Focal plane measuring device

Abstract

The focus surface measuring device provided by the application comprises an objective lens, a reflection module, a modulator, a photodetector, a mixer, a low-pass filter and a signal processor. The objective lens images an illumination light beam to the surface of a to-be-measured object and transmits a reflected light beam generated by the reflection of the illumination light beam on the to-be-measured object. The reflection module receives and transmits the reflected light beam and focuses the reflected light beam. The modulator applies modulation to the reflection module to form a modulated reflected light beam and outputs a signal after phase shift of a modulation source. The photodetector receives the modulated reflected light beam and outputs a light intensity signal. The mixer mixes the light intensity signal and the signal after phase shift of the modulation source and outputs a mixed signal. The low-pass filter filters the mixed signal and outputs a demodulated signal. The signal processor detects the demodulated signal and determines the defocus amount of the to-be-measured object according to the intensity of the demodulated signal. The focus surface measuring device only needs to use a single photodetector to measure the defocus amount of the to-be-measured object, so that the installation difficulty can be reduced and the influence of dark current on the measurement result can be reduced.

Description

Technical Field

[0001] This invention relates to the field of focal plane measurement technology, and in particular to a focal plane measurement device. Background Technology

[0002] Photolithography equipment is a machine that transfers a target pattern onto a substrate coated with a radiation-sensitive material (such as photoresist). In integrated circuit manufacturing, different patterns need to be transferred onto different layers of a substrate to achieve specific functions. The overlap between different layers of the substrate is called overlay, and the quality of overlay affects the performance of the chip. Therefore, metrology equipment is needed to control the overlay during the photolithography process. In order for the metrology equipment to accurately measure the overlay of the object under test (DUT), the area to be measured on the DUT needs to be on the focal plane of the objective lens. The focal plane of the DUT is locked by adjusting the relative position of the objective lens or the DUT. The accuracy of the focal plane locking depends on the accuracy of the measurement of the defocusing amount of the DUT.

[0003] Figure 1 This is a schematic diagram of a focal plane measuring device. Figure 1 As shown, in this focal plane measurement device, the illumination spot is imaged onto the surface of the object under test 525 through lens 510, beam splitter 515, and objective lens 520. The light spot reflected by the object under test 525 is split by beam splitter 530 after passing through objective lens 520 and beam splitter 515 and transmitted to aperture 535 and aperture 540 respectively. Photodetectors 545 and 550 measure the light signals passing through aperture 535 and aperture 540 respectively. Figure 2 for Figure 1 A schematic diagram of the confocal differential signal obtained by the focal plane measurement device. (See diagram below.) Figure 2 As shown, a confocal differential signal 580 is generated using a combination of signal 560 from aperture 535 and photodetector 545 and signal 570 from aperture 540 and photodetector 550. The defocusing amount of the object under test is then obtained using the confocal differential signal 580.

[0004] However, the aforementioned focal plane measuring device requires two photodetectors to be symmetrically installed when measuring the defocusing amount of the object under test, and relies on the light signals measured by the two photodetectors to generate a confocal differential signal. Therefore, it has high requirements for the installation accuracy of the two photodetectors, and the two photodetectors will increase the dark current and affect the measurement accuracy of the defocusing amount of the object under test. Summary of the Invention

[0005] This invention provides a focal plane measuring device that can measure the defocus of the object under test using only a single photodetector. This reduces the difficulty of installation and the influence of the dark current of the photodetector on the measurement results, thereby improving the measurement accuracy of the defocus of the object under test.

[0006] To achieve the above objectives, the focal plane measuring device of the present invention includes:

[0007] A light source used to provide a beam of light;

[0008] An objective lens is used to image the illumination beam onto the surface of the object under test and to transmit the reflected beam generated by the illumination beam reflected by the object under test;

[0009] A reflection module is used to receive and propagate the reflected beam, and to focus the reflected beam;

[0010] A modulator is used to apply modulation to the reflection module to change the optical path, so that the reflected beam becomes a modulated reflected beam, and the modulator outputs the phase-shifted signal of the modulation source.

[0011] A photodetector is used to receive the modulated reflected beam output by the reflection module and output a light intensity signal;

[0012] A mixer is used to receive the light intensity signal and the phase-shifted signal of the modulation source, and to mix the light intensity signal and the phase-shifted signal of the modulation source and output a mixed signal.

[0013] A low-pass filter is used to filter the mixed signal and output a demodulated signal; and

[0014] A signal processor is used to detect the demodulated signal and determine the defocusing amount of the object under test based on the intensity of the demodulated signal.

[0015] Optionally, the reflection module includes a focusing lens, a first pinhole aperture, and a second pinhole aperture; the reflected beam is focused between the first pinhole aperture and the second pinhole aperture after being processed by the focusing lens; the reflected beam passes through the first pinhole aperture and the second pinhole aperture and then illuminates the photodetector.

[0016] Optionally, the modulator modulates the focusing lens, or the modulator modulates both the first pinhole stop and the second pinhole stop simultaneously.

[0017] Optionally, the reflection module includes a second beam splitter and a first reflector; the reflected beam passes through the focusing lens, then through the second beam splitter to illuminate the first reflector, is reflected by the first reflector back to the second beam splitter, and is then reflected by the second beam splitter to the first pinhole aperture.

[0018] Optionally, the modulator modulates the first reflector.

[0019] Optionally, the reflection module includes a second reflector, and the reflected light beam is reflected by the second reflector and then illuminates the focusing lens; the modulator modulates the second reflector.

[0020] Optionally, the demodulated signal satisfies the formula Where S is the demodulated signal, x0 is the defocusing amount of the object under test at any position, I(x0) is the intensity of the reflected beam generated by the object under test at any position, and x m Let A be the modulation depth of the reflected beam, and let A be the modulation depth of the signal from the modulation source after phase shifting. The initial phase of the modulated reflected beam. The initial phase of the signal after phase shifting of the modulation source.

[0021] Optionally, the modulator includes a phase shifter for adjusting the initial phase of the modulated reflected beam, such that the demodulated signal satisfies the formula...

[0022] Optionally, the modulator modulates the reflected beam using a modulation method such as sinusoidal modulation, square wave modulation, triangular wave modulation, phase modulation, or frequency modulation.

[0023] Optionally, the focal plane position measuring device includes a workpiece stage; the signal processor has a focal plane feedback loop, which adjusts the position of the workpiece stage according to the focal plane compensation amount and the defocus amount of the object under test obtained by the signal processor.

[0024] Optionally, the objective lens is provided with a piezoelectric actuator; the signal processor has a focal plane feedback loop, which controls the piezoelectric actuator according to the focal plane compensation amount and the defocus amount of the object under test obtained by the signal processor, so as to adjust the focal length of the objective lens.

[0025] Optionally, the focal plane measuring device includes a first beam splitter, the illumination beam is reflected by the first beam splitter and enters the objective lens, and the reflected beam is irradiated onto the reflection module after passing through the objective lens and the first beam splitter.

[0026] In the focal plane measurement device of the present invention, an illumination source provides an illumination beam; an objective lens images the illumination beam onto the surface of the object under test and transmits a reflected beam generated by the reflection of the illumination beam by the object under test; a reflection module receives and propagates the reflected beam and focuses it; a modulator modulates the reflection module to change the optical path, so that the reflected beam becomes a modulated reflected beam, and the modulator outputs a phase-shifted signal from the modulation source; a photodetector receives the modulated reflected beam output by the reflection module and outputs a light intensity signal; a mixer receives the light intensity signal and the phase-shifted signal from the modulation source, mixes the light intensity signal and the phase-shifted signal from the modulation source, and outputs a mixed signal; a low-pass filter filters the mixed signal and outputs a demodulated signal; a signal processor detects the demodulated signal and determines the defocus amount of the object under test based on the intensity of the demodulated signal. In other words, the focal plane measurement device of the present invention utilizes modulation and demodulation technology, requiring only a single photodetector to measure the defocus amount of the object under test. This reduces the installation difficulty of the device and the influence of the dark current of the photodetector on the measurement results, thus improving the measurement accuracy of the defocus amount of the object under test. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a focal plane measuring device.

[0028] Figure 2 for Figure 1 A schematic diagram of the confocal differential signal obtained by the focal plane measurement device.

[0029] Figure 3 This is a schematic diagram of the focal plane measuring device according to an embodiment of the present invention.

[0030] Figure 4 This is a graph showing the relationship between the light intensity signal of the unmodulated reflected beam and the defocusing amount of the object under test.

[0031] Figure 5 This is a simulation diagram of the light intensity signal and demodulated signal according to an embodiment of the present invention.

[0032] Figure 6 This is a comparison diagram of the demodulated signal and the confocal differential signal.

[0033] Figure 7 This is a schematic diagram of the focal plane measuring device according to another embodiment of the present invention.

[0034] Figure 8 This is a schematic diagram of the focal plane feedback loop of the signal processor in one embodiment of the present invention. Detailed Implementation

[0035] The focal plane measuring device proposed in this invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of this invention will become clearer from the following description. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of this invention.

[0036] To reduce the installation difficulty of the focal plane measurement device and the influence of the dark current of the photodetector on the measurement results, and to improve the measurement accuracy of the defocusing of the object under test, this invention provides a focal plane measurement device. Figure 3 This is a schematic diagram of the focal plane measuring device according to an embodiment of the present invention. Figure 3 As shown, the focal plane measuring device in this embodiment includes: an illumination source 1, an objective lens 5, a reflection module, a modulator 13, a photodetector 11, a mixer 12, a low-pass filter 15, and a signal processor 16.

[0037] Illumination source 1 provides an illumination beam. Objective lens 5 images the illumination beam onto the surface of the object under test 6 and transmits a reflected beam generated by the reflection of the illumination beam from the object under test 6. Reflection module receives and propagates the reflected beam transmitted by objective lens 5 and focuses the reflected beam. Modulator 13 modulates the reflection module to change the optical path (i.e., the optical path of the reflection module), so that the reflected beam becomes a modulated reflected beam, and modulator 13 outputs a phase-shifted signal from the modulation source. Photodetector 11 receives the modulated reflected beam output by the reflection module and outputs a light intensity signal. Mixer 12 receives the light intensity signal and the phase-shifted signal from the modulation source, mixes the light intensity signal and the phase-shifted signal from the modulation source, and outputs a mixed signal. Low-pass filter 15 filters the mixed signal and outputs a demodulated signal. Signal processor 16 detects the demodulated signal and determines the defocusing amount of the object under test based on the intensity of the demodulated signal.

[0038] Specifically, such as Figure 3 As shown, the focal plane measuring device may include a field stop 2 and a collimating lens 3. The field stop 2 and the collimating lens 3 are sequentially arranged in the optical path after the illumination source 1. The aperture size of the field stop 2 determines the spot size of the illumination beam, and the collimating lens 3 collimates the illumination beam.

[0039] The focal plane measuring device also includes a first beam splitter 4. The illumination beam can pass through the collimating lens 3 and then onto the first beam splitter 4, and after being reflected by the first beam splitter 4, it enters the objective lens 5. The objective lens 5 projects the received illumination beam onto the surface of the object under test 6. The illumination beam that illuminates the surface of the object under test 6 is reflected by the object under test 6 to generate a reflected beam. The reflected beam can pass through the objective lens 5 and the first beam splitter 4 and then illuminate the reflection module.

[0040] As an example, such as Figure 3 As shown, the reflection module may include a focusing lens 7, a first pinhole aperture 9, and a second pinhole aperture 10. After being processed by the focusing lens 7, the reflected beam is focused between the first pinhole aperture 9 and the second pinhole aperture 10; that is, the focal plane of the focusing lens 7 is located between the first pinhole aperture 9 and the second pinhole aperture 10. After passing through the first pinhole aperture 9 and the second pinhole aperture 10, the reflected beam illuminates the photodetector 11; that is, the photodetector 11 is positioned in the optical path after the first pinhole aperture 9 and the second pinhole aperture 10.

[0041] It should be noted that when the defocusing amount of the object under test 6 changes, the focal plane position of the reflected beam between the first pinhole aperture 9 and the second pinhole aperture 10 changes, and the amount of light passing through the first pinhole aperture 9 and the second pinhole aperture 10 also changes, thereby changing the intensity of the light signal detected by the photodetector 11.

[0042] The reflection module may further include a second beam splitter 8 and a first reflector 14, which are disposed in the optical path between the focusing lens 7 and the first pinhole aperture 9. The reflected beam passes through the focusing lens 7, then through the second beam splitter 8, and illuminates the first reflector 14. After being reflected by the first reflector 14, the beam is reflected back to the second beam splitter 8, and then reflected again by the second beam splitter 8 to the first pinhole aperture 9.

[0043] like Figure 3 As shown, the reflection module may also include a second reflector 19, which is disposed between the first beam splitter 4 and the focusing lens 7. The reflected beam is reflected by the second reflector 19 and then shines onto the focusing lens 7.

[0044] In this embodiment, the modulator 13 can modulate some components of the reflection module to change the optical path, thereby modulating the reflected beam and forming a modulated reflected beam. Specifically, the modulator 13 can modulate the focusing lens 7; or, the modulator 13 can simultaneously modulate the first pinhole stop 9 and the second pinhole stop 10; or, the modulator 13 can modulate the first reflecting mirror 14; or, the modulator 13 can modulate the second reflecting mirror 19.

[0045] Taking the modulation of the first reflecting mirror 14 by the modulator 13 as an example, the first reflecting mirror 14 can be equipped with a vibration mechanism. The vibration mechanism can modulate the first reflecting mirror 14 according to the modulation signal output by the modulator 13, thereby converting the reflected beam through the first reflecting mirror 14 into a modulated reflected beam. For reflected beams of different wavelengths, the focal length of the focusing lens 7 is different. By changing the bias of the modulation signal, that is, by changing the vibration center of the first reflecting mirror 14, the focal point of the focusing lens 7 can be kept at the center position of the first pinhole aperture 9 and the second pinhole aperture 10.

[0046] The following example illustrates the principle (or process) of the focal plane measuring device in this embodiment for detecting and obtaining the defocus amount of the object under test, using the modulator 13 applying modulation to the first reflecting mirror 14.

[0047] Assume x is the defocusing amount of the object under test; I(x) is the intensity of the reflected beam generated by the object under test; the signal of the modulation source output by modulator 13 after phase shifting can be expressed as: Where A is the modulation amplitude of the signal after phase shifting from the modulation source, ω is the modulation frequency, and t is time. The initial phase of the signal after phase shifting of the modulation source.

[0048] Modulator 13 applies a sinusoidal modulation to the first reflecting mirror 14 (i.e., the optical path), which changes the intensity of the reflected beam. This intensity change is equivalent to a change in the defocusing amount of the object under test. The intensity of the modulated reflected beam... Where F is the intensity of the modulated reflected beam (i.e., F is the intensity signal), and x is the defocusing amount of the object under test. m Let ω be the modulation depth of the reflected beam, ω be the modulation frequency, and t be time. This represents the initial phase of the modulated reflected beam.

[0049] When x m When the linewidth is much smaller than I(x), performing a Taylor expansion of F at any position x0 yields the following expansion: Where x0 is the defocusing amount at any position, and I(x0) is the intensity of the reflected beam generated by the object under test at any position.

[0050] Mixer 12 mixes the light intensity signal and the phase-shifted signal from the modulation source, and the resulting mixed signal can be expressed as:

[0051]

[0052] Using the product-to-sum formula, we know that only the first derivative term will have a near-DC component after mixing. Therefore, the demodulated signal S obtained after the mixed signal is filtered by low-pass filter 15 is: Where x0 is the defocusing amount of the object under test at any position, and I(x0) is the intensity of the reflected beam generated by the object under test at any position. m Let A be the modulation depth of the reflected beam, and let A be the modulation depth of the signal from the modulation source after phase shifting. This represents the initial phase of the modulated reflected beam. The initial phase of the signal after phase shifting of the modulation source.

[0053] In this embodiment, the modulator 13 may further include a phase shifter, which is used to adjust the initial phase of the modulated reflected beam, so that the formula for the demodulated signal S is simplified to: This facilitates the determination of the defocus amount of the object under test based on the intensity of the demodulated signal. According to the expression for the demodulated signal, it is a signal proportional to the first derivative of the light intensity signal and the modulation depth. Since the demodulated signal is only related to the first derivative of the light intensity signal, the influence of the dark current of the photodetector can be reduced, thus helping to improve the measurement accuracy of the defocus amount of the object under test. As an example, a phase shifter can adjust the initial phase of the modulated reflected beam to be the same as the initial phase of the signal from the modulation source after phase shifting, or it can adjust the initial phase of the modulated reflected beam to be opposite.

[0054] The inventors conducted simulation tests on the optical path layout of the focal plane measuring device in this embodiment. Figure 4 This is a graph showing the relationship between the light intensity signal of the unmodulated reflected beam and the defocusing amount of the object under test. (Example) Figure 4 As shown, the intensity of the reflected beam received by the photodetector 11 is the convolution of the light spot formed by the illumination beam on the surface of the object under test and the point spread function of the subsequent optical path.

[0055] As an example, a Gaussian function was used to fit the light intensity signal to obtain a fitting curve. The modulation frequency was set to 40 kHz, the modulation depth to 0.5 μm, and the cutoff frequency of the low-pass filter 15 to 20 kHz. Simulations were performed on the light intensity signal (the light intensity signal corresponding to the modulated reflected beam) and the demodulated signal within a defocus range of (-50 μm, 50 μm). The results are as follows. Figure 5 As shown. According to Figure 5 It can be seen that the intensity of the demodulated signal is related to the defocusing amount of the object under test. When the signal processor 16 detects the intensity of the demodulated signal, it can obtain (determine) the defocusing amount of the object under test.

[0056] Figure 6 This is a comparison chart of the demodulated signal and the confocal differential signal. From Figure 6 It can be seen that the demodulated signal detected by the focal plane measuring device in this embodiment is... Figure 1Compared to the confocal differential signal detected by the focal plane measuring device shown, the slope is greater near the optimal focal plane (i.e., optimal defocus), for example, near a defocus of 0 μm, which is beneficial to improving the measurement accuracy of the defocus of the object under test.

[0057] It should be noted that the above explanation uses the example of modulator 13 using sinusoidal modulation to modulate the reflected beam to illustrate the principle (or process) of obtaining the defocus amount of the object under test. However, it is not limited to this; modulator 13 can also use square wave modulation, triangular wave modulation, phase modulation, or frequency modulation, or other modulation methods that can change the optical path, to modulate the reflected beam.

[0058] Figure 7 This is a schematic diagram of the focal plane measuring device according to another embodiment of the present invention. Figure 7 As shown, in this focal plane position measuring device, a piezo driver 18 can be installed on the objective lens 5; the signal processor 16 has a focal plane feedback loop, which controls the piezo driver 18 according to the focal plane compensation amount and the defocus amount of the test object obtained by the signal processor 16, so as to adjust the focal length of the objective lens 5, thereby realizing the automatic focusing function based on the measured defocus amount of the test object.

[0059] Figure 8 This is a schematic diagram of the focal plane feedback loop of a signal processor according to an embodiment of the present invention. (Reference) Figure 8 In this focal plane feedback loop, the input focal plane compensation value (offset) and the defocus amount of the test object obtained by the test are calculated and used as the input of the PID controller. The PID controller controls the piezoelectric actuator 18 according to the received calculation result.

[0060] In one embodiment, reference Figure 7 The focal plane position measuring device includes a workpiece stage 17, on which the object to be measured 6 is placed. The signal processor 16 has a focal plane feedback loop, which can adjust the position of the workpiece stage according to the focal plane compensation amount and the defocus amount of the object to be measured obtained by the signal processor, thereby adjusting the position of the object to be measured and realizing the automatic focusing function.

[0061] It should be noted that the focal plane compensation amount can be adjusted according to the needs of different processes, so that the test object is in different defocus states.

[0062] In the focal plane measurement device of this embodiment, the illumination source 1 provides an illumination beam; the objective lens 5 images the illumination beam onto the surface of the object under test 6 and transmits the reflected beam generated by the reflection of the illumination beam by the object under test 6; the reflection module receives and propagates the reflected beam and focuses the reflected beam; the modulator 13 modulates the reflection module to change the optical path, so that the reflected beam becomes a modulated reflected beam, and the modulator 13 outputs the signal of the modulation source after phase shifting; the photodetector 11 receives the modulated reflected beam output by the reflection module and outputs a light intensity signal; the mixer 12 receives the light intensity signal and the signal of the modulation source after phase shifting, mixes the light intensity signal and the signal of the modulation source after phase shifting, and outputs a mixed signal; the low-pass filter 15 filters the mixed signal and outputs a demodulated signal; the signal processor 16 detects the demodulated signal and determines the defocusing amount of the object under test according to the intensity of the demodulated signal. In other words, the focal plane measuring device in this embodiment utilizes modulation and demodulation technology, which only requires a single photodetector to measure the defocusing amount of the object under test. This reduces the difficulty of device installation and the influence of dark current of the photodetector on the measurement results, thereby improving the measurement accuracy of the defocusing amount of the object under test.

[0063] The above description is merely a description of preferred embodiments of the present invention and is not intended to limit the scope of the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A focal plane measuring device, characterized in that, include: A light source used to provide a beam of light; An objective lens is used to image the illumination beam onto the surface of the object under test and to transmit the reflected beam generated by the illumination beam reflected by the object under test; A reflection module is used to receive and propagate the reflected beam, and to focus the reflected beam; A modulator is used to apply modulation to the reflection module to change the optical path, so that the reflected beam becomes a modulated reflected beam, and the modulator outputs the phase-shifted signal of the modulation source. A photodetector is used to receive the modulated reflected beam output by the reflection module and output a light intensity signal; A mixer is used to receive the light intensity signal and the phase-shifted signal of the modulation source, and to mix the light intensity signal and the phase-shifted signal of the modulation source and output a mixed signal. A low-pass filter is used to filter the mixed signal and output a demodulated signal; as well as A signal processor is used to detect the demodulated signal and determine the defocusing amount of the object under test based on the intensity of the demodulated signal.

2. The focal plane measuring device as described in claim 1, characterized in that, The reflection module includes a focusing lens, a first pinhole aperture, and a second pinhole aperture; the reflected beam is focused between the first pinhole aperture and the second pinhole aperture after being processed by the focusing lens; the reflected beam passes through the first pinhole aperture and the second pinhole aperture and then illuminates the photodetector.

3. The focal plane measuring device as described in claim 2, characterized in that, The modulator modulates the focusing lens, or the modulator modulates the first pinhole stop and the second pinhole stop simultaneously.

4. The focal plane measuring device as described in claim 2, characterized in that, The reflection module includes a second beam splitter and a first reflector; the reflected beam passes through the focusing lens, then through the second beam splitter to illuminate the first reflector, is reflected by the first reflector back to the second beam splitter, and is then reflected by the second beam splitter to the first pinhole aperture.

5. The focal plane measuring device as described in claim 4, characterized in that, The modulator modulates the first reflector.

6. The focal plane measuring device as described in claim 2, characterized in that, The reflection module includes a second reflector, and the reflected light beam is reflected by the second reflector and then illuminates the focusing lens; the modulator modulates the second reflector.

7. The focal plane measuring device as described in claim 1, characterized in that, The demodulated signal satisfies the formula Where S is the demodulated signal, x0 is the defocusing amount of the object under test at any position, and I(x0) is the intensity of the reflected beam generated by the object under test at any position. m Let φ be the modulation depth of the reflected beam, A be the modulation depth of the signal after phase shifting of the modulation source, φ0 be the initial phase of the modulated reflected beam, and φ be the initial phase of the signal after phase shifting of the modulation source.

8. The focal plane measuring device as described in claim 7, characterized in that, The modulator includes a phase shifter, which is used to adjust the initial phase of the modulated reflected beam so that the demodulated signal satisfies the formula .

9. The focal plane measuring device as described in claim 1, characterized in that, The focal plane measuring device includes a workpiece stage; the signal processor has a focal plane feedback loop, which adjusts the position of the workpiece stage according to the focal plane compensation amount and the defocus amount of the object under test obtained by the signal processor.

10. The focal plane measuring device as described in claim 1, characterized in that, The objective lens is equipped with a piezoelectric actuator; the signal processor has a focal plane feedback loop, which controls the piezoelectric actuator based on the focal plane compensation amount and the defocus amount of the object under test obtained by the signal processor, so as to adjust the focal length of the objective lens.

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