Method of Producing a Cover Layer of Amorphous Carbon on a Substrate

1. Field of the Invention

The present invention relates to a method of producing a cover layer of amorphous carbon, which contains hydrogen and/or fluorine as an additional element, on a substrate. The invention also relates to a cover layer produced according to such a method.

2. Discussion of the Background

Cover layers of hydrogenated amorphous carbon (a--C:H) are used because of their great hardness. It is known to produce such cover layers by glow discharge at high frequencies from an acetylene gas atmosphere in conjunction with argon. Due to the low deposition rate, the production of cover layers according to this method is complicated, time consuming and expensive.

It has also been attempted to produce layers of amorphous carbon by means of direct current cathode sputtering using graphite as the target. The latter method is performed with aperture (diaphragm) systems (Journal of Non-Crystalline Solids 35 & 36 (1980), North Holland Publishing Company, pages 543-548).

It is an object of the present invention to develop a method of depositing cover layers of amorphous carbon on substrates faster and more economically than it has been feasible theretofore.

This is accomplished by the present invention by direct current magnetron cathode sputtering using a glass-like carbon target (or glassy carbon target) argon-hydrogen or argon-hydrogen-fluorocarbon atmosphere.

The method according to the invention permits the production of a transparent cover layer which has great hardness and low specific conductivity. Moreover, the layer is hydrophobic and resistant to alkaline solutions and to acids. At the appropriate power levels, the dc-magnetron process permits high deposition rates allowing the cover layers, preferably in a .mu.m thickness range, to be deposited quickly on the substrate.

As the target preferably glass-like carbon is employed whose microporosity, gas impermeability and hardness are approximately the same as those of glass. With glass-like carbon, cover layers can be produced which have very favorable characteristics. The purity of this material is very high compared to graphite. In particular, because of its low porosity (0 %), this material is better suited than graphite (porosity 20 %) for use in vacuum systems.

A cover layer according to the present invention is transparent, hydrophobic and has an electrical conductivity of about 10.sup.-12.OMEGA..sup.-1 cm.sup.-1. The optical band gap of the cover layer is greater than 2.7 eV. The cover layer has a Vickers hardness of 2000 or more.

A cover layer of the above described type can be employed on an electrophotographic recording material which may have an amorphous silicon layer as its carrier substance. Moreover, a cover layer of the above-described type can advantageously be applied to printed circuit boards to electrically insulate and protect them, and may also be applied to other carriers, for example, to image carriers and metals.

FIG. 1 is a cross-sectional view of an electrophotographic image carrier provided with a cover layer of amorphous carbon.

FIG. 2 is a graph of the conductivity of a cover layer as a function of the influx of hydrogen during deposition of the cover layer.

A cover layer 1 of amorphous carbon a-C:H which contains hydrogen and/or fluorine as additional element is produced on a substrate 2 in the manner to be described below. The term substrate, as used here, means a carrier material to which the cover layer can adhere. The substrate is, preferably, silicon (for example, the amorphous silicon of an electrophotographic recording material). Substrate 2 is disposed on a barrier layer 3 which is disposed on an aluminum carrier 4.

Cover layer 1 is produced on substrate 2 by direct current magnetron cathode sputtering. Glass-like coal is provided as the target.

The magnetron cathode sputtering can be produced in an argon-hydrogen atmosphere at a substrate temperature of room temperature to about 500.degree. C. The power density may be set between 1 W/cm.sup.2 and up to 1000 W/cm.sup.2. The hydrogen influx, which is regulated by way of flow meters, preferably lies between 1 and 200 sccm. At about 10 sccm, deposition rates of about 0.5 .mu.m/h can be realized.

Amorphous carbon can also be produced on substrate 2 by direct current magnetron cathode sputtering in an argonhydrogen-fluorocarbon atmosphere. The fluorocarbon employed is, in particular, the gaseous, non-toxic C.sub.2 F.sub.6. However, CF.sub.4 or C.sub.2 F.sub.4 may also be employed. The amorphous carbon layer containing fluorine (a--C:H:F) promises an even greater mechanical hardness since the C--F bond is stronger than the C--H bond.

Glass-like carbon is a type of carbon which in many characteristics is similar to glass. Glass-like carbon has a closed microporosity, is impermeable to gas and has a hardness corresponding to glass. Preferably, a glass-like carbon is employed whose apparent specific weight is in a range from 1.5 to 1.55 g/cm.sup.3 and whose open porosity is 0 %. Its gas permeability at room temperature is preferably <10.sup.-8 and its ash content is less than 400 ppm. The sulfur content of the glass-like carbon is about 50 ppm. The glasslike carbon may have a boron content of 2 ppm. The bending strength of the glass-like carbon is in a range from 600 to 1000 deca N/cm.sup.2 (1 deca N/cm.sup.2= 1 bar). Its crushing strength should be in a range from 1500 to 3000 deca N/cm.sup.2. A glass-like carbon can be employed which has a Shore hardness of 120 to 125 and a Vickers microhardness at 0.3 N of 150 to 225. The coefficient of thermal expansion is 3.2.times.10.sup.-6 per .degree. C. at 100.degree. C. and 3.5.times.10.sup.-6 per .degree. C. at 600.degree. C. The thermal conductivity may be, for example, in a range from 4 to 8 W per m.degree. C. The specific electrical resistance lies in a range from 45 to 55.times.10.sup.-4 .OMEGA. cm.

In the manufacture of the layer of amorphous carbon, an argon-hydrogen atmosphere or an argon-hydrogen-fluorocarbon atmosphere of 9.5.times.10.sup.-3 mbar pressure was set. The direct current power was 60 W. A layer was produced which had a band gap of 2.7 eV. The layer had a Vickers hardness of more than 2000. IR absorption measurements revealed CH vibration bands in the amorphous carbon. According to an X-ray structure analysis, the cover layer was amorphous. The refractive index of the layer lay in a range from 2.0 to 2.4 for a light wavelength of 633 nm while diamond has an index of 2.43, according to Landoldt-Bernstein "Optische Konstanten" [Optical Constants]).

FIG. 2 shows the specific conductivity of the cover layer 1 as a function of the hydrogen influx. As seen from the graph, a hydrogen influx in a range from 1 sccm to 10 sccm, results in a specific conductivity of about 10.sup.-12.OMEGA..sup.-1 cm.sup.-1. This value is a very low specific conductivity which practically corresponds to that of a good insulator. The cover layer produced in the above-described manner is transparent, hydrophobic, and resistant to alkaline solutions and to acids. Moreover, the cover layer is very hard and almost reaches the hardness of diamond. Cover layers of a thickness up to 10 .mu.m can be produced in a short time. The deposition rate is a function of the power; thus, for example, a deposition rate of 0.5 .mu.m/h can be realized with a d.c. power of about 1 W/cm.sup.2. Very high deposition rates can be realized with direct current powers of 200 W and higher, even in the kilowatt range.

For materials, such as selenium, whose properties are harmed at higher temperatures, substrate temperatures of 50.degree. C. may be selected. For relatively heat resistant materials, temperatures up to 400.degree. C. may be selected.

Doping gases may be added to the argon-hydrogen atmosphere or the argon-hydrogen-fluorocarbon atmosphere to produce n-conductive material or p-conductive material. A suitable doping gas for n-conductive material is phosphine (PH.sub.3). For p-conductive material, diborane (B.sub.2 H.sub.6) or trimethyl gallium (Ga(CH.sub.3).sub.3) is suitable.

Due to their many superior characteristics, the cover layers produced in the above-described manner are suitable for protecting numerous materials. One possible use is the covering of an electrophotographic recording material which may be selenium or silicon based. Another possible use is the coating of printed circuit boards, to insulate the printed circuit boards and protect them mechanically. Moreover, image carriers and metals may be coated. The cover layers may also be used for the production of antireflective coatings, for example on photodiodes.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.