Automatic focusing system, automatic focusing method and imaging optical system

Abstract

The invention provides an automatic focusing system, and relates to the technical field of semiconductor detection, and the system comprises a chopper which carries out the periodic shielding of two parallel light beams at a fixed frequency, so as to modulate the two parallel light beams into pulse light consistent with the chopping frequency; comprising a detector, the detector carries a synchronous demodulation circuit, pulse signals consistent with the chopping frequency are extracted through the synchronous demodulation circuit, continuous detection light is converted into pulse light with the specific frequency through the chopping modulation and synchronous demodulation technology, effective signals are separated from background noise in the frequency domain, and the detection efficiency is improved. Corresponding frequency signals are synchronously demodulated and extracted through a detector, interference of background light (direct current component) and other frequency noise of an imaging system is eliminated, and the signal purity is greatly improved; besides, through light path design, an automatic focusing light spot is arranged at the edge of a view field and is spatially separated from an imaging light spot covering the center of the view field, so that interference of an automatic focusing process on a core imaging area is fundamentally avoided.

Description

Technical Field

[0001] This application relates to the field of semiconductor detection technology, and in particular to an autofocusing system, an autofocusing method, and an imaging optical system. Background Technology

[0002] In the field of semiconductor inspection, as process precision continues to improve, the requirements for the accuracy of surface feature recognition in inspection equipment have significantly increased. This demand has driven the evolution of objective systems towards deep ultraviolet (193nm / 266nm), ultra-high magnification (≥150x), and ultra-small depth of field (≤200nm). At this point, fluctuations in the distance between the objective lens and the inspection surface (object surface) will lead to a decrease in imaging contrast, directly affecting the defect detection rate. Therefore, autofocus technology with nanometer-level precision and microsecond-level response has become the core bottleneck restricting inspection efficiency.

[0003] To avoid interference with the imaging system, current autofocus technologies generally employ a "different wavelength design," meaning the autofocus module uses a light source with a different wavelength than the imaging system. While this approach reduces optical crosstalk, it has two inherent drawbacks: (1) Aberration magnification problem: The optical elements of the deep ultraviolet objective (such as CaF2 lens) have been corrected for the 193nm wavelength with high precision, but the spherical aberration coefficient for 450nm visible light will increase by 5-8 times, causing the diameter of the focused signal spot to expand from 1μm to more than 5μm, and the distance measurement error is as high as 5%.

[0004] (2) Increased signal attenuation: The coating layer of the deep ultraviolet objective lens has extremely low transmittance to non-designed wavelengths, such as less than 20% transmittance to 850nm infrared light, resulting in an attenuation of more than 80% of the focusing signal light energy. Combined with the strong background light of the deep ultraviolet imaging system, the detector signal-to-noise ratio can drop below 8dB, causing frequent loss of focus.

[0005] To solve the above problems, adopting a "same-band design" (autofocus and imaging share the same wavelength) can avoid aberration and transmittance issues, but it will introduce new technical problems: (1) Imaging background light interference: The continuous illumination of the imaging system will be superimposed on the focusing signal as a strong background light, causing the detector to be unable to effectively distinguish the useful signal from the background interference, causing the autofocus module to misjudge the distance deviation.

[0006] (2) Deterioration of signal-to-noise ratio: The intensity of the imaging background light is usually 10-100 times that of the focusing signal light, which directly drowns out the weak focusing signal, resulting in a further reduction in signal-to-noise ratio, which cannot meet the requirements of high-precision measurement. Summary of the Invention

[0007] In view of this, the present invention provides an autofocus system that can filter out DC background light and other frequency noise generated by the imaging system.

[0008] To solve the above problems, this application adopts the following technical solution: This application provides an autofocus system, comprising: A chopper modulation unit includes a chopper that periodically blocks two parallel beams at a fixed frequency to modulate them into pulsed light with a chopper frequency consistent with the chopper frequency. The beam shaping and coupling unit includes a dual-aperture module and a zoom lens. The pulsed light is allowed to pass through the beam corresponding to the slit after passing through the two through holes of the dual-aperture module to form two circular light spots, which are then converged to the aperture surface by the zoom lens. The object surface reflection and signal return unit includes an objective lens and a reflector. The two circular light spots and the imaging illumination beam are focused onto the object surface by the objective lens, and the resulting autofocusing light spot is located at the edge of the objective lens's field of view, while the resulting imaging illumination light spot covers the entire central area of ​​the field of view. The reflected beams, after being reflected by the object surface, pass sequentially through the objective lens and the zoom lens, and then enter the reflector through the dual-aperture module. The reflector separates the two returning beams. The signal detection and processing unit includes detector A and detector B. Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through a synchronous demodulation circuit.

[0009] In some embodiments, a beam introduction and splitting unit is included, which includes a dual-slit module for splitting a portion of the imaging illumination beam into the two parallel beams.

[0010] In some embodiments, the beam modulation preprocessing unit includes a cylindrical lens and a single slit module located at the focal plane of the cylindrical lens, wherein the two parallel beams are incident on the single slit module after the cylindrical lens compresses their width.

[0011] In some embodiments, the fixed frequency is 1kHz-10kHz.

[0012] In some embodiments, the dual-slit module consists of two parallel slits, which spatially split the incident light beam into symmetrical dual beams.

[0013] In some embodiments, the dual-hole module is conjugate with the dual-slit module, and the size of the holes in the dual-hole module is set to correspond to the center-to-center distance of the dual-slit module.

[0014] In some embodiments, the aperture plane is conjugate with the object plane, and the position of the zoom lens can be dynamically adjusted to adjust the focal plane position of the focused spot on the object plane.

[0015] In some embodiments, detector A and detector B are two-quadrant or four-quadrant detectors.

[0016] In some embodiments, detectors A and B can read the amount of light spot movement in real time and calculate the amount of defocus on the object surface according to the triangulation method; when the chopper is blocked, detectors A and B read the background noise signal a1; when the chopper is turned on, detectors A and B read the signal light and noise signal a2, and the effective signal light energy is a = a2 - a1.

[0017] In some embodiments, the signal strength A measured by detector A and the signal strength B measured by detector B are S=(AB) / (A+B), where S is the normalized differential signal, which serves as a direct measure of the defocus amount.

[0018] In some embodiments, the reflector is a crescent reflector.

[0019] In some embodiments, the crescent-shaped reflector has a wedge-shaped reflective surface. The light beam emitted from the A aperture of the dual-aperture module is reflected by the object surface, returns through the B aperture of the dual-aperture module, and is reflected by the wedge-shaped reflective surface of the crescent-shaped reflector before being detected by the detector A. The light beam emitted from the B aperture of the dual-aperture module is reflected by the object surface, returns through the A aperture of the dual-aperture module, and is transmitted through the crescent-shaped reflector before being detected by the detector B.

[0020] This application also provides an autofocus method, comprising the following steps: Two parallel beams are periodically blocked at a fixed frequency, modulating the two parallel beams into pulsed light with the same chopping frequency. The pulsed light is allowed to pass through the corresponding slits after passing through apertures A and B to form two circular light spots, which converge to the aperture surface. The two circular light spots and the imaging illumination beam are focused on the object surface, so that the autofocusing light spot is located at the edge of the objective lens's field of view, and the imaging illumination light spot covers the entire central area of ​​the field of view. The reflected light beam, after being reflected by the object surface, is allowed to pass through the corresponding slit as a beam after passing through the A hole and the B hole and then separated; Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through the synchronous demodulation circuit.

[0021] In some embodiments, a light beam emitted from aperture A is reflected by the object surface, returns through aperture B, and is detected by detector A; a light beam emitted from aperture B is reflected by the object surface, returns through aperture A, and is detected by detector B.

[0022] This application also provides an imaging optical system, including an illumination source unit and the autofocus system. The illumination source unit is used to emit the imaging illumination beam. A portion of the imaging illumination beam is split into two parallel beams. The chopper periodically blocks the two incident parallel beams at a fixed frequency to modulate them into pulsed light consistent with the chopping frequency.

[0023] The present application adopts the above technical solution, and its beneficial effects are as follows: The autofocus system provided in this application includes a chopper that periodically blocks two parallel light beams at a fixed frequency to modulate them into pulsed light with the same chopping frequency; and a detector equipped with a synchronous demodulation circuit that extracts the pulse signal with the same chopping frequency. This application uses chopping modulation and synchronous demodulation technology to convert continuous detection light into pulsed light of a specific frequency, so that the effective signal is separated from background noise in the frequency domain. By extracting the corresponding frequency signal through synchronous demodulation by the detector, interference from background light (DC component) and other frequency noise of the imaging system is eliminated, greatly improving the signal purity. In addition, through optical path design, the autofocus spot is placed at the edge of the field of view, spatially separated from the imaging spot covering the center of the field of view, fundamentally avoiding interference of the autofocus process on the core imaging area. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the structure of the autofocus system provided in the embodiments of this application.

[0026] Figure 2 This is a schematic diagram of the chopper provided in the embodiments of this application.

[0027] Figure 3 This is a surface light spot distribution diagram provided in an embodiment of the present invention.

[0028] Figure 4 This is a schematic diagram of the structure of the reflector provided in an embodiment of the present invention.

[0029] Figure 5 A flowchart illustrating the steps of the autofocus method provided in this application embodiment. Detailed Implementation

[0030] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application. In the description of this application, it should be understood that the terms "upper", "lower", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified. To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments.

[0031] Please see Figure 1 This is a schematic diagram of the structure of the autofocus system provided in the embodiments of this application, including: a chopper modulation unit 10, a beam shaping and coupling unit 20, an object surface reflection and signal return unit 30, and a signal detection and processing unit. The technical solution for its implementation is described in detail below.

[0032] The chopper modulation unit 10 includes a chopper 11, which periodically blocks two incident parallel beams at a fixed frequency to modulate them into pulsed light with the same chopping frequency.

[0033] Please see Figure 2 This is a schematic diagram of the chopper 11 provided in this embodiment. The chopper 11 includes chopping blades 111 and a chopper circuit board 112, and a motor is provided inside the chopper circuit board 112.

[0034] Specifically, the chopper blade 111 is typically a disc-shaped structure made of metal or composite material, with one or more slits to allow light to pass through. The blade is small in size and lightweight, which is beneficial for high-speed rotation.

[0035] Specifically, the chopper circuit board 112 is used to control the speed and phase of the motor. The chopper circuit board 112 integrates a frequency feedback circuit (such as an encoder or Hall sensor) to ensure a stable chopping frequency. The motor inside the chopper circuit board 112 drives the chopper blades 111 to rotate at high speed. The motor type is typically a brushless DC motor or a stepper motor to achieve a stable and controllable speed.

[0036] It can be understood that the chopper 11 converts a continuous optical signal into a pulsed optical signal by periodically blocking the light beam through rotating blades. The modulation frequency f is determined by the blade rotation speed N (revolutions per second) and the number of slits m, f = N × m. The modulation frequency is controlled by adjusting the motor speed via a circuit board. Using multiple slits allows for higher frequencies at lower rotation speeds, reducing mechanical vibration. After the modulated optical signal is received by the detector, it can be demodulated using a lock-in amplifier. The chopper typically outputs a reference frequency signal (TTL or sine wave) for synchronization reference in the lock-in amplifier.

[0037] In this embodiment, the chopper 11 periodically blocks and modulates the signal light (two incident parallel beams) at a fixed frequency (e.g., 1kHz-10kHz). The smaller the slit, the smaller the blade, and the higher the frequency of the chopper 11, the higher the chopper modulation frequency and the higher the signal demodulation and response frequency.

[0038] The beam shaping and coupling unit 20 includes a dual-aperture module 21 and a zoom lens 22. After the pulsed light passes through the two through holes of the dual-aperture module 21, the beam corresponding to the slit is allowed to pass through to form two circular light spots, which are then converged to the aperture surface by the zoom lens.

[0039] In this embodiment, the dual-aperture module 21 has two through holes (e.g., with a diameter of 0.2 mm and a center-to-center distance of 1 mm) that only allow the light beams corresponding to the slits to pass through, forming two circular light spots and limiting the numerical aperture (NA) of the beam.

[0040] In this embodiment, the zoom lens 22 is used to converge the autofocus beam to the aperture stop, wherein the aperture stop is conjugate with the objective lens surface.

[0041] Furthermore, the zoom lens 22 can be moved back and forth to change the position of the focal plane of the focused spot, thereby changing the position of the autofocus focal plane. By adjusting the zoom lens 22, the position of the focused spot on the object surface can be dynamically adjusted, allowing the system to adapt to objectives of different magnifications or samples of different thicknesses, thus expanding the application range of the equipment.

[0042] The object surface reflection and signal return unit 30 includes an objective lens 31 and a reflector 32. Two circular light spots and an imaging illumination beam are focused on the object surface by the objective lens 31. The resulting autofocusing light spot is located at the edge of the objective lens's field of view, and the resulting imaging illumination light spot covers the entire central area of ​​the field of view. The reflected light beams reflected by the object surface pass sequentially through the objective lens 31 and the zoom lens 22, and then enter the reflector 32 through the dual-aperture module 21. The reflector separates the two returning light beams.

[0043] It should be noted that the beam converged by objective lens 31 includes two circular light spots and an imaging illumination beam. The two modulated circular pulse light spots and the imaging illumination beam are precisely focused onto the sample (object surface) after passing through objective lens 31.

[0044] Please see Figure 3 This is a distribution diagram of the object surface light spot provided in this embodiment. The autofocus light spot, consisting of two circular spots used for focusing and detection, is specifically positioned at the edge of the objective lens's field of view; the imaging illumination light spot covers and illuminates the central area of ​​the entire field of view.

[0045] It is understandable that the autofocus spot is positioned at the edge of the objective lens's field of view, while the imaging illumination spot covers the central area of ​​the field of view. This spatial physical isolation ensures that the autofocus detection spot does not intrude into the main imaging field of view, thus completely avoiding any interference with the subject's imaging quality during the autofocus process, and enabling parallel operation of focus detection and high-quality imaging.

[0046] It can be understood that the reflected light from the object surface includes imaging illumination light and autofocus light. The imaging illumination light, after being reflected / scattered by the sample, carries the sample's image information. The autofocus light forms two spots on the object surface, and its reflected light carries the positional information of the object surface's height. The beam reflected back from the object surface carries information about the sample being out of focus. It first returns along its original path, then passes through objective lens 31 and zoom lens 22 again, and then through the dual-aperture module 21 to form two returning beams that are incident on mirror 32. Mirror 32 separates the two returning beams.

[0047] The signal detection and processing unit 40 includes detector A 41 and detector B 42. Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through the synchronous demodulation circuit.

[0048] Specifically, the light beam emitted from aperture A of the dual-aperture module 21 is reflected by the object surface, returns through aperture B of the dual-aperture module 21, and is separated by the reflector 32 before being detected by detector A; the light beam emitted from aperture B of the dual-aperture module 21 is reflected by the object surface, returns through aperture A of the dual-aperture module 21, and is separated by the reflector 32 before being detected by detector B. Detectors A and B extract a pulse signal consistent with the chopping frequency through a synchronous demodulation circuit.

[0049] In this embodiment, the reflector 32 is a crescent reflector.

[0050] Please see Figure 4 This is a schematic diagram of the crescent-shaped reflector provided in this embodiment. The crescent-shaped reflector uses a wedge-shaped reflective surface. Based on the incident position of the two beams, it physically separates the returning beams after they cross, achieving spatial separation of the returning beams and ensuring that the A / B optical path signals are transmitted independently to their respective detectors. The beam emitted from aperture A of the dual-aperture module is reflected by the object surface and returns through aperture B of the dual-aperture module. This portion of the light is incident on the reflection half-region of the crescent-shaped reflector, precisely reflected, and guided to detector A. The beam emitted from aperture B of the dual-aperture module is reflected by the object surface and returns through aperture A of the dual-aperture module. This portion of the light is incident on the transmission half-region of the crescent-shaped reflector, directly passing through and being guided to detector B. When the sample defocuses, the position of the light spot on the object surface shifts relative to the sample, causing the signal intensity received by detectors A and B to change in opposite directions. This effectively amplifies the signal difference caused by defocusing while suppressing common-mode noise.

[0051] It is understood that this embodiment provides an ideal signal source for differential detection through a cross design where "light emitted from aperture A returns through aperture B, and light emitted from aperture B returns through aperture A," combined with a crescent-shaped reflector for physical separation. This design cleverly utilizes optical principles to achieve signal separation, has a compact structure, avoids the use of complex electronic beam splitters, and improves the mechanical stability and long-term reliability of the system.

[0052] It should be noted that the detector is divided into detector A and detector B, which are of the same type. The detector can read the amount of light spot movement in real time and calculate the defocusing amount of the object surface using triangulation.

[0053] Preferably, the detector can be a two-quadrant detector or a four-quadrant detector. By reading the frequency of the chopper and demodulating it, the detector separates the signal light and eliminates noise, effectively improving the signal-to-weight ratio of the system.

[0054] The following section uses the Quadrant Detector (QD) as an example to explain its specific implementation scheme in detail.

[0055] For example, detectors A and B are both four-quadrant detectors, each divided into four quadrants (A1, A2, A3, A4 and B1, B2, B3, B4).

[0056] A four-quadrant detector can measure the position of the light spot in the X and Y directions, but in this autofocus system, only one direction is of concern (i.e., the defocus direction, let's say the X direction).

[0057] The relationship between the spot position and the defocusing amount is calculated using the triangulation method: When the object is precisely focused, the two light spots are located at the center of the two detectors, respectively.

[0058] When the object surface defocuses, the position of the light spot on the detector will shift. The amount of defocusing is proportional to the amount of light spot shift.

[0059] For each four-quadrant detector, the position of the light spot in the X direction is obtained by calculating the signal difference between the four quadrants.

[0060] Taking detector A as an example, its position signal in the X direction is: S Ax = ((A1+A4)) (A2+A3) / (A1+A2+A3+A4) Similarly, the position signal of detector B is: S Bx = ((B1+B4)) (B2+B3) / (B1+B2+B3+B4) Note: We are only concerned with the X direction here, because the optical path design makes defocusing mainly cause movement in the X direction.

[0061] Calculation of defocus amount: Due to the cross-optical path design, when the object surface is out of focus, the two light spots move in opposite directions on the detector.

[0062] Therefore, the defocusing amount can be calculated by the difference between the position signals of the two detectors: Δ=k(S Ax S Bx Where k is the calibration coefficient, which is determined experimentally.

[0063] Another common method is to use sum-difference combinations: S = (S Ax S Bx ) / (S Ax +S Bx Then, S is converted to defocus amount using a calibration curve. This method can eliminate the effects of light intensity fluctuations.

[0064] The specific details for synchronous demodulation are as follows: Because the beam is modulated by the chopper, the detector outputs an AC signal. Therefore, synchronous demodulation of the signal in each quadrant is required. The synchronous demodulation circuit (or digital demodulation algorithm) extracts the signal component with the same chopping frequency and eliminates DC background light and noise at other frequencies.

[0065] The specific steps include: a. The chopper provides a reference frequency signal to the synchronous demodulation circuit.

[0066] b. For the signal in each quadrant, perform synchronous demodulation to obtain the amplitude of the AC component (i.e., the effective signal) in each quadrant.

[0067] c. Then use these amplitudes to calculate the position signal.

[0068] Real-time processing: The system uses a microprocessor or FPGA to read the demodulated signal in each quadrant in real time, and calculates the position signal and defocus amount. The system samples at multiples of the chopping frequency to ensure real-time tracking of the light spot movement.

[0069] Calibration: The system needs to be calibrated before actual use. By moving the object surface a known distance, the corresponding S value is recorded, and a curve (usually linear) is established between the defocus amount and the S value.

[0070] Understandably, four-quadrant detectors offer high resolution and high response speed. Differential measurements (two detectors) can eliminate common-mode noise, such as surface tilt and vibration. Synchronous demodulation effectively suppresses background light interference.

[0071] It should be noted that if a two-quadrant detector is used, the detector is only divided into left and right quadrants, making the calculation simpler. However, a four-quadrant detector can provide more dimensions of information and is suitable for more complex situations.

[0072] It is understood that in this embodiment, detectors A and B measure the position of the light spot, use the principle of triangulation, and calculate the defocusing amount of the object surface through differential and normalization processing, and combine synchronous demodulation technology to improve the signal-to-noise ratio.

[0073] Preferably, when the chopper is blocked, detector A and detector B read the background noise signal a1; when the chopper is turned on, detector A and detector B read the signal light and noise signal a2, and the effective signal light energy is a = a2 - a1.

[0074] Preferably, the signal strength A measured by detector A and the signal strength B measured by detector B are S=(AB) / (A+B), where S is the normalized differential signal, which serves as a direct measure of the defocus amount.

[0075] It should be noted that during autofocusing, the signal received by the detector includes not only the focusing signal light reflected from the object surface, but also background light from the imaging system (e.g., stray light from the illumination path, ambient light, etc.) and noise from the detector itself. This background and noise typically manifest as DC or low-frequency components, which can interfere with the detection of the focusing signal and reduce the signal-to-noise ratio. Further explanation follows: Chopper modulation: In the transmitting optical path, a chopper (mechanical or acousto-optic modulator, etc.) is set up to periodically block the signal light at a fixed frequency (e.g., 1kHz-10kHz).

[0076] When the chopper blades block the optical path, the signal light is blocked, and the detector receives only background noise (including the background light of the imaging system and the dark noise of the detector).

[0077] When the chopper blades open, the signal light passes through, and the detector receives the superposition of the signal light and background noise.

[0078] Signal reading: Let the detector reading when the chopper is blocked be a1 (background noise signal).

[0079] Let the detector reading when the chopper is turned on be a2 (signal light + background noise).

[0080] Effective signal extraction: Effective signal light energy a = a2 - a1.

[0081] This calculation eliminates DC background noise that does not change over time (because the background noise exists both when the chopper is on and off, and is canceled out by subtraction).

[0082] Furthermore, when the chopper is blocked: The signal read by detector A is a1_A. This includes all background interference reaching detector A, such as stray light from the imaging system, ambient light, dark current of the detector itself, and electronic noise.

[0083] The signal read by detector B is a1_B. This includes all background interference arriving at detector B.

[0084] Furthermore, when the chopper is turned on: The signal read by detector A is a2_A. This includes (the effective focused signal light + the background noise reaching detector A).

[0085] The signal read by detector B is a2_B. This includes (the effective focused signal light + the background noise reaching detector B).

[0086] Calculate the effective signals for each: Valid signal for channel A: a_A = a2_A - a1_A This calculation eliminates the background noise unique to the A-channel, resulting in a clean A-channel focused signal.

[0087] Valid signal for channel B: a_B = a2_B - a1_B This calculation eliminates the background noise unique to the B channel, resulting in a pure B-channel focused signal.

[0088] Then, the system uses the two "purified" valid signals a_A and a_B to perform the final difference calculation: S = (a_A - a_B) / (a_A + a_B).

[0089] It is understood that this embodiment forms a symmetrical dual optical path through a dual-aperture module and a crescent-shaped reflector, and uses the signal differential operation of detectors A and B (S=(AB) / (A+B)) to improve resolution and suppress pattern interference.

[0090] To accurately sample when the chopper is on and off, a synchronous demodulation circuit is required. This circuit operates synchronously with the chopper, ensuring that a2 is read when the chopper is on and a1 is read when the chopper is off.

[0091] Synchronous demodulation circuits typically include a reference signal with the same frequency as the chopper. Through phase-sensitive detection (lock-in amplification) technology, the signal component with the same frequency and phase as the reference signal, i.e. the effective signal, is extracted from the detector output signal.

[0092] Chopper frequency selection: Usually, 1kHz-10kHz is selected. This frequency range is far away from common noise frequencies (such as 100Hz / 120Hz power frequency interference) and can also meet the requirements of fast autofocus response.

[0093] Sampling timing control: It is necessary to precisely control the sampling timing to ensure that sampling occurs when the chopper is fully open and fully blocked, so as to avoid errors in the transition state.

[0094] Signal processing: Synchronous demodulation can be achieved using analog circuits, or digital signal processing (DSP) technology can be used to perform digital calculations after sampling by an ADC.

[0095] By modulating the signal light to convert it into an AC signal, an AC-coupled amplifier can be used, avoiding DC drift problems. Synchronous demodulation extracts only the signal with the same chopping frequency, suppressing noise at other frequencies (such as random noise and power frequency interference), greatly improving the signal-to-noise ratio.

[0096] Detector A and detector B operate as described above, obtaining their respective valid signals A and B. Then, dual-path differential processing is used: S=(AB) / (A+B) to further suppress common-mode noise and obtain a normalized defocus signal.

[0097] As can be understood, this embodiment is based on the triangulation principle of dual-optical-path measurement. It calculates the defocusing amount by detecting the relative movement of the two beams on the object surface, thus ensuring high detection accuracy in principle. The algorithm S = (A - B) / (A + B) is used for processing. This algorithm not only suppresses common-mode noise (such as power supply fluctuations and mechanical vibrations) but also eliminates signal intensity variations caused by factors such as changes in sample reflectivity and fluctuations in light source power through normalization processing, thereby achieving sub-micron level detection accuracy and high repeatability.

[0098] The autofocus system provided in this application converts continuous detection light into pulsed light of a specific frequency (e.g., 1kHz-10kHz) using a chopper, timestamping the effective signal. The synchronous demodulation circuit of the receiver detector extracts only the AC signal consistent with this frequency, thereby effectively filtering out the background light (DC component) of the imaging system and environmental noise of other frequencies. By employing the calculation method of "on-state signal (a2) - off-state signal (a1)," the background noise is directly subtracted in real time at the circuit level, ensuring that the extracted signal is a pure focusing error signal.

[0099] Please refer to the following: Figure 1 The autofocus system provided in this application also includes a beam introduction and splitting unit 50, which includes a dual slit module 51, the dual slit module being used to split a portion of the imaging illumination beam into the two parallel beams.

[0100] Furthermore, the dual-slit module 51 consists of two parallel slits (e.g., 0.2 mm wide × 2 mm long, with a center-to-center distance of 1 mm), which form symmetrical dual beams through spatial beam splitting, thereby changing the beam shape and reducing the beam size.

[0101] Furthermore, the dual-hole module 21 is conjugate with the dual-slit module 51, and the size of the holes in the dual-hole module 21 corresponds to the center-to-center distance of the dual-slit module 51. For example, the diameter of the holes is 0.2 mm, and the center-to-center distance is 1 mm.

[0102] The autofocus system provided in this application also includes a beam modulation preprocessing unit 60, which includes a cylindrical lens 61 and a single slit module 62 located at the focal plane of the cylindrical lens 61. Two parallel beams are compressed in width by the cylindrical lens 61 and then incident on the single slit module 62.

[0103] It is understandable that after the two parallel beams are compressed in width (reduced beam cross-sectional area) by the cylindrical lens 61, they are incident on the single slit module 62 located at the focal plane of the cylindrical lens 61, which filters out the high-order stray light generated by diffraction and retains only the 0th order main beam, ensuring that the beam entering the chopper has concentrated energy and a clear outline.

[0104] It is understood that this embodiment, through the synergistic effect of the dual-slit module 51, the cylindrical lens 61 and the single-slit module 62, compresses the beam to a tiny size (matching the width of the chopper blade), providing a basis for achieving high-frequency chopping (>1kHz) and avoiding inertial limitations caused by excessively large blade size.

[0105] Furthermore, the autofocus system provided in this application also includes a light source unit (not shown). The illumination beam provided by the light source unit couples a detection wavelength light from the illumination beam to the autofocus system. This beam first enters the dual-slit module and is split into two parallel vertical beams, i.e., two parallel beams entering the chopper; while other illumination imaging beams in the illumination beam are focused at the object surface by the objective lens to form an imaging illumination spot.

[0106] It is understandable that since the autofocus system and imaging system share a light source, there is no need to set up a separate light source for autofocus, which reduces the complexity and cost of the system. At the same time, a shared light source means that autofocus and imaging are consistent in wavelength, avoiding problems such as chromatic aberration caused by different wavelengths and ensuring consistency between focus detection and imaging. Using the same light source, the autofocus system couples a portion of the light from the illumination beam, so that the autofocus beam and the imaging beam are consistent in optical characteristics (such as coherence and polarization), avoiding additional optical interference or aberrations caused by introducing light sources with different characteristics. Furthermore, placing the autofocus spot at the edge of the objective lens's field of view, while the imaging illumination spot covers the central area of ​​the field of view, spatially separates autofocus and imaging. The imaging area (center of the field of view) is unaffected by the autofocus spot, thus ensuring image quality. Simultaneously, the autofocus spot detects in the edge region, without obstructing or interfering with sample observation and imaging. Although the autofocus spot is located at the edge of the field of view, weak focus signals can be effectively extracted and background noise suppressed through chopper modulation and synchronous demodulation techniques. Meanwhile, because the autofocus spot is separated from the imaging area, the strong light from the imaging illumination spot will not directly enter the autofocus detector, thus avoiding saturation and interference.

[0107] The autofocus system provided in this application converts continuous detection light into pulsed light of a specific frequency through chopper modulation and synchronous demodulation technology, thereby separating the effective signal from background noise in the frequency domain. The corresponding frequency signal is extracted by synchronous demodulation of the detector, eliminating interference from background light (DC component) and other frequency noise of the imaging system, and greatly improving signal purity. In addition, through optical path design, the autofocus spot is placed at the edge of the field of view, which is spatially separated from the imaging spot covering the center of the field of view, fundamentally avoiding interference of the autofocus process to the core imaging area.

[0108] Please see Figure 5 This application also provides an autofocusing method, which includes the following steps: Step S51: Periodically block the two parallel beams at a fixed frequency to modulate the two parallel beams into pulsed light with the same chopping frequency.

[0109] Step S52: The pulsed light is allowed to pass through the corresponding slits after passing through holes A and B to form two circular light spots, which are then converged onto the aperture surface. Step S53: The two circular light spots and the imaging illumination beam are focused on the object surface, so that the autofocusing light spot is located at the edge of the objective lens's field of view, and the imaging illumination light spot covers the entire central area of ​​the field of view; Step S54: The reflected light beam, after being reflected by the object surface, is allowed to pass through the corresponding slit as a beam after passing through the A hole and the B hole, and then is separated; Step S55: Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through the synchronous demodulation circuit.

[0110] Furthermore, the light beam emitted from aperture A is reflected by the object surface and returns through aperture B to be detected by detector A; the light beam emitted from aperture B is reflected by the object surface and returns through aperture A to be detected by detector B.

[0111] Please refer to the following: Figure 1 This embodiment takes the above-mentioned autofocus system as an example to explain its specific focusing method in detail. Its working principle is as follows: A detection wavelength light is coupled from the imaging illumination optical path of the imaging system to the autofocus system. This beam first enters the double slit module 51 and is split into two parallel beams (e.g., vertical stripes). After the two parallel beams are compressed by the cylindrical lens 61, they enter the single slit module 62 located at the focal plane of the cylindrical lens 61. The higher-order stray light generated by diffraction is filtered out, and only the 0th-order main beam is retained, ensuring that the beam entering the chopper 11 has concentrated energy and a clear outline.

[0112] The pre-processed light beam is incident on chopper 11, which periodically blocks the beam at a fixed frequency (1kHz-10kHz), modulating the continuous light into a pulsed light signal consistent with the chopping frequency. This design, through the synergistic effect of double slits, cylindrical lenses, and a single slit, compresses the light beam to a tiny size (matching the width of the chopper blades), providing a basis for achieving high-frequency chopping (>1kHz) and avoiding the inertial limitations caused by excessively large blade sizes.

[0113] The modulated pulsed beam, after passing through the first lens 12, is transmitted through the first beam splitter 13 and imaged onto the dual-aperture module 21. The dual-aperture module 21 is conjugate with the dual-slit module, and its two through holes only allow the beams corresponding to the slits to pass through, forming two circular light spots and limiting the numerical aperture of the beams. The beams passing through the dual-aperture module 21 are reflected by the second beam splitter 23 and the third beam splitter 24, and then converged by the zoom lens 22 and the fourth beam splitter 25 to the aperture stop (conjugate with the objective lens object plane). The focal plane position of the focused light spot on the object plane can be dynamically adjusted by adjusting the position of the zoom lens 22.

[0114] Two circular light spots and an imaging illumination beam are focused onto the detection surface (object surface) by the objective lens 31 through the imaging lens group 33. The autofocusing light spot is located at the edge of the objective lens's field of view and is spatially separated from the imaging illumination light spot to avoid interference with the imaging system. The reflected beam, after being reflected by the object surface, then passes sequentially through the objective lens 31, imaging lens group 33, fourth beam splitter 25, zoom lens 22, third beam splitter 24, and second beam splitter 23 into the dual-aperture module 21. After being reflected by the second lens 35 on the first beam splitter, it is incident on the reflector 32, which separates the two returning beams. Specifically: the beam exiting from aperture A of the dual-aperture module 21 is reflected by the object surface, returns through aperture B of the dual-aperture module 21, passes through the second lens 34, then through the fifth beam splitter 35, and is separated by the reflector 32 before being detected by the detector A; the beam exiting from aperture B of the dual-aperture module 21 is reflected by the object surface, returns through aperture A of the dual-aperture module 21, passes through the second lens 34, then through the fifth beam splitter 35, and is separated by the reflector 32 before being detected by the detector B.

[0115] Detector A and Detector B extract pulse signals consistent with the chopping frequency through a synchronous demodulation circuit. The detector converts the received optical signals into electrical signals, performs calculations according to a specified formula, and outputs the electrical signals as input signals for the closed-loop control system.

[0116] It should be noted that this embodiment only uses the aforementioned autofocus system as an example to illustrate its autofocus method. In practice, it is not limited to the aforementioned autofocus system. All autofocus methods mentioned in this application are within the scope of protection of this application.

[0117] The autofocus method provided in this application converts continuous detection light into pulsed light of a specific frequency through chopper modulation and synchronous demodulation technology, thereby separating the effective signal from background noise in the frequency domain. The corresponding frequency signal is extracted by synchronous demodulation of the detector, eliminating interference from background light (DC component) and other frequency noise in the imaging system, which greatly improves the signal purity. In addition, through optical path design, the autofocus spot is placed at the edge of the field of view, which is spatially separated from the imaging spot covering the center of the field of view, fundamentally avoiding interference of the autofocus process on the core imaging area.

[0118] This application also provides an imaging optical system, including an illumination source unit (not shown) and the above-mentioned autofocus system. The illumination source unit is used to emit an imaging illumination beam. A portion of the imaging illumination beam is split into two parallel beams. A chopper 11 periodically blocks the two incident parallel beams at a fixed frequency to modulate them into pulsed light consistent with the chopping frequency.

[0119] It is understandable that, since the autofocus detection light and the imaging illumination light have the same wavelength, stray light generated by the imaging optical path becomes the most significant background noise in focus detection. However, chopper modulation and synchronous demodulation techniques have an excellent suppression effect on this type of high-intensity background light with the same wavelength, fundamentally solving the interference problem within the system.

[0120] Furthermore, the autofocus spot is strictly limited to the edge of the objective lens's field of view, while the imaging illumination covers the center of the field of view. This physically isolates the focus detection from the main imaging in space, completely avoiding any obstruction or light pollution of the imaging field of view, ensuring the integrity and high contrast of the image. Chopper modulation converts the focus detection light into pulses of a specific frequency, while the imaging is usually continuous or at different frequencies. This frequency differentiation allows subsequent electronic processing to easily separate and extract the focus signal from the imaging signal.

[0121] It is understood that the imaging optical system provided in this embodiment, thanks to the aforementioned spatiotemporal isolation, enables continuous imaging of the sample while simultaneously performing real-time, online automatic focusing. This eliminates the need to pause imaging to perform focusing, significantly improving observation efficiency and making it particularly suitable for long-term recording of live samples or dynamic processes.

[0122] Furthermore, the detection light required by the autofocus system is directly taken from the same illumination source unit of the imaging system. This achieves true optical path homogeneity, eliminating the complex design of independent focusing light sources in traditional solutions. There is no need to configure separate light sources, drive circuits, and beam-combining optical components for the focusing function. This significantly reduces the number of system components, physical size, and overall complexity, while effectively reducing manufacturing costs and power consumption.

[0123] The above are merely preferred embodiments of this application, and only specifically describe the technical principles of this application. These descriptions are only for explaining the principles of this application and should not be construed as limiting the scope of protection of this application in any way. Based on this explanation, any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application, as well as other specific embodiments of this application that can be conceived by those skilled in the art without creative effort, should be included within the scope of protection of this application.

Claims

1. An automatic focusing system, characterized in that, include: A chopper modulation unit includes a chopper that periodically blocks two incident parallel beams at a fixed frequency to modulate them into pulsed light with the same chopper frequency. The beam shaping and coupling unit includes a dual-aperture module and a zoom lens. The pulsed light is allowed to pass through the beam corresponding to the slit after passing through the two through holes of the dual-aperture module to form two circular light spots, which are then converged to the aperture surface by the zoom lens. The object surface reflection and signal return unit includes an objective lens and a mirror. The two circular light spots and the imaging illumination beam are focused onto the object surface by the objective lens. The resulting autofocusing light spot is located at the edge of the objective lens's field of view, and the resulting imaging illumination light spot covers the entire central area of ​​the field of view. The reflected light beam, after being reflected by the object surface, passes sequentially through the objective lens, the zoom lens, and then enters the reflector through the dual-aperture module, where the reflector separates the two returning beams. The signal detection and processing unit includes detector A and detector B. Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through a synchronous demodulation circuit.

2. The autofocus system as described in claim 1, characterized in that, It includes a beam introduction and splitting unit, which includes a dual-slit module for splitting a portion of the imaging illumination beam into two parallel beams.

3. The autofocus system as described in claim 2, characterized in that, It includes a beam modulation preprocessing unit, which includes a cylindrical lens and a single slit module located at the focal plane of the cylindrical lens. The two parallel beams are compressed in width by the cylindrical lens and then incident on the single slit module.

4. The autofocus system as described in claim 1, characterized in that, The fixed frequency is 1kHz-10kHz.

5. The autofocus system as described in claim 2, characterized in that, The dual-slit module consists of two parallel slits, which split the incident light beam into symmetrical dual beams through spatial beam splitting.

6. The autofocus system as described in claim 5, characterized in that, The dual-hole module is conjugate with the dual-slit module, and the size of the holes in the dual-hole module is set to correspond to the center-to-center distance of the dual-slit module.

7. The autofocus system as described in claim 1, characterized in that, The aperture plane is conjugate with the object plane, and the position of the zoom lens can be dynamically adjusted. By adjusting the position of the zoom lens, the focal plane position of the focused spot on the object plane can be adjusted.

8. The autofocus system as described in claim 1, characterized in that, Detector A and detector B are either two-quadrant or four-quadrant detectors.

9. The autofocus system as described in claim 1, characterized in that, The detectors A and B can read the spot movement in real time and calculate the defocusing of the object surface according to the triangulation method. When the chopper is blocked, the detectors A and B read the background noise signal a1. When the chopper is turned on, the detectors A and B read the signal light and noise signal a2. The effective signal light energy is a = a2 - a1.

10. The autofocus system as described in claim 1, characterized in that, The signal strength A measured by detector A and the signal strength B measured by detector B are given by S = (AB) / (A+B), where S is the normalized differential signal, which serves as a direct measure of the defocus amount.

11. The autofocus system as described in claim 1, characterized in that, The reflector is a crescent reflector.

12. The autofocus system as described in claim 11, characterized in that, The crescent-shaped reflector has a wedge-shaped reflective surface. The light beam emitted from the A hole of the dual-hole module is reflected by the object surface, returns through the B hole of the dual-hole module, and is reflected by the wedge-shaped reflective surface of the crescent-shaped reflector before being detected by the detector A. The light beam emitted from aperture B of the dual-aperture module is reflected by the object surface, returns through aperture A of the dual-aperture module, is transmitted through the crescent-shaped reflector, and is detected by detector B.

13. An autofocusing method, characterized in that, Includes the following steps: Two parallel beams are periodically blocked at a fixed frequency, modulating the two parallel beams into pulsed light with the same chopping frequency. The pulsed light is allowed to pass through the corresponding slits after passing through apertures A and B to form two circular light spots, which converge to the aperture surface. The two circular light spots and the imaging illumination beam are focused on the object surface, so that the autofocusing light spot is located at the edge of the objective lens's field of view, and the imaging illumination light spot covers the entire central area of ​​the field of view. The reflected light beam, after being reflected by the object surface, is allowed to pass through the corresponding slit as a beam after passing through the A hole and the B hole and then separated; Detector A and detector B respectively detect the separated return light and extract the pulse signal consistent with the chopping frequency through the synchronous demodulation circuit.

14. The autofocusing method as described in claim 13, characterized in that, The light beam emitted from aperture A is reflected by the object surface, returns through aperture B, and is detected by detector A; the light beam emitted from aperture B is reflected by the object surface, returns through aperture A, and is detected by detector B.

15. An imaging optical system, characterized in that, The system includes an illumination source unit and the autofocus system of claim 1. The illumination source unit is used to emit the imaging illumination beam. A portion of the imaging illumination beam is split into two parallel beams. The chopper periodically blocks the two incident parallel beams at a fixed frequency to modulate them into pulsed light with a chopping frequency consistent with the chopping frequency.

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