal CCD camera in a heterodyne interference device, and to detect precise phase information from interference image intensity.SOLUTION: Two acoustic optical devices (AOD) are used to create two light waves having different frequencies and the two AODs are each driven with signals having two different frequency components. In this case, two beams having frequencies of the difference between two frequency components from the respective AODs overlap and generate a beat. When a frequency difference between two sets of beat light waves is set to approximately 100 Hz, the frequency of a beat image obtained by interference of the two sets of beat light waves becomes approximately 100 Hz. For the detected beat image, three or four beat images are detected in synchronization with the timing obtained by dividing one period of the beat image signal by three or four, and the detected image intensity is calculated to compute the phase information, thus computing a three-dimensional shape including height information of a sample.

Description

  The present invention relates to a configuration of a heterodyne interferometer for measuring a three-dimensional height shape of a sample in a submicrometer region based on the principle of heterodyne interferometry.

  In recent years, in many fields such as living organisms and industry, there is a growing need for measuring a minute height shape in the wavelength region or below the wavelength, and many measurements using an interferometer are performed. A commonly used interferometer causes two light waves of object light and reference light having the same optical frequency to interfere with each other, and calculates a phase distribution from the intensity distribution of the obtained interference fringes. This interference fringe is stationary in time, and a still image is detected. It is called homodyne interferometry in the sense of causing light of the same frequency to interfere, but generally the accuracy when calculating the phase from the intensity of the interference fringes is only a few tenths of the wavelength, and several tens of nanometers It was difficult to measure the height distribution of the region.

  Interferometry that enables higher-accuracy measurement with respect to the above homodyne interference, that is, heterodyne interferometry in which two light waves having different frequencies interfere with each other is well known. FIG. 6 shows the principle configuration of the conventional heterodyne interferometry, and the operation will be described. The laser light emitted from the laser light source 61 is divided into two directions by a beam splitter (BS) 611, and the laser light 612 that has passed through the BS 611 is irradiated onto the sample 62 whose height shape is to be measured. The laser light reflected by the sample 62 becomes object light 613. The other laser beam 614 reflected by the BS 611 is incident on the frequency shifter 63, and the frequency of the laser beam is converted. Reference numeral 64 denotes a frequency shifter drive signal unit that drives the frequency shifter 63 to set a frequency shift amount. The laser beam 615 whose frequency has been converted becomes reference light. Here, many acousto-optic elements are used for the frequency shifter 63, and the frequency shift amount is generally several tens MHz to several hundreds KHz.

  The frequency of the laser light emitted from the laser light source 61 is f0, the frequency of the object light 613 is f0, and the frequency of the reference light 615 is f1. That is, in heterodyne interference, it is necessary to set the frequencies of the object beam 613 and the reference beam 615 to different values. When the object beam 613 and the reference beam 615 are incident on the photoelectric conversion unit 65 made of a photodiode or the like, interference occurs and a photoelectrically converted signal is generated. This signal is a beat signal 616 and has a frequency f0-f1 which is the difference between the frequencies f0 and f1. The frequency of this difference is in the region of several hundreds KHz when low, and information such as the phase of the frequency in the optical region can be detected at a low frequency in the beat frequency region. This beat frequency is determined depending on the signal frequency from the frequency shifter drive signal unit 64.

  Since the phase of the beat signal 616 changes according to the height of the sample 62, detection of the phase is important. However, the phase cannot be detected only from a single beat signal. Therefore, it is necessary to use the comparison signal device 66, and the comparison signal 617 having the same frequency as the beat signal 616 is generated. Note that the phase of the comparison signal 617 is constant. The phase comparator 67 compares the phases of the beat signal 616 and the comparison signal 617, and detects the phase change of the object light 613 that changes according to the height of the sample 62. The shape calculation unit 68 calculates the height of the sample from the detected phase. Thus, in the conventional heterodyne interferometer, since the beat frequency is as high as several hundreds KHz or more, measurement as an image cannot be performed, and the phase between two electrical signals is detected.

  FIG. 7A shows a specific configuration example of the heterodyne interference apparatus. Laser light 700 emitted from the laser light source 70 enters the optical system 71. The frequency of the laser beam 700 is f0. The optical system 71 includes an acousto-optic element (hereinafter abbreviated as AOD) 72 as a frequency shifter for converting the frequency of laser light. The AOD 72 is driven by an AOD control circuit 720 and generates two beams of light separated into two having different frequencies. The frequencies of the two light beams are f1 and f2. The two-beam light is split in two directions by the beam splitter 725, and the two-beam light reflected by the beam splitter 725 is photoelectrically converted by the 75 light receivers 1 as reference light 73 to generate a reference signal 752. The two-beam light transmitted through the beam splitter 725 is irradiated and reflected on the sample 62 through the objective lens 727, and further reflected by the beam splitter 725, and is subjected to photoelectric conversion as the object light 74 by the light receiver 2 of 76, and the object light signal. 762 is generated. Each of the object light 74 and the reference light 73 is a two-beam light having a different frequency.

  The reference light signal 752 and the object light signal 762 are beat signals having the same frequency and the difference frequency f1-f2 between the two light beams. The phase φr of the reference light signal 752 is constant, but the phase φs of the object light signal 762 changes according to the height shape of the sample 62. The phase difference detection unit 77 compares the phase difference φs−φr between the object light signal 762 and the reference light signal 752 to detect the phase φs. The data processing unit 78 calculates the sample height information from the detected phase difference.

  The phase comparison operation will be described with reference light signal 752 and object light signal 762 shown in FIG. Both the frequencies of the two signals are f1-f2, and both are beat signals obtained by the interference of two light beams. Since the reference light signal 752 does not include sample information, the phase is constant. Since the phase of the object light 74 changes according to the height of the sample, the phase of the object light signal 762 changes. When the phase of the reference light signal 752 is set to 0, the detected phase difference corresponds to the phase of the object light. The relationship between the phase φ and the height is λ / 720 in the case of reflected light detection and λ / 360 in the case of transmitted light detection when the wavelength of the laser light is λ. In the case of a He—Ne laser having a wavelength of 633 nm, the phase of 1 degree is 0.88 nm for reflection and 1.8 nm for transmission. From the above, in the case of heterodyne interference, a high-sensitivity height shape that is several tens of times higher than that of conventional homodyne interference can be measured.

  The conventional heterodyne interferometer shown in FIG. 7 is configured to generate two beams of light from the AOD 72. FIG. 8A shows the signal system for driving the AOD 72 when two beams of light are generated from the AOD 72, and the operation will be described. The amplitude modulator 84 modulates the amplitude between the two signals with respect to the oscillator 82 that emits the oscillation frequency fa and the oscillator 83 that emits the oscillation frequency fm, and drives the AOD 72 with a signal having two frequency components fa ± fm. The laser beam 700 incident on the AOD 72 undergoes diffraction and frequency shift, and is split into two beam beams 85 and 86 traveling in different directions. The separation magnitude θm is proportional to the frequency fm, and the diffracted angle θa is proportional to the frequency fa. Further, the difference in frequency between the two light beams 85 and 86 is 2fm, and when the two light waves interfere with each other, a beat signal having a frequency of 2fm is obtained. The driving frequency of a normal AOD is about 40 MHz for fa and about 200 KHz for fm. Therefore, the frequency of the beat signal is about 400 KHz.

  FIG. 8B shows a state in which the two-beam light is irradiated on the sample whose three-dimensional shape is to be measured. As shown by 850 and 860, the intensity distribution of the two-beam light is often used in a state where the distance between the peak intensities is separated to about the diameter of each beam spot. Such a state is when the beat frequency is about several hundreds KHz. If there is a difference in height of Δh between the two points irradiated with the two-beam lights 850 and 860 focused on a minute spot diameter, a difference in optical path length occurs between the two beams, which is the beat signal. It is detected as a change in phase. Therefore, the heterodyne interferometer shown in FIG. 7 has a differential configuration for detecting a change in phase between two light beams irradiated on a sample, and is a point measurement method. Details of this technique are described in detail in the technical literature, “Optics”, Vol. 21, No. 5, (1992), pp 327 to pp 332, by the present inventors.

Problems to be solved by the invention

  A conventional heterodyne interferometer is a differential type that irradiates a sample with two light waves that are close to each other at different frequencies, converts the optical path difference between the two points irradiated with the spot light into a phase, and measures the height shape. It is a configuration. That is, point measurement. Therefore, in the case of a spread sample, it is necessary to scan two-beam light in a two-dimensional plane. In order to perform two-dimensional scanning, it is necessary to add a two-dimensional scanning optical system for performing scanning, which causes a problem that the optical apparatus is complicated and large. In addition, when the sample is mechanically moved two-dimensionally, there is a problem that the measurement time becomes long.

  Furthermore, when focusing on the beat signal, it is also a problem that the frequency of the beat signal is about 200 KHz or more. In order to increase the function of the heterodyne optical device from point measurement to surface measurement, it is necessary to detect a beat image with a CCD camera, for example. For this purpose, it is necessary to have a beat image in the response frequency band of the CCD camera, for example, about 50 Hz. Therefore, it is necessary to set the frequency of the beat signal generated from the AOD to a low frequency that is less than 1/1000 of the current frequency. In order to solve the above-mentioned problems, the present invention realizes a heterodyne interferometric optical device that realizes a low-frequency beat signal, for example, 100 Hz or less, and that can perform surface measurement using a normal CCD camera or the like. The object is to realize height shape measurement with short time and high measurement accuracy.

Means for solving the problem

  In order to achieve the above object, a heterodyne interferometer according to claim 1 of the present invention includes a first acoustooptic device that converts a laser beam emitted from a laser light source into a frequency of the laser beam, and a second one. The first and second acoustooptic elements are driven by signals of different frequencies output from the acoustooptic element driving unit and are incident on the acoustooptic elements of the first and second acoustooptic elements. Two light waves of object light and reference light having different frequencies are created, and the object light having the first frequency is reflected or transmitted by irradiating the sample whose height shape is to be measured, and the reference light having the second frequency. Heterodyne interference optical system that creates a beat image having a two-dimensional spread in which the interference light intensity changes in a sine wave form with respect to time having a frequency that is a difference between the object light and the reference light. When, An image detection unit comprising a camera that detects the beat image in response to a frequency band near the frequency of the beat image, and a plurality of the beat images in synchronization with a predetermined timing within a period of the beat image An image detection control unit that controls an image detection operation so as to detect sheets, an image processing unit that calculates the intensity of the detected images and calculates the phase of the beat image, and the image processing unit And a shape calculating unit that calculates the height shape of the object from the obtained phase.

  The invention according to claim 2 relates to the heterodyne interference device according to claim 1, wherein each of the object light and the reference light light waves generated by the heterodyne interference optical system has two different frequencies. Each of the object light and the reference light travels in a state in which the light waves constituting the two-beam light are substantially overlapped, and the difference between the two-beam lights. A beat light wave having a frequency is generated, and the beat image is generated by causing the object light and the reference light to interfere with each other. The invention described in claim 3 relates to the heterodyne interferometer according to claim 1, wherein the image detection control unit sets a timing obtained by dividing one cycle of the beat image into three or four, and according to the timing. The image detection unit is configured to detect 3 or 4 beat images.

Effect of the invention

  The heterodyne interferometer according to the present invention is a device for measuring a three-dimensional shape including a height shape by detecting a two-dimensional beat image with a commercially available CCD camera. The conventional heterodyne interferometer cannot measure the surface because the frequency of the beat image signal is about several hundreds KHz or more. Therefore, when measuring a wide sample, there is a problem that the measurement time becomes long. However, the present invention has an effect that the surface measurement is possible and the measurement time is greatly shortened. Furthermore, since the phase information is calculated by calculating the intensity of multiple images using the temporal phase shift method for the detected beat image, it is accurate without being affected by fluctuations in the intensity of the laser beam. Phase detection is possible, and the three-dimensional measurement accuracy is greatly improved. For the creation of the low-frequency beat image, two acousto-optic elements are used to create two sets of beat light waves, and the beat light waves interfere with each other to obtain a beat image. Since the frequency of the beat image can be easily changed by the frequency of the signal for driving the acousto-optic device, it can be freely set according to the response speed of the CCD camera used, and the adjustment can be simplified. is there.

  The heterodyne interferometer according to the present invention expands the measurement of the point position irradiated with the laser beam focused on a minute spot to the measurement of the surface, and can measure the image with a normal camera, for example, a CCD camera. It is the structure to do. For heterodyne interference, it is necessary to cause two light waves having different frequencies to interfere with each other. Two acousto-optic elements (AOD) are used to create two light waves with different frequencies. The AOD has a function of shifting the frequency of the laser light in accordance with the driving frequency. The two AODs are driven at different frequencies, and the frequency is different from the laser light (frequency f0) emitted from the laser light source. Create two light waves with f2.

  Light having a frequency of f1 is applied to the sample, and light reflected or transmitted by the sample is used as object light. The light with the frequency f2 is not irradiated on the sample, but is used as reference light, causing object light and reference light to interfere with each other. Since the object light and the reference light have different frequencies, a beat image in which the intensity changes sinusoidally at the difference frequency (f1-f2 = f3) is obtained by interference. This beat image has a two-dimensional spread, and the whole is detected by a camera such as a CCD. Since the frequency response of a commercially available CCD camera is about 100 Hz, the frequency of the beat image needs to be 100 Hz or less in order to detect a two-dimensional beat image. In order to obtain a beat frequency of 100 Hz or less, the AOD drive frequency difference needs to be 100 Hz or less, but since a normal AOD is driven at a frequency of about 50 MHz, a difference of about 100 Hz in the 50 MHz frequency region. It is difficult to put on. Therefore, in the present invention, the AOD is driven with a signal of two frequency components.

  When the first AOD (AOD1) is driven with a signal of two frequency components of fa ± f1, two-beam light with optical frequencies of f0 + fa + f1 and f0 + fa−f1 is obtained from AOD1. When the second AOD (AOD2) is driven with a signal of two frequency components of fa ± f2, two-beam light having optical frequencies of f0 + fa + f2 and f0 + fa−f2 is generated from the AOD2. These two sets of two-beam light are slightly separated, but substantially overlap and proceed. Therefore, the two-beam light as the object light and the two-beam light as the reference light interfere with each other to generate beat signals with beat frequencies 2f1 and 2f2. The frequency of the beat image obtained by causing these two sets of beat light waves to interfere is 2 (f1-f2).

  When the frequency fa is 50 MHz, if the frequency f1 is set to 5 KHz and the frequency f2 is set to 5 KHz + 50 Hz, the frequency of the beat image is 100 Hz. It is difficult to give a difference of 50 Hz to the high frequency fa, but it is easy to add a difference of 50 Hz to the low frequencies f1 and f2. By driving the AOD with such two frequency component signals and causing the two sets of beat signals to interfere with each other, a beat image having a frequency (100 Hz or less) lower than the frequency region of each beat signal can be set. Heterodyne interference for detecting a dimensional image can be realized.

  The beat image detected by the CCD camera includes object height (unevenness) information. In other words, the intensity of the beat image is analyzed because the optical path length of the two-beam light applied to the object changes according to the unevenness of the sample, and this is detected as a change in the intensity of the beat image as a phase change. This is very important. However, the phase cannot be detected from the intensity of only one beat image. That is, because the amplitude and bias intensity of the beat image intensity that changes in a sinusoidal shape with time are indefinite, it is impossible to determine at which phase the image is detected. Therefore, a plurality of images are detected in synchronization with a specific timing in one cycle of the beat image.

  A phase shift method is often used to calculate the phase of a sine wave signal. When calculating the phase from the intensity of the interference fringes stationary in time, the phase shift method calculates the phase from a plurality of image intensities detected at each spatial position by spatially moving the interference fringes. Also in the present invention, a plurality of images are detected at specific timings in a period of one cycle with respect to a beat image whose intensity changes in a sine wave shape with respect to time. At this time, the image is detected at a timing obtained by dividing one cycle of the beat image into three or four, that is, steps of π / 4 or π / 3. At this time, three or four beat images are detected. The detection of the above beat image is referred to as a temporal phase shift.

  Image analysis is performed according to a specific algorithm on the image detected by the temporal phase shift. What is important here is that even if the amplitude and bias intensity of the beat image intensity are unknown, by calculating between the multiple images, the unknown amplitude and bias intensity are canceled and an accurate phase value is obtained. It can be decided. If the phase can be determined, the height distribution of the sample can be detected from the wavelength and phase value of the laser beam.

  Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram illustrating the operation of the heterodyne interference apparatus according to the present invention. A laser light source 10 emits a laser beam 100 having a frequency f0. A heterodyne interference optical system 11 includes a first acoustooptic element (first AOD) 110, a second acoustooptic element (second AOD) 112, a beam splitter, and a large number of lenses. The laser light 100 is split in two directions by a beam splitter, one light is incident on the first AOD 110 and the other light is incident on the second AOD 112. The two acoustooptic elements are driven by the acoustooptic element drive unit 13 and converted into beams 102 and 104 having different frequencies, respectively. The beam 102 is irradiated through the beam splitter 114 onto the sample 12 whose height shape is to be measured. The laser beam reflected by the sample 12 is reflected by the same beam splitter 114 and enters the image detection unit 14 including a CCD camera or the like as object light. The same applies to the light transmitted through the sample 12 as well as the reflected light. The beam 104 passing through the second AOD is reflected by the mirror 116 and enters the image detection unit 14 as reference light.

  Since the frequencies of the object light and the reference light incident on the image detection unit 14 are different from each other, when the two lights interfere with each other, a beat signal having a frequency difference between the object light and the reference light is obtained. Further, since the object light and the reference light have a spread, the interfered light also has a spread. Therefore, the image detection unit 14 detects a broad beat image. The beat image changes in intensity in a sine wave shape with respect to time, and the detection timing of the beat image is determined by the image detection control unit 15. That is, the timing at which one cycle of the beat image is divided into N is determined. Usually, N = 3 or N = 4, so three or four beat images are detected. The operation of detecting an image by changing the timing at this time is a temporal phase shift.

  A plurality of beat images detected by the image detection unit 14 perform mutual calculation of image intensity by the image processing unit 16. This calculation is a commonly used image calculation based on phase shift, and calculates a phase corresponding to the image intensity. The shape calculation unit 17 converts the phase data detected by the image processing unit 16 into the height of the sample 12, and calculates a three-dimensional shape from the phase data of the entire image.

  FIG. 2 shows an example of the configuration of the acoustooptic device driving unit 13, and the frequency modulation operation of the laser light will be described. Reference numeral 20 denotes a DC voltage source, which generates a DC voltage V. A voltage controlled oscillator (VCO) 21 converts a DC voltage into an AC signal. When the DC voltage V is applied, an AC signal having a frequency f is generated. For example, when V = 0.5 volt, an AC signal having a frequency of 40 MHz is generated. A frequency f generated by the VCO 21 is applied to the 25 amplitude modulation circuits 1 and 26, respectively. Reference numerals 23 and 24 denote AC signal sources, which generate AC signals of frequencies m and n, respectively. The frequency m is input to the amplitude modulation circuit 1 and the frequency n is input to the amplitude modulation circuit 2. The amplitude modulation circuit 1 performs amplitude modulation between the AC signals f and m, creates a first AOD drive signal 252 having two frequency components with a frequency of f ± m, and drives the first AOD 110. The amplitude modulation circuit 2 performs amplitude modulation between the AC signals f and n, generates a second AOD drive signal 262 having two frequency components with a frequency of f ± n, and drives the second AOD 112.

  The first AOD 110 driven by the first AOD drive signal 252 generates two beams of light 272 and 274 separated into two. The second AOD 112 driven by the second AOD drive signal 262 generates two beams of light 282 and 284 separated into two. Each of the two sets of two-beam light is subjected to Doppler shift by ultrasonic waves traveling in the AOD, and the frequency of the light is modulated. Since this ultrasonic wave is generated according to the AOD drive signal, the optical frequency is modulated according to the drive signals 252 and 262. When the frequency of the laser light incident on the AOD is f0, the frequency of the beam 272 constituting the two-beam light is f0 + f + m, and the frequency of the beam 274 is f0 + fm. Therefore, the difference frequency between the two light beams 272 and 274 is 2 m. Similarly, the frequency of the beam 282 constituting the two-beam light is f0 + f + n, and the frequency of the beam 284 is f0 + f−n. Therefore, the difference frequency between the two light beams 282 and 284 is 2n. For example, when the frequencies m and n are set to 1000 Hz and 1050 Hz, 2m = 2000 Hz and 2n = 2100 Hz.

  Since the separation angle between the two sets of two-beam light is proportional to the frequencies m and n, the lower the frequencies m and n, the smaller the separation into two beams. Since a normal AOD has a separation angle of about 3 mrad at a frequency of about 200 KHz, the separation angle is about microrad at a frequency of about 1 KHz, and the light travels in a substantially overlapping state. When the frequencies m and n are about 1 KHz, the two beams 272 and 274 serving as object light and the two beams 282 and 284 serving as reference light each emit a beat frequency of about 2 KHz, and therefore when the object light and the reference light interfere with each other. The frequency difference 2 (mn) is the frequency of the beat image. When the frequencies m and n are set to 1000 Hz and 1050 Hz, 2 (mn) = 100 Hz, and a beat image having a low frequency enough to be detected by the CCD camera can be obtained.

FIG. 3 shows a beat image detection method. FIG. 3A shows the pixel position (x, y) of the beat image detected by the CCD camera. The image intensity of all pixels varies according to the amount of unevenness at each position of the sample, and the height is calculated by converting the intensity of all pixels into a phase. If the intensity of the point (x, y) is I (x, y),
I (x, y) = Ib (x, y) + Ia · cos (φ (x, y)) Here, Ib is the bias intensity, Ia is the amplitude, and φ is the phase. When calculating the phase from the intensity of the beat image, since the values of Ib and Ia are unknown, the phase cannot be calculated from only the intensity value of one image. Therefore, it is necessary to detect the phase by detecting a plurality of beat images and calculating the intensity between the beat images.

  FIG. 3B shows an image detection method with respect to the intensity time at the point (x, y) of the beat image. Reference numeral 32 denotes a temporal change in intensity at the point (x, y) of the beat image, and shows how the intensity changes in a sine wave shape. When a period of one cycle of the intensity signal 32 of the beat image is T, an image is detected at every timing when T is divided into four. Image 33 is detected at t = 0, image 34 is detected at t = T / 4, image 35 is detected at t = 2T / 4, and image 36 is detected at t = 3T / 4. The image intensities at the detected position (x, Y) are I0, I1, I2, and I3. 3B shows an example in which four beat images are detected at the timing when the period T is divided into four. As another example, three beat images are detected at the timing when the period T is divided into three. May be detected. In this case, a beat image is detected at timings t = 0, T / 3, and 2T / 3.

  FIG. 4 shows a calculation example when the phase is calculated from the detected intensity I0, I1, I2 and I3 of the beat image. Equation 1 determines the phase φ with the intensity of the four beat images at the point (x, y). Since image detection is performed at a timing obtained by dividing one cycle of the beat image signal into four, the phase shift amount is a step of π / 2. Equation 2 calculates the quotient of two sets of intensity differences for the four image intensities I0, I1, I2, and I3, and each of the bias intensity Ib and the amplitude Ia is cancelled. Therefore, tan (φ) is obtained by the calculation of (I3−I1) / (I0−I2), and the phase φ can be determined by Equation 3. The above is the phase calculation at the point (x, y) of the image, but if this calculation is performed for all the pixels, the phase distribution corresponding to the unevenness of the sample can be determined.

  Expressions 4 and 5 show relational expressions for converting the detected phase φ into the sample height. Expression 4 is a relational expression when the laser light applied to the sample is reflected, and Expression 5 is a relational expression when the sample is transmitted. Λ is the wavelength of the laser light, and h is the height. In the case of a He—Ne laser having a wavelength λ of 633 nm, in the case of reflection, one degree of phase is 0.88 nm at λ / 720. In the case of transmission, λ / 360 is equivalent to 1.76 nm.

  FIG. 5 shows a configuration example of the heterodyne interference optical system. The laser beam 100 emitted from the laser light source 10 has its beam diameter converted by the lens system 50 and is incident on the beam splitter (BS) 52. The laser light transmitted through the BS 52 is incident on the first acousto-optic device (AOD), undergoes diffraction and frequency shift action, and enters the sample 53 as object light, and the transmitted light is reflected by the reflection mirror. The laser beam reflected by the BS 52 is reflected by the reflection mirror 55, enters the second AOD 112, and is subjected to diffraction and frequency shift action to become reference light. In addition, although the object light was shown in the example which permeate | transmitted the sample 53, it is the same also when reflected by a sample.

  The object light and the reference light are superposed at the BS 56 and interfere to form a low-frequency beat image. The beat image has a size suitable for taking an image by converting the beam shape with the lens 57, and the image is detected by the CCD camera 58.

  As apparent from the above description, the present invention creates a two-dimensional beat image in a low frequency region detected by a normal CCD camera and detects an image at every specific timing of one cycle of the beat image signal. It is. In other words, both the object light and the reference light are used as beat signals, and a beat image with a difference frequency between the beat signals is created to reduce the frequency, and the phase shift method used in conventional static interference measurement Is applied to heterodyne interferometry for highly accurate phase detection.

  It is a block diagram explaining the structure and operation | movement of the heterodyne interference apparatus of this invention.   It is a figure showing the structure of the acoustooptic device drive part which drives the acoustooptic device of this invention.   (A) is a figure which shows the observation point when detecting an image with a CCD camera, (b) is a figure explaining the timing when a beat image is detected.   It is description of a type | formula when detecting a phase from image intensity | strength.   It is a figure showing the structure of a heterodyne interferometer.   It is a figure explaining the principle of the conventional heterodyne interferometry.   (A) is a figure explaining the structure of the conventional differential type heterodyne interferometer, (b) is a figure which detects the phase of a beat signal.   (A) is a figure which shows the structure when producing 2 beam light from the conventional acoustooptic device, (b) is a figure which shows a mode when 2 beam light is irradiated to the sample.

110 first acoustooptic element 112 second acoustooptic element 13 acoustooptic element drive unit 14 image detection unit 15 image detection control unit 16 image processing unit 17 shape calculation unit

Claims (3)

  Laser light emitted from a laser light source is incident on a first acoustooptic element and a second acoustooptic element for converting the frequency of the laser light, and the first and second acoustooptic elements are transmitted from an acoustooptic element drive unit. Driving with signals of different frequencies that are output to create two light waves of object light and reference light having different frequencies from each of the first and second acoustooptic elements, the object light having the first frequency is A sample whose height shape is to be measured is irradiated and reflected or transmitted, and the reference light having a second frequency and the object light are caused to interfere with each other and have a frequency that is the difference between the frequency of the object light and the reference light. From a heterodyne interference optical system that creates a beat image having a two-dimensional spread in which the light intensity changes in a sinusoidal shape with time, and a camera that detects the beat image in response to a frequency band near the frequency of the beat image An image detection unit that controls an image detection operation to detect a plurality of the beat images in synchronization with a predetermined timing within a period of one cycle of the beat image, and the detected An image processing unit that calculates the phase of the beat image by calculating the intensity between a plurality of images, and a shape calculation unit that calculates the height shape of the object from the phase obtained by the image processing unit. A heterodyne interferometer characterized by comprising:   Each of the object light and the reference light light waves created by the heterodyne interference optical system is composed of two beam lights that travel separately in two different frequencies, and each of the object light and the reference light is This is a state in which each light wave constituting the two-beam light travels in a substantially superposed state to generate a beat light wave having a frequency that is the difference between the two-beam light, and the object light and the reference light are caused to interfere with each other. The heterodyne interference device according to claim 1, wherein the beat image is created.   The image detection control unit sets a timing at which one cycle of the beat image is divided into three or four, and the image detection unit detects three or four beat images according to the timing. The heterodyne interferometer according to claim 1.

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