Chemistry: electrical and wave energy – Processes and products – Vacuum arc discharge coating
Reexamination Certificate
1999-01-25
2004-07-13
Cantelmo, Gregg (Department: 1745)
Chemistry: electrical and wave energy
Processes and products
Vacuum arc discharge coating
C204S298410, C204S298120, C204S298160, C204S298190
Reexamination Certificate
active
06761805
ABSTRACT:
The invention relates to a cathode arc source, in particular a filtered cathode arc source. This invention also relates to a graphite target and to a method of making a graphite target, in particular, a graphite target for use in a filtered cathode arc source.
The source of the invention is suitable for use with or without filtering apparatus, in particular a double bend filter duct as previously developed and described in International patent application no. PCT/GB96/00389, published as WO-A96/26531 by the same inventors.
Filtered cathode arc sources are used, inter alia, to produce thin diamond-like films on substrates, the diamond originating from a graphite target.
An acknowledged problem in prior art sources is that their intense arc spots produce large quantities of macroparticles that contaminate the plasma generated from the target and, in turn, the deposited films. The problem of reducing the number of macroparticles in the films is conventionally addressed by provision between the target and the substrate of means for filtering macroparticles from the plasma beam.
Known commercial apparatus incorporates a 45 degree angle bend to filter macroparticles from the beam. Apparatus described by the present inventors in International patent application no. PCT/GB96/00389 describes a double bend filter duct for the same purpose. While this latter apparatus is particularly efficient, it is desirable to provide alternative means for reducing macroparticles in the deposited films.
U.S. Pat. No. 5,468,363 by Falabella discloses a specific arrangement of cathode, anode and first and second coils in a cathodic-arc source. Falabella specifies that the cathode is located in the centre of a short solenoid coil. That is to say, the cathode station at which a target may be located is located at the centre of a coil producing a first magnetic field.
In the Falabella source, movement of the arc spot about and away from the target surface is a problem and it is necessary, in an attempt to constrain the arc to the end of the cathode, to limit the source to targets having sloped sides.
It is known to prepare commercial graphite targets by gluing graphite powder with a binding material, such as bitumen, and then hot pressing under high temperature and pressure to graphitise and harden the target. A known graphite target is from PURE TECH INC., which has a density of about 1.8 g/cm
3
. The use of this known graphite target produces a cathodic arc which is typically spot-like and uses a high current density. Using the known graphite target, a cathode arc spot usually has a diameter of about 1-10 cm. This type of arc spot produces a large amount of macroparticles which contaminate tetrahedral amorphous carbon (ta-C) films produced using known filtered cathode arc sources.
Another graphite target is made by Cambridge University and has a density of about 1.6 g/cm
3
, pressed at about 200-250 MPa, at room temperature, using graphite having a particle size of about 1 micron (10
−6
m). In use, this target produces very little plasma and, consequently, a very low or zero rate of deposition after the filter section.
It has also been observed that using known graphite targets, the surface of the target is consumed in an uneven manner, as the arc spot moves erratically about the surface of the target. This movement of the arc on the surface of the target is uncontrollable and the deposition rate falls when the spot approaches the edge of the target.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new design of cathode and anode and a new cathode target that, separately or in combination, eliminate or at least ameliorate some of the problems identified in the prior art. It is therefore an object of the invention to provide method and apparatus to generate an arc from a cathode target which in use emits fewer macroparticles compared to the prior art.
According to a first aspect of the invention there is provided a cathode arc source comprising a graphite cathode target and means for generating a magnetic field substantially normal to the target which has a point of zero field strength above the target surface.
In an embodiment of the invention, a cathode arc source for generating positive carbon ions from a cathode target, said ions being emitted in a direction substantially normal to a front surface of the cathode target, comprises a vacuum chamber and means for generating a magnetic field in the vacuum chamber, wherein the magnetic field has direction substantially normal to the front surface of the target and zero field strength at a position above the target and inside the vacuum chamber. The cathode arc source preferably comprises means for generating a first magnetic field proximal to the target and having a first field direction and means for generating a second magnetic field distal to the target and having a field direction opposite to that of the first magnetic field. The resultant magnetic field inside the vacuum chamber includes a point at which the field strength is zero in a direction substantially normal to the front surface of the cathode target.
Alternatively, the first and second magnetic fields are so positioned with respect to the target that the lateral field is enhanced in a region between the target and the substrate; the null point is preferably but not necessarily between the target and the substrate.
In use, the cathode arc source of the invention produces a beam of positive carbon ions having reduced numbers of macroparticles. The invention thus addresses the problem of how to remove macroparticles from a plasma of positive ions by control of a magnetic field, or a magnetic field resultant from two or more magnetic fields, within the vacuum chamber of the source so that fewer macroparticles are generated ab initio. Filtering of the plasma beam further to reduce macroparticles is an option and is a feature of preferred embodiments of the invention.
The invention thus addresses how to reduce macroparticle count at the point of macroparticle formation, rather than by removing them once formed.
In a particular embodiment of the invention, a cathode arc source for generating positive ions from a graphite cathode target comprises means for generating a magnetic field wherein:
(1) at a front surface of the target, field direction substantially normal to the front surface is towards the front surface;
(2) magnetic field strength in the direction substantially normal to the front surface decreases with increasing distance from the target to a point or region of zero field strength; and
(3) from the point or region of zero normal field strength, with increasing distance from the target, field direction is away from the front surface of the target.
In this arrangement, positive ions emitted from a front surface of the target pass first through a magnetic field whose direction is substantially opposite to the direction of the positive ions, secondly through a point or region in which the magnetic field strength in that direction is zero and thirdly through a magnetic field whose direction is substantially the same as that of the positive ions. The latter field conveniently can be used to steer the positive ions, through filtering apparatus for example, towards a substrate.
Alternatively, the field at the front surface of the target is away from the target towards the substrate, then there is a point or region of zero normal field and distally from the front surface the field direction is then reversed, i.e. towards the target.
Hereafter, a point or region of zero field strength in a direction substantially normal to the front surface of the target is also referred to as a null point.
In a preferred embodiment of the invention there is provided a cathode arc source comprising a cooled graphite cathode target, a cooled anode, an arc power supply, a substrate, means for generating positive ions in an arc between the cathode and the anode and means for generating a magnetic field in a direction substantially normal to the cathode target which field has zero field stren
Flynn David Ian
Shi Xu
Tan Hong Siang
Tay Beng Kang
Cantelmo Gregg
Filplas Vacuum Technology Pte. Ltd.
Sterne Kessler Goldstein & Fox P.L.L.C.
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