CN110877255A - Combined machining process for ultra-smooth machining of fused quartz optical surface - Google Patents

Combined machining process for ultra-smooth machining of fused quartz optical surface Download PDF

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Publication number
CN110877255A
CN110877255A CN201911257406.7A CN201911257406A CN110877255A CN 110877255 A CN110877255 A CN 110877255A CN 201911257406 A CN201911257406 A CN 201911257406A CN 110877255 A CN110877255 A CN 110877255A
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optical
polishing
processing
machining
roughness
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廖文林
张�浩
聂旭涛
聂徐庆
张逸飞
谢强
蒋国庆
万稳
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Institute Of Equipment Design & Test Technology Cardc
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B24GRINDING; POLISHING
    • B24BMACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B13/00Machines or devices designed for grinding or polishing optical surfaces on lenses or surfaces of similar shape on other work; Accessories therefor
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B24GRINDING; POLISHING
    • B24BMACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B1/00Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B24GRINDING; POLISHING
    • B24BMACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B1/00Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
    • B24B1/005Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes using a magnetic polishing agent

Abstract

The invention discloses a combined processing technology for ultra-smooth processing of a fused quartz optical surface, which comprises the following steps: preprocessing and processing at the early stage; high-efficiency magneto-rheological polishing of nanometer level surface shape precision; smoothing processing of medium-high frequency surface roughness; and (4) performing ultra-smooth polishing processing on the ion beam of the sub-nanometer precision optical surface. The processing technology provided by the invention can control surface/subsurface damage, surface shape precision and surface roughness in stages, realizes high-efficiency processing of converging full-band errors to sub-nanometer precision, and has the advantages of simple principle, easy realization and higher efficiency.

Description

Combined machining process for ultra-smooth machining of fused quartz optical surface
Technical Field
The invention relates to the technical field of ultra-precision machining of optical elements, in particular to a combined machining process for ultra-smooth machining of a fused quartz optical surface.
Background
Modern high-performance optical systems have higher and higher requirements on surface shape precision and surface quality of optical elements, and almost reach the physical limit of processing. The extreme ultraviolet lithography objective optical element is a very typical representative, and the requirement of simultaneously controlling the surface shape error of a full aperture scale, the roughness of a micron scale and the roughness of a nanometer scale in a nanometer/sub-nanometer range provides challenges for the surface shape manufacturing precision and the ultra-smooth surface processing of the optical element.
Because the fused quartz material has great advantages in the aspects of optical performance, material uniformity and the like, the fused quartz material is generally used as a preferred material for high-performance optical devices such as strong light optics, photoetching objective lenses and the like. However, the fused silica material contains large voids in addition to silicon and oxygen atoms in its internal composition, and is likely to be densified when a sufficiently large compressive stress is applied. In conventional polishing processes based on mechanical effects, the local action of the abrasive and the material is very likely to cause local material densification effects, particularly surface/sub-surface damage such as scratches, cracks, etc. In the course of ultra-precision machining of high performance optical elements, the generation of surface/sub-surface damage will seriously affect the surface shape precision and ultra-smooth surface generation of the optical elements.
In the process of processing optical elements in near-batch and mass production, the requirement of processing ultra-precise optical elements cannot be met only by a single technology, so the modern optical element processing usually adopts an integrated manufacturing mode. In the integrated manufacturing mode, the formation of high-precision, low-damage, high-efficiency and low-cost combined processing capability is a key problem, and the advantages of various processing technologies need to be integrated to optimize the whole combined process flow, but currently, few researches on the combined processing technology of the optical element with the sub-nanometer precision and the ultra-smooth surface are concerned.
Disclosure of Invention
The invention aims to solve the defects of the prior art and the problems of sub-nanometer surface shape precision and ultra-smooth surface generation of the fused quartz optical element, and further provides a technical scheme of a combined machining process for ultra-smooth machining of the fused quartz optical surface.
The scheme is realized by the following technical measures:
a combined processing technology for ultra-smooth processing of a fused quartz optical surface comprises the following steps:
(1) performing pretreatment processing in an early stage to ensure that the surface shape precision of the optical element is better than 1 mu m RMS;
(2) the high-efficiency magneto-rheological polishing of the nanometer level surface shape precision enables the surface shape precision of the optical element to be superior to 10nm RMS;
(3) smoothing processing of medium-frequency and high-frequency surface roughness is carried out, so that the medium-frequency surface roughness and the high-frequency surface roughness of the optical element are both superior to 0.2nm RMS;
(4) and performing ion beam ultra-smooth polishing processing on the sub-nanometer precision optical surface to ensure that the surface shape precision of the optical element surface is superior to 1.0nm RMS.
The scheme is preferably as follows: the step (1) comprises the following processing steps:
(1.1) carrying out pretreatment processing on the optical element by using a computer-controlled asphalt disc polishing method, removing surface damage obviously visible on the optical surface, and carrying out primary polishing processing on the surface shape error of the optical element;
(1.2) respectively detecting the surface shape precision and the surface quality of the optical element by using a wave surface interferometer and a digital microscope, and giving detection results of surface shape errors and surface damage; when the surface shape precision is better than 1 μm RMS and no obvious surface damage is observed, the pretreatment processing at the early stage is finished; otherwise, repeating the process from the step (1.1) to the step (1.2) until the processing result meets the requirement of surface shape precision.
The scheme is preferably as follows: the step (2) comprises the following processing steps:
(2.1) uniformly removing the surface material of the optical element by adopting magneto-rheological polishing, wherein nano-diamond micro powder is selected as a main polishing abrasive in the polishing process, and the uniform removal thickness is about 5 mu m;
(2.2) observing the surface quality of the optical element by using a digital microscope, and repeating the process of the step (2.1) if obvious surface damage exists on the optical surface; otherwise, entering the step (2.3);
(2.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into a magnetorheological polishing processing process software computer, and calculating to obtain the residence time required by magnetorheological polishing and the processed surface shape residual error so as to comprehensively evaluate the processing efficiency and the processing precision and preliminarily select a proper magnetorheological polishing removal function;
(2.4) forming a removal function close to the preliminary selection in the step (2.3) by adjusting polishing parameters such as the rotating speed, the magnetic field current, the flow and the like of the rheological polishing wheel, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by utilizing a magnetorheological polishing process software computer, generating a numerical control code required by a magnetorheological polishing machine tool by utilizing the residence time, and processing the optical element;
(2.5) detecting the face shape error by using the wave surface interferometer again, and finishing the magnetorheological polishing when the face shape precision is better than 10nm RMS; otherwise, repeating the step (2.3) to the step (2.4) until the machining result meets the requirement of surface shape precision.
The scheme is preferably as follows: the step (3) comprises the following processing steps:
(3.1) bringing the surface shape error of the optical element detected in the step (2.5) into a fairing polishing processing process software computer, obtaining a surface shape residual error after fairing processing through calculation, and evaluating the surface shape precision retention capacity in the fairing polishing process, so that proper fairing polishing parameters are selected;
(3.2) forming a numerical control code of a fairing polishing machine tool according to the fairing polishing parameter selected in the step (3.1), and fairing the optical element by using nano-scale cerium oxide micropowder as a main polishing abrasive in the polishing process, wherein the polishing time is 60 min;
(3.3) detecting the intermediate frequency surface roughness of the optical element by using a white light interferometer, and repeating the steps (3.1) and (3.2) when the intermediate frequency surface roughness is not controlled within 0.4nm RMS; otherwise, entering the step (3.4);
(3.4) selecting the same fairing polishing parameters as those in the step (3.2) to form a numerical control code of a fairing polishing machine tool, and using deionized water as a polishing abrasive to perform fairing processing on the optical element, wherein the polishing time is 30 min;
(3.5) respectively detecting the medium-frequency surface roughness and the high-frequency surface roughness of the optical element by using a white light interferometer and an atomic force microscope, wherein the medium-frequency surface roughness and the high-frequency surface roughness are both better than 0.2nm RMS, and the fairing polishing processing is finished; otherwise, repeating the step (3.4) until the intermediate frequency surface roughness and the intermediate frequency surface roughness reach the requirements.
The scheme is preferably as follows: the step (4) comprises the following processing steps:
(4.1) uniformly polishing and removing the surface material of the optical element by using low-energy ion beams, wherein the ion beams enter the fused quartz optical surface at a small angle in the processing process, and the uniformly removed thickness is about 100 nm;
(4.2) detecting the high-frequency surface roughness of the optical element by using an atomic force microscope, and repeating the step (4.1) when the high-frequency surface roughness is not controlled within 0.1nm RMS; otherwise, entering the step (4.3);
(4.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into an ion beam polishing process software computer, calculating to obtain a surface shape residual error after ion beam polishing, and evaluating the processed surface shape precision, thereby preliminarily selecting a proper ion beam polishing removal function;
(4.4) forming a removal function close to the preliminary selection in the step (4.3) by adjusting parameters such as the energy, the beam current and the like of the ion beam, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by using an ion beam polishing process software computer, generating a numerical control code required by an ion beam polishing machine tool by using the residence time, and processing the optical element;
(4.5) detecting the face shape error by using the wave surface interferometer again, and finishing the ion beam polishing processing when the face shape precision is superior to 1.0nm RMS; otherwise, repeating the step (4.3) to the step (4.4) until the machining result meets the requirement of surface shape precision.
The scheme is preferably as follows: the surface shape error of the optical element specifically refers to the surface error with the period of about 1mm to the full aperture size of the optical element.
The scheme is preferably as follows: the medium-frequency surface roughness of the optical element specifically refers to a surface error with a period of about 1 μm to 1 mm; the high-frequency surface roughness of the optical element specifically means a surface error having a period of less than 1 μm.
The scheme is preferably as follows: the low-energy ion beam in the step (4.1) is specifically that the ion energy used for ion beam polishing is in the range of 400eV to 800 eV; the ion beam small angle incidence in the step (4.1) specifically means that an included angle between an ion beam incidence direction and an optical surface normal direction is controlled within a range of 0-20 degrees.
The scheme is preferably as follows: the nano-scale diamond micropowder in the step (2.1) specifically refers to diamond particles with the particle size of less than 1000 nm.
The scheme is preferably as follows: the nano-scale cerium oxide micro powder in the step (3.2) specifically refers to cerium oxide particles with the particle size of less than 1000 nm.
The basic principle of the technical scheme provided by the invention is as follows: firstly, utilizing a traditional computer to control the polishing of an asphalt disc to preprocess an optical element to be processed, eliminating the obvious visible surface damage of the optical surface, and controlling the root mean square value (RMS) of the surface shape error of the optical element within 1 mu m; secondly, removing a layer of thicker material by utilizing the efficient processing capability of magnetorheological polishing to eliminate the subsurface damage covered by the surface layer and control the surface shape error within 10nm RMS; then, polishing the medium-high frequency surface roughness by utilizing smooth polishing, and controlling the medium-high frequency surface roughness within 0.2nm RMS under the condition of keeping the surface shape accuracy unchanged as much as possible; and finally, further improving the surface shape precision and the surface roughness by ion beam polishing, so that the surface shape precision, the medium-frequency surface roughness and the high-frequency surface roughness are respectively superior to 1.0nm RMS, 0.2nm RMS and 0.1nm RMS, and the full-band error of the optical element from the full-aperture surface shape precision to the nano-scale roughness is controlled in a sub-nanometer range of 1.0nm RMS.
The technical scheme has the beneficial effects that the ultra-smooth combined machining method of the sub-nanometer precision optical element is provided by combining the respective advantages of the traditional computer-controlled asphalt disc polishing, magneto-rheological polishing, smooth polishing and ion beam polishing, surface/sub-surface damage, surface shape precision and surface roughness can be controlled in stages, and the principle is simple, easy to implement and higher in efficiency. The combined machining method provided by the scheme can realize high-efficiency machining of consistent convergence of full-band errors, enables the surface shape error, the medium-frequency surface roughness and the high-frequency surface roughness of the optical surface to reach sub-nanometer precision simultaneously, and is widely applicable to machining of high-performance optical elements.
Therefore, compared with the prior art, the invention has prominent substantive features and remarkable progress, and the beneficial effects of the implementation are also obvious.
Drawings
FIG. 1 is a diagram showing the effect of surface shape errors after magnetorheological polishing.
FIG. 2 is a diagram showing the effect of surface shape error after smoothing polishing.
FIG. 3 is a graph showing the effect of medium frequency surface roughness after smoothing polishing.
FIG. 4 is a graph showing the effect of high frequency surface roughness after smoothing polishing.
FIG. 5 is a diagram illustrating the effect of surface shape error after ion beam polishing.
FIG. 6 is a graph showing the effect of medium frequency surface roughness after ion beam polishing.
Detailed Description
All of the features disclosed in this specification, or all of the steps in any method or process so disclosed, may be combined in any combination, except combinations of features and/or steps that are mutually exclusive.
Any feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving equivalent or similar purposes, unless expressly stated otherwise. That is, unless expressly stated otherwise, each feature is only an example of a generic series of equivalent or similar features.
Examples
The technical principle adopted by the embodiment is as follows: firstly, utilizing a traditional computer to control the polishing of an asphalt disc to preprocess an optical element to be processed, eliminating the obvious visible surface damage of the optical surface, and controlling the root mean square value (RMS) of the surface shape error of the optical element within 1 mu m; secondly, removing a layer of thicker material by utilizing the efficient processing capability of magnetorheological polishing to eliminate the subsurface damage covered by the surface layer and control the surface shape error within 10nm RMS; then, polishing the medium-high frequency surface roughness by utilizing smooth polishing, and controlling the medium-high frequency surface roughness within 0.2nm RMS under the condition of keeping the surface shape accuracy unchanged as much as possible; and finally, further improving the surface shape precision and the surface roughness by ion beam polishing, so that the surface shape precision, the medium-frequency surface roughness and the high-frequency surface roughness are respectively superior to 1.0nm RMS, 0.2nm RMS and 0.1nm RMS, and the full-band error of the optical element from the full-aperture surface shape precision to the nano-scale roughness is controlled in a sub-nanometer range of 1.0 nmRMS.
The specific operation process of the embodiment comprises the following steps:
1. pretreatment processing in an early stage:
(1.1) carrying out pretreatment processing on the optical element by using a computer-controlled asphalt disc polishing method, removing surface damage obviously visible on the optical surface, and carrying out primary polishing processing on the surface shape error of the optical element;
and (1.2) respectively detecting the surface shape precision and the surface quality of the optical element by using a wave surface interferometer and a digital microscope, and giving detection results of surface shape errors and surface damage. When the surface shape precision is better than 1 μm RMS and no obvious surface damage is observed, the pretreatment processing at the early stage is finished; otherwise, repeating the process from the step (1.1) to the step (1.2) until the processing result meets the requirement of surface shape precision.
2. High-efficiency magneto-rheological polishing of nanometer level surface shape precision:
(2.1) uniformly removing the surface material of the optical element by adopting magneto-rheological polishing, wherein nano-diamond micro powder is selected as a main polishing abrasive in the polishing process, and the uniform removal thickness is about 5 mu m;
(2.2) observing the surface quality of the optical element by using a digital microscope, and repeating the process of the step (2.1) if obvious surface damage exists on the optical surface; otherwise, entering the step (2.3);
(2.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into a magnetorheological polishing processing process software computer, and calculating to obtain the residence time required by magnetorheological polishing and the processed surface shape residual error so as to comprehensively evaluate the processing efficiency and the processing precision and preliminarily select a proper magnetorheological polishing removal function;
(2.4) forming a removal function close to the preliminary selection in the step (2.3) by adjusting polishing parameters such as the rotating speed, the magnetic field current, the flow and the like of the rheological polishing wheel, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by utilizing a magnetorheological polishing process software computer, generating a numerical control code required by a magnetorheological polishing machine tool by utilizing the residence time, and processing the optical element;
(2.5) detecting the face shape error by using the wave surface interferometer again, and finishing the magnetorheological polishing when the face shape precision is better than 10nm RMS; otherwise, repeating the step (2.3) to the step (2.4) until the machining result meets the requirement of surface shape precision.
3. Smoothing processing of medium-high frequency surface roughness:
(3.1) bringing the surface shape error of the optical element detected in the step (2.5) into a fairing polishing processing process software computer, obtaining a surface shape residual error after fairing processing through calculation, and evaluating the surface shape precision retention capacity in the fairing polishing process, so that proper fairing polishing parameters are selected;
(3.2) forming a numerical control code of a fairing polishing machine tool according to the fairing polishing parameter selected in the step (3.1), and fairing the optical element by using nano-scale cerium oxide micropowder as a main polishing abrasive in the polishing process, wherein the polishing time is 60 min;
(3.3) detecting the intermediate frequency surface roughness of the optical element by using a white light interferometer, and repeating the steps (3.1) and (3.2) when the intermediate frequency surface roughness is not controlled within 0.4nm RMS; otherwise, entering the step (3.4);
(3.4) selecting the same fairing polishing parameters as those in the step (3.2) to form a numerical control code of a fairing polishing machine tool, and using deionized water as a polishing abrasive to perform fairing processing on the optical element, wherein the polishing time is 30 min;
(3.5) respectively detecting the medium-frequency surface roughness and the high-frequency surface roughness of the optical element by using a white light interferometer and an atomic force microscope, wherein the medium-frequency surface roughness and the medium-frequency surface roughness are both better than 0.2nm RMS, and the fairing polishing processing is finished; otherwise, repeating the step (3.4) until the intermediate frequency surface roughness and the intermediate frequency surface roughness reach the requirements.
4. And (3) ion beam ultra-smooth polishing processing of the sub-nanometer precision optical surface:
(4.1) uniformly polishing and removing the surface material of the optical element by using low-energy ion beams, wherein the ion beams enter the fused quartz optical surface at a small angle in the processing process, and the uniformly removed thickness is about 100 nm;
(4.2) detecting the high-frequency surface roughness of the optical element by using an atomic force microscope, and repeating the step (4.1) when the high-frequency surface roughness is not controlled within 0.1nm RMS; otherwise, entering the step (4.3);
(4.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into an ion beam polishing process software computer, calculating to obtain a surface shape residual error after ion beam polishing, and evaluating the processed surface shape precision, thereby preliminarily selecting a proper ion beam polishing removal function;
(4.4) forming a removal function close to the preliminary selection in the step (4.3) by adjusting parameters such as the energy, the beam current and the like of the ion beam, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by using an ion beam polishing process software computer, generating a numerical control code required by an ion beam polishing machine tool by using the residence time, and processing the optical element;
(4.5) detecting the face shape error by using the wave surface interferometer again, and finishing the ion beam polishing processing when the face shape precision is superior to 1.0nm RMS; otherwise, repeating the step (4.3) to the step (4.4) until the machining result meets the requirement of surface shape precision.
In this embodiment, the above surface shape error of the optical element specifically refers to a surface error with a period of about 1mm to the full aperture size of the optical element;
in the present embodiment, the mid-frequency surface roughness of the optical element specifically refers to a surface error having a period of about 1 μm to 1 mm;
in the present embodiment, the above-mentioned high-frequency surface roughness of the optical element specifically refers to a surface error having a period of less than 1 μm;
in the present embodiment, the above-mentioned ultra-smooth surface specifically means that the high-frequency surface roughness is better than 0.1nm RMS;
in this embodiment, the nanometer level surface shape precision specifically means that the surface shape precision is better than 1.0nm RMS;
in this embodiment, the nano-scale diamond micropowder in step (2.1) specifically refers to diamond particles with a particle size of less than 1000 nm;
in this embodiment, the nano-sized cerium oxide fine powder in step (3.2) specifically refers to cerium oxide particles with a particle size of less than 1000 nm;
in the present embodiment, the low-energy ion beam in the step (4.1) specifically means that the ion energy used for the ion beam polishing process is in the range of 400eV to 800 eV;
in this embodiment, the small angle incidence of the ion beam in step (4.1) specifically means that the angle between the ion beam incidence direction and the normal direction of the optical surface is controlled in the range of 0 ° to 20 °.
In this embodiment, the optical element to be processed is a fused quartz concave spherical optical element, and the effective aperture is 135.7 mm. Firstly, a computer-controlled asphalt disc polishing method is adopted for pretreatment processing, the error of the polished surface shape is controlled to be within 1 mu m RMS, and no obvious damage is observed on the surface; secondly, removing the subsurface damage layer by utilizing magneto-rheological polishing, correcting the surface shape by using a removing function with high removing efficiency, and converging the surface shape error to 5.246nm RMS (refer to figure 1) so as to realize the efficient processing of the surface shape with nanometer precision; then, the surface shape error is corrected by smooth polishing, after polishing, the surface shape error is converged to 2.844nm RMS (see figure 2), and the medium-frequency surface roughness and the high-frequency surface roughness are respectively reduced to 0.202nm RMS (see figure 3) and 0.167nm RMS (see figure 4); and finally, performing final-stage polishing processing on the optical element by using ion beam polishing, wherein the surface shape precision reaches 0.368nm RMS (see figure 5) after polishing, and the medium-frequency surface roughness and the high-frequency surface roughness are respectively reduced to 0.204nm RMS (see figure 6) and 0.087nm RMS, so that the ultra-smooth processing of the sub-nanometer precision optical element is realized.
The invention is not limited to the foregoing embodiments. The invention extends to any novel feature or any novel combination of features disclosed in this specification and any novel method or process steps or any novel combination of features disclosed.

Claims (10)

1. A combined processing technology for ultra-smooth processing of a fused quartz optical surface is characterized by comprising the following steps: the method comprises the following steps:
(1) performing pretreatment processing in an early stage to ensure that the surface shape precision of the optical element is better than 1 mu m RMS;
(2) the high-efficiency magneto-rheological polishing of the nanometer level surface shape precision enables the surface shape precision of the optical element to be superior to 10nm RMS;
(3) smoothing processing of medium-frequency and high-frequency surface roughness is carried out, so that the medium-frequency surface roughness and the high-frequency surface roughness of the optical element are both superior to 0.2nm RMS;
(4) and performing ion beam ultra-smooth polishing processing on the sub-nanometer precision optical surface to ensure that the surface shape precision of the optical element surface is superior to 1.0nm RMS.
2. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 1, wherein the machining process comprises the following steps: the step (1) specifically comprises the following processing steps:
(1.1) carrying out pretreatment processing on the optical element by using a computer-controlled asphalt disc polishing method, removing surface damage obviously visible on the optical surface, and carrying out primary polishing processing on the surface shape error of the optical element;
(1.2) respectively detecting the surface shape precision and the surface quality of the optical element by using a wave surface interferometer and a digital microscope, and giving detection results of surface shape errors and surface damage;
when the surface shape precision is better than 1 μm RMS and no obvious surface damage is observed, the pretreatment processing at the early stage is finished; otherwise, repeating the process from the step (1.1) to the step (1.2) until the processing result meets the requirement of surface shape precision.
3. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 1, wherein the machining process comprises the following steps: the step (2) specifically comprises the following processing steps:
(2.1) uniformly removing the surface material of the optical element by adopting magneto-rheological polishing, wherein nano-diamond micro powder is selected as a main polishing abrasive in the polishing process, and the uniform removal thickness is about 5 mu m;
(2.2) observing the surface quality of the optical element by using a digital microscope, and repeating the process of the step (2.1) if obvious surface damage exists on the optical surface; otherwise, entering the step (2.3);
(2.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into a magnetorheological polishing processing process software computer, and calculating to obtain the residence time required by magnetorheological polishing and the processed surface shape residual error so as to comprehensively evaluate the processing efficiency and the processing precision and preliminarily select a proper magnetorheological polishing removal function;
(2.4) forming a removal function close to the preliminary selection in the step (2.3) by adjusting polishing parameters such as the rotating speed, the magnetic field current, the flow and the like of the rheological polishing wheel, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by utilizing a magnetorheological polishing process software computer, generating a numerical control code required by a magnetorheological polishing machine tool by utilizing the residence time, and processing the optical element;
(2.5) detecting the face shape error by using the wave surface interferometer again, and finishing the magnetorheological polishing when the face shape precision is better than 10nm RMS; otherwise, repeating the step (2.3) to the step (2.4) until the machining result meets the requirement of surface shape precision.
4. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 1, wherein the machining process comprises the following steps: the step (3) specifically comprises the following processing steps:
(3.1) bringing the surface shape error of the optical element detected in the step (2.5) into a fairing polishing processing process software computer, obtaining a surface shape residual error after fairing processing through calculation, and evaluating the surface shape precision retention capacity in the fairing polishing process, so that proper fairing polishing parameters are selected;
(3.2) forming a numerical control code of a fairing polishing machine tool according to the fairing polishing parameter selected in the step (3.1), and fairing the optical element by using nano-scale cerium oxide micropowder as a main polishing abrasive in the polishing process, wherein the polishing time is 60 min;
(3.3) detecting the intermediate frequency surface roughness of the optical element by using a white light interferometer, and repeating the steps (3.1) and (3.2) when the intermediate frequency surface roughness is not controlled within 0.4nm RMS; otherwise, entering the step (3.4);
(3.4) selecting the same fairing polishing parameters as those in the step (3.2) to form a numerical control code of a fairing polishing machine tool, and using deionized water as a polishing abrasive to perform fairing processing on the optical element, wherein the polishing time is 30 min;
(3.5) respectively detecting the medium-frequency surface roughness and the high-frequency surface roughness of the optical element by using a white light interferometer and an atomic force microscope, wherein the medium-frequency surface roughness and the high-frequency surface roughness are both better than 0.2nm RMS, and the fairing polishing processing is finished; otherwise, repeating the step (3.4) until the intermediate frequency surface roughness and the intermediate frequency surface roughness reach the requirements.
5. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 1, wherein the machining process comprises the following steps: the step (4) specifically comprises the following processing steps:
(4.1) uniformly polishing and removing the surface material of the optical element by using low-energy ion beams, wherein the ion beams enter the fused quartz optical surface at a small angle in the processing process, and the uniformly removed thickness is about 100 nm;
(4.2) detecting the high-frequency surface roughness of the optical element by using an atomic force microscope, and repeating the step (4.1) when the high-frequency surface roughness is not controlled within 0.1nm RMS; otherwise, entering the step (4.3);
(4.3) detecting the surface shape error of the optical element by using a wave surface interferometer, bringing the detected surface shape error into an ion beam polishing process software computer, calculating to obtain a surface shape residual error after ion beam polishing, and evaluating the processed surface shape precision, thereby preliminarily selecting a proper ion beam polishing removal function;
(4.4) forming a removal function close to the preliminary selection in the step (4.3) by adjusting parameters such as the energy, the beam current and the like of the ion beam, and accurately measuring the removal function under the polishing condition; accurately calculating residence time by using an ion beam polishing process software computer, generating a numerical control code required by an ion beam polishing machine tool by using the residence time, and processing the optical element;
(4.5) detecting the face shape error by using the wave surface interferometer again, and finishing the ion beam polishing processing when the face shape precision is superior to 1.0nm RMS; otherwise, repeating the step (4.3) to the step (4.4) until the machining result meets the requirement of surface shape precision.
6. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 2 or 3, wherein: the surface shape error of the optical element specifically refers to the surface error with the period of about 1mm to the full aperture size of the optical element.
7. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 1 or 4, wherein: the medium-frequency surface roughness of the optical element specifically refers to a surface error with a period of about 1 μm to 1 mm; the high-frequency surface roughness of the optical element specifically means a surface error having a period of less than 1 μm.
8. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 5, wherein the machining process comprises the following steps: the low-energy ion beam in the step (4.1) is specifically ion energy in a range of 400eV to 800eV used in ion beam polishing; the ion beam small angle incidence in the step (4.1) specifically means that an included angle between an ion beam incidence direction and an optical surface normal direction is controlled within a range of 0-20 degrees.
9. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 3, wherein the machining process comprises the following steps: the nano-scale diamond micro powder in the step (2.1) specifically refers to diamond particles with the particle size of less than 1000 nm.
10. The combined machining process for ultra-smooth machining of an optical surface of fused quartz according to claim 4, wherein the machining process comprises the following steps: the nano-scale cerium oxide micro powder in the step (3.2) specifically refers to cerium oxide particles with the particle size of less than 1000 nm.
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