Method of cleaning glass substrate for magnetic disk or semiconductor substrate

To propose a method of cleaning a surface of a glass substrate fabricated by a process of strengthening a surface thereof by alkaline ion exchange reaction by selectively removing alkaline metal on the surface by cleaning the surface by using an activated ionic water produced by electric polarization and as a result, to provide a glass substrate for a magnetic disk having a magnetic medium with insignificant corrosion and excellent S/N ratio, in cleaning a glass substrate for a magnetic disk using a glass substrate pulled up from a chemically strengthening treatment solution produced by an alkaline ion exchange reaction, after a final polishing step of fabricating the glass substrate for a magnetic disk, the glass substrate is cleaned by an activated anodically electrolyzed water produced by electric polarization to thereby selectively remove the alkaline metal at the vicinity of the surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to fabrication of a glass substrate for a 
magnetic disk used in a hard disk or the like used as a large scale record 
medium of a computer, particularly to a method of cleaning a glass 
substrate for a magnetic disk using a glass substrate pulled up from a 
chemically strengthening treatment solution produced by an alkaline ion 
exchange. 
2. Description of the Related Art 
Although conventionally, a substrate which uses an aluminum alloy and the 
surface of which is plated with nickel and phosphor has widely been used 
in recent years as a substrate for a magnetic disk, the demand for a glass 
substrate has been increasing owing to necessities of small size formation 
of, a hard disk drive, high recording density of a disk, as well as low 
flying height of a magnetic head to achieve the high density, promotion of 
impact resistance and the like in. 
Properties of withstanding centrifugal force caused by rotation of a disk 
drive and impact force caused by collision with a magnetic head and the 
like are required for a glass substrate for a magnetic disk and a glass 
substrate having large mechanical strength different from ordinary glass 
is necessary. 
In order to satisfy the above-described mechanical strength, there has been 
used a glass substrate of crystallized glass having a structure in which 
fine crystals are dispersed and strength is provided by strain caused by 
dispersion, or a glass substrate of an ion strengthening type in which 
after fabricating a glass substrate in a predetermined shape, an alkaline 
metal (for example, K.sup.+) having a large ionic radius is permeated to a 
surface of the glass substrate by thermal diffusion which provides large 
compressive stress caused by a difference in ionic radii at the surface of 
the glass substrate to thereby strengthen the glass substrate. 
Particularly, in recent years with high density recording of memory 
elements on a hard disk accompanied by using an MR head (magnetic 
resistance head) and a GMR head (giant magnetic resistance head), in 
correspondence therewith, it is necessary for the magnetic head to have a 
smooth face to a degree that a flying height the magnetic head above the 
surface of the disk substrate falls in a region of lower than 300 A 
(Angstrom). 
However, the size of crystals of the crystallized glass is in the magnitude 
of a micrometer order and therefore, there has been arisen a problem in 
which the roughness of the surface which originally is smooth becomes 
varied in the order of micrometers due to a difference in hardnesses of an 
amorphous matrix shape and crystals or a difference in chemical properties 
thereof. This gives rise to a problem in which when such a glass substrate 
is used in a magnetic disk, so-to-speak low flying height where a head of 
the MR head or the GMR head or the like is floated up proximate to the 
magnetic disk is difficult to carry out, or in respect of the head, in the 
case of so-to-speak narrow track formation where a memory track is formed 
in a narrow region with high density recording of the magnetic disk, 
so-to-speak modulation (adjustment node) is observed in reproduced output. 
In the case of a magnetic disk using the above-described crystallized 
glass by avoiding such a problem, it is difficult to provide the surface 
of the disk substrate with predetermined smooth face accuracy. 
As a result, at present, use of crystallized glass is shunned in a magnetic 
disk and strengthened glass produced by alkaline ion exchange has widely 
been used. However, according to a hard disk using strengthened glass 
produced by alkaline ion exchange, when a disk is driven in the case where 
the MR head or the GMR head is used, in respect of a limit in recording 
density, in view of medium noise, a magnetic memory layer on the surface 
of the disk substrate is made as thin as below 100 A (Angstrom) and 
further, a protective film formed on the surface is made as thin as about 
50 A (Angstrom) although conventionally, about 150 A (Angstrom) has been 
needed in order to reduce space loss owing to a medium of the head per se. 
Particularly, in respect of the medium noise, factors for determining the 
medium noise are surface smoothness of a glass substrate and moisture 
adsorbed to the glass substrate in a step of sputtering a magnetic film. 
That is, the glass substrate is cleaned immediately before entering a 
medium fabrication step and when an alkaline component is present on the 
surface of the glass substrate, moisture is liable to adsorb and the S/N 
(signal to noise) ratio that is one of medium properties is deteriorated 
by adsorbed moisture. 
Moreover, when an alkaline component is present on the surface of a glass 
substrate, the alkaline component is made to permeate into a magnetic film 
formed on the surface of the glass substrate thereby causing corrosion. 
When particularly, sodium ions are present among several alkaline 
components, in forming a film of a memory medium or after forming the 
film, difficulties are caused resulting in the worst state of 
deterioration of the memory medium layer or the like. 
Therefore, it has conventionally been proposed, in cleaning a glass 
substrate immediately before a medium fabrication step to clean the glass 
substrate by sulfuric acid and phosphoric acid solution immediately after 
chemical treatment of glass to thereby remove an alkaline component on the 
surface (Japanese Unexamined Patent Publication No. JP-A-9-22525) 
Further, although in fabricating a glass substrate having a surface 
roughness of about Ra 10-20 A (Angstrom), it is not so much necessary to 
take into consideration of the above-described presence of alkaline 
component, particularly, presence of sodium ions, in the case of a glass 
substrate having a surface roughness of Ra 5 A (Angstrom) or less that is 
needed in recent years, a final polishing step is necessary to provide a 
smaller surface roughness. However, when surface polishing is carried out 
on the glass substrate at a final step, in respect of the polished 
surface, as a result of polishing, a new surface is exposed and 
accordingly, the concentration of an alkaline component present on the 
surface of the glass substrate is not reduced. Further, when micro cracks 
which are fine cracks of the glass substrate are present, in the polishing 
step, a polishing solution or the like permeates the micro cracks and as a 
result, corrosion of a medium on the surface is caused in a magnetic disk 
composed of the glass substrate and constituting a product. 
Further, even when the surface is cleaned by using normal acid or the like 
to remove the alkaline component on the surface after the final polishing 
step, in this case, although the alkaline component can be removed, the 
surface roughness is deteriorated by a treatment using a solution 
including acid. Further, a new cleaning step is necessary to remove acidic 
component remaining on the surface of the glass substrate. Also, when 
cleaning by acid is carried out, in this case, a component mainly of acid 
remains on the surface, or permeates into micro cracks of the glass 
substrate and these substances remaining on the surface or permeating into 
micro cracks mix into a film of a magnetic recording medium in sputtering 
the magnetic recording medium resulting in deteriorating the SN ratio of 
the medium similar to the above-described case. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to obviate the above-described 
problems in the conventional technology. The inventive method comprises a 
step of carrying out an ion exchange reaction for a predetermined period 
of time in a molten alkaline ion salt bath after polishing a strengthened 
glass substrate of an alkaline ion exchange type to predetermined 
dimensions, a step of removing an alkaline salt adhered to the surface of 
the glass substrate by dissolving it by normal acid, a final polishing 
step of blowing a polishing solution to the surface until the surface 
roughness Ra of the glass substrate is reduced to 5 A (Angstrom) or less, 
a step of dipping the glass substrate for a predetermined period of time 
into an anodically electrolyzed water including hydronium ions at a 
predetermined concentration, a step of thereafter cleaning the glass 
substrate with high purity water and a step of drying the glass substrate 
after cleaning the glass substrate with high purity water. 
That is, according to a first aspect of the present invention, in a method 
of cleaning a glass substrate for a magnetic disk, the feature 
particularly resides in that a glass substrate for a magnetic disk using a 
glass substrate pulled up from a chemically strengthening treatment 
solution produced by alkaline ion exchange is cleaned by an activated 
ionic water produced by electric polarization to thereby remove 
selectively an alkaline metal on the surface of the substrate. 
Further, according to a second aspect of the present invention, in the 
method of cleaning a glass substrate for a magnetic disk in respect of the 
first aspect, the feature resides in that the activated ionic water is an 
anodically electrolyzed water. 
Further, according a third aspect of the present invention, in the method 
of cleaning a glass substrate for a magnetic disk in respect of the first 
aspect, the feature resides in that the activated ionic water is an 
anodically electrolyzed water having a hydrogen ion concentration of pH of 
5-6. 
Further, according to a fourth aspect of the present invention, in a method 
of making a glass substrate for a magnetic disk using a glass substrate 
pulled up from a chemically strengthening treatment solution produced by 
an alkaline ion exchange, the feature resides in that after a polishing 
step at a final fabrication stage of finishing the glass substrate to a 
predetermined surface roughness, the glass substrate is dipped for a 
predetermined period of time into an activated ionic water produced by an 
electric polarization to thereby selectively remove an alkaline component 
on the surface of the glass substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Firstly, pellets are produced by melting oxides having the following 
chemical compositions for glass of an alkaline ion exchange strengthening 
type. 
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SiO.sub.2 62.4 WT % 
Al.sub.2 O.sub.3 
3.0 
B.sub.2 O.sub.3 
1.1 
Na.sub.2 O 9.0 
K.sub.2 O 9.0 
MgO 3.0 
ZnO 12.0 
TiO.sub.2 0.6 
As.sub.2 O.sub.3 
0.2 
Sb.sub.s O.sub.3 
0.3 
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Next, produced pellets are compressed to a predetermined size by a hot 
press to provide a glass material substrate having no bubbles. Further, 
the substrate is fabricated into predetermined dimensions after processing 
it by machining of inner and outer peripheries, rough polishing and fine 
polishing. 
Next, the fabricated glass material substrate is subjected to an ion 
exchange reaction at temperature of 400.degree. C. for 3-5 hours in a 
molten salt including sodium nitrate and the like to thereby form a 
strengthened layer of about 40 micrometers on the surface of the glass 
material substrate. After forming the strengthened layer, the surface of 
the glass material substrate is polished to a surface roughness of Ra of 5 
A (Angstrom) or less by using a polishing agent of colloidal silica or the 
like. After polishing, the substrate is dipped into a solution of 
KMnO.sub.4 and is cleaned by flowing water to remove organic substances on 
the surface of the glass material substrate. 
The glass material substrate is dipped for a predetermined period of time 
into an anodically electrolyzed water (hereinafter, occasionally referred 
to also as "activated ionic water") having various ion concentrations and 
thereafter, the substrate is cleaned by high purity water, spin drying is 
carried out by rotating the glass material substrate per se, thereafter, 
the substrate is subjected to a corrosion resistance test at 80.degree. C. 
and 90% RH (Relative Humidity) for 10 days and the surface roughness of 
glass substrate is measured in respect of a maximum projection amount. 
Table 1 shows a state of treatment of the glass substrate by ionic water. 
TABLE 1 
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Glass substrate treatment by ionic water 
Ion con- Initial 
centration (pH) 
Dipping condition 
Rpoughness Ra 
______________________________________ 
No treatment 
3 18 
1 1 min.temperature 
12 248 
2 2 mins.emperature 
7 33 
3 4 1 min. 
24 
4 2 mins.C. 
3 16 
5 1 min.temperature 
3 17 
6 2 mins.emperature 
3 16 
5 1 min. C. 
5 23 
6 2 mins.C. 
6 94 
9 1 min.temperature 
7 31 
9 2 mins.C. 
5 32 
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In Table 1, notation Ra designates a center line average roughness at 
initial stage which is a value calculated by sampling a portion of a 
length of measurement from a roughness curve in a direction of the center 
line and arithmetically averaging the absolute value of a deviation of the 
center line at the sampled portion from the roughness curve and notation 
Rp designates a center line height which is one of parameters for 
evaluating wear resistance or the like which is a value calculated by 
sampling a portion of a length of measurement from the roughness curve in 
the center line direction indicating an interval between the center line 
of the sampled portion and a straight line passing a highest peak in 
parallel to the center line of the sampled portion. 
As is known from Table 1, even when the substrate is dipped into an 
anodically electrolyzed water having the ion concentration of PH of about 
5-6, the surface roughness of the glass substrate is not deteriorated by 
the treatment. 
Further, in respect of the glass substrate which has been treated 
ionically, a test is carried out at 80.degree. C. and RH 90% for 10 days 
which is a kind of references for evaluating a semiconductor whereby a 
maximum surface projection on the surface of the glass substrate is 
verified. 
Table 2 shows a result of verification. 
TABLE 2 
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Maximum projection amount after corrosion 
resistance test at 80.degree. C. & 90% RH for 10 days 
Ion con- Maximum projection 
centration (pH) 
Dipping condition 
amount A 
______________________________________ 
No treatment 
3400 
1 1 min. temperature 
608 
2 2 mins.temperature 
520 
3 5500.. C. 
4 6400ns. C. 
5 1 min. temperature 
200 
0 2 mins.temperature 
480 
5 2400.. C. 
6 6100ns. C. 
9 1 min. temperature 
3400 
9 3100ns. C. 
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Generally, when the surface of glass substrate of an alkaline ion exchange 
strengthening type is treated in a state of high pH ion concentration, the 
matrix is deteriorated and corrosion of the surface of the glass substrate 
is expedited. It is n from Table 2 that when the surface of the glass 
substrate is treated by an anodically electrolyzed water having a 
pertinent pH concentration, that is, an anodically electrolyzed water 
having a pH concentration (ion concentration) of 5-6 according to the 
embodiment, the glass the glass substrate is not deteriorated, an alkaline 
metal present on the surface of the glass substrate is removed and as a 
result, the corrosion resistance of the glass substrate is rather 
promoted. 
Further, the glass substrate treated by dipping into an anodically 
electrolyzed water having an ion concentration of about pH of 5-6, is 
provided with a low concentration of alkali on the surface of the 
substrate and therefore, the glass substrate is suitable for a high 
density magnetically recording medium having a small amount of adsorbed 
water. Hence, degassing measurement is carried out by using a quadrapole 
mass spectrometer in respect of a glass substrate which has been treated 
as described above and an untreated glass substrate and gas generation 
from the glass substrate is verified. According to the degassing 
measurement, substrates which have been cleaned by high purity water and 
dried by spin drying that is a normal pretreatment before a step of 
sputtering a magnetic disk, are heated in vacuum and are subjected to gas 
analysis by the quadrapole mass spectrometer. The result is shown in FIGS. 
1(a) and 1(b). 
FIGS. 1(a) and 1(b) show states of moisture adsorption present on the 
above-described surfaces by dipping glass substrates for magnetic disks 
into an anodically electrolyzed water which is carried out prior to 
sputtering the substrates. FIG. 1(a) is a graph showing gas generation in 
a case where no treatment by ionic water is carried out whereas FIG. 1(b) 
is a graph showing gas generation in a case where a glass substrate is 
treated by dipping it for 1 minute in an anodically electrolyzed water 
having pH 5. As known from FIGS. 1(a) and 1(b), in the case where no 
treatment by ionic water is carried out, significant gas generation is 
observed at the vicinity of temperature of 450.degree. C. whereas in the 
case where the glass substrate is dipped in anodically electrolyzed water 
having pH 5 for 1 minute, gas generation is considerably restrained. 
As described above, according to the method of cleaning a substrate for a 
magnetic disk of an ion exchange strengthening type using ionic water 
including hydronium ions, the concentration of an alkaline component on 
the surface of the glass substrate can be reduced without deteriorating 
the surface roughness and effectiveness can be expected in preventing 
corrosion of a substrate and promoting the SN ratio of a medium which have 
been the problems of this type of substrate and in preventing corrosion of 
a magnetically recording film. The ion concentration used can pertinently 
be varied in accordance with glass components and conditions of ion 
strengthening and polishing method. That is, according to the present 
invention, attention is paid to a phenomenon where diffusion of an 
alkaline metal in glass is significantly expedited by hydronium ions 
H.sub.3 O and an anodically electrolyzed water including a large amount of 
hydronium ions is used in cleaning a substrate for a magnetic disk of an 
ion exchange strengthening type. The anodically electrolyzed water is 
obtained by electrolysis of water, it is very active, it is selectively 
substituted for an alkaline component on the surface of the glass 
substrate, as a result, the anodically electrolyzed water can reduce the 
concentration of the alkaline component on the surface of the glass 
substrate. More characterizingly the substitution reaction in respect with 
the alkaline component is selectively carried out and accordingly, by 
setting preferably the concentration of the anodically electrolyzed water 
that is used pertinently, only the concentration of alkaline ions on the 
surface of the glass substrate can be reduced without deteriorating the 
surface roughness of the glass substrate which is a significant effect. 
Further, generally, molecular size of anodically electrolyzed water is 
small and therefore, the permeability of water into micro cracks in the 
glass substrate is excellent and the water is permeable easily into cracks 
and accordingly, even if the alkaline component is present in cracks, the 
alkaline component in the cracks can efficiently be removed. Furthermore, 
the amount of other base ions other than hydronium ions is comparatively 
small and therefore, after the treatment of cleaning the glass substrate 
by using the anodically electrolyzed water, no special detergent using 
acid or alkali is needed and cleaning by high purity water is sufficient 
whereby an effect dispensing with an extra cleaning step which has been 
needed in the conventional technology, is achieved.