Process for coating a substrate with an organic coating having gradated hardness

In an apparatus for coating a substrate with an organic substance by placing the substrate in a coating chamber containing a gaseous hydrocarbon compound, in which the hydrocarbon compound can be broken down by means of a cathode system having magnets, a plasma is formed in the vicinity of the substrate by applying a radiofrequency voltage to the cathode system while the substrate is held immediately in front of the cathode. The pressure in the coating chamber and the supply of the electric energy from the generator to the cathode are variable for the purpose of achieving a coating quality that will change during the build-up of the coating. For this purpose, electric motor-powered valve means are inserted into the gas supply line and into the pump suction line and are operated through a timing mechanism or a control apparatus which is connected to a vacuum gauge.

BACKGROUND OF THE INVENTION 
The invention relates to a process and an apparatus for coating a substrate 
with an organic coating by bringing the substrate into a coating chamber 
containing a gaseous hydrocarbon compound, in which the hydrocarbon 
compound can be decomposed by means of a cathode system having a magnet, 
while a plasma forming in the vicinity of the substrate can be excited by 
applying a radiofrequency voltage to the cathode system and holding the 
substrate directly in front of the cathode. 
It is known to apply a thin coating to thin substrate, such as data storage 
or sound recording media, by means of an ionized atmosphere or by 
plasma-supported continuous vapor depositing, radiofrequency energy being 
used to excite the plasma while the chemical reaction is concentrated in 
front of the cathode by means of a magnet system. Since the storage 
density of magnetic storage media is very greatly determined by the 
thickness of the magnetic coating and the distance of the recording head 
from this coating, very thin coatings and especially close head spacing 
are desired. Experience shows, however, that this also increases the 
danger of damage to the coating by contact of the head with the coating, 
caused, for example, by vibration or by dust particles. On the other hand, 
however, the wear on the magnet head, especially in start-stop operation, 
must be kept as low as possible. Since it is possible with the prior-art 
coating methods to produce only a single coating with a very homogeneous 
structure, the manufacturer of such media has to compromise with regard to 
the coating quality. 
The present invention sets for itself the task of creating a process and an 
apparatus which will be suitable on the one hand for producing a very thin 
coating which will be gentle on recording head, and on the other hand the 
task of producing a coating that will be comparatively insensitive to 
damage. 
SUMMARY OF THE INVENTION 
The pressure in the coating chamber and the feeding of the electric energy 
from the generator to the cathode are specifically controlled for the 
purpose of achieving a coating quality that will change during the 
creation of the coating, and the hardness, the coefficient of friction and 
the index of refraction of the coating will be given a specific gradation 
by increasing the pressure in the coating chamber or by reducing the 
energy input. 
In an apparatus suitable for the performance of this process, valve means 
are inserted into the gas line leading from the gas supply tank to the 
coating chamber and into the pump suction line connecting the vacuum pump 
to the coating chamber, whereby the pressure prevailing in the coating 
chamber and the throughput of the gas can be regulated. 
For this purpose the valve means inserted into the gas supply line and into 
the pump suction line are preferably operated hydraulically or 
pneumatically under the control of a timing mechanism. 
It is desirable for the power output of the generator connected to the 
electrical line supplying the cathode to be controllable by means of an 
electrically, hydraulically or pneumatically operated adjusting means 
under the control of a timing mechanism or of a control apparatus. 
In order to produce a coating with a gradated hardness, in which a softer 
surface coating follows onto a preceding harder protective coating, the 
timing mechanism and/or the control apparatus cooperate with a vacuum 
gauge which is in communication with the coating chamber and which 
produces the control signals corresponding to the pressure in the coating 
chamber and feeds them to the timing mechanism or to the control 
apparatus. 
Advantageously, the timing mechanism and/or the control apparatus cooperate 
with a flow meter which is inserted into the gas supply line between the 
gas tank and the valve means, and which produces signals corresponding to 
the flow of gas to the coating chamber and feeds them to the timing 
mechanism or to the control apparatus. 
In a preferred embodiment, the magnets producing a magnetic field in front 
of the cathode are disposed movably with their mounting on the cathode 
holder, and can be rotated by means of separate motors thereby achieving 
an especially uniform coating thickness.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The apparatus consists essentially of the coating chamber 1 with the 
cathode 2 disposed therein, the substrate 3 disposed above the cathode 2, 
the magnets 4 pertaining to the cathode 2, the electric power source 5, 
the connection 6 for the gas bottles 7 and 8, the flow meter 10 inserted 
into the gas feed line 20 with the magnetically operated valve means 11, 
the vacuum meter cell 13 connected by connection 12 to the coating chamber 
1, and an electric motor-powered valve means which is connected to the 
coating chamber 1 through a connection 14 for controlling the turbo vacuum 
pump 16, and the rotary piston vacuum pump 17 connected to the output 
thereof. 
After the substrate 3 has been admitted through a lock into the evacuated 
coating chamber 1, plasma tubes 18 and 19 form on the side of the 
substrate 3 facing away from the cathode 2, due to the decomposition of 
the monomer (e.g., acetylene gas) introduced through the connection 6, 
radiofrequency energy (e.g., 13.56 MHz) produced by the energy source 5 or 
the generator being applied to the cathode 2 for the excitation of the 
plasma. If desired, an inert gas, e.g., argon, can be mixed with the 
acetylene gas. By adjusting the process parameters the properties of the 
coating can be varied; in particular, the mechanical properties such as 
hardness and friction coefficient can be varied continuously in the course 
of the coating, so that a gradated hardness forms such that a softer 
surface coating follows a harder protective coating. With this gradation 
it is brought about that, while having optimum protective action for the 
substrate--e.g., a data storage medium, the wear on a magnet head 
cooperating with the data medium is minimized. 
A change in the input power produces a change of the energy of the 
particles impinging upon the substrate 3 and hence a structure change. 
Thus, gradations can be produced within the coating by shifting from one 
power level to the next during the coating. 
The substrate or data storage medium is best provided first with a 
relatively hard protective coating and, while this coating is being 
applied, the power and the pressure are varied such that the surface 
receives a coating which is softer and thus has good antifriction 
properties. In this manner, the hardest possible coating for the 
protection of the magnetic coating is combined in a single coating process 
with a softer coating of low friction coefficient that is especially 
desirable for start-stop operation. 
The example shown in FIG. 2, of a variant embodiment of a cathode 
arrangement, the dark-space gap a between the substrate 3 and the cathode 
2 is made vary narrow, so as to assure that no plasma cloud will form in 
the space between the substrate 3 and the cathode 2, i.e., the dark-space 
distance that establishes itself under the chosen process parameters 
(e.g., pressure and power) must not be exceeded. 
In FIG. 3 the dependency between the hardness of the coating and the 
electrical voltage applied during the coating process is shown in a graph. 
It can be seen that a hardness maximum is achievable at a power of about 
400 W. FIG. 4 shows that a minimum friction coefficient is established at 
about 1,000 W, and that at lower or higher power the friction coefficient 
approaches a maximum. As FIG. 5 shows, the refractive index also depends 
on the applied electric power, increasing as the power increases, until at 
about 2,000 W power it reaches a maximum level. In the graph in FIG. 6, it 
is shown that the refractive index and coating hardness also depend on the 
pressure prevailing in the coating chamber 1 during the coating process. 
Both of these values reach a maximum at a pressure of about 0.001 mbar.