Abstract:
A method and apparatus for forming a coating on a substrate. The system (1) comprises an electron-beam evaporator (5) to produce an evaporant (6) from a source material, a plasma generation chamber (3), within which a magnetoplasma is generated, and a magnetic field supply means (10) to apply a magnetic field (11) to the apparatus (1), to transport the magnetoplasma (60) to the substrate (12).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for coating a substrate, and in particular, to a method and apparatus which creates a magnetoplasma in the vicinity of an evaporated source material to assist the transport of an evaporant and/or ions of a background gas under the influence of an applied magnetic field for deposition on a substrate material. 
     DESCRIPTION OF THE PRIOR ART 
     The transporting of plasma by means of a magnetic field is well known in the literature, and is commonly used for forming thin film coatings in the case where the source of material is a cathodic arc (Aksenov et al, Soviet Journal of Plasma Physics 4 (1978), page 425). 
     The transportation of plasma by magnetic fields has also been described, in U.S. Pat. No. 4,810,935, entitled `Method and apparatus for producing large volume magnetoplasmas` by R W Boswell. In this case no source of evaporation was discussed, but rather the plasma was created in gases admitted to the process. 
     The use of a thermal evaporation source with a plasma in its vicinity is also well known as the `ion plating` process. That known process differs from the present invention by its use of electric fields only to transport the plasma to the substrate rather, than a magnetic field and an electric field, as in the invention. 
     The use of energetic ions to densify a film is also well known as Ion Assisted Deposition. That known process differs from the present invention by its use of a source of bombarding ions (plasma or ion source) not transported by a magnetic field and separate from the source of deposited material. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a method and apparatus for forming a coating on a substrate, which differs from those prior art methods and apparatus hereinbefore discussed. 
     The present invention also seeks to provide a method and apparatus for forming a coating on a substrate which has a wide variety of applications therefor, such as producing &#34;doped&#34; films or alloys of various materials, for example, semiconductor materials. 
     In one broad form, the present invention provides an apparatus for forming a coating on a substrate, comprising: 
     an electron-beam evaporator to produce an evaporant from a source material; 
     a plasma generation chamber, within which a magnetoplasma is generated from the said evaporant and (most usually) also from a background gas; and, 
     magnetic field supply means to apply a magnetic field to said apparatus, to transport said magnetoplasma to said substrate. 
     Preferably, said plasma generation chamber is supplied intermediate said evaporator and said substrate. 
     Preferably, the antenna is excited with radio frequency power, which is preferably in the frequency range 10 to 30 MHz. 
     Also preferably, each of said evaporator, said plasma generation chamber and said substrate are substantially axially concentric. 
     Most preferably, said plasma generation chamber is situated above said evaporator and below said substrate in a substantially vertical configuration. 
     In the preferred embodiment, said magnetic field acts to increase the density of said magnetoplasma and to increase the ionisation proportion of said evaporant and any background gases admitted to the chamber. 
     In another preferred embodiment the ionisation proportion of said evaporant is low or zero, and in the primary effect of the magnetic field is to transport ions of the said background gas to the surface to carry out the process of ion assisted deposition. 
     In another preferred embodiment the background gas contains precursors enabling the deposition of a doped film onto said substrate which originates from the background gas, while the dopant originates from the evaporant. 
     Most preferably, said evaporator is comprised of a crucible possibly having more than one compartment therein, each compartment housing a different source material, and whereby said evaporator is controlled to produce evaporant(s) from each source material, as desired. 
     In a preferred form, a bias voltage (DC or RF) is applied to said substrate. 
     Perhaps most preferably, said bias voltage is up to about 4 kV. 
     Preferably said substrate is heated or cooled. 
     In a most preferred form, a potential maximum in the plasma is located near to the evaporator such that said evaporant is largely ionised at a position at or above the potential maximum. 
     Perhaps most preferably, said magnetoplasma has an ion density greater than 10 11  /cc. 
     Preferably, said source material(s) is/are selected from boron, titanium. zirconium, carbon, hafnium, tungsten, silicon, germanium, chromium, tantalum, vanadium, magnesium, aluminium, manganese, nickel, copper, niobium, molybdenum, silver, indium, tin, yttrium, gold, any rare earth metal, gallium, calcium, strontium and barium. 
     Also preferably, said magnetoplasma is selected from the gases of argon, nitrogen, oxygen, hydrogen, methane, acetylene, phosphine, diborane, arsine, germane, silane, silicon tetrachloride, krypton, fluorine, trimethylgallium, aluminiumtrichloride, titaniumtetrachloride, teos and disilane. 
     Perhaps most preferably, said source material is boron and said magnetoplasma is comprised of a mixture of nitrogen and argon, such that said coating formed on said substrate comprises cubic boron nitride. 
     Perhaps most preferably, said source material is silicon and said magnetoplasma contains oxygen and/or nitrogen, such that said coating formed on said substrate comprises silicon dioxide or silicon nitride or a mixture of these. 
     Perhaps most preferably, said source material is germanium and said magnetoplasma comprises a mixture of silane and possibly oxygen, such that said coating formed on said substrate comprises germanium doped silicon or silicon dioxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the following detailed description of the preferred but non-limiting embodiment thereof, described in connection with the accompanying drawings, wherein: 
     FIG. 1 shows an schematic representation of the key components of an apparatus in accordance with the present invention; 
     FIG. 2 details more precisely an actual embodiment of the apparatus; 
     FIG. 3 shows a detailed view of the side of the apparatus shown in FIG. 2; and, 
     FIG. 4 shows a detailed view of the top of the apparatus shown in FIGS. 2 and 3. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout the drawings, like numerals will be utilised to represent similar features, except where expressly otherwise indicated. 
     The apparatus shown in the drawings, and as perhaps best illustrated in FIG. 1, and generally designated by the numeral 1, is comprised three core interconnected components, an evaporant producing area 2, a plasma generation chamber 3, and a deposition chamber 4. The evaporant chamber 2 basically comprises an electron beam evaporator 5, in which an electron beam is directed at a quantity of source material held in a crucible to produce an evaporant 6 from the source material. 
     The plasma generation chamber 3 comprises a gas inlet 7 to supply gas thereinto, and an antenna 9 for launching helicon waves. A magnetic field is sustained by field coils 10 which provide a magnetic field shown by field lines 11 which are essentially vertical and extend towards either end of the apparatus 1. The plasma is sustained by radiofrequency inductive excitation. A suitable design for the plasma source consists of a cylindrical silica tube to which an external antenna is attached of a type normally used for launching of helicon waves. 
     In the deposition chamber 4, a substrate which is to be coated 12, is located some distance above the plasma source and facing downwards. Field coils 13 assist the field coils 10 within the plasma generation chamber 3 to produce the magnetic field as shown by field lines 11. The substrate 12 may be biased with DC or RF potentials and heated or cooled to an appropriate temperature. 
     An important feature of the invention is that the maximum plasma potential in the plasma is located a distance above the evaporation source 5, which is sufficiently small that ions of evaporant 6 and/or background gases are largely polarised at or above the potential maximum. This is achieved by controlling/adjusting the pressure, plasma density, and magnetic field intensities to optimum values. 
     The conditions of the process which we have observed to lead to the formation of the cubic phase of boron nitride, c-BN, are listed in Table I, as follows: 
     
                       TABLE I______________________________________Conditions leading to c-BN formation______________________________________Electrical input to electron beam evaporator                   140 mA at 8 kVPower delivered to plasma source                   300 WDistance from boron source to bottom                   70 mmof plasma sourceLength of plasma source 300 mmInternal diameter of plasma source                   130 mmDistance from boron source to substrate                   600 mmrf voltage peak to peak applied to substrate                   1400 Vdc bias                 -360 VSubstrate temperature   260° C.Argon flow rate         3 sccmNitrogen flow rate      5 sccmTotal pressure          0.5 m TorrSubstrate material      siliconCurrent in field coils(top of helicon source) (field up)                   0.2A(bottom of helicon source) (field down)                   1AAverage diameter of field coils                   195 mmNumber of turns         250Coil centre to centre spacing                   210 mm______________________________________ 
    
     The conditions of the process which have been observed to lead to the formation of silicon dioxide are listed in table 2 as follows: 
     
                       TABLE II______________________________________conditions leading to the formation of silicon dioxide______________________________________Electrical input to the electron beam evaporator                   300 mA at 10 kVPower delivered to the plasma source                   800 WDistance from silicon source to bottom of                   70 mmplasma sourceLength of plasma source 150 mmInternal diameter of plasma source                   130 mmDistance from silicon source to substrate                   360 mmSubstrate temperature   &lt;200 C.Substrate dc bias       0 to -200 VOxygen flow rate        25 sccmTotal pressure          2 m TorrSubstrate material      siliconCurrent in fields coils(top of helicon source) 0.2A(bottom of helicon source)                   1AAverage diameter of field coils                   195 mmNumber of turns         250Coil center to centre spacing                   100 mm______________________________________ 
    
     To ionise some of the evaporant from the electron beam source and also to ionise any background gases present. The positive ions and electrons produced by the ionisation process are then transported to the vicinity of the substrate. A radiofrequency potential is applied to the substrate to alternatively attract the positive ions and the electrons. 
     The impact of the positive ions is used to produce desirable improvements in the properties of deposited films, and the impact of the electrons is used to neutralise any charges produced in the film by the positive ions. 
     Impact by positive ions may produce forms of materials which have useful properties by increasing their density as a result of the compaction effect of the ions. This process is known in the prior art as `ion assisted deposition`. An example of the use of this apparatus to achieve such a form of material, namely the cubic form of boron nitride, is provided as follows. Pure boron is used as a source of boron vapour in the electron beam evaporator. A mixture of argon and nitrogen gases is admitted to the apparatus. A selection of other materials may be chosen from the following non-exhaustive list. 
     
         ______________________________________Source Material    Background Gas______________________________________Boron              ArgonTitanium           NitrogenZirconium          OxygenCarbon             HydrogenHafnium            PhosphineTungsten           DiboraneChromium           Methane, acetyleneSilicon            ArsineGermanium          GermaneChromium           SilaneTantalum           Silicon TetrachlorideVanadium           KryptonMagnesium          FluorineAluminium          TrimethylgalliumManganese          AluminiumtrichlorideNickel             TitaniumtetrachlorideCopper             TeosNiobium            DisilaneMolybdenumSilverIndiumTinYttriumGoldany rare earth metalGalliumCalciumStrontiumBarium______________________________________ 
    
     Whilst particular embodiments of the present invention have been hereinbefore described, it will be appreciated that numerous variations and modifications will become apparent to the apparatus and method. It will also be appreciated that the apparatus and method may be utilised to form a wide variety of coatings of different characteristics on substrate materials. All such variations and modifications to the apparatus and method, and also to the applications for use of the apparatus and method including the different coatings provided thereby should be considered to fall within the scope of the invention as broadly hereinbefore described and as hereinafter claimed.