Abrading – Abrading process – Utilizing fluent abradant
Reexamination Certificate
2000-10-31
2003-05-27
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
Utilizing fluent abradant
C451S041000, C051S309000
Reexamination Certificate
active
06568995
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a method for cleaning a glass substrate and, more particularly, a glass substrate for use in a magnetic recording disc or a liquid crystal display, to which a high level of cleanliness is required.
In a multi-component glass substrate which is used as glass substrate for a magnetic recording disc or a liquid crystal display, a glass substrate is polished subsequent to formation process of the substrate, with using abrasive such as cerium oxide in order to ensure a high smoothness.
However, when the aforementioned glass substrate is polished with the abrasive, abrasive grains often remain sticking on the substrate surface firmly, which causes problems such as pinhole formation in subsequent processes. The abrasive grains firmly sticked to the substrate surface are very difficult to be removed by washing with water or neutral detergent. Consequently, the substrate has been washed with a liquid agent having etching ability, such as hydrofluoric acid (Japanese patent publication S50-45465A).
However, the hydrofluoric acid described in the above 50-45465 publication is still inadequate for giving a sufficiently clean glass substrate. This is because the glass substrate has a property to be positively charged in hydrofluoric acid solution, while various contamination particles including abrasive grains have a property to be negatively charged in hydrofluoric acid. Therefore, once removed contamination including abrasive grains can be adsorbed again onto the glass substrate, resulting in the glass substrate with insufficient cleanliness.
It is described in “Optical Glass (by Tetsuro Izumiya, published by Kyoritsu Shuppan) P. 165” that, when a glass substrate is washed with commercial alkaline detergent only, the contamination by the abrasive grains remains on the substrate surface, while complete removal of contamination accompanies glass corrosion, resulting in the rough substrate surface.
When the glass substrate with the rough surface, or with remaining abrasive grains, is layered with a magnetic film or a conducting film, the resulting disc may have difficulties in reading-out and writing-in, or it may give lettering errors due to discharging.
A magnetic recording device has been reduced in a distance between a magnetic head thereof and the substrate in pursuit of higher recording density, so that the contaminant particles on the substrate surface or the rough surface of the substrate possibly cause a head crush, that is, the head possibly collides with the particles or protrusions on the substrate surface during reading-out/writing-in operation. As a result, the demand for the cleanliness and smoothness of the substrate surface has increased.
Japanese patent publication H9-22885A describes a method to remove abrasive grains of cerium oxide on the substrate surface with using a washing solution containing sulfuric acid-hydrogen peroxide, hydrochloric acid-hydrogen peroxide, or nitric acid. In this method, the zeta-potential of the cerium oxide grains is changed by the above washing solution so that the remaining abrasive grains agglomerate to form larger particles which are easily removed in a subsequent scrubbing-washing process.
The purpose of the method described in the above 9-22885 publication is to agglomerate the cerium oxide particles by means of zeta-potential control in order to make them easily removable in the scrubbing-washing process. This is different from the purpose of the present invention in which cerium oxide removal is achieved by dissolution thereof by an action of a reducing agent and acid. Further, the scrubbing-washing process is not necessary in the method of a present invention, as the particles such as cerium oxide particles are removed by dissolution, while the scrubbing-washing process is indispensable in the method described in the 9-22885 publication.
In addition, the object of the present invention is in the cleaning of the glass substrate, while the object of the method described in the 9-22885 publication is in the cleaning of a semiconductor substrate, without any mention to the glass substrate.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to solve conventional problems as described above, and to provide a method of cleaning a glass substrate of high cleanliness for use in a magnetic disc and in a liquid crystal display, without occurrence of latent flaws and residual contaminant on the substrate surface.
The glass substrate of the present invention has a lanthanoid oxides quantity of less than 50×10
10
molecules/cm
2
remaining on the substrate surface.
The lanthanoid oxides include cerium oxide, lanthanum oxide, neodymium oxide, and praseodymium oxide. The lanthanoid oxides remaining on the substrate surface is particularly preferable in a range less than 0.5×10
10
molecules/cm
2
.
Such glass substrate having very small quantity of lanthanoid oxides remaining on the substrate surface is extremely preferable to be used as an information recording medium. Further, the substrate according to the present invention has a surface which is not excessively rough. This is also the reason why it is suitable as the information recording medium.
According to the method for cleaning the glass substrate of the present invention, the glass substrate polished with an abrasive containing lanthanoid oxides, and then the substrate is washed with a washing solution containing acid and reducing agent.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The glass composing the substrate of the present invention includes soda lime glass, aluminosilicate glass, borosilicate glass, aluminoborosilicate glass, and crystal glass from these kinds of glass. The aluminosilicate glass is preferable as a glass substrate for magnetic recording discs in respect of weathering property and cost. A preferable composition of aluminosilicate glass is as follows, expressed in molar fraction;
SiO
2
63-70
mol %
Al
2
O
3
6-12.5
mol %
Li
2
O
5-11
mol %
Na
2
O
6-14
mol %
K
2
O
0-2
mol %
TiO
2
0-5
mol %
ZrO
2
0-2.5
mol %
MgO
0-4.5
mol %
CaO
2-7.5
mol %
SrO
0-3
mol %
BaO
0-2
mol %
(The sum of MgO, CaO, SrO, BaO should be 2-12 mol %.)
The surface layer of the glass substrate may be modified. For example, the substrate may have a compression stressed surface layer formed by chemical strengthening treatment.
In polishing and washing processes which will be later explained in more detail, the substrate is finished to a surface roughness Ra less than 0.5 nm, and particularly less than 2.5 nm, and to an amount of lanthanoid oxides remaining on the surface of less than 50×10
10
molecules/cm
2
, and particularly less than 0.5×10
10
molecules/cm
2
. When Ra is greater than 0.5 nm, pointed protrusions called asperity are formed on the substrate surface. The substrate of the invention with the smooth surface and the very small quantity of lanthanoid oxides remaining on the surface is extremely preferable as an information recording medium.
In order to polish the substrate, an abrasive containing lanthanoid oxides is used. Examples of the abrasive include an abrasive containing cerium oxide as main component, and a powder abrasive which is produced by pulverizing calcinated natural ore bastnaesite consisting mainly of lanthanoid oxides (such as cerium oxide, lanthanum oxide, neodymium oxide, and praseodymium oxide).
There is no particular limitation in polishing method itself and various polishing apparatus can be used.
Subsequent to the polishing process, the substrate is washed with a washing solution containing acid and a reducing agent. There is no special limitation in the kind of acid, but in respect of dissolving ability to lanthanoid oxides, it is preferable to use at least one selected from nitric acid, sulfuric acid, hydrochloric acid, sulfamic acid, and phosphoric acid.
It is preferable to use one or more strong acids, particularly sulfuric acid, hydrochloric acid, or nitric acid. The acid concentration in the washing solution is preferably from 0.001 to 1
Mitani Kazuishi
Saito Yasuhiro
Hail III Joseph J.
Kanesaka & Takeuchi
Nippon Sheet GLass Co., Ltd.
Thomas David B.
LandOfFree
Method for cleaning glass substrate does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for cleaning glass substrate, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for cleaning glass substrate will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3022587