Automatic focusing system and thickness measuring system suitable for semitransparent wafer

Abstract

The invention discloses an automatic focusing system and a thickness measuring system suitable for a semitransparent wafer, and the automatic focusing system comprises the semitransparent wafer, an objective lens to be focused, a reflector, a laser light source, a first lens, a small-hole diaphragm, a programmable liquid crystal device, a collimating lens, a second lens, a third lens, a first detector, a second detector, and a motor. Accurate focusing of the objective lens is achieved through detection light spots of two detectors, and the thickness measuring system comprises a semitransparent wafer, a dispersion objective lens, a reflecting mirror, a white light source, an achromatic lens, a small-hole diaphragm, a programmable liquid crystal device, a collimating lens, a convergent lens, a slit, a lens and the detectors. According to the mapping relation between the transverse displacement on the detector and the thickness of the semitransparent wafer, the unknown thickness of the semitransparent wafer is measured, a spectrograph is not needed, and the cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of optical detection, and in particular relates to an automatic focusing system and a thickness measurement system suitable for a semi-transparent wafer. Background Art

[0002] The autofocus system is divided into active focus (AAF) and passive focus (PAF) based on the method. The former uses an additional optical path or structure to convert the focus information into a measurable physical quantity, usually optical information, for focusing. The latter mainly relies on digital image processing methods, directly using algorithms to perform a series of analyses through imaging conditions to help complete focusing. In recent years, with the continuous improvement of computer technology, autofocus technology for photography has shifted more to passive focus methods. Considering the needs of stability, modularity and convenience, active focus methods are currently generally used in industrial defect detection. As an important component of automated optical inspection (AOI), common specific applications include welding parts inspection, circuit board quality inspection, etc.

[0003] Semiconductor defect detection equipment has a fast detection speed. It is necessary to keep the focal plane of the optical system on the wafer surface while quickly scanning the wafer surface. Therefore, active focusing methods are more often used. Among them, the pupil segmentation method is the most widely used due to its low cost and high precision. At present, WDI, MSG, Prior, Evident and other companies have developed commercial modules using this method. The specific method is that the light beam emitted by the on-axis light source is blocked by an eccentric aperture, and the light on the unblocked side of the optical axis plane is collimated and converged on the wafer surface by the objective lens. Then the light beam is reflected by the wafer surface, propagates along the other side of the optical axis plane, and finally converges on the sensor. When the focal plane of the objective lens is at different positions on the wafer reflection surface, the light spot of the return light beam on the sensor will also change. For example, when the measured surface is located on the focal plane of the objective lens, the light spot on the sensor will be a point. When the measured surface is above the focal plane of the objective lens, the light spot on the sensor will be semicircular and located on the right side of the central axis of the sensor; when the measured surface is below the focal plane of the objective lens, the light spot on the sensor will be semicircular and located on the left side of the central axis of the sensor. The defocus amount can be determined by calculating the grayscale center of gravity of the return beam spot on the sensor. However, this method is not suitable for translucent wafers, as the reflection of the lower surface of the translucent wafer will have a greater impact on the focusing accuracy. Summary of the invention

[0004] In view of the above, the object of the present invention is to provide an automatic focusing system and a thickness measurement system suitable for translucent wafers, which can achieve precise focusing of the objective lens in a translucent wafer defect detection device and measure the thickness of the translucent wafer through a designed optical path.

[0005] To achieve the above-mentioned purpose of the invention, an embodiment provides an automatic focusing system suitable for a semi-transparent wafer, comprising: a semi-transparent wafer, an objective lens to be focused, a reflector, a laser light source, a first lens, a pinhole aperture, a programmable liquid crystal device, a collimating lens, a second lens, a third lens, a first detector, a second detector, and a motor;

[0006] The outgoing light of the laser light source is collimated by the first lens, and then passes through a pinhole diaphragm controlled by a motor to adjust the distance from the optical axis, so that the pinhole diaphragm acts as an off-axis light source to emit an off-axis light beam. The off-axis light beam passes through the programmable liquid crystal device, is collimated by a collimating lens, and is incident on a reflector. The reflector reflects the incident light to the objective lens and focuses it onto a translucent wafer.

[0007] When focusing on the upper surface of the semi-transparent wafer, the reflected light passing through the upper surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the second lens, and then converged by the second lens and received by the first detector to form a light spot. At the same time, the reflected light passing through the lower surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the third lens, and then converged by the third lens and received by the second detector to form a light spot.

[0008] When focusing on the lower surface of the semi-transparent wafer, the reflected light passing through the lower surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the third lens, and then converged by the third lens and received by the second detector to form a light spot. At the same time, the reflected light passing through the upper surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the second lens, and then converged by the second lens and received by the first detector to form a light spot.

[0009] The objective lens is adjusted by observing the light spot or light point to achieve focusing of the objective lens on the translucent wafer.

[0010] Preferably, the diameter of the pinhole aperture is 5-10 um, and the off-axis distance of the motor-driven pinhole aperture is 1-3 mm.

[0011] Preferably, the focal lengths of the first lens, the second lens, and the third lens are 50-75 mm, and the second lens and the third lens have the same focal length.

[0012] Preferably, the programmable liquid crystal device can display a circular light-transmitting area and a light-impermeable area, and outputs an off-axis light beam after passing through the programmable liquid crystal device. The laser light source is a laser LD with a wavelength of 660nm or 850nm.

[0013] To achieve the above-mentioned purpose of the invention, an embodiment of the present invention further provides a thickness measurement system suitable for a semi-transparent wafer, comprising: a semi-transparent wafer, a dispersive objective lens, a reflector, a white light source, an achromatic lens, a pinhole aperture, a programmable liquid crystal device, a collimating lens, a converging lens, a slit, a lens, and a detector;

[0014] The outgoing light of the white light source is collimated by an achromatic lens, and then passes through a small aperture diaphragm as an off-axis light source to emit an off-axis light beam. The off-axis light beam passes through a programmable liquid crystal device, is collimated by a collimating lens, and is incident on a reflector. The reflector reflects the incident light to a dispersive objective lens and converges the dispersed light of different colors onto different focal planes of a translucent wafer. Specifically, the long wave is focused on the lower surface of the translucent wafer, and the short wave is focused on the upper surface of the translucent wafer. There is a lateral displacement between the focus of the long wave light beam on the lower surface and the focus of the short wave light beam on the upper surface.

[0015] The short-wavelength light is reflected by the upper surface of the semi-transparent wafer and collimated by the dispersive objective lens again. It is then reflected by the reflector and passes through the converging lens to reach the slit. It is focused to a point at the slit position and passes through the slit without loss. The short-wavelength light reflected by other surfaces is blocked by the slit. Then, the light passing through the slit is converged on the detector after passing through the lens.

[0016] The long-wavelength light is reflected by the lower surface of the semi-transparent wafer and collimated by the dispersive objective lens again, because it has a lateral displacement with the short-wavelength light when propagating in the semi-transparent wafer. Then, it is reflected by the reflector and passes through the converging lens to reach the slit, where it is also focused to a point and has a lateral displacement with the convergence point of the short-wavelength light. The long-wavelength light then passes through the slit without loss, and the long-wavelength light reflected by other surfaces is blocked by the slit. Then, the light passing through the slit is converged on the detector after passing through the lens, and has a lateral displacement with the convergence point of the short-wavelength light on the detector.

[0017] The thickness measurement system is calibrated according to the mapping relationship between the lateral displacement on the detector and the thickness of the translucent wafer, and the known thickness of the translucent wafer. Then, the calibrated thickness measurement system is used to measure the unknown thickness of the translucent wafer according to the measured lateral displacement and the mapping relationship.

[0018] Preferably, the tilt angle of the light beam constrained between the pinhole aperture and the programmable liquid crystal device is in the range of 0-5°.

[0019] Preferably, the slit width is 15-25 um.

[0020] Preferably, the focal length of the achromatic lens is 25-50 mm, and the focal lengths of the collimating lens, the converging lens and the lens are all 50-75 mm; the lens images the slit size 1:1 to the detector.

[0021] Preferably, the mapping relationship between the lateral displacement on the detector and the thickness of the semi-transparent wafer is:

[0022] Assume that the wavelength range of the white light source includes λ a and λ b , the dispersion objective lens converges λ a and λ b At different depths, λ a The wavelength is the reflection wavelength of the semi-transparent wafer surface, λ b The wavelength is the reflection wavelength of the lower surface of the semi-transparent wafer, d is the thickness of the semi-transparent wafer, and λ is the thickness of the detector. a and λ b The lateral displacement l between the convergence points follows the following mapping relationship:

[0023]

[0024] where f c is the focal length of the dispersive objective lens, f f is the focal length of the converging lens, θ is the inclination angle between the optical axis determined by the pinhole aperture and the center of the light-transmitting part of the programmable liquid crystal device and the optical axis determined by the white light source and the achromatic lens, and the thickness d information is obtained from l;

[0025]

[0026] Preferably, the thickness measurement system is capable of simultaneously measuring the thickness of multiple surfaces of the translucent wafer.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The automatic focusing system and thickness measurement system provided by the present invention both realize precise focusing of the objective lens in the translucent wafer defect detection device by constructing a special optical path, thereby solving the problem that when focusing on a translucent wafer, the reflection of the lower surface of the translucent wafer causes low focusing accuracy in the current eccentric beam method. The thickness of the translucent wafer can also be measured by designing a special optical path without the need for a spectrometer, thereby reducing the thickness measurement cost and increasing the scope of use. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1is a structural schematic diagram of an automatic focusing system applicable to a translucent wafer provided in an embodiment focusing on the upper surface of the translucent wafer;

[0031] Figure 2 It is a structural schematic diagram of an automatic focusing system applicable to a translucent wafer provided in an embodiment focusing on the lower surface of the translucent wafer;

[0032] Figure 3 is a schematic diagram of the principle of a programmable liquid crystal device provided in an embodiment;

[0033] Figure 4 It is a schematic diagram of the structure of the autofocus system provided for comparison;

[0034] Figure 5 Schematic diagram of the light spot when the upper surface of the pure reflection wafer provided in the comparative example is located at (a) below the focal plane of the objective lens, (b) at the focal plane of the objective lens, and (c) above the focal plane of the objective lens;

[0035] Figure 6 Schematic diagram of the light spot when the upper surface of the semi-transparent wafer provided in the comparative example is located (a) below the focal plane of the objective lens, (b) at the focal plane of the objective lens, and (c) above the focal plane of the objective lens and the lower surface is located below the focal plane of the objective lens;

[0036] Figure 7 Schematic diagram of the structure of a thickness measurement system suitable for a semi-transparent wafer provided in an embodiment. DETAILED DESCRIPTION

[0037] To make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation methods described herein are only used to explain the present invention and do not limit the scope of protection of the present invention.

[0038] Example 1

[0039] In order to solve the technical problem that when focusing on a semi-transparent wafer, the reflection on the lower surface of the semi-transparent wafer will cause the focusing accuracy to be low, an embodiment provides an automatic focusing system suitable for a semi-transparent wafer, such as Figure 1As shown, it includes a semi-transparent wafer 108, an objective lens 107 to be focused, a reflector 106, a laser light source 101, a first lens 102, a pinhole aperture 103, a programmable liquid crystal device 104, a collimating lens 105, a second lens 109, a third lens 111, a first detector 110, a second detector 112, and a motor 113. Among them, the laser light source 101 is a laser LD with a wavelength of 660nm, the diameter of the pinhole aperture 103 is 10um, the motor 113 drives the pinhole aperture 103 to have an off-axis distance of 1mm, the focal length of the first lens 102 is generally 50mm, the focal lengths of the second lens 109 and the third lens 111 are equal, which is 75mm, and the second lens 109 and the third lens 111 have the same focal length.

[0040] like Figure 3 As shown, the programmable liquid crystal device 104 can display a circular light-transmitting area 22 and a light-impermeable area 21 , and an off-axis light beam is output after passing through the programmable liquid crystal device 104 .

[0041] Specifically, the laser light source 101 emits divergent outgoing light, which is collimated into a parallel light beam through the first lens 102. The divergence angle of the parallel light beam is small, and then passes through a pinhole aperture 103, and a divergent light beam with a small divergence angle is emitted from the pinhole. The position of the pinhole can be regarded as a point light source. The pinhole of the pinhole aperture 103 deviates from the optical axis of the first lens 102 and the laser light source 101 by a certain distance, so the light emitted from the pinhole can be regarded as an off-axis light source. The motor 113 is connected to the pinhole aperture 103 and can drive the distance of the pinhole aperture 103 from the axis. The off-axis light beam emitted from the pinhole is still an off-axis light beam after passing through the programmable liquid crystal device 104. The off-axis light beam is collimated into a parallel light beam by the collimating lens 105 and then incident on the reflector 106 and reflected by the reflector 106. The parallel light beam is on the right side of the optical axis and is inclined to the objective lens 107 at a certain angle with the optical axis. The objective lens 107 converges the inclined parallel light beam.

[0042] In one case, when the upper surface of the semi-transparent wafer 108 is at the focal point of the objective lens, the objective lens 107 converges the light beam and focuses it on a point on the upper surface of the semi-transparent wafer 108. The light beam reflected from the upper surface is on the other side of the optical axis of the objective lens 107, and then collimated by the objective lens 107 again. The collimated light beam is reflected by the reflector 106, and then converged by the second lens 109, and the focal point becomes a point on the first detector 110. Since the semi-transparent wafer 108 is semi-transparent, another part of the energy of the light beam converged by the objective lens 107 passes through the upper surface of the semi-transparent wafer 108 and is incident on the lower surface of the semi-transparent wafer 108, and a part of the energy is reflected by the lower surface of the semi-transparent wafer 108. Since the light beam converged by the objective lens 107 converges on the upper surface of the semi-transparent wafer 108, the light beam is a divergent light beam when it reaches the lower surface of the wafer. In addition, since the incident light beam is on one side of the optical axis of the objective lens 107 and the light beam is incident at an angle, there is a lateral offset between the convergence point of the incident light beam on the upper surface of the translucent wafer 108 and the center point of the convergence spot on the lower surface of the translucent wafer 108. The light beam reflected by the lower surface of the translucent wafer 108 then enters the objective lens 107. Since the lower surface of the translucent wafer 108 is not at the focus of the objective lens 107, the light beam is not a beam of parallel light after entering the objective lens 107. The light beam is then reflected by the reflector 106 and converged by the third lens 111. The third lens 111 and the second lens 109 have the same focal length. The light beam converged by the third lens 111 is received by the second detector 112 to form a light spot. The second detector 112 and the first detector 110 have the same lateral position. The objective lens is adjusted by observing the light spot or light point to achieve the focusing of the objective lens on the translucent wafer.

[0043] Another case, such as Figure 2As shown, when the lower surface of the semi-transparent wafer 108 is at the focal point of the objective lens. At this time, the convergent light beam of the objective lens 107 is not focused on a point on the upper surface of the semi-transparent wafer 108, but a light spot. The light beam reflected from the upper surface is on the other side of the optical axis of the objective lens, and then collimated by the objective lens 107 again. The collimated light beam is reflected by the reflector 106, and then converged by the second lens 109, and the focus point forms a light spot on the first detector 110. Since the semi-transparent wafer 108 is semi-transparent, another part of the energy of the convergent light beam of the objective lens 107 passes through the upper surface of the semi-transparent wafer 108 and is incident on the lower surface of the semi-transparent wafer 108. Because the lower surface of the wafer is at the focal point of the objective lens, there is a convergence point on the lower surface of the wafer. Part of the energy is reflected by the lower surface of the semi-transparent wafer 108. In addition, since the incident light beam is on one side of the optical axis of the objective lens and the light beam is incident at an angle, there is a lateral offset between the center of the convergent light spot of the incident light beam on the upper surface of the semi-transparent wafer 108 and the convergence point on the lower surface of the wafer 108. The light beam reflected by the lower surface of the semi-transparent wafer 108 then enters the objective lens 107. Since the lower surface of the semi-transparent wafer 108 is at the focus of the objective lens 107, the light beam is emitted as a beam of parallel light after entering the objective lens 107. The light beam is then reflected by the reflector 106 and converged by the lens 111. The third lens 111 and the second lens 109 have the same focal length. The light beam converged by the third lens 111 is received by the second detector 112. The second detector 112 and the first detector 110 have the same lateral position. What the second detector 112 receives is a light spot. Similarly, the objective lens is adjusted by observing the light spot or light spot to achieve the focusing of the objective lens on the semi-transparent wafer.

[0044] Comparative Example 1

[0045] Provide as Figure 4 The autofocus system shown is used as a comparative example. In this comparative example, the laser light source 201 emits a divergent light beam, which then passes through a light baffle 202. The light baffle 202 is placed on one side of the optical axis and blocks half of the light beam. Only the light beam on the other side of the optical axis can pass through the light baffle 202. The light beam passing through the light baffle 202 then passes through a collimating lens 203 to become a parallel light beam. The parallel light beam then continues to propagate along the other side of the optical axis, reaches the reflector 204, is reflected by the reflector 204, enters the objective lens 205, and is converged by the objective lens 205. The upper surface of the wafer 206 is placed on the focus of the objective lens 205, so it converges at a point on the upper surface of the wafer 206. After being reflected by the upper surface of the wafer 206, the light beam propagates along the other side of the optical axis. After passing through the objective lens 205, the light beam is collimated, and then reflected by the reflector 204 and passes through the collimating lens 204 again. After being converged by the collimating lens 204, it is reflected by the reflector 207 and converges on the detector 208.

[0046] The above is the case when the wafer 206 is a pure reflective wafer and is exactly at the focal plane of the objective lens. At this time, the light spot on the detector 208 is just a point. Figure 5When the focal plane of the objective lens is above the wafer 208, the light beam is as shown in (b). Figure 5 As shown in (a), the light spot focused on the wafer is on the right side of the system optical axis, and on the detector 208 is a semicircular light spot located on the right side of the detector 208. When the focal plane of the objective lens is below the wafer 208, the light beam is as shown in FIG. Figure 5 As shown in (c), the light spot converged on the wafer is on the left side of the system optical axis, and on the detector 208 there is a semicircular light spot, which is located on the left side of the detector 208. At this time, the focus is adjusted by calculating and determining the center of mass.

[0047] When the wafer 208 is a semi-transparent wafer, the light spot situation is very complicated. Figure 6 As shown, when the upper surface of the wafer 208 is below the focal plane of the objective lens, as shown in Figure 6 As shown in (a), the light beam incident on the upper surface of the wafer is a light spot located on the right side of the optical axis. At the same time, the light beam incident on the lower surface of the wafer 208 is a larger light spot, also on the right side of the optical axis. The detector 208 also detects two light spots, and the detected light spots are on the right side of the detector. When the upper surface of the wafer 208 is just located at the focal plane of the objective lens, as shown in FIG. Figure 6 As shown in (b), the light beam incident on the upper surface of the wafer is a point located on the optical axis. At the same time, the light beam incident on the lower surface of the wafer 208 is a spot on the right side of the optical axis. The detector 208 also detects a point and a large spot. The detected spot is on the right side of the detector and the spot is in the center of the detector. When the upper surface of the wafer 208 is above the focal plane of the objective lens and the lower surface of the wafer 208 is below the focal plane of the objective lens, as shown in FIG. Figure 6 As shown in (c), the light beam incident on the upper surface of the wafer is a light spot located on the left side of the optical axis. At the same time, the light beam incident on the lower surface of the wafer 208 is a light spot on the right side of the optical axis. The detector 208 also detects two light spots, which are respectively on both sides of the detector. There are other more complicated situations that are not listed one by one. It can be seen that the reflected light from the lower surface will interfere with the detection of the focus plane. The above detection method will be inaccurate or ineffective.

[0048] Example 3

[0049] In order to solve the technical problems of high cost and limited use of existing semi-transparent wafer thickness measurement due to the introduction of a spectrometer, an embodiment provides a thickness measurement system suitable for a semi-transparent wafer, such as Figure 7As shown, it includes a semi-transparent wafer 308, a dispersive objective lens 307, a reflector 306, a white light source 301, an achromatic lens 302, a pinhole diaphragm 303, a programmable liquid crystal device 304, a collimating lens 305, a converging lens 309, a slit 310, a lens 311, and a detector 312. Among them, the light beam inclination angle constrained between the pinhole diaphragm 303 and the programmable liquid crystal device 304 is within a range of 5°. The slit 310 has a slit width of 15um. The focal length of the achromatic lens 302 is 25mm, and the focal lengths of the collimating lens 305, the converging lens 309, and the lens 311 are all 50mm; the lens images the slit size 1:1 to the detector 312.

[0050] Specifically, the white light source 301 is a white light LED, which emits a divergent light beam, which is collimated by the achromatic lens 302 into a quasi-parallel light beam. The divergence angle of the parallel light beam is small, and then passes through a pinhole aperture 303. After the light beam passes through the pinhole aperture 303, the pinhole emits a divergent light beam with a small divergence angle. The position of this pinhole can be regarded as a point light source. The pinhole of the pinhole aperture 303 deviates from the optical axis of the achromatic lens 102 and the white light source 301 by a certain distance, so the light emitted from the pinhole can be regarded as an off-axis light source. The light beam emitted from the pinhole then passes through a programmable liquid crystal device 304, such as Figure 2 As shown. The programmable liquid crystal device 304 can display a circular light-transmitting area 22 and an opaque area 21. The light beam after passing through the programmable liquid crystal device 304 is an off-axis light beam, where the axis is the optical axis determined by the white light source 301 and the achromatic lens 302. The off-axis light beam is collimated into a parallel light beam by the incident collimating lens 305 and reflected by the reflector 306. The parallel light beam is on the right side of the optical axis and is obliquely incident on the dispersive objective lens 307 at a certain angle to the optical axis. The dispersive objective lens 307 has the function of separating the incident white light and focusing different colored lights on focal planes at different depths according to the different wavelengths of the colored lights. The objective lens 107 converges the inclined parallel light beams, and focuses the long waves on the bottom of the translucent material 308 and the short waves on the top of the translucent material 308, all of which are focused into one point, as shown in FIG. Figure 7 As shown in Figure 2. Because the incident light beam is tilted, there is a lateral displacement between the long-wavelength and the short-wavelength.

[0051] Specifically, the light of short wavelength is reflected by the upper surface of the semi-transparent wafer 308, and is collimated by the dispersive objective lens 307 again after passing through the dispersive objective lens 307, and then is reflected by the reflector 306 and passes through the converging lens 309 to reach the slit. The light beam is focused to a point at the slit position and passes through the slit without loss. The light of short wavelength reflected by other surfaces is blocked by the slit. Then, the light beam passes through the lens 311 and converges on the detector 312. The light of long wavelength is reflected by the lower surface, and is collimated by the dispersive objective lens 307 again after passing through the dispersive objective lens 307. Because there is a lateral displacement between the light of short wavelength and the light propagating in the semi-transparent material, the light beam is reflected by the reflector 306 and passes through the converging lens 309 to reach the slit. The light beam is focused to a point at the slit position and has a lateral displacement between the light convergence point of short wavelength and the light beam. Then, the light beam passes through the slit without loss. The light of long wavelength reflected by other surfaces is blocked by the slit. Subsequently, the light beam passes through lens 311 and converges on detector 312 , with a lateral displacement from the convergence point of the short wavelength on detector 312 .

[0052] In this way, a mapping relationship is obtained between the lateral displacement on the detector and the thickness of the semi-transparent wafer. The thickness measurement system is calibrated according to the mapping relationship and the known thickness of the semi-transparent wafer, and then the calibrated thickness measurement system is used to measure the unknown thickness of the semi-transparent wafer according to the measured lateral displacement and the mapping relationship. The system is not limited to measuring the thickness of two surfaces, but can measure the thickness of multiple surfaces at the same time.

[0053] Specifically, assuming that the wavelength range of the white light source includes λ a and λ b , the dispersion objective lens converges λ a and λ b At different depths, λ a The wavelength is the reflection wavelength of the semi-transparent wafer surface, λ b The wavelength is the reflection wavelength of the lower surface of the semi-transparent wafer, d is the thickness of the semi-transparent wafer, and λ is the thickness of the detector. a and λ b The lateral displacement l between the convergence points follows the following mapping relationship:

[0054]

[0055] where f c is the focal length of the dispersive objective lens, f f is the focal length of the converging lens, θ is the inclination angle between the optical axis determined by the pinhole aperture and the center of the light-transmitting part of the programmable liquid crystal device and the optical axis determined by the white light source and the achromatic lens, and the thickness d information is obtained from l;

[0056]

[0057] Compared with the existing structure that uses a dispersive objective lens for depth measurement, the thickness measurement system provided in the embodiment does not require an additional dispersive spectrometer, has a lower cost, and can measure thickness without moving the objective lens, and can measure the thickness of multiple layers at the same time.

[0058] The specific implementation methods described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An autofocus system suitable for a semi-transparent wafer, characterized in that: It includes: a semi-transparent wafer, an objective lens to be focused, a reflector, a laser light source, a first lens, a pinhole aperture, a programmable liquid crystal device, a collimating lens, a second lens, a third lens, a first detector, a second detector, and a motor; The outgoing light of the laser light source is collimated by the first lens, and then passes through a pinhole diaphragm controlled by a motor to adjust the distance from the optical axis, so that the pinhole diaphragm acts as an off-axis light source to emit an off-axis light beam. The off-axis light beam passes through the programmable liquid crystal device, is collimated by a collimating lens, and is incident on a reflector. The reflector reflects the incident light to the objective lens and focuses it onto a translucent wafer. When focusing on the upper surface of the semi-transparent wafer, the reflected light passing through the upper surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the second lens, and then converged by the second lens and received by the first detector to form a light spot. At the same time, the reflected light passing through the lower surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the third lens, and then converged by the third lens and received by the second detector to form a light spot. When focusing on the lower surface of the semi-transparent wafer, the reflected light passing through the lower surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the third lens, and then converged by the third lens and received by the second detector to form a light spot. At the same time, the reflected light passing through the upper surface of the semi-transparent wafer is collimated by the objective lens and then reflected by the reflector to the second lens, and then converged by the second lens and received by the first detector to form a light spot. The objective lens is adjusted by observing the light spot or light point to achieve focusing of the objective lens on the translucent wafer.

2. The automatic focusing system for a semi-transparent wafer according to claim 1, characterized in that: The diameter of the pinhole diaphragm is 5-10um, and the off-axis distance of the motor-driven pinhole diaphragm is 1-3mm.

3. The automatic focusing system for a semi-transparent wafer according to claim 1, characterized in that: The focal lengths of the first lens, the second lens, and the third lens are 50-75 mm, and the second lens and the third lens have the same focal length.

4. The automatic focusing system for a semi-transparent wafer according to claim 1, characterized in that: The programmable liquid crystal device can display a circular light-transmitting area and a light-impermeable area. After passing through the programmable liquid crystal device, an off-axis light beam is output. The laser light source is a laser LD with a wavelength of 660nm or 850nm.

5. A thickness measurement system suitable for a semi-transparent wafer, characterized in that: Includes: semi-transparent wafer, dispersive objective lens, reflector, white light source, achromatic lens, pinhole diaphragm, programmable liquid crystal device, collimating lens, converging lens, slit, lens, and detector; The outgoing light of the white light source is collimated by an achromatic lens, and then passes through a small aperture diaphragm as an off-axis light source to emit an off-axis light beam. The off-axis light beam passes through a programmable liquid crystal device, is collimated by a collimating lens, and is incident on a reflector. The reflector reflects the incident light to a dispersive objective lens and converges the dispersed light of different colors onto different depth focal planes of a translucent wafer. Specifically, the long wave is focused on the lower surface of the translucent wafer, and the short wave is focused on the upper surface of the translucent wafer. There is a lateral displacement between the focus of the long wave light beam on the lower surface and the focus of the short wave light beam on the upper surface. The short-wavelength light is reflected by the upper surface of the semi-transparent wafer and collimated by the dispersive objective lens again. It is then reflected by the reflector and passes through the converging lens to reach the slit. It is focused to a point at the slit position and passes through the slit without loss. The short-wavelength light reflected by other surfaces is blocked by the slit. Then, the light passing through the slit is converged on the detector after passing through the lens. The long-wavelength light is reflected by the lower surface of the semi-transparent wafer and collimated by the dispersive objective lens again, because it has a lateral displacement with the short-wavelength light when propagating in the semi-transparent wafer. Then, it is reflected by the reflector and passes through the converging lens to reach the slit, where it is also focused to a point and has a lateral displacement with the convergence point of the short-wavelength light. The long-wavelength light then passes through the slit without loss, and the long-wavelength light reflected by other surfaces is blocked by the slit. Then, the light passing through the slit is converged on the detector after passing through the lens, and has a lateral displacement with the convergence point of the short-wavelength light on the detector. The thickness measurement system is calibrated according to the mapping relationship between the lateral displacement on the detector and the thickness of the translucent wafer, and the known thickness of the translucent wafer. Then, the calibrated thickness measurement system is used to measure the unknown thickness of the translucent wafer according to the measured lateral displacement and the mapping relationship.

6. The thickness measurement system for semi-transparent wafers according to claim 5, characterized in that: The light beam tilt angle constrained between the pinhole aperture and the programmable liquid crystal device is within the range of 0-5°.

7. The thickness measurement system for semi-transparent wafer according to claim 5, characterized in that: The slit width is 15-25um.

8. The thickness measurement system for a semi-transparent wafer according to claim 5, characterized in that: The focal length of the achromatic lens is 25-50mm, and the focal length of the collimating lens, converging lens and lens is 50-75mm; the lens images the slit size 1:1 to the detector.

9. The thickness measurement system for semi-transparent wafer according to claim 5, characterized in that: The mapping relationship between the lateral displacement on the detector and the thickness of the semi-transparent wafer is: Assume that the wavelength range of the white light source includes λ a and λ b , the dispersion objective lens converges λ a and λ b At different depths, λ a The wavelength is the reflection wavelength of the semi-transparent wafer surface, λ b The wavelength is the reflection wavelength of the lower surface of the semi-transparent wafer, d is the thickness of the semi-transparent wafer, and λ is the thickness of the detector. a and λ b The lateral displacement l between the convergence points follows the following mapping relationship: where f c is the focal length of the dispersive objective lens, f f is the focal length of the converging lens, θ is the inclination angle between the optical axis determined by the pinhole aperture and the center of the light-transmitting part of the programmable liquid crystal device and the optical axis determined by the white light source and the achromatic lens, and the thickness d information is obtained from l; 10. The thickness measurement system for a semi-transparent wafer according to claim 5, characterized in that: The system is capable of measuring the thickness of multiple faces of a semi-transparent wafer simultaneously.

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