Enhanced macroparticle filter and cathode arc source

Details Enhanced macroparticle filter and cathode arc source Enhanced macroparticle filter and cathode arc source

2000-10-06

2003-01-28

McDonald, Rodney G. (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Vacuum arc discharge coating

C204S298410, C118S7230VE, C427S580000

Reexamination Certificate

active

06511585

ABSTRACT:

The present invention relates to deposition of coatings using a cathode arc source, to filtering of macroparticles from plasma in a cathode arc source, in particular using a magnetically enhanced macroparticle filter, and to substrate preparation prior to deposition and to control of deposition.
Cathode arc deposition of tetrahedral amorphous carbon, metallic, dielectric and other such coatings is known in the art and offers the potential for deposition of thin films of high quality. Applications in scratch resistant optical coatings and hard disc media coatings are but two of a wide range of proposed uses.
Hitherto, deposition of films by the cathode arc process has been limited to laboratory use, in general because of difficulties in the art in reliably depositing films that are free of or have acceptably low contamination by macroparticles large, neutral particles.
Provision of improved means for filtering macroparticles from the arc plasma have recently been described in WO-A-96/26531 and also in International patent application no. PCT/GB97/01992. Nevertheless, further improvements in filtering of macroparticles is desirable and will become more so as further applications for the deposited films arise.
The present inventors have identified problems with deposition of thin films using arc plasma, namely that the initial period of deposition can give a poor quality coating; it would therefore be desirable to avoid deposition using the plasma first emitted after striking an arc at a target. Likewise, the final period of deposition while the plasma density is falling, just before breakdown of the arc, can also give poor quality coatings, and it would be desirable to avoid deposition at this time. Known filtered cathode arc sources do not however have such a facility.
It is vital in some applications to be able to provide an accurate thickness of coating and to stop deposition when the desired thickness has been achieved.
However, there is a period of seconds after shutting down the arc during which there is a small residual amount of deposition—this can lead to a slightly greater coating thickness than is wanted. Existing apparatus does not enable deposition to be stopped suddenly.
Tetrahedral amorphous carbon coatings laid down using filtered cathode arc apparatus can be liable to tear or peel away from a substrate and, in an attempt to avoid or reduce this, pretreatment of the substrate to clean the surface that will receive the coating is optional, but can hitherto only be carried out either in apparatus separate from the deposition apparatus or in deposition apparatus incorporating an ion beam source in addition to a cathode arc source. The former inconveniently introduces an extra step into production of coatings, unattractive from a commercial point of view, as well as requiring two separate machines. Deposition apparatus with their own ion beam source are expensive as the ion beam source is a separate attachment to the chamber.
The present invention seeks to provide for further filtering of arc plasma and to overcome or at least ameliorate problems encountered with prior art cathode arc sources. It is therefore an object of the invention to provide method and apparatus for filtering macroparticles from plasma in a cathode arc source. A further object is to facilitate deposition of coatings on substrates that have been pretreated to promote adhesion of the coating thereto.
Accordingly, a first aspect of the invention provides a cathode arc source for depositing a coating on a substrate, said source comprising
an anode and a cathode station for a target,
a first filter means comprising a filter duct having at least one bend, and
first magnetic means for steering plasma through the filter duct for removal of macroparticles from the plasma;
wherein the apparatus comprises a second filter for further removal of macroparticles from the plasma, the second filter comprising a baffle, an aperture through which plasma can pass and second magnetic means for steering plasma through the aperture.
An advantage of the apparatus of the invention is that further filtering of macroparticles is provided which can result in further improvements in the quality of films, such as tetrahedral amorphous carbon films, deposited from the cathode arc source. The aperture is preferably closeable. The first aspect of the invention also provides the second filter in isolation from the rest of the apparatus.
In an embodiment of the invention, the second filter is located between the duct of the first filter and the substrate. In use, plasma containing positive ions and being contaminated by macroparticles is steered around the bend of the first filter and a proportion of macroparticles hit the side of the filter duct and are removed from the plasma. At the end of the duct closest to the substrate, that is to say after the plasma has passed through the bend, there is a relatively high density of the remaining macroparticles towards the outside of the duct. The plasma is however more dense in terms of positive ions towards the centre of the duct. The second filter of the invention thus is effective because its baffle is located towards the outside of the duct and thus macroparticles towards the outside of the duct hit the baffle and are prevented from travelling further towards the substrate.
A preferred embodiment of the source of the invention thus comprises a first filter duct attached to a chamber in which plasma is deposited on a substrate, wherein the second filter means is located at or near the exit of the filter duct and exerts its filtering effect before the plasma reaches the chamber.
It is further preferred that the second filter means is in the approximate form of a disc having a substantially circular aperture. As plasma in the duct is denser towards the duct centre and can be steered through a circular aperture this is a convenient configuration of the aperture to obtain high transmission of plasma through the second filter whilst filtering macroparticles from the plasma. The aperture shape may additionally be at least partially dictated by the cross-section of the filter duct. If this were not circular but oval-shaped, then an oval-shaped aperture or an aperture varying from circular might be appropriate.
A typical filter of the invention extends across the width of the filter duct. Macroparticles that hit the baffle of the disc fall back onto the side of the duct away from the substrate; after a period of use it will be preferred to clean the duct and/or the disc in this area, though a major part of the filtering is achieved by the first bend of the duct and consequently cleaning of the filter disc will not often be necessary.
In an embodiment of the invention shown in an example below, the filter disc is a substantially planar and circular disc of steel foil with a central aperture. In another embodiment of the invention shown in another example below the filter disc is not planar but comprises a baffle angle to deflect macroparticles away from the plasma. An advantage of this embodiment is that the purpose of the baffle it to reduce the number of macroparticle that reach the substrate. Some macroparticles when stopped by the baffle may be deflected into the plasma beam and thereafter carried towards the substrate by the plasma momentum. A baffle that is inclined to the plasma so as to deflect macroparticles away from the plasma and towards the side of the duct is thus a preferred component of the filter.
Generally, the disc should be thin and should not interfere with operation of the filter duct and should not interfere with the magnetic field that steers plasma through the filter duct. Typically, the disc is less than 2 mm thick, and is preferably less than 1 mm thick.
The aperture in the filter is for passage of plasma therethrough and may have a width from 10% to 50% the internal width of the filter duct, preferably from 25% to 40% the width of the filter duct at that point. The aperture will be designed according to its particular application and different sizes may be appropri

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