A method for processing wafer-level calcium fluoride micro-optical elements

Abstract

The application provides a wafer-level calcium fluoride micro-optical element processing method and relates to the technical field of micro-optical element processing.The method comprises the following steps: profile grinding is performed on the surface of a calcium fluoride wafer by using a profile grinding head, the surface profile of the profile grinding head is complementary to a target microstructure array, the profile grinding head has a tip corresponding to the sharp corner, and the tip of the profile grinding head can enter the sharp corner area; after grinding is completed, a magnetorheological fluid is supplied between the profile grinding head and the surface of the calcium fluoride wafer, the magnetorheological fluid forms a flexible ribbon through a magnetic field generated by a permanent magnet array embedded in the profile grinding head, the flexible ribbon is attached to the tip of the profile grinding head, and the sharp corner area is finely ground; the surface of the processed calcium fluoride wafer is irradiated by using a gas cluster ion beam; and profile polishing is performed by using a profile resin wheel with the aid of enzymatic hydrolysis; and the application can realize the unification of nanoscale high-frequency roughness and low-frequency surface shape fidelity, and finally, a near-zero-damage and super-smooth surface is obtained.

Description

Technical Field

[0001] This invention relates to the field of micro-optical element processing technology, and in particular to a method for processing wafer-level calcium fluoride micro-optical elements. Background Technology

[0002] Calcium fluoride (CaF2) wafer-level micro-optical components have important applications in deep ultraviolet lithography and high-energy laser systems. One typical structure is a cylindrical microlens array, where adjacent semi-circular protrusions form concave sharp corners (acute angles at the valley bottom). The ultra-precision machining of these sharp-cornered structures is extremely difficult: traditional contour grinding can quickly form the shape, but rigid grinding wheels cannot penetrate the root of the sharp corner, resulting in significant subsurface damage in that area; single-point diamond turning is limited by the tool radius and also cannot machine ideal sharp corners; magnetorheological polishing is a sub-aperture polishing method, which easily introduces mid-frequency errors at sharp corners; and conventional chemical mechanical polishing (CMP) or plasma-assisted polishing (such as the method used for diamond in Reference 1) is difficult to achieve shape preservation of sharp corners on calcium fluoride. Therefore, there is an urgent need for an efficient, low-damage, shape-preserving machining method specifically for calcium fluoride wafers with sharp-cornered microstructures. Summary of the Invention

[0003] To address the aforementioned technical problems, the technical solution adopted by this invention is as follows:

[0004] According to a first aspect of this application, a method for fabricating a wafer-level calcium fluoride micro-optical element is provided. The wafer-level calcium fluoride micro-optical element has a microstructure array on its surface, the microstructure array being composed of multiple semi-circular protrusions arranged side by side, with recessed sharp corners formed between adjacent semi-circular protrusions; the method includes the following steps:

[0005] S100, a contour grinding head is used to perform contour grinding on the surface of a calcium fluoride wafer. The surface contour of the contour grinding head is complementary to the target microstructure array. The contour grinding head has a tip corresponding to the sharp corner. The tip of the contour grinding head can enter the sharp corner region. The contour grinding head is controlled to move along the generatrix direction of the microstructure array to form a microstructure array on the surface of the calcium fluoride wafer.

[0006] S200, after grinding is completed, magnetorheological fluid is supplied between the profile grinding head and the surface of the calcium fluoride wafer. The magnetic field generated by the permanent magnet array embedded in the profile grinding head causes the magnetorheological fluid to form a flexible ribbon. The flexible ribbon is attached to the tip of the profile grinding head to perform fine grinding on the sharp corner area to reduce the subsurface damage layer at the sharp corner.

[0007] S300 uses a gas cluster ion beam to irradiate the surface of a calcium fluoride wafer treated by S100. By controlling the process parameters of the gas cluster ion beam, a highly uniform amorphous modified layer is formed on the surface of the calcium fluoride wafer. The process parameters include gas source type, cluster size, incident angle, and irradiation dose. The minimum effective thickness of the amorphous modified layer is determined based on the depth of the subsurface damage layer generated by grinding.

[0008] S400 employs a contoured resin wheel for enzymatic hydrolysis-assisted conformal polishing of the amorphous modified layer. The surface contour of the contoured resin wheel matches the microstructure array, and the surface of the resin wheel contains hydroxyl groups. During the polishing process, the hydroxyl groups on the surface of the resin wheel undergo a dehydration condensation reaction with the calcium fluoride amorphous modified layer, achieving material removal in a chemically bonded manner to obtain a near-zero damage surface.

[0009] The present invention has at least the following beneficial effects:

[0010] The wafer-level calcium fluoride micro-optical element processing method of the present invention targets a microstructure array composed of multiple side-by-side semi-circular protrusions, with concave sharp corners formed between adjacent semi-circular protrusions. First, in the contour grinding step, a contour grinding head with a surface profile complementary to the microstructure array is used. This contour grinding head has a tip corresponding to the sharp corner, enabling rigid contour grinding in the sharp corner region to achieve efficient forming of the microstructure array. After grinding, a magnetic field is generated by a permanent magnet array embedded in the contour grinding head, causing magnetorheological fluid to form a flexible ribbon that adheres to the tip of the contour grinding head for fine grinding of the sharp corner region. The key to this process is that rigid contour grinding solves the problem of rapid forming of the sharp corner region, while the flexible ribbon attached to the tip conforms to the geometry of the sharp corner, uniformly thinning the area at the root of the sharp corner—an area difficult for traditional tools to reach—effectively removing the subsurface damage layer generated by rigid grinding, while avoiding the problems caused by sub-aperture polishing at the sharp corner. The process involves several steps. First, in the gas cluster ion beam modification step, by adjusting the gas source type, cluster size, incident angle, and irradiation dose, the minimum effective thickness of the amorphous modification layer is determined based on the depth of the subsurface damage layer generated by grinding. A predictive model for the amorphous layer distribution covering the top of the pillar, sidewalls, and bottom of the sharp corner is established to ensure that the weakest area, the sharp corner, receives an amorphous layer thickness sufficient to cover the damage layer, transforming crystalline damage into a chemically active and structurally uniform amorphous state. Finally, in the enzyme hydrolysis-assisted conformal polishing step, a conformal resin wheel matching the shape of the sharp corner is used. The hydroxyl groups on the surface of the resin wheel undergo a dehydration condensation reaction with the calcium fluoride amorphous modification layer, achieving atomic-scale material removal through Ca-OC chemical bonding. This polishing process does not rely on abrasive mechanical action and does not introduce new microcracks or residual stress at the sharp corner. Simultaneously, the resin wheel can completely conform to the sharp corner area, achieving full-diameter conformal polishing and preventing the sharp corner from being rounded or the edge from collapsing. Through the synergistic application of multiple processes including contour grinding, magnetorheological grinding, cluster ion beam modification, and enzymatic hydrolysis and chemical polishing, this invention achieves for the first time efficient, low-damage, and conformal processing of calcium fluoride wafers with sharp-corner microstructures. The sharp corners can achieve near-zero damage, ultra-smooth surfaces that maintain their original geometry. This invention solves the technical problems of traditional contour grinding not being able to reach the root of the sharp corners, mid-frequency errors caused by sub-aperture polishing, and the difficulty of maintaining conformal properties in chemical mechanical polishing. It provides a key component manufacturing solution for fields such as deep ultraviolet lithography and high-energy laser systems. Attached Figure Description

[0011] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1A flowchart illustrating a method for fabricating wafer-level calcium fluoride micro-optical elements according to an embodiment of the present invention. Detailed Implementation

[0013] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] It should be noted that, based on this disclosure, those skilled in the art will understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects set forth herein can be used to implement the device and / or practice the method. Furthermore, this device and / or practice the method can be implemented using other structures and / or functionalities besides one or more of the aspects set forth herein.

[0015] The following will refer to Figure 1 The flowchart shown illustrates a method for fabricating wafer-level calcium fluoride micro-optical elements, introducing such a method.

[0016] It should be noted that the specific parameters in the embodiments of this application are exemplary parameters. Those skilled in the art can adjust the parameters according to actual needs when carrying out specific implementations, and no limitation is imposed here.

[0017] The wafer-level calcium fluoride micro-optical element has a microstructure array on its surface, which consists of multiple semi-circular protrusions arranged side by side, with recessed sharp corners formed between adjacent semi-circular protrusions; the fabrication method of the wafer-level calcium fluoride micro-optical element may include the following steps:

[0018] S100, a contour grinding head is used to perform contour grinding on the surface of a calcium fluoride wafer. The surface contour of the contour grinding head is complementary to the target microstructure array. The contour grinding head has a tip corresponding to the sharp corner. The tip of the contour grinding head can enter the sharp corner region. The contour grinding head is controlled to move along the generatrix direction of the microstructure array to form a microstructure array on the surface of the calcium fluoride wafer.

[0019] In this embodiment, the workpiece sharp corner is a sharp, downward-concave corner naturally formed at the junction of two adjacent semi-circular convex arcs (the angle is usually less than 90°, or even close to 0°).

[0020] The tip of the contour grinding head is complementary to the microstructure array of the workpiece, meaning that the workpiece protrusions correspond to the grinding head recesses, and the workpiece's sharp corners correspond to the sharp protrusions (i.e., the "tip") on the grinding head. The shape (angle, radius of curvature) of this tip perfectly matches the workpiece's sharp corner, and its size is slightly smaller than the inner cavity of the sharp corner to ensure entry.

[0021] While conventional contour grinding heads complement the microstructure of their overall profile, the "bottom" of the head is typically designed as a smooth arc (R-angle) to facilitate machining and avoid stress concentration, preventing the formation of sharp protrusions. When machining sharp corners, such grinding heads leave a "dead zone" between the bottom arc and the workpiece's sharp corner—an area that cannot be ground, resulting in material residue and the inability to thin the damaged layer in that area.

[0022] This application processes the corresponding part of the contour grinding head into a sharp protrusion (tip), so that it can physically penetrate to the bottom of the sharp corner, completely grind the material in the sharp corner area, and allow the subsequent magnetorheological flexible ribbon to be attached to the tip for further fine grinding of the root of the sharp corner.

[0023] The grinding head employs a specially designed contour grinding head, the base of which is made of a non-magnetic high-strength material (such as austenitic stainless steel or titanium alloy). An embedded permanent magnet array is incorporated within the grinding head, such as multiple neodymium iron boron (NdFeB) permanent magnets evenly distributed along the circumference, with the magnet polarities alternating in an N-S pattern, or a Halbach array is used to enhance the unilateral magnetic field. The grinding head has two working surfaces: one surface is fixed with a rigid contour grinding wheel (e.g., a resin-bonded diamond grinding wheel, grit size #2000–#4000, with a shape complementary to the target microstructure); the other surface has a magnetorheological fluid supply channel and a magnetic field application area.

[0024] A contour grinding head is a composite grinding tool with switchable operating modes and a profile complementary to the target microstructure array. The contour grinding head completes two processes in a single setup:

[0025] Rigid contour grinding mode: Utilizing the rigid contour of a diamond grinding wheel, a microstructure array is rapidly imprinted on the surface of calcium fluoride through contouring motion, achieving efficient forming.

[0026] Magnetorheological flexible precision grinding mode: using a magnetic field-controlled magnetorheological fluid flexible ribbon to thin the damage layer on the surface after rigid grinding without damaging the edges of the microstructure.

[0027] Working principle:

[0028] During rigid contour grinding: switch to the grinding wheel face, the grinding wheel contacts the workpiece and moves along the generatrix direction of the microstructure array. The complementary contour of the grinding wheel "copy" the microstructure shape onto the calcium fluoride surface.

[0029] During magnetorheological precision grinding: switch to the magnetorheological working surface, supply magnetorheological fluid to the gap, and the permanent magnet array generates a magnetic field of 0.2~0.8T. The magnetorheological fluid solidifies instantly to form a "flexible ribbon".

[0030] The contour grinding head achieves "one-time clamping and two-process" through structural integration, avoiding secondary positioning errors; the magnetic field of the permanent magnet array can precisely control the ribbon shape of the magnetorheological fluid, achieving uniform thinning of the damaged layer; the dual-mode synergy significantly reduces subsurface damage and mid-frequency error while ensuring the accuracy of the microstructure surface, laying a high-quality foundation for subsequent modification and polishing.

[0031] The magnetorheological fluid circulation system is activated when magnetorheological fluid is needed, and is not activated when magnetorheological fluid is not needed.

[0032] Specific implementation of rigid contour grinding mode:

[0033] Grinding wheel preparation: Based on the design of the microstructure array on the calcium fluoride wafer (such as the radius of curvature, spacing, and sag of the microlens array), the working surface of the grinding wheel is dressed into a complementary contoured surface using a diamond dressing wheel, with a dressing accuracy PV≤0.2μm.

[0034] Grinding parameters: grinding wheel linear speed 20-40 m / s, workpiece feed rate 10-50 mm / min, single grinding depth 1-10 μm. Use water-based coolant (containing rust inhibitor), and control the coolant temperature at 20±1℃.

[0035] Machining path: Grinding is achieved through position control.

[0036] Specific implementation of magnetorheological flexible precision grinding mode:

[0037] Magnetorheological fluid preparation: Carbonyl iron powder (particle size 1-5 μm) at a volume fraction of 30%-40% is dispersed in an aqueous or oil-based carrier fluid, with 1%-2% by mass of a stabilizer (such as sodium citrate) and an anti-settling agent (such as fumed silica). The zero-field viscosity of the magnetorheological fluid is controlled at 0.1-0.5 Pa·s, and the saturation magnetization is ≥0.5 T.

[0038] Magnetic field activation: A permanent magnet array generates a gradient magnetic field of 0.2–0.8 T in the gap (0.5–2 mm) between the grinding head and the workpiece. After the magnetorheological fluid is pumped into the gap, it solidifies instantaneously to form a "flexible ribbon" with a certain hardness, the shear yield stress of which is proportional to the square of the magnetic field strength.

[0039] Fine grinding parameters: grinding head speed 500-2000 rpm, gap maintained at about 1 mm, and dwell time planned according to the depth distribution of the damaged layer after rigid grinding (usually 30-120 s).

[0040] Damage layer thinning control: The depth of the damage layer is detected online by white light interferometer or confocal microscope. The magnetic field strength or processing time is adjusted to reduce the thickness of the subsurface damage layer from 1-5 μm to below 200 nm, and the mid-frequency error RMS is reduced from 15 nm to below 5 nm.

[0041] Typical example:

[0042] Workpiece: 4-inch calcium fluoride wafer, requiring a microlens array with a processing cycle of 100μm and a curvature radius of 0.5mm.

[0043] Rigid contour grinding: Using a #3000 diamond contour grinding wheel, the linear speed was 30 m / s, the feed rate was 30 mm / min, the grinding depth was 5 μm / pass, and a total of 3 passes were performed, with a total time of approximately 10 minutes. After machining, the microstructure surface shape PV was 0.4 μm, and the damage layer depth was approximately 3 μm.

[0044] Magnetorheological precision grinding: Magnetorheological fluid (carbonyl iron powder 35 vol%), magnetic field strength 0.5 T, grinding head speed 1000 rpm, gap 1 mm, residence time 90 seconds. After precision grinding, the depth of the damaged layer was reduced to 150 nm, and the mid-frequency error RMS was 4 nm.

[0045] This step involves in-situ switching between rigid contour grinding and magnetorheological flexible grinding using the same contour grinding head with an embedded permanent magnet array. This utilizes the efficient contouring motion of the rigid grinding wheel to rapidly form the microstructure array, while a magnetorheological flexible ribbon with a controllable magnetic field uniformly thins the subsurface damage layer and suppresses mid-frequency errors, achieving a synergy between efficient forming and low-damage grinding. Rigid grinding ensures surface accuracy and processing efficiency, while magnetorheological fine grinding avoids secondary clamping errors caused by changing grinding heads. Simultaneously, the adaptability of the flexible ribbon prevents impact damage to the microstructure edges from the rigid grinding head. After this step, the subsurface damage layer thickness of the microstructure array can be controlled to below 200 nm, and the mid-frequency error is significantly reduced, providing a high-quality surface substrate for subsequent modification and polishing.

[0046] S200, after grinding is completed, magnetorheological fluid is supplied between the profile grinding head and the surface of the calcium fluoride wafer. The magnetic field generated by the permanent magnet array embedded in the profile grinding head causes the magnetorheological fluid to form a flexible ribbon. The flexible ribbon is attached to the tip of the profile grinding head to perform fine grinding on the sharp corner area to reduce the subsurface damage layer at the sharp corner.

[0047] In this embodiment, the magnetic field is most concentrated at the tip of the contour grinding head (i.e., the sharp protrusion that complements the sharp corner of the workpiece). The magnetorheological fluid is attracted and accumulates around the tip, forming a thin and dense flexible polishing medium layer. This ribbon-like layer is not simply coated, but is stably "adsorbed" onto the tip surface under the action of the magnetic field and moves with the grinding head.

[0048] When the profile grinding head moves its tip into the sharp corner area between the two semi-circular protrusions of the workpiece, the flexible ribbon attached to the tip comes into contact with the root of the corner. Driven by the rotation or reciprocating motion of the grinding head, the carbonyl iron powder and abrasive in the ribbon exert a shearing effect on the surface of the corner, thereby removing a small amount of material and thinning the subsurface damage layer at that location.

[0049] The geometry of the contour-following grinding head's tip causes the magnetic field lines generated by the permanent magnet array to converge at that point, forming a localized high-gradient magnetic field (reaching over 0.8T). Carbonyl iron powder in the magnetorheological fluid is strongly attracted to the tip, forming stable "ribbon" aggregates.

[0050] Because the ribbon is flexible, when the tip of the grinding head enters the root of a sharp corner, the ribbon can be flattened and deformed to fit tightly against the inner surface of the sharp corner (including the side bevels and the bottom corner). This is something that rigid grinding wheels or rigid grinding heads simply cannot achieve.

[0051] When the grinding head rotates or reciprocates, the ribbon slides relative to the sharp corner surface. The magnetic particles and polishing particles in the ribbon maintain a certain pressure under the magnetic field, producing a controllable micro-cutting and frictional effect on the sharp corner surface, thinning the damaged layer through a plastic removal mechanism without generating new brittle cracks.

[0052] After conventional contour grinding, a layer of insufficiently ground material often remains at the root of sharp corners (because the rounded corner at the bottom of a rigid grinding wheel cannot reach this area), and the damaged layer is relatively deep. By attaching a flexible ribbon to the tip of the grinding head, it can "reach" into the deepest part of the sharp corner for supplementary fine grinding, making the depth of the damaged layer uniform across the entire diameter. Magnetorheological fine grinding is a deterministic polishing process; the size and pressure of the ribbon can be precisely controlled, avoiding the periodic mid-frequency errors that occur near sharp corners, as seen in sub-aperture polishing. The flexible ribbon is "gentler" than a rigid grinding wheel, removing the damaged layer without rounding or flattening the sharp corners, thus achieving conformal machining.

[0053] Furthermore, in S200, by optimizing the magnet layout and magnetic field strength distribution of the permanent magnet array, the morphology and pressure distribution of the flexible ribbon formed by the magnetorheological fluid are controlled to achieve uniform thinning of the subsurface damage layer.

[0054] In magnetorheological flexible precision grinding, the morphology and pressure distribution of the flexible ribbon (i.e., the magnetorheological fluid aggregate solidified under the action of a magnetic field) determine the uniformity of material removal.

[0055] In this embodiment, by optimizing the magnet layout (magnet shape, arrangement, polarity) and magnetic field strength distribution of the permanent magnet array (adjusted by magnet size, position, or auxiliary electromagnetic coil), precise control of the ribbon shape (such as flat-top, arc, saddle shape) and pressure distribution (uniform, slightly higher in the middle, slightly higher at the edges) is achieved, thereby uniformly thinning the subsurface damage layer across the entire aperture range.

[0056] The specific implementation is as follows:

[0057] Magnet layout design for permanent magnet arrays:

[0058] The permanent magnet array is embedded in the profile grinding head substrate, with the grinding head working surface (magnetorheological working surface) facing the workpiece. The magnet material is neodymium iron boron (NdFeB, grade N42~N52), with a nickel-plated surface for corrosion protection. Three typical layouts are listed below, and those skilled in the art can select or combine them according to the workpiece size and damage layer distribution.

[0059] (1) Ring Halbach array – generates a flat-top magnetic field (for uniform thinning)

[0060] Structure: Multiple magnets are arranged circumferentially on the working surface of the grinding head. The magnetization direction of each magnet is rotated by a certain angle (e.g., 8 magnets, each rotated 45°), forming a Halbach array. This array generates a uniform, high-intensity magnetic field on one side of the working surface of the grinding head, while the magnetic field on the other side is almost zero.

[0061] Specific parameters: Magnet dimensions 10mm×10mm×5mm (length×width×height), with 8 to 12 magnets evenly placed along the circumference of the 50mm diameter grinding head. Under Halbach arrangement, the magnetic field strength on the working surface can reach 0.6 to 0.8T, and the magnetic field fluctuation within a φ30mm range at the center of the working surface is <±5%.

[0062] Ribbon shape: flat-topped, meaning the pressure at the center and edge of the ribbon is consistent, suitable for wafers with uniformly distributed damage layers.

[0063] (2) Inner and outer double ring array - generates a convex pressure distribution (used for cases where the central damage is deep)

[0064] Structure: An inner ring magnet (20mm in diameter) is installed on the inner side of the grinding head working surface, and an outer ring magnet (45mm in diameter) is installed on the outer side. The two sets of magnets have opposite polarities (the inner ring's N pole faces outward, and the outer ring's S pole faces outward, or vice versa). The combined magnetic field distribution can be changed by adjusting the size ratio or relative distance between the inner and outer ring magnets.

[0065] Specific parameters: 6 inner ring magnets (8mm×8mm×5mm), 12 outer ring magnets (10mm×10mm×5mm). The distance between the inner and outer rings is 5mm. After optimization, the magnetic field strength at the center of the working surface is 0.7T, and at the edge it is 0.5T, forming a "protruding" pressure distribution with a high center and low edge.

[0066] Ribbon shape: convex shape (high center pressure), suitable for scenarios where the depth of the damage layer in the center of the wafer is greater than that at the edge (such as when there is a lot of heat generation and deep damage in the center area after rigid grinding).

[0067] (3) Gradient array – generates tilted pressure distribution (for wafer tilt or unilateral damage)

[0068] Structure: The magnets are not uniformly distributed along the circumference of the grinding head, with one side having denser and stronger magnets and the other side having sparser and weaker magnets; or wedge-shaped magnets (thick at one end and thin at the other end) can be used.

[0069] Specific parameters: On the side of the wafer with deeper damage, the number of magnets is increased by 50% or the magnet thickness is increased from 5mm to 8mm. The magnet layout is optimized through finite element simulation (such as COMSOL, Ansys Maxwell) so that the magnetic field strength gradually changes from 0.3T on one side to 0.7T on the other side.

[0070] Ribbon shape: Inclined pressure distribution, suitable for asymmetrical damage caused by workpiece clamping at an incline or rigid grinding.

[0071] Methods for controlling magnetic field intensity distribution:

[0072] In addition to the fixed arrangement of permanent magnets, the following control methods can be introduced to achieve dynamic or semi-dynamic adjustment:

[0073] (1) Movable magnet structure

[0074] A slider or rotating disk is installed inside the grinding head, allowing some magnets to move radially. As the magnets approach the working surface, the local magnetic field intensifies. A servo motor controls the magnet position, adjusting the magnetic field distribution in real time.

[0075] (2) Auxiliary electromagnetic coil

[0076] A small electromagnetic coil (0.5mm wire diameter, 100-200 turns, 0-2A current) is added to the permanent magnet array. The additional magnetic field generated by the electromagnetic coil can be superimposed on the permanent magnet magnetic field to achieve fine adjustment. For example, based on the basic magnetic field (0.5T) provided by the permanent magnet, the energized coil adds 0.1T in the central region to form the desired pressure distribution.

[0077] (3) Magnetic flux guide plate

[0078] A magnetically conductive material (such as a pure iron plate) is placed between the working surface of the grinding head and the magnet. The shape of the guide plate is designed (such as a central hole or an edge groove) to change the spatial distribution of the magnetic field lines, so that the magnetic field strength changes according to a predetermined curve.

[0079] The measurement and calibration of ribbon morphology and pressure distribution can be performed using the following methods:

[0080] Magnetic field measurement: Use a gaussmeter (such as Lake Shore 425) to measure the magnetic field strength at different locations (center, half radius, edge) on the working surface of the grinding head, and draw a magnetic field distribution map.

[0081] Pressure distribution measurement: A pressure-sensing diaphragm or thin-film pressure sensor array is used. The sensor is placed between the grinding head and the simulated workpiece (flat glass), and the same pressure and rotation speed as during polishing are applied to measure the static and dynamic pressure distribution.

[0082] Matching and adjusting process parameters:

[0083] Based on the measured pressure distribution, adjust the following parameters to optimize thinning uniformity:

[0084] If the center pressure is too high (center removal is too fast): ① Reduce the size of the inner ring magnet or increase the distance between the inner ring and the working surface; ② Increase the viscosity of the magnetorheological fluid (e.g., increase the volume fraction of iron powder to 38%–40%) to make the ribbon harder and the pressure easier to disperse; ③ Reduce the grinding head speed (e.g., from 1000 rpm to 600 rpm) to reduce shear stress.

[0085] If the edge pressure is insufficient (insufficient edge removal): ① Increase the number or thickness of the outer ring magnets; ② Reduce the gap (e.g., from 1mm to 0.7mm) to increase the edge magnetic field; ③ Extend the edge dwell time (e.g., through CNC path planning, make the grinding head stay in the edge area for 10% to 20% longer).

[0086] It should be noted that, based on the above description, and in combination with finite element magnetic field simulation software and conventional pressure testing methods, those skilled in the art can design permanent magnet array layouts suitable for different sizes and damage distribution characteristics, and achieve uniform thinning.

[0087] In this embodiment, by optimizing the magnet layout of the permanent magnet array (such as a ring-shaped Halbach array, an inner and outer double-ring array, or a gradient array) and the magnetic field strength distribution (adjustable by movable magnets, auxiliary electromagnetic coils, and guide plates), this solution achieves precise control over the morphology (flat-top, convex, inclined, etc.) and pressure distribution of the flexible ribbon formed by the magnetorheological fluid. Compared to a single uniform magnetic field, this method can customize the magnetic field according to the actual distribution of the depth of the damage layer on the wafer surface after rigid grinding (deep at the center and shallow at the edges, symmetrical or asymmetrical), so that the flexible ribbon is subjected to uniform or on-demand shear stress across the entire diameter range, thereby achieving uniform thinning of the subsurface damage layer. Experiments show that the uniformity of the damage layer thickness after optimization can be improved from ±40% to within ±10%, while avoiding excessive removal of the microstructure edges or insufficient removal of the central region, significantly improving the consistency and yield of wafer-level processing. This technical solution has a compact structure, is easy to integrate into the contour grinding head, and can achieve flexible control of the magnetic field distribution without replacing the grinding head.

[0088] Furthermore, in S100, a material removal function model is established, and the formulation of the magnetorheological fluid and the grinding process parameters are optimized based on the depth distribution of the subsurface damage layer after rigid contour grinding, so as to achieve quantitative removal of the damage layer.

[0089] In magnetorheological flexible precision grinding, the material removal rate is closely related to the magnetic field strength, gap, relative velocity, rheological properties of the magnetorheological fluid, and residence time. Setting parameters solely based on experience can easily lead to insufficient removal of the damaged layer (residual cracks affecting subsequent processes) or excessive removal (damaging surface accuracy and wasting processing time). In this embodiment, a material removal function model is established, expressing the removal rate as a function of controllable process parameters. Using the subsurface damage layer depth distribution after rigid grinding as input, the required residence time distribution or magnetic field distribution is calculated through deconvolution. Ultimately, the magnetorheological fluid formulation and process parameters are optimized to achieve quantitative and uniform removal of the damaged layer.

[0090] The specific implementation is as follows:

[0091] Step 1: Obtain the removal function of magnetorheological flexible precision grinding

[0092] Magnetorheological flexible precision grinding belongs to sub-aperture polishing. Its material removal characteristics need to be described by a removal function, rather than a simple constant removal rate. The removal function reflects the thickness distribution of material removed by the grinding head at various points on the workpiece surface per unit time. Magnetorheological removal functions are generally asymmetric D-shaped, comet-shaped, or horseshoe-shaped.

[0093] The specific calibration method is as follows:

[0094] A planar sample of calcium fluoride was prepared, with a surface flatness better than λ / 10.

[0095] Set process parameters such as magnetorheological fluid formulation (e.g., carbonyl iron powder volume fraction 35%, base fluid viscosity 0.3 Pa·s), magnetic field strength (e.g., 0.5 T), grinding head speed (e.g., 1000 rpm), and processing gap (e.g., 1 mm).

[0096] The grinding head is controlled to stay at a fixed position on the sample block for a certain period of time (e.g., 60 seconds) to form a small pit.

[0097] The three-dimensional morphology of the pit is measured using a white light interferometer or a profilometer, and its depth distribution is the removal function under this condition. The removal function is typically characterized by peak removal rate (nm / s) and feature width (mm).

[0098] By changing the magnetic field strength, rotation speed, gap, or magnetorheological fluid formula (which needs to be changed before processing), repeat the above steps to establish a removal function library under different parameter combinations.

[0099] Step 2: Measure the depth distribution of the subsurface damage layer after contour grinding.

[0100] After contour grinding, the depth of the subsurface damage layer may vary at different locations on the wafer (for example, the central region accumulates more grinding heat, resulting in deeper damage). Measurement methods can include:

[0101] Non-destructive testing: Optical coherence tomography (OCT) is used to scan the entire wafer aperture, and the depth distribution of subsurface damage is obtained by analyzing the backscattered signal. This method is fast, non-destructive, and suitable for online or in-situ inspection.

[0102] Fluorescence method: The wafer is immersed in a fluorescent dye. Damage cracks will absorb the dye and emit fluorescence under ultraviolet excitation. The depth of damage can be estimated by observing the fluorescence intensity through a confocal microscope. This method has high sensitivity and is suitable for rapid detection in engineering applications.

[0103] Destructive calibration: A sample is cut with a focused ion beam (FIB), and the crack depth in the cross-section is directly observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). This method offers the highest accuracy but is time-consuming and destructive to the sample, and is only used to calibrate the results of non-destructive testing.

[0104] In practice, full-aperture OCT scanning is preferred to obtain a depth distribution map of the damaged layers. The grid spacing can be set as needed (e.g., 5mm×5mm).

[0105] Step 3: Calculate the residence time distribution to achieve quantitative removal.

[0106] Objective: To increase the initial damage layer depth Thin to target value (e.g., 200nm), the thickness to be removed is:

[0107] ;

[0108] (x, y) represent the planar coordinates of the wafer surface. The material removal process of magnetorheological grinding can be approximated as a linear time-invariant system: Removed surface = Initial surface − (Removal function × Dwell time). Here, × denotes two-dimensional convolution. The removal function is known. and target removal amount The dwell time distribution is solved using deconvolution algorithms (such as Richardson-Lucy iteration or Fourier transform). .

[0109] The calculated residence time distribution is converted into CNC code to control the scanning speed of the grinding head on the wafer surface: regions with longer residence times have slower scanning speeds, and vice versa. The magnetorheological fluid formulation or magnetic field strength is not changed during processing; different removal amounts at different locations are achieved solely through speed variations.

[0110] After processing, the depth of the residual damage layer is measured again using OCT or fluorescence methods. If the depth across the entire diameter is less than... If the result is satisfactory, then it is acceptable; otherwise, based on the residual error distribution, the dwell time for supplementary processing is iteratively calculated until the requirements are met.

[0111] The above method enables the quantitative and controllable removal of subsurface damage layers by magnetorheological flexible precision grinding, avoiding both residual cracks and excessive grinding that could damage the surface shape.

[0112] S300 uses a gas cluster ion beam to irradiate the surface of a calcium fluoride wafer treated by S100. By controlling the process parameters of the gas cluster ion beam, a highly uniform amorphous modified layer is formed on the surface of the calcium fluoride wafer. The process parameters include gas source type, cluster size, incident angle and irradiation dose. The minimum effective thickness of the amorphous modified layer is determined based on the depth of the subsurface damage layer generated by grinding.

[0113] In this embodiment, a gas cluster ion beam (GCIB) processing system (e.g., commercially available equipment or a custom system) can be used. This equipment includes: a gas supply unit, a cluster generation chamber (forming clusters through adiabatic expansion), a cluster size selector (such as a rotating disk mass separator), accelerating electrodes, an ion beam scanning system, and a vacuum chamber. The gas clusters consist of hundreds to thousands of atoms / molecules, the accelerating voltage is 10–70 kV, and the cluster size (average number of atoms per cluster) is adjustable from 500 to 5000 atoms / cluster.

[0114] Furthermore, in S200, the gas source type includes pure inert gas clusters or mixed gas clusters containing reactive gases; the cluster size is controlled within a preset range by a cluster size selector; the incident angle is zoned and adjusted according to the morphological characteristics of the sharp corner region, column top, and sidewall of the microstructure array; the irradiation dose is quantitatively controlled according to the preset amorphous layer thickness to ensure that the amorphous layer thickness at the sharp corner meets the minimum effective thickness for covering the damaged layer.

[0115] Pure inert gas (such as Ar): The surface of calcium fluoride is disrupted by physical bombardment, forming an amorphous layer. Suitable for scenarios requiring rapid amorphization and moderate surface roughness.

[0116] Mixed gases (such as Ar + O2): O2 has a slight chemical etching effect, which can "polish" the surface while bombarding, making the amorphous layer smoother. Suitable for scenarios with high surface quality requirements.

[0117] In practice, you can choose according to your needs. For example, if you want efficiency, use pure Ar, and if you want a smooth finish, use Ar+O2 (e.g., 90% Ar+10% O2).

[0118] Cluster size can be controlled by a size selector. Clusters are particles formed by hundreds to thousands of atoms huddled together. The larger the size, the higher the energy carried by each cluster, the deeper it can penetrate during bombardment, and the thicker the amorphous layer.

[0119] Clusters are selected by controlling the electromagnetic field. The size is set between 1000 and 3000 atoms per cluster, depending on the required amorphous layer thickness.

[0120] The incident angle can be adjusted according to the shape of the microstructure, which includes column tops (horizontal planes), sidewalls (sloping planes), and valley bottoms (recesses). The effects of striking the microstructure with a vertical ion beam (0°) versus an angled beam (30°–60°) are different.

[0121] Column top: Vertical incidence, concentrated energy, and high amorphization efficiency.

[0122] Valley bottom: Usually vertical or at a small angle of incidence, but be aware of the blocking effect.

[0123] By rotating the workpiece stage or tilting the ion gun, different incident angles can be used for different areas to ensure that all surfaces are uniformly modified.

[0124] The irradiation dose can be quantitatively controlled based on the thickness of the target amorphous layer. The higher the dose, the thicker the amorphous layer; however, excessive bombardment can sputter away material. Experimental calibration can be performed first—under the same conditions, bombard the sample with different doses and measure the amorphous layer thickness to obtain a dose-thickness curve. Then, based on the desired amorphous layer thickness (e.g., 50 nm), find the required dose from the curve and set it directly.

[0125] By flexibly combining these four parameters, amorphous modified layers with consistent thickness and smooth surfaces can be obtained at different locations in the microstructure (pillar top, sidewall, valley bottom). Mixed gas makes the surface smoother, size selection allows for controllable thickness, partitioned angles ensure full coverage, and quantitative dosage avoids waste. Ultimately, the thickness deviation of the amorphous layer across the entire aperture is achieved to ≤±10%, providing an ideal interface for subsequent polishing.

[0126] Furthermore, in S200, an interaction model between the gas clusters and the calcium fluoride surface is established through molecular dynamics simulation, revealing the molecular dynamics process of atomic migration and bonding evolution on the calcium fluoride surface under ion beam irradiation of the gas clusters, and predicting the formation quality of the amorphous modified layer.

[0127] When gas clusters bombard the surface of calcium fluoride, the movement of atoms, the breaking of chemical bonds, and the formation of amorphous layers occur at the picosecond and nanoscale, processes that are impossible to observe experimentally. Molecular dynamics simulations act like a "super microscope," allowing the trajectory of each atom to be clearly seen on a computer.

[0128] By building models of calcium fluoride crystals and Ar clusters, and setting different cluster sizes, energies, and incident angles, simulations were run and the following could be observed:

[0129] Atomic migration: When clusters collide, surface atoms are "squeezed" apart, and the original lattice arrangement is disrupted.

[0130] Bonding evolution: The Ca-F bond breaks, atoms rearrange, and a disordered amorphous structure is formed.

[0131] Temperature and pressure distribution: The local temperature at the impact point soars to several thousand K, and the pressure is extremely high, which explains why the crystal lattice is destroyed.

[0132] Sputtering and damage: Some atoms are ejected (sputtering), and the rest form an amorphous layer. Simulations can provide the thickness, density, defect density, etc. of the amorphous layer.

[0133] Molecular dynamics simulations allow for the identification of optimal cluster ion beam parameters without extensive trial-and-error experiments. The simulations reveal the microscopic mechanisms of atomic migration and bonding evolution, helping to understand why certain parameters can lead to defects (such as cracks and voids) or inhomogeneities in amorphous layers. Translating these simulation results into process guidelines can significantly improve the density, thickness uniformity, and bonding strength of amorphous layers to the substrate, thereby ensuring modification quality and laying a solid foundation for subsequent polishing.

[0134] Furthermore, in S200, an amorphous layer distribution prediction model is established covering the top of the microstructure array pillars, the sidewalls, and the sharp bottom of the valleys, in order to determine the minimum effective thickness of the amorphous modified layer at different locations.

[0135] The microstructure has a column top (top plane), sidewalls (slopes), and valley bottom (bottom depression). When a gas cluster is bombarded by an ion beam, the energy and quantity of ions received at different locations vary.

[0136] Column top: Directly facing the ion beam, resulting in the strongest bombardment.

[0137] Sidewalls: Slanted towards the ion beam, resulting in energy dispersion and weaker bombardment.

[0138] Valley bottom: May be shielded, weakest from bombardment.

[0139] The result is uneven amorphous layer thickness: thick at the top of the pillars, medium on the sidewalls, and thin at the bottom. If the amorphous layer at the bottom of the valley is too thin, it cannot cover the grinding damage, causing problems in subsequent polishing. Therefore, it is necessary to build a model to calculate the amorphous layer thickness at each location in advance, ensuring that even the thinnest part is thick enough.

[0140] Amorphous layer distribution prediction models can be established through experimental calibration and geometric correction.

[0141] Calibration on planar samples: Planar calcium fluoride is bombarded with different doses, and the thickness of the amorphous layer is measured to obtain a "dose-thickness" standard curve. For example: dose →Thickness .

[0142] Considering the effect of the incident angle: With oblique incidence, the actual number of clusters received per unit area decreases (projection effect). Thickness ≈ ( (It is the angle between the incident direction and the surface normal). Sidewall Larger amorphous layers result in lower effective doses and thinner amorphous layers.

[0143] Considering the blocking effect: For deep trenches or high aspect ratio structures, the valley floor may not receive any ion beam at all. The actual proportion of ions reaching the valley floor needs to be calculated based on the microstructure's geometry (width, depth, sidewall inclination). This can be achieved using a simple geometric optics model or Monte Carlo simulation.

[0144] Final model form:

[0145] ;

[0146] ;

[0147] ;

[0148] in, , and These represent the thicknesses of the amorphous layer formed at the top of the column (the top plane of the microstructure), the sidewalls (the slope of the microstructure), and the valley bottom (the depression at the bottom of the microstructure), respectively.

[0149] After S100, the subsurface damage layer depth may vary at different locations (deeper damage at the pillar top, shallower damage at the sidewalls and valley bottom). However, to be on the safe side, the depth of the deepest damage can be used as a benchmark, requiring the amorphous layer thickness to be greater than the damage depth at that location. The minimum effective thickness at different locations can be determined through the following steps:

[0150] Measure or estimate the depth of residual damage layer at three locations: top of column, sidewall, and bottom of valley (e.g., 150 nm at top of column, 100 nm at sidewall, and 80 nm at bottom of valley).

[0151] The model predicts the thickness of the amorphous layer at three locations under a certain process parameter.

[0152] Ensure that the thickness of the amorphous layer at the thinnest location (usually the valley) is greater than the depth of damage at that location (allow a safety margin of 20%).

[0153] If the valley floor thickness is insufficient, increase the overall dose or use multi-angle irradiation (first irradiate the top of the column and the valley floor vertically, then irradiate the side walls at an angle).

[0154] This method allows for precise design of process parameters, ensuring that the amorphous layers at the top of the pillars, sidewalls, and bottom of the valleys are all greater than their respective required thresholds, while avoiding excessive thickness. This approach can completely cover the damaged layer while saving processing time, making it particularly suitable for complex microstructure arrays on high aspect ratio, large-size wafers.

[0155] S400 employs a contoured resin wheel for enzymatic hydrolysis-assisted conformal polishing of the amorphous modified layer. The surface contour of the contoured resin wheel matches the microstructure array, and the surface of the resin wheel contains hydroxyl groups. During the polishing process, the hydroxyl groups on the surface of the resin wheel undergo a dehydration condensation reaction with the calcium fluoride amorphous modified layer, achieving material removal in a chemically bonded manner to obtain a near-zero damage surface.

[0156] Furthermore, in S300, the surface contour of the contoured resin wheel matches the sharp corner shape of the microstructure array, and can completely fit the sharp corner area to achieve conformal polishing at the sharp corner.

[0157] High-modulus polymer materials, such as polyurethane (PU), polyester, or epoxy resin, are selected. The working surface of the resin wheel needs to be machined into a contour that complements the calcium fluoride microstructure array. For example, for an outwardly convex cylindrical microlens array, the working surface of the resin wheel is machined into an inwardly concave cylindrical array; for a grating structure, it is machined into a complementary tooth profile. Contouring can be performed using single-point diamond precision turning (accuracy PV ≤ 0.5 μm) or precision injection molding.

[0158] The conformal resin wheel is immersed in a buffer solution containing hydrolytic enzymes. Commonly used hydrolytic enzymes include: lipase (hydrolyzes polyester resins), protease (hydrolyzes polyamide resins), or cellulase (suitable for cellulose-based resins).

[0159] During the treatment, enzymes catalyze the hydrolysis of ester or amide bonds on the resin surface, generating a large number of polar functional groups such as hydroxyl groups (-OH). The treated resin wheel is rinsed three times with deionized water to remove residual enzyme solution, and then dried at room temperature or by blowing with nitrogen.

[0160] A contoured resin wheel is mounted on the spindle of a precision polishing machine, which features axial pressure control, speed adjustment, and oscillation functions. Calcium fluoride wafers are fixed to a vacuum chuck or bonded to a stage.

[0161] This step utilizes an enzymatically hydrolyzed conformal resin wheel. Under conditions free from abrasives and chemical corrosion, the hydroxyl groups on the resin wheel surface undergo a dehydration condensation reaction with the amorphous layer of calcium fluoride, achieving molecular or atomic-scale material removal through a chemically bond-dominated mechanism. This removal mechanism completely avoids the subsurface damage introduced by abrasive abrasion in traditional mechanical or chemical mechanical polishing. Furthermore, since the reaction occurs only at the interface between the amorphous layer and the resin wheel, when the amorphous layer is removed and crystalline calcium fluoride is exposed, the low chemical activity of the crystalline calcium fluoride automatically reduces the dehydration condensation reaction rate, creating a self-terminating effect and preventing over-polishing. The conformal resin wheel ensures the surface fidelity of the microstructure array, allowing the polished microlenses or gratings to maintain their designed contours. Ultimately, a near-zero-damage surface with a surface roughness Ra < 0.5 nm is obtained, meeting the stringent requirements of high-end applications such as deep ultraviolet lithography and high-energy laser systems for calcium fluoride micro-optical components.

[0162] Furthermore, in S300, the resin wheel is a contoured resin wheel whose surface profile matches the microstructure array on the surface of the calcium fluoride wafer to achieve conformal polishing.

[0163] Ordinary polishing wheels are flat and used to polish flat surfaces. However, the surface of calcium fluoride wafers is an array of microstructures (such as raised microlenses, or pits and gratings). If a flat wheel is used for polishing, it can only polish the top of the raised parts, and cannot reach the recessed parts at all. After polishing, the structure is deformed and the edges collapse.

[0164] Contouring resin wheels are made by machining the surface of a resin wheel into a shape that complements the microstructure of the workpiece. If the workpiece is a convex lens array, the surface of the resin wheel is made into an array of pits, with the radius of curvature and spacing of the pits perfectly matching those of the lenses. If the workpiece is a grating groove, the surface of the resin wheel is made into an array of convex teeth, with the tooth shape complementing that of the groove. In this way, during polishing, the resin wheel acts like a "stamp," perfectly conforming to every undulation of the workpiece, simultaneously contacting the top of the column, the sidewalls, and the bottom of the valley.

[0165] The use of a contour-following resin wheel enables the polishing tool to achieve complementary morphology and full-area contact with the microstructured workpiece. Compared to flat wheel or spherical polishing, this method can simultaneously perform conformal polishing on the pillar tops, sidewalls, and valleys of the microstructure, avoiding local over-polishing or under-polishing, maintaining the surface accuracy of the original microstructure, and achieving sub-nanometer level surface roughness. It is particularly suitable for high-density, high aspect ratio wafer-level micro-optical components, significantly improving yield and consistency.

[0166] Furthermore, in S300, by optimizing the process conditions of enzyme hydrolysis treatment, including the type and concentration of enzyme, reaction temperature and pH value, the degree of hydroxylation on the surface of the calcium fluoride amorphous modified layer is controlled, thereby controlling the dehydration condensation reaction rate and the material removal rate.

[0167] The dehydration condensation reaction (chemical bond removal) between the resin wheel and calcium fluoride has a reaction rate that depends on the number of hydroxyl groups (-OH) on the surface of the resin wheel. The more hydroxyl groups, the faster the chemical reaction and the faster the material removal. The number of hydroxyl groups can be controlled by enzymatic hydrolysis.

[0168] More hydroxyl groups → more vigorous dehydration condensation reaction → thicker amorphous layer removed per unit time → higher removal rate (e.g., 5 nm / min).

[0169] Fewer hydroxyl groups → slower reaction → lower removal rate (e.g., 1 nm / min).

[0170] When rapid removal of thick amorphous layers is required: use high enzyme concentration, optimal temperature / pH, and highly hydroxylated materials to achieve a high removal rate.

[0171] When precise control is required and over-discarding should be avoided: use low enzyme concentration or low temperature to reduce the removal rate and facilitate endpoint control.

[0172] It should be noted that those skilled in the art can use existing technology to optimize the ratio of resin wheel material to enzyme, control the reaction temperature and pH value, regulate the degree of hydroxylation on the surface of the resin wheel, and thus control the dehydration condensation reaction rate and material removal rate, which will not be elaborated here.

[0173] By systematically adjusting the resin wheel material, enzyme concentration, temperature, and pH value, the degree of hydroxylation on the resin wheel surface can be precisely controlled, thereby regulating the rate of dehydration condensation reaction and ultimately achieving continuously adjustable material removal rates (e.g., 1~5 nm / min). This ensures sufficient processing efficiency while reducing the rate near the polishing endpoint to avoid over-polishing, ensuring uniform removal of the amorphous layer across the entire aperture without damaging the substrate. This method does not change the polishing equipment; it can be achieved solely through treatment solution formulation and temperature control, making it simple, low-cost, and easy to implement.

[0174] Furthermore, in S300, the chemical bond formed in the chemical bond-dominated removal method is a Ca-OC bond.

[0175] Calcium fluoride (CaF2) has calcium ions (Ca) on its surface. 2+ ) and fluoride ions (F - After enzymatic hydrolysis, the resin wheel generates a large number of hydroxyl groups (-OH, i.e., oxygen atoms bonded to hydrogen) on its surface. When the resin wheel comes into contact with the amorphous layer of calcium fluoride, the oxygen atoms in the hydroxyl groups will chemically coordinate with the calcium ions on the surface of the calcium fluoride, removing a small molecule (water) and forming a Ca-OC chemical bond.

[0176] This removal method is abrasive-free, completely avoiding the cracks, dislocations, and residual stress introduced by abrasive scratching in traditional polishing. Since the Ca-OC bonds only occur between the resin wheel and the amorphous calcium fluoride, when the amorphous layer is completely removed and the crystalline calcium fluoride is exposed, the crystalline surface has low reactivity and is less prone to forming a large number of Ca-OC bonds. This causes the removal rate to automatically decrease, creating a self-terminating effect that protects the substrate from over-polishing. Ultimately, an atomically smooth, near-zero-damage surface is obtained.

[0177] Furthermore, although the steps of the method in this disclosure are described in a specific order in the accompanying drawings, this does not require or imply that the steps must be performed in that specific order, or that all the steps shown must be performed to achieve the desired result. Additional or alternative steps may be omitted, multiple steps may be combined into one step, and / or a step may be broken down into multiple steps.

[0178] While specific embodiments of the invention have been described in detail by way of examples, those skilled in the art should understand that the examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art should also understand that various modifications can be made to the embodiments without departing from the scope and spirit of the invention.

Claims

1. A method for fabricating wafer-level calcium fluoride micro-optical elements, characterized in that, The wafer-level calcium fluoride micro-optical element has a microstructure array on its surface, the microstructure array consisting of multiple semi-circular protrusions arranged side by side, with recessed sharp corners formed between adjacent semi-circular protrusions; the method includes the following steps: S100, a contour grinding head is used to perform contour grinding on the surface of a calcium fluoride wafer; the surface contour of the contour grinding head is complementary to the target microstructure array, the contour grinding head has a tip corresponding to the sharp corner, and the tip of the contour grinding head can enter the sharp corner region; the contour grinding head is controlled to move along the generatrix direction of the microstructure array to form a microstructure array on the surface of the calcium fluoride wafer. S200, after grinding is completed, magnetorheological fluid is supplied between the profile grinding head and the surface of the calcium fluoride wafer. The magnetic field generated by the permanent magnet array embedded in the profile grinding head causes the magnetorheological fluid to form a flexible ribbon. The flexible ribbon is attached to the tip of the profile grinding head to perform fine grinding on the sharp corner area to reduce the subsurface damage layer at the sharp corner. S300 uses a gas cluster ion beam to irradiate the surface of a calcium fluoride wafer treated by S100. By controlling the process parameters of the gas cluster ion beam, a highly uniform amorphous modified layer is formed on the surface of the calcium fluoride wafer. The process parameters include gas source type, cluster size, incident angle, and irradiation dose. The minimum effective thickness of the amorphous modified layer is determined based on the depth of the subsurface damage layer generated by grinding. The gas source type includes pure inert gas clusters or mixed gas clusters containing reactive gases. The cluster size is controlled within a preset range by a cluster size selector. The incident angle is zoned and controlled according to the morphological characteristics of the sharp corner regions, pillar tops, and sidewalls of the microstructure array. The irradiation dose is quantitatively controlled according to the preset amorphous layer thickness to ensure that the amorphous layer thickness at the sharp corners meets the minimum effective thickness for covering the damage layer. S400 employs a contoured resin wheel for enzymatic hydrolysis-assisted conformal polishing of the amorphous modified layer. The surface contour of the contoured resin wheel matches the microstructure array, and the surface of the resin wheel contains hydroxyl groups. During the polishing process, the hydroxyl groups on the surface of the resin wheel undergo a dehydration condensation reaction with the calcium fluoride amorphous modified layer, achieving material removal in a chemically bonded manner to obtain a near-zero damage surface.

2. The fabrication method of wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S200, by optimizing the magnet layout and magnetic field strength distribution of the permanent magnet array, and controlling the morphology and pressure distribution of the flexible ribbon formed by the magnetorheological fluid, uniform thinning of the subsurface damage layer is achieved.

3. The fabrication method of wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, A material removal function model was established, and the formulation of magnetorheological fluid and grinding process parameters were optimized based on the depth distribution of the subsurface damage layer after contour grinding to achieve quantitative removal of the damage layer.

4. The fabrication method of wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S300, an interaction model between gas clusters and the calcium fluoride surface is established through molecular dynamics simulation, revealing the molecular dynamics process of atomic migration and bonding evolution on the calcium fluoride surface under gas cluster ion beam irradiation, and predicting the formation quality of the amorphous modified layer.

5. The fabrication method of wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S300, an amorphous layer distribution prediction model is established covering the top of the microstructure array pillars, the sidewalls, and the bottom of the valleys, in order to determine the minimum effective thickness of the amorphous modified layer at different locations.

6. The fabrication method of the wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S400, the surface contour of the contoured resin wheel matches the sharp corner shape of the microstructure array, and can completely fit the sharp corner area to achieve conformal polishing at the sharp corner.

7. The fabrication method of wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S400, by optimizing the process conditions of enzyme hydrolysis treatment, including the type and concentration of enzymes, reaction temperature and pH value, the degree of hydroxylation on the surface of the calcium fluoride amorphous modified layer is controlled, thereby controlling the dehydration condensation reaction rate and the material removal rate.

8. The fabrication method of the wafer-level calcium fluoride micro-optical element according to claim 1, characterized in that, In S400, the chemical bond formed in the chemical bond-dominated removal method is a Ca-OC bond.

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