Processing method for obtaining fused quartz optical element with low surface metal impurity content

Abstract

The invention discloses a processing method for obtaining a fused quartz optical element with low surface metal impurity content, which sequentially comprises the steps of blank cutting, milling and grinding forming, laser pretreatment, fine grinding, rough polishing, acid pickling, two times of fine polishing and ion beam fine trimming. A silicon dioxide polishing solution is adopted for carrying out metal-pollution-free polishing, ultrasonic-assisted pickling is used for deeply removing metal impurities, and through the synergistic effect of zero-pressure fine polishing and non-contact ion beam fine trimming, the surface damage layer is finally removed, the surface shape precision is controlled, meanwhile, the content of the surface metal impurities is reduced to the ppb level, and the surface quality is improved. Particularly, the cerium element content is reduced by more than 90%, and an ultra-smooth surface with root-mean-square roughness superior to 0.5 nm is obtained. The method solves the technical problem that low surface roughness and low metal impurity content are difficult to consider in the traditional process, and is suitable for batch and stable manufacturing of high-end optical elements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical element manufacturing, and particularly relates to a processing method for obtaining a fused quartz optical element with low surface metal impurity content. BACKGROUND

[0002] Optical elements, especially fused quartz optical elements, are widely used in high-tech fields such as laser systems, high-energy physics devices, aerospace optical instruments and precision photoelectric equipment. These application scenarios have very high requirements for the surface quality of optical elements. Not only is a very low surface roughness required to ensure excellent optical performance, but the surface metal impurity content also needs to be strictly controlled to avoid problems such as reduced transmittance, increased scattering, non-uniform refractive index and reduced laser damage threshold caused by the introduction of impurities.

[0003] At present, the conventional processing technology of fused quartz elements mainly includes cutting, grinding and polishing processes, which often easily introduce metal element impurities. These impurities may come from the metal components of abrasives, polishing tools and processing equipment. Excessive metal element content will have a serious impact on the optical performance of the optical element, such as reducing the transmittance, increasing the scattering, affecting the refractive index uniformity of the optical element, and affecting the laser damage threshold of the optical element.

[0004] Although there have been attempts in the prior art to use non-metallic abrasives or tools to reduce metal contamination, there are still problems such as low processing efficiency, poor surface consistency and difficult control of surface shape precision. In addition, the traditional pickling and polishing process cannot effectively remove deep metal contamination while ensuring surface smoothness, especially in the treatment of subsurface defects.

[0005] Therefore, it is of great practical significance and practical value to develop an optical element processing technology that can ensure low metal impurities and processing efficiency and surface quality. SUMMARY

[0006] The present application provides a processing method for obtaining a fused quartz optical element with low surface metal impurity content, which optimizes the process flow, introduces key processes and controls process parameters to achieve a surface roughness better than 0.5 nm while controlling the surface metal impurity content to the ppb level, in order to solve the technical problems of easy introduction of metal impurities, insufficient control of surface and subsurface damage and difficulty in balancing high surface quality and low pollution control in the existing processing of fused quartz optical elements.

[0007] The technical scheme of the present application is as follows: A processing method for obtaining a fused quartz optical element with low surface metal impurity content, characterized in that the method comprises the following steps in sequence: A laser pre-treatment step: ablation of the surface of the milled and ground fused silica component with a pulsed CO2 laser to remove the sub-surface crack layer introduced by mechanical machining; A metal contamination free polishing step: at least one polishing of the fused silica component using a silica polishing liquid without metal abrasives; A deep cleaning etching step: etching of the fused silica component in a hydrofluoric acid based etching liquid to dissolve and remove metal contamination embedded in the surface of the fused silica component in the previous machining steps; A non-contact ion beam finishing step: ion beam bombardment of the surface of the fused silica component to remove residual contamination and reduce the surface roughness to sub-nanometer level in a non-contact manner.

[0008] Further, after the deep cleaning etching step, a two-stage fine polishing step is further included, which comprises: A first stage surface shape fine polishing: polishing with a set pressure using a pitch polishing pad and a silica polishing liquid to correct the surface shape accuracy; A second stage defect repair fine polishing: polishing under zero or micro pressure using a damping cloth polishing pad to remove the sub-surface defects and chemical residues exposed in the etching step; Further, the deep cleaning etching step is performed under the assistance of low frequency ultrasonic waves, and the fused silica component is fixed and carried by a PTFE fixture throughout the etching process.

[0009] Further, in the second stage defect repair fine polishing step, the repair time is about 5-10 min, the pressure is 0-3 kPa, and the rotation speed is 300-600 rpm, Further, before the laser pre-treatment step, further comprising: A blank cutting step: cutting of the fused silica blank into the drawing size using a wire cutting machine, wherein the cutting wire diameter is 0.10-0.25 mm and the cutting speed is 0.1-0.5 mm / min; A milling and grinding forming step: numerical control milling and grinding of the cut fused silica blank, wherein the grinding wheel rotation speed is 3000-4000 r / min, the 280# diamond wheel, and the removal depth d1 is 100-200 μm.

[0010] Further, in the laser pre-treatment step, the laser peak power is 20-25 W, the frequency is 1 kHz, the overlap rate is 85%, and the material removal depth d2 is 100-200 μm.

[0011] Further, in the deep cleaning pickling step, hydrofluoric acid and deionized water are mixed in a volume ratio of 1:2 to prepare a pickling solution, the fused quartz element is fixed on a polytetrafluoroethylene fixture and completely immersed in the pickling solution at an ambient temperature of 22±1°C, while being assisted by low-frequency ultrasonic waves at a frequency of 40-100 kHz, the etching time is controlled to be 2.5±0.5 min, then the element is taken out and cleaned using a high-pressure deionized water flow, and the deionized water has a resistance of ≥20 MΩ.

[0012] Further, in the non-contact ion beam finishing step, the ion source is argon ions, the incident angle of the ion beam is 60-75°, the ion beam energy is 500-800 eV, and the distance between the ion source and the element processing surface is 20-50 mm.

[0013] On the other hand, the present application also provides a fused quartz optical element obtained by the above method, characterized in that the root mean square roughness (RMS) of the surface is better than 0.5 nm, and the total content of low metal impurities on the surface is <8 ppb, reaching the ppb level, and the content of Ce element is only 1.2% of that in the traditional cerium oxide polishing process.

[0014] The technical effects of the present application are as follows: 1) By means of "laser pretreatment" and "non-contact ion beam finishing", the surface / subsurface damage from macroscopic to nanoscale is controlled. The laser pretreatment effectively removes the microcrack layer introduced by mechanical processing, and the ion beam finishing avoids the secondary pollution and damage that may be caused by traditional mechanical polishing, so that the surface roughness of the element is finally stabilized to be better than 0.5 nm, and the content of metal impurities on the surface is controlled at the ppb level, thereby solving the technical contradiction that high surface quality and low metal pollution are difficult to be achieved in the traditional process.

[0015] 2) By means of the synergistic scheme of "metal-pollution-free polishing" (silicon dioxide polishing liquid), "deep cleaning pickling", and "polytetrafluoroethylene fixture", the introduction path of metal impurities is cut off. In particular, the silicon dioxide polishing liquid is used to replace the traditional cerium oxide polishing liquid, so that the introduction of cerium element is eliminated. Experimental data show that, compared with the traditional process, the content of cerium element on the surface of the element can be reduced by more than 90% by the present application, thereby improving the damage threshold and long-term stability of the element in laser application.

[0016] 3) The deep cleaning pickling can etch and dissolve the residual metal pollutants in depth under the assistance of ultrasonic waves; in the two-stage fine polishing, the second stage adopts a unique configuration of zero / micro pressure + damping cloth to remove the subsurface defects and reaction residues exposed after pickling. Not only the metal impurities are deeply removed, but also the micro-damage possibly induced by pickling is repaired, so that an optimal balance between molecular-level cleanliness and atomic-level smoothness is achieved. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Flow chart of the processing method for obtaining a fused quartz optical element with low surface metal impurity content according to the present application. DETAILED DESCRIPTION

[0018] The present application will be further described in conjunction with examples, but should not be limited to the scope of transformation of the present application.

[0019] Please refer to Figure 1 , Figure 1 Flow chart of the processing method for obtaining a fused quartz optical element with low surface metal impurity content according to the present application, as shown in the figure, the processing method comprises the following steps: A) Blank cutting: using a wire cutting machine to cut the fused quartz blank into the size of the drawing, selecting the cutting wire diameter of 0.10-0.25mm and the cutting speed of 0.1-0.5mm / min; B) Milling and grinding: after the fused quartz element is cut by wire, the milling and grinding tolerance is set according to the drawing requirements. A numerical control milling and grinding machine is used for milling and grinding, the grinding wheel speed is selected to be 3000-4000r / min, the 280# diamond wheel is used, and the removal depth is d1, generally selected to be 100-200μm.

[0020] C) Laser pretreatment: after the quartz element is milled and ground, the surface is pretreated by laser to remove the surface defect layer, the pulse CO2 laser is selected for this process, the peak power is 20W-25W, the frequency is 1kHz, the overlap rate is 85%, the ablation processing is completed, and the removal depth is d2, generally selected to be 100-200μm.

[0021] D) Fine grinding: the processed surface of the fused quartz element after laser pretreatment is treated by abrasive wheel grinding to remove the ablation marks and part of the tool marks, the abrasive wheel grinding process includes W28# diamond fine grinding and W14# diamond fine grinding, the material removal depth of the 302# diamond fine grinding process is d3, generally selected to be 100-150μm, and the material removal depth of the 303# diamond fine grinding process is d4, generally selected to be 40-70μm.

[0022] E) Rough polishing: the fused quartz element after fine grinding is placed on a polishing disc with polyurethane polishing pad for polishing. The fused quartz element is fixed on the upper disc model by the method of asphalt point gluing, and is inverted on the polishing disc, an iron pen is inserted into the back hole of the upper disc model and pressure is applied to make the workpiece adhere to the polishing pad. After setting the processing position, feed amount and speed, the equipment is started, and silica polishing liquid is added at intervals for rough polishing, the rough polishing process removes a thickness of d5, generally selected to be 10-20μm.

[0023] F) Acid washing: Hydrofluoric acid solution is prepared according to the proportion of hydrofluoric acid and deionized water, and the fused quartz component is fixed on the clamp and immersed in the acid etching solution for etching. The material removal depth of the acid washing process is d6, generally selected from 0.2 to 2 μm. Among them, the hydrofluoric acid and deionized water are configured in a proportion of 1:2, and the operation is carried out in an environment of 22±1°C. After the fused quartz component is fixed on the polytetrafluoroethylene clamp, it is completely immersed in the acid etching solution, assisted by low-frequency 40-100 kHz ultrasonic waves to ensure uniform etching. The etching time is accurately controlled to be 2.5±0.5 min, and then it is quickly taken out and cleaned with high-pressure deionized water flow. The deionized water resistance is ≥20 MΩ.

[0024] G) Fine polishing 1: The core function of this process is to control and meet the surface accuracy of the quartz glass component. First, the fused quartz component after acid washing is placed in the workpiece ring of the polishing disc attached with polishing pitch for fine polishing processing. The polishing disc speed is set to 5-10 r / min, the ultra-precision hydraulic spindle system is 10-15 r / min, the radial reciprocating speed is 100-150 mm / min, the pressure is 50-100 N, and the polishing liquid is added at intervals. The single-side reflection surface of the quartz component is required to be better than 1-2λ, and the transmission surface is better than 1 / 3λ.

[0025] H) Fine polishing 2: The core function of this process is to control and meet the surface and subsurface defect indicators of the quartz glass component. The fused quartz component qualified in the fine polishing 1 process is placed in the workpiece ring of the polishing disc attached with damping cloth for fine polishing processing. The polishing disc speed is set to 5-10 r / min, the ultra-precision hydraulic spindle system is 10-15 r / min, the radial reciprocating speed is 100-150 mm / min, and the pressure is zero. The whole process takes 15-30 min to remove the subsurface defects of the fused quartz component exposed after acid washing and the residues generated in the acid washing process. Among them, the tooling does not use dispensing and disc attaching, but uses a special polytetrafluoroethylene mold due to the use of low metal impurities. In this process, the polishing pad is replaced by damping cloth, the smoothing time is about 5-10 min, the pressure is 1-3 kPa, and the number of revolutions is 300-600 rpm to remove the reactants remaining on the surface of the component during the acid washing process.

[0026] I) Ion beam finishing: For the fused quartz component after fine polishing, the component surface is non-contact processed by ion beam bombardment. High-energy ion beam bombardment is used to further remove the contaminants on the surface of the fused quartz, and finally a smooth surface with a root mean square roughness better than 0.5 nm and low metal impurity content is obtained. Among them, the ion source used is argon ion, the incident angle is 60-75°, the ion beam energy is 500-800 eV, and the processing distance is 20-50 mm. Finally, an ultra-smooth surface with a root mean square roughness (RMS) better than 0.5 nm and low metal impurity content is obtained.

[0027] Example: A piece of fused quartz blank with a size of 50mm x 50mm x 5mm is taken as the processing object, and the target is to process it into an optical element for a high-energy laser system, with a surface roughness of RMS <0.5nm and a surface metal impurity content (especially Ce, Fe, Cu, etc.) of less than 10 ppb.

[0028] The processing method of this embodiment specifically includes the following steps: A) Blank cutting: A wire cutting machine is used, the cutting wire diameter is selected to be 0.10-0.25mm, and the cutting speed is selected to be 0.1-0.5mm / min, and the fused quartz blank is cut to the design size of 52mm x 52mm x 5.2mm (reserving the subsequent processing allowance) B) Milling and grinding: A numerical control milling and grinding machine is used, a 280# diamond wheel is installed, the grinding wheel speed is selected to be 3000-4000r / min, and the fused quartz blank is milled and ground to the final size of 50mm x 50mm x 5mm through program control. The single-sided removal depth (d1) of this process is 150μm, aiming to quickly form and control the initial damage layer within a certain range.

[0029] C) Laser pretreatment: A pulsed CO2 laser is used to scan and process the entire optical surface after milling and grinding. The peak power is 20-25W, the frequency is 1kHz, and the spot overlap rate is 85%. The ablation process is completed, and the removal depth (d2) is 120μm, thereby completely removing the mechanical stress crack layer generated by the milling process and exposing the complete bulk material.

[0030] D) Fine grinding: The processed surface of the fused quartz element after laser pretreatment is treated with a loose abrasive wheel to remove ablation marks and part of the tool marks. The loose abrasive fine grinding process is divided into two stages: the first stage uses W28# diamond fine grinding, and the removal depth d3 is 100-150μm; the second stage uses W14# diamond fine grinding, and the removal depth d4 is 40-70μm. After the two-step fine grinding, the surface of the element is fine ground, and there is no visible laser scanning line.

[0031] E) Rough polishing: The fused quartz element after fine grinding is fixed to the upper disc mold by the method of applying pitch to the upper disc, and is inverted and placed on the polishing disc with polyurethane polishing pad. An iron pen is inserted into the hole at the back of the upper disc mold and pressure is applied to make the workpiece adhere to the polishing pad. After setting the processing position, feed amount and speed, the equipment is started, and silica polishing liquid is added at intervals for rough polishing. The rough polishing process removes a thickness of d5 of 10-20μm.

[0032] F) Acid washing: HF acid solution was prepared by mixing HF acid and deionized water at a volume ratio of 1:2. After the fused silica component was fixed on the clamp, it was immersed in the acid washing solution, and the whole container was placed in an ultrasonic cleaning tank (frequency 60 kHz). The etching time was accurately controlled at 2.5 minutes in a constant temperature environment of 22±0.5℃. After the etching was completed, the component was taken out, and a high-pressure deionized water gun with a pressure of 0.5 MPa and a resistivity of ≥20 MΩ was used for all-around flushing to terminate the reaction and remove the residual acid solution. The material removal depth d6 of the acid washing process is 0.2-2 μm, which can effectively dissolve and flush away the trace amount of metal contaminants that may be embedded on the surface in the previous processing.

[0033] G) Fine polishing 1: The core function of this process is to control and meet the surface shape accuracy of the fused silica component. After acid washing, the fused silica component is placed in the workpiece ring of the polishing disc attached with polishing pitch for fine polishing processing, the polishing disc speed is set to 5-10 r / min, the ultra-precision hydraulic spindle system is set to 10-15 r / min, the radial reciprocating speed is set to 100-150 mm / min, the pressure is set to 50-100 N, and the polishing is performed by adding silica polishing liquid at intervals. The single-sided reflective surface of the quartz component is required to be better than 1-2λ, and the transmission surface is required to be better than 1 / 3λ.

[0034] H) Fine polishing 2: The core function of this process is to control and meet the surface and subsurface defect index of the fused silica component. The fused silica component that meets the fine polishing 1 process is placed in the workpiece ring of the polishing disc attached with damping cloth for fine polishing processing, the polishing disc speed is set to 5-10 r / min, the ultra-precision hydraulic spindle system is set to 10-15 r / min, the radial reciprocating speed is set to 100-150 mm / min, and the zero pressure is applied. The process is performed for 15-30 minutes to remove the subsurface defects of the fused silica component exposed after acid washing and the residues generated during the acid washing process.

[0035] I) Ion beam finishing: The fused silica component after fine polishing 2 is sent to the vacuum chamber of the ion beam polisher, and the surface of the component is non-contact processed by ion beam bombardment. Argon ion source is used, and the working gas is 99.999% high-purity argon gas. The incident angle is set to 70°, the ion beam energy is set to 650 eV, the workpiece table speed is set to 5 r / min, and the ion source to workpiece distance is set to 30 mm. Through this non-contact processing, about 2 nm thick surface layer is uniformly removed by physical sputtering, finally removing any possible residual molecular level contaminants and obtaining an atomic level smooth surface.

[0036] The fused silica component processed in this embodiment is detected: 1. The root mean square roughness (RMS) is 0.38 nm measured by atomic force microscope (AFM).

[0037] 2. The PV value of the reflective surface is better than λ / 4 and the RMS value is better than λ / 40 by using Zygo interferometer measurement.

[0038] 3. The total metal impurity content of the surface is less than 8 ppb, reaching the ppb level, and the content of the contaminant Ce element is only 1.2% of that in the traditional cerium oxide polishing process, i.e. a decrease of about 98.8%, which is extremely significant, as measured by TOF-SIMS and ICP-MS analysis.

[0039] The embodiment fully proves that the processing method provided by the application can systematically solve the industry problem that it is difficult to simultaneously achieve an ultrasmooth surface of a fused quartz element and an extremely low metal impurity content, the process is stable and reliable, has high repeatability, and fully meets the application requirements of high-end optical systems.

[0040] The specific embodiments of the application are described above. It should be understood that the application is not limited to the specific implementation described above, and various modifications or changes can be made by those skilled in the art within the scope of the claims, which do not affect the essential content of the application. In the case of no conflict, the above embodiments and the features in the embodiments can be combined with each other.

Claims

1. A processing method for obtaining a fused quartz optical element having a low surface metal impurity content, characterized by, The method comprises the following steps in sequence: laser pretreatment step: using pulsed CO2 laser to ablate the surface of the milled and ground fused quartz element to remove the subsurface crack layer introduced by mechanical processing; metal contamination-free polishing step: using silica polishing liquid without metal abrasive to polish the fused quartz element at least once; deep cleaning acid pickling step: etching the fused quartz element in a hydrofluoric acid-based acid pickling solution to dissolve and remove the metal contaminants embedded on the surface of the fused quartz element in the foregoing processing steps; non-contact ion beam finishing step: using ion beam to bombard the surface of the fused quartz element to remove residual contaminants in a non-contact manner and reduce the surface roughness to sub-nanometer level.

2. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 1, characterized by, After the deep cleaning acid pickling step, a two-stage fine polishing step is further included, which comprises: first-stage surface shape fine polishing: using pitch polishing disc and silica polishing liquid to polish at a set pressure to correct the surface shape accuracy; second-stage defect repair fine polishing: using damping cloth polishing disc to polish under zero pressure or micro pressure conditions to remove the subsurface defects and chemical residues exposed in the acid pickling step.

3. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 1 or 2, characterized by, The deep cleaning acid pickling step is performed under the assistance of low-frequency ultrasonic waves, and the fused quartz element is fixed and carried by a polytetrafluoroethylene fixture throughout the acid pickling process.

4. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 2, characterized by, In the second-stage defect repair fine polishing step, the repair time is about 5-10 min, the pressure is 0-3 kPa, and the rotation speed is 300-600 rpm.

5. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 1, characterized by, Before the laser pretreatment step, the following steps are further included: blank cutting step: cutting the fused quartz blank into the size of the drawing using a wire cutting machine, wherein the cutting wire diameter is 0.10-0.25 mm and the cutting speed is 0.1-0.5 mm / min; milling and grinding forming step: numerically controlling the milling and grinding of the cut fused quartz blank, wherein the grinding wheel rotation speed is 3000-4000 rpm, the 280# diamond grinding wheel, and the removal depth d1 is 100-200 μm.

6. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 1, characterized by, In the laser pretreatment step, the laser peak power is 20-25 W, the frequency is 1 kHz, the overlap rate is 85%, and the material removal depth d2 is 100-200 μm.

7. The processing method for obtaining a fused quartz optical member with low surface metal impurity content according to claim 1, characterized by, In the deep cleaning acid pickling step, the acid pickling solution is prepared by mixing hydrofluoric acid and deionized water at a volume ratio of 1:2, the fused quartz element is fixed on the polytetrafluoroethylene fixture and completely immersed in the acid pickling solution at an ambient temperature of 22±1°C, while being assisted by low-frequency ultrasonic waves of 40-100 kHz, and the etching time is controlled to be 2.5±0.5 min before being taken out and cleaned using a high-pressure deionized water flow, wherein the deionized water resistance is ≥20 MΩ.

8. The processing method for obtaining a fused quartz optical member with a low surface metal impurity content according to claim 1, characterized by, In the non-contact ion beam finishing step, the ion source is argon ion, the incident angle of the ion beam is 60-75°, the ion beam energy is 500-800 eV, and the distance between the ion source and the processed surface of the element is 20-50 mm.

9. A fused silica optical element obtained by the method according to any one of claims 1 to 8, characterized in that The surface root mean square roughness (RMS) is better than 0.5 nm, and the total content of low metal impurities on the surface is <8 ppb, reaching the ppb level, wherein the content of Ce element is only 1.2% of that in the traditional cerium oxide polishing process.

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