Fluorinated diamond-like carbon protective coating for magnetic recording media devices

An improved wear-resistant protective coating for the surfaces of magnetic recording media devices that is formed of fluorinated diamond-like carbon and deposited by a plasma enhanced chemical vapor deposition process or other suitable methods to provide superior friction-reducing and stiction-reducing properties.

FIELD OF THE INVENTION 
The present invention generally relates to thin film magnetic recording 
media devices such as magnetic disks and magnetic heads and more 
particularly, relates to thin film magnetic recording media devices that 
have an improved wear-resistant, low friction and low stiction protective 
coating applied thereon and a method of applying such coating to magnetic 
recording media devices. 
BACKGROUND OF THE INVENTION 
In the design of thin film magnetic recording media devices, it is 
important to provide a protective coating on the uppermost surface of the 
device to assure durability and reliability. Hydrogenated diamond-like 
carbon (DLC) is a hard, wear-resistant material that has a relatively low 
friction coefficient. It has been used as a protective coating in magnetic 
recording media devices such as thin film magnetic disks and magnetic 
recording heads. 
For instance, U.S. Pat. No. 4,647,494 and U.S. Pat. No. 5,159,508 disclose 
the coating of a thin layer of hydrogenated carbon film onto a magnetic 
recording disk and a magnetic head slider, respectively. However, the 
method disclosed by both patents requires the application of an additional 
adhesion promoter layer onto the substrate before the final coating of the 
hydrogenated carbon can be applied. The patents therefore describe a 
two-stage deposition process. The tribological performance of these 
devices must be improved through the use of liquid lubricants on the 
surface of the protective DLC coating. In a modem recording device with 
reduced head-to-disk distance for increased recording densities, 
elimination of the extra lubricant layer is desirable. The removal of the 
liquid lubricant is also desirable for the elimination of capillary forces 
and meniscus formation which can cause increased stiction. The application 
of liquid lubricants to magnetic disk surfaces requires several processing 
steps, the elimination therefore reduces the manufacturing costs of such 
disks. 
One method to eliminate the need of liquid lubricant is to further reduce 
the friction coefficient of the DLC coating. Such a method is disclosed by 
Miyake et al., in J. Tribol. Trans. ASME 113 (1991) 384. Approximately one 
micron thick of silicon-containing carbon films are first deposited by 
electron cyclotron resonance deposition and then the specimen surface is 
fluorinated by exposure to a CF.sub.4 plasma. It was shown that surface 
fluorination of DLC can reduce the friction and microwear of DLC films. 
Since the fluorination process is performed subsequent to the deposition 
of the DLC film, the fluorination is limited to the uppermost layer of the 
coating. As the wear of the coating removes the fluorinated layer, its 
lubricating advantage is lost after a relatively short wear time. The 
advantage of fluorination could be extended if it occurs throughout the 
entire thickness of the protective layer to maintain wear resistance. 
Fluorinated DLC films have been previously prepared by others. For 
instance, Seth et al., reported in Thin Solid Films, 230 (1993) 90 that 
high fluorine content of films leads to a large drop in density which 
indicates a comparatively open structure of the films. The films were 
found to be extremely soft and had no wear-resistance. 
It is therefore an object of the present invention to provide a fluorinated 
DLC protective coating for magnetic recording media devices that does not 
have the shortcomings of other conventional protective coatings. 
It is another object of the present invention to provide a fluorinated DLC 
protective coating for magnetic recording media devices that does not 
require the use of additional liquid lubricants on its surface in order to 
provide adequate wear-resistance. 
It is a further object of the present invention to provide a fluorinated 
DLC protective coating for magnetic recording media devices that has 
superior wear-resistance throughout its entire coating thickness such that 
its wear-resistance property does not deteriorate with the wear of the 
uppermost layer. 
It is another further object of the present invention to provide a 
fluorinated DLC protective coating for magnetic recording media devices 
that can be applied directly to the uppermost surface of the devices 
without an intermediate adhesion promoter layer. 
It is yet another object of the present invention to provide a fluorinated 
DLC protective coating for magnetic recording media devices that has 
superior wear-resistance and reduced stiction properties. 
It is yet another further object of the present invention to provide a 
fluorinated DLC protective coating for magnetic recording media devices 
that can be deposited in a plasma enhanced chemical vapor deposition 
chamber at temperatures below 250.degree. C. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a fluorinated diamond-like carbon 
protective coating for magnetic recording media devices that can be 
applied by a deposition method of those used in depositing diamond-like 
carbon films is provided. 
In the preferred embodiment, magnetic recording disks or heads are coated 
with a hard coating of fluorinated diamond-like carbon (FDLC) to provide 
wear-resistance and reduced friction and stiction. The FDLC films are 
prepared by plasma enhanced chemical vapor deposition (PECVD) on 
negatively biased substrates from mixtures of fluorinated hydrocarbons 
with hydrogen, preferably from fluorinated hydrocarbons with a large ratio 
of fluorine to carbon in the molecule such as hexafluorobenzene (C.sub.6 
F.sub.6) or pentafluorobenzene (C.sub.6 HF.sub.5). Fluorinated carbon 
films having superior wear-resistance can be obtained through the right 
combination of a reactant gas mixture, plasma parameters and a bias 
voltage of the substrate. Since the films are fluorinated throughout the 
entire thickness, removal of surface layers through wear does not change 
the composition and the tribological properties of the protective film. 
In an alternate embodiment, the FDLC film is deposited in combination with 
a thin intermediate layer of non-fluorinated diamond-like carbon (DLC). 
For instance, the thickness of the DLC layer can be approximately 4 nm 
combined with a 6 nm thickness of the top FDLC layer. 
The present invention is further directed to a method of depositing 
fluorinated diamond-like carbon protective coatings or two-stage DLC/FDLC 
coatings on magnetic recording media devices by a plasma enhanced chemical 
vapor deposition technique.

DETAILED DESCRIPTION OF THE PREFERRED AND THE ALTERNATE EMBODIMENTS 
The present invention provides a fluorinated diamond-like carbon protective 
coating for magnetic recording media devices that can be deposited by a 
plasma enhanced chemical vapor deposition method. 
After the surface of a magnetic recording device is first prepared for 
coating, the device is loaded into a parallel plate plasma reactor and 
electrically connected to become one of the electrodes. After the reactor 
is pumped to a preset negative pressure, a suitable mixture of reactant 
gases of fluorinated hydrocarbon vapors and hydrogen is flowed into the 
reactor, while the pressure inside the reactor is controlled at a 
desirable value within the range between 30 mTorr and 300 mTorr. A DC or 
RF power is then applied to the electrodes of the reactor to ignite a 
plasma such that the device to be coated becomes negatively biased 
relatively to ground or to other parts of the reactor. The device is kept 
in the plasma until the required thickness of coating is obtained. A 
desirable coating thickness is in the range between 3 nm and 30 nm. 
Referring initially to FIG. 1, where a schematic of an enlarged 
cross-sectional view of the upper layer 12 of a recording device 10 is 
covered by a protective layer of wear-resistant FDLC 14. In a preferred 
embodiment of the invention, the thickness of the single FDLC layer 14 is 
approximately 10 nm. The film is deposited by a reactant gas mixture of 
C.sub.6 F.sub.6 at a flow rate of 0.8 sccm and H.sub.2 at a flow rate of 
16 sccm. The chamber pressure during the reaction is maintained at 100 
mTorr and the plasma is sustained with a DC power supply operated under 
voltage control. The device is connected to a bias voltage of -800 V DC. 
An alternate embodiment of the present invention is shown in FIG. 2. A 
schematic of an enlarged cross-sectional view of the upper layer 22 of a 
recording device 20 is coated by an intermediate layer 24 of 
wear-resistant DLC and an upper layer 26 of wear-resistant FDLC. In this 
embodiment, the thickness of the DLC layer 24 is approximately 4 nm 
combined with a thickness of the FDLC layer 26 of approximately 6 nm. In 
this alternate construction of the protective coating layers, a suitable 
thickness for the DLC layer is in the range between about 2 nm to 10 nm, 
while a suitable thickness for the top FDLC layer is in the range between 
3 nm and 30 nm. The intermediate layer 24 of DLC can be deposited at 
similar conditions as that used in the deposition of the FDLC layer but 
replacing the gas mixture with 10 sccm of cyclohexane (C.sub.6 H.sub.12). 
FIG. 3 shows a schematic of an enlarged cross-sectional view of a magnetic 
recording disk 30 having a disk substrate 32 coated by a magnetic layer 34 
which is in turn coated by the present invention FDLC layer 36. FIG. 4 
shows a schematic of an enlarged cross-sectional view of a magnetic 
recording head 40 having a support section 42 including active read or 
write devices 44 that are coated by a present invention FDLC protective 
coating layer 46. 
In another example of the deposition of FDLC films, the deposition process 
is carried out by using a reactant gas mixture of C.sub.6 F.sub.6 at a 
flow rate of 0.8 sccm and H.sub.2 at a flow rate of 16 sccm. The chamber 
pressure is maintained during the reaction at 30 mTorr while an RF power 
of 50 watts is applied to the electrode holding the device to be coated 
such that a bias voltage of about -350 V DC is obtained. A DLC coating can 
be deposited under similar conditions by replacing the gas mixture with 10 
sccm of cyclohexane (C.sub.6 H.sub.12). 
The deposition temperature used in all examples is maintained at below 
250.degree. C. Under these deposition conditions, a deposition rate up to 
70 nm/min can be obtained. 
The protective films have been wear tested against a steel ball in a 
pin-on-disk tribotester. It was found that the wear resistance of the FDLC 
films is similar to that of non-fluorinated DLC films. For instance, using 
a pin made of 410C ball bearing steel of a diameter of 0.8 cm at a load of 
11 gm, the depth wear rate is 0.07 to 0.5 nm/thousand rotations. However, 
the stiction property of the FDLC film is reduced compared to that of the 
DLC film, i.e. about 30% of that of the non-fluorinated DLC. 
While the present invention has been described in an illustrative manner, 
it should be understood that the terminology used is intended to be in a 
nature of words of description rather than of limitation. 
Furthermore, while the present invention has been described in terms of a 
preferred and an alternate embodiment thereof, it is to be appreciated 
that those skilled in the art will readily apply these teachings to other 
possible variations of the invention. For instance, other layer 
constructions of the DLC and FDLC films may be used and other coating 
methods may be employed to deposit the films while substantially achieving 
the same desirable results of the present invention.