JPS6113950B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polishing method for polishing the surface of a workpiece into a mirror surface.

In recent years, materials for electronic components have a disordered crystal structure,
Or, many require complete mirror finishing with no residual stress. Semiconductor materials such as silicon single crystals are subjected to chemical polishing as a final processing, taking advantage of the fact that they are highly pure single crystals, but many materials used industrially are polycrystalline materials. , and is not unrelated to the presence of impurities. If such materials are chemically polished, the anisotropy of the crystals may cause steps to occur at grain boundaries, and impurities may remain or be eluted, making it impossible to produce a mirror surface.

Conventionally, metals such as rolling rolls are mirror-finished because the material surface is removed while being made amorphous during processing, and the crystal structure of the finished surface is completely destroyed. . In such cases, the material has advantages such as increased hardness and improved fatigue fracture strength, and was a suitable processing method for its intended use.

However, in materials for electronic components, it is necessary to generate a mirror surface without changing the properties of the material itself, and conventional mirror polishing methods are incomplete.

The present invention provides a polishing method capable of easily producing a nearly perfect mirror surface for such purposes.

The present invention is particularly effective for materials for electronic components that require a mirror surface without disordered crystal structure or residual stress, that is, a mirror surface without a process-altered layer, as described above. These are high magnetic permeability ferrites such as Mz-Zn ferrite and Ni-Zn ferrite, and metal magnetic materials such as sendust, which are used as head materials.

In the following, using ferrite as an example, a case in which the present processing method is applied to machining the gap surface of a magnetic head will be described in detail.

Ferrite is a metal oxide magnetic material that is widely used as a material for electronic components, but high magnetic permeability ferrites such as Mn-Zn ferrite and Ni-Zn ferrite have excellent high frequency characteristics, so they are used in magnetic heads. It is used as a magnetic core material, and its processing requires extremely high precision. In particular, in recent years, as magnetic recording technology has made remarkable progress and high-density recording and reproduction with a recording wavelength of 1 μm or less has been put into practical use, the requirements have become increasingly severe. What is particularly important in a magnetic head is the mirror finish on the surface facing the gap that constitutes the magnetic gap, but as the recording wavelength becomes shorter, the width of the magnetic gap also becomes narrower, and recently it has become less than 0.3 μm. Furthermore, this mirror surface must also be a perfect mirror surface.

In particular, the magnetic properties of ferrite are sensitive to crystal structure and stress, and if the processing method is incorrect, significant deterioration of the properties will occur.

The gap surface of ferrite for magnetic heads is polished to a mirror surface by lapping or polishing, but what has been commonly used is a lap made of a soft metal such as tin or lead, and polished with diamond having an average grain size of 3 μm or less. This is a method of polishing into particles.
In this case, the surface roughness is less than 0.05 μm, and the magnetic gap is beautifully sharp when viewed macroscopically, but if you look closely at the 0.3 μm gap, you can see that it is an average gap as shown in Figure 1. long
Compared to 0.3 μm, a surface roughness of 0.05 μm is a size that cannot be ignored, and will deteriorate the frequency characteristics of the magnetic head. In addition, in Figure 1,
1 is a non-magnetic spacer for forming a gap;
For example, it is glass made by vapor deposition, sputtering, etc., and 2 and 2' are ferrite cores.

In addition, many reports have been made regarding processing alteration of ferrite due to conventional polishing methods and the resulting deterioration of magnetic properties, and we ourselves have reported
It has been confirmed that there is a processing deterioration depth of approximately 1.5 m.
As an example, Mn-Zn single crystal ferrite (100)
Figure 2 shows the state of the process-affected layer on the surface. The range from the machined surface to a depth of 0.02 to 0.03 μm is an amorphous layer A also called Beilby layer, and it is thought that the ferrite has lost its magnetism in this part. Below that, there is a layer B with finer crystals, reaching a depth of approximately 0.1 μm. Further below that, a strained layer C with crystal defects such as dislocations and slips reaches a depth of 0.3 μm, and below that is a crystal structure layer D that can be regarded as almost a bulk. The magnetism of ferrite also recovers through the crystalline refinement layer B and the strained layer C, but even when it reaches the bulk layer D, it does not return to the original magnetism (for example, magnetic permeability) of ferrite. This is because a large compressive residual stress acts on the process-affected layer, and a tensile stress that balances this acts on the bulk layer, causing elastic strain.

Now, the gap made up of surfaces with such a processing-affected layer is effectively expanding, and the magnetic sharpness of the gap edge has been lost, which affects the high frequency characteristics of the magnetic head. is deteriorating.

In other words, the gap surface of a magnetic head for high-density recording such as a VTR should have surface roughness that is relatively negligible compared to the gap length.
Surface roughness 0.01μm for 0.3μm gap
It is required that there be almost no deterioration due to processing. As a concrete method to achieve this, it is common to use ultrafine abrasive grains such as alumina, but in our experiments, polishing using alumina hardly improves processing deterioration. As another means, removing the alteration caused by processing using chemical or electrochemical methods has been considered, and has been put into practice in some cases. However, in general, crystals have different chemical removal rates depending on their faces, and in the case of polycrystals, large steps occur at grain boundaries, so these methods are not effective for polycrystalline ferrite.
For single-crystal ferrite, chemical etching after polishing, or making the polishing liquid acidic and simultaneously applying polishing with abrasive grains and chemical etching, is effective in removing the damaged layer.
The surface roughness actually deteriorates. Furthermore, regarding Ni-Zn ferrite, the technology for growing single crystals has not yet been developed, so methods that are effective only for single crystals cannot be used.

There are several other problems with chemical processing, the first of which is the problem of sag at the edges. Although it is not noticeable when the amount of removal is small, the shape accuracy deteriorates when so-called deep etching is performed. The second problem is the presence of impurities in ferrite.Compared to other electronic component materials such as Si, ferrite generally has extremely high impurities, and there are many impurities that cannot be removed by chemical etching. For example, with Mn--Zn single crystal ferrite, it is extremely difficult to completely prevent platinum from being mixed into the crucible. Even in polycrystalline ferrite such as hot-pressed ferrite, precipitation of impurities at grain boundaries is unavoidable. Such impurities are generally not removed by a ferrite etchant and remain as protrusions after etching. Needless to say, this protrusion becomes an obstacle in forming a gap of about 0.3 μm, and deteriorates the accuracy of the gap length. The third problem is that when ferrite is polished as the core of a magnetic head, the surface facing the gap often has a composite structure of ferrite and glass, and when the ferrite is etched, the glass part becomes convex. Put it away. Therefore, it is necessary to perform further etching of the glass, and repeating various types of etching increases the possibility of defects occurring.

Next, the processing method of the present invention will be explained.

FIG. 3 shows an example of a polishing apparatus used in the processing method of the present invention. A disc-shaped lap 3 is rotationally driven by a rotating shaft 4, and the outer periphery of the lap 3 is surrounded by a cylindrical outer wall 5 to constitute a liquid container. A polishing liquid 6 is placed here to perform submerged polishing. It's getting old. The ferrite 7 of the workpiece is bonded to a sample holder 8 with heat-softening resin or the like, and the sample holder 8 is connected to a rotating shaft 9 supported by a bearing 12, and the ferrite 7 is connected to a rotating shaft 9 supported by a bearing 12. Rotationally driven. The rotating shaft 9 is not restrained in the axial direction in the bearing 12, and when it is stationary, the ferrite 7 is in contact with the lap 3 due to its own weight, but when the lap 3 and sample holder 8 rotate, the ferrite 7 is in contact with the lap 3 due to the fluid pressure of the polishing liquid 6. As a result, a dynamic pressure fluid bearing condition is created between the lap 3 and the ferrite 7, and the sample holder 8 floats up. Note that, if necessary, pressure may be applied from above the gear 10 using a spring or the like, or a load may be added. The device is constructed with a highly rigid base 14, allowing highly accurate rotational movement.
Although Fig. 3 shows an example of a device in which the sample holder 8 is forcibly driven and performs planetary motion between the sample and the lap 3, there are other ways in which the sample and the lap can be moved even if a correction wheel type surface polishing device is used. A similar effect can be obtained if a fluid bearing state of dynamic pressure is realized during this period. In order to realize this dynamic pressure fluid axis, it is effective to increase the flow rate or increase the viscosity of the polishing liquid 6. Specifically, a direct means of increasing the flow velocity is to increase the rotational speed of the lap 3 or the sample holder 8, but in general, high-speed motion tends to cause deterioration of accuracy in surface polishing equipment. Undesirable. As another means,
It is effective to provide grooves on the surface of the lap 3 and to increase the surface roughness of the lap 3.
One of the characteristics of this processing method is that the lap 3 and ferrite 7 do not come into direct contact during processing.
Unlike normal polishing equipment, the roughness of the lap surface does not affect the finished surface.

The polishing liquid 6 is a suspension of fine MgO particles mixed with distilled water, and preferably has a small average particle size, and those with an average particle size of about 0.1 μm are easily available. Increasing the concentration of MgO improves processing efficiency, but increases the likelihood of surface sagging. Therefore, an appropriate concentration should be selected depending on the purpose of processing, but for polishing the gap surface of a magnetic head, it is best to keep it at 5% by weight or less. Furthermore, in order to stabilize the condition of the hydrodynamic bearing, it is effective to dissolve a thickener such as glycerin in distilled water as a means of increasing the viscosity of the polishing fluid 6. However, increasing the amount of glycerin increases the machining efficiency. tends to decrease.

If solid particles other than MgO fine particles are mixed into the polishing liquid 6, it will be difficult to create a mirror surface, so careful attention must be paid to maintaining the cleanliness of the atmosphere in which the distilled water is controlled.More importantly, the material of the lap 3 Processed and controlled. In polishing methods that use diamond abrasive grains, etc., the abrasive grains used are
The flatness of the lap 3 is managed and maintained, and there is no need to limit the initial processing method for the lap surface. However, since this processing method causes very little wear on the lap 3, it is necessary to improve the flatness at the initial stage. It is necessary to keep it. It is also necessary to prevent impurities from remaining after processing the lap surface. In order to satisfy these conditions, the wrap 3 is made of a soft metal such as tin, aluminum, or brass, and is cut using a lathe.

Next, processing examples of the present invention will be explained. Figure 4 shows the surface roughness of the machined surface of Mn-Zn single crystal ferrite, and Rmax is 10 Å or less. Considering that the lattice constant of ferrite is 8 to 9 Å, it is difficult to expect a mirror surface higher than this. The measuring instrument used was Taylor Hobson Talystep.

FIG. 5 shows the case of Ni--Zn polycrystalline ferrite. In this case, anisotropy in processing speed of crystal planes appears, resulting in a step of 20 to 30 Å at grain boundaries. However, the surface roughness inside the grain is still
It is 10 Å or less. In the case of Mn-Zn polycrystalline ferrite, this grain boundary step was approximately 20 Å.
In order to examine the crystallinity of these machined surfaces, we took an electron diffraction image and found that Kikuchi lines clearly appeared.
It was found that the surface had the same crystallinity as the surface treated with chemical etching. In addition, when we polished the side surface of the μ-ring using this processing method and measured its magnetic permeability, we found that
It was confirmed that the value was almost that of the bulk, and that there was almost no residual stress. That is, with this processing method, no processing deterioration was detected.

In addition, in the Mn-Zn single crystal ferrite used in Figure 4, platinum was precipitated with an average diameter of 2 to 3 μm, but when observed with a differential interference microscope, it was seen as a very slight depression, and the surface roughness was measured. In this case, the width of the stylus tip is 2.5 μm.
Therefore, the dent could not be detected, but it is estimated that 50
The concavity is less than Å, so it is not a problem at all when constructing a gap. Also in the case of the Ni--Zn polycrystalline ferrite shown in FIG. 5, no deterioration of the mirror surface due to impurities precipitated at the grain boundaries was observed.

FIG. 6 is a processed surface of Mn-Zn polycrystalline ferrite filled with glass to form a composite structure. The glass used was lead glass with a Bitkers hardness of 500 Kg/mm ² . At the boundary between glass and ferrite, approximately
Although there is a step difference of 200 Å, the ferrite part is convex, so there is no problem in forming the gap in this case as well. Furthermore, if we pay attention to the edges of the ferrite parts, we can see that they have sharp edges, which shows that this processing method has no sag and has excellent shape accuracy.

FIG. 7 shows the surface roughness of the processed surface of Sendust. In this case, although it is a polycrystalline metal, very few grain boundaries are observed, and even with the inclusion of "undulations" that may be caused by impurities, the surface roughness is very low.
It is 20 Å or less.

In addition, in order to investigate materials that are effective for this processing method,
Processing of various materials was carried out. Regarding metals,
In addition to Sendust, which has already been mentioned, there are few materials that have been tested due to the need for a perfect mirror surface without processing deterioration, but basically the softer the metal, the lower the quality of the machined surface and it is difficult to call it a mirror surface, but stainless steel We confirmed that it is possible to polish steel to a surface roughness of approximately 20 Å.

Regarding ceramics, ferrite to Al ₂ O ₃
Many materials can be processed, and the surface roughness can be
A mirror surface of less than 50 Å was obtained. However, even in the case of ceramics, it was not possible to obtain a highly mirrored surface with low Vickers hardness.

A feature of this processing method is the use of MgO fine particles as an abrasive, but other abrasive particles have their own problems. For example, Al ₂ O ₃ , SiO ₂ ,
For hard particles such as Cr ₂ O ₃ , the surface roughness is 100 Å.
It was impossible to do the following, and it was also impossible to eliminate processing deterioration. In the case of soft particles such as Fe ₂ O ₃ and Fe ₃ O ₄ , it was possible to minimize processing deterioration, but in polycrystalline materials, grain boundary steps occur and it is difficult to obtain a perfect mirror surface. I couldn't do it. Furthermore, in the case of soft particles such as CaCO ₃ and ZnO, although a processing effect is recognized, the processing speed is extremely slow and cannot be used industrially.

That is, as mentioned above, only MgO was capable of producing an unaltered mirror surface, regardless of whether it is polycrystalline or single crystalline. One of the points to be careful about in this machining method is to prevent direct contact between the lap and the workpiece, and if the membrane of the hydrodynamic bearing mentioned above breaks, it will cause large scratches on the machined surface. , the lap will also be damaged. This processing method promises to produce a highly unaltered mirror surface as long as the hydrodynamic bearing is stable.

A video head was manufactured using the Mn-Zn single crystal ferrite explained in Figure 4, and its characteristics were compared with a head made using conventional processing methods. As a result, no difference was observed between the two in the low frequency range, but in the high frequency range. This processing method was clearly superior. i.e. 5MHz
The output at (corresponding to a recording wavelength of 1 μm) was 2 to 3 dB higher in the head produced by this processing method. This is due to the fact that this processing method provides an unaltered mirror surface, but furthermore, the processed surface of this processing method is an extremely clean surface, even when various cleaning methods are used with conventional processing methods. Defects due to dirt that could not be removed did not become a problem in the case of this processing method.

As described above, the present invention is particularly effective when used for processing magnetic materials for magnetic heads, and makes it possible to stably and easily manufacture high-performance magnetic heads.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the details of the gap part of the magnetic head by the conventional processing method, Fig. 2 is a diagram showing the processed damaged layer by the conventional processing method, and Fig. 3 is an example of the processing equipment used in the processing method of the present invention. The side sectional view and FIGS. 4 to 7 are views showing examples of processed surfaces by the processing method of the present invention, respectively. 3... Wrap, 5... Cylindrical outer wall, 6... Polishing liquid, 7... Workpiece, 8... Sample holder, 9... Rotating shaft, 10, 11... Gear, 13... Rotating drive shaft.

Claims (1)

[Claims] 1. In a processing liquid in which MgO fine particles are suspended in water,
The machining surface of the workpiece and the lap are caused to move relative to each other, and a dynamic pressure fluid bearing state is formed between the machining surface and the lap using machining fluid as a working fluid, and the machining is performed while maintaining the machining surface and the lap in a non-contact state. A polishing method characterized in that the machined surface of the workpiece is machined into a mirror surface by colliding MgO fine particles in a liquid with the machined surface.