Substrate for a magnetic recording medium having laser beam absorption characteristic and a magnetic recording medium using the substrate

A substrate for a magnetic recording medium. The substrate is a non-magnetic brittle material having a substantially flat surface on which laser marks having a height in a range of 10-500 .ANG. are formed in a landing zone. The laser marks are formed by irradiating a pulsed laser beam having a wavelength capable of being absorbed in the non-magnetic brittle material within a range of substantially circular form having a waist diameter D .mu.m, and which has a physical property of 0.5<E.sup.v /(D/20).sup.2 <6 .mu.J where E.sup.v .mu.J represents a pulse energy for initiating vaporization of the glass substrate by irradiation of the pulsed laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a substrate for a magnetic recording 
medium and a magnetic recording medium using the substrate. 
2. Discussion of Background 
There has been a demand for a magnetic recording medium (hereinbelow, 
referred to as a disk) having a high recording density, along with which 
it has been necessary to reduce a flying height of a magnetic head 
(hereinbelow, referred to as a head). In recent years, a flying height of 
a head from a disk surface of 500 .ANG. or lower, desirably, 300 .ANG. or 
lower is required. Accordingly, it is necessary that the disk surface is 
extremely flat in a data zone where the recording/reading of data are 
effected. 
On the other hand, in a contact-start-stop (CSS) method, a head takes off 
and lands on a disk surface when the rotation of the disk is started or 
stopped. The disk surface is not smooth, but there are minute projections 
and recesses called a texture formed to prevent the head from sticking on 
the disk surface at the time of taking off and landing. 
In order to satisfy both requirements of preventing the head from sticking 
and of the head taking off with a low flying height while a high recording 
density is maintained, it is necessary to form the texture of minute 
projections and recesses wherein a dispersion in the height of the 
projections is small. However, it is not easy to form such texture. 
Accordingly, there is proposed a disk wherein the disk surface is divided 
into a zone in which the head rests at the time of stopping the disk 
(i.e., a landing zone) and a zone for effecting recording/reading of data 
(i.e., a data zone), and the texture is formed only in the landing zone 
while the data zone is left smooth. 
The landing zone is formed in an inner peripheral area or an outer 
peripheral area with a predetermined width on a surface of a doughnut-like 
disk. The area other than the landing zone in the disk surface is 
substantially occupied by the data zone. 
To form the texture in the disk surface, the texture is formed in a surface 
of a substrate to be used for the disk. Although a magnetic layer and 
other required layers are formed on the substrate, the texture formed in 
the substrate should correctly be transferred on the disk surface. In a 
case of forming the landing zone in the disk surface, the texture can be 
formed in the substrate surface at the region just below the landing zone 
in the disk surface (it is referred to as a landing zone of substrate). In 
this case, it is unnecessary to form the texture in the data zone in the 
disk and the data zone has a smooth surface. Accordingly, the 
corresponding area in the substrate (it is referred to as a data zone of 
substrate) is also smooth, and such material is suitable for a substrate 
for a magnetic disk, which is made of a non-magnetic brittle material such 
as glass having a feature of smoothness in its surface. Accordingly, a 
substrate for a magnetic recording medium which is formed of such material 
and has the texture having minute projections and recesses wherein a 
dispersion in the height of the projections is small, is envisioned. 
In forming the texture in the surface of the substrate, there have been 
proposed a mechanical method using an abrasive material on a substrate of 
aluminum alloy with NiP plating or a glass substrate, or a chemically 
treating method to a glass substrate or the like. However, these methods 
have a disadvantage that the height of projections is not uniform thereby 
preventing the lowering of a flying height of the head. 
Besides the above-mentioned methods, U.S. Pat. No. 5,062,021, U.S. Pat. No. 
5,108,781 and Japanese Unexamined Patent Publication JP-A-8-106630 propose 
a method of forming a texture by using a laser. Further, a paper "A New 
Laser Texturing Technique for High Performance Magnetic Disk Drives" IEEE 
Trans. Mag., Vol.31, pp2946-2951,1995 describes related art. 
In these documents, metal or alloy is used as the material to be processed 
by laser. Further, in these documents, the mechanism of forming a texture 
by applying a laser to such material is as follows. On irradiating laser, 
there takes place a temperature distribution in an irradiated region with 
the result of a distribution of surface tension in the radiated area, 
whereby there occurs a re-arrangement of compositions in correspondence to 
the surface tension, and solidification follows. In these prior art 
documents, there is no consideration of a substrate constituted by a 
brittle material such as a glass substrate, a glass ceramics substrate or 
a carbon substrate. 
EP 0652554A discloses a method of forming a texture by irradiating a laser 
beam on the surface of a brittle material having a thermal shock fluence 
threshold level. However, there is no special proposal to control the 
height of minute projections and the depth of minute recesses in order to 
form a uniform texture wherein a dispersion of the height of the 
projections is small. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a substrate for a 
magnetic recording medium which is formed of glass, glass ceramics, 
ceramics or another non-magnetic brittle material and in which a texture 
comprising minute projections and recesses can be formed by irradiating a 
pulsed laser beam, wherein a dispersion of the height of the projections 
is small. 
It is a further object of the present invention to provide a magnetic 
recording medium comprising the above-mentioned substrate on which a 
magnetic layer and other required layers are formed. 
In accordance with the present invention, there is provided a substrate for 
a magnetic recording medium which comprises a non-magnetic brittle 
material having a substantially flat surface in which a plurality of laser 
marks are formed each having a projection of a height H in a range of 
10-500 .ANG. in at least a part of it, the laser marks being formed by 
irradiating a pulsed laser beam having a wavelength capable of being 
absorbed in the non-magnetic brittle material within a range of 
substantially circular form having a waist diameter D .mu.m, and which has 
a physical property of 0.5&lt;E.sub.v /(D/20).sup.2 &lt;6 .mu.J where E.sub.v 
.mu.J represents a pulse energy value for initiating vaporization of the 
substrate by irradiation of the pulsed laser beam. 
In the present invention, the substrate for a magnetic recording medium, 
which is made of a non-magnetic brittle material, is referred to as a 
non-magnetic brittle substrate. 
In this specification, the non-magnetic brittle substrate includes a 
substrate of non-magnetic brittle material such as glass, glass ceramics, 
ceramics or the like, or a substrate comprising a non-magnetic brittle 
material having a thickness sufficient to form laser marks, which is 
laminated on another material. 
The pulsed laser beam used for the present invention means the beam 
constituted by the repetition of a pulse. In this specification, the 
energy of a pulse is referred to as an amount of a pulse energy of the 
pulsed laser beam, or a pulse energy value, or simply, a pulse energy. 
The pulsed laser beam used for the present invention may be one having a 
wavelength capable of being absorbed by the non-magnetic brittle material 
constituting the substrate, for example, the 4th harmonics of a YAG laser, 
the 4th harmonics of a YLF laser or the 4th harmonics of a YVO.sub.4 laser 
or the like. 
The laser beam used for the present invention is substantially circular in 
a cross-sectional surface which is perpendicular to an optical axis on the 
surface of the substrate, and the laser beam has a Gaussian distribution 
or a distribution similar to the Gaussian distribution wherein the 
intensity of the energy of the laser beam is the largest at the center and 
attenuates in the radial direction. A value two times as large as the 
radius at which the intensity of the energy is l/e.sup.2 of the intensity 
at the center, is called a waist diameter and is represented as D .mu.m. A 
symbol e represents the base of a natural logarithm. 
In the present invention, the optical axis of the pulsed laser beam to be 
irradiated is substantially perpendicular to the surface of the substrate. 
The laser marks referred to in this description mean lifted portions formed 
by irradiating the pulsed laser beam onto the surface of the non-magnetic 
brittle substrate having a substantially flat surface or portions wherein 
the shape of the surface of the substrate is changed which also include 
the lifted portions. The laser marks are formed dispersibly in the surface 
of the substrate to thereby form a texture. 
It is preferable that each of the laser marks is in a substantially 
projected form in the direction out of the substrate, or it has a 
peripheral portion which is in a substantially projected form in the 
direction out of the substrate and it has a concave portion at the center 
of the projected portion. 
In order to avoid a sticking phenomenon, to improve durability to CSS and 
to reduce a flying height of the head, the height H of each of the 
projections as the laser marks should be 10-500 .ANG., more preferably, 
10-300 .ANG.. It is further preferable that when a dispersion of the 
height H is represented by a standard deviation .sigma., .sigma. is 5% or 
lower of an average value of H, in particular, 3% or lower. 
The substrate used for the present invention should have an amount of pulse 
energy E.sub.v to initiate vaporization when the energy of the pulsed 
laser is applied wherein E.sub.v is smaller than 6 .mu.J and larger than 
0.5 .mu.J. When E.sub.v is 6 .mu.J or more, the dispersion of the height H 
of the projections becomes large. On the other hand, when E.sub.v is 0.5 
.mu.J or less, vaporization will occur before a sufficient height H of the 
projections can be formed and a height of 10 .ANG. or more can not be 
obtained. 
In the present invention, the landing zone and the data zone are formed in 
the substrate for a magnetic recording medium, and the pulsed laser beam 
is irradiated specifically on the landing zone to form the laser marks 
whereby a flying height of the head in the data zone is reduced and a 
danger of the sticking in the landing zone is eliminated simultaneously. 
Further, the present invention provides a magnetic recording medium in 
which a magnetic layer and other required layers are formed on the 
above-mentioned substrate made of a non-magnetic brittle material. 
The height and the shape of the laser marks formed in the surface of the 
substrate can be retained in the outermost surface of the magnetic 
recording medium through a barrier layer, an under layer, a magnetic 
layer, a protective layer, a lubricant layer and so on which are formed on 
the substrate. For instance, the height and the shape of the laser marks 
formed in the landing zone of the substrate constitute a texture as a form 
of projections having substantially the same height and substantially the 
same shape in the landing zone of the magnetic recording medium. The 
texture contributes to reduce a frictional force between the magnetic 
recording medium and the head and to prevent the sticking of the head. 
Description will be made as to the shape of the laser marks formed in 
response to an amount of pulse energy when the pulsed laser beam is 
applied to a brittle substrate such as a glass substrate or a glass 
ceramic substrate. 
When an amount of the pulse energy of the pulsed laser beam is increased, 
the temperature at irradiated portions is increased whereby lifting of the 
irradiated portions in the substrate surface starts (FIG. 3). This is 
called a simple lifting type. 
When the pulsed laser beam having a higher energy is subsequently 
irradiated and the energy reaches E.sub.v, vaporization initiates in the 
irradiated portions of material, and a recessed portion appears at almost 
the center of the top of the lifting portion, where the intensity of the 
irradiated energy is the highest (FIG. 4). This is called a sink type. The 
energy value of the pulse at the transition from the simple lifting type 
to the sink type corresponds to the energy E.sub.v to initiate 
vaporization of the substrate. When the pulsed laser beam having a further 
higher energy is irradiated, the vaporizing area expands and sinking 
progresses with the result that each of the laser marks has a shape that 
the peripheral portion remains lifted in a substantially projected form 
and a recess is formed at its central portion (FIG. 5). This is called a 
crater type. 
Each of the laser marks has a projecting portion in at least a part of it 
and the height H of the projecting portion is in a range of 10-500 .ANG.. 
The height H of the projecting portion is referred to as the height of 
laser mark or simply, the height. FIGS. 3 to 5 show the height H of 
various projections wherein an arbitrary scale is used for each of the 
Figures, therefor, the scales are not common in FIGS. 3 to 5. In FIGS. 4 
and 5, the height H indicates the height of peripheral portions. 
When a pulsed laser beam is irradiated onto a non-magnetic brittle 
substrate, there occur changes such as temperature rise, expansion, 
lifting and so on due to the absorption of the energy of the pulsed laser 
beam at and around the irradiated portions in the substrate. The rate of 
change depends on the wavelength and the energy of the laser. Further, 
they are related to the properties and the compositions of the substrate. 
In the present invention, the rate of absorbing a laser energy by the 
substrate is called the sensitivity of the substrate to the laser energy. 
When the rate of absorbing is high, the sensitivity is high. 
According to a result of study, the width of the energy of the laser beam 
at a time from occurrence of the lifting to the initiation of vaporization 
is very narrow in a substrate which has not particularly been treated to 
increase the sensitivity to the laser energy, and the rate of a change in 
the height of the lifting portion to a change in the energy of the 
irradiated pulsed laser beam is steep. Accordingly, it is necessary to 
strictly control the variation of an irradiated energy in order to control 
the height of the projections within a predetermined range. 
On the other hand, it is known that a laser power varies with respect to 
time. A severe control of variation of an applied energy is restricted by 
power stability of a laser to be used. A requirement of strictly 
controlling the variation of the applied energy results an excessively 
severe request in a production step and reduction of yield, and therefor, 
it is uneconomical. Formation of the texture with use of a laser in which 
variation of the power is strictly controlled may be possible under a 
strictly controlled site such as a R & D site. However, it is not 
realistic in an industrial beam. 
Further, according to a result of study by the inventor, when a 
commercially available glass substrate which is not treated so as to 
increase the sensitivity to a laser is used, the height H of projections 
as laser marks reaches about 1 .mu.m, which is remarkably high in 
comparison with 500 .ANG. or less, preferably, 300 .ANG. or less as a 
target height. Accordingly, it is very difficult for a commercially 
available glass substrate to control the height in a very low range. 
Further, when the commercially available glass substrate is irradiated by 
a laser beam, it exhibits a remarkable change of expansion and a lifting 
which causes cracks and reduction of strength. 
In accordance with the present invention, there is provided a substrate 
which is made of a non-magnetic brittle material such as glass, glass 
ceramics or the like, and which has a high sensitivity to a pulsed laser 
energy and initiates vaporization on the application of a lower energy of 
a pulsed laser beam having a wavelength capable of being absorbed by the 
material. 
In the present invention, the substrate made of glass can be selected from 
those suitable for magnetic recording medium in the group constituting of 
soda-aluminosilicate glass, sodalime silicate glass and alkali-containing 
borosilicate glass. The light absorbing agent used to increase the 
sensitivity to a pulsed laser may be, Fe.sub.2 O.sub.3, CeO.sub.2, V.sub.2 
O.sub.5, TiO.sub.2 and the like and any combination of these compounds can 
be used for an ultraviolet laser beam used in the present invention. 
Such a substrate allows the formation of laser marks having a height H in a 
range of 10-500 .ANG. with a small dispersion by the irradiation of the 
pulsed laser. Accordingly, a substrate having a texture suitable for high 
density recording and a magnetic recording medium having such substrate 
can be obtained.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 is a diagram showing a cross sectional view of an embodiment of a 
magnetic recording medium according to the present invention wherein 
reference numeral 1 designates a non-magnetic brittle substrate having a 
substantially flat surface, which has a physical property of 0.5&lt;E.sub.v 
/(D/20).sup.2 &lt;6 .mu.J where E.sub.v .mu.J represents the energy for 
initiating vaporization when a pulsed laser having a wavelength .lambda. 
is irradiated with a waist diameter D .mu.m on the substrate. For the 
non-magnetic brittle substrate, glass, glass ceramics or ceramics is 
desirable. However, another non-magnetic brittle substrate such as a 
carbon substrate to which the same mechanism of forming the laser marks 
applies can be used. Further, a substrate composed of a composite material 
wherein the above-mentioned material is laminated on another kind of 
material may be used. 
Further, it is desirable that the surface of the substrate is 
mirror-finished to have a fine roughness of less than 10 .ANG. prior to 
the irradiation of the laser beam in order to avoid an undesirable 
physical contact with the head and to control the dispersion of the height 
of the laser marks in a narrow range. 
In FIG. 1, a number of laser marks 3 of a height of 10-500 .ANG., 
preferably, 10-300 .ANG. are formed in the surface of the non-magnetic 
brittle substrate. When the height H is less than 10 .ANG., the prevention 
of the sticking of the head by the laser marks can not be expected. The 
upper limit of the height of the laser marks is determined by a flying 
height of head required in this technical field because the height of the 
laser marks directly determines the lowest fly height for the head. From 
the viewpoint of increasing a magnetic recording density, it is required 
for the upper limit to be 500 .ANG., preferably, 300 .ANG.. 
Reference numeral 2 designates a magnetic layer of a Co based ferromagnetic 
alloy of 50-500 .ANG. thickness, which is formed by a sputtering method, 
or a vacuum metallizing method or the like. In the substrate structure of 
the present invention, at least a magnetic layer is formed on the 
non-magnetic brittle substrate. However, a barrier layer having a 
corrosion resistance property and/or an under layer for controlling the 
crystal growth of the magnetic layer may be formed between the 
non-magnetic brittle substrate and the magnetic layer. For the barrier 
layer, Cr or Ti based material can be used. Further, for the underlayer, 
NiP or Cr based material can be used. 
A protective layer and/or lubricant layer may be formed on the magnetic 
layer 2. The protective layer may be a carbon layer, a hydrogenated carbon 
layer, a carbon nitride layer, a hydrogenated carbon layer containing 
nitrogen, a carbide layer such as TiC, SiC or the like, or an oxide layer 
such as ZrO.sub.2. For the lubricant layer, a film of a perfluoropolyether 
type lubricant having a thickness of 5-50 .ANG. is used, for example. 
The shape of the laser marks 3 formed in the brittle substrate is 
maintained in the outermost surface of the magnetic recording medium 
through a barrier layer, an under layer, a magnetic layer, a protective 
layer and a lubricant layer formed thereon, as shape-changing portions 30 
having the substantially same shape as the laser marks 3, thus, a texture 
is formed in the front surface of the magnetic recording medium by which a 
frictional force between the head and the disk is reduced. 
FIG. 2 shows an example of the structure of an apparatus capable of 
irradiating a pulsed laser beam onto the non-magnetic brittle substrate to 
form laser marks as a texture. Numeral 4 designates a YAG Q switched 
pulsed laser device. The device includes therein a non-linear optical 
element whereby a pulsed laser energy 15 having a wavelength of 532 nm 
which is the second harmonics of the fundamental wave (wavelength: 1064 
nm) of YAG is generated. 
The generated energy is introduced to the non-linear optical element 6 by 
means of a reflection mirror 5, and a pulsed laser energy 16 of 4th 
harmonics (wavelength: 266 nm) is generated by wavelength conversion. 
Besides the YAG laser, any laser having a wavelength capable of being 
absorbed by the material to be processed may be used. A solid state laser 
such as YLF, YVO.sub.4 or the like, an excimer laser, an argon laser can 
be selected. When the non-magnetic brittle substrate is glass, the glass 
has strong absorption nature in an ultraviolet region and an infrared 
region of more than 3 .mu.m, it is preferable to use the 4th harmonics of 
the YAG laser, the YLF laser, and the YVO.sub.4 laser, or the excimer 
laser or the second harmonics of a visible light region of the argon laser 
whose wavelengths fall in these regions. Further, a CO.sub.2 laser may be 
used. 
The laser beam 16 is introduced through an attenuator 7 for controlling 
irradiated energy, a reflection mirror 8 and an aperture 9 for shaping the 
laser beam into a desired shape to an expander 10 which controls the laser 
beam to have a predetermined distribution on the surface to be recorded. 
Then, the laser beam is introduced onto the surface of the brittle 
substrate 14 through a galvano mirror (X axis) 11, a galvano mirror (Y 
axis) 12 and an objective lens 13. The pulsed laser beam 16 is deflected 
by driving the galvano mirror (X axis) 11 and the galvano mirror (Y axis) 
12 to be dispersively irradiated onto the surface of the substrate which 
is fixed. As a result, a number of laser marks are dispersively formed in 
a concentric form, a spiral form or a desired pattern on the substrate by 
controlling the movement of the galvano mirrors movable in two axes X and 
Y in association with a repetition frequency of the pulsed laser beam. 
Formation of a number of laser marks may be realized by other than the 
method of deflecting the laser beam by means of the above-mentioned 
galvano mirrors 11, 12. For example, laser marks can dispersively be 
formed in a spiral form or a concentric form by fixing the position of a 
laser beam and rotating a spindle on which the brittle substrate is 
mounted while the spindle is moved in the radial direction in association 
with the rotation of the spindle. In this case, however, the laser beam 
source can be moved in the radial direction instead of moving the spindle. 
The dispersively formed laser marks can be obtained by another method other 
than using the Q switch pulsed laser 4 shown in FIG. 2. For example, a 
pulsed laser beam obtained by modulating a continuous wave laser with an 
EOM or an AOM can be used. Further, a number of laser marks can be formed 
at a time by irradiating a laser light to the substrate by means of a mask 
having a predetermined pattern. 
It is supposed that a lifting portion is formed in the substrate according 
to the mechanism as follows. When irradiation of the pulsed laser beam is 
effected in a time determined by a width of a pulse, an irradiated area of 
a glass material is heated to a high temperature and is expanded in volume 
(reduction of the density). Since the area not subjected to the 
irradiation of the laser beam which is around the irradiated area has a 
large heat capacity, the irradiated area is rapidly cooled at a rate which 
can not maintain the thermal equilibrium. As a result, after cooling, the 
irradiated area takes a volume slightly larger than the original volume 
(i.e., a density smaller than the original density), thus, a lifting 
portion is formed. Accordingly, a lifting portion extending outside with 
respect to the surface of the substrate is considered to be the total sum 
of an increase in the volume of the irradiated area which is heated by the 
laser beam. 
When a pulsed laser beam having a higher energy is irradiated, temperature 
at a part of the irradiated area exceeds a vaporization temperature of the 
material, and vaporization of the material is initiated. At this stage, a 
process of forming a lifting portion due to the increase of volume and a 
process of vaporization of the material at an elevated temperature portion 
in a part of the irradiated area (at the central portion in a case of 
using a focused laser) occur simultaneously, whereby a sink type or a 
crater type laser mark which has a lifting portion at the peripheral 
portion and a recess at the central portion, can be obtained. 
Although a crater type laser mark similar to the above-mentioned is 
obtainable in a case of using a NiP alloy, it is considered that the 
formation of the crater type laser mark is resulted from a volume 
rearrangement of fused material due to a distribution of surface tension 
which is generated by a temperature distribution by the irradiation of the 
laser beam, and the mass before and after the formation of the laser mark 
is balanced. On the other hand, in the case of the glass material used for 
the present invention, the crater type laser mark is resulted from the 
volume expansion and the volume vaporization, and the mass before and 
after the formation of the laser mark is not balanced, and on the 
contrary, the mass after the formation is decreased. 
A paper "Laser Texture on Alternative substrate Disks (E. Teng, W. Goh, and 
A. Eltoukhy StorMedia Inc., 1996 Intermag)" describes a lifting portion 
forming mechanism caused by irradiating a laser beam on glass ceramics in 
which there is a proposal of expansion of volume due to a change from 
crystallization (higher density) to a non-crystallization (lower density) 
by the application of a laser energy. 
The inventor of this application thinks that the mechanism of forming a 
lifting portion in a glass ceramics material in which crystal regions 
exist in a non-crystal region derives from either the reduction of density 
due to the material being subject to a rapid cooling step and/or volume 
expansion due to the change from crystallization to non-crystallization. 
Anyway, the main cause of the formation of laser marks in a brittle 
material such as glass, glass ceramics, or the like is due to expansion of 
volume in an area to which a laser beam is irradiated to elevate 
temperature. Since the expansion of the volume at this area is closely 
related to the depth of penetration of the applied laser energy into the 
substrate, it seems necessary to make the depth of penetration of the 
applied energy to be small in order to prevent forming of excessively 
large laser marks. 
The depth of penetration of the laser energy into the substrate varies 
depending on the sensitivity of the substrate to the laser energy. In a 
substrate having a lower sensitivity to the laser energy, the laser energy 
incident on the substrate is not substantially absorbed at or near the 
substrate surface. Accordingly, the depth of the area subjected to heating 
and the resultant expansion is large with the result that the height of a 
lifting portion is remarkably high, and it is difficult to control the 
laser mark to be a height of 10-500 .ANG., preferably, 10-300 .ANG.. In 
addition, since a higher pulse energy is needed (an arrow mark B in FIG. 
7) for initiating lifting in the substrate, an increase in the height of 
the laser mark in correspondence to a value of the pulse energy is steep 
as shown in FIG. 7. Accordingly, in addition to the difficulty in 
controlling the height of the laser mark to be a predetermined height, the 
dispersion of the height of the laser marks is large when there is 
variation of the energy of laser. 
On the other hand, in a substrate having a higher sensitivity to the laser, 
the laser energy incident onto the substrate is substantially absorbed at 
or near the substrate surface. Accordingly, the depth of penetration of a 
laser beam, i.e., the depth of the area subjected to heating and the 
resultant expansion by the laser beam is small with the result that the 
height of a lifting portion can be controlled to be a height of 10-500 
.ANG., preferably, 10-300 .ANG.. A pulse energy (an arrow mark A in FIG. 
6) by which lifting initiates in the substrate, is low, and a rate of an 
increase of the height of the laser mark to an increase of pulse energy 
value is mild as shown in FIG. 6. Accordingly, a dispersion of the height 
of the laser marks can be made small even though there is variation of 
laser pulse energy. 
Now, the present invention will be described in detail with reference to 
Examples. However, it should be understood that the present invention is 
by no means restricted by such specific Examples. 
Laser processing was effected to various kinds of non-magnetic brittle 
substrates with use of the laser texture processing apparatus shown in 
FIG. 2 in which the 4th harmonics of a YAG laser (wavelength .lambda.=266 
nm) was used as a pulsed laser beam. 
In Examples 1, 2 and 3 and Comparative Examples 2 and 3, disk-like glass 
substrates were used as substrates for hard disks, which were prepared by 
incorporating different amounts of Fe.sub.2 O.sub.3 and CeO.sub.2 as a 
light absorbing agent to commercially available soda-aluminosilicate glass 
in order to increase their sensitivity to the above-mentioned wavelength 
of laser beam. The content (% by weight) of the light absorbing agents for 
each of the substrates is shown in Table 1. The glass substrate of 
Comparative Example 2 has the least content, and the content is increased 
in the order of Comparative Example 3, Example 1, Example 2 and Example 3, 
which has the largest content. 
TABLE 1 
______________________________________ 
Comparative 
Examples Examples 
1 2 3 2 3 
______________________________________ 
Fe.sub.2 O.sub.3 
2 3 7 0.5 0.9 
CeO.sub.2 
2 3 7 0.5 0.9 
______________________________________ 
Accordingly, the sensitivity to the above-mentioned wavelength of laser 
beam is the lowest in Comparative Example 2 and the highest in Example 3. 
The glass substrate in Comparative Example 1 was of another commercially 
available soda-aluminosilicate glass and did not contain the light 
absorbing agent, and therefor, the sensitivity is lower than the glass 
substrate of Comparative Example 2. 
Generally, the sensitivity of a substrate to a laser beam can be expressed 
by an absorption coefficient of the substrate with respect to the 
wavelength of the laser beam. The absorption coefficient is obtained by 
measuring a transmittance to the substrate having a known thickness. In 
measuring the transmittance of the 4th harmonics of the YAG laser beam, it 
was very slight in the glass substrate of 0.635 nm thickness in 
Comparative Example 1, and the absorption coefficient could not be 
obtained. In the measurement of the transmittance of each glass substrate 
of Examples 1, 2 and 3 and Comparative Examples 2 and 3 each having a 
thickness of 0.635 nm, the absorption coefficient could not be obtained as 
well because the transmittance was below the limit of detection. Further, 
even in a case of a thickness of 100 .mu.m, the absorption coefficient 
could not be obtained. Such results of measurements show that the almost 
of the irradiated laser energy is supposed to be absorbed in a portion 
having a depth of 100 .mu.m in Examples 1, 2 and 3 and Comparative 
Examples 2 and 3. 
Thus, since it was found to be difficult to express the sensitivity of the 
substrates to the laser beam by means of the absorption coefficient, the 
inventor has decided to identify the sensitivity of glass substrates by 
using a pulse energy value E.sub.v (.mu.J) which can initiate vaporization 
in the substrates when a pulsed laser beam is irradiated. Since material 
having a higher sensitivity to a laser beam absorbs the laser energy 
strongly, vaporization is initiated at a lower pulse energy. On the other 
hand, since material having a lower sensitivity has difficulty absorbing 
the laser energy, a higher pulse energy is needed to initiate 
vaporization. 
Hereinbelow, a value E.sub.v /(D/20).sup.2 (.mu.J) which is obtained by 
standardizing a pulse energy for initiating vaporization with respect to a 
waist diameter of 20 .mu.m is used as a sensitivity of a substrate 
material to a pulsed laser beam. Namely, a substrate material having a 
larger E.sub.v /(D/20).sup.2 has a lower sensitivity to an irradiated 
pulsed laser beam. On the other hand, a substrate material having a 
smaller E.sub.v /(D/20).sup.2 has a higher sensitivity to the laser beam. 
First, a pulsed laser beam is irradiated with a pulse energy of specified 
value to a glass substrate to thereby form a plurality of laser marks on 
the glass substrate. Each laser mark is formed by the application of each 
pulse. When a pair of galvano mirrors is deflected for each pulse, a group 
of laser marks with certain intervals is formed. In these Examples, the 
intervals of the laser marks were 60 .mu.m in the radial direction and the 
circumferential direction. Then, by gradually changing the value of the 
pulse energy and by repeating a step of forming laser marks in 
correspondence to respective values of pulse energy, laser marks which 
were in response to the pulse energy of each of the values were formed on 
the glass substrate. The pulse energy was changed by changing the peak 
value of the pulsed laser beam, but without changing the pulse width. 
The height of the laser marks formed by the application of various values 
of pulse energy was measured with use of an optical surface profile 
measuring device (tradename: ZYGO, manufactured by ZYGO Company). Further, 
the shape of the laser marks corresponding to various pulse energy values 
was observed with the measuring device, and a value E.sub.v (.mu.J) for 
initiating vaporization was obtained by a pulse energy value at which the 
shape of the laser marks was changed from the simple lifting type (FIG. 3) 
to the sink type (FIG. 4). 
Process parameters in laser processing are as follows: 
Pulse repetition rate=5 kHz, 
Laser pulse width=65 nsec, 
Focal length of objective lens=150 mm, and 
Waist diameter D=10 .mu.m. 
FIG. 8 shows a relation of irradiated pulsed energy values and the height 
of the laser marks in Examples 1 and 3, and Comparative Examples 1 and 2. 
Examples 1 and 3 correspond to the diagram of FIG. 6 and Comparative 
Examples 1 and 2 correspond to the diagram of FIG. 7. The slopes of curves 
for Examples 1 and 3 are mild, and those for Comparative Examples 1 and 2 
are steep. 
Table 2 shows data on the pulse energy value E.sub.v (.mu.J) for initiating 
vaporization, the vaporization initiating energy E.sub.v 
(.mu.J)/(D/20).sup.2 standardized with respect to a waist diameter of 20 
.mu.m, the height of laser marks H.sub.v (.ANG.) when an irradiated pulse 
energy is E.sub.v (.mu.J), an irradiated pulse energy value E.sub.250 
(.mu.J) to obtain a height of laser marks of 250 .ANG., and the allowed 
variation of irradiated pulse energy .DELTA.E/E.sub.250 to make a height H 
within a range of .+-.5%. i.e., 250 .ANG. .+-.13 .ANG.. In Table 2, 
.DELTA.E=(E.sub.263 -E.sub.237) wherein E.sub.263 represents an irradiated 
pulse energy to obtain a height of 263 .ANG. and E.sub.237 represents an 
irradiated pulse energy to obtain a height of 237 .ANG.. 
.DELTA.E/E.sub.250 indicates allowable variation (%) of an irradiated 
pulse energy required for a variation in the height of the laser marks 
within .+-.5%. A larger value means that material used has a higher 
tolerance to a variation of laser power, and therefor, a higher value is 
preferable. 
TABLE 2 
______________________________________ 
E.sub.v E.sub.v /(D/20).sup.2 
H.sub.v E.sub.250 
.DELTA.E/E.sub.250 
.mu.J .mu.J .ANG. .mu.J 
% 
______________________________________ 
Example 1 
1.1 4.4 550 0.70 3.4 
Example 2 
0.5 2.1 260 0.51 9.8 
Example 3 
0.4 1.7 230 0.55 8.4 
Comparative 
13.0 51.8 8240 8.62 0.4 
Example 1 
Comparative 
3.1 12.5 4010 1.87 0.5 
Example 2 
Comparative 
2.4 9.6 1580 1.17 0.9 
Example 3 
______________________________________ 
In a case of the YAG laser as used in the Examples, for instance, the width 
of variation of the laser power is about 2% in terms of the fundamental 
wavelength. Further, by converting the wavelength to the 2nd harmonics, 
the 4th harmonics and the like, stability in the laser power is known to 
decrease. Table 2 shows that there is a difficulty in controlling a 
dispersion of the height of the laser marks because .DELTA.E/E.sub.250 is 
less than 2% in Comparative Examples 1, 2 and 3. On the other hand, 
.DELTA.E/E.sub.250 is largely beyond 2% in Examples 1, 2 and 3 which 
indicate an improvement with respect to controllability of the dispersion 
of the height of the laser marks. Although the same light absorbing agents 
as in Examples 1, 2 and 3 is contained in Comparative Examples 2 and 3, 
the content is not sufficient, and it is found that the controllability of 
the height of the laser marks is not sufficient. Further, when laser marks 
having a height of 250 .ANG. are to be formed, a required pulse energy in 
Example 3 is 0.55 .mu.J, however, it exceeds the pulse energy E.sub.v 
(0.42 .mu.J) for initiating vaporization, and the shape of the laser marks 
of 250 .ANG. shows the sink type as shown in FIG. 4. On the other hand, in 
Examples 1 and 2, the irradiated pulsed energy to form the laser marks of 
250 .ANG. high does not exceed E.sub.v and it provides the simple lifting 
type as shown in FIG. 3. 
In Examples 1, 2 and 3, there was found no fine cracks in the substrates, 
and usable textures were obtained. 
In Comparative Example 1, the height H.sub.v at E.sub.v reaches 8000 .ANG.. 
It is understood that when a substrate material in which the sensitivity 
to laser energy has not in particular been increased is used, laser marks 
having a height of about 1 .mu.m are formed as described in EP 062554A. It 
is unnecessary for the texture in a disk to have such height. It is really 
required to form laser marks having a height of 500 .ANG. or less, 
preferably, 300 .ANG. or less with a small dispersion and to obtain a 
texture constituted thereby. Although it is theoretically possible to form 
laser marks of a height of 250 .ANG. by suitably selecting an irradiated 
pulse energy in Comparative Example 1, there is a large change of the 
height of the laser marks in correspondence to a change of the irradiated 
energy. Accordingly, a change of the height of the laser marks becomes 
large in response to a slight change of the irradiated laser energy, and 
it is very difficult to obtain a texture in which the height of the laser 
marks is well controlled. 
Table 3 shows measured values, with use of an AFM (atomic force 
micrometer), of the height (.ANG.) of 6 laser marks which are optionally 
selected in each of Examples 1, 2 and 3 and Comparative Examples 1 and 3 
in Table 2, and each dispersion thereof. In Examples 1, 2 and 3, there was 
found that control of the height was so good that the dispersion of the 
height is small. On the other hand, in Comparative Examples 1 and 3, there 
was a very large dispersion of the height. In Comparative Example 1, in 
particular, as an increase of the height of the laser marks in response to 
an increase of the pulse energy is steep, adjustment of the pulsed energy 
was too difficult to form laser marks of a height of 500 .ANG. or less. In 
order to reduce a flying height of the head in response to an increase of 
the recording density of a magnetic recording medium, it is necessary to 
form laser marks in response to a required flying height for the head 
wherein a dispersion of the height is small as possible. It is necessary 
that .sigma. is within 5%, desirably within 3% of the average value of the 
height H where .sigma. represents a dispersion of the height. 
As shown in Table 2, in Examples 1, 2 and 3, E.sub.v /(D/20).sup.2 is 
larger than 0.5 .mu.J and smaller than 6 .mu.J. Further, as shown in Table 
3, the standard deviation a of the height H of the laser marks is within 
5% with respect to the average value of H. Especially, in Examples 2 and 
3, the standard deviation is within 3%. 
TABLE 3 
______________________________________ 
Comparative 
Example Examples 
1 2 3 1 3 
______________________________________ 
Pulsed energy (.mu.J) 
0.85 0.52 0.52 9.9 1.26 
Height 
Measured 1 476 258 225 2622 522 
of volume 2 489 249 238 1984 485 
laser 3 471 251 232 3536 415 
marks 4 446 267 225 4678 448 
(.ANG.) 5 436 259 237 2755 435 
6 486 262 223 2047 465 
Average value 
467 258 230 2934 462 
Maximum 489 267 7238 4678 522 
value 
Minimum 436 249 223 1964 415 
value 
.sigma. 22 7 7 1026 38 
.sigma./Average 
4.6 2.7 2.9 35 8.2 
value 
______________________________________ 
Table 4 shows results of forming laser marks in substrates of Examples 4 
through 6 and Comparative Examples 4 and 5. A pulsed laser beam used was 
the 4th harmonics of a YAG laser (wavelength .lambda.=266 nm). 
The substrates used in Examples 4, 5 and 6 are respectively the same as 
those used in Examples 1, 2 and 3, and the substrates used in Comparative 
Examples 4 and 5 are respectively the same as those used in Comparative 
Examples 2 and 3. 
Process parameters in laser processing are as follows (only the waist 
diameter is different from that in Examples 1 through 3 and Comparative 
Examples 2 and 3): 
Pulse repetition rate=5 kHz, 
Laser pulse width=65 nsec, 
Focal distance of objective lens=150 mm, and 
Waist diameter D=20 .mu.m. 
TABLE 4 
______________________________________ 
E.sub.v E.sub.v /(D/20).sup.2 
H.sub.v E.sub.250 
.DELTA.E/E.sub.250 
.mu.J .mu.J .ANG. .mu.J 
% 
______________________________________ 
Example 4 
4.0 4.0 420 2.51 4.4 
Example 5 
2.2 2.2 300 2.05 9.2 
Example 6 
1.8 1.8 210 2.74 10.4 
Comparative 
16.9 16.9 3190 0.6 0.6 
Example 4 
Comparative 
8.6 8.6 1150 1.8 1.8 
Example 5 
______________________________________ 
In Table 4, Comparative Examples 4 and 5 show that there is a problem in 
controlling the height of the laser marks because .DELTA.E/E.sub.250 
values do not reach 2%. On the other hand, in Examples 4, 5 and 6, 
.DELTA.E/E.sub.250 values largely exceed 2%, and there is an improvement 
on controllability of the laser marks. Although the substrates of 
Comparative Examples 4 and 5 contain the same light absorbing agents as in 
Examples 4, 5 and 6, the content is insufficient, and which reveals that 
effect of the agents on the controllability of the height of the laser 
marks is insufficient. When laser marks having a height of 250 .ANG. are 
formed, a required irradiated pulsed energy in Example 6 is 2.7 .mu.J, 
which is beyond a pulsed energy value E.sub.v (1.8 .mu.J) for initiating 
vaporization. In this case, the shape of the laser marks of the 250 .ANG. 
shows the sink type as shown in FIG. 4. On the other hand, in Examples 4 
and 5, the irradiated pulsed energy for forming laser marks of 250 .ANG. 
does not exceed E.sub.v. In this case, the shape is the simple lifting 
type as shown in FIG. 3. 
Table 4 shows that in Examples 4, 5 and 6, E.sub.v /(D/20).sup.2 values are 
respectively larger than 0.5 .mu.J and smaller than 6 .mu.J. 
In the above-mentioned Examples, the 4th harmonics of a YAG laser 
(wavelength: 266 nm) is used as the laser beam. Since the absorption band 
of the ordinary glass material is in an ultraviolet region and an infrared 
region of more than 3 .mu.m, it is easier to increase the sensitivity to a 
laser beam having a wavelength falling in these regions. However, the 
present invention is not limited to using such laser beam, and any 
combination of a laser wavelength and the sensitivity of a substrate which 
realizes the condition of 0.5&lt;E.sub.v /(D/20).sup.2 &lt;6 .mu.J may be 
utilized. 
Table 5 shows a result of forming laser marks in Example 7 and Comparative 
Example 6. 
In Example 7 and Comparative Example 6, substrates made of glass ceramics 
(crystalline glass) which have different compositions from each other are 
used. The glass ceramics are such material that crystallized glass 
particles exist in amorphous glass. 
The pulsed laser beam used was the 4th harmonics of a YAG laser beam 
(wavelength .lambda.=266 nm). 
Process parameters in laser processing with the pulsed laser are as 
follows: 
Pulse repetition rate=5 kHz, 
Laser pulse width=65 nsec, 
Focal length of objective lens=150 mm, and 
Waist diameter D=20 .mu.m. 
TABLE 5 
______________________________________ 
E.sub.v E.sub.v /(D/20).sup.2 
H.sub.v E.sub.250 
.DELTA.E/E.sub.250 
.mu.J .mu.J .ANG. .mu.J 
% 
______________________________________ 
Example 7 
2.1 2.1 400 1.38 4.0 
Comparative 
20.0 20.0 1750 12.3 1.0 
Example 6 
______________________________________ 
In Table 5, since .DELTA.E/E.sub.250 is less than 2% in Comparative Example 
6, it is understood that there is a problem in controlling the height of 
the laser marks. On the other hand, in Example 7, .DELTA.E/E.sub.250 
largely exceeds 2%, and therefor, there is found an improvement of 
controllability of laser marks. The substrate of Comparative Example 6 
contains V.sub.2 O.sub.5 as a light absorbing agent. However, the content 
is not sufficient, and it is understood that the effect on controllability 
of the height of the laser marks is insufficient. Further, the substrate 
of Example 7 contains TiO.sub.2 as a light absorbing agent. In a case of 
forming laser marks of a height of 250 .ANG. in Example 7, a required 
irradiated pulse energy is 1.38 .mu.J which is less than E.sub.v (2.1 
.mu.J), the pulse energy for initiating vaporization. In this case, the 
shape of the laser marks of 250 .ANG. was of the simple lifting type as in 
FIG. 3. 
In Table 4, E.sub.v /(D/20).sup.2 in Example 7 is larger than 0.5 .mu.J and 
smaller than 6 .mu.J. From the above, an improvement in controllability of 
forming laser marks can be recognized even in a substrate made of a glass 
ceramics material as far as it satisfies 0.5 .mu.J&lt;E.sub.v /(D/20).sup.2 
&lt;6 .mu.J in the same manner as the glass material. 
According to the present invention, the following excellent advantages can 
be provided. 
(1) Laser marks constituting a texture can be formed with high precision so 
that the height falls in a small range (i.e., 500 .ANG. or less, 
preferably, 300 .ANG. or less) whereby a low flying height of a magnetic 
head is obtainable. 
(2) A dispersion of the height of laser marks constituting a texture can be 
suppressed to be small whereby a lower flying height of a magnetic head 
and durability in CSS can be simultaneously satisfied thereby improving in 
recording capacity. 
(3) With use of the substrate of the present invention, it is possible to 
use a laser beam of small power to thereby reduce cost and space in 
production line. 
(4) By increasing a pulse repetition rate, a time for forming the texture 
can be shortened to thereby increase productivity. However, when a Q 
switch pulse solid state laser such as YAG is used and if the pulse 
repetition rate is increased, energy per pulse is decreased, which makes 
it difficult to form laser marks. Use of the substrate according to the 
present invention makes it possible to form a texture with a lower energy. 
Accordingly, productivity can be increased by increasing the pulse 
repetition rate. 
(5) A tolerance to variation of an irradiated laser beam energy is large, 
and production in an industrial scale for magnetic recording medium is 
easy. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.