A wide-range, high-precision autofocus device based on a rectangular amplitude mask

Abstract

A large-range, high-precision autofocus device based on a rectangular amplitude mask, belonging to the field of optical detection technology, aims to solve the problem of incompatibility between accuracy and linear range in existing autofocus methods. It includes: a semiconductor laser, a collimating beam expander, a rectangular amplitude mask, a λ / 2 waveplate, a polarizing beam splitter, a λ / 4 waveplate, a wavelength beam splitter, a microscope objective, a sample, a tube lens, a focusing CCD, a computer, a motor control module, and an electric displacement stage. The rectangular amplitude mask is a square chrome-plated glass plate with a rectangular ring-shaped cutout structure missing one long side in the central area. By replacing the traditional knife edge and ring-shaped DOE with the rectangular amplitude mask, the centroid of the light spot on the CCD changes at different rates on the x-axis and y-axis sections. The lower rate of change on the x-axis allows for coarse autofocus over a large linear range, while the higher rate of change on the y-axis allows for fine autofocus over a small linear range. The combination of these two methods enables fine autofocus over a large linear range.

Description

Technical Field

[0001] This invention belongs to the field of optical detection technology, specifically a large-range, high-precision autofocus device based on a rectangular amplitude mask. Background Technology

[0002] Currently, Micro LED wafer inspection equipment generally employs microscopic inspection. The eccentric beam method is an active autofocusing method used in microscopes. This method primarily relies on establishing a relationship function between the centroid value of the spot on the CCD and the defocus position to achieve autofocus. Calculating the spot requires a good spot size; however, due to the diffraction of the system laser and the influence of complex system assembly, a good linear relationship is often difficult to achieve, causing the system's accuracy to exceed the depth of focus of the microscope objective. Furthermore, calculating only a single centroid of the spot cannot simultaneously meet the requirements of accuracy and linear range.

[0003] See Optics Express, where JIANLIN DU et al. published a paper titled "Large-range and high-precision autofocus method based on an annular DOE for a laser directwriting system". This scheme uses a 1 / 4-inch knife-edge and annular DOE to modulate the beam into a 3 / 4-inch ring. The defocusing amount is obtained by calculating the centroid of the spot in the x and y directions. Because the spot is a 3 / 4-inch ring, this technical scheme has the problem of incompatibility between accuracy and linear range. Summary of the Invention

[0004] To address the incompatibility between accuracy and linear range in existing autofocus methods, this invention provides a high-precision autofocus device with a wide range based on a rectangular amplitude mask. Furthermore, a waveplate is incorporated to modulate the beam polarization state, effectively reducing stray light in the system. This device offers high accuracy, a large linear range, and a simple structure that is easy to assemble and adjust.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A large-range, high-precision autofocus device based on a rectangular amplitude mask is characterized by comprising: a semiconductor laser, a collimating beam expander, a rectangular amplitude mask, a λ / 2 waveplate, a polarizing beam splitter, a λ / 4 waveplate, a wavelength beam splitter, a microscope objective, a sample, a tube lens, a focusing CCD, a computer, a motor control module, and an electric displacement stage.

[0007] The semiconductor laser, collimating beam expander, rectangular amplitude mask, λ / 2 waveplate and polarizing beam splitter are arranged coaxially in sequence.

[0008] The wavelength beam splitter, λ / 4 wave plate, polarizing beam splitter prism, tube lens and focusing CCD are arranged coaxially in sequence;

[0009] The wavelength spectrometer, microscope objective, and sample are arranged coaxially in sequence.

[0010] The semiconductor laser emits a near-infrared laser beam with a wavelength of 780 nm. After passing through a collimating beam expander, the beam is incident on a rectangular amplitude mask. The rectangular amplitude mask adjusts the beam cross-section shape into a rectangular ring missing one long side. The beam from the rectangular ring is then incident on a λ / 2 waveplate, where it is modulated into vertically vibrating linearly polarized light. This vertically vibrating linearly polarized light is then incident on a polarizing beam splitter and reflected by the beam splitter. After passing through a λ / 4 waveplate, the linearly polarized light is converted into right-handed circularly polarized light, which is then incident on a wavelength beam splitter and reflected onto a microscope objective. The beam is then focused onto the sample by the microscope objective and reflected back to the microscope objective by the sample surface. Right-handed circularly polarized light is converted into left-handed circularly polarized light. This left-handed circularly polarized light is then reflected by a wavelength splitter and passes through a λ / 4 waveplate, where it is modulated into horizontally vibrating linearly polarized light. This horizontally vibrating linearly polarized light is transmitted through a polarizing beam splitter and a tube lens, and then received by a focusing CCD. Simultaneously, the microscope objective is fixed on an electrically driven stage that can move along the optical axis. The focusing CCD is connected to a computer, which transmits the light spot information to the computer. The computer is connected to a motor control module, which processes the light spot information and manipulates the motor control module. The motor control module is connected to the electrically driven stage, causing the microscope objective to move axially along the optical axis, thus achieving automatic focusing.

[0011] The rectangular amplitude mask is a square chrome-plated glass plate with a rectangular ring-shaped hollow structure in the middle area that is missing one long side.

[0012] The fast axis direction of the λ / 2 waveplate is not fixed. By rotating the λ / 2 waveplate, the linearly polarized light incident on the polarizing beam splitter becomes a vertically vibrating s-wave.

[0013] The fast axis of the λ / 4 waveplate is at 45° to the vibration direction of the vertically vibrating linearly polarized light (i.e., s-wave).

[0014] The wavelength splitter has high reflectivity for near-infrared beams with a wavelength of 780nm and high transmittance for visible light.

[0015] The beneficial effects of this invention are:

[0016] This invention utilizes a rectangular amplitude mask instead of the traditional knife edge and ring DOE, so that the centroid of the light spot on the CCD changes at different rates on the x-axis and y-axis sections. The low rate of change on the x-axis allows for coarse autofocus over a large linear range, while the high rate of change on the y-axis allows for fine autofocus over a small linear range. The combination of the two enables fine autofocus over a large linear range. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the large-area high-precision autofocus device based on a rectangular amplitude mask according to the present invention;

[0018] Figure 2 This is a schematic diagram of the rectangular amplitude mask described in this invention;

[0019] Figure 3 This is a schematic diagram of the light spot captured by the focusing CCD11 under defocus conditions according to the present invention;

[0020] Figure 4 This is a schematic diagram of the light spot shape collected by the focusing CCD when the sample is located at different positions near the focal plane of the microscope objective in this invention;

[0021] The components include: 1. Semiconductor laser, 2. Collimating beam expander, 3. Rectangular amplitude mask, 4. λ / 2 waveplate, 5. Polarizing beam splitter, 6. λ / 4 waveplate, 7. Wavelength beam splitter, 8. Microscope objective, 9. Sample, 10. Tube lens, 11. Focusing CCD, 12. Computer, 13. Motor control module, and 14. Electric displacement stage. Detailed Implementation

[0022] The present invention will now be described in further detail with reference to the accompanying drawings.

[0023] like Figure 1 As shown, the large-range, high-precision autofocus device based on a rectangular amplitude mask includes: a semiconductor laser 1, a collimating beam expander 2, a rectangular amplitude mask 3, a λ / 2 waveplate 4, a polarizing beam splitter 5, a λ / 4 waveplate 6, a wavelength beam splitter 7, a microscope objective 8, a sample 9, a tube lens 10, a focusing CCD 11, a computer 12, a motor control module 13, and an electric displacement stage 14.

[0024] like Figure 2 As shown, the rectangular amplitude mask 3 is a square chrome-plated glass plate with a rectangular ring-shaped cutout structure missing one long side in the central area. This structure modulates the cross-sectional shape of the laser beam, so that the light spot collected on the focusing CCD 11 is as... Figure 3 As shown, the shape is also a rectangular ring missing one long side, and the centroid position of the light spot has a linear relationship with the defocusing amount. Because the long and short sides of the rectangular amplitude mask 3 are of different sizes, the centroid i along the x-axis of the light spot...cog With the y-axis centroid j +cog The rate of change is different, and the centroid of the x-axis i cog The centroid j of the y-axis corresponds to the short side of the rectangular amplitude mask 3. +cog Corresponding to the long side of the rectangular amplitude mask 3, the centroid j of the y-axis is... +cog The rate of change is greater than the x-axis centroid i cog The rate of change. And along the y-axis, it has two centroid positions, the distance between the two centroid positions is j. TC The rate of change is the y-axis centroid j +cog The rate of change is twice that of the centroid, so the distance j between the two centroid positions along the y-axis is... TC The rate of change of the centroid is greater than twice the rate of change of the x-axis centroid. The rate of change of the centroid is positively correlated with focusing accuracy; the greater the rate of change, the higher the accuracy. Coarse focusing is achieved over a wide range using the x-axis centroid with a low rate of change, and coarse focusing is achieved using the distance j between the two centroid positions along the y-axis with a high rate of change. TC By combining precise focusing within a small range with high-precision autofocus over a large linear range, it is possible to achieve high-precision autofocus.

[0025] The semiconductor laser 1, collimating beam expander 2, rectangular amplitude mask 3, λ / 2 waveplate 4, and polarizing beam splitter 5 are arranged coaxially in sequence.

[0026] Wavelength beam splitter 7, λ / 4 wave plate 6, polarizing beam splitter 5, tube lens 10 and focusing CCD 11 are arranged coaxially in sequence.

[0027] The wavelength spectrometer 7, the microscope objective 8, and the sample 9 are arranged coaxially in sequence.

[0028] The semiconductor laser 1 emits a near-infrared laser beam with a wavelength of 780 nm. After passing through the collimating beam expander 2, the beam is incident on a rectangular amplitude mask 3. The rectangular amplitude mask 3 adjusts the beam cross-sectional shape into a rectangular ring missing one long side. Then, the beam is incident on a λ / 2 waveplate 4, where it is modulated into vertically vibrating linearly polarized light. This vertically vibrating linearly polarized light is incident on a polarizing beam splitter 5, reflected by the prism, and then passes through a λ / 4 waveplate 6, which converts the linearly polarized light into a right-handed circularly polarized beam. The polarized light is then incident on the wavelength beam splitter 7 and reflected by the wavelength beam splitter 7 onto the microscope objective 8. The beam is focused onto the sample 9 by the infinite conjugate microscope objective 8 and reflected back to the microscope objective 8 by the surface of the sample 9. At this time, the right-hand circularly polarized light becomes left-hand circularly polarized light. Then, the left-hand circularly polarized light is reflected by the wavelength beam splitter 7 and passes through the λ / 4 waveplate 6 again. It is modulated by the λ / 4 waveplate 6 into horizontally vibrating linearly polarized light. The horizontally vibrating linearly polarized light is transmitted through the polarizing beam splitter prism 5 and then through the tube lens 10, and then received by the focusing CCD 11. Meanwhile, the microscope objective 8 is fixed on the motorized stage 14 that can move along the optical axis. The focusing CCD 11 is connected to the computer 12. The focusing CCD 11 transmits the spot information to the computer 12. The computer 12 is connected to the motor control module 13. The computer 12 processes the spot information and manipulates the motor control module 13. The motor control module 13 is connected to the motorized stage 14, so that the microscope objective 8 moves axially in the optical axis direction to achieve autofocus.

[0029] The fast axis direction of the λ / 2 waveplate is not fixed. By rotating the λ / 2 waveplate, the linearly polarized light incident on the polarizing beam splitter becomes a vertically vibrating s-wave.

[0030] The fast axis of the λ / 4 waveplate is at 45° to the vibration direction of the vertically vibrating linearly polarized light (i.e., s-wave).

[0031] The wavelength beam splitter 7 achieves high reflectivity for near-infrared beams with a wavelength of 780nm and high transmittance for visible light.

[0032] The computer 12 processes the light spot information and manipulates the motor control module 13. The specific steps are as follows: the position where the absolute value of the light spot centroid is the smallest is the positive focus position. The electric displacement stage 14 is controlled to move uniformly in the positive defocus and negative defocus directions at a distance of 10 μm. The computer 12 records the light spot position and calculates the light spot centroid value. First, the collected light spot is subjected to median filtering to reduce the influence of diffraction. Then, the Otsu method is used to calculate the adaptive segmentation threshold of the image. The area below the threshold is the background area, and the gray value becomes 0. The area above the threshold is the target area, and the gray value remains unchanged. Figure 4This is a schematic diagram of the spot shape acquired by the focusing CCD when the sample is located at different positions near the focal plane of the microscope objective. The computer 12 calculates the single-row centroid value i of the spot after image processing according to the following formula. cog With respect to the centroid value j of the single-row y-axis TC Computer 12 performs a linear fit between the obtained defocus amount and the corresponding centroid value, and records the information of the fitted straight line.

[0033]

[0034] Where i = 1...j max , i is the line number, i max It is the total number of rows, j c P is the y-coordinate of the image center. ij This represents the grayscale value of the pixel at coordinates (i, j) in the image.

[0035]

[0036]

[0037] j TC =j +cog -j -cog

[0038] Where j = 1...j max j is the column number, j max It is the column number of the row, i c The x-coordinate of the image center.

[0039] Using the centroid value j of a single row y TC The size of the defocus is calculated using the value of the centroid of a single line x. cog The sign of j is used to determine the defocus direction. TC Substituting the value into the fitted linear equation above, we can obtain the magnitude of the defocusing amount, i. cog When i is positive, it indicates that the defocus direction is negative. cog When the value is negative, it indicates that the defocus direction is positive. After obtaining the defocus information, the computer 12 manipulates the motor control module 13 to drive the electric displacement stage 14, so that the microscope objective 8 moves axially in the optical axis direction to complete the autofocus.

Claims

1. A large-range, high-precision autofocus device based on a rectangular amplitude mask, characterized in that its... include: Semiconductor laser (1), collimating beam expander (2), rectangular amplitude mask (3), λ / 2 waveplate (4), polarizing beam splitter (5), λ / 4 waveplate (6), wavelength beam splitter (7), microscope objective (8), sample (9), tube lens (10), focusing CCD (11), computer (12), motor control module (13) and electric displacement stage (14); The semiconductor laser (1), collimating beam expander (2), rectangular amplitude mask (3), λ / 2 waveplate (4) and polarizing beam splitter (5) are arranged coaxially in sequence; The wavelength beam splitter (7), λ / 4 wave plate (6), polarizing beam splitter (5), tube lens (10) and focusing CCD (11) are arranged coaxially in sequence; The wavelength beam splitter (7), the microscope objective (8), and the sample (9) are arranged coaxially in sequence; The semiconductor laser (1) emits a near-infrared laser beam with a wavelength of 780 nm. After passing through a collimating beam expander (2), the beam is incident on a rectangular amplitude mask (3). The rectangular amplitude mask (3) adjusts the beam cross-section shape to a rectangular ring missing one long side. Then, the beam of the rectangular ring is incident on a λ / 2 waveplate (4) and modulated by the λ / 2 waveplate (4) into vertically vibrating linearly polarized light. The vertically vibrating linearly polarized light is incident on a polarizing beam splitter (5) and reflected by the polarizing beam splitter (5). After passing through a λ / 4 waveplate (6), the λ / 4 waveplate (6) converts the linearly polarized light into right-hand circularly polarized light. Then, it is incident on a wavelength beam splitter (7) and reflected by the wavelength beam splitter (7) onto a microscope objective (8). The beam is focused by the microscope objective (8) onto the sample (9) and reflected back to the microscope objective (8) by the surface of the sample (9). At this time, the right-hand circularly polarized light becomes Left-handed circularly polarized light is reflected by a wavelength beam splitter (7) and then passes through a λ / 4 waveplate (6), and is modulated by the λ / 4 waveplate (6) into horizontally vibrating linearly polarized light. The horizontally vibrating linearly polarized light is transmitted through a polarizing beam splitter (5) and then through a tube lens (10), and is then received by a focusing CCD (11). At the same time, the microscope objective (8) is fixed on an electric displacement stage (14) that can move along the optical axis. The focusing CCD (11) is connected to a computer (12), and the focusing CCD (11) transmits the spot information to the computer (12). The computer (12) is connected to a motor control module (13), and the computer (12) processes the spot information and manipulates the motor control module (13). The motor control module (13) is connected to the electric displacement stage (14), so that the microscope objective (8) moves axially in the optical axis direction to achieve automatic focusing.

2. The large-range, high-precision autofocus device based on a rectangular amplitude mask according to claim 1, characterized in that, The rectangular amplitude mask (3) is a square chrome-plated glass plate with a rectangular ring-shaped hollow structure in the middle area that is missing one long side.

3. The large-range, high-precision autofocus device based on a rectangular amplitude mask according to claim 1, characterized in that, The fast axis direction of the λ / 2 waveplate (4) is not fixed. By rotating the λ / 2 waveplate (4), the linearly polarized light incident on the polarizing beam splitter becomes a vertically vibrating s-wave.

4. The large-range, high-precision autofocus device based on a rectangular amplitude mask according to claim 1, characterized in that, The fast axis of the λ / 4 waveplate (6) is at 45° to the vibration direction of the vertically vibrating linearly polarized light, i.e., the s-wave.

5. The large-range, high-precision autofocus device based on a rectangular amplitude mask according to claim 1, characterized in that, The wavelength splitter (7) has high reflectivity to near-infrared laser beams with a wavelength of 780nm and high transmittance to the visible band.

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