Magnetic disc with low-friction glass substrate

A thin-film medium formed on a low-friction glass substrate. The substrate is micro-roughened by plasma etching, under conditions which produce a desired surface density of zero crossings and depth of microscopic surface irregularities.

FIELD OF THE INVENTION 
The present invention relates to a thin-film magnetic disc having a glass 
substrate, and to a method of texturing the substrate to achieve a disc 
with a low-friction surface. 
BACKGROUND OF THE INVENTION 
Thin-film hard disc magnetic media are widely used for on-line data storage 
in computers. There are several magnetic and surface properties which are 
important to the recording density, as well as to disc lifetime and wear 
characteristics. The most important of these are: 
(1) Magnetic remanence, which determines the signal amplitude which can be 
read from an isolated pulse stored in the medium--the greater the 
remanence, the greater the signal amplitude which can be detected in a 
reading operation. 
(2) Coercivity, defined as the magnetic field required to reduce the 
remanence magnetic flux to 0, i.e., the field required to erase a stored 
bit of information. Higher coercivity in a medium allows adjacent recorded 
bits to be placed more closely together without mutual cancellation. Thus, 
higher coercivity is associated with higher information storage density. 
(3) Bit shift or peak shift, a phenomenon which refers to the broadening 
between voltage peaks which occurs in the read voltage waveform It is 
desired to achieve low bit shifting, inasmuch as bit shifting limits the 
resolution at which adjacent peaks can be read, and thus places an upper 
limit on recording density. 
(4) Flying height, i.e., the distance which a read/write head floats above 
the spinning disc. Less overlap of voltage signals in adjacent magnetic 
domains in the disc occurs as the read/write head is moved closer to the 
disc surface, allowing recording density to be increased. The flying 
height is limited principally by surface irregularities in the disc. 
(5) Surface imperfections and irregularities. Such can lead to excessive 
interactions between the disc and the head, and limit the flying height of 
the head which can be safely employed. 
(6) Stiction, or static friction, defined as the frictional contact between 
the disc and read/write head when the head is parked on the disc and the 
disc first begins to rotate. Low stiction reduces wear on the disc and 
head with repeated stop/start operations. 
(7) Surface durability, which provides increased disc lifetime and reduced 
head wear. 
Heretofore, discs having high coercivity and remanence characteristics have 
been prepared by sputtering a thin magnetic film on a metal substrate, 
typically an aluminum substrate. Prior to sputtering, the substrate is 
textured by grinding, typically using a rotary abrasive pad placed off 
center with respect to the surface of the spinning substrate. The purpose 
of the texturing is create a roughened surface characterized by submicron 
surface irregularities. The roughened surface reduces stiction between the 
disc and head by reducing surface contact between the two. 
Before texturing, the metal substrate is plated with a selected alloy, such 
as nickel/phosphorus, to achieve a requisite surface hardness. The plated 
disc is then polished to remove surface nodules which form during the 
plating process. Because the nodules have varying degrees of hardness, the 
polishing step tends to leave surface irregularities in the form of 
surface depressions or mounds. 
The sputtering operation used to produce the thin magnetic film is 
preferably carried out by first sputtering a chromium underlayer onto the 
substrate surface, then sputtering a cobalt-based magnetic thin film over 
the underlayer. A protective, lubricating carbon overcoat may then be 
applied over the thin-film layer by sputtering. The resulting disc can 
have high coercivity and remanence properties, as described in co-owned 
U.S. Pat. No. 4,816,127, and good wear and lubricity properties, as 
described in co-owned U.S. patent applications Ser. No. 341,550 filed Apr. 
21, 1989, and Ser. No. 341,705, filed Apr. 21, 1989. 
Despite the favorable magnetic and surface-wear properties which can be 
achieved in the above-described thin-film magnetic disc formed on a metal 
substrate, the recording density of the disc is limited in flying height 
by the irregularities on the disc surface (due to surface irregularities 
in the plated metal substrate). The best flying head distances which have 
been achieved with metal-substrate discs is about 6 .mu.inches. 
It is possible to reduce flying height, and therefore increase recording 
density, by forming a thin-film magnetic layer on a smooth-surfaced 
substrate, such as a highly polished glass or ceramic substrate. However, 
this approach has generally been limited by difficulties which have been 
encountered in (a) achieving a high-coercivity, high-remanence, low bit 
shift magnetic film on a glass substrate by sputtering, and (b) texturing 
the substrate to produce a low-stiction disc surface The former limitation 
has been addressed in co-owned U.S. patent application Ser. No. 408,655 
filed Sept. 19, 1989. The second limitation is the subject of the present 
application. 
SUMMARY OF THE INVENTION 
It is therefore one general object of the present invention to provide a 
method of texturing the surface of a glass substrate which can be 
controlled to achieve a selected surface density of nm-range surface 
irregularities, preferably with a selected depth of irregularities, which 
have been found to be necessary to produce a minimum coefficient of static 
friction, or stiction, as measured with a read/write head placed against 
the surface of a thin-film disc. 
The invention includes, in one aspect, a method of microroughening the 
surface of a glass substrate to achieve a low coefficient of static 
friction in a thin-film disc. The method employs plasma etching to create 
the desired nm-range surface irregularities in the glass surface. The 
plasma is produced by radio frequency glow discharge of an etchant gas 
containing between 4 and 70 mole percent O.sub.2 and between 30 and 96 
mole percent of a normally inert halocarbon gas. The plasma is allowed to 
interact with the glass surface for a period sufficient to produce a 
surface density of zero crossings, measured along any axis in the surface 
plane, of at least about 40/mm. 
In a preferred embodiment, the gas plasma is further allowed to interact 
with the glass surface for a period sufficient to produce a peak-to-valley 
distance, defined as the maximum depth of the nm-range irregularities on 
the surface of glass, of at least about 12-13 nm, i.e., about 0.5 
.mu.inch. 
The method is preferably carried out under conditions such that the desired 
density of surface irregularities and the depth of irregularities is 
achieved after about 3-30 minutes, and more typically 3-15 minutes of 
exposure of the glass surface to the plasma. The RF power level is between 
about 300-600 watts. 
Also forming part of the invention is a glass substrate formed by the 
method of the invention. The substrate is characterized by a surface 
density of nm-range surface irregularities, measured along any axis in the 
surface plane, of at least about 40/mm. The depth of the surface 
irregularities is preferably such that the surface of the thin-film disc, 
after sputtering a thin-film magnetic layer and a carbon overcoat on the 
glass surface, of at least about 12-13 nm. 
In yet another aspect, the invention includes a thin-film magnetic disc 
composed of (a) the above micro-roughened glass substrate, (b) a chromium 
underlayer formed on the substrate surface, (c) a thin-film magnetic layer 
formed on the underlayer, and (d) a carbon overcoat. The magnetic disc 
preferably has a coefficient of static friction, as determined by the 
friction force in a mini-composite 10 g read/write head placed on the 
disc, when rotation of the disc is initiated, between about 2-3 grams. 
These and other objects and features of the present invention will become 
more fully apparent when the following detailed description of the 
invention is read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
Details of the glass micro-roughening method of the invention are given in 
Section I below. The surface features of the micro-roughened glass surface 
can be characterized by a surface density of zero crossings and a 
peak-to-valley depth of nm-range irregularities, and these parameters can 
be controlled to minimize the static frictional coefficient of a thin-film 
medium formed on the micro-roughened substrate, as described in Section 
II. Section III describes a thin-film magnetic disc formed by sputtering a 
magnetic thin film on the substrate, and characterized by a low static 
coefficient of friction. 
I. Glass Micro-Roughening Method 
FIGS. 1 and 2 show, in schematic view, an plasma etching apparatus 10 which 
is suitable for use in practicing the glass substrate micro-roughening 
method of the invention. The apparatus includes an enclosed, 
pressure-tight chamber 12 having conductive inner walls 11. The apparatus 
includes gas-flow and pressure controls (not shown) for maintaining a 
selected pressure of a given gas composition introduced into the chamber. 
A metallic fixture 16 in the chamber has a mandrel 18 constructed to 
support glass substrates, such as substrate 20, with both sides of each 
substrate exposed. The fixture may be insulative or formed of a conductive 
material such as aluminum. The apparatus also includes a radio-frequency 
(RF) alternating current power source 22 which can be adjusted to a 
selected power level and automatically matched to the impedance of the 
reactor. 
The configuration of the apparatus shown in FIG. 1 is a "cage" 
configuration in which the substrates are enclosed within a metal cage 14 
whose sides are formed of a conductive-metal screen which allows plasma 
gas formed within the chamber to pass freely into and out of the interior 
of the cage. One of the RF electrodes in the apparatus is connected to 
cage 14, and the other is connected to the conductive walls of the chamber 
and is grounded, as shown. With application of a radio frequency voltage 
across the electrodes, in the presence of an ionizing gas, a primary 
plasma or glow discharge is formed in the chamber region 12a outside cage 
14, such that the glass substrate is exposed to a secondary plasma etching 
effect. 
The configuration of the apparatus shown in FIG. 2 is a "shelf" 
configuration in which the substrates are positioned between two metallic 
shelves 24, 25. The anodic and cathodic electrodes of the power source are 
connected to the two shelves, as shown. With application of a radio 
frequency voltage across the two electrodes, in the presence of an 
ionizing gas, a primary plasma or glow discharge is formed in the entire 
chamber, such that the glass substrate is directly exposed to a primary 
plasma etching effect. 
One plasma etching apparatus which is suitable for use in the present 
invention is a Branson Model 4055 gas plasma system. 
The glass substrate used in the invention is preferably a highly polished 
sodalime or aluminosilicate glass substrate having conventional thin-film 
disc dimensions, typically about 0.05 inches thick and 3.5 inches in 
diameter. Smooth-surfaced substrates of this type are available from Hoya 
(Japan), Graverbel (Belgium), and Pilkington (United Kingdom). Example 1 
below describes the micro-roughening method applied to both sodalime glass 
and aluminosilicate glass. 
The substrate is placed within the apparatus 10, and the chamber of the 
apparatus is filled with a gas capable of forming a glow discharge plasma 
in a radio-frequency field. The gas is preferably composed of between 
about 4-70 mole percent oxygen and between about 30-96 mole percent of a 
halocarbon gas. One preferred gas composition, employed in Example 1B, 
contains 96 mole percent halocarbon and 4 mole percent O.sub.2. 
One preferred halocarbon gas is carbon tetrafluoride, CF.sub.4, which is a 
normally inert gas that does not react directly with Si at any temperature 
up to the CF.sub.4 boiling point of T=1685.degree. K. However, if an 
electrical discharge is initiated in CF.sub.4, atomic fluorine is produced 
as a reaction product (shown in the reactions below), and the activated 
fluorine atoms react spontaneously with the silicon at room temperature 
and above to form a volatile gas, SiF.sub.4, which will not be decomposed 
in the plasma. Other replacements for CF.sub.4 are C.sub.2 F.sub.6, 
CHF.sub.3, C.sub.3 F.sub.8, CF.sub.4 +H.sub.2 and SF.sub.4. 
Possible ionization and etching reaction which may occur in an O.sub.2 /CF4 
gas mixture are shown below. 
EQU O.sub.2 e.sup.- .fwdarw.O++O*+e.sup.-, 
EQU CF.sub.4 +e.sup.- .fwdarw.CF.sub.3 +F*+e.sup.-, 
EQU CF.sub.4 +e.sup.- .fwdarw.CF.sub.n *+(4-n)F*-+ne.sup.- (n=1,2,3,) 
EQU Si-O+e.sup.- .fwdarw.Si*+O+e.sup.-, 
EQU Si*+4F.fwdarw.SiF.sub.4 (volatile). 
As seen from the reactions above, electrons produced by ionization of both 
O.sub.2 and the halogenated gas produce halogen radicals (F.sup.*) which 
react with Si atom on the glass surface to remove Si atoms by 
volatilization of SiF.sub.4 product formed on the surface. 
More generally, the gas composition used in the present invention is 
capable, under ionization conditions, of reacting with free Si atoms on 
the glass surface, to scavenge the free Si and to produce a volatile 
product gas, such as SiF.sub.4, that is unlikely to redeposit on the 
adjacent glass surface. 
By contrast, processes such as reactive ion beam etching (RIBE) and ion 
milling are unsuitable for this purpose because they produce non-volatile 
reaction products that permit redeposition and trenching to occur, as 
discussed in S. Wolf and R. H. Tauber in Silicon Processing for the VLSI 
Era, Vol. I. Lattice Press, 1986, pp. 539-585). 
For O.sub.2 present in the gas composition, the etch rate generally 
increases up to about 20 mole percent, beyond which a decrease in etch 
rate may be observed. See S. M. Sze (editor), VLSI Technology, McGraw-Hill 
Book Co., 198, Chap. 8 by C. J. Mogab, pp. 324-326.). 
The plasma etch process uses moderately high gas pressures, of the order of 
0.2-1 Torr, preferably about 0.4 Torr, at room temperature, which allows a 
relatively isotropic etch process to occur at the surface of the glass. 
The RF power source is set to a power level sufficient to produce plasma 
glow discharge within the chamber, and preferably at a power level between 
about 300-600 watts. These settings apply to both the cage and shelf 
configurations, although higher power levels may be required in the cage 
configuration, for comparable etch times. 
According to an important feature of the invention, the plasma etching is 
carried out for a period which is sufficient to achieve desired surface 
characteristics related to (a) density of zero crossings and (b) 
peak-to-valley distances of nm-range irregularities formed on the surface 
by the plasma etching process. 
These surface parameters, which will be described in detail in Section II 
below, can be appreciated from the surface irregularity changes which 
occur during plasma etching, as illustrated in FIGS. 3A-3C. The 
microscopic relief of a highly polished glass substrate surface prior to 
treatment is seen in FIG. 3A. With isotropic plasma etching, nm-range 
surface irregularities, such as seen at 26, 28 in FIG. 3B, are produced. 
During the initial stages of etching the surface irregularities show a 
time-dependent increase in peak-to-valley depth and number of crossings 
through a centerline, shown in dotted line at 30 in FIG. 3B. The 
centerline is the mean "depth" at which the integrated peak and valley 
areas are equal. The position of this line is also shown in FIG. 3A and 
3C. 
With continued etching, the peaks of the irregularities become etched 
preferentially, producing the more flattened surface irregularities seen 
in FIG. 3C. The surface irregularities in this figure have lower 
peak-to-valley depths and a lower density of zero-crossings. 
The time of exposure needed to achieve the desired zero-crossing and 
peak-to-valley depth will depend on gas pressure and composition, RF power 
level in the plasma chamber, and whether a shelf or cage configuration is 
employed. At the gas pressure, composition, and power settings described 
above, typical etching times are between about 3-30 minutes, and 
preferably between about 3-15 minutes. As noted above, optimal plasma etch 
times for the shelf configuration (where the substrate is exposed to the 
primary plasma) are generally shorter than those for the cage 
configuration, at comparable gas composition and power conditions. 
The optimal time of etching can be determined readily for any specified gas 
composition and power settings. This is done by exposing a series of 
identical glass substrates to the selected plasma etch conditions for 
increasing time increments of, for example, 5 minute, for a 3-30 minute 
interval. The zero-crossing and peak-to-valley values of each substrate 
are then determined, as described in Section II for each substrate or for 
a thin-film medium formed on the substrate. After identifying the plasma 
etch time which gives highest zero-crossing and/or peak-to-valley values, 
the exposure time can be refined by looking at, for example, a number of 1 
minute incremental times on either side of the previously selected etching 
time. The examples below illustrate plasma etching times employed under a 
variety of etching conditions, and the surface properties which were 
observed under each set of conditions. 
II. Low-Friction Surface Parameters 
As noted above, FIG. 3B shows a representative cross-sectional view of a 
surface at the sub-micron level, after a selected period of plasma 
etching. 
One surface parameter which was examined is the linear density of zero 
crossings, Z.sub.c. Quantitatively, Z.sub.c is defined by 
EQU Z.sub.C =(N.sub.max +N.sub.min)/L 
Where N.sub.max and N.sub.min are the number of peaks and valleys, 
respectively, over a length L along any axis. 
A second surface parameter is the distance, normal to the surface, between 
the highest peak and the lowest valley within a given area, e.g., 250 
.mu..sup.2, of the surface. 
A third surface parameter which may be measured is the surface density of 
summits, defined simply as the number of peaks of surface irregularities, 
such as irregularities 26, 28 in FIG. 3B, over a given area or along a 
given length L of a surface axis. Since a surface irregularity may not 
necessarily cross the center line, the linear density of summits will 
generally be greater than the linear density of zero-crossings. 
The three surface parameters above can be measured by interferometer, in 
which the heights at many positions over the surface of the substrate is 
measured, and these coordinates are used to construct a three-dimensional 
topographic map of the surface. The coordinates are then used to calculate 
(a) arithmetic mean roughness, (b) linear density of zero-crossings, (c) 
maximum peak-to-valley depth, and (d) surface area density of summits. 
The interferometry measurements and calculations can be performed by 
commercially available interferometers, equipped with known microcomputer 
capability for calculating the above four surface parameters. One 
interferometer which is suitable for this purpose is a phase-shifting 
interferometer, Model TOPO-3D by WYKO Co. (Tuscon, Ariz.). 
Since much of the light which is directed against the glass substrate 
surface in an interferometry measurement is lost as transmitted light, the 
interferometer typically must be equipped with sensitive optics, in order 
to obtain reliable surface height measurements. This sensitivity can be 
achieved in the above interferometer. 
Alternatively, the surface features can be measured by metallizing the 
surface, for example, with a thin sputtered metal layer before making the 
surface measurements. One standard approach which has been used herein 
involves forming a thin-film magnetic disc, including a carbon overcoat, 
on the glass substrate, and making the surface measurements on the 
finished disc. One preferred disc, and sputtering method for making the 
disc, are described in Section III below. This approach to determining the 
surface features of a substrate assumes that (a) the surface features of 
the substrate are substantially preserved in the sputtered layers, and (b) 
the semi-transparent carbon overcoat does not significantly alter the 
surface height measurements made by interferometry. The advantage of this 
approach, as will now be seen, is that surface parameters related to the 
glass substrate can be directly correlated with frictional coefficient 
values measured on a coated magnetic thin film disc. 
Glass substrates were micro-roughened by plasma etching under various gas 
composition, gas pressure, RF power level and exposure time conditions, on 
sodalime or aluminosilicate glass substrates. After plasma etching, the 
substrates were sputtered to form, successively, a chromium underlayer, a 
cobalt-based magnetic thin film, and a carbon overcoat, according to the 
sputtering methods detailed in Section III. Each disc was then examined by 
interferometry, as above, to determine mean surface roughness height, 
linear density of zero crossings, peak-to-valley depth, and density of 
summits on the disc surface. 
FIG. 6 is a schematic view of a device 35 used in measuring the static 
coefficient of friction of a carbon-overcoated surface on a disc, such as 
a disc 32. The device used in this test is a "Dysan" tester for measuring 
the stiction and friction coefficient of a disc. 
Briefly, the device includes a motor-driven rotor, indicated at 34, for 
rotating the disc at a low speed, typically about 1 rpm. A standard 
10-gram mini-composite read/write head 36 can be switched between a 
contact position (solid lines) and a non-contact or unloaded position 
(dotted lines) with respect to the disc. This head, in turn, is coupled to 
a force transducer 38 which measures the force F (in the circumferential 
direction) applied to the head upon rotation of the disc. 
FIG. 4 shows a scatter plot of stiction coefficient measurements (expressed 
in grams of force applied to transducer 38) as a function of linear 
density of zero crossings, based on stiction coefficient and zero-crossing 
measurements made on the several discs whose substrates were prepared 
under various plasma etching conditions, as described above. The points 
numbered 1-4 in the figure (and in FIG. 5), correspond to discs whose 
substrates were prepared as described Examples 1A, 1B, 2, and 1C, 
respectively. In the range between 20-40 zero-crossings, there is a 
general trend toward lower striction coefficient with higher linear 
density of zero crossings, but also considerable scatter in this area. In 
the range above about 40/mm, there is a strong correlation between linear 
density of zero crossings and low stiction coefficient, with values of 
about 2 grams. 
FIG. 5 shows a scatter plot of stiction coefficient measurements as a 
function of peak-to-valley depth, for the same discs. In the range less 
than about 12-13 nm peak-to-valley depth, there is a general trend toward 
lower stiction coefficient with greater depth, but also considerable 
scatter in this area. In the range above 12-13 nm, there is a strong 
correlation between peak-to-valley depth and low stiction coefficient, 
again with values of about 2 grams. 
A similar scatter plot of stiction coefficient as a function of summit 
density (not shown) gave no clear correlation between density of summits 
and stiction coefficient. 
It will be appreciated from the foregoing that the substrate 
micro-roughening method of the invention is carried out under conditions 
which produce a linear density of zero-crossings in the nm-range 
irregularities on the disc surface of at least 40/mm, as measured either 
in the substrate or a thin-film disc formed on the substrate. In a 
preferred embodiment of the invention, the etching conditions are also 
selected to produce a peak-to-valley depth of the surface irregularities 
of at least about 12-13 nm. 
III. Thin-Film Disc 
FIG. 7 shows an enlarged, fragmentary cross-sectional view of a thin-film 
magnetic disc 40 formed in accordance with the invention. The disc 
generally includes a glass substrate 42 which has been micro-roughened as 
above to have a linear density of zero crossings of at least about 40/mm, 
and a peak-to-valley depth of at least about 12-13 nm. 
The three layers formed on the substrate include a chromium underlayer 43, 
a thin-film magnetic layer 44, and a carbon overcoat 46. These layers are 
preferably formed by sputtering, according to known methods (e.g., U.S. 
Pat. No. 4,816,127). Briefly, the substrate is placed in a conventional 
sputtering apparatus and moved through a succession of sputtering chambers 
designed for sputtering onto the substrate (a) a chromium underlayer, to a 
thickness of about 400 to 4,000 .ANG. (b) a thin-film magnetic layer, to a 
thickness of about 300-1,500 .ANG., and a carbon overcoat of about 250-600 
.ANG.. The chromium underlayer is preferably deposited in a two-step 
sputtering operation, as described in the above-noted U.S. patent 
application for "High-Coercivity Disc with Glass Substrate". The thin-film 
layer is preferably a cobalt-based alloy containing, in one embodiment, 
1-10% tantalum, 10-16% chromium, and 60-85% cobalt, and in another 
embodiment, 2-10% chromium, 20-28% nickel, and 70-88% cobalt. 
The nm-range surface characteristics of the disc were examined by 
interferometry and for stiction coefficient, as described above. As 
detailed above, low stiction coefficients (less than about 2 grams) were 
observed for all discs in which the linear density of zero crossings was 
greater than about 40/mm, and where the peak-to-valley height was greater 
than about 12-13 nm. 
From the foregoing, it can be appreciated how various objects and features 
of the invention have been met. The glass micro-roughening method is 
rapid, controllable, according to selected etching conditions, and 
reproducible. In particular, the plasma etching conditions can be selected 
to achieve zero-crossing and maximum peak-to-valley depth which insure low 
stiction. 
The following examples illustrate glass surface microroughening under a 
variety of substrate, gas composition, pressure, power, and etching time 
conditions, and the surface characteristics which were achieved under the 
various conditions. The examples are intended to illustrate, but in no way 
limit, the method of the invention. 
EXAMPLE 1 
Plasma Etching in a Shelf Configuration 
A group of glass substrates were placed in the shelf configuration on a 
fixture within the chamber of a Branson Model 4055 gas plasma system 
(Hayward, Cal.). The RF power supply in the machine was connected between 
the cage and the interior walls of the chamber. Thus the primary plasma 
was formed in the region outside the cage. 
A. Sodalime glass substrate 
95 mm sodalime glass substrates, 1.27 mm thick, were obtained from 
Graverbel (Belgium). The discs were placed on the shelf of the etching 
machine, and the chamber was filled with an etchant gas containing 50 mole 
percent each of CF.sub.4 and O.sub.2, at a total pressure of 0.4 Torr. The 
RF power supply was set 500 Watts, and the plasma formed in the chamber 
was allowed to interact with the glass substrate for 10 minutes. 
The glass substrates were used in forming thin-film media by sputtering a 
chromium underlayer on each glass substrate, to a final underlayer 
thickness of about 500 .ANG., and a thin-film magnetic disc to a final 
magnetic layer thickness of about 500 .ANG.. A carbon overcoat having a 
final thickness of about 300 .ANG. was sputtered over the magnetic layer. 
The sputtering method for forming the disc generally follows the procedure 
described above. 
The depth of density of nm-range irregularities in the disc surfaces were 
determined by interferometry, using a WYKO interferometer TOPO-3D. The 
average values calculated for the disc reflecting the depth and density of 
nm-range surface irregularities on the glass substrates were: linear 
zero-crossing density of 24/mm and maximum depth peak-to-valley of about 
9.8 nm. 
The static coefficient of friction of the thin-film disc was measured using 
a standard ten-gram mini-composite head. The force exerted on the head as 
the discs were rotated from a stationary position was measured at 9 grams, 
given a stiction coefficient defined as the observed force/load on the 
head equal to 0.9. The point for the group of discs is indicated by 1 in 
FIGS. 4 and 5. 
B. A group of 95 mm sodalime glass discs was obtained from Pilkington 
(United Kingdom). Plasma etching conditions were identical to those used 
in Example 1A. A thin-film medium having a carbon overcoat was formed on 
the substrate as above, and the density and depth of nm-range surface 
irregularities were measured by interferometry, as above. The linear 
zero-crossing density of nm-range irregularities at the disc surface was 
measured at 22/mm, with a maximum peak-to-valley depth of about 12 nm. The 
coefficient of static friction, measured as above, was about 0.49. This 
disc group is represented by point 2 in the FIG. 4 and FIG. 5 plots 
C. Aluminosilicate Glass 
A group of 130 mm aluminosilicate glass discs were obtained from Hoya 
(Japan). Plasma etching conditions were similar to those in Example 1A, 
except that the gas mixture contained 96 mole percent CF.sub.4 and 4 mole 
percent O.sub.2, and the total etching time was 5-15 minutes. 
A thin-film medium having a carbon overcoat was formed on the substrate as 
above, and the density and maximum peak-to-valley depth of nm-range 
surface irregularities were measured by interferometry, as above. The 
average value linear zero-crossing density of nm-range irregularities at 
the disc surfaces was measured at 43/mm, with an average maximum 
peak-to-valley depth of about 19 mm. The average coefficient of static 
friction, measured as above, was about 0.22. This group of discs is 
represented by point 4 in the FIG. 4 and FIG. 5 plots. 
EXAMPLE 1 
Plasma Etching in a Cage Configuration 
A group of 95 mm sodalime glass discs was obtained from Pilkington (United 
Kingdom). Plasma etching was done in a larger system with a barrel-type 
chamber (Model 7150) in a cage configuration. The plasma condition used 
was: (a) the etchant gas contained 30 mole percent CF and 70 mole percent 
O.sub.2 at a total pressure of 0.3 Torr, (b) the RF power source was 
adjusted to a power output of 750 watts, and (c) plasma etching was 
carried out for 10 minutes. 
A thin-film medium having a carbon overcoat was formed on each of the 
substrates as above, and the density and depth of nm-range surface 
irregularities were measured by interferometry, as above. The linear 
zero-crossing density of nm-range irregularities at the disc surface was 
measured at 44/mm, with a maximum peak-to-valley depth of about 18 nm. The 
coefficient of static friction, measured as above, was about 0.26. This 
disc group is represented by point 3 in the FIG. 4 and FIG. 5 plots. 
Although preferred embodiments of the invention have been described, it 
will be apparent to those skilled in the art that various changes and 
modifications can be made without departing from the invention.