JPH08323604A - Abrasive method for sic, and manufacture of optical element - Google Patents

Abstract

PURPOSE: To form an extremely smooth surface, by abrading SiC by using abrasive liquid containing a diamond abrasive grain having a specific range mean grain diameter. CONSTITUTION: A polycrystal diamond abrasive grain, having a mean grain diameter of 1μm (1-3μm) of 0.025wt.% (0.2wt.% or less), is put into purified water of 2 liter to be stirred, and a hexametaphosphoric acid sodium fine powder, as dispersant, of 2wt. parts (1-2 wt. parts) is added to a diamond abrasive grain of 100wt. parts in abrasive liquid to obtain abrasive liquid 7. Both of an abrasive pan 1 having a pitch surface 6 having a penetration of 15 (5-20) and an article 2 to be worked retained by an article to be worked holder 3 are dipped into a vessel 8 filled with the abrasion liquid 7 to abrade CVD-SiC material. While an abrasive pan rotating shaft 9 is rotated at 7rpm, an oscillation shaft 4 for supporting the article 2 is oscillated at 5 cycles/minute. Abrasion pressure is 30KPa (66Kpa or less). Consequently, an extremely smooth surface, having a surface roughness of 0.15nm RMS, can be obtained without a pit and an abrasive passing trace on a surface to be worked.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for polishing the surface of a SiC (silicon carbide) material, more specifically CVD-S.
The present invention relates to a polishing method very suitable for polishing a mirror optical element or the like having a surface made of iC (SiC formed by a CVD method) to form a highly accurate surface.

[0002]

2. Description of the Related Art Conventionally, a CVD-SiC material is used as a mirror for high-energy short-wavelength light because of its excellent physical properties despite its high price. The shape of these mirrors is usually a simple shape such as a flat surface, a cylindrical surface, a spherical surface, or a toroidal surface. A typical manufacturing method of these optical elements is a step of grinding a sintered substrate of β-SiC into a final shape, and CV is applied to the substrate.
The step of forming an α or β-SiC dense film by the D method, the step of forming the surface by grinding again, and the polishing to reduce the shape error, waviness, surface roughness, etc. to improve the surface quality and to improve the mirror. It consists of a finishing process.

In this final step (finishing step),
For example, a polishing dish having a diameter approximately the same as that of the mirror material (in the case of trying to create a cylindrical surface, a polishing dish having the same radius of curvature as the cylindrical surface, and the unevenness being opposite) is C
It is moved relative to the VD-SiC mirror substrate, and polishing is performed by interposing a polishing liquid in which a polishing agent such as chromium oxide fine powder, silica fine powder, and diamond fine powder is dispersed in water, and a cylindrical surface having a predetermined curvature is mirror-finished. Go. When polishing a flat surface, a flat polishing dish larger than the mirror material is usually used. As a polishing device, a normal oscillating polishing machine is used when polishing a flat surface or a spherical surface, and when polishing a surface having a non-axisymmetric shape such as a cylindrical or toroidal shape, the lower axis and the upper axis are independent of each other. A cylindrical polishing machine that swings in the orthogonal direction is used.

[0004]

In these mirrors, the shape accuracy is important, but at the same time, the surface roughness is 0.3n.
A super smooth surface of the order of mRMS is desired. However, with respect to the surface roughness, the generation mechanism thereof has not been sufficiently elucidated. Only smooth surfaces up to about 4 nm and Rmax of 4 nm are obtained.

In the prior art, when processing an SiC optical element which requires ultra-smooth surface roughness, a processor who is skilled in finishing polishing polishes an abrasive such as colloidal silica, chromium oxide or diamond. While observing the state of the surface, the conditions were finely changed, and polishing was performed to deal with it.
However, this method requires a long processing time and a high processing cost. Furthermore, in the prior art, the surface roughness is 0.5 to
Even if a smooth surface of about 2 nm RMS can be formed over time, for example, it is possible to reliably obtain an ultra smooth surface (surface roughness of about 0.3 nm RMS or less) which is often required as a short wavelength optical element. It was difficult.

The object of the present invention is to obtain a surface roughness of 0.3 nm RM.
An object of the present invention is to provide a SiC polishing method and an optical element manufacturing method capable of forming an ultra-smooth surface of about S.

[0007]

The above object can be achieved by the present invention described below.

A method for polishing SiC, characterized in that a polishing liquid containing diamond abrasive grains having an average particle diameter of 1 to 3 μm is used.

A method for manufacturing an optical element having a surface made of SiC, comprising a step of polishing with a polishing liquid containing diamond abrasive grains having an average particle diameter of 1 to 3 μm.

[0010]

The present invention is characterized by using a polishing liquid containing diamond abrasive grains having an average grain size of 1 to 3 μm. When this polishing liquid is used to polish a spherical surface of a CVD-SiC material with a full-face dish polishing tool of approximately the same size as the surface to be processed, pits are generated on the surface to be processed, and abrasive grain passage marks are generated. It is possible to obtain a highly reliable, highly accurate and uniform super-smooth spherical surface without causing a deterioration factor of the polished surface. Therefore, it becomes unnecessary to finely change the polishing conditions and to carry out the polishing process while observing the state of the polished surface, and the work becomes very easy. Further, since the polishing conditions can be kept constant, the processing efficiency can be improved and the processing cost can be reduced.

Further, for example, when aspherical mirrors are polished from a CVD-SiC material with a polishing tool smaller than the surface to be processed, pits, abrasive grain passing marks, etc. can be similarly prevented and highly reliable and highly uniform. A super smooth aspherical surface is obtained. Further, the present invention is a highly versatile method that can be carried out regardless of the shape of the surface to be processed, since good results can be obtained only by setting the polishing liquid and the polishing tool material.

According to the knowledge of the present inventors, the main reason for the deterioration of surface roughness during polishing of CVD-SiC, which is a highly brittle polycrystalline material, is that pits (crystals) are caused by the difference in hardness between crystal faces. The low hardness portion of the grain is selectively removed by polishing to generate mosaic pits) and the appearance of abrasive grain passage marks.

In the conventional method, when a full-scale polishing tool having substantially the same size as the surface to be processed is used, it is empirically known as a countermeasure against the deterioration factor of polishing that the polishing is carried out at a relatively large polishing removal rate. It was However, as described above, the reason for the deterioration of the surface roughness is pits or abrasive grain passage marks, and increasing the polishing removal rate is nothing but increasing the material removal action of the abrasive on the surface to be processed. As a result, the appearance of abrasive grain passage marks increases on the surface to be polished, and the surface roughness is finished. Further, if the polishing removal rate is lowered, the appearance of abrasive grain passage traces can be prevented, but pits are generated on the polished surface. Therefore, it is considered that a sufficient super smooth surface could not be obtained only by adjusting the polishing removal rate known empirically as in the conventional method.

Therefore, the present inventors have made various studies on polishing conditions and the like in order to find out a polishing method that can easily obtain an ultra-smooth surface, and have completed the present invention as a result. Hereinafter, of the experiments conducted for this study, representative ones will be described.

<Experiment 1: Study on Average Particle Size of Diamond Abrasive Grains> In the prior art, for example, in the case of ultra-smooth polishing of quartz glass, the smaller the average particle size of the abrasive grains dispersed in the polishing liquid, the smaller the average particle size. It is known that the surface roughness is good. On the other hand, the present inventors have found that the CVD-S
When performing iC ultra-smooth polishing, it is not necessary that the grain size of the abrasive grains is simply small, and that diamond should be used as the abrasive grains and the average grain size should be within a specific range. I found out from.

First, CVD with a diameter of 50 mm and a thickness of 10 mm
-Five test pieces of SiC material were flat-polished to a surface roughness as good as possible by a conventional method, and an average surface roughness was measured and recorded by a non-contact type surface roughness measuring device.

Next, to 2 liters of purified water ,. The average particle size is 0.25 μm ,. Average particle size 0.5 μm ,. Average particle size 1 μm ,. Average particle size 3 μm ,. 5 with an average particle size of 5 μm
The test piece was partially polished by a polishing tool smaller than the surface to be processed, using each of the five types of polishing liquids obtained by stirring 0.025% by weight of polycrystalline diamond abrasive grains of each type.
The polishing conditions were such that the tool diameter was 16 mm, the penetration of the pitch used for the tool was 15, the polishing pressure was 25 KPa, and the tool oscillation was ± 4 mm and 5 Hz. The tool should move in the direction perpendicular to the swing.
Reciprocal scanning was performed in a 16 mm section at 53 mm / sec.

As a result of this polishing, the average particle size of 0.25 μm
In the polishing with the diamond abrasive grains of m, the deterioration of the surface roughness due to mosaicization of the polished surface (generation of pits) was remarkable, and the surface roughness after 90 minutes was 0.75 nm RMS. In polishing with diamond abrasive grains having an average particle diameter of 0.5 μm, deterioration of the surface roughness due to mosaicization of the polished surface was observed in some places, and the surface roughness after 90 minutes was 0.4 nm RMS. In polishing with diamond abrasive grains having an average grain size of 1 μm and an average grain size of 3 μm, the super smoothing of the polished surface proceeded, and the surface roughness after 90 minutes was 0.2 nm RMS.
However, when the average particle size is 3 μm, the precipitation phenomenon of the abrasive grains tends to occur, so caution is required during long-time processing. In the case of polishing with diamond abrasive grains having an average particle diameter of 5 μm, the appearance of abrasive grain passing traces on the polished surface is increased, and mosaicing of the polished surface is not observed, but the surface roughness is 0.4 nmR after 90 minutes.
It was MS, and the precipitation phenomenon of the abrasive grains remarkably occurred. For reference, polishing was also attempted with diamond abrasive grains having an average grain size of 6 μm, but the tool pitch was worn and the tool life was short and inappropriate under the conditions of this experiment. Among these, typical abrasive grains have an average particle size of 3 points (0.25 μm, 1 μm, 3
μm) results are shown as a graph in FIG.

From the above results, when performing ultra-smooth polishing of CVD-SiC, deterioration of the polishing surface due to the appearance of abrasive grain passage marks and mosaicization of the polishing surface due to the average particle diameter of the diamond abrasive grains in the polishing liquid It was confirmed that the frequency of deterioration of the surface roughness due to was different, and at the same time, it was confirmed that the average particle size range should be 1 μm to 3 μm in order to reasonably obtain a super smooth surface.

<Experiment 2: Study on Polishing Pressure> In the prior art, for example, in the case of ultra-smooth polishing of quartz glass,
It is known that polishing pressure has little effect on surface roughness. On the other hand, the present inventors have found from the results of the following experiments that the polishing pressure and the surface roughness have a correlation when performing ultra-smooth polishing of CVD-SiC.

Under the same conditions as in Experiment 1, except that the average grain size of the diamond abrasive grains was 1 μm and the polishing pressure was. 1
2 KPa ,. 25 KPa ,. 45 KPa ,. 66
Polishing was performed using four types of KPa.

The result of this polishing is shown as a graph in FIG. In the case of 12 KPa, it took a long time to improve the surface roughness, and it was about 0.3 nm RMS in 120 minutes. In the case of 25 KPa and 45 KPa, the surface roughness is rapidly improved, and it is 0.3 nm after 90 minutes of processing.
The RMS level was reached, and super smooth polishing proceeded smoothly. Of 6
At 6 KPa, the surface roughness improves initially, but 0.5n
It was confirmed by the normalski microscope observation that the deterioration of the surface roughness started after the improvement to about mRMS and the deterioration of the surface roughness due to the mosaicization of the polished surface was remarkable.

From the above results, it can be confirmed that when performing ultra-smooth polishing of CVD-SiC, the degree of improvement in surface roughness differs depending on the degree of polishing pressure, and at the same time, the polishing pressure range that gives good results is 25 to 45 under the conditions of this experiment
It was confirmed to be KPa.

<Experiment 3: Examination on Penetration of Pitch>
In the prior art, for example, in the case of ultra-smooth polishing of quartz glass, it is known that the softer the pitch material of the polishing tool, the smoother the surface roughness obtained. On the other hand, when performing ultra-smooth polishing of CVD-SiC, the present inventors do not necessarily want only the pitch material to be soft, and the following experiment shows that a pitch with a penetration of a specific range is suitable. We found it from the results.

Under the same conditions as in Experiment 1, except that the diamond abrasive grains had an average particle size of 1 μm and the pitch penetration (the smaller the penetration, the harder the hardness was). 2 ,. 5,
． 15 ,. 20 ,. Polishing was carried out with 5 types of No. 25 and No. 25.

As a result of this polishing, in the case of needle penetration of 2,
When the pitch material was hardened, the surface roughness deteriorated with respect to the initial value. When this was observed with a Normalski microscope, many abrasive grain passage marks were found on the polished surface. In case of the penetration degree of 5, the surface roughness is improved, and it is 0.3 nm RM after 90 minutes of processing.
It became about S. That is, super smooth polishing proceeded smoothly. When this was observed with a Normalski microscope, some traces of passage of abrasive grains were found on the polished surface. Penetration of 15,
With a penetration of 20, the surface roughness improved steadily to 0.2
It reached about nmRMS. When this was observed with a Normalski microscope, the polished surface was smoothed. Penetration of 25
In the case of No. 4, mosaicing of the polished surface had begun to occur, and the surface roughness was about 0.4 nm RMS.

From the above results, it was confirmed that the pitch penetration (hardness of the pitch material) is preferably about 5 to 20 when performing ultra-smooth polishing of CVD-SiC.

<Experiment 4: Examination of Crystallinity of Diamond Abrasive Grains> The inventors of the present invention have found that when performing ultra-smooth polishing of CVD-SiC, the diamond abrasive grains are preferably polycrystalline rather than single crystal. Was found from the results of the following experiments.

Under the same conditions as in Experiment 1, except that the diamond abrasive grains had an average particle diameter of 1 μm, Single crystal diamond abrasive grains ,. Polycrystalline diamond abrasive grains,
And polishing was performed.

As a result of this polishing, in the case of the single crystal diamond abrasive grain, the surface roughness was rapidly improved up to about 0.3 nm RMSD, but the rate of improvement thereafter was slow. When this was observed with a Normaski microscope, the appearance of abrasive grain passage marks was observed on the polished surface. In the case of the polycrystalline diamond abrasive grain of No. 3, the surface roughness was improved, and it was about 0.2 nm RMS after 90 minutes of processing. That is, ultra-smooth polishing proceeded smoothly. When this was observed with a Normaski microscope, the polished surface was smoothed.

From the above results, it can be confirmed that the degree of improvement in the surface roughness is different due to the crystal structure of the diamond abrasive grains when performing ultra-smooth polishing of CVD-SiC, and at the same time, the polycrystalline diamond is better. It was confirmed that it gave good results.

The results obtained in Experiments 1 to 4 are considered to be based on the following reasons.

The CVD-SiC material has a Knoop hardness of 2800.
It is a very hard material of ~ 3500, and is a material having a strong chemical covalent bond and stability. Therefore, it has poor workability and is conventionally known as a difficult-to-process material. This Si
When C is polished with diamond abrasive grains, both of them are materials with strong covalent bonds and stable, and therefore polishing removal proceeds by purely mechanical cutting accumulation. In addition, since a pitch having an appropriate viscoelastic property is generally used for polishing, the abrasive grains are held (embedded) at the pitch to perform polishing motion on the SiC surface. At this time, since there is not much difference in hardness between the two, the abrasive grains need sharpness, but since the abrasive grains are held at the pitch, it is considered that the sharpness depends on the grain size of the abrasive grains (from the pitch surface). The amount of protrusion of the abrasive grain controls the sharpness). If the particle size of the abrasive grains is too large, the amount of protrusion of the abrasive grains will increase, the appearance of abrasive grain passage marks will increase, and a super smooth surface cannot be obtained. This is similar to the case of polishing an optical material other than SiC, and the smaller the grain size of the abrasive grains, the smoother the surface roughness can be obtained. However, when polishing SiC, Experiment 1
As shown in, it is not necessary to simply reduce the particle size. This is because if the grain size of the abrasive grains is too small, the amount of protrusion becomes insufficient, and it becomes difficult to uniformly cut each crystal of high hardness and polycrystalline CVD-SiC.
This is because a phenomenon (mosaicization of the polished surface) of removing only the crystal having relatively low hardness in the polycrystal of iC occurs. From these points, an ultra-smooth surface is obtained when the average grain size of the abrasive grains is set appropriately, that is, when the average grain size of the diamond abrasive grains is set in an appropriate range (1 μm to 3 μm) as in the present invention. It is thought to be done.

The average grain size of the abrasive grains also has a correlation with the polishing pressure, the hardness of the pitch material, the crystallinity of the abrasive grains, and the like. For example, when the polishing pressure is too high, the embedding of the abrasive grains in the pitch is promoted, and as a result, the same effect as that of using the smaller-diameter abrasive grains is exhibited. From this point, the polishing pressure is preferably 66 KPa or less. Further, as shown in Experiment 2, about 25 to 45 KPa is optimal. Also, for example, if the hardness of the pitch material is too low or too high,
The degree of embedding of abrasive grains in the pitch is affected. From this point, the range of pitch penetration is 5 as shown in Experiment 3.
It is preferably about 20. Further, for example, when the single crystal diamond is used as the abrasive grains, crushing is likely to occur during polishing, and as a result, an edge having a small radius of curvature scans the SiC surface, and thus the abrasive grain passage marks are generated on the surface to be polished. It tends to be seen frequently. On the other hand, when polycrystalline diamond is used, since it does not have a cleavage plane and is tough, an abrasive grain passage mark will not occur if an appropriate abrasive average grain size is selected. From this point, as shown in Experiment 4, it is preferable to use polycrystalline diamond.

In general, when polishing a flat surface or a spherical surface in a liquid using a flat plate, it takes several tens of hours for processing, so that diamond abrasive grains may precipitate. For the purpose of preventing this, the average particle size of the abrasive grains is preferably 1 to 3 μm, and more preferably 1 μm as in Experiments 1 to 4. For the same reason, the amount of diamond abrasive grains in the polishing liquid is preferably 0.2% by weight or less, more preferably about 0.025% by weight as in Experiments 1 to 4. Similarly, for the purpose of stabilizing the dispersibility in the polishing liquid, it is desirable to add a dispersant to the polishing liquid.

When performing ultra-smooth polishing of an aspherical surface using a small-diameter tool, it is necessary to maintain the dispersion stability of abrasive grains more than that of the above-described polishing of a flat surface or spherical surface using a full-face dish. From this point as well, it is desirable to add a dispersant to the polishing liquid. As the dispersant, for example, sodium hexametaphosphate is desirable. Further, as the amount of the dispersant added,
1 to 2 parts by weight is desirable with respect to 100 parts by weight of the polishing agent in the polishing liquid. By adding a dispersant, the precipitation of abrasive grains can be prevented and stable removal can be achieved even during long-term polishing. The solvent for the polishing liquid is usually water, preferably purified water.

When processing an aspherical mirror by CVD-SiC, a shape-correction polishing method using a polishing tool having a diameter smaller than that of the mirror to be processed has been used. The formation of pits, the appearance of abrasive grain passage marks, etc. constantly occur, and the quality required due to these defects on the surface of the workpiece cannot be satisfied. It is considered that such a problem is because the setting of the polishing abrasive grains and the polishing pressure amount is not proper, and the average particle diameter of the abrasive particles which is too small and the polishing pressure which is too high deteriorates the polishing surface. On the other hand, according to the present invention, even when performing ultra-smooth polishing of an aspherical surface using a small-diameter tool, the polishing liquid is made to have an appropriate configuration in advance for SiC that is the material to be polished, and an appropriate polishing pressure and pitch material A good polished surface can be obtained by using as a tool, and even for aspherical surfaces that could not be processed with satisfactory quality in the conventional technology, it is possible to perform excellent smoothing with no defects on the polished surface. You can do it.

The "average particle size" of the diamond abrasive grains referred to in the present invention is a value obtained by a particle size distribution measuring method such as a sedimentation method based on the Stokes fluid resistance law or a centrifugal sedimentation method.
This is the so-called Stokes diameter. In this measuring method, diamond abrasive grains are uniformly dispersed in a medium (pure water), and the subsequent sedimentation state of the abrasive grains is measured by a change in the transmittance of a light beam that traverses the medium. In the following examples, Shimadzu's Shimadzu centrifugal sedimentation type particle size distribution analyzer SA-CP is used.
It measured using the 3rd type.

In this apparatus, the concentration change of the sample liquid (suspension of diamond abrasive grains) at a fixed position from the upper surface of the medium is measured along with the elapsed time from the start of sedimentation,
At that time, the ratio of the particles causing the concentration change is obtained.
This is because the mass (size) of the particle and the sedimentation velocity have the following relationship, and the time of the sedimentation start time and the change in concentration at a certain height (determined from the change in light transmittance) This is because the mass distribution can be calculated.

[0040]

[Equation 1] u: Settling velocity of particles θ: Time required for particles to settle for a certain distance ρ _p : Particle density ρ _l : Medium density η: Viscosity coefficient of medium ω: Rotational angular velocity β: Rotational angular acceleration R: From rotation center to particle Distance D _p : Stokes particle size

[0041]

The present invention will be described in more detail with reference to the following examples.

<Example 1> A mirror element for an excimer laser was polished by a CVD-SiC material having a diameter of 50 mm and a thickness of 10 mm as follows. For this application, it is required to polish one surface to an ultra smooth surface so as to have a surface roughness of 0.2 nm RMS or less, and there are defects such as pits, scratches, latent scratches, and fog on the polished surface. Don't

First, as a preprocessing, the CVD-SiC material was flat-polished to a surface roughness as good as possible by a conventional method. Next, 0.025% by weight of polycrystalline diamond abrasive grains having an average particle size of 1 μm was added to 2 liters of purified water and stirred, and sodium hexametaphosphate fine powder as a dispersant was added to 100 parts by weight of diamond abrasive grains in the polishing liquid. 2 parts by weight, 24
After stirring for a time, a polishing liquid was obtained.

Using this polishing solution, a final polishing was carried out using the polishing apparatus shown in FIG. That is, penetration 1
Both the polishing dish 1 having the pitch surface 6 of 5 and the workpiece (mirror material) 2 held by the workpiece holder 3 are submerged in the container 8 filled with the polishing liquid 7, and the normal horizontal swing method is used. Polishing proceeded with relative motion slower than the polishing conditions. The polishing dish rotation shaft 9 was rotated at 7 rpm by a rotation mechanism (not shown), while the swing shaft 4 supporting the workpiece 2 was swung at 5 cycles / minute by a swing mechanism (not shown). The polishing pressure was set to 30 KPa by a pressing mechanism (not shown), and polishing was performed unattended for 16 hours.

When the polished surface after the above polishing was observed, no deterioration of the polished surface such as pits on the surface to be machined and development of abrasive grain passage marks during processing was observed, and the surface roughness An ultra-smooth surface of 0.15 nm RMS was obtained.

Example 2 Length 750 mm, Width 100 m
The mirror element for soft X-rays was polished with a CVD-SiC material having a thickness of 15 mm and a thickness of 15 mm. The busbar radius is 440m, and the shape is 2
It is a toroidal of 00 mm. As in Example 1, for this application, the surface was polished to a super smooth surface with a surface roughness of 0.2 nm.
It is required to be less than MRS, pits on the polished surface,
Defects such as scratches and fog should not be present.

First, as a pre-processing, the CVD-SiC material was polished to a surface roughness as good as possible by a conventional method. next,
0.02% by weight of polycrystalline diamond abrasive grains having an average particle diameter of 3 μm was added to 40 liters of purified water and stirred, and 0.16 g of sodium hexametaphosphate fine powder as a dispersant (based on 100 parts by weight of diamond abrasive grains in the polishing liquid). 2 parts by weight) and stirred for 24 hours to obtain a polishing liquid.

Using this polishing solution, final polishing was carried out using a cylindrical polishing machine. FIG. 4 is a schematic diagram showing this cylindrical polishing machine, (a) is a front view thereof, and (b) is a side view thereof. This polishing machine obtains relative motion of polishing by swinging both the workpiece 12 and the polishing tool 11 in the orthogonal direction by the swinging crank portions 14 and 15, respectively. The workpiece 12 is fixedly held on the work table 20 by the workpiece fixture 13.
Further, the pitch surface 16 of the polishing tool 11 has a shape similar to that of the processed surface of the work piece 12 and is shaped in a concavo-convex manner, and the portion overlapping the work piece 12 is in contact. The penetration of the pitch part is 20. The polishing liquid is supplied by a polishing liquid supply unit (not shown).
From the polishing liquid supply nozzle 17 to the polishing unit at all times, and then returned from the polishing liquid drain 11 of the container 18 to the polishing liquid supply unit.

The polishing conditions in this example are as follows:
The work table 20 on which No. 2 is placed was oscillated at 17 cycles / minute, and the oscillating crank portion 15 supporting the polishing tool 11 was oscillated at 20 cycles / minute, and the polishing load was set to 25 KPa due to the weight of the polishing tool and the weight. Then, it was left unattended for 16 hours.

After observing the polished surface after the above-mentioned polishing, polishing such as pits on the surface to be processed, occurrence of abrasive grain passing traces (latent scratches) in pre-processing, generation of fog (polishing burn), etc. No deterioration of the surface was observed, and an ultra-smooth surface having a surface roughness of about 0.15 nm RMS was obtained.

<Embodiment 3> Next, an embodiment in which a highly accurate aspherical surface is formed using a small-diameter tool by the polishing method of the present invention will be described in detail.

In a shape correction polishing system for generating and polishing a highly accurate aspherical surface, first, an error shape with respect to the design shape of the surface to be processed is measured using a high accuracy shape measuring machine, and an error shape map for the surface to be processed is obtained. create. By using this error shape map, the removal amount per unit time of the small diameter tool that swings at a constant speed, and the removal shape, the residence time distribution on the work surface of the small diameter tool that minimizes the residual error shape is calculated. . The calculation of this residence time distribution is performed on a computer. The calculation method is generally called a deconvolution method. This operation is an inverse operation of the convolution method (convolutional integration method), and a method of simulating actual removal is known.

In this simulation method, the error shape and the unit removal shape of the small-diameter tool are displayed in a matrix in an array of each scale, and the unit removal shape matrix is superimposed on the error shape matrix in the predetermined tool scanning direction to obtain the error shape matrix. The unit removal shape matrix element is subtracted from each element, and when a negative value does not occur in all calculated elements (when excessive removal does not occur), the third matrix is the residence time distribution integration matrix (error shape matrix and Of the arrays of the same scale, all the array elements are initially 0.) Among the elements in which the center of the unit removal shape matrix is present in the current calculation, the unit time 1
Is calculated, the unit removal shape matrix is shifted by one element in the planned tool scanning direction on the error shape matrix, and the determination as to whether removal is possible is repeated, and the tool scanning is simulated until the removal cannot be performed over the entire area. It is a thing.
The tool scanning speed in the predetermined tool scanning direction is calculated from the obtained residence time distribution matrix. This is, for example, a unit time (N) (unit: unit length (L) obtained by accumulating a unit length (L) determined from the size (L2) of one element of the error shape matrix (residence time distribution matrix) into each residence time distribution matrix element. Can be divided by, for example, seconds).

A tool that oscillates at a constant speed according to the obtained tool scanning speed matrix is raster-scanned on the work surface. At this time, the workpiece is fixedly held in the polishing liquid, and raster scanning polishing is performed in the polishing liquid with a small diameter tool.

In the case of this embodiment, the work piece has a length of 270 mm in the generatrix direction, a radius of curvature of the generatrix of 490 m, and a length of 70 in the sagittal direction.
It is a CVD-SiC toroidal mirror aspherical mirror having a diameter of 2 mm and a sagittal radius of curvature of 2 m. It takes about 12 hours for a 16 mm small diameter tool to scan the entire surface to be processed while realizing the residence time distribution obtained as a result of the above calculation.

The polishing liquid used in this example is purified water 20.
10 g (0.05% by weight) of polycrystalline diamond abrasive grains having an average particle size of 1 μm per liter, 0.2 g of sodium hexametaphosphate fine powder as a dispersant (2 parts by weight per 100 parts by weight of diamond abrasive grains in the polishing liquid) In addition, it was obtained by stirring for 24 hours, then leaving it for another 24 hours, and passing it through a 5 μm cartridge filter. This polishing liquid is supplied to the polishing section from the tank outside the polishing device by the polishing liquid supply pump, and excess polishing liquid is discharged to the tank outside the polishing device on the machine by the drain pump.

Using this polishing liquid, a small-diameter tool using an asphalt pitch having a penetration of 15 and a diameter of 16 mm is used at a polishing pressure of 25 KPa and at a frequency of 5 Hz in the direction orthogonal to the scanning direction.
After oscillating ± 2 mm for 1 axis and performing raster scanning with a feed pitch of 1 mm, a total processing time of about 55 hours / 3 pass polishing was performed, and an aspherical surface shape as planned was obtained.

When the polished surface was observed after the above-described polishing, no deterioration of the polished surface such as pits and development of abrasive grain passage marks was observed on the surface to be processed, and the surface roughness was 0.16n.
An ultra smooth surface of about mRMS was obtained.

<Embodiment 4> Next, an embodiment in the case of forming a highly accurate aspherical surface by a polishing machine controlled by an NC apparatus by the polishing method of the present invention will be described in detail.
FIG. 5 is a diagram showing an operation flow of the polishing apparatus 30 according to this embodiment, and FIG. 6 is a diagram showing the polishing apparatus 30.

In this embodiment, first, the work piece 36 is processed.
(Hereinafter, referred to as a work) is set on the work chuck and fixed (step S0). With the measurement scanning pattern (data D1) determined by this work shape, the shape measurement is performed starting from the measurement start point and using the scan line which is a locus of a weaving weave shape within the measurement range (step S1). Resultant work shape data (data D
From 4), the previously obtained error to be reproduced, that is, the system error (data D2) is subtracted (step S2), the design shape (data D3) of the workpiece is fitted to this (curve fit), the difference from the design shape, That is, the error shape (data D5) is obtained (step S3).

Next, it is judged whether or not this error shape has reached the target accuracy (step S4), and if it has already reached the target accuracy, the process ends and the work is removed (step S7). If not yet reached, the deconvolution calculation operation (step S) is a calculation operation for obtaining the residence time distribution from the unit removal shape (data D6) and the error shape.
5) is performed to obtain a residence time distribution (data D8). The NC polishing operation is performed based on the residence time distribution, the polishing scan pattern (data D7), and the design shape (data D3) (step S6). Then, the shape measurement of step 1 is performed again.

FIG. 7 shows an operation flow relating to the NC polishing operation of step S6.

First, in preparation for polishing, the Y table 32 is moved to the polishing section P, and the tilting device 40 is lowered at the first polishing position to bring the polishing head 50 into contact with the work. Next, the scanning speed is determined from the current position using the residence time distribution (data D8) (step S1).
9). Next, its scanning speed and scanning pattern (data D7)
The scanning position after the synchronization time ΔT is calculated from (step S
20). Next, it is determined whether the scanning is completed (step S2
1), if not finished, design shape (data D3)
The polishing position and the attitude are calculated from (step S22), and the tool position, that is, the position in the work normal direction by the distance of the output value D of the sensor described above is corrected (step 23), and X,
The six-axis target positions of Y, θ, Z1, Z2, and Z3 are calculated (step S24), and the target positions are transmitted to the subordinate computer (step S25). The lower-level computer synchronously moves the 6-axis according to the method described above according to the transmitted target position of the 6-axis (step S26), and transmits the tool position, that is, the output value D of the sensor to the upper-level computer (step 18). .

In the case of this embodiment, the workpiece has a length of 270 mm in the generatrix direction, a radius of curvature of 490 m in the generatrix, and a length of 70 in the sagittal direction.
It is a CVD-SiC toroidal mirror aspherical mirror having a diameter of 2 mm and a sagittal radius of curvature of 2 m.

First, the work 36 is attached to the work chuck in the tab 35 on the θ table 34. This work chuck fixes the work by vacuum suctioning the bottom surface of the work. Next, the XYθ table is set along the Y-axis to the shape measuring unit M.
To stop the XYθ table at the origin position of the shape measuring system. Further, the θ table 34 is fixed at the origin of its rotation. Next, the shape of the surface of the workpiece to be machined (hereinafter referred to as the machined surface) is measured according to the operation flow shown in FIG. The measurement is started by inputting each parameter and inputting a measurement start command. At this time, the parameters to be input include the measurement start position, the end position, the shape measurement region,
The measurement data acquisition interval, the scanning speed of the stylus, the measurement scanning pattern (data D1 in FIG. 5) including the stylus pressure, and the design shape of the processed surface.

Next, the above-described deconvolution (step S5 in FIG. 5), that is, the calculation for controlling the tool at the time of correction polishing is performed. First, the error shape of the surface to be processed (data D5 in FIG. 5) is read into the calculation area of the host computer. Further, the polishing removal shape (unit removal shape) per unit time (data D6 in FIG. 5) of the polishing tool used in the correction polishing is read into the calculation area of the host computer.

This unit removal shape is obtained in advance on a test piece having a material shape similar to that of the surface to be machined at a constant position for a known time under the same tool and polishing conditions as those used in the actual correction polishing. It is obtained by performing polishing, measuring the shape of the obtained polishing depression, and converting the shape per unit time. At this time, for example, if the known time is 600 seconds and the unit time is 1 second, the depth of the polishing recess obtained by polishing at a fixed position for 600 seconds may be multiplied by 1/600. Further, these two data groups are composed of three-dimensional data of (x, y, z) per point. x,
y and z are meshes with equal intervals, and z represents error data or a unit removal shape. By using these two data groups read in the calculation area of the host computer, the deconvolution calculation of the residence time distribution of the tool on the surface to be processed is performed.

The tool residence time distribution (data D8 in FIG. 5) obtained by this calculation means the tool polishing at each point necessary for polishing and removing the error shape in the corrected polishing region of the surface to be processed. It is a two-dimensional representation of time. That is, the sum of these represents the total polishing time required for one correction polishing of the surface to be processed.

Next, the XYθ table is moved to the polishing processing section P and stopped at the origin of the polishing processing section P. The Z tilt arm of the polishing portion P is attached with a uniaxial swinging type polishing head 50, and a pitch tool having a diameter of 16 mm is fixed to the tool holding portion at the tip of the polishing head 50. The polishing head 50 oscillates at the designated frequency according to a command from the host computer. In this embodiment, the swing stroke is 8 mm and the swing frequency is 5 Hz. This is a preferable condition for removing an error (shape error, ripple) in the intermediate frequency region from a low frequency in the spatial frequency distribution of the error shape. Further, a polishing liquid supply device (not shown) provided outside the polishing device of this embodiment supplies the polishing liquid into the tab 35 on the θ table 34 in response to a command from the host computer, so that the surface of the workpiece is polished. Cover with and prepare for polishing.

When a command is input to the high-order computer for the correction polishing preparation, the high-order computer reads the data regarding the shape of the surface to be processed and the residence time distribution into the calculation area. here,
The polishing start position, the tool scanning pattern, the number of scans, and what percentage of the total polishing time obtained from the residence time distribution are input to the host computer. In the present embodiment, the scan pattern of the tool is a continuous weave scan, and the scan count is 3
It was time.

When the high-order computer is instructed to start the correction polishing, as described above, the X table 33 and the Y table 32 are used.
Respectively move and stop so that the tool at the tip of the Z tilt arm is positioned at the polishing start position of the surface to be processed. After that, the Z tilt arm descends, and when the signal from the vertical position sensor of the polishing tool in the polishing head 50 reaches a predetermined value, the Z tilt arm is stopped to descend. That is, it is detected that the tool has come into contact with the surface to be processed and the relative position of the tool within the polishing head 50 has risen, and the lowering of the Z tilt arm is stopped. After that, a command for generating a predetermined polishing load is sent to the constant pressure device 20 in the polishing head 50, and the pitch tool is pressed against the corrected polishing start position of the surface to be polished with a predetermined polishing load. In this embodiment, the polishing load is 25 KPa.

After confirming that the tool has been pressed to a predetermined load, the host computer starts swinging the polishing head 50 to start the correction polishing. As described above, the scanning of the polishing tool is performed on the x and y axes. The host computer calculates the tool scanning speed from the data of the residence time distribution and the intervals between the points x and y of the data. For example, assuming that the x, y data is at 1 mm intervals, the residence time at any position (x1, y1) is 40 sec, 25% of that is actually processed, and the number of scans is 4, this The scanning speed V (x1, y1) 0.5 mm before and after the position (x1, y1) is V (x1, y1) = 1 / {(4
0 × 0.25) / 4} = 0.4 (mm / sec). While performing such a calculation, as described above, the host computer obtains the target coordinates of each axis for each synchronization time ΔT, transfers the target coordinates to the lower computer, and the lower computer controls each axis, so that the correction polishing progresses. To do.

When the first correction polishing is completed, the polishing head 50 is isolated upward, the polishing liquid is drained from the tub 35, and the work is dried. After that, the xyθ table is moved to the shape measuring unit M, and the second shape measurement is performed. The alternate repetition of the shape measurement and the correction polishing is completed or continued depending on whether the shape of the work is within the tolerance with respect to the design shape. In this working example, this operation was repeated three times, and the flat shape accuracy PV could be processed to 0.06 μm. During this time, the shape is measured in the area of 300 × 100 mm at a pitch of 1 mm, and the polishing is 300 ×.
A 100 mm section was raster scanned with a 1 mm pitch.

The polishing liquid used in this example is the same as that used in Example 3. Using this polishing liquid, using a pitch with a penetration of 15 and a polishing pressure of 25 KPa, oscillating ± 2 mm uniaxially at 5 Hz in the direction orthogonal to the scanning direction, and raster scanning at a feed pitch of 1 mm, Processing time Approximately 55 hours / 3-pass polishing processing resulted in the expected aspherical shape. No deterioration of the polished surface such as generation of pits or appearance of abrasive grain passage marks was observed on the surface to be processed, and the surface roughness was 0.
An ultra-smooth surface of about 16 nm RMS was obtained.

[0075]

According to the present invention described above, for example, CVD-SiC, which is a highly brittle polycrystalline material, has high reliability without causing deterioration of the polishing surface such as generation of bits and appearance of abrasive grain passage marks. And super smooth surface (surface roughness 0.3nmR
It can be polished to MS or less). Furthermore, the present invention is a highly versatile method applicable to spherical surfaces, flat surfaces, and aspherical surfaces.

Further, the conventional complicated process of advancing the polishing process while finely changing the polishing conditions and observing the state of the polished surface is not required, and the work is facilitated. Further, since the polishing conditions can be kept constant, the processing efficiency can be improved and the processing cost can be reduced.

Further, from the viewpoint of preventing the precipitation of the polishing agent, the CVD-S which requires a long time such as 10 to 20 hours.
This is particularly effective when the iC material is aspherically polished with a small diameter tool.

[Brief description of drawings]

FIG. 1 is a graph showing the results of Experiment 1 for three representative particle sizes.

FIG. 2 is a graph showing the results of study example 2.

3 is a diagram showing a polishing machine used in Example 1. FIG.

FIG. 4 is a view showing a cylindrical polishing machine used in Example 2.

FIG. 5 is a flowchart showing an operation flow of a polishing apparatus according to a fourth embodiment.

FIG. 6 is a view showing a polishing apparatus used in Example 4.

FIG. 7 is a flowchart showing an operation flow relating to the NC polishing operation of step S6 of FIG.

[Explanation of reference numerals] 1 polishing dish 2 surface to be processed 6 polishing dish pitch surface 7 polishing liquid 11 polishing tool 12 workpiece 17 polishing liquid supply nozzle 21 polishing liquid drain 36 workpiece

Claims (9)

[Claims] 1. A method of polishing SiC, which comprises using a polishing liquid containing diamond abrasive grains having an average grain size of 1 to 3 μm. 2. The polishing method according to claim 1, wherein a pitch having a penetration of 5 to 20 is used. 3. The polishing method according to claim 1, wherein the polishing pressure is 66 KPa or less. 4. The polishing method according to claim 1, wherein the diamond abrasive grains are polycrystalline. 5. The polishing method according to claim 1, wherein the amount of diamond abrasive grains in the polishing liquid is 0.2% by weight or less. 6. The polishing method according to claim 1, wherein the polishing liquid further contains 1 to 2 parts by weight of a dispersant with respect to 100 parts by weight of the polishing agent. 7. A method of manufacturing an optical element having a surface made of SiC, comprising the step of polishing with a polishing liquid containing diamond abrasive grains having an average particle diameter of 1 to 3 μm. Method. 8. The method of manufacturing an optical element according to claim 7, wherein the surface made of SiC is a film formed by a CVD method. 9. A surface roughness of 0. 1 is obtained by polishing with a polishing liquid containing diamond abrasive grains having an average grain size of 1 to 3 μm.
9. A smooth surface having a thickness of 3 nm RMS or less is formed.
A method for manufacturing the optical element according to claim 1.