JP2000053450A - Method for manufacturing substrate for information recording medium and substrate for information recording medium - Google Patents

Abstract

(57) [Summary] [Problem] To have a smooth surface (or a smooth surface in which the protrusions are reduced) without protrusions due to residual abrasive or the like on the surface,
To provide an information recording medium substrate and a method for manufacturing the same, which can reduce the flying height of a head. SOLUTION: In a method for producing a substrate for an information recording medium by subjecting a disc-shaped substrate to a final polishing process using a polishing liquid and a polishing pad, the polishing agent concentration of the polishing liquid is gradually or stepwise reduced. A method for producing a substrate for an information recording medium, wherein the final polishing is performed with the polishing agent concentration being substantially zero.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a substrate for an information recording medium (for example, a magnetic disk, an optical disk, a magneto-optical disk, etc.) and a substrate for an information recording medium.

[0002]

2. Description of the Related Art There are two types of materials for substrates currently in practical use as substrates for magnetic disks, which are examples of substrates for information recording media, such as aluminum and glass. Further, the material of the glass substrate includes amorphous glass and crystallized glass. The manufacturing process of a hard disk (an example of an information recording medium) using an aluminum substrate includes, for example, the following five processes (aluminum rolling plate process, blanking process, substrate process, Ni-P plating process, media process). Is done.

[0003] First, in an aluminum rolled plate process, an aluminum raw material is melted and cast, which is hot rolled to a plate thickness of about 5 mm, and then cold rolled to finish it to a predetermined thickness. The rolled aluminum plate is coiled and transferred to a blanking process. In the blanking process, an aluminum disk made by punching an aluminum rolled plate to a predetermined inner and outer diameter is sandwiched between iron or aluminum spacers, and annealed while applying pressure to eliminate warpage, undulation and distortion of the plate I do. Then, a blank is manufactured by adjusting the dimensions of the inner and outer diameters and adjusting the shape of the inner and outer circumferences.

[0004] In the substrate process, after performing rough grinding and finish grinding on both surfaces of the blank using a sponge grindstone, an annealing process is performed to remove distortion caused by the grinding. Then, cleaning is performed to remove abrasive grains and dirt attached to the blank surface to obtain a substrate. In the Ni-P plating process, as pretreatment, degreasing (removal of oils and fats on the substrate surface with a non-erosive degreasing agent), etching (removal of a non-uniform natural protective film), and zinc substitution treatment (substitution using substitution reaction) Formation of a zinc film on the straight surface).

[0005] Next, the substrate is immersed in a plating solution, and a Ni-P plating layer is deposited on the surface of the substrate by a reduction reaction. Here, deposition is performed until the thickness of the plating layer becomes about 12 μm. Then, the disk on which the Ni-P plating layer is formed is sandwiched between polyurethane polishing cloths, and while the alumina-based abrasive is injected, the surface of the plating layer is 1-2
Finish polishing with a polishing thickness of about μm.

The polishing of the magnetic disk substrate is performed by using, for example, a double-side polishing machine (polishing apparatus) having a schematic configuration shown in FIG. After the finish polishing, the substrate is washed to remove the abrasive remaining on the surface of the plating layer, thereby obtaining a magnetic disk substrate made of aluminum.

Finally, a hard disk is completed by forming a magnetic recording film and the like on the magnetic disk substrate in the media process. On the other hand, a hard disk (an example of an information recording medium) using a glass substrate is manufactured by, for example, the following steps. First, a blank called a blank is rounded to the desired outer diameter, then a hole for the inner diameter is made,
Lapping (roughing, primary lapping) of the blank surface using fine powder of silicon carbide.

After the inner and outer diameters are processed to predetermined dimensions and shapes, the blank surface is wrapped again (secondary lapping). In addition, abrasives and stains attached to the blank are removed by ultrasonic cleaning. Next, the blank surface after the second lapping is polished on both sides with an abrasive (cerium oxide) to a predetermined flatness (3 to 5 μm).
After finishing, cleaning is performed to remove abrasives and the like remaining on the surface.

The polishing is performed using, for example, a double-side polishing machine (polishing apparatus) having a schematic configuration shown in FIG. When the substrate is an amorphous glass containing sodium or the like, cleaning is performed after chemical strengthening by low-temperature ion exchange treatment. Next, a glass magnetic disk substrate (substrate) can be obtained by inspecting for surface defects.

Finally, a hard disk using a glass substrate is completed by forming a magnetic recording film and the like on the magnetic disk substrate (media process).

[0011]

On the surface of the magnetic disk substrate manufactured by the conventional manufacturing method, the abrasive used in the final polishing step remains as projections without being completely removed. In the magnetic disk substrate made of glass, in addition to the presence of the protrusions caused by the remaining abrasive used in the final polishing step, there are also protrusions on the substrate surface generated in the low-temperature ion exchange treatment step.

If the protrusions remain on the substrate surface when the magnetic recording film is formed, there is a problem that a high quality magnetic recording film cannot be obtained. Also, considering that the flying height of the head corresponding to the surface recording density of 10 Gbit / in2 is about 20 nm, the remaining of the protrusions on the substrate surface significantly impairs the smoothness of the substrate. There is a problem because the possibility of contact with the substrate is increased.

The present invention has been made in view of such a problem, and has a smooth surface (or a smooth surface in which the protrusions are reduced) without protrusions due to the remaining abrasive or the like on the surface.
An object of the present invention is to provide an information recording medium substrate and a method for manufacturing the same, which enable a low flying height of a head.

[0014]

Therefore, the present invention firstly provides a method for producing a substrate for an information recording medium by subjecting a disc-shaped substrate to a finish polishing process using a polishing liquid and a polishing pad. A method for producing a substrate for an information recording medium, comprising: performing final polishing by gradually or stepwise decreasing the abrasive concentration of a polishing liquid, and finally performing final polishing with the abrasive concentration being substantially zero. (Claim 1) "is provided.

The present invention is also directed to a method for manufacturing a substrate for an information recording medium by subjecting a disc-shaped substrate to a finish polishing process using a polishing liquid and a polishing pad. A polishing step before polishing in which the polishing agent concentration (or the fluctuation range of the polishing agent concentration) of the polishing liquid is substantially constant, and the polishing agent concentration of the polishing liquid is reduced, and finally the polishing agent concentration is substantially reduced. A method of manufacturing a substrate for an information recording medium, characterized in that the method is performed by a polishing step at a later stage of performing polishing with zero.

Further, the present invention has a third feature that "finally polishing is performed by interrupting the supply of the polishing liquid containing the polishing agent and supplying only water as the polishing liquid." 3. The method according to claim 1 or 2 (claim 3). Further, the present invention fourthly provides "a low-temperature ion exchange treatment is performed on the disk-shaped substrate before or after the finish polishing is performed.
(Claim 4). "

[0017] Fifthly, the present invention provides a method according to claim 4, characterized in that, when a low-temperature ion exchange treatment is performed after the finish polishing, the finish polishing is further performed again. )"I will provide a. The present invention sixthly provides "the manufacturing method according to claims 1 to 5, wherein the polishing pad is made of artificial leather suede."

Further, the present invention provides, in a seventh aspect, a substrate for an information recording medium manufactured by the method according to claims 1 to 6,
A substrate for an information recording medium having a surface roughness of R max 100 angstroms or less and Ra 5 angstroms or less measured by an atomic force microscope in a measurement area of 10 μm square (Claim 7) "
I will provide a.

[0019]

As described above, the abrasive used in the final polishing step remains as projections on the surface of the information recording medium substrate manufactured by the conventional manufacturing method without being completely removed. I have. Most of the abrasive attached to the substrate surface in the polishing step can be removed by washing the substrate.

However, since the abrasive (projections) which cannot be completely removed even by washing the substrate remains on the surface of the substrate, and the size of the remaining abrasive is minute, it is difficult to find by visual observation of the substrate. Have difficulty. Further, in the glass information recording medium substrate, in addition to the presence of the protrusions due to the remaining abrasive used in the final polishing step,
As described above, there are also protrusions on the substrate surface generated in the low-temperature ion exchange processing step.

In general, the substrate is cleaned even after the low-temperature ion exchange process, but it is extremely difficult to remove all the projections on the substrate surface generated by the low-temperature ion exchange process by cleaning. If the projections remain on the substrate surface when the magnetic recording film is formed, a high quality magnetic recording film cannot be obtained, and the smoothness of the substrate will be significantly impaired, and the possibility of contact between the head and the substrate will be reduced. The problem of increasing the size arises.

Then, as a result of intensive studies by the present inventors,
The present inventors perform a final polishing by gradually or stepwise reducing the polishing agent concentration of the polishing liquid when performing the final polishing with the polishing liquid and the polishing pad on the disk-shaped substrate. A polishing step in which a final polishing is performed with the abrasive concentration being substantially zero (claim 1), or a polishing step in which polishing is performed with the abrasive concentration of the polishing liquid (or the fluctuation range of the abrasive concentration) substantially constant; If the final polishing is performed by lowering the polishing agent concentration of the polishing liquid and finally polishing the substrate at a polishing agent concentration of substantially zero (claim 2), the polishing agent on the substrate surface can be removed. It has been found that it can be removed to such an extent that the problem does not occur (claims 1 to 6).

As a method of finally performing the final polishing with the abrasive concentration being substantially zero, for example, the supply of the polishing liquid containing the abrasive is cut off, and the final polishing is performed by supplying only water as the polishing liquid. A method can be mentioned (claim 3). When the supply of the polishing liquid containing the polishing agent is interrupted and only water is supplied as the polishing liquid, the water is not circulated in the polishing apparatus (double-sided polishing machine), but is supplied in a flowing manner. The polishing agent concentration of the polishing liquid supplied between the pad and the workpiece (disc-shaped substrate) necessarily decreases gradually.

If the time for supplying only water as the polishing liquid is increased, the workpiece (disc-shaped substrate) is physically rubbed with the polishing pad in the presence of water.
The abrasive on the substrate surface can be removed to such an extent that the above-mentioned problem does not occur. Although the cause of such an abrasive removal effect is not clear, when water is supplied to the substrate, the formation of a hydrated film on the substrate surface is promoted, and the polishing pad physically acts on this hydrated film, and water is supplied. It is expected that small foreign matters protruding from the substrate surface in the sum film and the hydrate film are removed.

The PH value of the water used for the final polishing of the substrate according to the present invention is preferably 4 to 10. As the water, pure water, ion-exchanged water, tap water and the like can be used, but a buffer solution may be used in combination. Further, an aqueous solution in which an acid, an alkali, and a salt are dissolved may be used. Thus, according to the finish polishing according to the present invention, while minimizing the physical force acting directly on the substrate as much as possible, the removal of the soft hydrated film generated on the substrate surface and the removal of the protruding minute foreign matter are promoted, Indicates the surface roughness of the substrate without damaging the substrate
The R max value can be favorably (reduced).

According to the present invention (claims 1 to 6), the surface has a smooth surface having no large protrusions due to the remaining abrasive or the like (or a smooth surface having reduced protrusions),
An information recording medium substrate capable of reducing the flying height of the head is obtained. The finish polishing of the substrate according to the present invention is effective not only for the information recording medium substrate after the low-temperature ion exchange treatment, but also for the information recording medium substrate before the ion exchange treatment. Claims 4 and 5).

Further, the finish polishing of the substrate according to the present invention is also effective for a substrate for an information recording medium which is not subjected to an ion exchange treatment. As the polishing pad used for the final polishing of the substrate according to the present invention, a polyurethane pad or the like can be used, but it is preferable that the polishing pad is made of artificial leather suede. The reason is not clear, but with such a configuration, without damaging the substrate,
The effect of favorably (reducing) the Rmax value indicating the surface roughness of the substrate is particularly large.

The purpose of the finish polishing according to the present invention is to remove at least protrusions on the surface of the substrate, and the above purpose can be achieved even if the thickness removed by the finish polishing is about 100 angstroms or less per one surface of the substrate. Can be. The measurement of the surface accuracy of the information recording medium substrate obtained by the manufacturing method according to the present invention (claims 1 to 6) can be performed using an atomic force microscope.

That is, if a 10 μm square region on the substrate surface is scanned using an atomic force microscope equipped with a probe having a radius of curvature smaller than several tens of nanometers, minute surface irregularities and projections can be obtained. Can be measured. The radius of curvature of the stylus in the stylus type surface roughness measurement is several hundred nm or more, and is not suitable for measurement of minute surface irregularities, protrusions, etc., and the obtained measurement value includes a considerable error. It is not preferable.

According to the manufacturing method according to the present invention (claims 1 to 6), for example, the surface is 10 μm
A substrate for an information recording medium (claim 7) having a main surface having a surface roughness of R max 100 angstroms or less and Ra 5 angstroms or less measured in the measurement area of □ is obtained. When the substrate having a smooth surface according to the present invention is used as a substrate for an information recording medium, and the recording medium is formed on the substrate, the flying height of the head can be suppressed to a low level. be able to.

That is, according to the present invention, the head (for example,
The flying height of the magnetic head can be reduced, and the density of the information recording medium (for example, a magnetic disk) can be increased (particularly, the linear recording density can be increased). In addition, it is possible to realize a high-density information recording medium (for example, a magnetic disk), and to reduce the number of disks mounted on a disk device (a hard disk drive device, for example, a magnetic disk drive device) and to reduce the size of the disk diameter. In addition, the device can be downsized.

In particular, if the performance of the hard disk drive is improved, the size is reduced, and the space is saved, the performance of a personal computer or the like on which the hard disk drive is mounted can be improved. Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method for manufacturing a substrate for an information recording medium according to the following embodiment is a method for manufacturing a substrate for an information recording medium by subjecting a disk-shaped substrate to finish polishing. A polishing step in which a polishing step is performed before polishing by making the polishing agent concentration (or the fluctuation range of the polishing agent concentration) of the polishing liquid substantially constant when performing finish polishing with a polishing liquid and a polishing pad;
Finish polishing is performed by lowering the polishing agent concentration of the polishing liquid, and finally by setting the polishing agent concentration to approximately zero and performing a polishing process at a later stage in which polishing is performed.

Then, as a method of finally performing the final polishing with the abrasive concentration being substantially zero, the supply of the polishing liquid containing the abrasive is interrupted, and the final polishing is performed by supplying only water as the polishing liquid. The method is adopted. In addition, when the supply of the polishing liquid containing the polishing agent is shut off and only water is supplied as the polishing liquid, the water is supplied in a flowing manner without being circulated in the polishing apparatus (double-side polishing machine). Therefore, the polishing agent concentration of the polishing liquid supplied between the polishing pad of the polishing apparatus and the workpiece (disc-shaped substrate) necessarily decreases gradually, and the time for supplying only water as the polishing liquid is reduced. If you make it longer,
The workpiece (disk-shaped substrate) is physically rubbed with a polishing pad in the presence of water, and as a result, the abrasive on the substrate surface can be sufficiently removed. <Example 1> The substrate for an information recording medium of the present example had a surface roughness of Rmax 56 angstroms and Ra3.9 angstroms measured in an area of 10 μm square by an atomic force microscope, and the surface was polished. It has a smooth surface without large projections due to the residual agent.

The manufacturing process will be described below. First, an aluminum magnetic disk substrate (disc-shaped) on which a Ni-P plating layer is formed is prepared and polished (by setting the polishing agent concentration of the polishing liquid or the fluctuation range of the polishing agent concentration to be approximately constant). (Final polishing before polishing). That is, a polishing pad made of polyurethane or artificial leather suede is attached to the upper and lower platens made of cast iron of a double-sided polishing machine (polishing device), and between the upper and lower polishing platens to which the polishing pad is fixed. And bringing the magnetic disk substrate into close contact with the substrate,
The polishing liquid was supplied to the pad and the substrate polishing surface in a circulating manner, and the substrate was rotated and slid to simultaneously polish both surfaces (plating layer) of the substrate.

Here, as the polishing liquid, a solution obtained by dissolving alumina with water to a concentration of 5% by weight was used, and as the polishing pad, a polishing pad having grooves of 5 mm width at intervals of 10 mm was used. Polishing conditions are: polishing pressure: 30-100g / c
m ² , lower platen rotation speed: 5-40 rpm, upper platen rotation speed: 5-40 rp
m, polishing time: 8 to 15 minutes.

Next, the polishing step is performed by lowering the polishing agent concentration of the polishing liquid on both surfaces of the substrate which has been subjected to the polishing processing in the preceding step, and finally making the polishing agent concentration substantially zero. Finish polishing was further applied. That is, the supply of the polishing liquid containing the polishing agent is interrupted, and only water is supplied as the polishing liquid in a flowing manner, so that the final polishing at the later stage is performed on both surfaces of the substrate subjected to the final polishing at the preceding stage. Further processing was applied.

Polishing conditions are as follows: polishing pressure: 30 to 100 g / c
m ² , lower platen rotation speed: 5-40 rpm, upper platen rotation speed: 5-40 rp
m, polishing time: 2 to 10 minutes. When the main surface of the magnetic disk substrate subjected to each finish polishing was observed in a measurement area of 10 μm square using an atomic force microscope, the surface roughness was Rmax 56 angstroms and Ra3.9 angstroms.

For comparison, in the case where the same aluminum magnetic disk substrate is subjected to the conventional method of performing only the substrate cleaning after the above-mentioned final polishing, the processed magnetic disk Observation of the main surface of the substrate for use in a measurement area of 10 μm square using an atomic force microscope revealed that the surface roughness was Rmax 180 angstroms and Ra8 angstroms.

Further, while protrusions (foreign matter) remained on the main surface of the magnetic disk substrate that had been subjected to the above-described conventional method, the main surface of the substrate that had been subjected to the finish polishing process of the present embodiment. No protrusion (foreign matter) was particularly observed on the surface. Therefore, it was apparent that the finish polishing according to the present example was effective in smoothing the surface of the aluminum magnetic disk substrate. <Example 2> The substrate for an information recording medium of the present example had a surface roughness Rmax of 55.1 angstroms and Ra2.9 angstroms measured on an atomic force microscope in a measurement area of 10 μm square, and the surface was polished. It has a smooth surface without large projections due to the residual agent.

The manufacturing process will be described below. First, a magnetic disk substrate (disc-shaped) made of aluminosilicate glass is prepared and subjected to polishing (before polishing is performed before polishing with the polishing agent concentration of the polishing liquid or the fluctuation range of the polishing agent concentration being substantially constant). Finish polishing). That is, a polishing pad made of artificial leather suede is attached to an upper platen and a lower platen made of cast iron of a double-sided polishing machine (polishing device), and the magnetic pad is fixed between the upper and lower polishing platens to which the polishing pad is fixed. The disc substrate was brought into close contact with the pad and the polishing liquid was supplied to the polishing surface of the substrate in a circulating manner, and the substrate was rotated and slid to simultaneously polish both surfaces of the substrate.

As the polishing liquid, a polishing liquid made of cerium oxide dissolved in water to a concentration of 2% by weight was used, and a polishing pad made of artificial leather suede was prepared by inserting grooves of 5 mm width at intervals of 10 mm. (FIG. 2) was used. Polishing conditions are: polishing pressure: 30-100 g / cm ² , lower platen rotation speed: 5
~ 40rpm, upper platen rotation speed: 5 ~ 40rpm, polishing time: 8 ~ 1
5 minutes.

Next, on the both surfaces of the substrate which has been subjected to the polishing in the preceding stage, the polishing agent concentration of the polishing liquid is reduced, and finally the polishing is performed with the polishing agent concentration being substantially zero. Finish polishing was further applied. That is, the supply of the polishing liquid containing the polishing agent is interrupted, and only water is supplied as the polishing liquid in a flowing manner, so that the final polishing at the later stage is performed on both surfaces of the substrate subjected to the final polishing at the preceding stage. Further processing was applied.

The polishing conditions are as follows: polishing pressure: 30 to 100 g / c
m ² , lower platen rotation speed: 5-40 rpm, upper platen rotation speed: 5-40 rp
m, polishing time: 2 to 10 minutes. When the main surface of the magnetic disk substrate subjected to each finish polishing was observed in a measurement area of 10 μm square using an atomic force microscope, the surface roughness was Rmax 45.6 angstroms and Ra 2.4 angstroms. Also, no protrusion (foreign matter) was particularly observed on the main surface of the substrate.

Next, the magnetic disk substrate that had been subjected to each finish polishing was washed and then immersed in a molten salt of potassium nitrate to perform a low-temperature ion exchange treatment. That is, a compressive stress layer was formed on the substrate surface by ion-exchanging sodium ions in the substrate glass with potassium ions.
After cleaning the substrate subjected to the low-temperature ion exchange treatment, the main surface of the substrate is cleaned to a surface of 10 μm using an atomic force microscope.
When observed in the measurement area of m □, the surface roughness was R max55.1
It was on-question and Ra 2.9 on-question.

From this, it is understood that the value of the surface roughness becomes larger than the value of the surface roughness before the treatment (Rmax 45.6 angstroms, Ra 2.4 angstroms) by performing the low-temperature ion exchange treatment. This suggests that projections on the main surface of the substrate occur even in the low-temperature ion exchange treatment step.

However, the surface roughness of the main surface of the substrate when the low-temperature ion exchange treatment is performed without performing the finish polishing according to the present embodiment on the magnetic disk substrate is as follows:
As a result of observation with an atomic force microscope in a measurement area of 10 μm square on the surface, Rmax was 128 angstroms and Ra 3.8 angstroms, and foreign substances as projections were also observed. Therefore, it was apparent that the finish polishing according to the present example was effective in smoothing the surface of the magnetic disk substrate.

[0048]

As described above, the present invention (Claim 1)
According to 6), the information recording has a smooth surface without large projections due to the remaining abrasive or the like (or a smooth surface in which the projections are reduced), and enables a low flying height of the head. A medium substrate is obtained. According to the production method of the present invention (claims 1 to 6), for example, the surface roughness of the main surface measured in an area of 10 μm square by an atomic force microscope is R
An information recording medium substrate having a maximum of 100 Å or less and a maximum of 5 Å or less can be obtained.

If the substrate having a smooth surface according to the present invention is used as a substrate for an information recording medium and a recording medium is formed on the substrate, the flying height of the head can be suppressed low.
It is possible to cope with high density information recording. That is, according to the present invention, the flying height of the head (for example, a magnetic head) can be reduced, and the density of an information recording medium (for example, a magnetic disk) can be increased (particularly, the linear recording density can be increased).

Further, the density of the information recording medium (for example, a magnetic disk) can be increased, and the disk device (a hard disk drive device, for example, a magnetic disk drive device)
It is possible to reduce the number of disks to be mounted on the disk and to reduce the size of the disk diameter, and to reduce the size of the apparatus. In particular, if the performance of the hard disk drive is improved, the size is reduced, and the space is saved, the performance of a personal computer or the like on which the hard disk drive is mounted can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a double-side polishing machine.

FIG. 2 is a plan view of a polishing pad according to the present invention.

[Explanation of symbols]

 1 ・ ・ Substrate for magnetic disk 2 ・ ・ Abrasives 3 ・ ・ Upper surface plate 4. ・ Lower surface plate

Claims (7)

[Claims] 1. A method of producing a substrate for an information recording medium by subjecting a disc-shaped substrate to a final polishing process using a polishing liquid and a polishing pad, wherein the polishing agent concentration of the polishing liquid is gradually or stepwise. A method for manufacturing a substrate for an information recording medium, comprising: performing final polishing at a reduced concentration; 2. A method for producing a substrate for an information recording medium by subjecting a disc-shaped substrate to a final polishing process using a polishing liquid and a polishing pad, wherein the final polishing process comprises: Or a fluctuation range of the polishing agent concentration) and a polishing step in the previous stage of performing polishing with substantially constant, reducing the polishing agent concentration of the polishing liquid,
A method of manufacturing a substrate for an information recording medium, which is finally carried out by a polishing step at a later stage in which polishing is performed with the abrasive concentration being substantially zero. 3. The method according to claim 1, wherein the supply of the polishing liquid containing the polishing agent is finally stopped, and the final polishing is performed by supplying only water as the polishing liquid. . 4. The method according to claim 1, wherein a low-temperature ion exchange treatment is performed on the disk-shaped substrate before or after the finish polishing. Method. 5. The method according to claim 4, wherein when the low-temperature ion exchange treatment is performed after the finish polishing, the finish polishing is performed again. 6. The method according to claim 1, wherein the polishing pad is made of artificial leather suede. 7. A substrate for an information recording medium manufactured by the method according to claim 1, wherein the main surface has a surface roughness of R m measured in an area of 10 μm square by an atomic force microscope.
An information recording medium substrate with an ax of 100 Å or less and a Ra of 5 Å or less.