Rectangular filtered vapor plasma source and method of controlling vapor plasma flow

Abstract

The invention provides an arc coating apparatus having a steering magnetic field source comprising steering conductors (62, 64, 66, 68) disposed along the short sides (32c, 32d) of a rectangular target (32) behind the target, and a magnetic focusing system disposed along the long sides (32a, 32b) of the target (32) in front of the target which confines the flow of plasma between magnetic fields generated on opposite long sides (32a, 32b) of the target (32). The plasma focusing system can be used to deflect the plasma flow off of the working axis of the cathode. Each steering conductor (62, 64, 66, 68) can be controlled independently. In a further embodiment, electrically independent steering conductors (62, 64, 66, 68) are disposed along opposite long sides (32a, 32b) of the cathode plate (32), and by selectively varying a current through one conductor, the path of the arc spot shifts to widen the erosion corridor. The invention also provides a plurality of internal anodes, and optionally a surrounding anode for deflecting the plasma flow.

Description

FIELD OF INVENTION [0001] This invention relates to apparatus for the production of coatings in a vacuum. In particular, this invention relates to a vacuum arc coating apparatus having a rectangular filtered cathodic or magnetron arc source providing improved arc spot scanning, and a plasma focusing and filtering system. BACKGROUND OF THE INVENTION [0002] Many types of vacuum arc coating apparatus utilize a cathodic arc source in which an electric arc is formed between an anode and a cathode plate in a vacuum chamber. The arc generates a cathode spot on a target surface of the cathode, which evaporates the cathode material into the chamber. The cathodic evaporate disperses as a plasma within the chamber, and upon contact with one or more substrates coats the substrates with the cathode material, which may be metal, ceramic etc. An example of such an arc coating apparatus is described in U.S. Pat. No. 3,793,179 issued Feb. 19, 1974 to Sablev, which is incorporated herein by reference...

AI Technical Summary

Benefits of technology

[0034] The steering conductors may be disposed in front of or behind the target surface of the cathode plate. When disposed in front of the target surface the steering magnetic field lines intersect with the target surface at an obtuse angle, which obviates the motion-limiting effect of the acute angle principle and allows the arc spot to move over a larger region of the target surface, thereby further increasing the size of the erosion zone.

[0036] Increasing the current through one steering conductor increases the strength of the magnetic field generated by that conductor relative to the strength of the magnetic field of the steering conductor along the opposite side of the cathode plate, shifting the magnetic field on the opposite side of the cathode plate transversely. Selective unbalancing of the steering conductor currents can thus compensate for the magnetic influence of the focusing conductors, and if desired can increase the effective breadth of the erosion zone to thus provide more uniform erosion of the target surface and a larger area within the coating chamber in which the cathodic evaporate is dispersed at concentrations sufficient to uniformly coat the substrates.

[0038] The present invention further provides means for restricting the cathode spot from migrating into selected regions of the target evaporation surface outside of the desired erosion zone. The invention accomplishes this by providing a shield at floating potential positioned over one or more the selected regions of the target evaporation surface, which prevents the arc spot from forming in or moving into the shielded region. In one preferred embodiment the shield is positioned immediately above the region of the target surface in the vicinity of the anode, spaced from the target surface. The evaporation zone is thus restricted to the area of the target surface surrounding the shield, protecting the anode from deposition of cathodic evaporate and providing better distribution of cathodic evaporate over the substrates to be coated, which results in more uniform coatings over a greater coating area.

[0039] The shield can be used to keep arc spots away from any region where the negative conductor traverses the cathode, which represents the lowest voltage drop relative to the anode. Where a large anode or multiple anodes are provided the shield restrains the arc spot from migrating into the region of the cathode located beneath an anode, where most of evaporated material would be trapped by the anode rather than flowing to the substrate.

[0040] Further, the presence of the shield allows the target evaporation surface of the cathode plate to be constructed of a combination of the coating material and another material, for example an expensive coating material such as titanium or platinum in the desired erosion zone and an inexpensive material such as steel outside of the erosion zone. The steel portion of the target surface may be shielded by a floating shield of the invention to prevent cathode spot formation and motion thereon, and thus prevent contamination of the coating while optimizing utilization of the expensive coating material.

[0042] The invention further provides a magnetic focusing system which confines the flow of plasma between magnetic fields generated on opposite sides of the coating chamber. This prevents the plasma from contacting the surface of the housing, to avoid premature deposition, and increases the concentration of plasma in the vicinity of the substrate holder.

Problems solved by technology

However, in a large surface area cathode arc coating apparatus of this type a significant portion of the target evaporation surface of the cathode plate goes largely unused, due to the scanning pattern of arc spots which follows certain physical principles:

These effects result in a limited erosion zone relative to the available area of the target surface of the cathode plate, reducing the life of the cathode and dispersing cathodic evaporate into the coating chamber in non-uniform concentrations.

However, it is not possible to construct the cathode plate so that the desired coating material is located only in the erosion zone, since the arc spot will occasionally stray out of the erosion zone and if the target surface is not entirely composed of the selected coating material the cathodic evaporate from outside the desired erosion zone will contaminate the coating on the substrates.

In these cases the arc spot tends to move chaotically over the target surface of the cathode and the cathodic evaporate accordingly disperses in random locations and non-uniform concentrations within the coating chamber, rendering uniform coating of the substrates improbable.

This random motion also causes the arc spot to move off of the target surface of the cathode and causes undesirable erosion of non-target portions of the cathode plate, for example the side edges.

This results in contamination of the coating by the ring material, and ultimately in ring failure.

In self-steering cathodic arc sources it is not possible to use external magnetic fields in the vicinity of the target surface of the cathode.

It is therefore not possible in such an apparatus to use a plasma-focusing magnetic field, as the influence of the focusing magnetic field makes the distribution of cathodic spots on the working surface of the cathode irregular and non-uniform.

Any external magnetic field, for example for focusing or deflecting the arc plasma flow, interferes with the self-sustained magnetic field generated by the cathode and anode current conductors and disrupts the self-steering character of the cathode spot.

However, the absence of magnetic focusing reduces the efficiency of the coating process and impairs the quality of substrate coatings, because the content of the neutral component (macroparticles, clusters and neutral atoms) in the region of the substrates, and thus in the substrate coating, increases.

A cathode in this type of plasma source rapidly becomes concave due to evaporative decomposition, and its useful life is therefore relatively short.

Moreover, since the evaporation surface of the cathode becomes concave in a relatively short time it is practically impossible to use a high voltage pulse spark igniter in such a design, so that a mechanical igniter must be used which lowers working reliability and stability.

Accordingly, self-steering arc plasma sources tend to use the target surface inefficiently and the cathode thus has a relatively short useful life.

This increases the size of the region within the coating chamber in which coating can occur.

However, this still significantly limits the area of the target surface of the cathode which is available for erosion, because this type of arc coating apparatus creates a stagnation zone in the region where the tangential component of the magnetic field is strongest The cathode spot eventually settles in the stagnation zone, tracing a retrograde path about the erosion zone and creating a narrow erosion corridor on the target surface.

This limits the uniformity of the coating on the substrates and reduces the working life of the cathode.

Also, filtration in this apparatus is poor, since the substrates are directly exposed to droplets and macroparticles entrained in the cathodic evaporate.

This results in an undesirable period where the magnetic field is zero; the arc is therefore not continuous, and is not controlled during this period.

Consequently this ‘pseudo-random’ steering method cannot consistently produce reliable or reproducible coatings.

However, the mechanical adaptations required for such a system make the apparatus too complicated and expensive to be practical.

A disadvantage of this approach is that the arc spot will occasionally migrate from the selected erosion zone to another part of the cathode target surface where the intensity of the transverse magnetic field is small and, there being no means available to return the arc spot to the desired erosion zone, will stagnate in the low magnetic field region.

However, this patent does not address the problem of magnetically steering an arc spot around a rectangular cathode plate in the presence of a deflecting magnetic field.

This design has the drawback of low target utilization rate and inefficient energy consumption (P. Robinson and A. Matthews, Characteristics of a Dual Purpose Cathodic Arc / Magnetron Sputtering System, Surface and Coating Technology, 43 / 44 (1990) 288-298, which is incorporated herein by reference).

The setback of this approach is an extremely low utilization rate of the target when it is running in arc mode due to cathodic arc spots having a tendency to settle or stay in a location near the area where the tangential component of the arch-shape planar magnetron-type magnetic field is strongest.

These designs also suffer from a low ionization rate of the plasma vapor flow due to a high degree of confinement provided by the balance magnetron magnetic field.

This design allows for better utilization of cathode target material, but still suffers from a relatively small arc steering area.

It also confines most of the ionized plasma flow on or near the target surface, which significantly reduces the ionization rate of vapor plasma flux and results in reduced coating adhesion.

One disadvantage of this design is that the focusing coil is not adapted to the shape of the magnetron target.

This will result in a high non-uniformity of magnetron plasma flow when applied to rectangular magnetron targets.

This results in an increase of coating defects, which is especially detrimental for such a precision applications as metal interconnect copper, and magnetic media coatings and other semiconductor and optical coatings.

It also reduces the functional properties of hard wear-resistant coatings for cutting tools, increases the coefficient of friction in coatings designated for low friction applications, and decreases the corrosion resistance of both decorative and protective coatings.

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