Plasma Processing Method and Apparatus

The present invention relates to plasma processing method and apparatus more particularly, it relates to, but not exclusively, a method of adhering a protective film of carbonaceous material to comparatively soft substrates to provide a transparent electrically insulating coating thereon.

Hard, thin films of diamondlike carbon have been deposited on substrates in a number of ways. By "diamondlike carbon" is meant carbon with exceptional hardness, e.g. which can neither be cut with a razor blade, nor scratched by rubbing with steel wool. The chemical bonding appears to be dominated by sp.sup.3 tetrahedral diamondlike bonding as opposed to the sp.sup.2 trigonal bonding of graphite. Diamondlike carbon films may or may not give evidence of crystallinity by x-rays.

Japanese patent application No. sho 56-146936 describes an example, in which carbon deposition is carried out while the deposited carbon material is subject to the attack of accelerated ions at the same time so that soft portions are selectively removed to leave comparatively hard material. This technique is excellent to increase the hardness of the carbon film thus deposited. Substrates to be coated in accordance with this technique, however, have to be given bias voltages in order to cause sputtering action thereon. For this reason, the advantage of this technique is diminished when the substrates are made of insulating materials.

It is therefore an object of the present invention to provide a plasma processing apparatus and method of a new type.

In order to accomplish the above and other objects and advantages, a pair of auxiliary electrodes are provided in a vacuum chamber in the both sides of a substrate such as a glass substrate to be treated, besides another pair of electrodes for producing plasma gas in the vacuum chamber. A relatively low frequency voltage is applied between the auxiliary electrodes in order to cause the bombardment of ions of plasma upon the substrate.

By the plasma treatment, hard films can be deposited since the bombardment of ions tends to selectively remove soft portions of the material deposited during the deposition process. When carbonaceous films are formed by use of a carbon compound gas, the hardness and the resistivity can be controlled by adding a fluorine compound gas. The fluorine contained in the carbon film deposited functions also to enhance the water repellant property of the film. Such carbon films are suitable for protective coatings on glass windows of vehicles.

Other substrates suitable for treatment by the present invention are made of, for example insulating materials having resistivities not lower than 10.sup.15 ohm centimeters, such as polyesters, alkyd resins, oil-free alkyd resins, unsarurated polyesters, acrylic resins and amino-resins. Particularly, suitable organic material used in manufacture of motorcars are acrylic lacquers, acrylic melanins and block acrylic urethanes.

This invention can be better understood from the following detailed description when read in conjunction with the drawing in which

FIG. 1 is a schematic cross sectional diagram showing a plasma CVD apparatus in accordance with one embodiment of the present invention.

FIGS. 2(A) and 2(B) are vertical and horizontal cross sectional views showing a window glass for motor cars which has been coated with a carbon protective film in accordance with the present invention.

FIG. 2(C) is a schematic cross sectional view showing a modification of the embodiment shown in FIGS. 2(A) and 2(B).

FIGS. 3(A) and 3(B) are horizontal and perspective cross sectional views showing a cylinder coated with a carbon protective film in accordance with the present invention.

FIGS. 4(A) and 4(B) are perspective and cross sectional views showing a rule coated with a carbon protective film in accordance with the present invention.

FIGS. 5(A) and 5(B) are perspective and cross sectional views showing a set square coated with a carbon protective film in accordance with the present invention.

Referring now to FIG. 1, a chemical vapor deposition apparatus is illustrated. The apparatus comprises a vacuum chamber 9 defining a deposition space therein, an evacuation system 25 comprising a rotary pump 23 and a turbo molecular pump 22 which are connected to the chamber 7 through a valve 21, a gas supply system 30 comprising four gas feeding lines, each being provided with a flow meter 29 and a valve 28 connected to the reaction chamber 7, a pair of mesh electrodes 3-1 and 3-2 made of alminum and located at the upper and lower positions of the inside of the depositino space, a power supply 40 for supplying energy to the mesh electrodes 3-1 and 3-2, a plurality of substrate hoders 20 for holding substrates 1, the holders 20 being connected to the chamber 7 through capacitors 19, a plurality of A1 mesh electrodes 50 (13-n, 13-n') each being interposed between adjacent substrates, a bias voltage applying means 17 for applying an AC voltage between adjacent ones of the electrodes 50. The reaction chamber 7 is provided with a gate valve 9 through which substrates to be coated are disposed in the chamber.

The energy supply 40 comprises a first power source 15-1 which supplies an AC voltage to the mesh electrode 3-1 through a matching device 16-1 comprising a LCR circuit, a second power source 15-2 which supplies an AC voltage to the mesh electrode 3-2 through a matching device 16-2 comprising a LCR circuit, and a phase adjuster 26 connected between the first and second powder sources 15-1 and 15-2. Both the first and second power sources have grounded at terminals 5-1 and 5-2. The bias applying device 17 is provided with first and second AC voltage sources 17-1 and 17-2, which supply AC voltages between adjacent electrodes 13-n and 13-n'. One terminals of the voltage sources 17-1 and 17-2 are ground at 5-3.

In operation, after evacuating the chamber 7, a carbon compound gas is introduced into the deposition space 8 at 0.001 to 1 Torr. For example, the carbon compound gas consists of C.sub.2 F.sub.6 and C.sub.2 H.sub.4 in the ratio of from 1:4 to 4:1, e.g. 1:1 and introduced so that the pressure in the reaction chamber 7 becomes 0.5 Torr. The transparency and resistivity of the carbon material can be controlled by changing the amount of fluorine contained in carbon material to be deposited through the adjustment of the introduction rate of C.sub.2 F.sub.6 to C.sub.2 H.sub.4. The temperature in the deposition space is no higher than 150.degree. C. Alternate voltages of from 1 MHz to 100 MHz, e.g. 13.56 MHz, are applied to the mesh electrodes 3-1 and 3-2 from the first and second power sources 15-1 and 15-2. The phase difference between them is adjusted to be 0.degree. or 180.degree. by means of the phase adjuster 26. By virtue of the high frequency electric voltage, the carbon compound gas is converted into plasma state and carbon deposition is carried out on the substrates. The deposition speed is from 100 to 1000 .ANG./min. The thickness of the deposited carbon films may be 0.1 to 8 micrometers on flat surfaces and 1 to 3 micrometers on projected surfaces. At the same time, an AC voltage is applied between adjacent electrodes 13-n and 13-n' in order to induce an electric field normal to each substrate. The frequency of the AC voltage between the electrodes 13-n and 13-n' is selected to be in the range of from 10 Hz and 100 KHz, e.g., 50 Hz. At such comparatively low frequencies, ions of the plasma can follow the electric field and bombard against the substrate surface on which carbon deposition is being carried out. As a result, the hardness of the carbon material deposited becomes high, e.g. 600 to 6000 kg/mm.sup.2. The resistivity of the carbon is for example from 1.times.10.sup.6 to 5.times.10.sup.12 ohm centimeter, tipically from 1.times.10.sup.7 to 1.times.10.sup.11 ohm centimeter. The optical energy band gap is no narrower than 1.0 eV, typically 1.5 to 5.5 eV. The carbon material includes hydrogen at 30 atom % or less and fluorine at 0.3 to 10 atom%.

Carbonaceous films were coated on several substrates in accordance with the present invention. FIGS. 2(A) and 2(B) are horizontal and vertical cross sectional views showing a curved glass or plastic pane 1 to be fit in the front window of a motor car coated with a carbonaceous film 45 of 0.1 to 8 micrometers. The whole surface of the glass pane 1 is coated with a carbon film in accordance with the present invention. This film can be coated only one side of the pane. In this case, the panes treated in the apparatus shown in FIG. 1 are disposed in parallel with each other in order to make pairs as illustrated in FIG. 2(C). FIGS. 3(A) and 3(B) illustrate a carbon coating formed on a cylindrical substrate. FIGS. 4(A) and 4(B) illustrate a carbon coating formed on a plastic rule 1. The thickness of the coating 41 is 0.1 to 8 micronmeters for example. FIGS. 5(A) and 5(B) illustrate a carbon coating formed on a set square. The thickness of the coating is 0.5 micrometer.

While several embodiments have been specifically described by way of examples, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departing from the scope of the invention as defined by the appended claims. For example, the present invention is usuful when front, side or rear windows or side mirrors are coated with carbon protective films.