Femtosecond laser implicit cutting depth high-precision control system based on confocal interference ranging

Abstract

The invention belongs to the technical field of semiconductor material processing, and particularly relates to a femtosecond laser implicit cutting depth high-precision control system based on confocal interference ranging, and the method comprises the following steps: S1, cleaning, fixing and leveling a SiC crystal ingot; s2, the pulse width, the repetition frequency, the power, the scanning speed and the scanning interval of femtosecond laser are set, and a laser focus is accurately positioned at the position, away from the upper surface by the target thickness, in the crystal ingot; s3, controlling a laser beam to scan in the crystal ingot in a spiral or chessboard type path to form a continuous modified layer; s4, mechanical force is applied to the crystal ingot subjected to hidden cutting processing, so that the crystal ingot is broken along the modified layer, and the substrate and the crystal ingot parent body are separated; and S5, carrying out plasma cleaning and surface detection on the stripped substrate. The core problems that in the prior art, material loss is large, thermal damage is serious, and machining precision is low can be solved, and efficient and high-precision preparation of the SiC substrate is achieved.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor material processing technology, specifically relating to a high-precision control system for femtosecond laser hidden cutting depth based on confocal interferometric ranging. Background Technology

[0002] As a wide bandgap semiconductor material, SiC possesses excellent properties such as a large bandgap, high breakdown electric field, high thermal conductivity, and fast electron saturation drift velocity, making it irreplaceable in applications such as new energy vehicles, rail transportation, smart grids, and aerospace. The fabrication of SiC substrates is a core step in the industrialization of SiC devices, and one of its key steps is to peel SiC ingots into single-crystal substrates of a specific thickness.

[0003] Currently, SiC ingot exfoliation mainly employs traditional mechanical cutting (such as internal circular cutting and wire sawing) and conventional laser cutting technologies. Mechanical cutting suffers from high cutting losses (typically 15%-20%), high surface roughness (Ra generally greater than 1μm), and susceptibility to cracks and stress damage, requiring complex subsequent grinding and polishing processes, which increases production costs and reduces production efficiency. Conventional laser cutting (such as nanosecond lasers), due to its long pulse width, generates a significant heat-affected zone (typically greater than 50μm) when interacting with SiC material, leading to defects such as molten recast layers and microcracks at the cut edges, severely affecting the electrical and mechanical properties of the substrate. Summary of the Invention

[0004] The purpose of this invention is to provide a high-precision control system for femtosecond laser hidden cutting depth based on confocal interferometric ranging, which can solve the core problems of high material loss, severe thermal damage and low processing accuracy in traditional technologies, and realize efficient and high-precision preparation of SiC substrates.

[0005] The specific technical solution adopted by this invention is as follows: A high-precision stripping method for femtosecond laser hidden cutting depth based on confocal interferometric ranging includes the following steps: S1: Cleaning and leveling the SiC ingot; S2: Set the pulse width, repetition frequency, power, scanning speed and scanning interval of the femtosecond laser, and precisely position the laser focus at the distance from the target thickness on the upper surface of the ingot; S3: Control the laser beam to scan inside the ingot in a spiral or checkerboard pattern to form a continuous modified layer; S4: Apply mechanical force to the ingot after the hidden cutting process to cause it to fracture along the modified layer, thereby separating the substrate from the ingot matrix; S5: Perform plasma cleaning and surface inspection on the stripped substrate.

[0006] In S2, the femtosecond laser has a pulse width of 50-200 fs, a repetition frequency of 100-500 kHz, a laser power of 2-5 W, a scanning speed of 300-800 mm / s, and a scanning interval of 0.5-1 μm; the positioning accuracy of the laser focus is ±1 μm.

[0007] In step S3, during the processing, laser reflection and transmission signals are collected in real time, and the laser power and scanning speed are dynamically adjusted. When the continuity of the modified layer reaches more than 95%, the hidden cutting process is stopped.

[0008] A high-precision femtosecond laser hidden-cut depth stripping device based on confocal interferometric ranging includes: Ingot fixing and leveling module, used to fix and level SiC ingots; The femtosecond laser generator module is used to generate the femtosecond laser pulses required for processing. The optical focusing and scanning module is used to focus and guide femtosecond laser pulses into the interior of the crystal ingot for scanning; The real-time monitoring module is used to monitor the interaction state between the laser and the material, as well as the processing procedure. The stripping execution module is used to apply mechanical force to the ingot to separate the substrate; The central control module uses a PLC controller to connect to and control all the above modules.

[0009] The optical focusing and scanning module includes a beam expander, a focusing lens, a two-dimensional scanning galvanometer, and a focus positioning unit; the beam expander has a magnification of 2-5 times; the focusing lens has a numerical aperture of 0.3-0.5; the two-dimensional scanning galvanometer has a scanning range of 0-300mm×300mm and a scanning accuracy of 0.1μm; the focus positioning unit is driven by piezoelectric ceramic and has a positioning accuracy of ±1μm.

[0010] The ingot fixing and leveling module includes a vacuum adsorption worktable, a three-point support precision leveling mechanism, and a laser interferometer flatness detection unit; the stripping execution module includes a vacuum chuck, an array of 8-16 precision pins, and a pressure control system.

[0011] The real-time monitoring module includes a laser reflection detector, a transmission detector, and a high-speed camera; the laser reflection detector has a detection accuracy of 0.01mW, and the transmission detector has a response time of less than 1ns, used to monitor the interaction state between the laser and the material in real time; the high-speed camera has a shooting frame rate of 1000-5000 frames / second.

[0012] A high-precision control system for femtosecond laser hidden depth based on confocal interferometric ranging includes: The optical path unit is used to guide femtosecond laser pulses and focus them inside the SiC ingot to form a modified layer; The dual-mode confocal interferometric ranging unit is optically coupled to the processing optical path unit and is configured to perform a high-precision surface positioning mode before processing to determine the reference position of the ingot surface, and to perform a high-speed real-time focus tracking mode during processing to monitor the surface position changes caused by processing. The control and processing unit is connected to the dual-mode confocal interferometric ranging unit and is configured to process the ranging signal and calculate the error of the femtosecond laser focus relative to the target depth in real time based on an indirect depth control model. A focus closed-loop control unit, connected to the control and processing unit, is configured to drive an actuator based on the depth error to compensate for the relative positional deviation between the focus and the ingot. The time-gated synchronization unit is connected to the processing optical path unit and the dual-mode confocal interferometric ranging unit, respectively, and is configured to generate a synchronization timing signal so that the sampling time of the dual-mode confocal interferometric ranging unit is staggered from the pulse emission time of the femtosecond laser.

[0013] The dual-mode confocal interferometric ranging unit includes a differential confocal measurement unit and a white light interferometric measurement unit; In the high-precision surface positioning mode, the differential confocal measurement unit is activated to scan and lock the absolute position of the upper surface of the SiC ingot with nanometer-level precision. In the high-speed real-time focus tracking mode, the white light interferometry unit is activated to indirectly track the depth of the internal laser focus by measuring the relative change in the position of the upper surface of the ingot.

[0014] The implementation logic of the indirect depth control model is as follows: Before processing, the initial upper surface position of the ingot is obtained through a surface positioning mode; Set the target concealment depth and calculate the absolute position of the focal target; During processing, the current position of the upper surface of the crystal ingot is obtained in real time through focus tracking mode; Calculate the current estimated position of the focus; The depth error is obtained by comparing the current estimated position of the focus with the absolute position of the target.

[0015] The technical effects achieved by this invention are as follows: This invention employs laser hidden cutting technology, which achieves peeling by forming a modified layer inside the crystal ingot. The cutting loss rate is reduced from 15%-20% in traditional mechanical cutting to less than 5%, which greatly improves material utilization and reduces production costs.

[0016] This invention minimizes thermal damage. The ultrashort pulse characteristics of femtosecond lasers allow the material to absorb energy before thermal diffusion occurs, thus completing the modification process. The heat-affected zone is much smaller than that of nanosecond lasers, effectively avoiding the formation of molten recast layers and microcracks, and improving the electrical properties and mechanical strength of the substrate.

[0017] This invention offers high processing precision. Through the synergistic effect of the focus positioning unit and the real-time monitoring system, the hidden cutting depth control error is small, the thickness uniformity (TTV) of the substrate after peeling is small, and the surface roughness (Ra) is small, thus meeting the precision requirements of high-end SiC devices for the substrate.

[0018] This invention is highly efficient and adaptable. The scanning speed of the two-dimensional scanning galvanometer is significantly improved compared to the traditional wire saw cutting efficiency. The device can be adapted to SiC ingots of different sizes, and substrates of different thicknesses can be peeled off by adjusting the laser parameters, making it widely applicable. Attached Figure Description

[0019] Figure 1 This is a flowchart of the peeling method of the present invention; Figure 2 This is a block diagram of the stripping device of the present invention; Figure 3 This is a block diagram of the control system of the present invention. Detailed Implementation

[0020] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0021] like Figure 1 As shown, the femtosecond laser hidden-cut depth high-precision stripping method based on confocal interferometric ranging includes the following steps: S1: Cleaning and leveling the SiC ingot; select n-type or p-type SiC single crystal ingots with a diameter of 4-8 inches and a length of 50-200 mm. First, use ultrasonic cleaning technology, sequentially using acetone, ethanol, and deionized water to clean the surface of the ingot to remove surface oil and impurities, with each cleaning time being 10-15 minutes; then fix the ingot on a vacuum adsorption worktable, and use a precision leveling device to ensure that the flatness error of the upper surface of the ingot is less than 0.5 μm; S2: Set the pulse width, repetition rate, power, scanning speed, and scanning interval of the femtosecond laser, and precisely position the laser focus inside the ingot at the target thickness from the upper surface; use a femtosecond laser source with a pulse width of 50-200 fs, a repetition rate of 100-500 kHz, and a center wavelength of 1030 nm. Based on the target peeling thickness, set the laser focus position: position the focus inside the ingot at the target thickness from the upper surface, with a focus positioning accuracy of ±1 μm; simultaneously set the laser power to 2-5 W, the scanning speed to 300-800 mm / s, and the scanning interval to 0.5-1 μm. S3: Control the laser beam to scan inside the ingot in a spiral or checkerboard pattern to form a continuous modified layer; start the femtosecond laser source and drive the laser beam to scan inside the ingot along a preset path through a two-dimensional scanning galvanometer; the scanning path adopts a spiral or checkerboard pattern, and the overlap rate between adjacent scanning lines is 50%-70% to ensure the formation of a continuous modified layer inside the ingot; during the processing, the laser reflection signal and transmission signal are collected through a real-time monitoring system, and the laser power and scanning speed are dynamically adjusted. When the continuity of the modified layer reaches more than 95%, the hidden cutting process is stopped. S4: Apply mechanical force to the ingot after the hidden cutting process to cause it to fracture along the modified layer, thus separating the substrate from the ingot body; transfer the ingot to the peeling device and use a mechanical force-assisted peeling method: fix the bottom of the ingot with a vacuum chuck, and apply a uniform pressure of 0.5-1MPa to the edge of the ingot using a precision pin to cause the ingot to fracture along the internal modified layer, thus separating the substrate from the ingot body; during the peeling process, observe the fracture process in real time with a high-speed camera to ensure a smooth peeling surface; S5: Perform plasma cleaning and surface inspection on the stripped substrate; use plasma cleaning technology to remove residual debris from the surface, with a cleaning time of 5-10 minutes; then use an atomic force microscope to inspect the substrate surface roughness and a laser interferometer to inspect the substrate thickness uniformity, ensuring that the substrate surface roughness Ra is less than 0.5μm and the thickness uniformity TTV is less than 2μm.

[0022] Example 1 500μm thick substrate prepared by exfoliation of 4-inch SiC ingot S1: Select a 4-inch n-type 4H-SiC crystal ingot, 100mm in length. Clean it using ultrasonic cleaning, followed by 12 minutes each of acetone, ethanol, and deionized water; fix it on a vacuum adsorption stage and precisely level it to ensure the surface flatness error is 0.3μm. S2: Femtosecond laser pulse width 100fs, repetition frequency 300kHz, center wavelength 1030nm, power 3W, scanning speed 500mm / s, scanning spacing 0.8μm; the focal point is located at 500μm from the upper surface, with a positioning accuracy of ±1μm; S3: The two-dimensional scanning galvanometer adopts a spiral scanning path with an overlap rate of 60%; it monitors the reflection signal in real time and stops processing when the continuity of the modified layer reaches 96%, with a processing time of 8 minutes. S4: Vacuum chuck fixes the bottom of the ingot, 12 precision pins apply 0.8MPa pressure, and the modified layer is fractured and peeled off. The fracture process is monitored by a high-speed camera. The peeling time is 2 minutes. S5: After plasma cleaning for 8 minutes, the surface roughness Ra was measured by AFM and the thickness uniformity TTV was measured by laser interferometer and was 1.5 μm, which met the requirements.

[0023] Example 2 1mm thick substrate fabricated by exfoliation of 6-inch SiC ingot S1: Ingot pretreatment: 6-inch p-type 6H-SiC ingot, 150mm in length, ultrasonically cleaned for 15 minutes each, and the flatness error after leveling is 0.4μm; S2: Parameter settings: Pulse width 150fs, repetition frequency 400kHz, power 4W, scan speed 600mm / s, scan spacing 1μm; focus position at 1mm; S3: Checkerboard scanning path, overlap rate 70%, stops when the continuity of the modified layer reaches 97%, processing time 15 minutes.

[0024] S4: 16 ejector pins apply 1MPa pressure, peeling time 3 minutes; S5: After plasma cleaning for 10 minutes, the Ra value was measured to be 0.4 μm and the TTV value was 1.8 μm, which met the requirements.

[0025] like Figure 2 As shown, the femtosecond laser hidden-cutting depth high-precision stripping device based on confocal interferometric ranging includes: The ingot fixing and leveling module is used to fix and level SiC ingots; it includes a vacuum adsorption stage, a precision leveling mechanism, and a flatness detection unit; the adsorption force of the vacuum adsorption stage is 0.8-1.2MPa, ensuring that the ingot is firmly fixed; the precision leveling mechanism adopts a three-point support structure with a leveling accuracy of 0.1μm; the flatness detection unit uses a laser interferometer to detect the flatness of the upper surface of the ingot in real time. The femtosecond laser generator module is used to generate the femtosecond laser pulses required for processing; it includes a femtosecond laser, a laser power regulator, and a pulse controller; the output pulse width of the femtosecond laser can be adjusted within the range of 50-200 fs, and the repetition frequency can be continuously adjusted within the range of 100-500 kHz; the adjustment accuracy of the laser power regulator is 0.1W; the pulse controller can achieve precise synchronization of the pulse signal. The optical focusing and scanning module is used to focus and guide femtosecond laser pulses into the interior of the crystal ingot for scanning. It includes a beam expander, a focusing lens, a two-dimensional scanning galvanometer, and a focus positioning unit. The beam expander has a magnification of 2-5 times; the focusing lens has a numerical aperture of 0.3-0.5, ensuring the laser focus diameter is less than 1μm; the two-dimensional scanning galvanometer has a scanning range of 0-300mm × 300mm and a scanning accuracy of 0.1μm; the focus positioning unit uses piezoelectric ceramic drive, with a positioning accuracy of ±1μm. A real-time monitoring module is used to monitor the interaction state between the laser and the material, as well as the processing procedure. It includes a laser reflection detector, a transmission detector, and a high-speed camera. The laser reflection detector has a detection accuracy of 0.01mW, and the transmission detector has a response time of less than 1ns, enabling real-time monitoring of the laser-material interaction. The high-speed camera has a frame rate of 1000-5000 frames per second, used to observe the peeling process. The stripping execution module is used to apply mechanical force to the ingot to separate the substrate; it includes a vacuum chuck, a precision pin array, and a pressure control system. The suction area of ​​the vacuum chuck is matched to the diameter of the ingot; the precision pin array has 8-16 pins, evenly distributed at the edge of the ingot; the pressure control system has a pressure adjustment range of 0-2 MPa and an adjustment accuracy of 0.01 MPa. The central control module, using a PLC controller, connects to and controls all the above modules, enabling the setting of processing parameters, automated control of the processing process, and real-time data acquisition and analysis. The central control module is equipped with a human-machine interface that can intuitively display the processing status and detection data.

[0026] like Figure 3 As shown, the femtosecond laser hidden depth high-precision control system based on confocal interferometric ranging includes: The optical path unit is used to guide femtosecond laser pulses and focus them inside the SiC ingot to form a modified layer; The dual-mode confocal interferometric ranging unit is optically coupled to the processing optical path unit. It is configured to perform a high-precision surface positioning mode before processing to determine the reference position of the ingot surface, and to perform a high-speed real-time focus tracking mode during processing to monitor the surface position changes caused by processing. The dual-mode confocal interferometric ranging unit includes a differential confocal measurement unit and a white light interferometric measurement unit. In the high-precision surface positioning mode, the differential confocal measurement unit is activated to scan and lock the absolute position of the upper surface of the SiC ingot with nanometer-level precision. In high-speed real-time focus tracking mode, the white light interferometry unit is activated to indirectly track the depth of the internal laser focus by measuring the relative change in the position of the upper surface of the ingot; The detection optical path of the dual-mode confocal interferometric ranging unit and the processing optical path of the processing optical path unit are coaxially combined through a dichroic mirror and focused on the SiC ingot through the same confocal objective lens. The control and processing unit, connected to the dual-mode confocal interferometric ranging unit, is configured to process ranging signals and, based on an indirect depth control model, calculate in real time the error of the femtosecond laser focus relative to the target depth. The implementation logic of the indirect depth control model is as follows: Before processing, the initial upper surface position of the ingot is obtained through a surface positioning mode; Set the target concealment depth and calculate the absolute position of the focal target; During processing, the current position of the upper surface of the crystal ingot is obtained in real time through focus tracking mode; Calculate the current estimated position of the focus; The depth error is obtained by comparing the current estimated position of the focus with the absolute position of the target. The control and processing unit also integrates an adaptive digital filtering algorithm to suppress vibration noise in the processing environment; and has a built-in temperature drift compensation model for software compensation of thermal deformation during long-term processing. The focus closed-loop control unit, connected to the control and processing unit, is configured to drive an actuator based on the depth error to compensate for the relative positional deviation between the focus and the ingot. The time-gated synchronization unit, connected to both the processing optical path unit and the dual-mode confocal interferometric ranging unit, is configured to generate a synchronization timing signal, ensuring that the sampling time of the dual-mode confocal interferometric ranging unit is staggered from the pulse emission time of the femtosecond laser. The time-gated synchronization unit is configured to generate a measurement enable window delayed by the laser pulse within each or every N femtosecond laser pulse cycles. The detector of the dual-mode confocal interferometric ranging unit samples the signal only within this measurement enable window, thereby avoiding interference from the plasma induced by the femtosecond laser pulse on the measurement light.

[0027] The above description is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained in this invention are implemented according to conventional methods in the art unless otherwise specified or limited.

Claims

1. A femtosecond laser hidden incision depth high-precision peeling method based on confocal interferometric distance measurement, characterized in that, The method comprises the following steps: S1: cleaning and fixing and leveling the SiC crystal ingot; S2: setting the pulse width, repetition frequency, power, scanning speed and scanning interval of the femtosecond laser, and accurately positioning the laser focus point at the target thickness inside the crystal ingot from the upper surface; S3: controlling the laser beam to scan in a spiral or chessboard path inside the crystal ingot to form a continuous modified layer; S4: applying mechanical force to the crystal ingot after the hidden cutting process is completed, so that the crystal ingot is broken along the modified layer, and the separation of the substrate and the crystal ingot body is realized; S5: plasma cleaning and surface detection of the peeled substrate.

2. The peeling method according to claim 1, characterized by: In the S2, the pulse width of the femtosecond laser is 50-200 fs, the repetition frequency is 100-500 kHz, the laser power is 2-5 W, the scanning speed is 300-800 mm / s, and the scanning interval is 0.5-1 μm; the positioning accuracy of the laser focus point is ±1 μm.

3. The peeling method according to claim 1, characterized by: In the S3, during the processing, the laser reflection signal and the transmission signal are collected in real time, and the laser power and the scanning speed are dynamically adjusted; when it is monitored that the continuity of the modified layer reaches more than 95%, the hidden cutting process is stopped.

4. A femtosecond laser incision depth high-precision peeling device based on confocal interferometric distance measurement, for realizing the peeling method of any one of claims 1-3, characterized in that, It comprises: a crystal ingot fixing and leveling module for fixing and leveling the SiC crystal ingot; a femtosecond laser generation module for generating the femtosecond laser pulses required for processing; an optical focusing and scanning module for focusing and guiding the femtosecond laser pulses to scan inside the crystal ingot; a real-time monitoring module for monitoring the interaction state of the laser and the material and the processing process; a peeling execution module for applying mechanical force to the crystal ingot to separate the substrate; a central control module using a PLC controller to connect and control all the above modules.

5. The peeling apparatus according to claim 4, characterized by: The optical focusing and scanning module comprises an expander mirror, a focusing lens, a two-dimensional scanning galvanometer and a focus positioning unit; the expansion ratio of the expander mirror is 2-5 times; the numerical aperture of the focusing lens is 0.3-0.5; the scanning range of the two-dimensional scanning galvanometer is 0-300 mm×300 mm, and the scanning accuracy is 0.1 μm; the focus positioning unit is driven by a piezoelectric ceramic, and the positioning accuracy is ±1 μm.

6. The peeling apparatus according to claim 4, characterized by: The crystal ingot fixing and leveling module comprises a vacuum adsorption workbench, a three-point support type precision leveling mechanism and a laser interferometer flatness detection unit; the peeling execution module comprises a vacuum suction cup, an array composed of 8-16 precision thimbles and a pressure control system.

7. The peeling apparatus according to claim 4, characterized by: It comprises a laser reflection detector, a transmission detector and a high-speed camera; the detection accuracy of the laser reflection detector is 0.01 mW, and the response time of the transmission detector is less than 1 ns, which is used to monitor the interaction state of the laser and the material in real time; the shooting frame rate of the high-speed camera is 1000-5000 frames / second.

8. A femtosecond laser incision depth high-precision control system based on confocal interferometry distance measurement, integrated with the peeling device of any one of claims 4-7, characterized in that, It comprises: a processing light path unit for guiding the femtosecond laser pulses and focusing them inside the SiC crystal ingot to form a modified layer; a dual-mode confocal interference distance measurement unit optically coupled with the processing light path unit, configured to execute a high-precision surface positioning mode before processing to determine the reference position of the crystal ingot surface, and execute a high-speed real-time focus tracking mode during processing to monitor the surface position change caused by processing; a control and processing unit connected to the dual-mode confocal interferometric ranging unit, configured to process the ranging signal and calculate in real time a depth error of the femtosecond laser focal point relative to a target depth according to an indirect depth control model; a focal point closed-loop control unit connected to the control and processing unit, configured to drive an actuator according to the depth error to compensate for the relative position deviation between the focal point and the ingot; a time-gated synchronization unit connected to the machining light path unit and the dual-mode confocal interferometric ranging unit respectively, configured to generate a synchronization timing signal to make the sampling time of the dual-mode confocal interferometric ranging unit and the pulse emission time of the femtosecond laser staggered with each other.

9. The control system of claim 8, wherein the dual-mode confocal interferometric ranging unit includes a differential confocal measurement unit and a white light interferometric measurement unit; in the high-precision surface positioning mode, the differential confocal measurement unit is started to scan and lock the absolute position of the upper surface of the SiC ingot with nanometer-level precision; in the high-speed real-time focal point tracking mode, the white light interferometric measurement unit is started to indirectly track the depth of the internal laser focal point by measuring the relative change of the upper surface position of the ingot.

10. The control system of claim 8, wherein: The implementation logic of the indirect depth control model is as follows: Before processing, the initial upper surface position of the ingot is obtained through the surface positioning mode; set the target depth of cut, and calculate the target absolute position of the focal point; in the processing, the current position of the upper surface of the ingot is obtained in real time through the focal point tracking mode; calculate the current estimated position of the focal point; by comparing the current estimated position of the focal point with the target absolute position of the focal point, the depth error is obtained.

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