High-surface-shape-precision sheet optical element machining method based on double-face polishing

Abstract

The invention discloses a high-surface-shape-precision sheet optical element machining method based on double-face polishing. The high-surface-shape-precision sheet optical element machining method comprises the following steps that two first optical sheets with the same thickness and the radius-thickness ratio larger than 10 are machined; preparing a sacrificial layer on one surface of each first optical thin sheet; enabling the two first optical sheets to form a composite bonding body through the sacrificial layer; double-sided thinning and chemical mechanical polishing are carried out on the outer surface of the composite bonding body, so that the total thickness of the composite bonding body is reduced to the target thickness, and meanwhile, the preset surface shape precision and surface roughness requirements are met; carrying out de-bonding on the polished composite bonding body, and separating to obtain two independent second optical sheets; and removing sacrificial layer residues on the surface of the second optical sheet to obtain the final sheet optical element with high surface shape precision. The problems that in the double-face polishing process of the thin sheet optical element, the thin sheet optical element is prone to fragmentation, and high surface shape precision machining is difficult to achieve can be solved, and thin sheet optical element machining with high surface shape precision, high yield and high efficiency is achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of precision optical processing, and particularly relates to a high-surface-accuracy wafer optical element processing method based on double-sided polishing. BACKGROUND

[0002] Wafer optical elements have important applications in high-end equipment and scientific research fields due to their ultra-thin structure and large diameter-thickness ratio. In aerospace and national defense, light and thin windows and filters are important elements of airborne / spaceborne optical systems; in high-end scientific research devices, large-diameter thin quartz plates are important components of particle detectors; in laser technology, Yb:YAG crystal wafers are core elements for achieving high-power laser output as disc laser gain media; in the semiconductor industry, thin film windows and filters of extreme ultraviolet (EUV) lithography machines are beneficial to reducing light absorption and equipment size and weight; in addition, wafer optical elements also have important applications in emerging fields such as flexible electronics and photonics, terahertz technology, and are key elements supporting the development of high-end equipment, cutting-edge research and new generation information technology, with broad application prospects and market potential.

[0003] Compared with traditional optical elements, wafer optical elements have a large diameter-thickness ratio due to their extremely thin thickness (tens to hundreds of microns) and large diameter, poor rigidity, and are prone to stress deformation, which brings great challenges to their ultra-precision optical processing. Chemical mechanical polishing (CMP) is the mainstream technology for obtaining high-surface-accuracy and ultra-smooth non-damage surface optical elements. Compared with traditional optical elements with moderate diameter-thickness ratio, the chemical mechanical polishing surface of large diameter-thickness ratio wafer optical elements is directly affected by the stress of the upper disc, which easily leads to uncontrollable surface of the wafer element, so the single-sided upper disc chemical mechanical polishing scheme is difficult to achieve high-surface-accuracy processing of wafer optical elements. Currently, the double-sided polishing scheme has become the main method for processing wafer optical elements because it does not need to bond the upper and lower discs, avoiding the surface distortion caused by the bonding stress of the wafer element. In the double-sided polishing process, the workpiece is placed in the carrier hole of a device called "star wheel", which rotates around the polishing machine axis while rotating between the upper and lower polishing discs, thereby driving the wafer optical element to move along a specific polishing path between the upper and lower polishing discs, and finally achieving high-precision double-sided synchronous polishing of the workpiece.

[0004] However, when the thickness of the wafer optical element is reduced to the order of hundreds of microns or even sub-hundreds of microns, and the diameter-thickness ratio is greater than 50, the traditional double-side polishing scheme exhibits a series of deficiencies and defects in the processing of wafer elements: first, when the thickness of the wafer element is significantly reduced, its structural stiffness (ability to resist deformation) will decrease significantly, and the edge of the wafer optical element will continuously collide with the inner wall of the rigid star wheel during the polishing process, which will easily cause the edge of the element to collapse, crack, or even break; second, in order to ensure effective contact between the wafer element and the upper and lower polishing discs during double-side polishing, the thickness of the star wheel must be slightly smaller than the thickness of the workpiece, so when processing ultra-thin workpieces, the star wheel itself is also made very thin, which is low in strength and easy to deform and damage, further affecting the processing stability and making it difficult to achieve high surface shape accuracy processing of wafer elements. In the prior art, a scheme of using a composite star wheel (such as embedding a plastic or rubber buffer layer in the inner wall of a blue steel metal star wheel) is usually used to alleviate the collision problem, but this scheme has limited effect on wafer elements with a diameter-thickness ratio of hundreds of microns or less, as the edge strength of the wafer element is too low, and slight collision can still cause failure; at the same time, the insufficient strength of the ultra-thin star wheel makes the processing process unstable, with low yield, and it is impossible to achieve high surface shape accuracy and high efficiency wafer optical element processing. Therefore, there is an urgent need for a processing method that can solve the problems of wafer optical elements in the double-side polishing process, such as easy breakage, poor process stability, and difficulty in achieving high surface shape accuracy processing. SUMMARY

[0005] The purpose of the present application is to provide a high surface shape accuracy wafer optical element processing method based on double-side polishing, which can solve the problems of wafer optical elements in the double-side polishing process, such as easy breakage and difficulty in achieving high surface shape accuracy processing, and achieve high surface shape accuracy, high yield, and high efficiency wafer optical element processing.

[0006] To achieve the above-mentioned purpose, the present application provides a high surface shape accuracy wafer optical element processing method based on double-side polishing, comprising the following steps:

[0007] S1: According to the preset requirements of surface shape accuracy and surface roughness, process two first optical wafers with the same thickness and a diameter-thickness ratio greater than 10;

[0008] S2: Prepare a sacrificial layer on one surface of each first optical wafer;

[0009] S3: Bond the two first optical wafers face to face through the sacrificial layer to form a composite bonded body;

[0010] S4: Double-side thinning and chemical mechanical polishing are performed on the two outer surfaces of the composite bonded body to reduce the total thickness of the composite bonded body to the target thickness, while achieving the preset surface shape accuracy and surface roughness requirements;

[0011] S5: the polished composite bonding body is debonded to separate two independent second optical wafer;

[0012] S6: removing the residual of the sacrificial layer on the surface of the second optical wafer to obtain a final high surface shape precision wafer optical element.

[0013] As a further scheme of the present application: in step S1, the first optical wafer is obtained by double-sided chemical mechanical polishing processing, and the surface shape precision PV value is better than λ / 10@632.8nm, and the surface roughness Ra value is better than 0.5nm.

[0014] As a further scheme of the present application: in step S2, the material of the sacrificial layer is one of magnesium fluoride, silicon dioxide and photoresist, and the thickness of the sacrificial layer is 1nm to 100nm.

[0015] As a further scheme of the present application: in step S3, the face-to-face bonding is realized by using a normal temperature bonding process.

[0016] As a further scheme of the present application: in step S4, double-sided chemical mechanical polishing is used, and the thickness of the wandering star wheel is less than the final target thickness of the composite bonding body and greater than the target thickness of the second optical wafer.

[0017] As a further scheme of the present application: in step S5, one of the methods of thermal slip method, laser irradiation method and chemical etching method is used for debonding.

[0018] As a further scheme of the present application: in step S6, ion beam etching or wet etching is used to remove the residual of the sacrificial layer.

[0019] As a further scheme of the present application: the thickness of the second optical wafer is 50μm to 300μm, and the diameter-thickness ratio is greater than 50.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] Significantly improve the processing yield: by temporarily bonding two wafer optical elements into a thick workpiece for double-sided optical processing, the bending and impact resistance of the workpiece during double-sided polishing is greatly enhanced, which is beneficial to solve the problem of cracking and fragmentation in the traditional double-sided polishing process.

[0022] Improve the surface shape precision and surface quality: the combination of thick workpiece and thick wandering star wheel makes the polishing process more stable, and the pressure and movement are more uniform, which can obtain high-quality wafer elements with high surface shape precision (PV better than λ / 10@632.8nm) and low surface roughness (Ra better than 0.5nm).

[0023] High processing efficiency: the final finishing of two components can be completed at the same time in one polishing process, and the production efficiency is significantly improved compared with single piece processing method.

[0024] Wide application range: the method is suitable for processing thin sheet optical elements of laser crystals, glass, silicon, silicon carbide, sapphire and other materials. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The figure is a process flow diagram of the method of the application.

[0026] Figure 2 The figure is an interference fringe pattern diagram of the final surface of the Si thin sheet optical element after debonding, measured by a laser interferometer based on the high surface shape precision thin sheet optical element processing method based on double-sided polishing,

[0027] Figure 3 The figure is a roughness parameter diagram of the final surface of the Si thin sheet optical element after debonding, measured by a white light interferometer based on the high surface shape precision thin sheet optical element processing method based on double-sided polishing.

[0028] Figure 4 The figure is an interference fringe pattern diagram of the final surface of the Si thin sheet optical element processed based on the traditional double-sided polishing scheme, measured by a laser interferometer,

[0029] Figure 5 The figure is a roughness parameter diagram of the final surface of the Si thin sheet optical element processed based on the traditional double-sided polishing scheme, measured by a white light interferometer.

[0030] Figure 6 The figure is an interference fringe pattern diagram of the final surface of the Yb:YAG thin sheet optical element after debonding, measured by a laser interferometer based on the high surface shape precision thin sheet optical element processing method based on double-sided polishing,

[0031] Figure 7 The figure is a roughness parameter diagram of the final surface of the Yb:YAG thin sheet optical element after debonding, measured by a white light interferometer based on the high surface shape precision thin sheet optical element processing method based on double-sided polishing.

[0032] Figure 8 The figure is an interference fringe pattern diagram of the final surface of the Yb:YAG thin sheet optical element processed based on the traditional double-sided polishing scheme, measured by a laser interferometer.

[0033] Figure 9 The figure is a roughness parameter diagram of the final surface of the Yb:YAG thin sheet optical element processed based on the traditional double-sided polishing scheme, measured by a white light interferometer. DETAILED DESCRIPTION

[0034] The application will be further described below with reference to the accompanying drawings. The application will be further described below with reference to the accompanying drawings.

[0035] As Figure 1 shown, a high surface accuracy wafer optical element processing method based on double-sided polishing, comprising the following steps:

[0036] S1: according to the surface accuracy and surface roughness preset requirements, processing two first optical wafers with the same thickness and a diameter-thickness ratio greater than 10;

[0037] Specifically, the first optical wafer is obtained by double-sided chemical mechanical polishing, and the surface accuracy PV value is better than λ / 10@632.8nm, and the surface roughness Ra value is better than 0.5nm.

[0038] S2: preparing a sacrificial layer on one surface of each first optical wafer;

[0039] Specifically, the material of the sacrificial layer is magnesium fluoride, silicon dioxide or photoresist, and the thickness is 1nm to 100nm.

[0040] S3: face-to-face bonding of two first optical wafers through the sacrificial layer to form a composite bonded body;

[0041] Specifically, the face-to-face bonding is realized by using normal temperature bonding process.

[0042] S4: double-sided thinning and chemical mechanical polishing of the two outer surfaces of the composite bonded body, so that the total thickness of the composite bonded body is reduced to the target thickness, and the preset surface accuracy and surface roughness requirements are reached;

[0043] Specifically, double-sided chemical mechanical polishing is adopted, and the thickness of the wandering star wheel is less than the final target thickness of the composite bonded body and greater than the target thickness of the second optical wafer.

[0044] S5: separating the two independent second optical wafers by debonding the polished composite bonded body;

[0045] Specifically, one of the methods of thermal slip method, laser irradiation method and chemical etching method is used for debonding.

[0046] S6: removing the residual sacrificial layer on the surface of the second optical wafer to obtain the final high surface accuracy wafer optical element.

[0047] Specifically, ion beam etching or wet etching is used to remove the residual sacrificial layer; the thickness of the second optical wafer is 50μm to 300μm, and the diameter-thickness ratio is greater than 50.

[0048] Working principle: The core principle of the present application is to change the mechanical properties of the element in the processing process by the element bonding-unbonding method in the traditional optical double-sided polishing process, so as to break through the problem that the large diameter-thickness ratio thin sheet optical element is difficult to realize high surface shape precision processing in the traditional double-sided polishing processing. Specifically, first, two pieces of initial thin sheet optical elements which have realized high quality optical surface processing (surface shape precision PV is better than λ / 10@632.8nm, surface roughness Ra is better than 0.5nm) are bonded face to face through a nanometer thick sacrificial layer at room temperature to form a composite bonded body with significantly increased overall thickness and rigidity; then, the composite bonded body is used as a thickened and significantly enhanced rigid 'pseudo thick workpiece', and high-efficiency and stable double-sided chemical mechanical polishing is carried out by using its excellent bending and impact resistance, so as to effectively avoid the problems such as fragmentation and surface shape distortion of the ultra-thin element in the traditional double-sided polishing due to insufficient rigidity, and more solid polishing disc such as a wandering star wheel can be used to ensure the stable realization of high surface shape precision and ultra-smooth surface processing of the thin sheet optical element. After polishing, the composite body is separated by low-stress unbonding technology, and the sacrificial layer is removed, and finally two independent high surface shape precision thin sheet optical elements are obtained.

[0049] Example 1:

[0050] Process 24 pieces of high surface shape precision ultra-smooth surface silicon (Si) thin sheet optical elements with a diameter of 50mm and a final target thickness of 200μm (diameter-thickness ratio 250).

[0051] Step S1: Firstly, 24 Si wafer optical elements with diameter of 50 mm, thickness of 1.1 mm and ratio of diameter to thickness of about 45 were prepared as first optical wafers, and were divided into two groups according to 12 wafers per group; in order to ensure high surface accuracy of the first wafers, a diamond pellet modified polyurethane polishing disc was used as a polishing disc of a double-sided polishing machine, and four double-sided polishing star wheels were prepared, and three Si wafer optical elements to be processed were evenly placed in each star wheel; a double-sided polishing method was used to precisely polish the 12 Si wafer optical elements in one group; during the polishing process, the polishing path was controlled by controlling the rotation speed and revolution speed of the star wheel; the rotation speed was controlled in the range of 10-50 rpm, and the revolution speed (sun wheel speed) was controlled in the range of 5-30 rpm; combined with the control of the polishing pressure of the upper and lower polishing discs, chemical mechanical polishing was performed for 1 hour by using CeO2 (particle size 500 nm) polishing liquid with a concentration of 5%; then the surface shape and roughness of the processed Si wafer optical elements were detected by using a laser interferometer every 15 minutes; the rotation speed and revolution speed of the star wheel were adjusted according to the detection results, until the whole surface shape accuracy of the Si wafer optical elements was less than λ / 10@632.8 nm and the high surface quality processing was realized; then further chemical mechanical polishing was performed for 0.5 hours by using silicon sol (particle size 100 nm) polishing liquid with a concentration of 3%; then the surface shape and roughness of the processed Si wafer optical elements were detected by using a laser interferometer and a white light interferometer every 15 minutes; the rotation speed and revolution speed of the star wheel were also adjusted according to the detection results, until the whole surface shape accuracy of the Si wafer optical elements was less than λ / 10@632.8 nm and the surface roughness Ra was less than 0.5 nm, and the high surface quality processing was realized; after the processing was completed, the other 12 Si wafer optical elements were processed by using the same method, and finally the first Si optical wafers with diameter of 50 mm, thickness of 1.0 mm and ratio of diameter to thickness of 50 were realized.

[0052] Step S2: A 5 nm thick MgF2 film was prepared as a sacrificial layer on one surface of each Si first optical wafer by using electron beam evaporation film plating, and the sample table temperature was controlled in the range of 60-100°C during the evaporation process, so as to reduce the stress introduced due to mismatch of thermal expansion coefficients and avoid distortion of the surface shape of the optical wafer.

[0053] Step S3: After the 24 pieces of Si wafer elements with MgF2 sacrificial layer are plated, they are sequentially cleaned with acetone, ethanol, and deionized water to remove possible organic and inorganic contaminants on the surface of the Si wafer optical element; after drying, the first optical wafer sample is placed in the reaction chamber of a plasma cleaning machine, and the surface of the side plated with the MgF2 film is activated using a 1:1 mass fraction Ar / O2 mixed gas; during activation, the radio frequency power is set to 150 W, the mixed gas flow is adjusted to maintain a pressure of 2-5 Pa in the reaction chamber, and the activation time is 3 minutes; immediately after activation, the activated surfaces of the Si wafer optical elements plated with the MgF2 film are bonded together in pairs in a clean environment to achieve pre-bonding, and the pre-bonded assembly is then placed in an oven for annealing for 10 hours to produce 12 pieces of Si wafer composite bonded body with a diameter of 50 mm and a thickness of 2 mm.

[0054] Step S4: The 12 pieces of composite bonded body are first placed in 4 star wheels with a thickness of 1 mm, 3 pieces in each star wheel, and a diamond grinding liquid with a particle size of 10 μm is used to grind and thin both outer surfaces (i.e., the original non-bonding surface) of the composite bonded body using a double-sided polishing machine to reduce the total thickness of the composite bonded body to 1.1 mm (i.e., about 550 μm per piece); then 4 star wheels with a thickness of 380 μm are used, and diamond polishing liquids with particle sizes of 6 μm, 3 μm, and 1 μm are sequentially used to double-side thin and polish the two outer surfaces of the composite bonded body using a double-sided polishing machine to reduce the total thickness of the composite bonded body to 420 μm (i.e., about 210 μm per piece); on this basis, the composite bonded body is further double-side chemically mechanically polished using a CeO2 polishing liquid with a concentration of 5% (particle size 500 nm) and a silicon sol polishing liquid with a concentration of 3% (particle size 100 nm); during polishing, the polishing path is controlled by controlling the rotation and revolution speeds of the star wheels, the rotation speed is controlled in the range of 10-50 rpm, and the revolution speed is controlled in the range of 5-30 rpm; in combination with the control of the polishing pressure of the upper and lower polishing discs, the two outer surfaces of the composite bonded body are processed with high surface shape accuracy and low surface roughness; during processing, the surface shape and roughness of the processed Si composite bonded body element are detected every 15 minutes using a laser interferometer and a white light interferometer, and the rotation and revolution speeds of the star wheels are adjusted according to the detection results until the Si composite bonded body wafer element with a total thickness of 400 μm, a surface shape accuracy of PV<λ / 10@632.8 nm, and a surface roughness of Ra<0.5 nm is achieved.

[0055] Step S5: The precisely processed composite bonded body is placed in an oven and heated to 300°C, and the difference in thermal expansion coefficients between the MgF2 sacrificial layer and silicon causes shear slip at the interface to achieve debonding; after completion, the oven temperature is slowly reduced to room temperature to obtain independent Si wafer optical elements with a diameter of 50 mm and a thickness of 200 μm.

[0056] Step S6: etching the debonded Si wafer optical element using a low-energy Ar ion beam etching device. First, the debonded Si wafer optical element is firmly mounted on a sample table to ensure good thermal contact. The chamber is pumped to high vacuum (<1.0 x 10-3 Pa) to avoid interference with the etching process and contamination of the sample surface by residual gas. The working gas is high-purity argon gas with a purity of 99.999%. The beam energy is controlled at 300 eV. When the characteristic spectral line intensity of Mg or F significantly decreases and stabilizes during the etching process, it indicates that the MgF2 layer has been etched clean. After etching is complete, the Si wafer element is ultrasonically cleaned in isopropanol and deionized water in a super-clean environment to remove redeposited particles generated during etching, and then dried with high-purity N2. Finally, a high-quality Si wafer optical element with a diameter of 50 mm and a thickness of 200 μm is obtained, which has high surface shape accuracy (PV < λ / 10 @ 632.8 nm) and low surface roughness (Ra < 0.5 nm). The final surface shape of the debonded Si wafer optical element is measured by a laser interferometer Figure 2 as shown. The final surface roughness parameters of the debonded Si wafer optical element are measured by a white light interferometer as shown. Figure 3

[0057] ​Comparative experiment: 24 pieces of silicon (Si) wafer optical elements with a diameter of 50 mm and a final target thickness of 200 μm (diameter-thickness ratio of 250) were processed by using the traditional double-sided polishing scheme. Since the double-sided polishing equipment used in the experiment can only process 12 pieces of wafer optical elements with a diameter of 50 mm at a time, the 24 pieces of Si wafer optical elements with a diameter of 50 mm and a thickness of 0.3 mm (diameter-thickness ratio of about 45) were divided into two groups of 12 pieces each and processed in turn. In order to achieve high surface shape accuracy of the wafer, the diamond pellet-shaped polyurethane polishing disc after finishing was also used as the polishing disc of the double-sided polishing machine, and four star wheels with a thickness of 180 μm were prepared, and three pieces of Si wafer elements to be processed were uniformly placed in each star wheel. The double-sided polishing method was used to polish the 12 pieces of Si wafer elements, and the wafer elements were polished by using diamond polishing liquid with a particle size of 1 μm, CeO2 polishing liquid with a concentration of 5% (particle size of 500 nm) and silicon sol polishing liquid with a concentration of 3% (particle size of 100 nm) in turn. During the polishing process, the polishing path was adjusted by controlling the rotation and revolution speeds of the star wheels, the rotation speed was controlled in the range of 10-40 rpm, the revolution speed was controlled in the range of 5-30 rpm, and the surface shape accuracy of the wafer elements was adjusted by combining the adjustment of the polishing pressure of the upper and lower polishing discs. The surface shape and roughness of the processed Si wafer elements were detected by using a laser interferometer and a white light interferometer every 15 minutes, and the rotation and revolution speeds of the star wheels and the polishing pressure were also adjusted according to the detection results. It was found during the polishing process that the Si wafer elements were very prone to edge cracking, and the micro debris generated by the cracking significantly deteriorated the surface roughness of the wafer elements. After the processing of the two groups of 24 pieces of Si wafer elements was completed, only 5 pieces did not crack. In addition, due to the thinness and poor rigidity of the star wheels, it was difficult to achieve high-precision adjustment of the surface shape of the wafer during the polishing process. The surface shape interferogram of the Si wafer optical element measured by the laser interferometer is shown in FIG. 8. Figure 4 As shown in FIG. 8, the surface shape is very irregular. The final surface roughness parameters of the Si wafer optical element measured by the white light interferometer are shown in FIG. 9. Figure 5 As shown in FIG. 9, obvious scratches can be observed.

[0058] Example 2:

[0059] 80 pieces of Yb:YAG wafer optical elements with a diameter of 20 mm and a final target thickness of 150 μm (diameter-thickness ratio of about 133) were processed to have high surface shape accuracy and super-smooth surfaces.

[0060] Step S1: First, 80 pieces of Yb:YAG thin sheet optical elements with a diameter of 20 mm, a thickness of 510 μm, and a diameter-thickness ratio of ~ 39 were prepared as the first optical thin sheets, and were divided into two groups according to 40 pieces per group; in order to ensure high surface shape precision processing of the first optical thin sheets, a diamond pellet modified polyurethane polishing disc was used as a polishing disc of a double-sided polishing machine, and four double-sided polishing star wheels with a thickness of 450 μm were prepared, 10 pieces of Yb:YAG thin sheet optical elements were uniformly placed in each star wheel, and a double-sided polishing method was used to precisely polish 40 pieces of Yb:YAG thin sheet optical elements in one group, the polishing path was controlled by controlling the rotation and revolution speeds of the star wheels during the polishing process, the rotation speed was controlled in the range of 10-50 rpm, the revolution speed (sun wheel speed) was controlled in the range of 5-30 rpm, the polishing pressure of the upper and lower polishing discs was controlled, Al2O3 (particle size 500 nm) polishing liquid with a concentration of 5% was used for chemical mechanical polishing for 1 hour, then the surface shape and roughness of the processed Yb:YAG thin sheet optical elements were detected by a laser interferometer and a white light interferometer every 15 minutes, the rotation and revolution speeds of the star wheels were adjusted according to the detection results, until the high surface quality processing of the Yb:YAG thin sheet optical elements was realized, the whole surface shape precision PV < λ / 10@632.8 nm, and the surface roughness Ra < 0.5 nm; then the same method was used to double-sided chemical mechanical polish the other 40 pieces of Yb:YAG thin sheet optical elements, and finally the processing of 80 pieces of Yb:YAG first optical thin sheets with a diameter of 20 mm, a thickness of 500 μm, and a diameter-thickness ratio of 40 was realized.

[0061] Step S2: The Yb:YAG first optical thin sheets were sequentially cleaned by ultrasonic cleaning with acetone, ethanol, and deionized water to remove possible organic and inorganic contaminants on the surface; after cleaning, the thin sheet elements were placed in an oven at 60°C for 30 minutes to remove the adsorbed water vapor on the surface and enhance the surface hydrophilicity; then the surface of the thin sheet was treated by O2 plasma at a power of 100 W for 2 minutes to further improve the surface hydrophilicity of the thin sheet optical element; after the treatment, a uniform layer of low shrinkage photoresist was coated on the surface of the thin sheet optical element by using a spin coater, the rotation speed was set to 6000-8000 rpm, and the thickness of the photoresist layer was about 60 nm.

[0062] Step S3: In a clean environment, the 80 pieces of Yb:YAG first optical thin sheets coated with photoresist were bonded together two by two, placed on a hot plate and heated uniformly at 90°C for 2 minutes, and then slowly cooled to room temperature to realize the preparation of a Yb:YAG composite bonded body with a diameter of 20 mm and a thickness of 1 mm.

[0063] Step S4: 40 pieces of the composite bonding body were first placed in 4 star wheels with a thickness of 500 μm, 10 pieces in each star wheel, diamond grinding liquid with a particle size of 10 μm was used, and double-sided polishing machine was used to grind and thin the two outer surfaces (i.e. the original non-bonding surface) of the composite bonding body, so that the total thickness of the composite bonding body was thinned to 520 μm (i.e. about 260 μm per piece); then 4 star wheels with a thickness of 280 μm were used, diamond polishing liquid with particle sizes of 6 μm, 3 μm and 1 μm were used in turn, and double-sided polishing machine was used to thin and polish the two outer surfaces of the composite bonding body, so that the total thickness of the composite bonding body was thinned to 320 μm (i.e. about 160 μm per piece); on this basis, the composite bonding body was further polished by double-sided chemical mechanical polishing with Al2O3 (particle size 500 nm) polishing liquid with a concentration of 5%, and the polishing path was controlled by controlling the rotation and revolution speeds of the star wheels during the polishing process, the rotation speed was controlled in the range of 10-50 rpm, the revolution speed was controlled in the range of 5-30 rpm, and the polishing pressure of the upper and lower polishing discs was controlled, so as to process the two outer surfaces of the composite bonding body with high surface shape precision and low surface roughness; during the processing, the surface shape and roughness of the processed Yb:YAG composite bonding body were detected by laser interferometer and white light interferometer every 15 minutes, the rotation and revolution speeds of the star wheels were adjusted according to the detection results, until the Yb:YAG composite bonding body with a whole thickness of 300 μm, a surface shape precision of PV < λ / 10@632.8 nm and a surface roughness of Ra < 0.5 nm was realized.

[0064] Step S5: The composite bonding body after precision processing was placed on the processing platform of the laser debonding equipment, and the Yb:YAG composite bonding body was debonded by using 355 nm ultraviolet laser with a pulse width of 30 ns and a repetition rate of 20 kHz. During the processing, CCD vision system was used to ensure that the laser focus was accurately aligned with the Z-axis position of the photoresist bonding layer, the pulse energy density was set to 100 mJ / cm 2 , and the scanning speed was set to 10 mm / s to scan the whole bonding surface of the Yb:YAG composite bonding body, and after the scanning was completed, the independent Yb:YAG wafer optical element with a diameter of 20 mm and a thickness of 150 μm was separated by using vacuum chuck.

[0065] Step S6: After debonding, the surface of the Yb:YAG wafer optical element still has some pyrolytic photoresist residues, so the wafer optical element is soaked and cleaned using N-methyl pyrrolidone (NMP) solvent to remove the photoresist residues. After completion, the Yb:YAG wafer optical element is ultrasonically cleaned using isopropyl alcohol and deionized water, and then dried with high-purity N2. Finally, a high-quality Yb:YAG wafer optical element with a diameter of 20 mm and a thickness of 150 μm, a high surface shape precision (PV < λ / 10 @ 632.8 nm), and a low surface roughness (Ra < 0.5 nm) is obtained. The final surface shape interference fringe pattern of the Yb:YAG wafer optical element after debonding is measured by a laser interferometer Figure 6 as shown in FIG. 6. The final surface roughness parameters of the Yb:YAG wafer optical element after debonding are measured by a white light interferometer as shown in FIG. 7. Figure 7 as shown in FIG. 6. The final surface roughness parameters of the Yb:YAG wafer optical element after debonding are measured by a white light interferometer as shown in FIG. 7.

[0066] Comparative experiment: 80 pieces of Yb:YAG wafer optical elements with diameter of 20 mm and final target thickness of 150 μm (diameter-thickness ratio ~ 133) were processed by using the traditional double-sided polishing scheme to obtain high surface shape precision ultra-smooth surface. Since the double-sided polishing equipment used in the experiment can only process 60 pieces of 20 mm diameter wafer optical elements at a time, 80 pieces of Yb:YAG wafer optical elements with diameter of 20 mm and thickness of 510 μm (diameter-thickness ratio ~ 39) were divided into two groups according to 40 pieces per group and processed in turn. In order to realize the high surface shape precision processing of the wafer, the diamond pellet modified polyurethane polishing disc was also used as the polishing disc of the double-sided polishing machine, and four star wheels with a thickness of 200 μm were prepared, 10 pieces of wafer optical elements to be processed were evenly placed in each star wheel. The double-sided polishing machine was used to precisely thin and polish 40 pieces of Yb:YAG wafer optical elements by using diamond polishing liquid with particle size of 6 μm and 3 μm in turn, and the total thickness of the Yb:YAG wafer optical elements was thinned to 210 μm. On this basis, star wheels with a thickness of 130 μm were used, and Yb:YAG wafer optical elements were double-sided chemical mechanical polished by using diamond polishing liquid with particle size of 3 μm and 1 μm and Al2O3 polishing liquid with a concentration of 5% (particle size 500 nm) in turn. During the polishing process, the polishing path was adjusted by controlling the rotation and revolution speeds of the star wheel, the rotation speed was controlled in the range of 10-20 rpm, and the revolution speed was controlled in the range of 5-20 rpm. Combined with the adjustment of the polishing pressure of the upper and lower polishing discs, the surface shape precision of the wafer optical elements was adjusted. The surface shape and roughness of the processed wafer optical elements were detected by using the laser interferometer and the white light interferometer every 15 minutes. Similarly, the rotation and revolution speeds of the star wheel and the polishing pressure were adjusted according to the detection results. It was found that the Yb:YAG wafer elements were very easy to crack at the edge and even the whole wafer elements cracked when the thickness was less than 200 μm during the polishing process. The small debris generated by the cracking significantly deteriorated the surface roughness of the wafer elements. Finally, only 19 pieces of Yb:YAG wafer elements did not crack after the processing of the two groups of 80 pieces of Yb:YAG wafer elements. In addition, due to the thinness and poor rigidity of the star wheel, it was difficult to realize the high precision adjustment of the wafer surface shape during the polishing process. The final surface shape interferometric pattern of the Yb:YAG wafer optical elements processed based on the traditional double-sided polishing scheme was measured by the laser interferometer as shown in Figure 8 The final surface roughness parameter diagram of the Yb:YAG wafer optical elements processed based on the traditional double-sided polishing scheme was measured by the white light interferometer as shown in Figure 9

[0067] The above specific embodiments further illustrate the purpose, technical scheme and advantages of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.​

Claims

1. A method for fabricating high-precision thin-film optical elements based on double-sided polishing, characterized in that, Includes the following steps: S1: Based on the preset requirements for surface shape accuracy and surface roughness, process two first optical thin films with the same thickness and a diameter-to-thickness ratio greater than 10; S2: Prepare a sacrificial layer on one surface of each first optical sheet; S3: Two first optical thin films are bonded face-to-face through a sacrificial layer to form a composite bond; S4: Perform double-sided thinning and chemical mechanical polishing on the two outer surfaces of the composite bond to reduce the total thickness of the composite bond to the target thickness, while achieving the preset surface accuracy and surface roughness requirements. S5: Debond the polished composite bond to separate two independent second optical sheets; S6: Remove the sacrificial layer residue from the surface of the second optical sheet to obtain the final high-precision thin-film optical element.

2. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S1, the first optical thin film is obtained by double-sided chemical mechanical polishing, and its surface shape accuracy PV value is better than λ / 10@632.8nm, and its surface roughness Ra value is better than 0.5nm.

3. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S2, the sacrificial layer is made of one of magnesium fluoride, silicon dioxide, or photoresist, and the thickness of the sacrificial layer is from 1 nm to 100 nm.

4. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S3, a room temperature bonding process is used to achieve face-to-face bonding.

5. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S4, double-sided chemical mechanical polishing is used, where the thickness of the planetary wheel is less than the final target thickness of the composite bond and greater than the target thickness of the second optical sheet.

6. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S5, one of the following methods is used for debonding: thermal slip method, laser irradiation method, or chemical etching method.

7. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, In step S6, the sacrificial layer residue is removed by ion beam etching or wet etching.

8. The method for processing high-precision thin-film optical elements based on double-sided polishing according to claim 1, characterized in that, The thickness of the second optical thin film is 50 μm to 300 μm, and the aspect ratio is greater than 50.

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