Composite coatings

Details Composite coatings Composite coatings

2000-10-03

2002-05-14

Jones, Deborah (Department: 1773)

Coating processes

Coating by vapor, gas, or smoke

Carbon or carbide coating

C428S408000, C427S122000, C427S470000, C427S577000

Reexamination Certificate

active

06387443

ABSTRACT:

The present invention relates to composite coatings, in particular such coatings comprising carbon and another metallic element. The present invention relates also to methods of making composite coatings, materials for making the coatings and substrates coated with the coatings.
Amorphous silicon-carbon alloys (a−Si
1.x
C
x
) have attracted much recent attention not only due to the composition dependent variability of their optical band gap but also because of their important role as intermediate layers for the growth of diamond films on crystalline silicon and non-diamond substrate. Several attempts have been made to deposit (a−Si
1.x
C
x
) films using existing thermal chemical vapour deposition (CVD) or plasma assisted CVD techniques, but these techniques involve high deposition temperatures which may destroy or damage many substrate materials. Also, known CVD techniques have used metal organic compounds, undesirable due to their toxicity.
It is known to deposit hard thin films, such as tetrahedral amorphous carbon (ta-C), using a filtered cathode arc (McKenzie et al 1991, Fallon et al 1993, Martin et al 1988). These ta-C films have interesting and useful properties, such as extreme hardness (~70 Gpa), thermal stability, high electrical resistivity, wide Tauc optical band gap (~2.5eV), smooth surface and low friction, and transparency in wide spectral range because of the high sp
3
fraction of carbon atoms (up to 87%) in the film.
However, the high internal stress in the films can limit their applications, especially when it is desired to deposit a relatively thick film, as the film may flake away from the substrate.
In order to reduce the internal stress of ta-C films, and in an attempt to improve adhesion of thick films of this type, different modifications have been made, such as nitrogen incorporation into the films. However, whilst the internal stress can be reduced a little, this is not sufficient to enable significant increases in usable film thickness. In addition, there are disadvantages to incorporation of nitrogen into these films as so doing can harm many of the mechanical properties of the films.
Metal-containing diamond-like-carbon (DLC) materials are known potentially to have useful electrical and mechanical properties, wear resistance and friction (Dimigen et al 1987). It has been reported that such films containing certain low percentages of metals can have comparable wear resistance and friction coefficient with the a-C:H films, and may have better adhesion to the substrate. Introducing certain metal elements such as aluminum into the DLC films may reduce film stress, but only at the unacceptable expense of its mechanical properties, such as hardness and Young's modulus.
It is therefore an object of the invention to provide composite coatings that solve or at least ameliorate the aforementioned problems. In particular it is an object of specific embodiments of the invention to provide composite coatings that exhibit reduced stress, thus enabling deposition of relatively thick coatings whilst retaining acceptable hardness.
Accordingly, the present invention provides, in a first aspect, a method of applying a coating to a substrate using a cathode arc source, comprising:
generating an arc between a cathode target and an anode of the source; and
depositing positive target ions on the substrate to form the coating,
wherein the coating is a composite of at least first and second elements and the target comprises said at least first and second elements.
Thus, the invention enables the production of composite coatings from targets used in a cathode arc that contain two or more coating components. It is an advantage of the method that composite films can easily be produced using the filtered cathode arc process, and without the need for introduction of gaseous compounds into the arc vacuum chamber. Composite films were previously made using, for example, a graphite target and hydrocarbon gas, SiH
4
gas or a metal organic compound in vapour form introduced typically close to the substrate. The resultant films had high hydrogen content and suffered from poor mechanical properties. The method of the invention avoids the necessity for gaseous components and enables production of films that have lower hydrogen contents than and improved mechanical properties than possible hitherto. Films of the invention typically have a hydrogen content of 20% or less, preferably 10% or less, and in specific embodiments of the invention substantially hydrogen-free coatings are produced.
It is a further option for the method to deposit a coating that is a composite of at least first, secondhand third elements and wherein the target comprises said at least first, second and third elements. Alternatively, the coating can be a composite of at least first, second and third elements and the target comprises said at least first and second elements and the method comprises introducing the third element into the coating in a gaseous or liquid form.
It is envisaged that the method of the invention is of application without limit to the choice of target materials. Specifically, the method has successfully been carried out using a target that comprises carbon, producing a coating of a composite comprising tetrahedral amorphous carbon. The target preferably contains, as second element, a metal other than carbon. The target should be electrically conducting, so other target materials may be chosen that are non-metallic, provided that the target is sufficiently conducting to be used as a cathode target in a cathode arc deposition apparatus. Where the second element is a metal it is suitable selected from titanium, nickel, chrome, aluminum, silicon and tungsten. Reference to element is intended to be reference to the element whether present in elemental or ionic or compound form.
In a particularly preferred embodiment of the invention the method comprises depositing a layer of a composite film of carbon and silicon, suitably using a target which contains at least 40% carbon, the remainder being substantially silicon. The composite Si—C film obtained has uses in the semiconductor field. Also the Si—C film obtained can be used for its improved trabelogical properties of reduced stress and high hardness compared to known DLC and DC-based films.
In a further particularly preferred embodiment of the invention the method comprises depositing a layer of a composite film of carbon and aluminium, suitably using a target which contains at least 80% carbon, the remainder being substantially aluminum.
The use of composite targets has the advantage that it is possible according to the invention to deposit coatings that have a high proportion of sp
3
bonds. It is preferred that the deposited coating has an sp
3
content of at least 60%, more preferably at least 70%, and in specific embodiments of the invention sp
3
percentages of 80% and above are achievable.
The invention additionally provides in the first aspect a method of depositing a composite coating of at least first and second elements, comprising:
generating an arc between an anode and a cathode target, wherein the cathode target comprises said first and second elements, so as to generate positive ions of said first and second elements; and
depositing said ions on a substrate to form the composite coating.
The target used in the method can comprise carbon and the composite coating comprise tetrahedral amorphous carbon having an sp
3
content of at least 70%.
In a second aspect of the invention there is provided a composite coating comprising tetrahedral amorphous carbon and a metallic element other than carbon, the composite coating having an sp
3
content of at least 60%. The sp
3
content in preferred coatings is at least 70%.
In embodiments of the invention a composite coating comprises 99.9-80% carbon and 0.1-20% aluminium. These have been found to exhibit particularly desirable properties as more specifically set out in the examples below.
In further embodiments of the invention a composite coating comprises 99.9-40% carbon and 0

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