Coaxial automatic focusing method

Abstract

The invention relates to the technical field of wafer processing, and discloses a coaxial automatic focusing method, which comprises the following steps: forming a damage point on the surface of a wafer by using a laser focus; the focusing F axis is moved upwards in the Z direction, a target adjusting module is adjusted, observation is performed through a coaxial charge-coupled device until the target is imaged clearly, and the difference value between the coordinate of the focusing F axis and the coordinate of the focusing F axis of a damage point formed by focusing of a laser focus on the surface of the wafer when the cross-shaped target forms a clear image in air above the surface of the wafer is recorded; in the wafer machining process, the focusing F axis is driven to move in the Z direction, image matching is carried out through the coaxial charge-coupled device so as to identify the coordinate position f2'of the focusing F axis of clear imaging of the target, and an actual wafer machining point is obtained based on interpolation. According to the invention, switching between confocal and non-confocal of the laser focus and the cross-shaped target focus can be realized, and the problem that the brightness of the front electrode of the ultrathin wafer interferes with automatic focusing pattern recognition in a visual confocal scheme can be solved.

Description

Technical Field

[0001] This invention relates to the field of wafer fabrication technology, and more specifically to a coaxial autofocusing method. Background Technology

[0002] With the increasing popularity of mini-LED direct-view display technology, the demand for pixel density is rising, and the upstream demand for miniaturization of mini-LED wafers is becoming increasingly apparent. As the size of mini-LED wafers continues to shrink, their thickness also decreases, leading to increased light transmittance. However, especially when laser-processing wafers with a thickness of 60µm or less, the front electrode of the mini-LED can interfere with optical focusing during laser processing, making it impossible to achieve fast and accurate focusing. At the same time, laser autofocus modules are costly, which is not conducive to cost reduction and efficiency improvement in mass industrial production.

[0003] In existing optical focusing technologies, a crosshair reticle with specific markings is typically introduced. A coaxial optical path design precisely correlates the image of the reticle with the focal point of the processing laser in space. This allows for indirect and rapid precise laser focus positioning by judging the clarity of the reticle markings on the wafer surface. This system couples and separates the visible light markings generated by a point source illuminating the reticle with the processing optical path of an infrared picosecond laser within the imaging tube using dichroic mirrors, ensuring they share the same near-infrared achromatic objective and focus on the same point. When the reticle markings are most clearly observed on the CCD, it indicates that the wafer surface is precisely located on the focal plane of the objective lens. At this point, the laser focal point also precisely coincides with the wafer surface, thus solving the problem of rapid and precise laser focusing during the stealth dicing of ultra-thin wafers.

[0004] However, the existing technical solution has the following shortcomings: Since the image of the reticle projected on the wafer surface is confocal with the laser focus, there is no interference when dealing with opaque objects such as silicon, ceramics or thick transparent objects. However, when dealing with mini-LED wafers with sapphire substrates of 60um and below or ultra-thin glass wafers, the chip electrodes on the front side will form shadows of varying brightness on the back side through the substrate, affecting the visual recognition of the reticle pattern and causing focusing failure. Summary of the Invention

[0005] To address the problems of traditional optical focusing based on the co-focusing of laser focus and visual focus, and the failure of focusing caused by interference from the front chip electrodes of the wafer when imaging ultra-thin transparent wafers with crosshairs, this invention proposes a coaxial autofocus method.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A coaxial autofocus method, applied to wafer fabrication, employs a coaxial focusing system comprising a focusing F-axis and a coaxial imaging module fixed on the focusing F-axis, a coaxial charge-coupled device, a point light source, and a near-infrared achromatic objective lens for laser; the coaxial imaging module includes a target and a target adjustment module for adjusting the Z-axis position of the target; the method includes: Move the focusing F-axis and use the laser focus formed by the laser near-infrared achromatic objective lens to form a damage point on the wafer surface, and record the coordinate position f1 of the focusing F-axis at this time; Turn on the point light source, and focus the light after passing through the target and the laser near-infrared achromatic objective lens. Adjust the pose of the coaxial charge-coupled device so that the center of the target in the coaxial charge-coupled device coincides with the damage point, and make the laser focus coaxial with the coaxial charge-coupled device. Move the focusing F-axis upward along the Z-direction and adjust the target adjustment module. Observe through the coaxial charge-coupled device until the target image is clear. Record the coordinate position f2 of the focusing F-axis at this time and calculate the deviation value Δf from f1. Δf = f1 - f2. During wafer fabrication, the focusing F-axis is driven to move along the Z-axis, and image matching is performed through the coaxial charge-coupled device to identify the coordinate position f2' of the focusing F-axis with clear target imaging. Then, the focusing F-axis is controlled to move to the position f2'+Δf. At this position, the laser focus formed by the laser through the near-infrared achromatic objective lens is the actual wafer fabrication point.

[0007] In one embodiment, the coaxial imaging module, the coaxial charge-coupled device, and the point light source constitute an automatic focusing system; The coaxial focusing system includes a laser slicing system; the laser slicing system and the automatic focusing system share a laser near-infrared achromatic objective lens, and also include an ultrafast infrared laser, a processing stage, and a horizontal motion platform; the wafer is placed on the processing stage, and the horizontal motion platform can drive the processing stage to translate in the X and Y directions; The laser emitted by the ultrafast infrared laser is coupled by a dichroic mirror into the laser-using near-infrared achromatic objective lens in the automatic focusing system, and focused to form the laser focal point.

[0008] In one embodiment, the coaxial imaging module further includes a plano-convex lens, a reflecting aluminum mirror, a first achromatic cemented doublet lens, a semi-transparent and semi-reflective mirror, a second achromatic cemented doublet lens, and a triaxial micro-motion slide; the triaxial micro-motion slide can drive the coaxial charge-coupled device to perform X-axis and Y-axis displacement, and can rotate along the Z-axis; The light from the point source passes sequentially through a plano-convex lens, a target, a reflecting aluminum mirror, a first achromatic cemented doublet, a semi-transparent mirror, and a near-infrared achromatic objective lens for lasers, forming a target image on the wafer surface. The light corresponding to the target image is reflected by the wafer and then passes sequentially through a near-infrared achromatic objective lens for lasers, a semi-transparent mirror, and a second achromatic cemented doublet lens, before being imaged in a coaxial charge-coupled device.

[0009] In one embodiment, the focal length f of the first achromatic cemented doublet lens is... d The range is: 30.6mm≤f d The optical path distance L2 between the center point of the first achromatic cemented doublet and the center point of the target is ≤70.1mm, and the optical path distance L3 between the first achromatic cemented doublet and the entrance pupil of the near-infrared achromatic objective lens for lasers is 55mm≤L3≤175mm.

[0010] In one embodiment, the focusing method can be applied to the hidden cleaving of sapphire wafers, silicon wafers, glass wafers and silicon carbide wafers with a diameter of 60µm or less.

[0011] In one embodiment, the target is matched with the field of view of the coaxial charge-coupled device, the area of ​​the target occupies 1 / 6 of the field of view of the coaxial charge-coupled device, the thickness of the target pattern is less than 1 μm, and the line width of the cross mark set on the target is controlled to be less than 0.15 mm.

[0012] In one embodiment, the target adjustment module is a hollow sleeve with a boss inside; the target is placed inside the target adjustment module and on the boss; it also includes a threaded pressure ring and a Z-axis adjustment thread disposed inside the target adjustment module; the threaded pressure ring is placed on the target to press and fix the target; the pitch d of the Z-axis adjustment thread is in the range of 0.7mm≤d≤0.9mm, and is used to cooperate with the threads of the coaxial imaging module housing and the rubber head set screw to realize target fixation and Z-axis adjustment.

[0013] Compared with the prior art, the beneficial technical effects of the present invention are: In the coaxial autofocus solution of the present invention, the laser focus and the crosshair target focus can be switched between cofocus and non-cofocus, which can be used to solve the problem of interference between the brightness of the front electrode of ultra-thin wafers below 60um and the automatic focusing pattern recognition of the visual cofocus solution; for silicon wafers and other opaque or low transparency wafers, whose surfaces are free from interference, the present invention can also realize a visual cofocus autofocus solution for these wafers with free surface interference; in addition, the present invention can also effectively solve the problem of high cost when adding a laser focusing module. Attached Figure Description

[0014] Figure 1This is a flowchart of a coaxial autofocus method according to the present invention.

[0015] Figure 2 This is a side view of the coaxial focusing system of Embodiment 1 and Embodiment 2 of the present invention.

[0016] Figure 3 These are top views and enlarged views of the cross-shaped target structure in Embodiments 1 and 2 of the present invention.

[0017] Figure 4 This is a schematic diagram of the target adjustment module in Embodiment 1 and Embodiment 2 of the present invention.

[0018] Figure 5 This is a side view of the hidden cutting machine structure of Embodiment 1 and Embodiment 2 of the present invention.

[0019] Figure 6 These are isometric views of the hidden cutting machine structure of Embodiment 1 and Embodiment 2 of the present invention.

[0020] Figure 7 This is a side view and enlarged view of the non-confocal application of the coaxial focusing system according to Embodiment 1 of the present invention.

[0021] Figure 8 This is a side view and enlarged view of the coaxial focusing system in confocal application according to Embodiment 2 of the present invention.

[0022] Figure 9 This is a schematic diagram illustrating the laser focus acting on the interior of a wafer in Embodiments 1 and 2 of the present invention.

[0023] Figure 10 This is a schematic diagram of an ultrathin wafer according to Embodiment 1 of the present invention.

[0024] Figure 11 This is a schematic diagram of a silicon wafer or conventional wafer before and after dicing, according to Embodiment 2 of the present invention.

[0025] In the picture: 1. Infrared ultrafast laser; 2. Dichroic mirror; 3. Near-infrared achromatic objective lens for lasers; 4. Wafer; 41. Laser focus; 42. Crosshair target focus; 43. Hidden cutting modification layer; 431. Microcracks after hidden cutting modification; 44. Wafer iron ring; 45. Wafer protective film; 46. Ultrathin sapphire wafer; 461. Grain; 462. Wafer electrode; 47. Silicon wafer.

[0026] 5. Process the platform; 6. Horizontal motion platform; 61. X-axis motion platform; 62. Y-axis motion platform; 7. Coaxial imaging module; 71. Three-axis micro-motion slide; 72. Second achromatic cemented doublet lens; 73. Semi-transparent mirror; 74. Plano-convex lens; 75. Crosshair target; 751. Target adjustment module; 7511. Threaded pressure ring; 7512. Boss; 7513. Z-axis adjustment thread; 752. Crosshair mark; 753. Target boundary mark; 76. Reflecting aluminum mirror; 77. First achromatic cemented doublet lens; 8. Coaxial charge-coupled devices; 9. Point light source; 10. Focusing F-axis; 101. Focusing F-axis mounting plate; 11. Marble base; 12. External optical path system. Detailed Implementation

[0027] A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0028] like Figure 1 As shown, a coaxial autofocus method is applied to wafer processing. The coaxial focusing system includes a focusing F-axis and a coaxial imaging module fixed on the focusing F-axis, a coaxial charge-coupled device, a point light source, and a near-infrared achromatic objective for lasers. The coaxial imaging module includes a target and a target adjustment module for adjusting the Z-axis position of the target. The method includes the following steps: S1, move the focusing F-axis, and use the laser focus formed by the laser near-infrared achromatic objective lens to form a damage point on the wafer surface, and record the coordinate position f1 of the focusing F-axis at this time; S2, turn on the point light source, the light is focused and imaged after passing through the target and the laser near-infrared achromatic objective lens, adjust the pose of the coaxial charge-coupled device so that the center of the target in the coaxial charge-coupled device coincides with the damage point, and make the laser focus coaxial with the coaxial charge-coupled device; S3, move the focusing F-axis upward along the Z direction and adjust the target adjustment module. Observe through the coaxial charge-coupled device until the target image is clear. Record the coordinate position f2 of the focusing F-axis at this time and calculate the deviation value Δf from f1. Δf = f1 - f2. S4. During the wafer processing, the focusing F-axis is driven to move along the Z direction. Image matching is performed through the coaxial charge-coupled device to identify the coordinate position f2' of the focusing F-axis with clear target imaging. Then, the focusing F-axis is controlled to move to the position f2'+Δf. At this position, the laser focus formed by the laser near-infrared achromatic objective lens is the actual wafer processing point.

[0029] Example 1: For example, to cut ultra-thin sapphire wafers smaller than 60µm, please refer to [link / reference]. Figure 8, Figure 9 Because the hidden etch process is highly sensitive to the depth of the hidden etched modified layer 43 formed inside the wafer by focusing the laser focal point, a precise laser focal point is required to effectively form the modified microcracks 431 on the wafer surface. Furthermore, because the front-side die electrode 462 of the ultra-thin sapphire wafer 46 can penetrate the wafer and create light and dark interference on the back surface, it affects the automatic visual focus finding and prevents automatic focusing. This embodiment proposes a coaxial focusing system and method, including steps S10, S20, S30, S40, and S50: Step S10 includes the following steps: Step S101: Refer to Figure 2 , Figure 5 The automatic focus system consists of a coaxial imaging module 7, a coaxial charge-coupled device (CCD) 8, and a point light source 9, which are fixed on the focusing F-axis 10.

[0030] Step S102: Refer to Figure 2 , Figure 5 The coaxial imaging module 7 is controlled by a three-axis micro-motion slide 71 and consists of a second achromatic cemented doublet lens 72, a semi-transparent and semi-reflective mirror 73, a plano-convex lens 74, a cross-shaped target 75, a target adjustment module 751, a reflecting aluminum mirror 76, a first achromatic cemented doublet lens 77, and a housing. The coating of the semi-transparent and semi-reflective mirror is for visible light.

[0031] Step S103: Refer to Figure 3 , Figure 4 The substrate of the cross-shaped target 75 is made of high-transparency optical glass, such as K9 glass.

[0032] The target pattern consists of a crosshair 752 and a target boundary marker 753. To eliminate the impact of the target pattern thickness on the imaging depth of field error, the crosshair 752 and the target boundary marker 753 are made by sputtering coating, resulting in an extremely thin thickness of approximately 1µm. The target size occupies approximately 1 / 6 of the visual range of the coaxial charge-coupled device 8. When different pixel coaxial charge-coupled devices are selected, the displayed size of the crosshair target received on the coaxial charge-coupled device will slightly deviate. The actual size of the crosshair target is approximately 0.8 × 0.8 mm. The line width of the target pattern is approximately 0.15 mm.

[0033] The crosshair target 75 is fixed inside the target adjustment module 751. The target adjustment module is made of a hollow aluminum alloy sleeve with an inner diameter slightly larger than the size of the crosshair target glass. It has an internal boss 7512 for holding the crosshair target. The crosshair target is pressed and fixed by a threaded pressure ring 7511. The Z-axis adjustment thread 7513 is made of fine thread and is used to mate with the threads of the coaxial imaging module housing and the rubber head set screw to achieve Z-axis up and down adjustment and fixation. Specifically, according to... Figure 2 ,4 5. The coaxial imaging module housing and the Z-axis adjustment thread 7513 are connected via fine-pitch threads. Rotating the Z-axis adjustment thread 7513 clockwise or counterclockwise lowers or raises the distance between the crosshair target within the target adjustment module and the object-side principal point of the first doublet lens, thereby changing the focal point position of the crosshair target image. To ensure the adjusted device remains secure, there is a threaded hole on the side where the coaxial imaging module housing connects to the target adjustment module. A rubber-tipped set screw is screwed in to lock the target adjustment module like a pin. The rubber-tipped set screw has a built-in rubber pad to prevent damage to the fine-pitch threads on the target adjustment module.

[0034] The pitch d range of the Z-direction adjusting thread 7513 is: 0.7mm≤d≤0.9mm.

[0035] Step S104: See Figure 5 , Figure 6 The automatic focusing system is fixed on the marble base 11 via the focusing F-axis fixing plate 101. The focusing F-axis 10 is perpendicular to the processing stage 5. Since the wafer thickness is generally no more than 2mm, the focusing F-axis movement stroke does not need to be very long, generally within 5mm. A lead screw servo displacement device with closed-loop compensation of grating ruler can be used, with a repeatability of 1um and a movement speed of up to 20mm / s.

[0036] The horizontal motion platform 6 includes a Y-axis motion platform 62 fixed on the marble base 11, and an X-axis motion platform 61 fixedly connected to the moving end of the Y-axis motion platform 62. The processing stage 5 is fixedly connected to the moving end of the X-axis motion platform 61. The Y-axis motion platform 62 can drive the processing stage 5 to move in the Y direction, and the X-axis motion platform 61 can drive the processing stage 5 to move in the X direction. The horizontal motion platform 6, the infrared ultrafast laser 1, and the external optical path system 12 are fixed on the marble base 11.

[0037] Among them, the X-axis motion platform 61 and the Y-axis motion platform 62 are both composed of linear motors and high-precision guide rails, with closed-loop feedback of grating rulers and repeatability of 1µm.

[0038] Step S20: Refer to Figure 7 Based on the focusing F-axis position space, select a second achromatic cemented doublet 72 and a first achromatic cemented doublet 77 with appropriate focal lengths; typically, select a focal length f. t For the second achromatic cemented doublet lens 72 with a focal length of 100mm, choose a focal length of f. dThe first achromatic cemented doublet 77 has a diameter of 50mm. The optical path distance L1 between the second achromatic cemented doublet 72 and the target surface of the coaxial charge-coupled device 8 is approximately 80mm. The optical path distance L2 from the first achromatic cemented doublet 77 to the crosshair target is 24.5mm ≤ L2 ≤ 55mm. The distance between the second achromatic cemented doublet 72 and the entrance pupil of the near-infrared achromatic objective lens 3 for lasers is approximately 180mm. The optical path distance L3 between the center of the first achromatic cemented doublet 77 and the entrance pupil of the near-infrared achromatic objective lens for lasers is approximately 150mm.

[0039] The focal length f of the first achromatic cemented doublet lens 77 is... d Depending on the size of the coaxial imaging module and the space available for the focusing F-axis position, the selectable focal length range is: 30.6mm ≤ f d Within the range of ≤70.1mm, the adjustable range of the distance between the first achromatic cemented doublet lens 77 and the crosshair target is: 24.5mm≤L2≤55mm. The range of the distance L3 between the first achromatic cemented doublet lens 77 and the entrance pupil of the near-infrared achromatic objective lens for laser is: 55mm≤L3≤175mm.

[0040] Step S30 includes the following steps: Step S301: Refer to Figure 6 , Figure 10 Turn on the spotlight 9 and coaxial charge-coupled device 8, place the ultrathin sapphire wafer 46 on the processing stage 5 and turn on the vacuum.

[0041] The ultrathin sapphire wafer 46 is attached to the wafer iron ring 44 by a wafer protective film 45. The ultrathin sapphire wafer is composed of individual grains 461 with extremely small grain size, with a length and width of approximately 70×100um.

[0042] Because the wafer is very thin, the grain electrode 462 will create light and dark interference on the back surface of the wafer through the sapphire.

[0043] Step S302: See Figure 6 , Figure 7 Next, the horizontal motion platform 6 is moved so that the wafer is directly below the laser near-infrared achromatic objective lens 3. The focusing F-axis 10 is moved so that the point light source is collimated by the plano-convex lens 74 and illuminates the crosshair target 75. The light then enters the laser near-infrared achromatic objective lens 3 through the reflecting aluminum mirror 76, the first achromatic doublet lens 77, the visible light semi-transparent semi-reflective mirror 73, and the dichroic mirror 2, illuminating the approximate surface of the ultrathin sapphire wafer 46.

[0044] Step S303: See Figure 7 , Figure 10Next, the approximate surface image of the ultrathin sapphire wafer is received by the target surface of the coaxial charge-coupled device 8 through a laser near-infrared achromatic objective lens 3, a dichroic mirror 2, a semi-transparent and semi-reflective mirror 73, and a second achromatic cemented doublet lens 72. The dichroic mirror coating transmits visible light but reflects infrared light.

[0045] Step S304: See Figure 5 , Figure 7 Next, the ultrafast infrared laser 1 is turned on and a small power such as 0.1W is set. The infrared laser is coupled into the infrared achromatic objective lens 3 through the external optical path system 12 and the dichroic mirror 2. It is basically optically coaxial with the coaxial imaging module 7. The laser is focused by the laser near-infrared achromatic objective lens 3 to form a laser focal point.

[0046] Step S305: See Figure 5 , Figure 7 Next, the focusing F-axis is finely adjusted up and down along the Z-axis so that the laser focus is exactly applied to the wafer surface, forming a damage point to determine the position of the laser focal plane, and the current focusing F-axis coordinate f1 is recorded, thus obtaining the laser focus on the wafer surface.

[0047] Step S40 includes the following steps: Step S401: Refer to Figure 6 , Figure 7 , Figure 10 The horizontal motion platform 6 is moved so that the laser draws a cross on the surface of the ultrathin sapphire wafer 46 and then the laser is turned off, using the damage cross as a reference.

[0048] Step S402: See Figure 7 Next, adjust the three-axis micro-motion slide 71 on the coaxial imaging module. The coaxial charge-coupled device is rotated along the Z direction by the three-axis micro-motion slide 71, so that the target imaging angle of the coaxial charge-coupled device and the laser scribing position are horizontal in the X direction.

[0049] Step S403: See Figure 7 Next, the three-axis micro-motion slide 71 on the coaxial imaging module is adjusted so that the coaxial charge-coupled device is translated along the X and Y directions by the three-axis micro-motion slide 71, so that the center point of the target surface of the coaxial charge-coupled device coincides with the center of the cross-shaped damage point formed by the laser on the surface of the wafer 4. That is, the laser focus and the coaxial charge-coupled device are completely coaxial, so that the position displayed on the target surface of the coaxial charge-coupled device 8 is the position where the laser focus scribing is applied.

[0050] Step S50 includes the following steps: Step S501: Refer to Figure 7The point light source 9 is collimated by the plano-convex lens 74 and illuminates the cross target 75. The light then enters the laser near-infrared achromatic objective lens 3 through the reflecting aluminum mirror 76, the first achromatic doublet lens 77, the semi-transparent and semi-reflective mirror 73, and the dichroic mirror 2. The cross target 75 is then focused and imaged by the laser near-infrared achromatic objective lens 3.

[0051] Step S502: See Figure 7 Then, the image formed is received by the target surface of the coaxial charge-coupled device 8 through the laser near-infrared achromatic objective lens 3, dichroic mirror 2, semi-transparent and semi-reflective mirror 73, and second achromatic doublet lens 72.

[0052] Step S503: See Figure 5 , Figure 7 Next, at the laser focus 41, the focusing F axis is moved upward by 20µm along the Z axis. The target adjustment module 751 is adjusted, and the image of the cross target is observed through the coaxial charge-coupled device 8 until a clear image of the cross target is presented on the coaxial charge-coupled device 8. The target adjustment module 751 is fixed, and the current clear position coordinate f2 of the cross target on the focusing F axis is recorded, which gives the cross target focus position 42.

[0053] Step S504: See Figure 7 The difference Δf between the laser focus 41 and the crosshair target focus 42 is 20µm. The Δf value is generally very short; excessive adjustment will affect the imaging quality of the crosshair target 75, and will not exceed 25µm. The difference Δf between the laser focus 41 and the crosshair target focus 42 is written into the upper computer's scribing software configuration.

[0054] Step S505: See Figure 5 , Figure 7 The host computer dicing software drives the focusing F-axis to move up and down within a certain range, and receives and matches the image of the camera target surface through the coaxial charge-coupled device 8, identifies the image of the clear cross target, and obtains the focusing F-axis coordinate f2 where the clear target is located.

[0055] Step S506: See Figure 5 , Figure 7 Next, the focusing F-axis is moved downwards by Δf along the Z-direction based on coordinate f2 to obtain the actual focal point of the laser, thereby achieving high-precision and fast autofocus.

[0056] The above solves the problem of interference from the brightness and darkness of the front electrode on the back surface of ultra-thin wafers of 60um and below, which prevents the automatic finding of the laser focus using a crosshair target and laser confocal optical system. By using the coordinate difference Δf formed by the crosshair target focus and the laser focus, the interference of the brightness and darkness of the wafer electrode on the visual recognition of the crosshair target when imaging on the back surface of the wafer is avoided. The function of automatic focusing is achieved by identifying the clear position of the crosshair target through a coaxial charge-coupled device.

[0057] Example 2: For example, regarding the cutting of silicon wafers, please refer to [link / reference]. Figure 8 , Figure 10 Because the hidden etch process is highly sensitive to the depth of the hidden etched modified layer 43 formed inside the wafer by focusing the laser focal point, a precise laser focal point is required to effectively form the modified microcracks 431 on the wafer surface. Furthermore, because the silicon wafer 47 is opaque to visible light, the crosshair target can directly image on the wafer surface. This invention proposes a coaxial autofocus method, comprising steps S11, S21, S31, S41, and S51: Step S11 includes the following steps: Step S111: See Figure 2 , Figure 5 The automatic focus system consists of a coaxial imaging module 7, a coaxial charge-coupled device (CCD) 8, and a point light source 9, which are fixed on the focusing F-axis 10.

[0058] Step S112: See Figure 2 , Figure 5 The coaxial imaging module 7 is controlled by a three-axis micro-motion slide 71 and consists of a second achromatic doublet lens 72, a semi-transparent semi-reflective mirror 73, a plano-convex lens 74, a crosshair target 75, a target adjustment module 751, a reflecting aluminum mirror 76, a first achromatic doublet lens 77, and a housing. The coating of the semi-transparent semi-reflective mirror is visible light.

[0059] Step S113: See Figure 3 , Figure 4 The cross-shaped target 75 substrate is made of high-transparency optical glass, such as K9 glass.

[0060] The target pattern consists of a crosshair 752 and a target boundary marker 753. To eliminate the impact of the target pattern thickness on the imaging depth of field error, the crosshair 752 and the target boundary marker 753 are made by sputtering coating, resulting in an extremely thin thickness of approximately 1µm. The target size occupies approximately 1 / 6 of the visual range of the coaxial charge-coupled device 8. When different pixel coaxial charge-coupled devices are selected, the displayed size of the crosshair target received on the coaxial charge-coupled device will slightly deviate. The actual size of the crosshair target is approximately 0.8 × 0.8 mm. The line width of the target pattern is approximately 0.15 mm.

[0061] The crosshair target 75 is fixed inside the target adjustment module 751. The target adjustment module is made of a hollow aluminum alloy sleeve with an inner diameter slightly larger than the size of the crosshair target glass. It has a boss 7512 inside for placing the crosshair target. The crosshair target is pressed and fixed by a threaded pressure ring 7511. The Z-axis adjustment thread 7513 is made of fine thread and is used to cooperate with the coaxial imaging module housing thread and the rubber head set screw to realize Z-axis up and down adjustment and fixation.

[0062] The pitch d range of the Z-direction adjusting thread 7513 is: 0.7mm≤d≤0.9mm.

[0063] Step S114: See Figure 5 , Figure 6 The automatic focusing system is fixed on the marble base 11 via the focusing F-axis fixing plate 101. The focusing F-axis 10 is perpendicular to the processing stage 5. Since the wafer thickness is generally no more than 2mm, the focusing F-axis movement stroke does not need to be very long, generally within 5mm. A lead screw servo displacement device with closed-loop compensation of grating ruler can be used, with a repeatability of 1um and a movement speed of up to 20mm / s.

[0064] The processing platform 5 is fixed on the vehicle-mounted plate of the X-axis motion platform 61, and the X-axis motion platform 61 and the Y-axis motion platform 62 are orthogonally stacked in the X and Y directions. The horizontal motion platform 6, the infrared ultrafast laser 1, and the laser appearance optical path system 12 are fixed on the marble base 11.

[0065] The horizontal motion platform consists of a linear motor and a high-precision guide rail, with closed-loop feedback from a grating ruler and a repeatability accuracy of 1µm.

[0066] Step S21: See Figure 8 Based on the focusing F-axis position space, select a second achromatic cemented doublet 72 and a first achromatic cemented doublet 77 with appropriate focal lengths; typically, select a focal length f... t For the second achromatic cemented doublet lens 72 with a focal length of 100mm, choose a focal length of f. d The first achromatic cemented doublet 77 has a diameter of 50mm. The optical path distance L1 between the second achromatic cemented doublet 72 and the target surface of the coaxial charge-coupled device 8 is approximately 80mm. The optical path distance L2 from the first achromatic cemented doublet 77 to the crosshair target is 24.5mm ≤ L2 ≤ 55mm. The distance between the second achromatic cemented doublet 72 and the entrance pupil of the near-infrared achromatic objective lens 3 for lasers is approximately 180mm. The optical path distance L3 between the center of the first achromatic cemented doublet 77 and the entrance pupil of the near-infrared achromatic objective lens for lasers is approximately 150mm.

[0067] The focal length f of the first achromatic cemented doublet lens 77 is... d Depending on the size of the coaxial imaging module and the space available for the focusing F-axis position, the selectable focal length range is: 30.6mm ≤ f d Within the range of ≤70.1mm, the adjustable range of the distance between the first achromatic cemented doublet lens 77 and the crosshair target is: 24.5mm≤L2≤55mm. The range of the distance L3 between the first achromatic cemented doublet lens 77 and the entrance pupil of the near-infrared achromatic objective lens for laser is: 55mm≤L3≤175mm.

[0068] Step S31 includes the following steps: Step S311: See Figure 6 , Figure 11 Turn on the spotlight 9 and the coaxial charge-coupled device 8, place the silicon wafer 47 on the processing stage 5 and turn on the vacuum.

[0069] The silicon wafer 47 is attached to the wafer iron ring 44 by a wafer protective film 45.

[0070] Step S312: See Figure 6 , Figure 8 Next, the horizontal motion platform 6 is moved so that the wafer is directly below the laser near-infrared achromatic objective lens 3.

[0071] Step S313: See Figure 8 , Figure 11 Next, the focusing F-axis 10 is moved to collimate the point light source through the plano-convex lens 74, illuminating the crosshair target 75. The light then passes through the reflecting aluminum mirror 76, the first achromatic cemented doublet 77, the semi-transparent mirror 73, and the dichroic mirror 2, entering the near-infrared achromatic objective lens 3 for laser illumination. This image then illuminates the approximate surface of the silicon wafer 47. The image of the approximate surface of the wafer 47 is received by the target surface of the coaxial charge-coupled device 8 through the near-infrared achromatic objective lens 3, the dichroic mirror 2, the semi-transparent mirror 73, and the second achromatic cemented doublet 72. The dichroic mirror is coated to transmit visible light but reflect infrared light.

[0072] Step S314: See Figure 5 , Figure 8 Next, the ultrafast infrared laser 1 is turned on and set to a relatively low power, such as 0.1W. The infrared laser is coupled into the infrared achromatic objective lens 3 via the laser external optical path system 12 and the dichroic mirror 2, and is optically coaxial with the coaxial imaging module 7. The laser is focused by the laser near-infrared achromatic objective lens 3 to form a laser focal point.

[0073] Step S315: See Figure 5 , Figure 8 Next, the Z-axis is finely adjusted up and down to adjust the focus F-axis. The laser focus is applied to the wafer surface to form a damage point, which determines the position of the laser focal plane. The current focusing F-axis coordinate f1 is recorded, thus obtaining the laser focus on the wafer surface.

[0074] Step S41 includes the following steps: Step S411: See Figure 6 , Figure 8 , Figure 11 The horizontal motion platform 6 is moved so that the laser draws a cross on the surface of the silicon wafer 47 and then the laser is turned off, using the damage cross as a reference.

[0075] Step S412: See Figure 8 Next, the three-axis micro-motion slide 71 on the coaxial imaging module is adjusted so that the coaxial charge-coupled device is rotated along the Z direction, making the target imaging angle of the coaxial charge-coupled device and the laser scribing position horizontal in the X direction.

[0076] Step S413: See Figure 8 Next, the three-axis micro-motion slide 71 on the coaxial imaging module is adjusted so that the coaxial charge-coupled device is translated along the X and Y directions by the three-axis micro-motion slide 71, so that the center point of the target surface of the coaxial charge-coupled device coincides with the center of the cross-shaped damage point formed by the laser on the surface of the wafer 4. That is, the laser focus is completely coaxial with the coaxial charge-coupled device, so that the position displayed on the target surface of the coaxial charge-coupled device 8 is the position where the laser focus scribing is applied.

[0077] Step S51 includes the following steps: Step S511: See Figure 8 The point light source 9 is collimated by the plano-convex lens 74 and illuminates the cross target 75. The light then enters the laser near-infrared achromatic objective lens 3 through the reflecting aluminum mirror 76, the first achromatic doublet lens 77, the semi-transparent and semi-reflective mirror 73, and the dichroic mirror 2. The cross target 75 is then focused and imaged by the laser near-infrared achromatic objective lens 3.

[0078] Step S512: See Figure 8 Then, the image formed is received by the target surface of the coaxial charge-coupled device 8 through the laser near-infrared achromatic objective lens 3, dichroic mirror 2, semi-transparent and semi-reflective mirror 73, and second achromatic cemented doublet lens 72.

[0079] Step S513: See Figure 5 , Figure 8 Next, the target adjustment module 751 is adjusted, and the image of the crosshair target is observed through the coaxial charge-coupled device 8 until a clear image of the crosshair target is presented on the coaxial charge-coupled device 8. The target adjustment module 751 is then fixed, and the current clear position coordinate f2 of the crosshair target on the focusing F-axis is recorded, which is the focus position 42 of the crosshair target. Because the focusing F-axis did not move when the target adjustment module 751 was adjusted, the laser focus and the crosshair target focus are confocal, i.e., f1=f2.

[0080] Step S514: See Figure 8 The difference Δf between the laser focus 41 and the crosshair target focus 42 is 0um. Write the difference Δf = 0um between the laser focus 41 and the crosshair target focus 42 into the upper computer dicing software configuration.

[0081] Step S516: See Figure 5 , Figure 8The host computer dicing software drives the focusing F-axis to move up and down within a certain range. Through the coaxial charge-coupled device 8, the camera target surface receives and matches the image, identifies the clear image of the crosshair target, and obtains the focusing F-axis coordinate f2 where the clear target is located, which is the focus of the laser, thereby achieving high-precision and fast autofocus.

[0082] The above describes the process of autofocusing by using the crosshair target focus and the laser focus to achieve co-focusing. When the coaxial charge-coupled device identifies a clear crosshair target, the coordinates corresponding to the F-axis of the focusing axis are the positions of the laser focus, thus completing the autofocusing process.

[0083] This invention selects a suitable achromatic cemented doublet lens to make the crosshair target image deviate from the laser focus, slightly shorter than the laser focus or confocal with the laser focus, which is adjustable. This allows the image of the crosshair target to be formed in the air above the chip surface, or imaged on the chip surface. By moving the focusing F-axis, the image of the crosshair target in the field of view becomes clear. Through observation using a coaxial charge-coupled device, the difference between the focusing F-axis coordinate when the crosshair target forms a clear image in the air above the wafer surface and the focusing F-axis coordinate of the damage point formed by the laser focus on the wafer surface is recorded. This value is set in the dicing software configuration. The software uses the coaxial charge-coupled device for image matching, moves the focusing F-axis to match a clear image of the crosshair target. According to the wafer thickness and transparency requirements, the focusing F-axis is moved by a pre-set fixed deviation (the fixed deviation is set to 0 when confocal with the laser). The focusing F-axis coordinate of the clear crosshair target image plus the fixed deviation value from the laser focal plane is the actual focus of the laser, thereby achieving high-precision and fast autofocus. This eliminates the interference of the front electrode of the transparent wafer on the reticle imaging; a three-axis micro-motion slide is added at the connection between the coaxial charge-coupled device and the coaxial optical path. By adjusting the X and Y displacement stages in the three-axis micro-motion slide, the coaxial charge-coupled device is moved so that the laser scribing trajectory is completely coincident with the center of the target surface of the coaxial charge-coupled device, thereby realizing micron-level wafer processing; an achromatic cemented doublet lens is added between the crosshair target and the coaxial main optical path to ensure that the micro-motion of the crosshair target within the focal imaging range does not affect the imaging quality, thereby realizing the flexible application of the crosshair target and the laser focal point as confocal or non-confocal.

[0084] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0085] It should be understood that although the steps in the flowcharts of the accompanying drawings are shown sequentially as indicated by the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some of the steps in the flowcharts of the accompanying drawings may include multiple steps or stages, which are not necessarily completed at the same time, but may be executed at different times, and the execution order of these steps or stages is not necessarily sequential, but may be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0086] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0087] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0088] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A coaxial auto-focusing method applied to wafer processing, characterized in that, The coaxial focusing system comprises a focusing F shaft, a coaxial imaging module fixed on the focusing F shaft, a coaxial charge-coupled device, a point light source and a near-infrared achromatic objective for laser; the coaxial imaging module comprises a target and a target adjusting module for adjusting the Z-direction position of the target; the method comprises: moving the focusing F shaft, forming a damage point on the wafer surface by the laser focus formed by the near-infrared achromatic objective for laser, and recording the coordinate position f1 of the focusing F shaft at this time; turning on the point light source, focusing the light after passing through the target and the near-infrared achromatic objective for laser, adjusting the position of the coaxial charge-coupled device, and making the center of the target in the coaxial charge-coupled device coincide with the damage point, so that the laser focus is coaxial with the coaxial charge-coupled device; moving the focusing F shaft upward along the Z direction and adjusting the target adjusting module, and observing through the coaxial charge-coupled device until the target image is clear, recording the coordinate position f2 of the focusing F shaft at this time, and calculating the deviation value Δf of f1, Δf = f1-f2; during the wafer processing, driving the focusing F shaft to move along the Z direction, and matching the image through the coaxial charge-coupled device to identify the coordinate position f2' of the focusing F shaft where the target image is clear, and then controlling the focusing F shaft to move to the position of f2'+Δf, at which the laser focus formed by the near-infrared achromatic objective for laser is the actual wafer processing point.

2. A coaxial autofocus method according to claim 1, characterized in that, The coaxial imaging module, the coaxial charge-coupled device and the point light source form an automatic focusing system; The coaxial focusing system comprises a laser hidden cutting system; the laser hidden cutting system shares the near-infrared achromatic objective for laser with the automatic focusing system, and further comprises an ultrafast infrared laser, a processing stage and a horizontal motion platform; the wafer is placed on the processing stage, and the horizontal motion platform can drive the processing stage to move in the X direction and the Y direction; The laser emitted by the ultrafast infrared laser is coupled into the near-infrared achromatic objective for laser in the automatic focusing system by a dichroic mirror, and is focused to form the laser focus.

3. A method of on-axis auto-focusing according to claim 1 or 2, characterized in that, The coaxial imaging module further comprises a plano-convex lens, a reflective aluminum mirror, a first achromatic doublet lens, a half-mirror, a second achromatic doublet lens and a three-axis micro-motion stage; the three-axis micro-motion stage can drive the coaxial charge-coupled device to move in the X direction and the Y direction, and can rotate along the Z direction; The light of the point light source passes through the plano-convex lens, the target, the reflective aluminum mirror, the first achromatic doublet lens, the half-mirror and the near-infrared achromatic objective for laser in sequence to form a target image on the wafer surface; the light corresponding to the target image is reflected by the wafer, and then passes through the near-infrared achromatic objective for laser, the half-mirror and the second achromatic doublet lens in sequence to form an image in the coaxial charge-coupled device.

4. A coaxial autofocus method according to claim 3, characterized in that The focal length f of the first achromatic doublet lens d The focal length f of the first achromatic doublet lens d The focal length f of the first achromatic doublet lens The optical path distance L2 between the center point of the first achromatic doublet lens and the center point of the target ranges from 24.5 mm to 55 mm, and the optical path distance L3 between the first achromatic doublet lens and the near-infrared achromatic objective entrance pupil for laser ranges from 55 mm to 175 mm.

5. A coaxial autofocus method according to claim 1, wherein, The focusing method can be applied to the hidden cutting processing of sapphire wafers, silicon wafers, glass wafers and silicon carbide wafers with a thickness of less than 60 um.

6. A coaxial autofocus method according to claim 1, wherein The target matches the field of view of the target surface of the coaxial charge-coupled device, the area of the target accounts for 1 / 6 of the visual range of the target surface of the coaxial charge-coupled device, the thickness of the target pattern is within 1 um, and the line width of the cross mark arranged on the target is controlled within 0.15 mm.

7. A coaxial autofocus method according to claim 1, wherein The target adjusting module is a hollow sleeve, and a boss is arranged in the sleeve; the target is arranged in the target adjusting module and placed on the boss; a threaded compression ring arranged in the target adjusting module and a Z-direction adjusting screw are further arranged; the threaded compression ring is placed on the target and used for compressing and fixing the target; the pitch d of the Z-direction adjusting screw ranges from 0.7 mm to 0.9 mm, and is used for cooperating with the coaxial imaging module shell thread and the rubber head top screw to realize target fixing and Z-direction adjusting.

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