High-precision polishing method for titanium sapphire crystal

Abstract

The invention discloses a titanium sapphire crystal composite polishing technology integrating chemical mechanical polishing, magneto-rheological polishing and ion beam polishing, and relates to the field of laser crystal ultra-precision machining. According to the method, the technical bottlenecks that the subsurface damage of the titanium sapphire crystal is difficult to control and the surface shape precision and the surface roughness are difficult to consider at the same time due to factors such as high Mohs hardness and doping concentration are solved through a three-stage synergistic process of chemical mechanical polishing pre-shaping, magneto-rheological polishing low-damage surface trimming and ion beam polishing atomic-scale fine trimming. The surface roughness Ra of the titanium sapphire crystal processed through the method is smaller than or equal to 0.1 nm, the surface shape precision PV is smaller than or equal to lambda / 20 (lambda = 632.8 nm), the subsurface damage layer depth is smaller than 5 nm, and the strict requirements of an intense laser system for crystal elements are met.

Description

Technical Field

[0001] This invention relates to the field of ultra-precision machining technology for laser crystals, and in particular to a high-precision polishing method for titanium sapphire (Ti:Al2O3) crystals, which is applicable to the high-precision machining of crystals used in the laser field. Background Technology

[0002] Titanium sapphire (Ti:Al2O3) crystals have become core components of tunable lasers and ultra-intense, ultrafast lasers due to their wide emission bandwidth, high thermal conductivity, and stable physicochemical properties. However, their high Mohs hardness of 9 and uneven doping concentration distribution make them prone to defects such as subsurface scratches and cracks during processing. This makes it difficult to achieve high-precision surface shapes and low roughness in titanium sapphire crystals, especially large-sized ones. This often leads to severe wavefront distortion, a decreased laser damage threshold, and deterioration of laser beam quality, severely restricting their application in high-end optical systems.

[0003] Traditional titanium sapphire crystal polishing techniques have significant limitations: while chemical mechanical polishing (CMP) alone can achieve global planarization, the mechanical forces easily create a subsurface damage layer with a depth >20nm; magnetorheological polishing, based on the Bingham fluid plastic shear principle, can achieve low-damage processing, but the "spot" contact mode leads to low polishing efficiency and makes it difficult to eliminate mid-to-high frequency surface shape errors; ion beam polishing can achieve atomic-level surface quality, but its extremely low removal rate (typically <1nm / s) makes it difficult to use alone for damage layer removal. The cold processing method for titanium sapphire surfaces proposed by Fujian Fujing Technology Co., Ltd. (CN109551312A) achieves a surface quality of 10 / 5 defect (S / D) and λ / 8 flatness (surface shape), but it does not solve the problem of subsurface damage control. Studies of pure CMP show that the optimal process can only reduce surface roughness to 0.262nm, and the surface shape accuracy is difficult to exceed λ / 8. Therefore, developing a composite polishing technology that can synergistically leverage the advantages of multiple technologies while balancing efficiency and precision has become an important requirement in the field of titanium sapphire crystal processing. Summary of the Invention

[0004] To address the technical bottlenecks in existing titanium sapphire polishing technologies, such as the difficulty in coordinating "damage layer removal—surface shape correction—ultra-smooth surface construction," the inability of a single process to simultaneously achieve low damage and high efficiency, and the tendency for abrupt changes in surface quality when multiple processes are combined, this invention provides a surface polishing method for titanium sapphire (Ti:Al2O3) crystals.

[0005] The technical solution of the present invention is as follows: A high-precision polishing method for titanium sapphire crystals mainly includes the following steps: S1 Macroscopic Damage Removal: The cut titanium sapphire wafer is surface-ground to quantitatively remove a thickness of 30±5μm to eliminate the macroscopic crack layer introduced by cutting. S2 Chemical Mechanical Polishing Pre-shaping: The wafer processed in step S1 is chemically mechanically polished until the surface roughness Ra is ≤5nm and no sand pits are visible under a 200x microscope, forming a uniform surface suitable for low-stress fine finishing. S3 Magnetorheological Polishing Low-Damage Finishing: The wafer processed in step S2 is subjected to magnetorheological polishing. By controlling the magnetic field and shear rate, deterministic material removal is achieved, resulting in a removal amount of 0.5-1μm on the crystal surface, and its surface shape accuracy is corrected to PV≤λ / 8, where λ=632.8nm; S4 Ion Beam Polishing Atomic-Level Refinement: The wafer processed in step S3 is polished with an ion beam. The surface is bombarded with an argon ion beam in a vacuum environment to achieve atomic-level material removal of 50-100nm and obtain an ultra-smooth surface. In this process, step S2 provides a substrate with initial surface convergence and consistent chemical properties for step S3, step S3 provides a transition surface with high surface accuracy and extremely shallow subsurface damage for step S4, and step S4 is used to finally eliminate any nanoscale surface undulations and amorphous layers that may remain in step S3. The three-step process works together to achieve the processing target of surface roughness Ra≤0.1nm, surface accuracy PV≤λ / 20 and subsurface damage layer depth<5nm.

[0006] Furthermore, the abrasive used in step S1 is boron carbide.

[0007] Furthermore, in step S2, the polishing slurry used is a silica sol polishing slurry with pH=9.0-10.0, wherein the particle size of the silica sol is 80-100nm and the mass concentration is 20-30%.

[0008] Furthermore, the silica sol polishing solution contains 0.5-1.0 wt% sodium hydroxide as a pH adjuster and 0.1-0.2 wt% polyethylene glycol as a dispersant.

[0009] Furthermore, in step S2, the polishing disk rotates at 25-35 rpm, and the polishing pressure applied to the wafer is 0.05-0.1 MPa.

[0010] Furthermore, in step S3, the gradient magnetic field strength in the polishing area is 0.2-0.5T, and the linear velocity of the polishing wheel is 8-12m / s.

[0011] Furthermore, in step S3, the mechanical behavior of the magnetorheological polishing medium is dynamically adjusted with the shear frequency, which ranges from 5 to 20 Hz.

[0012] Furthermore, in step S3, the ion beam energy is 300-500 eV, the beam current density is 0.1-0.3 mA / cm², and the scanning rate is 5-10 mm / s.

[0013] The present invention also provides a titanium sapphire crystal element, characterized in that it is manufactured by the method described above, and has a surface roughness Ra≤0.1nm, a surface shape accuracy PV≤λ / 20, and a subsurface damage layer depth<5nm.

[0014] Compared with the prior art, the present invention has the following advantages: 1. Strong damage control capability: The multi-stage process gradually reduces the amount of material removed (micrometer level → submicrometer level → nanometer level), and the depth of the subsurface damage layer is reduced from >30nm in traditional processes to <5nm. 2. Excellent surface quality: The final surface roughness Ra≤0.1nm and the surface accuracy PV≤λ / 20 significantly exceed the roughness and surface accuracy levels of existing technologies; 3. High processing efficiency: The combination of multi-stage process modes for rapid removal shortens the overall processing cycle by more than 60% compared to a single polishing mode; 4. Wide adaptability: It can process titanium sapphire wafers with a maximum diameter of ~Ø300mm to meet the needs of different scale application scenarios. Attached Figure Description

[0015] Figure 1 This is the surface roughness test result (two-dimensional planar image) of a titanium sapphire crystal using a high-precision polishing method.

[0016] Figure 2 This is the surface roughness test result (3D image) of a titanium sapphire crystal using a high-precision polishing method.

[0017] Figure 3 These are the surface shape accuracy test results (front surface) of a titanium sapphire crystal using a high-precision polishing method. Figure 4 This is the test result of the surface shape accuracy of a titanium sapphire crystal using a high-precision polishing method (reverse surface).

[0018] Figure 5 These are the test results of the reflective surface shape accuracy of titanium sapphire crystals using traditional chemical mechanical polishing methods.

[0019] Figure 6 The results are from a transmission surface accuracy test of a titanium sapphire crystal using a traditional chemical mechanical polishing method. Detailed Implementation

[0020] The present invention will be further described below with reference to embodiments, but should not be construed as limiting the present invention.

[0021] Example 1: Polishing of a Ø225mm×70mm titanium sapphire crystal Wafer cleaning: Take the cut Ø225mm, 70mm thick titanium sapphire wafer and clean it.

[0022] Waxed Patch: Carnauba wax is used to bond and fix the titanium gemstone wafer; Rough grinding stage: Place the wafer on the worktable of the thinning machine, add boron carbide abrasive and use a grinding wheel to grind the surface of the wafer, removing 28μm of titanium sapphire surface.

[0023] Chemical mechanical polishing (CMP) pre-shaping: The processed wafer is mounted on the pressure head and placed on a CNC polishing machine for chemical mechanical polishing pre-shaping. The silica sol pH is adjusted to 9, particle size to 90nm, concentration to 28wt.%, polishing disc speed to 28rpm, polishing pressure to 0.08MPa, and the wafer surface roughness Ra to 4.5nm.

[0024] Magnetorheological polishing (MRF) low-damage surface finishing: The processed wafer is transferred to the worktable of a magnetorheological polishing machine. A magnetorheological shear-hardened polishing medium is used. A gradient magnetic field of 0.3T is applied through a multi-axis linkage magnetorheological machine tool. The polishing wheel linear speed is 9m / s. The mechanical behavior of the magnetorheological polishing medium is adjusted with the shear frequency in the range of 10-18Hz to achieve a crystal surface removal amount of 0.9μm and a surface shape accuracy of PV of 0.12λ.

[0025] Ion beam polishing (IBF) atomic-level finishing: The wafer is transferred to the stage of the ion beam polishing machine, and the cavity vacuum is adjusted to 5 × 10⁻⁶. -4 Pa, ion beam energy = 400 eV, beam current density = 0.3 mA / cm², scan rate = 7 mm / s, ultimately achieving a wafer removal depth of 70 nm.

[0026] The polished titanium sapphire wafer is placed on a plate and subjected to ultrasonic cleaning and vacuum drying to complete the polishing process.

[0027] The final measured surface roughness Ra = 0.095 nm (see appendix). Figure 1 Appendix Figure 2 The full-aperture reflective surface accuracy PV=0.045λ and the subsurface damage layer depth=4.2nm.

[0028] Example 2: Polishing of a Ø180mm×60mm titanium sapphire crystal This embodiment has the same technical conditions as Embodiment 1. The final measured surface roughness Ra = 0.092 nm, full-aperture reflective surface accuracy PV = 0.043λ, and subsurface damage layer depth = 4.0 nm. These results demonstrate that the method of this invention has good consistency and repeatability on wafers of different thicknesses.

[0029] Example 3: Polishing of a Ø150mm×50mm titanium sapphire crystal In this embodiment, the magnetic field was adjusted to 0.5T in magnetorheological polishing, and the ion beam energy was adjusted to 350eV in ion beam polishing. All other conditions were consistent with those in Example 1. The final measured surface roughness Ra was 0.085nm, the full-aperture reflective surface accuracy PV was 0.039λ, and the subsurface damage layer depth was 4.0nm.

[0030] Example 4: Polishing of a Ø50mm×20mm titanium sapphire crystal A Ø50mm×20mm titanium sapphire wafer was selected. During magnetorheological polishing, the magnetic field was adjusted to 0.5T. During ion beam polishing, the ion beam energy was adjusted to 350eV, the beam current density to 0.2mA / cm², and the scanning rate to 5mm / s. Other parameters were as described in Example 1. The final processing results were: surface roughness Ra = 0.090nm, and positive surface shape accuracy PV = 0.043λ (see Appendix). Figure 3 The reverse surface accuracy PV = 0.032λ (see appendix) Figure 4 ), Subsurface damage layer depth = 3.8 nm.

[0031] Comparative Example 1: Single Chemical Mechanical Polishing The same type of titanium sapphire wafers were processed using only the coarse grinding and CMP processes described in Example 1 until the CMP-limited surface state was achieved.

[0032] Comparative Example 2: The CMP and MRF processes from Example 1 are used, omitting the IBF final repair step.

[0033] The results of Example 1 and the comparative example are compared in the table below: As shown in the table above, while single CMP (Comparative Example 1) can achieve a certain degree of flatness, it suffers from poor surface roughness, extremely low surface shape accuracy, and introduces severe subsurface damage, failing to meet the requirements of high-end optical applications. The two-step CMP+MRF method (Comparative Example 2) significantly improves both roughness and surface shape compared to single CMP, demonstrating the effectiveness of MRF for deterministic shaping. However, its surface quality (Ra=0.7nm) and final surface shape (PV=0.12λ) still show a significant difference from an atomically smooth surface, and the subsurface damage (32nm) remains relatively deep, indicating that a micro-defect layer still exists after the MRF process and needs to be removed by IBF. This is because MRF itself has inherent limitations (such as the "kissing" phenomenon and dielectric residue), which are precisely the limitations that IBF can most effectively address.

[0034] The three-stage composite method of this invention (Example 1) is comprehensively superior in all indicators. The key is the introduction of IBF as the final finishing step, which achieves a leap from nanoscale roughness to sub-nanometer roughness, advances the surface accuracy to the λ / 20 level, and controls the subsurface damage layer depth to an extreme level below 5nm.

[0035] The specific embodiments and comparative examples of the present invention have been described above, but are merely examples, and the present invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications and substitutions to the present invention are also within the scope of the present invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of the present invention should be covered within the scope of the present invention.

Claims

1. A high-precision polishing method for titanium sapphire crystals, characterized in that, The steps are as follows: S1 Macroscopic Damage Removal: The cut titanium sapphire wafer is surface-ground to quantitatively remove a thickness of 30±5μm to eliminate the macroscopic crack layer introduced by cutting. S2 Chemical Mechanical Polishing Pre-shaping: The wafer processed in step S1 is chemically mechanically polished until the surface roughness Ra is ≤5nm and no sand pits are visible under a 200x microscope, forming a uniform surface suitable for low-stress fine finishing. S3 Magnetorheological Polishing Low-Damage Finishing: The wafer processed in step S2 is subjected to magnetorheological polishing. By controlling the magnetic field and shear rate, deterministic material removal is achieved, resulting in a removal amount of 0.5-1μm on the crystal surface, and its surface shape accuracy is corrected to PV≤λ / 8, where λ=632.8nm; S4 Ion Beam Polishing Atomic-Level Refinement: The wafer processed in step S3 is polished with an ion beam. The surface is bombarded with an argon ion beam in a vacuum environment to achieve atomic-level material removal of 50-100nm and obtain an ultra-smooth surface. In this process, step S2 provides a substrate with initial surface convergence and consistent chemical properties for step S3, step S3 provides a transition surface with high surface accuracy and extremely shallow subsurface damage for step S4, and step S4 is used to finally eliminate any nanoscale surface undulations and amorphous layers that may remain in step S3. The three-step process works together to achieve the processing target of surface roughness Ra≤0.1nm, surface accuracy PV≤λ / 20 and subsurface damage layer depth<5nm.

2. The method according to claim 1, characterized in that: The abrasive used in step S1 is boron carbide.

3. The method according to claim 1, characterized in that: In step S2, the polishing slurry used is a silica sol polishing slurry with pH=9.0-10.0, wherein the particle size of the silica sol is 80-100nm and the mass concentration is 20-30%.

4. The method according to claim 3, characterized in that: The silica sol polishing solution contains 0.5-1.0 wt% sodium hydroxide as a pH adjuster and 0.1-0.2 wt% polyethylene glycol as a dispersant.

5. The method according to claim 1, characterized in that: In step S2, the polishing disk rotates at 25-35 rpm, and the polishing pressure applied to the wafer is 0.05-0.1 MPa.

6. The method according to claim 1, characterized in that: In step S3, the gradient magnetic field strength in the polishing area is 0.2-0.5T, and the linear velocity of the polishing wheel is 8-12m / s.

7. The method according to claim 1, characterized in that: In step S3, the mechanical behavior of the magnetorheological polishing medium is dynamically adjusted with the shear frequency, which ranges from 5 to 20 Hz.

8. The method according to claim 1, characterized in that: In step S3, the ion beam energy is 300-500 eV, the beam current density is 0.1-0.3 mA / cm², and the scanning rate is 5-10 mm / s.

9. A titanium-sapphire crystal element, characterized in that, It is manufactured by the method of any one of claims 1 to 8, and has a surface roughness Ra≤0.1nm, a surface shape accuracy PV≤λ / 20, and a subsurface damage layer depth<5nm.

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