LOCAL POLISHING METHOD, LOCAL POLISHING DEVICE, AND CORRECTIVE POLISHING APPARATUS USING THE LOCAL POLISHING DEVICE

- The University of Tokyo

Provided is a local polishing technique suitable for corrective polishing. Press polishing is performed while supplying a polishing solution between a work and a work-polishing rotating tool locally pressed against the work, the polishing solution having abrasive grains composed of organic particles with an average particle size of 5 μm or more dispersed in a liquid. The rotating tool is made of an elastic material.

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Description
TECHNICAL FIELD

The present invention relates to a local polishing method and a local polishing device, which can be suitably used for corrective polishing.

BACKGROUND ART

In machining of an optical element such as an optical lens, for example, corrective polishing is performed. In the corrective polishing, a work (workpiece) is entirely scanned while a rotating tool, which is capable of local polishing, is being pressed against the work at the same time of supplying polishing slurry composed of fine abrasive grains with an average particle size of approximately 1 μm between the work and the rotating tool. In the corrective polishing, a static machining mark is obtained by polishing a material of the same quality as the work in advance without scanning the rotating tool, and a shape of the static machining mark per unit time is obtained. Then, corrective machining to a desired shape is performed by a process of performing deconvolution (deconvolution integral) calculation for this unit machining shape and a shape error (correction target shape) on the work to calculate a residence time (scanning speed) of the rotating tool and then scanning the rotating tool along a distribution of the residence time (for example, refer to Patent Literature 1).

In the corrective polishing, the scanning is performed as mentioned above. Accordingly, it is important that a local machining amount by the rotating tool is linear with time and stable fora long period of time. However, in the conventional corrective polishing, there has been a problem that it is difficult to obtain a stable machining amount since the rotating tool itself that is locally pressed against the work is worn and pressing force is liable to fluctuate. Moreover, since a surface texture of the tool largely affects transportation and retention of the fine abrasive grains, discharge of chips, and the like, the rotating tools are used for the polishing after the surface texture is prepared by pretreatment such as truing (shape formation) and dressing (toothing). However, such pretreatment causes a decrease in machining efficiency and an increase in cost. In addition, since the rotating tool is easy to wear as described above, it is necessary to frequently perform a maintenance operation on the surface shape, which causes a further decrease in efficiency and a further increase in cost.

On the other hand, in recent years, a non-contact elastic emission machining (EEM) method and a polishing method using a magnetic fluid as a tool have also been being used (for example, refer to Patent Literature 2). In the EEM method, the rotating tool is not actively pressed against the work, but a gap equal to or larger than the particle size of the fine abrasive grains is maintained between the rotating tool and the work, and the fine abrasive grains in the polishing slurry flowing therebetween and the surface of the work are chemically bonded to each other and are subjected to adhesion removal, whereby precision polishing is performed. However, these non-contact machining methods require a circulation device that thoroughly controls viscosity and concentration of the polishing slurry, and equipment thereof tends to become large in size. Moreover, in order to speed up a flow of the slurry passing through the gap, it is necessary to set an outer diameter of an outer peripheral surface of the rotating tool, which faces the work, to a predetermined value or more, and also to set a rotation speed of the rotating tool to a predetermined value or more, and there also occurs a certain limit to a correctable spatial resolution.

The correctable spatial resolution in the corrective polishing relates to the size of the unit machining shape. For example, when undulations with a period of 1 mm are corrected, it is obvious that the undulations cannot be corrected unless a size of the unit machining shape is 1 mm or less. A non-contact corrective polishing device currently in widespread use has a limit of spatial resolution of approximately 1 mm. Further, even in research on the corrective polishing technique, which aims to a higher resolution, a spatial resolution therein exhibits up to approximately 0.3 mm. On the other hand, an optical element using a short wavelength light source such as X-ray is required to have accuracy in the order of single nanometer; however, in an X-ray optical element separately developed by the inventors of the present invention, an effect of wave surface error due to an undulation region with a spatial wavelength of approximately 0.1 to 0.3 mm is confirmed. The non-contact corrective polishing technique cannot cope with correction of such small undulations of the spatial wavelength.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-22005

Patent Literature 2: Japanese Examined Patent Application Publication No. H2-25745

SUMMARY OF INVENTION Technical Problems

In view of the above-mentioned circumstances, what the present invention seeks to solve is to provide a local polishing technique that is suitable for the corrective polishing, the local polishing technique preventing cost from increasing with a simple structure, enabling a device to be miniaturized, also stabilizing a machining amount by the local polishing, and also being capable of obtaining a high spatial resolution.

Solutions to Problems

As a result of diligent examinations to solve the above-mentioned problems, the inventors of the present invention found the following. That is, the fact that a tool itself is worn and an effect of the surface texture of the tool increases in the conventional local polishing using pressing is caused by the fact that a rotating tool 11 comes into direct contact with a work 9 since the rotating tool 11 is pressed against a machining target surface 90 of the work 9 while interposing fine abrasive grains 81 with an average particle size of 1 μm or less therebetween as illustrated in FIG. 1A. Then, the inventors of the present invention conceived that, if coarser abrasive grains 81 with an average particle size of 5 μm or more were used as illustrated in FIG. 1B, then the direct contact between the rotating tool 11 and the work 9 and the wear of the rotating tool 11 due to the direct contact would be prevented, the surface texture of the tool surface would not affect the machining, and the machining amount could be stabilized while preventing cost from increasing with a simple structure. Then, the inventors of the present invention proceeded with further examinations.

The inventors of the present invention first performed press polishing with a rotating tool using silica, as polishing abrasive grains, having an average particle size of 14 μm. As a result, machining stability was confirmed. However, there occurred a problem that surface roughness after the polishing was greatly deteriorated (see the result of Comparative example 1 of Raster scan machining test 1 to be described later).

On the other hand, from the Stokes equation, it is seen that a sedimentation rate of particles is proportional to a square of a particle size. That is, dispersibility also becomes a bottleneck in consideration of use of polishing abrasive grains with a particle size of 10 μm or more, which is set this time. Since a density of usually used polishing abrasive grains is 2 to 8 g/cm3, it is difficult to improve the dispersibility as long as the solution is pure water (density: 1 g/cm3). Actually used aggregated silica had poor dispersibility, and it was very difficult to handle the aggregated silica in terms of storage and re-stirring though the aggregated silica was stabilized in the slurry circulation.

The inventors of the present invention considered using, as abrasive grains, monodispersive particles with an average particle size of 5 μm or more and a low density, apart from the general concept of “polishing abrasive grains” based on metal oxides and metal carbides, and then conceived to use organic particles. For example, particles of urethane, acrylic, styrene and the like, which are polymer materials, are produced by an emulsion polymerization method. These have a substantially spherical shape, and it is possible to produce those having a particle size of 5 to 10 μm or more. Further, the organic particles are resin. The organic particles are inexpensive, have a low density, have good dispersibility, and also have good detergency after polishing so as to be solved in an organic solvent such as acetone. As described above, a variety of advantages are considered in polishing.

Then, as a result of actually performing the press polishing with acrylic particles (average particle size: 10 μm), excellent machining stability was obtained, and at the same time, machining in which surface roughness was maintained was able to be achieved, and the present invention was completed.

That is, the present invention includes the following inventions.

(1) A local polishing method including press polishing performed while supplying a polishing solution between a work and a work-polishing rotating tool locally pressed against the work, the polishing solution having abrasive grains composed of organic particles with an average particle size of 5 μm or more dispersed in a liquid. Here, the average particle size refers to a median diameter in a particle size distribution measured by the laser diffraction/scattering method.

(2) The local polishing method according to (1), in which the rotating tool is made of an elastic material.

(3) The local polishing method according to (1) or (2), in which the liquid is pure water or a liquid containing water as a main component.

(4) The local polishing method according to any one of (1) to (3), in which the rotating tool includes: a rotating body; a shaft body that has a tip end provided with the rotating body and is long in an axial direction around which the rotating body is rotated; and a rotation support portion that supports the shaft body on a base end side thereof for allowing the shaft body to rotate around an axis center thereof, and the rotating body is pressed, at an outer circumferential surface thereof, against the work to curve the shaft body, and elastic restoring force of the curved shaft body causes the rotating body to be pressed and urged against the work.

(5) The local polishing method according to any one of (1) to (4), in which an outer diameter of a polishing action region on an outer circumferential surface of a rotating tool, the outer circumferential surface facing the work, is set to 5.0 mm or less.

(6) The local polishing method according to any one of (1) to (5), in which the organic particles are acrylic particles or urethane particles.

(7) A local polishing device including: a work-polishing rotating tool locally pressed against a work; and machining solution supply means for supplying, between the rotating tool and the work, a polishing solution in which abrasive grains composed of organic particles with an average particle size of 5 μm or more are dispersed in a liquid.

(8) The local polishing device according to (7), in which the rotating tool is made of an elastic material.

(9) The local polishing device according to (7) or (8), in which the liquid is pure water or a liquid containing water as a main component.

(10) The local polishing device according to any one of (7) to (9), in which the rotating tool includes: a rotating body; a shaft body that has a tip end provided with the rotating body and is long in an axial direction around which the rotating body is rotated; and a rotation support portion that supports the shaft body on a base end side thereof for allowing the shaft body to rotate around an axis center thereof, the rotating body is pressed, at an outer circumferential surface thereof, against the work to curve the shaft body, and elastic restoring force of the curved shaft body causes the rotating body to be pressed and urged against the work.

(11) The local polishing device according to any one of (7) to (10), in which an outer diameter of a polishing action region on an outer circumferential surface of a rotating tool, the outer circumferential surface facing the work, is 5.0 mm or less.

(12) The local polishing device according to any one of (7) to (11), in which the organic particles are acrylic particles or urethane particles.

(13) A corrective polishing device, in which the local polishing device according to any one of (7) to (12) is used.

Advantageous Effects of Invention

According to the above-described invention of the present application, the wear of the rotating tool is prevented in the local press polishing, the surface texture of the tool surface does not affect the machining, either, and pretreatment and maintenance of the tool surface can be omitted. Then, local polishing, which obtains excellent machining stability while preventing cost from increasing with such a simple structure as described above, and can also maintain surface roughness at the same time, can be achieved, and can be preferably used as the corrective polishing.

Moreover, when the rotating tool is made of an elastic material, the work is polished by a rolling action of the abrasive grains between the rotating tool and the work, thus enabling the further improvement in surface roughness. In addition, the elastic deformation of the rotating tool stabilizes the pressing force, thus enabling the further improvement in machining stability.

Moreover, when the liquid that allows the organic particles as abrasive grains to be dispersed therein is pure water or a liquid containing water as a main component, the dispersibility of the organic particles is improved, and the machining stability is further improved. In addition, when the work is silicon, glass or the like, a soft hydrated film made of water is formed on a surface thereof, whereby the removal is further promoted, and the surface roughness can also be improved.

Moreover, the rotating tool is composed of: a rotating body; a shaft body that has a tip end provided with the rotating body and is long in an axial direction in which the rotating body is rotated; and a rotation support portion that supports the shaft body on a base end side thereof for allowing the shaft body to rotate around an axis center thereof, the rotating body is pressed, at an outer circumferential surface thereof, against the work to curve the shaft body, and elastic restoring force of the curved shaft body causes the rotating body to be pressed and urged against the work. In this case, the pressing force of the rotating tool is stabilized, thus enabling the further improvement in machining stability.

When the outer diameter of a polishing action region on an outer circumferential surface of the rotating tool, which faces the work, is set to 5.0 mm or less, local polishing with a higher resolution is enabled.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an explanatory view for explaining a machining principle.

FIG. 1B is an explanatory view for explaining the machining principle.

FIG. 2 is a front view illustrating a local polishing device according to a typical embodiment of the present invention.

FIG. 3 is a perspective view of the local polishing device viewed from diagonally below.

FIG. 4 is an explanatory view illustrating a main part of the local polishing device.

FIG. 5 is a front view illustrating a local polishing device according to another embodiment.

FIG. 6 is an explanatory view illustrating a main part of the local polishing device.

FIG. 7 is an explanatory view illustrating a main part of a rotating tool.

FIG. 8A is a surface observation image of a static machining mark according to Example 1.

FIG. 8B is a surface observation image of a static machining mark according to Example 8.

FIG. 8C is a surface observation image of a static machining mark according to Example 9.

FIG. 9 is a surface observation image of a static machining mark according to Comparative example 1.

FIG. 10A is a graph illustrating a relationship between a machining amount and a machining time according to Example 1.

FIG. 10B is a graph illustrating a relationship between a machining amount and a machining time according to Comparative example 1.

FIG. 11 is an explanatory view illustrating a method of a raster scan machining test.

FIG. 12 is a surface observation image of a raster scan machining result according to Comparative example 1.

FIG. 13 is a surface observation image of surface roughness before raster scan machining.

FIG. 14A is a surface observation image of surface roughness after raster scan machining according to Example 1.

FIG. 14B is a surface observation image of surface roughness after raster scan machining according to Example 2.

FIG. 14C is a surface observation image of surface roughness after raster scan machining according to Example 8.

FIG. 15A is a surface observation image of surface roughness after raster scan machining according to Comparative example 1.

FIG. 15B is a surface observation image of surface roughness after raster scan machining according to Comparative example 2.

FIG. 16A is a graph illustrating a relationship between a machining amount and pressing force on a synthetic quartz glass substrate of Example 1.

FIG. 16B is a graph illustrating a relationship between a machining amount and pressing force on a silicon substrate of Example 1.

FIG. 17 is a surface observation image of a raster scan machining result on the silicon substrate according to Example 1.

FIG. 18 is a graph illustrating a relationship between a machining amount and a particle size of abrasive grains.

FIG. 19 is a graph illustrating a relationship between a machining amount and an abrasive grain concentration.

FIG. 20 is a figure of an ideal machining result (ideal machining amount calculation result) and a surface observation image illustrating an actual machining result according to Corrective polishing test 1.

FIG. 21 is a comparison diagram between cross sectional profiles (machining amounts) at the respective centers, in the horizontal direction, of the ideal machining result and the actual machining result, according to Corrective polishing test 1.

FIG. 22 is surface observation images of surface roughnesses before and after Corrective polishing test 1.

FIG. 23 includes a figure of an ideal machining result (ideal machining amount calculation result), and a surface observation image of an actual machining result according to Corrective polishing test 2.

FIG. 24 is surface observation images of surface roughnesses before and after Corrective polishing test 2.

 DESCRIPTION OF EMBODIMENTS

Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

As illustrated in FIGS. 2 to 4, a local polishing device 1 according to a typical embodiment of the present invention includes: a work-polishing rotating tool 11 locally pressed against a work 9, and machining solution supply means 12 for supplying, between the rotating tool 11 and the work 9, a polishing solution 8 in which abrasive grains composed of organic particles with an average particle size of 5 μm or more are dispersed in a liquid.

As illustrated in FIG. 1B, a machining principle is that the abrasive grains 81 in the polishing solution 8 supplied between the rotating tool 11 and the work 9 are sandwiched therebetween and roll on the surface of the work 9, so that the surface of the work 9 is polished. Therefore, a polishing action region of an outer circumferential surface of the rotating tool 11, which faces the work 9, does not need to adjust a surface texture thereof, and in this example, is composed of an elastic body made of an elastic material that can capture the abrasive grains 81 easily.

The device illustrated in FIGS. 2 to 4 is configured as a type of device for polishing such a plate-shaped work 9, and is provided with a work holding mechanism 13 (X-axis stage 41, Y-axis stage 42, Z-axis stage 43) that holds the work 9 with the machining target surface 90 facing downward so as to be capable of moving the work 9 in XYZ-directions at an upward position. Further, at a position substantially directly below the work 9 held by the work holding mechanism 13, provided as the machining solution supply means 12 is a machining solution injection unit 14 including an injection nozzle 30 that injects the polishing solution 8 directly above with a gap s1 into which the rotating tool 11 is inserted.

In this example, the machining target surface 90 of the plate-shaped work 9 is flat; however, even if the machining target surface 90 is a curved surface such as a surface of a lens, the curved surface can be dealt with by moving the work 9 three-dimensionally by the work holding mechanism 13. If the work holding mechanism 13 is provided with the rotating mechanism (rotating stage; θ stage) that rotates the work 9 while holding the same, then a posture of the work can be changed in addition to the position thereof, and a degree of freedom in machining can be further enhanced.

In this example, a support base 23 that supports the rotating tool 11 in an inclined state is provided with a mechanism that supports a rotation support portion 22 so that an angle thereof is adjustable, and can more flexibly deal with the shape of the machining target surface 90. Thus, the degree of freedom in machining is enhanced. Operations of these work holding mechanism 13 and support base 23 are automatically controlled by a computer (not shown), whereby the local polishing device 1 can be used as a corrective polishing device that automatically scans the machining target surface.

The machining solution injection unit 14 as the machining solution supply means 12 is composed of: the injection nozzle 30; a recovery tank 31 around the injection nozzle 30, which receives the polishing solution 8 injected from the nozzle, hits a work's machining target surface 90 and drops; and a pump 35 that supplies the polishing solution 8, which is received and recovered into the recovery tank 31, again to the injection nozzle 30 and injects the polishing solution 8 upward. During machining, the polishing solution 8 circulates between the injection nozzle 30, the work's machining target surface 90, the recovery tank 31, and the pump 35.

The polishing solution 8 is injected and supplied to the works machining target surface 90 by the machining solution injection unit 14 in this way, whereby the polishing solution 8 can be stably and efficiently supplied to the local polishing region of the machining target surface 90, and stable polishing for a long time can be achieved with a small amount of polishing solution. In addition, the local polishing device 1 can be used regardless of the shape and size of the work, which also contributes to cost reduction. In particular, the polishing solution 8 can be supplied evenly in all directions by being injected from directly below the machining target surface 90 in the form of a fountain, a machining rate is stabilized more, and an amount of water can also be reduced.

The rotating tool 11 is made of an elastic material. Specifically, the rotating tool 11 is composed of: a rotating body 20 made of an elastic material such as rubber; a shaft body 21 that has a tip end provided with the rotating body 20 and is long in an axial direction in which the rotating body 20 is rotated; and a rotation support portion 22 that rotates the shaft body 21 around an axis center thereof while supporting the same on a base end side. Then, an outer circumferential surface of the rotating body 20 is pressed against the work 9, whereby the shaft body 21 is curved, and the rotating body 20 is pressed and urged against the work 9 by the elastic restoring force of the curved shaft body 21.

As illustrated in FIG. 7, preferably, the rotating body 20 is configured so that an outer diameter d2 of a polishing action region 201 on an outer circumferential surface 20a that faces the work 9 (that is, the diameter d2 is a maximum diameter in the region) is 5.0 mm or less. In the rotating body 20, a diameter of the rotating body (the diameter is a maximum diameter of the outer circumferential surface thereof) is set to be smaller than a diameter of the conventional rotating tool. As a shape of the rotating body 20, various shapes such as a spherical shape, a partial spherical shape, a ring shape having a circular cross section (toroidal shape), and the like can be adopted. Then, the abrasive grains interposed between the rotating body 20 and the work's machining target surface 90 are rolled on the machining target surface 90 while being pressed by the rotating body 20 to polish the machining target surface 90, and the surface roughness is further improved.

Since the rotating body 20 is made of elastic materials in this way, the pressing force for pressing the machining target surface 90 through the abrasive grains 81 is stabilized, in addition, the abrasive grains 81 can be firmly held and rolled on the machining target surface 90, so that machining stability is improved. As a specific elastic material, it is preferable to use fluorine rubber. Fluorine rubber has a small friction of coefficient, and is chemically stable when PH adjustment of the machining solution is considered.

Moreover, the rotating body is set to have a small diameter as described above, whereby it is considered that force for local pressing will be exerted against the work's machining target surface 90, energy applied to the intervening abrasive grains for pressing the abrasive grains per unit area will be increased, and the machining rate will also increase. Further, a small unit machining shape is obtained, and an excellent spatial resolution is also obtained.

As the shaft body 21, a shaft of metal such as stainless steel, which has a cross section smaller in diameter than the rotating body 20, can be used. Other materials may be naturally used as long as the materials are long and flexible in the axial direction. In this example, a tip end portion of the shaft body 21 is fitted and fixed into the toroidal-shaped rotating body 20, and a base end portion of the shaft body 21 is fixed to the rotation support portion 22, whereby the rotating tool 11 is configured. An electric motor or the like can be used for the rotation support portion 22.

The shaft body 21 is extended diagonally from a position diagonally below a gap s1 between the work 9 and the nozzle 30 so that the rotating body 20 on the tip end thereof is pressed against the machining target surface 90. The base end portion is rotatably supported by the rotation support portion 22 provided at the position located below. The axial direction of the shaft body is defined to be an axial direction at a position of the tip end provided with the rotating body when the shaft body is slightly curved by the pressing against the work. In this example, the shaft body is provided at an angle of approximately 45 degrees with respect to the normal line of the machining target surface 90; however, the angle is not limited to that angle. Such a structure in which the rotating body 20 is inserted into the gap s1 by the shaft body 21 from diagonally below is adopted as in this example, whereby, when the machining target surface is a curved surface (for example, a free curved surface), it is easy to machine the work while rotating the same. When the shaft body 21 is placed in parallel to the machining target surface 90, it is difficult to rotate and machine the work in this way.

As in this example, the flexible shaft body 21 is used, and the work 9 is machined while the rotating body 20 is pressed and urged against the work 9 by the elastic restoring force of the curved shaft body 21. In this way, a positional relationship between the rotating tool 11 and the work 9 may be slightly deviated due to the shape of the machining target surface 90. Even in such a situation, the shaft body 21 is only elastically deformed by that amount, a large fluctuation in the pressing force can be avoided, and the pressing force is kept substantially constant, whereby a stable machining amount is obtained. This means that strict accuracy is not required for the stages 41, 42 and 43 (work holding mechanism 13) (for example, an accuracy of approximately 10 μm is sufficient even in nano-level corrective polishing of a lens).

The liquid of the polishing solution 8 is preferably pure water or a liquid containing water as a main component in terms of dispersibility of organic particles. Various organic particles can be adopted, and those having a density close to 1 g/cm3, such as acrylic particles, urethane particles, and styrene particles, which are made of polymer materials, are particularly preferable. Among them, urethane and acrylic (both densities are 1.2 g/cm3) are more preferable. Organic particles have a density closer to 1 g/cm3 than metal oxide particles which are general abrasives, and are easy to disperse without precipitating. Organic particles of different materials may be mixed.

Moreover, an average particle size of the organic particles is preferably 5 μm or more and 30 μm or less.

Then, referring to FIGS. 5 and 6, a description will be given of an embodiment of another device configuration according to the present invention, which is suitable when the surface (rotating surface) of a work having a rotating body shape is used as a machining target surface. Here, the surface of the work includes an outer circumferential surface of a cylindrical work such as a rod lens, an outer circumferential surface of a conical or truncated cone-shaped work, and an inner/outer circumferential surface of a cylindrical work.

Like the device 1 of the above-mentioned typical embodiment, a local polishing device 1A according to this embodiment includes: the rotating tool 11 locally pressed against the work 9; and the machining solution supply means 12 for supplying, between the rotating tool 11 and the work 9, the polishing solution 8 in which abrasive grains composed of organic particles with an average particle size of 5 μm or more are dispersed in a liquid. The local polishing device 1A polishes the surface of the work 9 according to the same machining principle.

The work holding mechanism 13 is provided with a mechanism that rotates a columnar or cylindrical work around an axis center thereof together with XYZ stages (not shown). In the machining of the inner circumferential surface/outer circumferential surface of the columnar or cylindrical work, it is not necessary to move the work horizontally to a large extent. Accordingly, in this embodiment, a machining tank 32 having an upper end opening, which houses the polishing solution 8, is provided as the machining solution supply means 12. In addition, the work 9 held by the work holding mechanism 13 is immersed from above through the opening of the machining tank 32, and in a similar way, the rotating tool 11 is immersed from diagonally above through a gap between the opening and the work. Then, the machining target surface 90 as the outer circumferential surface of the work is polished in the liquid by the rotating tool 11.

In order to stir the polishing solution 8 in the machining tank 32, the machining tank 32 is installed on a magnetic stirrer 33, and a stirrer 34 provided on the bottom of the machining tank 32 performs stirring by low-speed rotation.

Besides, since the configuration of the rotating tool 11, the configurations of the polishing solution 8 and the organic particles 81 contained therein, and other configurations are the same as those of the above-mentioned typical embodiment, the same structures are denoted by the same reference numerals, and a description thereof is omitted.

While the embodiments of the present invention have been described above, the present invention is not at all limited to these embodiments, and it is a matter of course that the present invention can be implemented in various forms without departing from the spirit of the present invention.

EXAMPLES

Hereinafter, results of various tests performed using polishing slurry of Examples 1 to 9 and Comparative examples 1 to 4 will be described.

(Polishing Slurry)

As shown in Table 1 below, 13 types of polishing slurry of Examples 1 to 9 and Comparative examples 1 to 4 were prepared.

TABLE 1 Average Abrasive grain particle size concentration Material (μm) (vol %) Liquid Example 1 acrylic 10 14.3 pure water Example 2 acrylic 15 14.3 Example 3 acrylic 15 16.7 Example 4 actylic 10 16.7 Example 5 acrylic 30 16.7 Example 6 acrylic 15 8.3 Example 7 acrylic 15 25 Example 8 urethane 15 14.3 Example 9 urethane 15 14.3 Fluorinert FC-43 Comparative silica 14 8.5 pure water example 1 Comparative acrylic 3 14.3 example 2 Comparative acrylic 3 16.7 example 3 Comparative no no 0 example 4 abrasive abrasive grains grains

(Static Machining Mark Test 1)

Static machining mark tests in which the scanning of the tool was stopped were performed, with using the four types of polishing slurry of Examples 1, 8 and 9 and Comparative example 1 and the local polishing device according to the above-mentioned typical embodiment illustrated in FIGS. 2 to 4 and 7, under machining conditions (rotation speed of the rotating tool, pressing force thereof, machining time) in Table 2 shown below. Note that details of the local polishing device are as follows.

    • Rotating body of rotating tool: fluorine rubber with toroidal shape; diameter (d1): 3 mm
    • Shaft body of rotating tool: stainless steel shaft of φ 1 mm
    • The above rotating tool was installed so that the axial direction of the shaft body defined 55 degrees with respect to the normal line of the machining target surface, and the outer diameter (d2) of the polishing action region was set to 2.2 mm. Here, though the axial direction was slightly curved due to the pressing of the rotating tool against the work, the axial direction is defined to be the axial direction at the position of the tip end provided with the rotating body.
    • Motor of rotation support portion of rotating tool: a motor capable of controlling the rotation speed in a range of 50 to 4000 rpm

Moreover, the work was made of a synthetic quartz glass substrate, the work's machining target surface was flat, and the rotating tool was pressed by lowering the stage by a predetermined amount after the rotating body came into contact with the machining target surface.

TABLE 2 Rotation Pressing Machining Static speed force time machining (rpm) (N) (min) mark result Example 1 2000 0.012 5 FIG. 8A Example 8 350 0.012 5 FIG. 8B Example 9 2000 0.012 30 FIG. 8C Comparative 350 0.012 1 FIG. 9 example 1

As seen from measurement results of a scanning white interferometers in FIGS. 8A to 8C and FIG. 9, also in the case of each example (FIGS. 8A, 8B and 8C) where the liquid of the polishing slurry was pure water/fluorinert and the organic abrasive grains were acrylic/urethane, the local machining was achieved as in the case (FIG. 9) of the polishing slurry of Comparative example 1 using general polishing abrasive grains. It was confirmed that an inert perfluoro compound (fluorinert) was also effective in addition to water as a liquid of the general polishing slurry.

(Static Machining Mark Test 2)

A test of confirming a change, depending on the machining time, in the machining amount of the static machining mark was performed, with using the two types of polishing slurry of Example 1 and Comparative Example 1 and the same local polishing device and the same type of work (synthetic quartz glass substrate with a flat work's machining target surface) as in the above static machining mark test 1, under machining conditions (rotation speed of the rotating tool, pressing force thereof) in Table 3 shown below.

TABLE 3 Rotation speed Pressing force Machining (rpm) (N) amount result Example 1 2000 0.012 FIG. 10A Comparative 600 0.006 FIG. 10B example 1

From graphs of FIGS. 10A and 10B, it is seen that the machining amount is proportional to the machining time in each of Example 1 and Comparative example 1. It was confirmed that the machining amount of the static machining marks, which was required for the corrective polishing, was proportional to the time.

(Raster Scan Machining Test 1)

An area with 2.5 mm square was raster-scanned, as illustrated in FIG. 11, in steps of 10 μm, with using the five types of polishing slurry of Examples 1, 2 and 8 and Comparative Examples 1 and 2, and the same local polishing device and the same type of work (synthetic quartz glass substrate with a flat work's machining target surface) as in the above static machining mark test 1, under the machining conditions (rotation speed of the rotating tool, pressing force thereof, machining time) of Table 4 shown below. As a result, in each of the examples and the comparative examples, the same raster scan removal as in FIG. 12 (Comparative example 1) was confirmed. Moreover, the surface roughness of each example was evaluated using an RMS value of 0.187 mm×0.14 mm using a scanning white interferometer.

TABLE 4 Rota- tion Pressing Machining speed force time (rpm) (N) (hour) Result Example 1 2000 0.012 2 RMS 0.169 nm FIG. 14A Example 2 2000 0.012 2 RMS 0.225 nm FIG. 14B Example 8 1700 0.012 2 RMS 0.368 nm FIG. 14C Comparative 350 0.012 1 RMS 0.488 nm FIG. 15A example 1 Comparative 2000 0.012 2 RMS 0.557 nm FIG. 15B example 2 Before — — — RMS 0.175 nm FIG. 13 machining

In the case of Comparative example 1 (silica particles) and Comparative example 2 (acrylic particles with an average particle size smaller than 5 μm), the surface roughness has deteriorated considerably after the machining as seen from comparison of FIGS. 15A and 15B with FIG. 13 showing a state before the machining. On the other hand, in the case of Examples 1, 2 and 8 (acrylic particles/urethane particles with an average particle size of 10 μm or more), relatively good surface roughness is maintained even after the machining as seen from similar comparison of FIGS. 14A, 14B and 14C with FIG. 13 showing the state before the machining.

(Static Machining Mark Test 3)

FIGS. 16A and 16B show results of machining two types of works (synthetic quartz glass substrate and silicon substrate, each of which has a flat work's machining target surface) while changing the pressing force using the polishing slurry of Example 1 and the same local polishing device as in the above static machining mark test 1.

As seen from the respective graphs of FIGS. 16A and 16B, the machining amount is proportional to the pressing force.

(Raster Scan Machining Test 2)

An area with 1.0 mm square in the work (silicon substrate with a flat work's machining target surface) was raster-scanned in steps of 10 μm under machining conditions of 2000 rpm as the rotation speed of the rotating tool, 0.006 N as the pressing force of the rotating tool, and 29 minutes as the machining time, with using the polishing slurry of Example 1 and the same local polishing device as in the above static machining mark test 1. FIG. 17 shows measurement results of a scanning white interferometer.

From the results shown in FIG. 17, it was demonstrated that it was possible to also machine a silicon substrate like the machining (FIG. 12) for the glass substrate.

(Static Machining Mark Test 4)

The work (synthetic quartz glass substrate with a flat machining target surface) was machined, with using: the totally five types of polishing slurry of Examples 3 to 5 and Comparative example 3, which are different from one another only in average particle size, and of Comparative example 4 that did never contain abrasive grains but contained only pure water; and the same local polishing device as in the above static machining mark 1, under the same machining conditions (1600 rpm as the rotation speed of the rotating tool, 0.012 N as the pressing force of the rotating tool, and one minute as the machining time). FIG. 18 shows results of the machining.

As shown in the graph of FIG. 18, the polishing amount increased up to a particle size of 15 μm; however, the machining amount decreased at the particle size of 30 μm.

(Static Machining Mark Test 5)

The work (synthetic quartz glass substrate with a flat machining target surface) was machined, with using: the four types of polishing slurry of Examples 3, 6 and 7 and Comparative example 4, which are different from one another only in abrasive grain concentration; and the same local polishing device as in the above static machining mark test 1, under the same machining conditions (1600 rpm as the rotation speed of the rotating tool, 0.012 N as the pressing force of the rotating tool, and one minute as the machining time). FIG. 19 shows results of the machining.

As shown in the graph of FIG. 19, as the abrasive grain concentration increased, the machining amount also increased. It was found that, as the concentration was further increased, the increase in the machining amount slowed down.

(Corrective Polishing Test 1)

It was tested whether a shape error with a period of 0.1 mm in the work (synthetic quartz glass substrate with a flat machining target surface) could be correctively polished, with using the polishing slurry of Example 1 and the same local polishing device as in the above static machining mark test 1.

Any target shape with a width of 0.1 mm was prepared, and deconvolution calculation thereof was conducted with a unit machining shape calculated based on static machining marks obtained from the static machining mark test 1 of Example 1, whereby a residence time distribution was calculated. Scanning according to the residence time distribution was conducted on the synthetic quartz glass flat substrate, and measurement was performed with a scanning white interferometer. As a result, a shape extremely close to the ideal was able to be produced (FIGS. 20 and 21).

Moreover, as shown in FIG. 22, the surface roughness region was also almost unchanged and maintained a state before the machining. The surface roughness was evaluated using an RMS value of 0.187 mm×0.14 mm.

(Corrective Polishing Test 2)

It was tested whether a shape error with period of 0.15 mm in a work (columnar lens made of φ 10 mm synthetic quartz glass in which an outer circumferential surface was a machining target surface) could be correctively polished, with using the polishing slurry of Example 1 and the column or cylinder machining local polishing device according to the above-mentioned embodiment illustrated in FIGS. 5 and 6.

Like the above corrective polishing test 1, any target shape with a width of 0.15 mm was prepared, and deconvolution calculation thereof was conducted with the unit machining shape obtained from the aforementioned static machining mark test 1 of Example 1 as in Corrective polishing test 1, whereby a residence time distribution was calculated. Scanning according to the residence time distribution was conducted on such a machining target surface, and measurement was performed with a scanning white interferometer. As a result, a shape extremely close to the ideal was able to be produced (FIG. 23). Moreover, as shown in FIG. 24, a result that the surface roughness region was also almost unchanged was obtained. The surface roughness was evaluated using an RMS value of 0.187 mm×0.14 mm.

In each of the corrective polishing tests, as a result, a spatial resolution of the ideal target shape was able to be obtained, and the surface roughness region was also able to be maintained. From the above result, the corrective polishing with a desired 0.1 mm periodic shape was achieved. This method, which combines a rotating tool and relatively large-diameter organic particles, can be defined as a corrective polishing method with high correction spatial resolution and high stability. In particular, this method is considered to be a sufficiently useful technique in the development of high-precision optical elements.

REFERENCE SIGNS LIST

    • s1 Gap
    • d1 Diameter
    • d2 Outer diameter
    • 1 Local polishing device
    • 8 Polishing solution
    • 9 Work
    • 11 Rotating tool
    • 12 Machining solution supply means
    • 13 Work holding mechanism
    • 14 Machining solution injection unit
    • 20 Rotating body
    • 20a Outer circumferential surface
    • 201 Polishing action region
    • 21 Shaft body
    • 22 Rotation support portion
    • 23 Support base
    • 30 Injection nozzle
    • 31 Recovery tank
    • 32 Machining tank
    • 33 Magnetic stirrer
    • 34 Stirrer
    • 35 Pump
    • 41 X-axis stage
    • 42 Y-axis stage
    • 43 Z-axis stage
    • 81 Abrasive grains
    • 90 Machining target surface

Claims

1: A local polishing method comprising press polishing performed while supplying a polishing solution between a work and a work-polishing rotating tool locally pressed against the work, the polishing solution having abrasive grains composed of organic particles with an average particle size of 5 μm or more dispersed in a liquid.

2: The local polishing method according to claim 1, wherein the rotating tool is made of an elastic material.

3: The local polishing method according to claim 1, wherein the liquid is pure water or a liquid containing water as a main component.

4: The local polishing method according to claim 1, wherein the rotating tool comprises:

a rotating body;
a shaft body that has a tip end provided with the rotating body and is long in an axial direction around which the rotating body is rotated; and
a rotation support portion that supports the shaft body on a base end side thereof for allowing the shaft body to rotate around an axis center thereof, and
the rotating body is pressed, at an outer circumferential surface thereof, against the work to curve the shaft body, and elastic restoring force of the curved shaft body causes the rotating body to be pressed and urged against the work.

5: The local polishing method according to claim 1, wherein an outer diameter of a polishing action region on an outer circumferential surface of a rotating tool, the outer circumferential surface facing the work, is set to 5.0 mm or less.

6: The local polishing method according to claim 1, wherein the organic particles are acrylic particles or urethane particles.

7: A local polishing device comprising:

a work-polishing rotating tool locally pressed against a work; and
machining solution supply section that supplies, between the rotating tool and the work, a polishing solution in which abrasive grains composed of organic particles with an average particle size of 5 μm or more are dispersed in a liquid.

8: The local polishing device according to claim 7, wherein the rotating tool is made of an elastic material.

9: The local polishing device according to claim 7, wherein the liquid is pure water or a liquid containing water as a main component.

10: The local polishing device according to claim 7, wherein the rotating tool comprises:

a rotating body;
a shaft body that has a tip end provided with the rotating body and is long in an axial direction around which the rotating body is rotated; and
a rotation support portion that supports the shaft body on a base end side thereof for allowing the shaft body to rotate around an axis center thereof, and
the rotating body is pressed, at an outer circumferential surface thereof, against the work to curve the shaft body, and elastic restoring force of the curved shaft body causes the rotating body to be pressed and urged against the work.

11: The local polishing device according to claim 7, wherein an outer diameter of a polishing action region on an outer circumferential surface of a rotating tool, the outer circumferential surface facing the work, is 5.0 mm or less.

12: The local polishing device according to claim 7, wherein the organic particles are acrylic particles or urethane particles.

13: A corrective polishing device, wherein the local polishing device according to claim 7 is used.

Patent History
Publication number: 20210331283
Type: Application
Filed: Aug 7, 2019
Publication Date: Oct 28, 2021
Applicants: The University of Tokyo (Tokyo), NATSUME OPTICAL CORP. (Iida-shi, Nagano)
Inventors: Hidekazu MIMURA (Tokyo), Hirokazu HASHIZUME (Iida-shi), Yusuke MATSUZAWA (Iida-shi)
Application Number: 17/264,511
Classifications
International Classification: B24B 13/01 (20060101); B24B 41/04 (20060101);