JPS5847308B2 - Surface polishing method - Google Patents

Description

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

To pressurize the surface of the workpiece, use a tool with a cutting edge, a rotating cutting blade, etc. for cutting power, a grinding wheel for grinding, etc., and inject free fine particles between the lap and the workpiece to create a lap. So-called mechanical machining methods such as lapping, which creates intersecting motion between the lap and the workpiece while applying pressure, have been known for a long time. Techniques such as ultrasonic power, which uses energy to violently collide with the surface of a workpiece, have been successively developed.

However, these processes are always based on plastic deformation, which is caused by destruction accompanied by the activation of dislocations and cracks existing in the workpiece material due to a large stress field, and therefore the phenomenon accuracy of plastic deformation is dependent on the finished surface accuracy. Therefore, the development of higher precision processing technology is desired in order to produce equipment with higher performance and higher reliability.

By the way, since all materials are composed of atomic bonds, if the unit of fracture in the force OI can be made on 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, electron beam force Loe processing, ion beam processing methods, and the like have been intensively studied.

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, which melts and evaporates the material atoms. Alternatively, the sputtering phenomenon in which atoms are ejected by elastic collisions of ions is used, but in any case, the energy necessary to separate the atoms is applied by some method, and both f
It can be said that this method utilizes a physical phenomenon that can, in principle, perform force U-work in units of the atomic order.

However, the actual power of these methods in IJ engineering is
Such good accuracy has not been achieved.

The reason for this seems to be that in electrochemical processing, selective processing is performed due to the non-uniformity of the material and the distribution of internal microscopic defects, while in physical processing, the supplied energy is extremely large and it is difficult to melt. In addition, electrons and ions from the tool penetrate beneath the surface of the material, causing thermal deterioration or numerous point defects in the machining direction or inside, which impairs the physical properties of the machined surface layer. There must be something there.

Therefore, in order to selectively remove surface atoms without damaging the physical properties of the surface layer, mechanical processing energy is supplied using a tool as small as possible, but larger than electrons or ions. It can be seen that this is rather advantageous.

Of course, in conventional mechanical force methods such as cutting, grinding, and lapping, the range of the stress field exerted by the cutting edge of the tool is too large, resulting in fracture accompanied by plastic deformation due to the behavior of dislocations and cracks that occur within the material. After all, damage to the physical properties of the processed surface layer cannot be avoided.

There, it becomes electrons and ions. If it is possible to mechanically apply energy to the ideal crystalline region of the defect distribution interval of the material using a tool that is sufficiently large compared to the cutting edge of a tool such as lapping and sufficiently small compared to the cutting edge of a tool such as lapping, etc., it is possible to cause defects. It is conceivable that processing close to the atomic order would be possible.

Based on this idea, there is a processing method in which a mixed flow of air and fine particles is accelerated by a rotating disk and collides with the workpiece, or a process in which fine powder is suspended in an alkaline liquid and approaches the surface of the workpiece. A machining method using a rotating ball that utilizes the bearing condition 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 polishing method with improved processing accuracy.

In liquid honing, a machining fluid containing grains suspended in a liquid is injected onto the surface of the workpiece at an angle of inclination of 40 to 50 degrees using the force of compressed air. When machining a workpiece surface by applying a liquid to it, in order to avoid dislocation defects in the workpiece material and cause it to break, the particles should act at an angle as close to parallel to the workpiece surface as possible. is desirable.

In order to improve machining efficiency, it is of course necessary to impart as much kinetic energy as possible to the fine powder and grains so that they 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. It will be done.

However, when using a fluid flow as a means for accelerating powder and grains, the smaller the processing unit (and grains), the more special consideration must be given to the flow state.

That is, when the flow path of the fluid is determined as described above,
As the fluid velocity increases, the flow state changes from laminar to turbulent.

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, in order to cause the particles to cross the workpiece surface as parallel as possible (at an angle as close as possible), it is desirable that the flow state of the fluid be laminar. This conflicts with the proposition of imparting as much kinetic energy as possible, and ultimately improving machining efficiency.

A fluid (770 working fluid) in which fine powder and particles are suspended is flowed in a laminar flow state between the workpiece processing surface and an opposing surface that is parallel to each other at a narrow interval, and the suspension and particles create a surface finish. or the method of carrying out the polishing is, therefore, unless by some other means imparting large kinetic energy to the grains themselves without creating turbulence, or by opening the grains (
It is easy to understand that in this case, there is no industrially advantageous precision surface polishing method from the point of view of processing efficiency.

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 toward the anode, and this phenomenon is called electrophoresis, and the migration speed υ of the particles at this time is expressed by the formula υ−εc
It is well known that it is given by E/μ (where ε and μ are the dielectric constant and viscosity of the solvent; C is the zeta potential of the particles; and E is the magnitude of the electric field).

By the way, for example, SIO2, 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 S io2 is very high and the basicity of water is also high.

Even in ethyl alcohol, a typical example of a non-aqueous solvent,
Although the basicity of alcohol is lower than that of water, since the acidity of 5102 is high, it is also negatively charged.

In addition, in the case of Al2O3, 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, but from the point of view of dielectric constant (when the dielectric constants of colloidal particles and solvent are different, the dielectric constant The dielectric constant of alcohol is much higher than that of Al2O3 (a large force causes a positive charge), and an actual test shows that Al2O3 is negatively charged in alcohol.

When examining the charged state of particles in this way, it was found that particles are generally negatively charged in water, non-aqueous liquids, and liquids called DIM.

Therefore, the fine powder and particles suspended in the fluid have the property of being electrophoretically attracted to and moving toward the anode.

In addition, it is elementary knowledge in physics that when parallel plate condensers store electric charge, an electric field is generated in the space between the plates, and that the electric field is induced by changes in the magnetic field.

Therefore, when there is a facing surface with a slight gap between it and the machined surface, if an electric field or magnetic field is applied there through the working fluid in which particles are suspended, the machined surface that becomes the anode side, and the facing surface An electrophoresis phenomenon occurs in which grains are attracted to one side of the space, and the density of grains increases on either side of the space, which in turn causes the number of grains to increase as it acts on the surface, leading to improved processing efficiency. It turns out.

Thus, the present inventors have developed a system in which a machining liquid in which fine powder and particles are suspended in a fluid is passed between a machining surface and an opposing surface disposed very close to and parallel to the machining surface in a laminar flow state. There is a technique for finishing or polishing the surface of a workpiece by using a method of finishing or polishing the surface of a workpiece, and an electric double layer exists on the surface of particles dispersed in a liquid, and when an electric or magnetic field is applied to the colloidal particle system, the particles have a surface charge (
This utilizes the phenomenon of electrophoresis due to the force exerted by the field, and combines this with a technology that concentrates the grains on the processed surface or the opposing surface and controls the number of grains when acting on the processed surface to improve the efficiency of the grains. This led to the completion of the present invention.

That is, according to the present invention, a workpiece placed stationary on a polisher in a fluid suspension of fine abrasive particles is floated by the dynamic pressure of the fluid, and the workpiece is placed on the polisher surface and the force ['' A laminar flow of the fluid suspension is formed between the workpiece and the workpiece, and at the same time (this laminar flow is passed under the influence of an electric field or a magnetic field, particles in the fluid are electrophoresed on the workpiece surface or polisher surface side f). An ultra-precision surface polishing method is provided, which is characterized by controlling the number of abrasive fine particles that exert this attraction force on the surface of an object.

In order to create a laminar flow state of 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 particles). A method is used in which a workpiece placed stationary on a polisher is floated by the dynamic pressure of the fluid by rotation of the polisher in a processing liquid, and a laminar flow of a fluid suspension is formed between the polisher and the workpiece. It will be done.

It has been known for a long time that if a stationary flat support plate of an object is rapidly moved in the horizontal direction in a fluid, the object will rise above the flat support plate due to the dynamic pressure of the fluid. No attempt has been made to use it for polishing the surface of objects.

This means that even if an extremely narrow gap is created between the floated workpiece and the support polisher surface, and a layer of machining fluid is created between them, the amount of energy required to move the support polisher surface alone is insufficient. It is thought that the kinetic energy of the particles is insufficient and a sufficient polishing effect cannot be obtained.

Therefore, the present inventors applied an electric field or a magnetic field to the laminar liquid between this processed surface and the opposing polisher surface, and electrophoretically attracted the particles in the fluid to the workpiece surface or the opposing polisher surface. The intention was to increase the number of particles acting on the material, thereby increasing processing efficiency.

Dispersed particles in a liquid generally have an electric charge or a particle radius,
It is well known that the dielectric constant of a solvent is proportional to the zeta potential, and that when a charged particle exists in a uniform electric field, an electrostatic force that is directly proportional to the strength of the electric field and the electric charge acts on the particle. It is that you are.

If the machined surface and the opposing surface are held parallel with a certain gap, and the particles are negatively charged, they will be moved between them parallel to the machined surface, 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 of E=V/h is generated between the machined surface and the opposing surface, so an electrostatic force acts on the grain in the direction of the machined surface, causing υ.

yields a velocity of motion. In addition, gravity acts on the grains, and
Since a motion velocity of υ is also generated, in the direction perpendicular to the machined surface, this υ.

The velocity of motion υ is given as the resultant force of υ8 and υ8.

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 υ is also given, and the relative velocity of the grain with respect to the machined surface is υ
It turns out that is expressed by.

In other words, when a machining fluid containing particles is passed in a laminar flow state between the machined surface and the opposing surface held in parallel, and a DC current is applied between the machined surface and the opposing surface to create an electric field, υ is It is constant when the laminar flow state is maintained, and υ is constant depending on the machining fluid, so for grains, υ.

It is clear that a relative velocity with respect to the machined surface is given that is proportional to .

The stronger the electric field, the more the grain is electrostatically drawn toward the machined surface, i.e., the grain's υ.

Since the motion speed is constant, and the distance between the machined surface and the opposing surface is constant and the electric field is proportional to the applied voltage,
It is easily understood that when electrophoresis is performed by controlling the applied voltage, the relative velocity of the grains to the machined surface is uncontrolled.

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 is understood that the amount of processing can also be controlled by controlling the strength of the electric field, that is, the voltage.

Next, let us consider the case where the grains are concentrated on the opposing surface, that is, when the DC voltage V is applied at a distance such that the opposing surface has a positive electrode and the processed surface or other electrode has a negative electrode.

In this case, E=
An electric field of V/h is generated, which causes the grains to be pulled toward the opposing 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 when the electric field becomes stronger and concentrates on the opposing surface, the number of particles increases. It can be seen that as the force increases, the grain density increases as it acts on the machined surface, and that the bonding force between grains and the opposing surface is influenced by this electrostatic force and gravity.
These bonds can be easily broken if the carcass grains come into contact with the machined surface and receive force from it, and the energy given by the carcass grains to the machined surface is proportional to this bond force (considering that it is proportional to the electric field) It is easy to understand that by increasing the voltage and increasing the electrostatic force, the energy of the grains increases when applied to the machined surface, and in the end, controlling the strength of the electric field can control the amount of processing or the surface treatment effect. It is something that connects.

The workpiece placed stationary on the polisher is held parallel to the polisher surface at a narrow distance at a position where its gravity and the buoyant force due to the fluid dynamic pressure caused by the rotation of the polisher are balanced. The polisher and the polisher can be easily charged positively or negatively, for example, by using a suitable brush or by providing a separate electrode in the fluid suspension and applying a DC voltage between the electrode and the polisher. It is also extremely easy to place a coil electrode outside the container and apply the laminar magnetic field to the entire fluid suspension under a strong magnetic field.

As mentioned above, in general, in a fluid suspension (processing fluid) of grains, the grains are negatively charged, regardless of whether the fluid is aqueous or non-aqueous.

Therefore, by applying an electric or magnetic field, the particles tend to be electrophoretically bent toward the anode and concentrated densely on the anode surface.

However, when water is used as the liquid on the cutting edge, consideration must be given to the fact that the ionized dust on the surface of the grains becomes ionized, becomes an electrolyte solution, and is adsorbed to the anode surface by an electrode reaction. .

Therefore, below (in this case, cases where the solvent of the processing fluid is aqueous and non-aqueous) will be considered separately.

I) When using a non-aqueous solvent In this case, there is no need to consider the ionization and ionization of the grains, so even if either the cutting surface or the polisher surface serves as an anode, the fine grains on the anode side are electrically charged. When the particles are electrophoretically attracted to each other on the surface, the density of the particles increases, and when the particles eventually act on the surface to be processed, the number of particles also decreases, and an improvement in processing efficiency is expected.

To apply an electric field or a magnetic field to the laminar flow, the cutting surface and the polisher surface in the machining fluid can be directly connected to a DC power terminal via a brush, the machining surface and the polisher surface can be placed under the influence of a strong magnetic field, or the polisher surface can be directly connected to the DC power terminal via a brush. A method in which an electrode is placed in the machining liquid facing the surface, and the electrode and polisher surface are connected to a DC power terminal, and a laminar flow affected by the electric field is passed through the space between the cutting surface and the polisher surface. It is possible to carry out the method of the invention in various ways.

Therefore, the following experiment was conducted using the apparatus shown in Figure 1 of the attached figure, in which the simplest electrode and polisher surface were directly connected to the terminal of a DC power source.

In the apparatus shown in Fig. 1, 1 is a double cylindrical container made of a non-conductive material for containing the processing liquid; 2 is a metal polisher rotated within the container 1 by a rotating shaft 3; 4 is a processing liquid. Object holder; 5 is a workpiece held in the workpiece holder and is to be placed stationary on a metal polisher at the beginning of operation; 6 is an electrode; 7 is a brush; 8 is a DC power source; 9 is a voltmeter; 10 is a The ammeter, 11, represents the force u superfluid (fluid suspension of abrasive particles).

5 volumetric A1203 (8 μm) in container 1 of this experimental equipment
Add the alcohol solution as a processing liquid and use the brass polisher 2.
Above, 20X held in acrylic workpiece holder 4
A Si single crystal wafer between 20×0.8 was left still as the workpiece 5, and the rotating shaft 3 was rotated at 300 rl) II (94 m/min) to levitate the blade edge material 5 together with the holder 4 by fluid dynamic pressure. A laminar flow state of the machining liquid is created between the cutting edge material and the polisher surface, and the polisher side is used as an anode, and the electrode 6
An electric field is applied between the polisher surface and the electrode by applying a DC voltage of 0 to 500 V between the polisher surface and the electrode to cause electrophoresis in the grains of the laminar flow, and the processing effect at that time is expressed as the processing amount per hour. Figure 2 shows the results of the investigation in relation to the applied voltage.

As you can see from the figure, if you increase the voltage and strengthen the electric field,
The grains in the machining fluid concentrate on the opposing polisher surface, and when they act on the machining surface, the effect of increasing the grain density and increasing the amount of machining is noticeable.

In the apparatus shown in Fig. 1, a vertically movable brush is used in place of the electrode 6, and a DC voltage is applied between the workpiece and the polisher by directly applying voltage to the workpiece with or without a conductive workpiece holder. If you use the method of applying voltage, if the workpiece is positively charged and negatively charged by electrophoresis, the grains can be attracted to the workpiece surface, increasing the kinetic energy of the grains or acting on the workpiece surface. It is then very easy to understand that by increasing the number of grains, surface scratches or polishing can be controlled.

2) When water is used as a processing liquid solvent In this case, the degree of ionization on the surface of the amber grains is too high, and if a voltage is applied directly to the workpiece, an electrode reaction will occur on the surface of the amber grain and the ionized grains will is electrically neutralized on the surface and adsorbed thereon, eventually interfering with processing.

Therefore, in this case, a device such as that shown in FIG. 1, in which an electrode is placed in addition to the workpiece, should be selectively used.

After a laminar flow is created between the upper surface of the cutting edge and the opposing polisher surface, it is easy to cause the particles in this laminar flow to undergo an electrophoretic phenomenon and attract the particles to the processed surface or the opposing polisher surface. The present invention is not limited to placing the laminar flow under an electric field as described above, but the same effect can also be achieved by applying a magnetic field.

That is, the present invention can also be practiced with an apparatus such as that shown in FIG.

In the illustrated apparatus, 21 is a magnetic pole; 22 is a workpiece holder; 23 is a workpiece; 24 is a processing fluid; 25 is a polisher; 2
6 is a rotating shaft; 27 is a coil; 28 is a container.

A double cylinder is arranged around the rotating shaft 26, a coil 27 is wound around the inner cylinder, and a current is passed through the outer surface of the inner cylinder and the inner surface of the outer cylinder as magnetic poles 21. A magnetic field is created and the polisher 25 rotates to levitate the workpiece 23 together with the holder 22 from the polisher surface,
This system causes electrophoresis in the grains in the laminar flow of machining liquid that is formed between the surface of the workpiece and the polisher surface, and controls the movement of the grains under the influence of a magnetic field.

Using the apparatus shown in Figure 3, a solution containing A1203 (2 μm) and grains at volume fractions of 5 and 5r02 (0.02 μm) mixed in 2 volume distilled water was used to process Si single crystals (001) and A1203 (2 μm).
Using a sample wrapped with It 203 & 2000 grains, rotate the polisher at 300 rpl',
A laminar flow is formed between the sample processing surface and the polisher surface, and at the same time
~1. When we examined the influence of the magnetic field through the coil current of the OA, we found that there was no significant increase in the amount of machining as with the electric field. The machined surface was treated for 1 hour using only laminar flow.
As shown in the figure, the processed surface was treated with a current of 0.5 A through a coil for 1 hour, and the processed surface was treated with a current of 0.5 A through a coil for 1 hour. As a result of the added motion, a force U effect proportional to the strength of the magnetic field was observed, resulting in markedly directional blade marks.

In applying electric and magnetic fields, we have only explained the trick of applying a DC voltage above for the sake of simplicity, but it is possible to perform electrophoresis by using alternating current instead of direct current and subjecting the laminar flow to the influence of an electric or magnetic field whose direction changes. It should be understood that controlling the movement of the grains is also encompassed by the present invention.

[Brief explanation of drawings]

Fig. 1 is a vertical cross-sectional view of the apparatus used to carry out the method of the present invention, and Fig. 2 is a 5i (ooi) mechanically processed sample using the apparatus shown in Fig. 1. A diagram showing the relationship between applied voltage and processing amount per hour in the case of processing, FIG. 3 is a longitudinal cross-sectional view of the apparatus used when carrying out the method of the present invention in yet another embodiment, and FIGS. FIG. 4 is a diagram showing processing marks when a Si wafer is processed using the apparatus shown in FIG. 3; 1...Container, 2...Polisher, 3...
...Polisher rotating shaft, 4... Workpiece holder, 5... Pressure object, 6... Electrode, 7...
...Brush, 8...Power supply, 11...
- Processing fluid (fluid suspension of abrasive particles).

Claims (1)

[Claims] 1. In a fluid suspension of fine abrasive particles, a workpiece placed on a polisher is floated by the dynamic pressure of the fluid by rotation of the polisher, and the suspension is placed between the polisher surface and the workpiece. Forming a laminar flow, and at the same time subjecting the laminar flow to the influence of an electric or magnetic field, particles within the fluid are electrophoretically attracted to the workpiece surface or polisher surface, thereby controlling the number of abrasive fine particles acting on the workpiece surface. Characteristic surface polishing method.