JP2000308950A - Polishing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To be not going to remain an deteriorated layer to deteriorate perfor mance as an optical element on a surface of a working surface by uniformly polishing and removing an overall area of the working surface in a short period of time after providing a specified shape and specified surface roughness at the time of polishing and machining the optical element made from fluoride crystal material. SOLUTION: A polishing tool 10 is constituted by affixing an elastic body 10b of chloroprene rubber, etc., of about 3-5 mm thick of a stainless steel-made circular tool shank 10a having a diameter of about 1/10 of the maximum diameter of a working surface of a working article and molding an asphalt pitch 10c penetration of which is about 5-20 degrees on it. Polishing liquid to be used for short time uniform polishing is to be polishing liquid dispersing diamond abrasive grains an average drain diameter of which is less than 1/4 μm in a saturated aqueous solution of a crystal material constituting the working article with purified water as a solvent. This polishing liquid which is used in the previous process can be used as it is. As it is possible to keep polishing conditions constant, it is possible to improve working performance and to reduce working cost, too.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polishing method for polishing a high-precision optical element.
The present invention relates to a polishing method for polishing a high-precision optical lens made of a fluoride crystal material such as aF ₂ or MgF ₂ .

[0002]

_2. Description of the Related Art A high-precision optical lens made of a fluoride-based crystal material such as CaF ₂ or MgF ₂ has a good transmittance over an extremely wide wavelength band. Used as optical elements.

The following two types of polishing methods are employed for polishing a high-precision optical lens made of a fluoride crystal material such as CaF ₂ , depending on its shape.

[0004] The first is polishing of a spherical surface or a flat surface by a normal traverse polishing machine. A polishing plate having substantially the same diameter as the lens material to be created, having the same radius of curvature, and formed in the opposite direction to the surface of the polishing liquid is used as a polishing liquid. In this state, polishing is performed by making a relative movement with the lens material in the state of interposing, to finish a predetermined spherical surface or a flat surface.

[0005] Another method is a method of polishing an aspherical surface, in which a shape correction polishing is performed using a polishing tool having a smaller diameter than a surface to be processed.

However, in these polishing methods, CaF ₂
Of numerous pits on the surface to be polished of an optical element made of a fluoride-based crystal material such as MgO or MgF ₂ , development of traces of abrasive grains during pre-processing, polishing or fogging due to the formation of CaO and MgO (cloudiness)
Troubles occur on the surface of the surface to be polished, for example, and the required quality cannot be satisfied.

The present inventor has proposed a polishing method described in Japanese Patent Application Laid-Open No. 08-19943 in order to solve these problems. That is, for polishing an optical lens made of a fluoride crystal material such as CaF ₂ or MgF ₂ , a polishing liquid in which diamond abrasive grains are dispersed in a saturated aqueous solution of a crystal material such as CaF ₂ using purified water as a solvent is effective. By performing polishing in this polishing liquid, elution of the crystal material constituting the workpiece is suppressed, and polishing can be performed for a long time. Therefore, in aspheric polishing using a small-diameter polishing tool, In addition, it is possible to prevent the polished surface from being deteriorated, and to obtain a highly accurate and uniform smooth surface.

[0008]

However, as described above, when polishing an optical lens made of a fluoride-based crystal material such as CaF ₂ or MgF ₂ , the saturation of the crystal material such as CaF ₂ using purified water as a solvent. By performing polishing in a polishing solution in which diamond abrasive grains are dispersed in an aqueous solution, even if the surface roughness of the surface to be processed can be smoothed,
The surface layer of the surface to be processed is deteriorated, and when irradiating with extreme ultraviolet light or the like, light absorption considered to be caused by the deteriorated layer on the surface occurs, and the performance may deteriorate in a short time. .

Accordingly, the present invention has been made in view of the above-mentioned unsolved problems of the prior art,
An object of the present invention is to provide a polishing method for polishing an optical element made of a fluoride-based crystal material such as CaF ₂ without leaving a deteriorated layer that degrades the performance of the optical element on the surface of the surface to be processed. .

[0010]

The inventor of the present invention has made intensive studies on the alteration of the surface layer of the surface to be processed such as an optical element, and as a result, the altered layer that absorbs extreme ultraviolet rays can be determined only by measuring the surface roughness. Is difficult to perform, and there is a correlation between the formation of the deteriorated layer that affects the extreme ultraviolet rays and the standing time in the polishing liquid. It has been found that by performing polishing and removal of the entire surface to be processed with a uniform thickness of 5 nm or more in a short period of time, it is possible to remove a harmful altered layer and obtain an optical surface with a predetermined performance. The present invention has been completed.

That is, in the polishing method of the present invention, when an optical element made of a fluoride-based crystal material is polished, after a predetermined shape and a predetermined surface roughness are obtained, the entire surface of the surface to be processed is further removed. Are uniformly polished and removed in a short time.

In the polishing method of the present invention, it is preferable to uniformly remove 5 nm or more of the entire surface to be processed within 15 minutes for uniform polishing removal in a short time.

[0013] In the polishing method of the present invention, in the case of uniform polishing removal in a short time, a polishing liquid in which diamond abrasive grains having an average particle diameter of 1/4 µm or less are dispersed in purified water is compared with a polishing liquid. It is preferable to use a polishing tool having a sufficiently small polishing surface.

[0014] In the polishing method of the present invention, the polishing tool may have a penetration of 5 to 20 pitches or a foamed PVF.
Preferably, the diameter of the polishing tool is about 1/10 of the maximum diameter of the surface to be processed.

In the polishing method of the present invention, it is preferable that the polishing liquid is a polishing liquid in which diamond abrasive grains are dispersed in a saturated aqueous solution of a crystal material constituting the workpiece using purified water as a solvent.

[0016]

According to the polishing method of the present invention, when polishing an optical element made of a fluoride crystal material such as CaF ₂ ,
After a given shape and a given surface roughness are obtained,
By uniformly polishing and removing the entire surface of the surface to be processed in a short time, and more specifically, a saturated aqueous solution of a crystal material such as CaF ₂ using purified water as a solvent is used to remove diamond having an average particle size of 1/4 μm or less. Polishing liquid in which abrasive grains are dispersed, penetration is 5
By using a 20-pitch or a small-diameter polishing tool made of a foamed PVF material and uniformly polishing and removing 5 nm or more of the entire surface of the work surface in a short time of 15 minutes or less, Ca
An optical element made of a fluoride crystal material such as F ₂ can be polished to an ultra-smooth surface, and the generation of an altered layer on the surface that degrades the performance of the optical element can be prevented. Further, the aspherical shape of the optical element can be polished with high accuracy, and a harmful deteriorated layer can be removed to obtain an optical surface having a predetermined performance.

[0017]

Embodiments of the present invention will be described with reference to the drawings.

The shape correction polishing process for creating and polishing a highly accurate aspheric surface according to the present invention will be described with reference to the process chart shown in FIG.

First, an error shape with respect to a design shape of a work surface is measured using a high-precision shape measuring instrument (S1), and an error shape map file for the work surface is created (S1).
2). A removal amount per unit time and a removal shape file of the small-diameter tool oscillating at a constant speed are created (S3), and the remaining error shape is minimized using the removal amount, the removal shape file, and the error shape map file. The residence time distribution of the small tool on the work surface is calculated (S4). Handling of these data and calculation of the residence time distribution are performed on a computer. The calculation method is generally a method called deconvolution.
This is an inverse operation of the convolution method (convolution integration method), and is known as a method of simulating actual removal. 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 along the predetermined tool scanning direction, and the error shape matrix element is used. In the case where the unit removal shape matrix elements are respectively subtracted and no negative value occurs in all calculated elements (when excessive removal does not occur), the third step is performed.
Of the residence time distribution integration matrix (an array of the same size as the error shape matrix, and all of the array elements are initially set to 0), the center of the unit removal shape matrix is added in the current calculation. The unit time 1 is added to the elements that have been removed, the unit removal shape matrix is shifted by one element in the predetermined tool scanning direction on the error shape matrix, and it is determined whether removal is possible again. Simulates a tool scan until is no longer possible. From the obtained residence time distribution matrix, a tool scanning speed in a predetermined tool scanning direction is calculated and used as a file (S5). This is, for example, the size (L) of one element of the error shape matrix (residence time distribution matrix).
^The unit length (L) determined in ² ) may be divided by the unit time (N) integrated in each residence time distribution matrix element (the unit is, for example, seconds).

A tool that swings at a constant speed in accordance with the matrix of the obtained tool scanning speed is raster-scanned on the surface to be processed to perform shape creation polishing (S6). FIG. 2 shows an example of a raster scanning pattern on the surface to be processed. Where F
Is a raster pattern that is the scanning and polishing locus of the tool.
This pattern is set so that the tool completely exits from the actual effective beam diameter when the workpiece is used. At this time,
The workpiece 1 is fixed and held in the polishing liquid 3 by using the polishing apparatus shown in FIG. Thereafter, the error shape is measured again (S7), and an error shape map file is created (S8), and the pass / fail of the design tolerance is determined (S9). If the design tolerance is not satisfied yet, the error shape map is used. The residence time distribution is calculated again (S4). This loop (S4 to S9) is repeated until the pass / fail judgment for the design tolerance is passed. The passed workpiece is subjected to uniform polishing within a short time, which is a feature of the present invention (S10), and the processing is terminated (S10).
11).

Next, an embodiment in which the workpiece is polished using the polishing apparatus shown in FIG. 3 will be described.

The workpiece 1 is a convex CaF ₂ aspherical lens having a diameter of 226 mm and an approximate radius of curvature of 480 mm. The polishing liquid 3 used here is composed of 20 g of purified water, 80 g of diamond abrasive grains having an average particle diameter of 0.2 μm as a polishing agent, and fine powder of a crystal material (for example, CaF ₂ ) constituting a workpiece. 0.5 g and add them to 24
After stirring for a period of time, the mixture is left for 24 hours, passed through a 0.5 μm cartridge filter, and stored in the tank 4 outside the polishing apparatus. The polishing liquid 3 in the tank 4 is supplied by a polishing liquid supply pump 5 into a tub 7 to which a workpiece on the polishing apparatus is fixed. Excess polishing liquid is discharged from the tub 7 on the polishing apparatus to the tank 4 outside the polishing apparatus by the drain pump 6.

As a polishing tool, an elastic body (chloroprene rubber: rubber hardness: 30 degrees) 2b having a thickness of 3 mm for conforming to a stainless steel tool shank 2a, and a thickness of 1 mm
A small-diameter polishing tool 2 having a diameter of 16 mm and an asphalt pitch (penetration of 10 degrees) 2c is used. The small-diameter polishing tool 2 is pressed in the direction of arrow D with a load of 500 gf, and while moving in the direction of arrow E with a swinging stroke of 8 mm and a period of 5 Hz, the residence time distribution obtained as a result of the above calculation is applied to the entire surface to be processed. Scanning is performed while the workpiece 1 is being polished.

As described above, by repeating the shape measurement and the shape creation polishing, a predetermined aspherical shape and a predetermined surface roughness are obtained. Polishing is performed.

As shown in FIG. 4, a polishing tool 10 used for short-time uniform polishing which is a feature of the present invention is made of stainless steel having a diameter of about 1/10 of the maximum diameter of the surface to be processed of the workpiece. 3-5m thick circular tool shank 10a
An elastic body 10b of about m in length, such as chloroprene rubber, is adhered, and an asphalt pitch 10c having a penetration of 5 to 20 degrees is formed thereon. The polishing liquid used for the short-time uniform polishing is prepared by adding a diamond abrasive having an average particle diameter of 1/4 μm or less to a saturated aqueous solution of a crystalline material (for example, CaF ₂ ) constituting a workpiece using purified water as a solvent. This is a dispersed polishing liquid. Note that the polishing liquid used in the previous step can be used as it is.

Specifically, a 3 mm thick chloroprene rubber material 10b is stuck on a stainless steel circular tool shank 10a having a diameter of 20 mm, and an asphalt pitch 10c having a penetration of 15 degrees is formed thereon. Small diameter polishing tool 1
0 was set. This polishing tool 10 is used, and the polishing liquid used in the previous step is used as it is. Polishing was performed under the swinging condition of the polishing tool with a stroke of 10 mm, a frequency of 2 Hz, and a polishing load of 500 gf. The scanning conditions in the direction orthogonal to the swinging direction are as follows: a scanning speed of 15 mm / sec, a feed pitch of 4 mm, and a processing time per pass as short as about 15 minutes. By passing this step twice, uniform polishing removal of 5 nm was performed.

As described above, the surface roughness of the work surface obtained by uniform polishing removal of evenly polishing 5 nm or more of the entire work surface in a polishing time of 15 minutes or less is 0.4 nm on average.
RMS, and no harmful altered layer was observed on the surface.

As described above, according to the present invention, it is possible to polish an optical element made of a fluoride-based crystal material such as CaF ₂ into an ultra-smooth aspherical shape, and at the same time, it is possible to suppress the generation of a harmful altered layer on the surface. became.

As shown in FIG. 5A, the polishing tool used for short-time uniform polishing according to the present invention has a diameter of 20 mm.
5m thick stainless steel circular tool shank 11a
m of foamed PVF (polyvinyl formal) material 11b, and asphalt pitch 1 having a penetration of 15 degrees
The polishing tool 11 formed with 1c may be used.

Further, as shown in FIG. 5B, the polishing tool used for the short-time uniform polishing according to the present invention has a diameter of 20 m.
5m thick stainless steel circular tool shank 12a
The polishing tool 12 to which the foamed PVF material 12b of m is attached may be used.

As described above, the tool using the foamed PVF material for the polishing tool is particularly suitable when the shape change of the aspherical surface is large. This is because the foamed PVF material softens in the polishing liquid,
This is because a large shape change can be followed in a short time and good surface roughness can be obtained.

[0032]

As described above, according to the present invention,
It is possible to polish a fluoride-based crystal material such as CaF ₂ into an ultra-smooth surface and to prevent the formation of an altered layer on the surface, and it is also versatile and can be applied to aspheric surfaces. As described above, it is possible to prevent the polished surface deterioration phenomenon that deteriorates the performance as an optical element, and to obtain an optical surface having a predetermined performance with high reliability and high accuracy and a uniform ultra-smooth surface. Is particularly effective when polishing an aspherical shape with a small-diameter polishing tool. Further, since the polishing conditions can be kept constant, the processing efficiency can be improved and the processing cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a process chart showing a flow of a shape correction polishing process for creating and polishing a highly accurate aspheric surface according to the present invention.

FIG. 2 is a diagram illustrating a raster scanning pattern on a surface to be polished;

FIG. 3 is a schematic view of a polishing apparatus for polishing a workpiece such as an optical element.

FIG. 4 is a schematic view showing a configuration of a polishing tool used for short-time uniform polishing in the present invention.

FIGS. 5A and 5B are schematic views each showing another configuration of a polishing tool used for short-time uniform polishing in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Workpiece 2 (Small diameter) polishing tool 2a Tool shank 2b Elastic body 2c Asphalt pitch 3 Polishing liquid 4 Tank 5 Polishing liquid supply pump 6 Drain pump 7 Tab 10 Polishing tool 10a Tool shank 10b Chloroprene rubber material 10c Asphalt pitch 11 Polishing Tool 11a Tool shank 11b Foamed PVF material 11c Asphalt pitch 12 Polishing tool 12a Tool shank 12b Foamed PVF material

Claims (8)

[Claims] 1. When polishing an optical element made of a fluoride-based crystal material, after a predetermined shape and a predetermined surface roughness are obtained, the entire area of the surface to be processed is further uniformly polished in a short time. A polishing method characterized by removing. 2. The polishing method according to claim 1, wherein, during uniform polishing removal in a short time, the entire area of the surface to be processed is uniformly polished and removed by 5 nm or more within 15 minutes. 3. A polishing liquid in which diamond abrasive grains having an average particle diameter of 1/4 μm or less are dispersed in purified water and a polished surface sufficiently smaller than a surface to be processed is used for uniform polishing removal in a short time. 2. A polishing tool comprising:
Or the polishing method according to 2. 4. The polishing method according to claim 3, wherein the polishing tool has a penetration of 5 to 20 pitches. 5. The polishing method according to claim 3, wherein the polishing tool is made of a foamed PVF material. 6. The polishing method according to claim 3, wherein a diameter of the polishing tool is about 1/10 of a maximum diameter of the surface to be processed. 7. The polishing liquid according to claim 3, wherein said polishing liquid is a polishing liquid in which diamond abrasive grains are dispersed in a saturated aqueous solution of a crystalline material constituting said workpiece using purified water as a solvent. The polishing method according to any one of the above. 8. The polishing method according to claim 1, wherein the fluoride-based crystal material is CaF ₂ .