CN107052913B - RB-SiC optical element polishing process processing methods - Google Patents

Abstract

The invention discloses a polishing process method for RB-SiC optical components. The method first uses inductively coupled plasma polishing technology to realize fine grinding and polishing of RB-SiC optical components, and then utilizes radio frequency magnetron sputtering surface planarization technology in the optical Deposit a nanoscale planarization layer on the surface, and finally use ion beam polishing technology to assist free radical microwave plasma polishing technology to achieve ultra-smooth surface processing of optical components. Compared with the prior art, the present invention uses plasma polishing technology to replace the traditional optical processing method, combined with nanoscale planarization layer preparation technology and ion beam modification polishing technology, which greatly shortens the cost of medium and large diameter RB‑SiC components. The processing cycle has established a new method of polishing RB‑SiC optical components with the characteristics of high efficiency, high precision and low loss.

Description

RB-SiC optical component polishing process processing method

technical field

The present invention relates to the technical field of ultra-smooth precision machining on the surface of optical elements, in particular to a polishing process method for RB-SiC optical elements.

Background technique

In recent years, with the vigorous development of the aerospace industry and the rapid progress of social production, human beings have become more and more enthusiastic about exploring the space field. Aerospace technology has made significant contributions to human observation of space, research on the earth and the entire vast space. Many countries have begun to attach importance to the research of aerospace technology and space optics. In order to meet the requirements of use, large-scale space systems such as space telescopes and remote sensing reconnaissance cameras need to have a high enough resolution and a large enough aperture, but the increase of the aperture, the corresponding It will increase the weight of the entire optical system. Therefore, it is not only necessary to improve the imaging quality of the optical system, but also to reduce its weight to reduce the launch cost. People put forward certain requirements for the material and manufacturing process of the space optical system reflector.

As a new type of space mirror processing material, silicon carbide (SiC) has a series of advantages such as low density, high strength and elastic modulus, small thermal expansion coefficient, good thermal conductivity, and high chemical stability. Large-diameter, lightweight mirror base material. Since the end of the 1970s, the United States, Germany, Japan and other developed countries have done a lot of research on the application of SiC materials to mirror substrates, and accumulated rich research experience. my country started relatively late in this regard. In recent years Some research results have also been obtained.

As a method for preparing SiC mirrors, the reaction sintering method has a simple process and low sintering temperature, and is the preferred method for preparing large-diameter and complex-shaped reaction-bonded silicon carbide (RB-SiC) materials.

RB-SiC is a material with a two-phase structure composed of Si and SiC (Si content is about 14%). Due to the difference in physical properties between the two, the removal rate of Si during the polishing process is faster than that of SiC, resulting in RB-SiC After polishing, the surface quality of SiC decreases, the roughness increases, the reflectivity decreases, and the scattering phenomenon is serious, which cannot meet the requirements of high-quality ultra-smooth optical systems at all.

The existing RB-SiC polishing method is a series of RB-SiC surface modification deposition process methods based on the Changchun Institute of Precision Optical Mechanical Physics and the Institute of Chemical Physics of the Chinese Academy of Sciences (invention patent: authorization number 200710159200.1), the core of which is, The intermediate process is beneficial to the coating method to deposit Si films (or other material modification layers) of several microns to tens of microns on the SiC surface.

At present, in the field of this material, a RB-SiC processing method based on magnetron sputtering modification has been mainly developed. The specific process is as follows:

The first step is to use the traditional grinding method to grind SiC, so that the surface roughness Ra of the material can reach below 10nm (or RMS below 20nm), which can be used for optical interferometry;

The second step is to use the magnetron sputtering method to deposit tens of microns of Si film on the surface of RB-SiC. The core of this method is that the application of RB-SiC is mainly to achieve high optical reflection. Therefore, on the RB-SiC substrate Deposit a sufficiently thick Si modified layer. Compared with RB-SiC, the Si film has a single material, low hardness and easy processing;

In the third step, the Si thin film deposited on the RB-SiC is polished using the CNC small tool polishing method to achieve ultra-smooth surface processing and surface shape correction.

The existing traditional optical processing methods have the following problems. First, the processing efficiency is low and the cycle time is long. Second, it is difficult to achieve an ultra-smooth surface (roughness RMS less than 1nm, surface shape RMS below λ/5). This is because: the deposition rate of magnetron sputtering is not high, and the deposition of tens of microns thick Si layer results in longer processing time. Second, magnetron sputtering deposits Si thin films. The roughness will deteriorate, usually from 7~8nm to more than ten nanometers.

In view of this, how to design an ultra-smooth surface processing method that can reduce the surface roughness of RB-SiC optical components and improve surface quality, especially how to design a surface polishing processing method that is easy to operate and easy to implement, can effectively control the surface of optical components. Surface shape, inhibit surface and sub-surface damage, reduce surface optical loss.

Contents of the invention

Aiming at the degradation of the surface quality of the material caused by polishing RB-SiC by the existing traditional mechanical polishing method, the surface roughness is large, the subsurface damage of the material surface is serious, especially, the ultra-long processing cycle and the extremely low processing efficiency, the present invention The purpose of the invention is to provide a processing method for polishing the RB-SiC optical element and overcome the problems existing in the existing polishing method for the material.

In order to achieve the above object, the technical solution adopted in the present invention is:

The RB-SiC optical element polishing process processing method includes the following steps:

Step 1. Firstly, the RB-SiC blank is polished and etched to realize the precision grinding of the optical element, so that the surface roughness value of the optical element converges to within 20 nanometers;

Step 2, using radio frequency magnetron sputtering (RF-MS) to deposit a nanoscale planarization layer on the surface of the RB-SiC optical element;

Step 3, use the radical microwave plasma source technology (RPS) to polish the planarization layer deposited on the surface of the RB-SiC substrate, use the radical plasma technology to confine the plasma within the plasma source body, and pass through the vacuum chamber Conductance control, forming a large area of uniform active radicals, making the active radicals chemically react with the material of the planarization layer, and realizing ultra-smooth polishing of the surface of optical components;

Step 4: Use ion beam modification polishing technology (IBF) to modify and polish the planarization layer on the surface of the optical element, and realize the surface shape modification of the optical element by removing the surface with high certainty.

The ICP etching and polishing device in the step 1 has a background vacuum of 2.0×10 ^-4 Pa and a working vacuum of 0.5-10 Pa.

When the initial blank surface roughness RMS>100nm, the bias power is maintained at 100~150W, and the reactive gas uses high-purity carbon tetrafluoride (purity 99.99%).

For the initial RB-SiC workpiece with RMS>100nm, the RF power is 150W, the bias power is 150W, the etching gas flow rate is 25~30sccm, and the working pressure is 2~5Pa.

In the step ⁴ , a 13.56MHz radio frequency ion source is used, and the three-dimensional motion control system is used to control the processing trajectory and residence time of the ion source. 99.99%) is the working gas.

In step 2, the working vacuum degree is 1.2Pa, and the target power density is controlled at 5~10W/cm ² .

In step 3, the conductance of the vacuum gas and the pumping speed of the vacuum pump are controlled to form an active base area with a uniformity of <5%. The typical vacuum degree during polishing is 50-100Pa.

In step 4, the ion beam energy is controlled below 800eV, the beam spot size is controlled below 10mm, and the etching efficiency is 1.0×10 ^-3 ~0.02mm ³ /min.

The method of the present invention has the following advantages:

1. This processing method greatly shortens the processing cycle of existing processing methods and improves efficiency. This is mainly reflected in the fact that in the initial polishing stage, the invention creatively directly applies the ICP plasma etching processing method to the surface processing of the SiC rough billet. Through process optimization, the SiC surface can be roughened by adjusting the ratio of fluorine-containing gas and oxygen. The thickness is from RMS100~200nm to RMS10~20nm, and the surface shape is better than 4 microns, which improves the processing efficiency and completely solves the difficulty of processing SiC materials with high hardness by traditional methods. This method can shorten the processing cycle by 1/3~1/2;

2. The idea of the modification method in the traditional process is based on the fact that RB-SiC is brittle and hard to process, and the material composition is complex (including Si and SiC in different proportions). Due to its inconsistent composition, all in small tool polishing or traditional During the grinding process, the removal efficiency of Si and SiC in different phases is different, and it is almost impossible to achieve smooth surface processing. Therefore, a modification method was proposed, the core of which is to deposit a sufficiently thick Si layer on the surface of RB-SiC , as long as its bonding strength with the substrate RB-SiC film can be guaranteed, the subsequent method is only to polish the silicon layer, and does not involve the polishing of the substrate RB-SiC material. Therefore, the modification method must make the deposited silicon layer thick enough , to avoid subsequent processing to the substrate. In the magnetron sputtering deposition process of the present invention, only a 100-500nm Si planarization layer needs to be plated, while the modified method needs to be plated with a 2-200μm layer. Due to the low deposition rate of the intermediate frequency magnetron sputtering, this process link, the processing efficiency of the method adopted by the present invention is also much better than the existing processing method;

3. The purpose of using magnetron sputtering to deposit a silicon layer in the traditional process is surface modification, while the purpose of using magnetron sputtering to deposit a silicon layer in the present invention is to planarize. When the film thickness is within hundreds of nanometers, after film deposition The surface quality of the material can be optimized to a certain extent, mainly manifested in the reduction of the surface roughness and the reduction of the intermediate frequency processing error. The patent of the present invention is based on this consideration. During the film deposition process, the process parameters are optimized and adjusted to ensure Under the condition of the bonding strength of the film base, the surface roughness of the material can be optimized to below RMS2nm by plating a 100~500nm thin film layer;

4. Use plasma to realize the excitation of fluorine-containing reactive gas (note that ionization is not required), and use these active groups to act on the surface of the material. Since the action of active ions or other ions is suppressed, no physical damage will occur during the polishing process. Sputter etching, but only use the chemical reaction between Si and SiC and fluorine-containing active groups to achieve large-area uniform etching and polishing, effectively suppressing the cracking of the film layer caused by the thermal mismatch between the planarization layer and the optical element substrate or fall off, and because its reaction rate is determined by the concentration of active groups, it is easy to implement the etching precision of the surface planarization layer at the nanometer level, and further reduce the surface roughness of the material on the basis of the previous steps To below RMS1nm;

5. The ion beam polishing modification method is a relatively mature method used in the non-contact polishing modification method at present. This method cannot be directly used for the polishing modification of RB-SiC materials. The reason is that RB-SiC is different. The sputtering efficiency of the phases is different, which leads to the deterioration of the surface shape during the polishing process. However, in the present invention, due to the use of magnetron sputtering to deposit a planarization layer, the ion beam polishing modification method can directly act on the surface of the Si film, realizing the surface shape maintenance of large-area optical elements;

6. The polishing methods adopted in the present invention are all based on plasma polishing and ion beam polishing, both of which are non-contact processing. There is no contact stress and strain in the etching process, and full-caliber high-precision roughness can be achieved. While improving the surface shape and surface quality, it can achieve high-efficiency shaping and polishing of optical components, greatly improving polishing efficiency and reducing time costs. For brittle and hard materials such as SiC, this completely avoids damage to the surface and subsurface of components by traditional processing methods or contact processing methods such as small tools, and will not cause stress problems.

Description of drawings

Readers will have a clearer understanding of various aspects of the present invention after reading the detailed description of the present invention with reference to the accompanying drawings. in,

Fig. 1 is the schematic diagram of processing technology flow in the embodiment of the present invention;

2 is a schematic diagram of the working principle of ICP etching in an embodiment of the present invention;

3 is a schematic diagram of the principle of radio frequency magnetron sputtering in an embodiment of the present invention;

4 is a schematic diagram of the principle of using a free radical plasma source in an embodiment of the present invention;

Figure 5 is a graph of the surface roughness test results of optical elements under different processes;

Among them, a is the surface roughness test diagram of the reaction sintered silicon carbide substrate after preliminary ICP etching, b is the surface roughness test diagram of the silicon planarization layer deposited on the surface of the reaction sintered silicon carbide substrate, and c is the RPS etching planarization Surface roughness test chart after layer.

Detailed ways

In order to make the technical content disclosed in this application more detailed and complete, reference may be made to the accompanying drawings and the following various specific embodiments of the present invention.

A polishing process method for RB-SiC optical components, comprising the following steps:

Step 1. Firstly, the RB-SiC blank is polished and etched to realize the precision grinding of the optical element, so that the surface roughness value of the optical element converges to within 20 nanometers;

Step 2, using radio frequency magnetron sputtering (RF-MS) to deposit a nanoscale planarization layer on the surface of the RB-SiC optical element;

Step 3, use the radical microwave plasma source technology (RPS) to polish the planarization layer deposited on the surface of the RB-SiC substrate, use the radical plasma technology to confine the plasma within the plasma source body, and pass through the vacuum chamber Conductance control, forming a large area of uniform active radicals, making the active radicals chemically react with the material of the planarization layer, and realizing ultra-smooth polishing of the surface of optical components;

Step 4: Use ion beam modification polishing technology (IBF) to modify and polish the planarization layer on the surface of the optical element, and realize the surface shape modification of the optical element by removing the surface with high certainty.

The ICP etching and polishing device in the step 1 has a background vacuum of 2.0×10 ^-4 Pa and a working vacuum of 0.5-10 Pa.

When the initial blank surface roughness RMS>100nm, the bias power is maintained at 100~150W, and the reactive gas uses high-purity carbon tetrafluoride (purity 99.99%).

For the initial RB-SiC workpiece with RMS>100nm, the RF power is 150W, the bias power is 150W, the etching gas flow rate is 25~30sccm, and the working pressure is 2~5Pa.

In the step ⁴ , a 13.56MHz radio frequency ion source is used, and the three-dimensional motion control system is used to control the processing trajectory and residence time of the ion source. 99.99%) is the working gas.

In step 2, the working vacuum degree is 1.2Pa, and the target power density is controlled at 5~10W/cm ² .

In step 3, the conductance of the vacuum gas and the pumping speed of the vacuum pump are controlled to form an active base area with a uniformity of <5%. The typical vacuum degree during polishing is 50-100Pa.

In step 4, the ion beam energy is controlled below 800eV, the beam spot size is controlled below 10mm, and the etching efficiency is 1.0×10 ^-3 ~0.02mm ³ /min.

The specific implementation manners of various aspects of the present invention will be further described in detail below with reference to the accompanying drawings.

The surface flattening processing method of the reaction sintered silicon carbide optical element involved in the present invention can realize the flattening processing effect of the surface of the reaction sintered silicon carbide optical element, and the roughness can be less than 1 nm.

See Figure 1:

The present invention combines ICP plasma polishing technology, radio frequency magnetron sputtering planarization layer deposition technology (RF-MS), free radical plasma source polishing technology (PR2) and ion beam modification polishing technology to construct a complete set of Polishing process. The basic idea of this process is to use plasma technology to realize non-destructive polishing of RB-SiC optical components, to overcome the stress and damage problems introduced by traditional optical processing methods, and the second is to use physical vapor deposition method to planarize the surface of optical components. The idea of layer, while improving the surface roughness of the optical element, makes the surface material of the optical element uniform, thus making ion beam modification and polishing possible, and the thickness of the nano-level planarization layer ensures the efficiency of the process cycle.

In order to achieve the above object, the specific scheme of the patent of the present invention is:

1) First, use ICP polishing and etching technology to realize the etching process of RB-SiC blank samples. For optical components with different roughness, focus on the bias power and reaction gas flow process parameters to make the surface roughness of optical component blank samples The value converges below 20 nanometers to realize the fine grinding and polishing process of optical component samples.

2) Afterwards, use radio frequency magnetron sputtering technology to deposit a Si planarization layer on the surface of the RB-SiC workpiece after fine grinding and polishing. Adjust to determine the appropriate deposition rate (15nm/min~20nm/min), through the rotation of the optical element, realize the uniform deposition of the Si planarization layer, and reduce the surface roughness value of the optical element to below 2nm. According to the requirements of the forming process, the thickness of the planarization layer is controlled at 100~500nm.

3) The third step is to use RPS etching and polishing technology to etch and polish the planarization layer deposited on the surface of RB-SiC. This method can realize the polishing of large-area (> Ф 300mm caliber) complex surfaces. RPS technology passes 2.45GHz The microwave excitation source makes the _N2 gas ionize to form plasma. Since no extraction electrode is applied, the plasma is confined inside the plasma source. At this time, high-purity fluorine-containing gas and oxygen and other active gases are introduced into the plasma Activated under the action of the ionized active gas, a small amount of ionized active gas is still confined inside the plasma source, while a large number of activated active radicals form a large-area uniform area under the control of vacuum conductance, directly acting on the optical The surface of the component is polished through the chemical reaction between the active radical gas and Si, and the surface roughness is finally optimized to less than 1nm.

4) Finally, use ion beam polishing technology to perform ion beam modification and polishing on the RB-SiC optical element after RPS polishing, control the removal function by adjusting the characteristic parameters of the ion source, and then use the difference between the actual surface shape and the ideal surface shape to obtain The residence time of each point on the surface of the component, through the three-dimensional motion control system, realizes the surface shape correction of the chemical component, and the corrected surface shape can reach 1/5~1/10 λ .

Example 1:

1) ICP etching: see Figure 2, the background vacuum of the vacuum chamber of the ICP etching equipment is 2.0×10 ^-4 Pa, and the working vacuum is controlled at 0.5~10Pa. Put the reaction sintered silicon carbide sample (RMS at 209.72nm) with a diameter of 150mm and a thickness of 10mm on the base of the vacuum chamber of the ICP etching equipment, and open the pre-pumping valve and the front-stage valve in sequence. When the pressure value of the composite vacuum gauge drops to At 5Pa, turn on the molecular pump and press the start button. When the speed of the molecular pump reaches 400r/min, open the high valve and close the pre-pumping valve at the same time. When the pressure value of the composite vacuum gauge is lower than 10 ^-1 Pa, turn on the gas flowmeter Switch the switch of the CF ₄ gas flowmeter to the valve control position, adjust the knob of the flowmeter, set the flow rates of the two gases to 25sccm respectively, then open the main gas valve, adjust the high valve, let the air pressure stabilize at 1Pa, and pass the time controller to etch The time is set to 3000s. At this time, turn on the RF power supply and the bias power supply at the same time, adjust the matching knob, and set the RF power and bias power to 150W respectively. After the etching is completed, turn off the main gas valve and the gas flow meter switch in sequence. , Molecular pump stop button, until the molecular pump speed drops to 0, then close the high valve and front valve, after cooling for two minutes, press the inflation valve button, after the inflation is completed, press the open button, take out the etched sample for Surface roughness test, the test result RMS value is 16.702nm.

2) Deposit planarization layer by radio frequency magnetron sputtering:

A Si planarization layer was plated on the reaction sintered silicon carbide substrate after ICP etching by using radio frequency magnetron sputtering technology. Referring to Fig. 3, general-purpose radio frequency magnetron sputtering equipment is used, the sputtering target used is a high-purity silicon target with a purity of 99.9995%, the working gas is determined to be argon with a purity of 99.99%, and the radio frequency is 13.56MHz. The background vacuum was evacuated to 8.0×10 ^-4 Pa, and then the argon flow rate was set to 40 sccm, and the working pressure was set to 1.2 Pa. At the same time, the pre-coated substrate was rotated to the position above the corresponding sputtering target by rotating the sample stage. The target base distance is 80mm; turn on the RF power supply, adjust the RF power density to 10W/cm ² , pre-sputter for 15 minutes after discharge, and then close the baffle, the thickness of the deposited film is 386nm; turn off the RF power supply, intake valve and molecular pump The baffle valve between the vacuum chamber and the vacuum chamber was annealed for 15 minutes under pressure, the ionization gauge was closed, the inflation valve was opened, and the sample was taken out after inflation, and the RMS value of the surface roughness was tested to be 1.6599nm.

3) RPS etching and polishing: see Figure 3, evacuate the vacuum chamber to a background vacuum of 2.0×10 ^-3 Pa, feed 500Sccm high-purity N ₂ (purity 99.999%), turn on the 2.45GHz microwave power supply, and set the microwave power supply to At 1.5KW, after stabilizing for 15 minutes, reduce the N ₂ flow rate to 100Sccm, feed 450Sccm CF ₄ (purity 99.99%) and 30Sccm O ₂ (purity 99.999%), adjust the vacuum gas pumping speed, and set the vacuum degree at 60Pa. The surface of the optical element was polished, and when the Si planarization layer was removed by 150nm, the RMS value of the surface roughness after etching the planarization layer was tested to be 0.79706nm. Among them, the magnetic field lines distributed in the vertical direction confine electrons near the surface of the target, prolong their trajectory in the plasma, and increase the probability of electrons participating in gas molecule collisions and ionization processes.

4) Ion beam polishing: see Figure 4, set typical ion source working parameters: background vacuum 1.0×10 ^-4 Pa, RF power 180W, beam voltage 500V, accelerating grid voltage 100V, argon (99.99%) flow rate 5Sccm , the working vacuum degree is 5.0×10 ^-2 Pa, the diameter of the beam spot is controlled at Ф 10~5mm, and the removal efficiency is determined to be controlled at 5.0×10 ^-3 ~0.01mm ³ /min, and finally the surface shape correction of the optical element is realized. to 1/10λ.

In this embodiment, a 13.56MHz radio frequency ion source is used, and the three-dimensional motion control system is used to control the processing trajectory and residence time of the ion source. The background vacuum of the vacuum chamber used is 1.0×10 ^-4 Pa. 99.99%) is the working gas.

See Figure 5, which uses Taylor Hobson's Taly Surf CCI2000 non-contact white light interferometer to measure the surface roughness of the sample after ICP fine grinding and polishing, magnetron sputtering planarization deposition, and RPS polishing, among them, Figure 5 (a) is the roughness of the test point on the surface of the sample after ICP fine grinding and polishing. It can be seen from the figure that the RMS has dropped from 209.72nm of the blank to RMS16.702nm. After plating a layer of silicon planarization layer, the surface of the sample is rough The surface roughness of RB-SiC dropped to RMS1.6599nm (as shown in Figure 5(b)), and after RPS polishing was finally used, the surface roughness of RB-SiC dropped to 0.79706nm, down to below 1nm, successfully achieving ultra-smooth surface processing.

The above is a preferred embodiment of the present invention, but for those of ordinary skill in the art, its content is not limited to the examples. By reading the description of the present invention, some changes and changes can be made without departing from the principles of the present invention. , All technical solutions under the idea of the present invention are within the protection scope of the claims of the present invention.

Claims (8)

1. The RB-SiC optical element polishing process machining method comprises the following steps:

   step 1, firstly, polishing and etching RB-SiC blank materials to realize precise grinding of optical elements, so that the roughness value of the surfaces of the optical elements is converged within 20 nanometers;

   step 2, depositing a nanoscale planarization layer on the surface of the RB-SiC optical element by utilizing a radio frequency magnetron sputtering technology (RF-MS);

   step 3, polishing the planarization layer deposited on the surface of the RB-SiC substrate by utilizing a free radical microwave plasma source technology (RPS), limiting plasma in a plasma source body by utilizing the free radical plasma technology, and forming large-area uniform active free radicals by controlling the conductance of a vacuum chamber, so that the active radicals and the planarization layer material perform chemical reaction to realize the ultra-smooth polishing processing of the surface of the optical element;

   and 4, utilizing an ion beam shape-modifying polishing technology (IBF) to modify and polish the surface planarization layer of the optical element, and realizing the surface shape modification of the surface of the optical element by removing the surface with high certainty.
2. 2. The RB-SiC optical element polishing process machining method according to claim 1, characterized in that: the polishing etching in the step 1 is an ICP etching polishing device with a background vacuum of 2.0 multiplied by 10^-4Pa, and the working vacuum is controlled to be 0.5-10 Pa.
3. 3. The RB-SiC optical element polishing process machining method according to claim 2, characterized in that: when the initial blank surface roughness RMS is more than 100nm, the bias power is maintained at 100-150W, and the reactive gas is carbon tetrafluoride with the purity of 99.99%.
4. 4. The RB-SiC optical element polishing process machining method according to claim 3, characterized in that: aiming at an initial RB-SiC workpiece with RMS of more than 100nm, the radio frequency power is 150W, the bias power is 150W, the etching gas flow is 25-30 sccm, and the working pressure is 2-5 Pa.
5. 5. The RB-SiC optical element polishing process machining method according to claim 4, wherein: in the step 4, a 13.56MHz radio frequency ion source is adopted, a three-dimensional motion control system is used for controlling the processing track and the residence time of the ion source, and the background vacuum of a vacuum chamber is 1.0 multiplied by 10^-4Pa, argon with the purity of 99.99 percent is used as working gas.
6. 6. RB-SiC optical element polishing according to claim 5The technical processing method is characterized in that: in the step 2, the working vacuum degree is 1.2Pa, and the target power density is controlled to be 5-10W/cm²。
7. 7. The RB-SiC optical element polishing process machining method according to claim 6, characterized in that: and 3, forming an active base region with the uniformity of less than 5% by controlling the vacuum gas conductance and the pumping speed of a vacuum pump, wherein the typical vacuum degree is 50-100 Pa during polishing.
8. 8. The RB-SiC optical element polishing process machining method according to claim 7, characterized in that: step 4, controlling the ion beam energy below 800eV, controlling the beam spot size below 10mm, and controlling the etching efficiency to be 1.0 multiplied by 10^-3~0.02mm³/min。