WO2010026425A1

WO2010026425A1 - Gas discharge electron source

Info

Publication number: WO2010026425A1
Application number: PCT/GB2009/051118
Authority: WO
Inventors: Mervyn Howard Davis
Original assignee: Nordiko Technical Services Limited
Priority date: 2008-09-05
Filing date: 2009-09-03
Publication date: 2010-03-11
Also published as: EP2319065A1; TW201027584A; JP2012502419A; GB0816240D0

Abstract

A plasma bridge neutraliser (10) comprising: a cathode (2); an anode (1) configured to be arranged annularly with respect to the cathode and spaced therefrom; means to cause electrons to be generated by the cathode by glow discharge; and an extraction plate (3) having at least one aperture (25) therein through which the generated electrons can, in use, be extracted.

Description

GAS DISCHARGE ELECTRON SOURCE

The present invention relates to a plasma bridge neutraliser. In particular, it relates to a plasma bridge neutraliser for injecting an electron flux into an ion beam from outside the beam.

In various technologies, very thin films are required. These thin films are generally of the thickness of a few microns or even a few nanometers. Ion beams are often used in the fabrication or processing of these films. The use of ion beams is well known but generally involves the illumination of substrates, or parts thereof, by a stream of energetic positive ions. Where the ion beam impacts the surface of the substrate or target, the surface will become charged. In circumstances where the substrate or target being impacted is conductive, the surface charging is not a problem.

Many end-uses require either wholly or partially insulating substrate or target, to be processed and illuminated by a positive ion beam. La these circumstances, electrical charge can accumulate. This accumulation of charge can lead to various problems.

In particular, the accumulation of the charge on an insulating surface can lead to unipolar arcing which can cause damage and particle generation. Where the devices are sensitive, dissipation of the accumulated electrical charge through the device can lead to irreversible damage. In addition, since the illuminating ion beam comprises a stream of positively charged ions, accumulated charge on the surface of the device can lead to non-uniformity of illumination thereby impeding the process and ultimately resulting in unwanted distortions and reduction in quality of the product. In the worst situations, complete failure of the systejtn can result.

In an attempt to overcome the above problems an electron source is used to introduce mobile electrons into the ion beam. These electrons have a mass which is four orders of magnitude smaller than that of the positive ions and are therefore readily transported within the ion beam, and act to effectively neutralise any charge accumulated at the surface illuminated by the ion beam.

l A variety of methods for generating the mobile electrons are known. The basic method of generating these electrons is by thermionic emission from a filament immersed within the positive ion beam. Whilst this method is useful in some applications, it is not generally used commercially since there are issues with the device being coated becoming contaminated by unwanted particles emitted from the filament. In addition, the life of the filament can be unacceptably short for commercial applications. Indeed, the lifetime can be completely unpredictable.

In an attempt to address the problems associated with immersed filaments, remote devices known as plasma bridge neutralises are used in commercial applications. These devices are used to inject an. electron flux into the positive ion beam from outside the boundary of said beam. The term "plasma bridge" arises from the plasma plume that bridges the space between the electron source and the ion beam boundary when the device is effectively coupled to the ion beam. Known plasma bridges differ from the immersed filaments of the basic method in that the filament is isolated in a discrete can from which a stream of electrons are extracted through a small aperture by the application of an electrical bias. Whilst this arrangement successfully addresses the problems associated with filament contamination, it does not solve the problem of unpredictability of the filament lifetime.

An alternative approach for addressing the above problems is to use a filament-less plasma bridge neutraliser. These neutralisers rely on glow discharge techniques to generate the electron beam. Methods of generating a glow discharge from which electrons may be extracted include dc hollow cathode, radio-frequency (RF) or microwave excitation. Since these arrangements do not use filaments the problems associated with short filament life are obviated. However, whilst these systems address this problem, they suffer from their own disadvantages and drawbacks.

In commercial applications whichever excitation technique is selected, the operation must be reliable and predictable. In addition, spacial constraints within the deposition equipment limit the size of the apparatus that can be used for the plasma bridge neutraliser. There is therefore a dichotomy to be addressed between spatial constraints and the ability to provide the required level of electron flow. In this connection, it should be noted that the neutraliser needs to provide an output of at least 25OmA while providing reliable, continuous operation for a period of many hundreds of hours.

It is therefore desirable to provide an alternative, preferably improved, plasma bridge neutraliser which overcomes the problems associated with known arrangements.

Thus according to the present invention, there is provided a plasma bridge neutraliser comprising: a cathode; an anode configured to be arranged annularly with respect to the cathode and spaced therefrom; means to cause electrons to be generated by the cathode by glow discharge; and an extraction plate having at least one aperture therein through which the generated electrons can, in use, be extracted.

Whilst the cathode will generally be of circular cross-section and the anode will be a conventional annulus therearound, it will be understood that any cross-section of cathode may be used and that the anode may similarly be of any suitable configuration. In view of this for the purposes of this application, the term "annularly" should be construed as meaning something of any configuration which surrounds the sides of the cathode.

The anode and cathode may be included within a housing or, in a preferred arrangement, the anode may form the housing.

The neutralizing of the present invention provides for the stimulation and sustainability of an annular hollow cathode discharge. By maintaining this annular discharge heat is readily dissipated to the anode.

hi one arrangement, the anode will extend around the sides of the cathode and around the end of the cathode remote from the extraction plate. The extraction plate may be connected directly to the anode.

The anode may be formed from any suitable material. In one arrangement, it will be formed from stainless steel. The cathode may be formed from any suitable material. It will generally be formed from a refractory metal. In one arrangement it may be formed from titanium. The extraction plate can be may be made from any suitable material. The material will generally be a soft, refractory, metal. One suitable material would be tantalum. The use of the soft material assists in the formation of a modist gas tight seal. Washers may be used, which may be made from graphite, to improve the seal.

The anode and the cathode may each be of any suitable size. In one arrangement, the cathode will be smaller in length than the anode such that when assembled, the cathode extends forwardly of the cathode such that there is a space between the cathode and the extraction plate. The relative sizes of the anode to the cathode may be any suitable value. In one arrangement, the ratio of the surface area of the anode exposed to glow discharge to that of the cathode is from about 3:1 to about 2:1, preferably about 2.4:1 to about 2.5:1. However, the cathode will erode in operation and as this occurs, the ratio will increase and may become in the region of from about 4:1. The ratio of the masses of the anode to the cathode will depend on the material used but for the embodiment in which the anode is formed from stainless steel and the cathode is formed from titanium, the ratio may be in the region of 20:1.

In one arrangement, the cathode may be from about 20 to about 24 mm in diameter, preferably about 22 mm diameter and the anode may have an external diameter of from about 40 to about 44 mm, preferably about 42 mm. Where anodes and cathodes having non-circular cross-sections are used, the sizes will generally be of similar nominal sizes to those given above.

Any suitable length of anode and cathode may be used. LQ one arrangement, the cathode may be from about 8 mm to about 12 mm long with about 10 mm being particularly preferred. The anode may be from about 36 to about 40 mm long with about 38 mm being particularly preferred.

The plasma bridge neutraliser may be located within a tube which will allow the neutraliser to be installed in thin film deposition apparatus. The tube, which may be of any convenient cross- section, may be of any suitable size but may have an internal diameter slightly larger than the external diameter of the neutralizer of the present invention and may therefore be of the order of from about 48mm to about 52mm with about 50mm being particularly preferred. The overall length of the tube, which will house not only the neutralizer of the present invention but also the ancillary equipment for controlling and operation thereof, may be in the region of from about 350 to about 375mm, preferably from about 360 to about 370 mm. It will therefore be acknowledged that the spatial problems associated with prior art devices have been addressed.

The cathode may be solid but in a preferred arrangement will have a plurality of apertures running longitudinally therethrough. Such cathodes are known as hollow cathodes. The apertures may be of any suitable size and configuration, hi one arrangement the apertures may be circular in cross-section and may have a diameter in the region of about 5 mm.

The spacing between the anode and the cathode may be of any suitable size. Generally it will be selected to be small to promote the internal hallow glow discharge occurring solely within the cathode itself. However, at very small spacings, of the order of less than about lmm, problems may arise. Material moves around inside the assembly due to the affects of sputtering from one surface to another. This leads to an accumulation of debris, often as dendritic needles, which will bridge the gap between the two electrodes. In order to reduce this, the gap will generally be of the order of from about lmm to about 10 mm with gaps in the region of about 5mm being particularly preferred.

The internal surfaces of the neutralizer assembly may be roughened to encourage material adhesion. Roughening may be achieved by any suitable means. In one arrangement, bead blasting may be used to roughen the surfaces. This technique is common practice in the preparation of vacuum deposition equipment.

The cathode will generally be provided with a high tension supply, normally dc, which is needed to excite the cathode sufficiently to cause discharge to occur and the electrons to be formed. This supply may be provided via a metallic bolt which passes through the anode and into the cathode. The bolt may be formed of any suitable material. Stainless steel may be used but it may degrade in use and eventually fail. In a preferred arrangement, titanium or a similar metal may be used.

Where the anode extends around the end of the cathode remote from the extraction nlate one nr more insulators may be used to separate the anode from the cathode both electrically and physically. The insulators may be formed from any suitable material. Ceramic materials may be particularly suitable. Suitable ceramics include alumina, porcelain or steatite. Where an insulating material is required which has good thermal conductivity aluminium nitride may be used. Anodised aluminium may also be used to isolate the anode from ground since the anode bias is relative low, typically of the order of about 24V.

Washers and/or seals may also be used. The material from which washers and/or seals are formed will be selected to withstand the environment within the neutraliser. Graphite may be a suitable material.

A washer and/or space may be used to create a small crevice between the surfaces of the cathode and any insulator which may be present. In operation, the insulator may become coated with conductive material. The use of a crevice, or undercut, will prolong the time it takes to make a conductive path between the anode and cathode and thereby increase the life of the neutraliser.

The arrangement of the present invention not only meets the spatial requirements but also provides a high electron flux output which reduces the risk of overheating and melting of one or more components and which has substantially longer life than has been achievable heretofore. Without wishing to be bound by any theory, it is believed that the key to the longevity of operation is the means of removing heat. In the arrangement of the present invention, the cathode is well isolated and loses heat primarily through radiation. By arranging the configuration such that there is an annular discharge, a substantial proportion of the heat load is on the anode which can more readily dissipate heat, both through radiation from its body and any support plate and through conduction. Heat from the anode conducted back to a supporting chamber in the tube where present which behaves as a heat sink. Thus the arrangement allows for heat loss by radiation, conduction and convention. By enabling this path for heat dissipation it is possible to maintain a high electron output without the risk of overheating which may cause the cathode to melt.

Where a support plate is used, it will need to take the physical weight of the neutralizer and carry the anode bias. The support plate will generally be earthed. The support plate may be formed from a ceramic material. Suitable ceramic materials include those identified above in connection with insulators. The support plate may be at anode potential and tied back to an earthed support flange, and vacuum interface, using bolts which pass through insulators. These may be made from anodised aluminium since it provides electrical isolation but offers relatively high thermal conduction.

In operation, the neutraliser emits a plume of electrons from one or more apertures in the extraction plate. Suitable apertures include those having a diameter of about lmm. A single aperture may be used as the current density for electrons is high when compared to that for positive ions. In one arrangement, the extraction plate may include multiple apertures. Where multiple apertures are used, they may be located to line up with corresponding apertures in the cathode. For example, four apertures can be arranged in an alignment with four apertures in the cathode. In one arrangement, apertures may be aligned with the annular space around the periphery of the cathode.

It is also possible to mount the cathode in a manner that permits electron extraction from more than one face of the anode. Where used, this arrangement has the advantage that it readily allows the neutraliser to provide art electron flux directed into two, or more, discrete ion beams simultaneously. The apparatus may be configured such that the extraction plate provides emission from facets displaced at angles to one another other than parallel. Each emitting facet may be configured with a single, or with multiple, emission apertures. It will be understood that the detailed design of the cathode and anode can be manipulated to suite the geometry into which the electron plume needs to be ejected.

It will be understood that for a complete process module the budget for the total gas load can present difficulties. It is therefore advantageous in certain circumstances to minimise the gas load from the neutraliser. It will be understood that in the prior art arrangements using an immersed filament, there is the advantage that the immersed filament operates using the background gas and does not require a dedicated gas flow. Since the neutraliser of the present invention relies on the generation of electrical glow discharge there is a need for a gas flow and suitable means for allowing ingress of gas will be provided. In principle, the only exit from the anode can be the emission aperture in the extraction plate. In practice, the aperture plate may not form an hermetic seal nor will the cathode feedthrough. It is therefore desirable to maximise the sealing at these areas while balancing cost requirements.

The present invention will now be described, by way of example, by reference to the accompanying drawings in which:

Figure 1 is a schematic drawing of a cross-section through the neutralizer of the present invention;

Figure 2 is a more detailed cross-section through the neutralizer of the present invention;

Figure 3 illustrates the neutralizer of the present invention located in a tube for insertion in the deposition apparatus with ancillary apparatus illustrated;

Figure 4 is front end view of the arrangement of Figure 2;

Figure 5 is an illustration of some examples of arrangements for apertures in the extraction plate;

Figure 6 is a schematic illustration of the plasma bridge neutraliser emitting an electron plume in two directions; and

Figure 7 is a schematic representation of the seal between the extraction plate and the anode. s schematically illustrated in Figure 1, the plasma bridge neutraliser of the present invention comprises a stainless steel anode 1, a titanium cathode 2 and a tantalum aperture extraction plate 3. The extraction plate 3 is connected to the anode 1 via suitable fixings 4. The extraction plate illustrated has a single aperture 25 through which the electrons will be emitted. A space 5 extends around the cathode and above and below it. A titanium bolt 6 is used to hold the anode 1 and cathode 2 in position. This bolt serves two functions. It supports the cathode in position and connects the high tension supply to the cathode. Insulators 7, 8 and 9 are present to separate the anode 1 from the cathode 2 both electrically and physically.

Washers may be used, not shown in Figure 1, to take up the space. The washers will also create the crevice between the surfaces of the cathode and the insulator.

In the illustration of Figure 2, the washers 11, 12 and 13 are shown. The crevice formed by washer 13 is also visible. A gas inlet 14 is provided.

Figure 3 shows the neutraliser 10 located in the tube 15. The gas inlet 14 is connected to a gas feedthrough 16 and 17 which are connected via a flexible hose 18. A safety switch 19 is provided. A support bar 20 is located within the tube 15. Clamps 21 and 22 are provided and these include seals 23 and 24.

The tube 15 viewed from the front end through which electrons are emitted is illustrated in Figure 4.

Figure 5 is an illustration of some examples of aperture locations on the extraction plate.

hi an alternative embodiment, the plasma bridge neutraliser 10 of the present invention is illustrated schematically in Fig 6. In the arrangement, 2 extraction plates 3a and 3b are shown each with a corresponding aperture 25a and 25b.

Electron plumes 30a and 30b are emitted from their respective apertures.

Detail of the extraction plate 3 connected to the anode 1 by fixing 4 is illustrated in more detail in Figure 7. In this arrangement a clamp ring 50 is used to extend around the anode such that clamping occurs around the complete periphery of the anode. This is illustrated further in Figure 4.

Claims

1. A plasma bridge neutraliser comprising: a cathode; an anode configured to be arranged annularly with respect to the cathode and spaced therefrom; means to cause electrons to be generated by the cathode by glow discharge; and an extraction plate having at least one aperture therein through which the generated electrons can, in use, be extracted.

2. A plasma bridge neutraliser according to Claim 1 wherein the anode forms the housing for the neutraliser.

3. A plasma bridge neutraliser according to Claim 1 or Claim 2 wherein the anode is formed from stainless steel.

4. A plasma bridge neutraliser according to any one of Claims 1 to 3 wherein the cathode is formed from titanium.

5. A plasma bridge neutraliser according to any one of Claims 1 to 4 wherein the extraction plate is formed from tantalum.

6. A plasma bridge neutraliser according to any one of Claims 1 to 5 wherein the ratio of the surface area of the anode exposed to glow discharge to that of the cathode is from about 3:1 to about 2:1, preferably about 2.4: to about 2.5:1.

7. A plasma bridge neutraliser according to any one of Claims 1 to 6 wherein the cathode has a plurality of apertures running therethrough.

8. A plasma bridge neutraliser according to any one of Claims 1 to 7 wherein a metallic bolt passes through the anode and into the cathode.

9. A plasma bridge neutraliser according to Claim 8 wherein the bolt is formed from titanium..

10. A plasma bridge neutraliser according to any one of Claims 1 to 9 wherein insulators are provided to separate the anode from the cathode electrically and physically.