PLASMA SOURCE

Abstract: The invention which is the subject of this application relates to the provision of improvements to a plasma source which can be used, for example, in conjunction with magnetrons with metal targets for creating reacted metal layers on substrates such as, for example, ophthalmic lens. The improvements to the plasma source allow the plasma created to be controlled such that the flow of plasma from the plasma source is controlled as it passes towards the coated substrates. The ability to control the path of the plasma allows the plasma itself to be denser in form and, furthermore, reduces the area over which the plasma reacts thereby reducing wastage of materials while increasing the effectiveness of the plasma created.

Description: PLASMA SOURCE The invention which is the subject of this application is a plasma source for use in the generation of a plasma and typically, but not exclusively, for use in the preparation of coatings on substrates and particularly ophthalmic coatings for lens. Plasma sources are used extensively in the preparation of optical coatings on lenses and particularly, but not exclusively, to cause, for example, a reaction on a previously deposited metal such as the oxidation of metal layers which have previously been applied to the surface of the lens by sputtering. One particular area of use is in the manufacture of a multi-layered coated lens wherein a metal layer is applied to the lens to be coated and the same is then oxidized by introducing oxygen and activating the plasma source to react the metal layer and so on with further or alternate layers of metal until the multi-layered coating is formed. Activating the plasma source and the magnetic field causes a plasma to be generated into the vacuum chamber in which the lens are held whereupon the action of the plasma on the metal layers on the lens causes same to be oxidized. There are several forms of plasma source currently utilised, one of which is known as an unbalanced magnetron whereby a gas supply, such as an oxygen supply, is provided adjacent to the magnetron, which, when activated, generates electrons and these electrons, in conjunction with the oxygen, causes a plasma to be created and emitted into the vacuum chamber and, upon impact of the plasma with the lens outer layer, oxidation of the layer is caused. Furthermore, this type of magnetron can be used in conjunction with an oxygen and argon mixture and the plasma created is impacted onto uncoated lenses to effectively clean the surface of the same prior to the coating process commencing. This form of unbalanced magnetron plasma source is extensively used in ophthalmic coating apparatus to advantage as it is found to create a plasma containing a mixture of, for example, oxygen species when oxygen or oxygen containing gas is supplied, and a generation of a plasma of this type is of benefit in the preparation and improvement of the structure of the coating on the lens. However, problems do occur and one problem is that good adhesion of coating layers to the lens cannot always be obtained and another is that a sufficiently high rate of oxidation cannot be obtained to allow rapid deposition of layers onto the substrate. This, in combination with the fact that the energy of the plasma is relatively low, means that the subsequent oxides formed may not be of the desired quality and that problems can be encountered with respect to the adhesion of the layers to the lens. Thus, although the unbalanced magnetron produces a plasma of sufficient volume, the fact that the emission and direction of emission from the magnetron cannot be controlled, and the energy and flux of the same is relatively low, causes the effect of the plasma to be diluted. In particular the magnetron does not easily provide sufficient energy and flux from the plasma to allow good adhesion of the coatings to the lens and additionally the activity level of the plasma is lower than required. An alternative form of device for use in the formation in the coating of the lens is disclosed in various papers, such as in the Journal of Vacuum Science and Technology A, 2nd Series, Volume 10 No. 4, Part 2 - July/August 1992, entitled PLASMA ION ASSISTED DEPOSITION: A PROMISING TECHNIQUE FOR OPTICAL COATING by S Pongratz and A Zolla, wherein there is disclosed a plasma ion deposition source which includes a chamber in which is mounted a filament cathode and an anode means and, positioned adjacent thereto, an ion gun. In operation, a gas such as argon is introduced into the chamber surrounding the filament and the filament electrically activated to create a DC voltage between the anode and cathode to create a glow discharge while, at the same time, oxygen is introduced into the vacuum chamber. The ion gun is activated to discharge electrons into the chamber and the plasma created causes further oxidation of the metal oxide layer which is simultaneously applied to the lens to coat the same. Major disadvantages with this arrangement are, firstly, that a filament is required to be provided to allow the ion gun to be operable and, during operation, this filament is eroded at an extremely fast rate, for example, with a tungsten filament failure can occur due to erosion within 30 minutes of commencement of operation, and hence, new filaments are required to be fitted frequently which is expensive and time consuming. Commercial exploitation of this form of device in any coating apparatus and particularly in relatively small scale coating apparatus, is impractical and uneconomic. A further disadvantage is that only energetic ions and fast atoms, rather than mixtures of actuated species of ions, are impacted onto the lens coating and hence the growth of the coatings and composition of the same is impaired. A further problem of conventional plasma sources is that if the bodies of the sources are made from aluminium, or a similar metal, at least part of the body can be eroded by arcing during operation and the eroded material is subsequently deposited on the lens as micro droplets which can cause a haze effect on the lens coating. The aim of the present invention is to provide a plasma source which is operable without the need for a filament and which is provided in a form whereby the pattern of the plasma created and the area of generation of the same can be controlled and dictated by the shape of the plasma source. A further aim is to provide the plasma source in a manner whereby erosion of the surfaces is reduced and hence the quality of the coating on the lens is improved by all or any combination of the improvements detailed herein. The present invention relates to a plasma source for use in generation of a plasma wherein said source includes a support structure in which is formed a magnetron and side walls to define a plasma creating area; a means for introducing a gas into said area and a means for activating said magnetron to create a plasma in said area and at least the side walls are provided with a series of magnets thereon to control the emission of the plasma created from the plasma source. Typically the base is provided with a gas inlet to allow the gas to be introduced into the area which is in the form of a cavity. In one embodiment the inlet is mounted substantially in the centre of the magnetron base or alternatively, the inlet or inlets can be positioned to introduce the gas into the cavity at particular locations. One possibility is that the gas inlet is of an annular shape. In one embodiment the gas is oxygen and in another embodiment the gas is a mixture of oxygen and argon and preferably the gas pressure is between 1-5 mTorr. In a preferred embodiment the side walls are in the form of a cylinder with the base provided at one end to form what can be described as an open-ended barrel shape. Typically the plasma source operates on the principles of an unbalanced magnetron. Alternatively, the side walls can be shaped to define a box or a linear shape or any suitable geometrical shape, or indeed, in one embodiment the side walls can be angled inwardly to form an exit aperture for the plasma which is significantly smaller than the base such as, for example, a slot. In whichever form, the exit aperture is formed to optimise the uniformity of the reaction created by the plasma which is emitted by the source. The shape of the exit aperture is therefore dependent upon the particular requirements and the required plasma application pattern for the particular use and coating process for which the plasma source is provided. In one embodiment the side wall comprises a plurality of upstanding portions at spaced apart intervals and thus it should be understood that the side wall does not have to be a continuous wall around the periphery of the magnetron base. The criteria for the spacing of the said side walls is determined by the fact that the space between the same should not be so great that the control effect on the plasma is reduced. In a further aspect of the invention the magnetron base is provided with additional magnets to a conventional base and the provision and location of the additional magnets assists in the confinement of the plasma created and thereby improves the properties of the plasma in terms of density, floating potential and the like. In a further aspect of the invention the magnetron base is textured and/or shaped to improve the generation of plasma and the control of same in the plasma source of the invention. Typically the plasma source of the invention is utilised on the principles of operation of an unbalanced magnetron of conventional shape and form and therefore the plasma source of the current invention provides improvements in the efficiency and performance but does not require any additional operation requirements. Typically the plasma is created by the introduction of the gas into the cavity and the supply of controlled electrical power to the plasma source to create a negative bias of the plasma source in relation to a vacuum chamber in which the plasma source is mounted and the gas inlet is not earthed. Thus a feature of the plasma source is the magnetic confinement of the plasma created and the directional control of the gas flow through the source which is achieved by providing the plasma source in the form claimed. Furthermore, as the plasma source of the invention provides a means for magnetically and physically enclosing the plasma by confining the gas within the plasma creating area defined by the cavity of the plasma, thus a more intense plasma is created and this increases the efficiency of use of the gas which is introduced into the cavity or plasma creating area. In a preferred embodiment at least the core of the plasma source base and side walls are formed from aluminium or aluminium alloy as it is relatively cheap, easy to machine, non-magnetic and a good thermal conductor and, in this embodiment, at least the base is preferably coated with a layer of titanium facing into the cavity. Preferably the faces of the side walls facing into the cavity are also coated in titanium. In an alternative embodiment a metal or metal alloy or compound having similar properties as titanium i.e. high melting and low sputter yield in oxygen, can be used to coat the surfaces defined in the cavity thereby reducing arcing and material erosion. In a further aspect of the invention it is preferred that whatever material is used to form the surfaces of the side walls and base, at least part of the same is covered in an insulating material such as, for example, plastic insulating tape as this is found to reduce and indeed prevent arcing occuring within the plasma source during operation and thus reduces damage occuring to the magnetron during operation and the deposition of eroded material onto the articles to be coated. Typically the side walls of the plasma source are provided with a series of pockets formed at spaced intervals for the location of magnets therein and said magnets can be either permanent or electromagnets. Typically electrons generated in the cavity are moved in a substantially annular path in an area around the cavity adjacent the side walls and the magnets are spaced so as to maintain the electrons in this path and prevent substantial leakage of the electrons from the path. The introduction of a gas into the cavity and into the path of the electrons causes a plasma of ions and electrons to be created and this is emitted from the cavity under the influence of the magnets and side walls and within an area substantially defined by the cross sectional area of the cavity. The position of the magnets can be used to optimise the plasma distribution. In one preferred embodiment there is a provided a network of channels through the support structure of the plasma source and through which air or water or other coolant fluid is passed during operation of the plasma source to cool the plasma source. In one embodiment the plasma source is used in conjunction with apparatus for coating substrates in the form of ophthalmic lenses held in a vacuum chamber into which the plasma source protrudes. Typically the plasma source is used in conjunction with at least one magnetron which is used to provide a sputtered metal coating onto the lens and activation of the plasma source causes the plasma to be treated as described above and hence the plasma impacts on the metal layer on the lens to create a reaction thereon. In one specific embodiment, two metal sputtering magnetrons and a plasma source, are provided in a vacuum chamber and operation of the same in a specific controlled manner allows a multi-layer coating to be formed on the lens. A typical lens which can be coated using apparatus incorporating the plasma source of the invention includes CR39 lens, lens with a thermally cured hard coat resin on lens with a UV cured hard coat resin. In a further aspect of the invention there is provided apparatus for the coating of ophthalmic lens to form a multi- layered coating, said apparatus including a vacuum chamber and, in said vacuum chamber, a lens holder adapted to rotate in said chamber, at least one magnetron with a metal target for the sputtering of the metal onto the lens to form a metal layer thereon and a plasma source and wherein the said plasma source includes a base portion with side walls depending therefrom and power supply means to said plasma source and a means for the introduction of a gas into the area defined between the side walls and base to create a plasma in said source and control the path of exit of same from the source onto the lens. Operation of the apparatus so described causes a plasma to be created in a vacuum chamber to react with the metal coating on said lens to form the required coating. Typically the apparatus includes two magnetrons with two different metal targets with one magnetron in operation at any one time to sputter deposit a layer of metal onto the lens and the plasma source is operated during alternation of the magnetrons to cause each metal layer to be reacted and therefore create a multi-layer coating of reacted metal on the substrates. Typically the gas introduced into the chamber is either of oxygen or an oxygen argon mixture. Typically the oxygen gas is introduced for the purpose of forming the sputtered metal layer into an oxide. Preferably the oxygen argon mixture is introduced to create a plasma which acts to clean and prepare the lens surface prior to the sputtering of metal thereon. Other gases could be used for example, nitrogen to produce nitride films, hydrocarbons or fluorinated hydrocarbons to produce carbon/carbide or fluorocarbon containing layers and these provide other potential applications of the plasma source. A specific embodiment of the invention is now described with reference to the accompanying drawings, wherein:- Fig. 1 illustrates a plan view of the plasma source according to the invention; and Fig. 2 illustrates a sectional elevation of the plasma source along line A-A of Fig. 1. Referring firstly to Fig. 1, the plasma source 2 according to the invention is shown and is held at negative voltage with respect to the vacuum chamber in which it is held. The plasma source comprises a magnetron base 4 called a reflector (so called because it reflects electrons into the magnetic field) which is circular in this embodiment and substantially planar and, around the periphery of same, there is provided side wall 6 and, defined between the side wall 6 and the base 4 is an area or cavity in which the magnetic field will be generated. At the centre of the base 4 in this embodiment is provided an inlet 8 which leads to a gas supply and allows a gas, on its own or in a gas mixture, to be introduced into the area or cavity 10 defined by the base 4 and the side wall 6. In alternative embodiments the gas inlet aperture may be annular to allow the gas to be introduced into the cavity 10 substantially adjacent to the side wall 6. The base 4 and side wall 6 are mounted on a support structure 11 comprising a cooling body and an insulator 12. The support structure 11 includes service facilities such as cooling water inlets and outlets and seals for sealing the vacuum and water within the vacuum source. Fig. 2 illustrates a cross sectional view along lines A-A of the plasma source and the same reference numerals for the same parts are used as with Fig. 1 with the cooling body referred to and shown by the numeral 14. The side wall 6 is shown in more detail and is formed from an aluminium alloy body 16 and, in this embodiment, the inside surface of the wall of the cylinder i.e. the wall facing inwardly and towards the cavity 10 may be of aluminium, or of aluminium coated with titanium. The base 4 may be coated with, or made from, titanium and may be covered with an insulating layer of a suitable polymer such as, for example, a self-adhesive insulating tape 19. The side walls 6 are also provided with a series of pockets 20 at spaced intervals. The pockets are in the shape of longitudinal channels each of which passes substantially along the length of the side wall. In each pocket there is provided a number of magnets 22 which can be either permanent or electro-magnetic type. Sufficient magnets are provided so that the same are of sufficient power and the magnets are spaced so that the same are sufficiently close to cause the electrons generated in the cavity 10 to pass around the cavity in path 31 defined by the shape of the side walls 6, in this case an annular path such that the magnets act as a trap to maintain the electrons in this path to prevent the electrons from leaking from the cavity 10. The plasma source support structure 11 includes a cooling water inlet 24 and outlet 26 which are spaced apart and joined by a series of channels which cause the water supply to pass into the support structure and adjacent to, but separated from, at least the base 4 to allow the same to be cooled during the operation of the plasma source 2 but separated to prevent corrosion of the aluminium structure and magnets. The plasma source is conventionally used for the treatment of metal coatings on substrates such as lenses and the location apertures 28 allow the plasma source 2 to be bolted into the wall of a vacuum chamber and depend into the same such that the cavity is in connection with the chamber and the plasma created can then be applied in the desired path to lenses held within the vacuum chamber. In operation, the plasma source is started to operate by the switching on of a controlled power supply which causes electrons and ions to be generated in the cavity and the electrons, under the control of the magnets in the side walls, move in an annular path around the cavity 10 and adjacent the side wall 6. The introduction of a gas, such as oxygen, into the cavity 10 causes plasma to be created which passes around the side walls to create an annular discharge. As the plasma continues to be generated, the same is emitted from the plasma source and is directed towards the lenses to be coated in the vacuum chamber and impacts on the lenses which are held at a set distance from the plasma source to react the surfaces thereof and create the desired layer structure. It is found that by using the plasma source of the invention that a denser and higher flux plasma is created and also that the plasma has a higher floating potential. This is due to the fact that a reduction of the electrons which can leak from the source in an uncontrolled manner is achieved by the provision of the side wall 6 with magnets therein as part of the plasma source which act to constrain and control the motion of the electrons in the cavity, the negative species of the electron cloud then trap the positive ions so forming in conjunction with the gas, a plasma which is shaped by the magnetic field which is defined by the magnets in the side walls of the plasma source. Activated oxygen species which are neutral will predominantly be formed where the plasma is densest but are free to travel as they are not influenced by the magnetic or electrostatic fields. The discharge of the plasma source is provided in a direction substantially in the plane of the central axis 30 of the plasma source and substantially over an area 32, the size of which is substantially defined by the cross sectional area of the outlet of the cavity 10 which can be larger, smaller or the same or of a different shape to the base as desired and this area is substantially maintained until the plasma contacts the lens. The control of the path of the electrons in the cavity allows the plasma to be controlled and directed more accurately towards the coated lens, hence the plasma has higher energy and therefore has a greater and improved effect on the treatment of the metal layer on the lens than is possible by using conventional unbalanced magnetron plasma sources. The present invention provides a plasma source which, in use, generates a plasma which has improved energy and density and, as a result of this, the coating created by using a plasma source of this type is found to be more stable in terms of structure and longevity. This allows lenses which previously could not be coated using this form of conventional apparatus to be coated as the adhesion and structure of the coatings is improved to a sufficient extent. Furthermore the apparatus of the invention does not require the use of a filament and hence considerable savings are made in terms of operating costs and furthermore the coating of the relevant surfaces in a suitable material, such as titanium, ensures improved coating quality.

Claims: CLAIMS 1. A plasma source for use in generation of a plasma wherein said source includes a support structure in which is formed a base and side walls to defind a plasma creating area; a means for introducing a gas into said area and a means for creating a plasma in said area and at least the side walls are provided with a series of magnets therein to control the emission of the plasma created from the plasma source. 2. A plasma source according to claim 1 wherein the base is provided with a gas inlet to allow the gas to be introduced into the cavity defined as the plasma coating area. 3. A plasma source according to claim 2 wherein the gas inlet is mounted substantially in the centre of the base. 4. A plasma source according to claim 2 wherein a plurality of gas inlets are located on the base and/or side walls of the plasma source. 5. A plasma source according to claim 1 wherein the gas introduced is oxygen and/or with argon mixed therewith. 6. A plasma source according to claim 1 wherein the side walls are shaped so that the same are in the form of an open ended cylinder with the base at one end thereof. 7. A plasma source according to claim 1 wherein the source operates on the principles of an unbalanced magnetron with the inside of the side wall provided as the magnetron. 8. A plasma source according to claim 1 wherein the side walls comprise a plurality of upstanding portions spaced apart around the periphery of the base to form, in conjunction the side wall of the plasma source. 9. A plasma source according to claim 1 wherein the base is textured and/or shaped and/or may contain magnets therein or adjacent thereto. 10. A plasma source according to any of the preceding claims wherein a supply of controlled electrical power is provided to the plasma source to create a negative bias of the plasma source in relation to the vacuum chamber in which the plasma source is mounted. 11. A plasma source according to any of the preceding claims wherein a series of magnets are located at spaced intervals around the side wall. 12. A plasma source according to claim 11 wherein upon operation of the plasma source, the magnets serve to confine and maintain an electron-rich plasma substantially adjacent the side wall. 13. A plasma source according to claim 12 wherein the gas inlet is located to introduce gas into the cavity and into the path of the electrons to cause a plasma of ions and electrons to be created. 14. A plasma source according to any of the preceding claims wherein the support structure includes a series of channels through which a coolant fluid can be passed during operation of the plasma source to cool the same. 15. A plasma source according to any of the preceding claims wherein the plasma source is used in conjunction with apparatus for coating substrates in the form of opthalmic lenses held in a vacuum chamber. 16. A plasma source according to claim 15 wherein a plasma source is used in conjunction with at least one magnetron which is provided with a metal target to sputter metal onto the substrate to be coated. 17. Apparatus for the coating of a plurality of substrates to form a multilayered coating thereon, said apparatus including a vacuum chamber and, in said vacuum chamber, a substrates holder adapted to rotate or reciprocate in said chamber, at least one magnetron with a metal target for the sputtering of metal onto the substrates to form a metal layer thereon and a plasma source to generate a plasma to react the metal layer and wherein the said plasma source includes a base portion with a side wall depending therefrom and power supply means to said plasma source and a means for the introduction of a gas into the area defined between the side wall and base of the plasma source. 18. Apparatus according to claim 17 wherein the same includes two magnetrons with two different metal targets with one magnetron in operation at any one time to sputter deposit a layer of metal onto the substrates and the plasma sources operated during alternation of operation of the magnetrons to cause each metal layer to be reacted and therefore create a multilayer of reacted metal on the substrates. 19. A plasma source according to any of the preceding claims wherein the core of the base and side walls are formed from aluminium or aluminium alloy. 20. A plasma source according to claim 19 wherein at least the inner surface of at least the base and/or side walls is formed from titanium. 21. A plasma source according to any of the preceding claims wherein the faces of the base and/or side walls facing the cavity of the plasma source are covered or at least partially covered with an insulating material. 22. Apparatus for the coating of a plurality of substrates, said apparatus including a vacuum chamber, and, in said vacuum chamber, a substrate holder adapted to rotate and/or reciprocate in the said chamber, and a plasma source to generate a plasma to react with a gas introduced into the chamber to cause a film to be deposited on the substrates and a power supply means to said plasma source and wherein the said plasma source includes a base portion with a side wall depending therefrom and a means for the introduction of the gas into the area defined between the side wall and the base of the plasma source. 23. A plasma source according to claim 22 wherein the gas introduced is a hydrocarbon or fluorinated hydrocarbon gas to produce a carbon/carbide or fluorocarbon containing layer on the substrates. 24. A plasma source and apparatus as hereinbefore described with reference to the accompanying drawings.

Inventors: J AFER HUSSAIN, SPENCER ALARIC GRAHAM, WALLS JOHN MICHAEL, WAUGH ALLEN ROBERT, WHITE NORMAN HENRY, J'AFER, HUSSAIN, SPENCER, ALARIC, GRAHAM, WALLS, JOHN, MICHAEL, WAUGH, ALLEN, ROBERT, WHITE, NORMAN, HENRY
Applicants: AFER HUSSAIN J, APPLIED VISION LTD, SPENCER ALARIC GRAHAM, WALLS JOHN MICHAEL, WAUGH ALLEN ROBERT, WHITE NORMAN HENRY, J'AFER, HUSSAIN, SPENCER, ALARIC, GRAHAM, WALLS, JOHN, MICHAEL, WAUGH, ALLEN, ROBERT, WHITE, NORMAN, HENRY
Application year: 1996
Main classification: H01J37
IPC codes: H01J37