JPS5946739B2 - Surface polishing method - Google Patents

Description

[Detailed description of the invention] The present invention relates to a novel precision surface polishing method.

For surface processing of workpieces, cutting using a tool with a cutting edge or rotating cutting blade, etc., and grinding using a stone, etc.

0 So-called machining methods such as rappi/gu have been known for a long time, in which loose fine particles and grains are injected between the lap and the workpiece, applying pressure to the lap and causing intersecting motion between the lap and the workpiece. Techniques such as plasting, which sprays abrasive particles onto the surface of a workpiece, and ultrasonic machining, which uses ultrasonic waves to violently impinge on the surface of a workpiece, have also been developed.

However, these processes are based on destruction and plastic deformation, which involve the activation of dislocations and cracks existing in the workpiece material due to a large stress field, so the accuracy of the plastic deformation phenomenon may be affected by the finished surface. In order to produce higher-performance, more reliable equipment, it is desired to develop higher-precision processing technology. By the way, all materials are composed of atomic bonds, so if the unit of destruction during processing could be made to the atomic order,
The finished surface accuracy should be able to approach the atomic order.
As such removal processing methods on the atomic order, electrochemical processing methods such as electrolytic polishing and chemical polishing, or electron beam processing,
Io/beam processing methods have been intensively researched. In the former, the energy required to separate the bonds between atoms is provided electrochemically or chemically, and in the latter, the kinetic energy of electrons is converted into thermal energy to melt or evaporate the material atoms. The method uses the sputtering phenomenon in which atoms are ejected by the elastic 05 collision of ions, but in any case, the energy necessary to separate the atoms is applied by some method. Both methods can be said to utilize physical phenomena that can, in principle, be processed in units of atomic order. However, in actual processing using these methods, such good accuracy has not been achieved. This seems to be due to the fact that electrochemical processing requires selective processing due to the non-uniformity of the material and the distribution of internal microscopic defects.On the other hand, physical processing requires extremely large amounts of supplied energy. The melting phenomenon takes precedence, and electrons and ions from the tool penetrate beneath the surface of the material, resulting in thermal alteration or numerous point defects on the machined surface or inside, which impairs the physical properties of the machined surface layer. It's probably something that's happening. Therefore, in order to selectively remove surface atoms without damaging the physical properties of the processed surface layer, it is necessary to mechanically supply processing energy using a permanently minute tool, but larger than electrons or ions. It can be seen that this is rather advantageous. Of course, in conventional mechanical processing methods such as cutting, grinding and wrapping, the range of stress field exerted by the tool cutting edge is too large, resulting in fracture accompanied by plastic deformation due to the behavior of dislocations and cracks that already exist within the material, resulting in processing problems. Damage to the physical properties of the surface layer cannot be avoided. Therefore, using tools (e.g., fine grains) that are sufficiently large compared to electrons and ions and sufficiently small compared to cutting edges of tools such as wrapping, mechanical energy is applied to the ideal crystalline region of the defect distribution interval of the material. If possible, it is thought that processing close to the atomic order that does not involve defects would become possible. Based on this idea, a processing method in which a mixed flow of air and other fine particles is accelerated by a rotating disk and collides with the workpiece, or a processing method in which fine powder particles are suspended in an alkaline liquid and approaches the workpiece surface to check the bearing condition. A machining method using a rotating ball was proposed. However, even with these methods, the accuracy cannot be said to be sufficient, and there is still a need for an ultra-precision surface polishing method with improved processing accuracy. In liquid honing, a machining fluid containing carcasses suspended in a liquid is heated by compressed air to
The spray is applied to the workpiece surface at an angle of inclination of 50°, but when machining is performed by applying a machining liquid containing suspended fine powder and grains to the workpiece surface, it is difficult to avoid dislocation defects in the workpiece material and cause fracture. It is desirable that the grains act at an angle as close to parallel to the machined surface of the workpiece as possible.

In addition, in order to increase machining efficiency, it is of course necessary to give as much kinetic energy as possible to the fine powder and grains and have it act on the surface of the workpiece. Therefore, it is natural to consider a method in which a machining fluid, which is a low-viscosity fluid suspension of fine powder and grains, is passed at high speed between the machining surface of the workpiece and an opposing surface provided in parallel with an extremely narrow interval. . However, when using a fluid flow as a means for accelerating powder and grains, the smaller the processing unit (and grain), the more special consideration must be given to the flow state. That is, when the width of the fluid flow path is determined as described above, as the fluid velocity increases, the flow state changes from laminar flow to turbulent flow. In laminar flow, the flow condition is uniform in the flow direction, and particles in the fluid do not move in the direction perpendicular to the wall surface, but in turbulent flow, the flow is turbulent and the movement of particles is not uniform. Therefore, the grains are attached to the workpiece surface.
In order to make the particles act at an angle as close to parallel as possible, it is desirable that the flow state of the fluid be laminar, and that the flow rate of the machining fluid has its own certain limits, and that it imparts as much kinetic energy as possible to the grains. This almost conflicts with the proposition of increasing machining efficiency. A fluid (processing fluid) in which fine powder and particles are suspended is caused to flow in a laminar flow state between the workpiece processing surface and an opposing surface provided in parallel with a narrow interval, and the suspension and particles improve the surface finish or finish. Therefore, the method of carrying out the polishing is not suitable in terms of processing efficiency, unless some other means is used to impart large kinetic energy to the grains themselves without creating turbulence, or to control the grains. It is easily understood that this method cannot be used as an industrially advantageous precision surface polishing method.

Generally, when two electrodes are inserted into a liquid in which sand grains or colloidal particles are suspended, and a DC voltage is applied between them, the particles move.When the particle side is negatively charged and the solution side is positively charged, the particles move. This phenomenon is called electrophoresis, and the migration speed v of the particles at this time is expressed by the formula v=εHE/
It is well known that it is given by μ (where ε and μ are the dielectric constant and viscosity of the solvent; h is the zeta potential of the particles; and E is the magnitude of the electric field). By the way, SiO2, for example, which is widely used as abrasive grains for grinding tools, is negatively charged in water, which is a typical low-viscosity fluid. This is because the acidity of SiO2 is very high and the basicity of water is also high. Even in ethyl alcohol, which is a typical example of a non-aqueous solvent, the basicity of alcohol is lower than that of water, but since the acidity of SiO2 is high, it is also negatively charged. In addition, in the case of Ae2O3, the acidity is not so high, so from that point of view it is difficult to say that it will be negatively charged in alcohol. The larger one is positively charged) The alcohol is Ae2O3
The dielectric constant is much higher than that of Ae2O3, and actual tests show that Ae2O3 is negatively charged in alcohol. When we investigated the charging state of the particles in this way, we found that the particles are generally negatively charged in water, drainage liquid, and a fluid called machining fluid. Therefore, the fine powder and grains suspended in the fluid have the property of being electrophoretically attracted to and moving toward the anode. Furthermore, it is elementary knowledge in physics that when electric charge is stored in a parallel plate capacitor, an electric field is generated in the space between the plates, and that an electric field is induced by changes in the magnetic field. Therefore, if there is an opposing surface with a slight gap between it and the processed surface, and if an electric field is applied there through the processing liquid in which particles are suspended, either the processed surface or the opposing surface will become the anode side. An electrophoretic phenomenon occurs in which grains are attracted to the surface of the space, and the grain density increases on either side of the space, and when it acts on the machined surface, the number of grains increases, leading to improved machining efficiency. It turns out. Thus, the present inventors passed a machining liquid in which fine powder and grains were suspended in a fluid in a laminar flow state between a machining surface and an opposing surface disposed very close to and parallel to the machining surface, thereby machining. A technique for finishing or polishing the surface of an object, and when applying an electric field to the colloidal particle system by applying an electric double layer force to the surface of particles dispersed in a liquid, the particles generate a force from the field due to the surface charge. The present invention has been completed by utilizing the phenomenon of electrophoresis caused by electrophoresis, and combining this with technology that concentrates particles on the processed surface or the opposing surface and controls the number of particles when acting on the processed surface to improve processing efficiency. I've reached it. That is, according to the present invention, a workpiece holder is floated by buoyancy in a fluid suspension of fine abrasive particles, a polisher is provided at a distance from the holder, and the machined surface of the workpiece attached to the workpiece holder is Holding the polisher surface parallel to the polisher, and applying rotational motion in opposite directions to the workpiece holder and the polisher to form a laminar flow of the suspension in the space created by the processed surface and the polisher surface. , applying a DC voltage to an electrode and a polisher provided in the suspension at a location other than the workpiece; and an abrasive that acts on the surface of the workpiece by attracting abrasive fine particles onto the polisher surface by electrophoresis. An ultra-precision surface polishing method is provided that includes controlling the number of particulates. In order to create a laminar flow state for the processing liquid of the particle suspension between the workpiece surface and the opposing surface that are held in parallel with a narrow interval, the present invention uses a fluid suspension of the abrasive fine particles (and grains). Means is used to impart rotational motion in opposite directions to the surface of the workpiece and the polisher surface (opposing surface), which are sufficiently close to each other and maintained at equal distances in the liquid, that is, the machining fluid. It is of course possible to create a laminar flow of the machining fluid between the two surfaces held in parallel, and various other methods can be considered, but the The viscosity of
Or, many factors such as the workpiece, polisher material, dimensions, etc. are involved. Externally injecting machining fluid between these surfaces to create effective laminar flow is fairly complex. On the other hand, by rotating the machined surface and the polishing surface, which are held parallel to each other in a suspension of grains, in opposite directions using separate rotating shafts, Creating a laminar flow state is an extremely effective and easy method since it is easy to adjust the distance between the processing surface and the opposing surface and the speed of the rotating surface. The distance between the machined surface of the workpiece and the opposing polisher surface must be at least t larger than the particle size so that the particles of the processing fluid can pass through, but from the viewpoint of processing efficiency, etc. It is desirable that it be as narrow as possible, taking into account the type and concentration of the processed product, the carcass grains, the suspension fluid, the desired processing effect, and the strength of the electric field for electrophoresis, which will be described later.・Simple experimental operation The optimum range can be selected depending on each case. For this purpose, it is preferable to make the distance between the processed surface and the polished surface freely adjustable by, for example, moving the rotating shaft. According to a preferred embodiment of the present invention, the machined surface and the polisher surface, which are held parallel to each other with a narrow gap in the processing fluid, are rotated in opposite directions by their respective rotating shafts, most preferably by a group of eccentric rotating shafts. By rotating one of these rotation axes, the distance between the machining surface and the polisher surface is adjusted by 2, and an effective laminar flow of machining liquid is created therebetween. The rotational speeds of the processed surface and the polisher surface are appropriately selected depending on the workpiece and processing fluid. When the machining surface and the polisher surface, which are held parallel to each other at a narrow interval in the machining fluid, are given rotational motion in opposite directions, and an effective laminar flow of the machining fluid is formed between them, The processed surface is continuously subjected to an abrasive action due to the suspension and particles, and the processed amount increases in proportion to the processing time (see attached Figures 2 to 5).

As already mentioned, in polishing using this laminar flow machining fluid, it is impossible to further increase the flow velocity of the fluid because it will cause turbulence, so in the method of the present invention, next, this machining surface and the opposing polisher surface are An electric field is applied to the laminar flow between the fluid and the fine particles in the fluid are electrophoretically attracted to the workpiece surface or polisher surface side, increasing the number of abrasive fine particles acting on the workpiece surface, thereby significantly improving machining efficiency. It is intended to increase the number of people. Dispersed particles in a liquid generally have an electric charge that is proportional to the particle radius, the solvent permittivity, and the zeta potential, and that the presence of charged particles in a uniform electric field causes the particles to have an electric charge that is proportional to the electric field strength. It is well known that there is an electrostatic force that is directly proportional to the force and charge.

If the machined surface and the opposing surface are held parallel with a certain gap h, and the particles are negatively charged, they will be moved parallel to the machined surface between them, and a direct current will be applied so that the machined surface is positively charged and the opposing surface is negatively charged. Assume that a voltage v is applied. Then, an electric field E=VA is generated between the machined surface and the opposing surface, so that an electrostatic force acts on the grain in the direction of the machined surface, thereby producing a motion velocity of VE. In addition, since gravity acts on the grain and a motion velocity of -Va is also generated, a motion velocity Vy as the resultant force of this VE and Va is ultimately given in the direction perpendicular to the machined surface. Since a force is acting on the other grain to move parallel to the machined surface at the same speed as the fluid, a lateral motion velocity of Vx is also given, and the relative velocity v of the grain with respect to the machined surface is expressed as It turns out that it will be done.

In other words, when a machining fluid containing particles is passed in a laminar flow state between the machined surface and the opposing surface that are held parallel, and a DC current is applied between the machined surface and the opposing surface to create an electric field, Vx
is constant when the laminar flow state is maintained, and Va is constant due to the machining fluid, so it is clear that the grains are ultimately given a relative velocity with respect to the machined surface that is proportional to VE. As the electric field becomes stronger, the force that electrostatically attracts the grains toward the machined surface, that is, the VE motion velocity of the grains, increases. Since it is proportional to the applied voltage, it is easy to understand that when electrophoresis is performed by controlling the applied voltage, the relative speed of the grains to the machined surface can be controlled. Furthermore, if we consider that the amount of processing is proportional to the kinetic energy when the grain comes into contact with the processed surface, it will be understood that the amount of processing can also be controlled by controlling the strength of the electric field, that is, the voltage. Next, we will consider the case where the particles are concentrated on the opposing surface, that is, the case where a DC voltage v is applied at a distance h such that the opposing surface has a positive charge and the processed surface or other electrode has a negative charge. In this case, E=v/h between the processing surface or other electrode and the opposing surface.
An electric field is created, which pulls the grains towards the opposite surface. When the particles are concentrated on the opposing surface in this way, they move together with the opposing surface due to the frictional force with the opposing surface and act on the machined surface, but as the electric field becomes stronger and concentrates on the opposing surface, the number of particles increases. It can be seen that the grain density increases as it acts on the machined surface as it increases, and that the bonding force between opposing surfaces and grains, and between grains is influenced by this electrostatic force and gravity. Bonds are easily broken when the grains come in contact with the machined surface and receive force from it, and considering that the energy given by the grains to the machined surface is proportional to this bonding force, it is possible to increase the electric field, or voltage. The more the electrostatic force is increased, the more the energy of the particles applied to the processed surface increases, and the ability to control the strength of the electric field is easily linked to controlling the amount or surface treatment effect. be understood. As mentioned above, in general, in a fluid suspension (processing fluid) of carcasses, regardless of whether the fluid is aqueous or non-aqueous,
and the grains are negatively charged.

Therefore, by application of an electric field, the grains are electrophoretically drawn towards the anode and tend to concentrate densely on the anode surface. However, when water is used as the processing fluid, consideration must be given to the fact that the surface of the grains has a high degree of ionization, and the grains are ionized and become an electrolyte solution, which is adsorbed to the anode surface in an electrode reaction. When water is used as a processing liquid solvent, the surface of grains has a high electrolyte, so when a voltage is applied directly to the processed sample, an electrode reaction occurs on the surface of the processed sample, and when ionized, the grains become electrically Neutralize and improve your strength? This will cause it to stick and interfere with machining.

Therefore, in this case, it is desirable to provide an electrode other than the workpiece and apply a DC voltage between the electrode and the polisher. The present inventors have confirmed that the method of the present invention when water is used as a processing fluid solvent can be effectively carried out by using the apparatus shown in FIG. 1 and by applying an electric field with the polisher side as the anode. . The apparatus shown in FIG. 1 is a cylindrical container 1 for containing a machining fluid made of a non-conductive material and having an axial opening in the center of the bottom; A disk-shaped metal polisher 2 with a smooth outer circumferential side surface for vertical movement along the inner wall; a metal polisher 2 is joined at the center of the bottom surface to give rotational movement to the polisher, and passes through an axial opening in the bottom of the container and vertically downward. a metal polisher rotating shaft 3 extending from the bottom surface of the shaft to which the workpiece 5 can be held so that the surface to be processed is parallel to the polisher surface, and a plastic sample holder 6 which is floated in the processing liquid 4 by buoyancy;
In order to rotate the sample holder in the opposite direction to the polisher, the holder rotation shaft 7 extends vertically upward from the center of the holder and is located eccentrically from the polisher rotation axis. Means for moving the shaft up and down for adjustment (not shown)
; Electrode 8 provided in the machining liquid at a location other than the workpiece, facing the polisher; Means for applying voltage from a DC power supply 10 to the polisher rotating shaft 3 and the electrode 8 (power supply 10; brush 9; voltmeter 11); ; consists of an ammeter 12).

In carrying out the method of the present invention using this device, the workpiece is held on the bottom of the sample holder so that its machined surface is parallel to the polisher surface, and the rotation axis of the sample holder is placed between the machined surface and the polisher surface. After the sample holder rotation axis and the polisher rotation axis are rotated in opposite directions, the sample holder rotation axis and the polisher rotation axis are rotated in opposite directions to create a gap between the processing surface and the polisher surface. After the desired laminar flow of the working fluid is created, a DC voltage is applied to the polisher and the electrode so that the polisher surface becomes the anode. In the following specific example to demonstrate the effects of the method of the present invention, the above-mentioned apparatus including a brass disk-shaped polisher with an outer diameter of 80 mm, an acrylic sample holder, and a brass electrode with an eccentric distance of 44 mm between the vertical rotation axis and the sample holder was used. 20 Si single crystal wafers were processed under the following processing conditions: rotation speed 300 rpm, polisher rotation speed 100 rpm, voltage 0 to 500 V, gap 10 to 30 μm.
A sample of Y20×0.8 knob was pre-processed for wrapping finish with Ae2O3 Waterfall 2000 and grains, but the (001) and (】1】) sides were processed using water suspensions of various concentrations of SiO2 and Ae2O3 as the processing fluid. Processed.

The results of the experiment are shown in Figures 2-7. Figure 2 shows Ae, O, (
Si (11
1) The results were obtained by applying a voltage of 0 to 200 V to the surface with a gap (hereinafter referred to as the gap) of 10 μm between the surface and the polisher surface, and examining the relationship between the amount of processing and the voltage. As is clear from the figure, as the voltage increases and the electric field becomes stronger, the amount of processing gradually increases. Figure 3 shows 5% by volume of Ae2O (10 μm) and Si
Using a liquid solution in which O2 (0.02 μm) was suspended in 2% by volume water, a gap of 20 μm-(:Si(001) surface was
Figure 4 shows the relationship between the strength of the current and the amount of processing when processing using 0V direct current, and Figure 4 shows the relationship between the current strength and the amount of processing using 0V direct current.
6μm) Water suspension, gap 30μm 1 voltage 200V S
These are the results of investigating the relationship between the strength of current and the amount of processing when processing the i(001) plane. These results show that the amount of processing increases in proportion to the current. In this way, when water is used as a suspending solvent for grains and Ae, O3 and grains are used, the processing efficiency is greatly increased by implementing the method of the present invention by applying a DC voltage with the polisher side as an anode. It is clear that the amount increases with increasing current/voltage. The apparatus shown in Figure 1 is particularly useful, but not limited to, in the case of water-based machining fluids, and is particularly useful when using non-aqueous solvents to attract grains to the polisher side. In order to make it clearer that this is also an extremely effective treatment means, Fig. 5 shows 5 volume% Ae2O3.
Relationship between processing amount and voltage when processing a Si (001) surface using a (8 μm) alcoholic liquid as the processing fluid, using the apparatus shown in Figure 1 with one side of the polisher as an anode, and increasing the voltage with a gap of 20 μm. showed that. As can be seen from the figure, it will be understood that if the voltage is increased and the electric field is strengthened, an extremely excellent effect can be obtained even when one side of the polisher is used as an anode. Next, a case will be considered in which SiO2 is used as grains and water is selected as a solvent.

The combination in this case has a high degree of ionization and has the strongest tendency to negatively charge the grains. Figure 6 shows SiO2(2
Using a machining fluid containing 5% by volume of water (μm), a gap of 20
The relationship between the amount of processing and the voltage is shown when the Si (1) 1) surface is processed while increasing the voltage in μm. As the figure shows, the amount of processing does not increase even when the voltage increases, so it seems that the effects of the invention are not being obtained. However, Figure 7 shows the relationship between the surface roughness h (μm) (continuous indicator surface roughness measuring machine [Tokyo Seimitsu], measured with a stylus tip radius of 5 μm and a measuring force of 4 mN or less) under the same conditions and the voltage. As shown, the roughness of the machined surface gradually decreases as the voltage increases,
Also, microphotograph measurements show that as the voltage increases, the processed surface changes from a lapped surface to a polysynched surface, and the object of the invention of ultra-precision surface polishing is certainly achieved. Water 11C. When SiO2 and grains are dispersed, the degree of ionization is large and the amount of charge of SlO2 is too large, so that if they gather on the polisher surface, the grains will stick there and will not be separated. As the voltage increases, the number of particles increases as they gather on the polisher surface, and it easily reaches a saturation state, so that the amount of processing does not increase any further. However, as the number of grains increases as they adhere to the polisher surface, the contact angle of the grains with the machined surface decreases, and the roughness of the machined surface decreases proportionally, which is the reason for the results shown in Figures 6 and 7. It is thought that. In any case, even in the case of a machining liquid using water as a solvent, the object of the invention can be achieved by the method of the present invention, which involves applying a direct current voltage to the polisher side as an anode. In this way, the method of the present invention rotates the processing surface and the opposing surface in opposite directions in a fluid suspension of grains to create a laminar flow of the suspension in the narrow gap formed by these surfaces, thereby removing the material from the workpiece. At the same time, an electric field is applied to the laminar flow to electrophoretically concentrate the grains on the polished surface, and when applied to the processed surface, the number of grains is controlled and ultra-precision surface polishing is achieved. do. In the present invention, since the workpiece holder is floated in the fluid suspension by buoyancy, particles attracted onto the polisher surface by electrophoresis will elastically move against the machined surface of the workpiece held in the workpiece holder. As a result, flattening of the machined surface by precision polishing can be promoted without difficulty.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a polishing device used to carry out the method of the present invention, and FIG. 2 is a diagram showing the influence of voltage on the processing amount when a Si wafer is processed using the device shown in FIG. , Figure 3 is a diagram similar to Figure 2 showing the relationship between the amount of machining and current, Figure 4 is a diagram similar to Figure 2 showing the relationship between the amount of machining and current, and Figure 5 is a diagram showing the relationship between the amount of machining and voltage. Figure 6 is a diagram similar to Figure 2 showing the relationship between processing amount and voltage. Figure 7 is a diagram similar to Figure 2 showing the relationship between processing amount and voltage. FIG. 3 is a diagram showing the relationship between voltage and surface roughness of a machined surface. 1... Container, 2... Polyshear (opposing surface), 3... Polyshear rotating shaft, 4...-7
J liquid (fluid suspension of fine abrasive particles), 5...-
7J Emono, 6... Workpiece holder, 7...
...Holder rotation axis, 8...Electrode, 10...
...DC.

Claims (1)

[Claims] 1. A processing holder is floated by buoyancy in a fluid suspension of fine abrasive particles, a polisher is provided at a distance from the holder, and a polisher is attached to the processing surface of a workpiece attached to the workpiece holder. The workpiece holder and the polisher surface are held parallel to each other, the workpiece holder and the polisher surface are held parallel to each other, and the workpiece holder and the polisher are given rotational motion in opposite directions to create the workpiece surface and polisher surface. forming a laminar flow of the suspension in a space; applying a DC voltage to an electrode and a polisher provided at a location other than the workpiece during the suspension; and transferring fine abrasive particles to the polisher surface by electrophoresis. A surface polishing method that involves controlling the number of abrasive particles that act on the surface of a workpiece by attraction. 2. The surface polishing method according to claim 1, wherein the processed surface and the polisher surface are held parallel to each other at a narrow interval that is at least the particle diameter of the suspended abrasive fine particles.