Abstract:
A method for forming a conductive region on a first portion of a substrate, the method comprising exposing the first portion to a filtered beam of substantially fully ionised metallic ions under a pulsed, modulated electrical bias. The method uses FCVA (Filtered Cathodic Vacuum Arc) techniques to generate the filtered ion beam and permits the formation of a conformal metal coating, even in high aspect ratio vias and trenches. The method also permits the in-filling of vias and trenches to form conductive interconnects. Particular examples concern the deposition of copper ions.  
     Also disclosed are an adapted FCVA apparatus for depositing metals on substrates and control apparatus for controlling ion beams impacting upon substrates, the control apparatus being suitable for incorporation within existing filtered on beam sources.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the deposition of a thin layer of material onto a substrate using a vapour deposition technique, and preferably using FCVA (Filtered Cathodic Vacuum Arc) techniques.  
         BACKGROUND TO THE INVENTION  
         [0002]    In microelectronics, and in particular in the semiconductor industry, the use of aluminium as an interconnect material has been declining. Copper has become the preferred interconnect material. The use of aluminium is being phased out for two main reasons: aluminium&#39;s relatively high resistivity (2.65×10 −4  ohm.cm) and electromigration.  
           [0003]    The use of copper in place of aluminium overcomes both the resistivity and electromigration problems to some extent. Resistivity in copper is lower than that of aluminium (1.67×10 −6  ohm.cm) and the higher mass and higher melting point of copper (1085° C. compared to 660° C. for Al) results in the lower susceptibility to electromigration. As a result higher current densities can be allowed at a given packing density provided copper interconnects replace aluminium ones.  
           [0004]    During the 1980&#39;s, a typical feature size in an integrated circuit was of the order of a micron (˜1 μm). Devices then were provided with either a single layer or a double layer of aluminium alloy metallisation. The relatively large feature size meant that contacts could be formed from the aluminium, which had low resistance, and that any conductive connector formed between conductive layers was sufficiently wide to allow coverage using sputtered aluminium (such conductive connectors are commonly referred to as ‘vias’).  
           [0005]    More recently, the trend has been towards smaller feature sizes (0.13 microns as of 2001) and to multilevel metallisation. Copper metallisation has replaced aluminium as the metallisation of choice. A viable alternative to copper is silver.  
           [0006]    The increased number of layers (as many as eight layers in logic chips) places additional pressure upon the reliability of vias that are themselves decreasing in width. There is a trend toward holes (either trenches or vias) of increasing aspect ratio, where aspect ratio may be defined as the ratio of the depth of a hole to the narrowest width of that hole. Often the most pronounced reliability requirements are in the lower metal layers where the pitch and aspect ratio conditions are most demanding.  
           [0007]    Copper is not a perfect material however, it does have an inconveniently high diffusion rate which makes direct deposition of a copper layer on a dielectric substrate impractical. Diffusion of copper into a substrate, for example silicon or SiO 2 , can create deep-level defects, electrical leakage between metal lines and at the very least reduces the resistivity of the substrate. Typically, a diffusion barrier layer is deposited over regions where copper is to be deposited, thereby preventing copper particles from degrading the properties of the substrate. Materials used in the barrier layer are generally refractory metals or their nitrides: for example Ta(N), Ti(N), W(N), or a multilayer of a combination of these materials.  
           [0008]    Rather than etch a selected portion of a complete metal layer (subtractive metal etch patterning), the preferred method for patterning is the so-called damascene patterning process. Via and/or metal line patterns are etched into a dielectric layer. These etched holes are then inlaid with copper. When both types of hole are filled during the same deposition step, the patterning process is termed a dual damascene process.  
           [0009]    The deposition of copper on integrated circuit substrates conventionally comprises of a number of steps, as illustrated in FIG. 1A. A diffusion barrier is first laid down upon the substrate  102 . A thin seed layer of copper is then formed over the barrier generally using a dry process (for example chemical vapour deposition (CVD) or sputtering)  104 . To cover the seed layer conformally, an electroless copper plating technique is often employed  108 , and before electroless copper plating (from an ionic solution often including copper sulphate) can take place the substrate for deposition must generally be activated  106  by a catalytic surface (gold, platinum or palladium). During the activation process  106 , the substrate is exposed to a solution of precursors of the catalyst, including often highly toxic reagents such as HF, HCl, and acetic acid, and the catalytic activation layer precipitates onto the substrate. Once activated, the substrate can be plated using the electroless technique  108 . The enhanced conductive seed layer can then be used in a per process, electro copper plating (ECP)  110 . To remove excess copper deposits a process known as chemical mechanical polishing (CMP) is employed  112 . Typically, this involves the use of a polishing pad combined with a slurry compound. CMP therefore entails considerable mechanical stresses upon the substrate and metal.  
           [0010]    Thin films of material are increasingly used in a large number of technologies ranging from electronics to optics to metallurgical coatings. Ideally, the functional properties of a thin film should be equivalent or superior to those of the bulk material. These properties are largely determined by the parameters employed in the deposition process. However, whatever the final application, there are properties that are desirable in all types of thin films. These properties include good adhesion to the substrate, reproducibility and sufficient mechanical strength and hardness. Further constraints are imposed depending upon the final application, for example, good optical properties for optical films, high microhardness for wear-resistant layers and dense porous-free films for corrosion-resistant coatings.  
           [0011]    The formation of a conformal thin seed layer of copper is particularly acute when applied to the filling in of trenches or vias in the substrate (see FIG. 1B). As the reader will appreciate, conformal is a term of art. A conformal coating uniformly coats the substrate surface irrespective of the contours of the surface (i.e. vertical, horizontal or sloping).  
           [0012]    The semiconductor industry currently uses a variety of vacuum coating process falling broadly into two types: physical vapour deposition (PVD) and (plasma enhanced) chemical vapour deposition ((PE)CVD). Both types of deposition technique are characterised by the comparatively high temperature required and by the often toxic, raw materials and by-products generated.  
           [0013]    The PVD process mainly consists of evaporation and sputtering. Vacuum evaporation is a relatively simple process involving a system with a vacuum chamber, vacuum pumping system, a power supply and heating element, and a fixture to position the part over the source. Heat could also be generated by the more expensive electron beam method.  
           [0014]    Sputtering is a well-known and versatile thin film deposition process. Material is sputtered from a source (target) onto a substrate. The process can produce many kinds of materials in a partial vacuum environment. Compared to evaporation, sputtering produces denser films with better adhesion to the substrate. It can also be scaled up easily.  
           [0015]    Conventional magnetron sputtering has a low efficiency of ionization from energetic collision between the electrons and gas atoms leaving a high proportion of neutral particles. Most of the coating species reaching the wafer surface and trenches are not ionized and lack a preferred direction (in other words, the velocity distribution of the sputtered neutrals is substantially isotropic). The broad distribution of arrival angles and energy creates a seed layer deposition pattern in which the coating species preferentially coats the upper portion of the trench walls (a so-called overhang) with very little material reaching the bottom of the trench or the lower portions of the trench walls (see FIG. 2A).  
           [0016]    Today&#39;s semiconductor industry has adopted a version of magnetron sputtering assisted by an inductively coupled plasma (ICP). An electromagnetic coil is immersed in a plasma and an rf signal is applied to the coil thereby creating a secondary plasma that ionises a significant proportion of the sputtered neutrals.  
           [0017]    ICP assisted techniques seek to enhance the ionization rate and the directionality of incident coating species by comparison with the conventional magnetron sputtering deposition technique. ICP techniques provide a narrower distribution of arrival angles, which improves the covering the bottom of deep contact holes. Coating of the trench floor and overhang are reduced, however they are still present (see FIG. 2B).  
           [0018]    The CVD process is a method of forming dense structure parts or coatings using the decomposition of relatively high vapour pressure gases. Gaseous compounds of the materials to be deposited are transported to a substrate surface where a thermal reaction/deposition occurs. By-products of the reaction are then expelled from the system. The process can generate high purity coatings and is a good conformal process for covering complicated objects. However, the process temperature is usually high (200-1000° C.). With few exceptions, the precursors are corrosive, hygroscopic, air sensitive and toxic. For this reason, the CVD process is generally carried out in closed systems.  
           [0019]    The quality of CVD and PVD films is open to considerable improvement and alternative processes are under continual development. One alternative, the Cathodic Arc Deposition process, has received a lot of attention in recent years because of its ability to generate a much higher degree of ionisation than conventional technologies. Using this technique results in films that bond better and have higher density. This technique is normally used to apply wear-resistant coatings to machine tools. However, the substantial distribution of relatively large, typically neutral, multi-atom clusters (often referred to as macroparticles) associated with the process limits its usefulness. Macroparticles are characteristically particles visible under an optical microscope in a film deposited using cathodic arc methods.  
           [0020]    During the past few years, a new deposition system, called the Filtered Cathodic Vacuum Arc (FCVA), has been developed. As the name suggests, FCVA includes a filtration step that substantially reduces the number of macroparticles carried with the plasma, thereby facilitating the production of nigh quality coatings and films, including Tetrahedral Amorphous Carbon (ta-C) and Al 2 O 3  films. During deposition a plasma beam with neutral atoms and macroparticles is emitted from a cathodic arc spot by the cathodic vacuum arc process. The unwanted macroparticles and neutrals are then filtered out by cross-magnetic and electric fields. Only ions within a well-defined energy range are allowed to reach the substrate. High-quality films can be produced by this technique which satisfies the general requirements for all applications—that the deposited films adhere well on a substrate and have good properties that are predictable and reproducible.  
           [0021]    Compared to the conventional PVD and CVD techniques, FCVA produces 100% ionised coating species with well-defined and tuneable energy. The technique produces some unique films, such as ta-C film and nigh quality Al 2 O 3  film, that can not be produced by other techniques.  
           [0022]    Filing or coating vias and trenches with copper presents problems for all known deposition techniques. It is therefore an object of the invention to overcome or at least ameliorate the aforementioned problems.  
         SUMMARY OF THE INVENTION  
         [0023]    According to one aspect of the present invention, there is provided a method for forming a conductive region on a first portion of a substrate, the method comprising:  
           [0024]    generating a pulsed, modulated electrical bias signal;  
           [0025]    applying said bias signal proximate the substrate; and  
           [0026]    exposing said first portion to a filtered beam of substantially fully ionised metallic ions under influence of said bias signal.  
           [0027]    The bias signal applied in the second step may be applied either to a conductor in the neighbourhood of the substrate or directly to the substrate itself.  
           [0028]    A filtered beam has a narrow energy distribution and a small range of incidence angles. The last step therefore ensures that trenches or vias in the first portion of the substrate are filled from the bottom upwards, with substantially uniform side wall coating and with a reduced overhang.  
           [0029]    The metallic ions are preferably copper ions. Advantageously, the average degree of ionisation is approximately +2.5e. The method has many advantages in that adhesion, nucleation and conformity properties of the copper layer are each much improved over the prior art methods. Furthermore, using a pulsed modulated DC signal to bias FCVA process means that at least some of the stages of copper deposition can be omitted.  
           [0030]    Examples of substrates that may be coated in accordance with the present invention include: a polymer (a liquid epoxy or a resin layer), silicon, silicon nitride, or silicon dioxide. In addition to deposition of metallic ions, the present invention can be adapted, through choice of a modulation pattern in the bias signal, to permit polishing and/or etching of the deposited surface.  
           [0031]    The step of generating the bias signal advantageously comprises: providing a signal generator having configurable signal characteristics, said signal characteristics including frequency, duty cycle, voltage and deposition time; and configuring the signal characteristics in accordance with a deposition programme. Conveniently this step also includes providing a deposition programme store having a plurality of deposition programmes stored thereupon, determining which of said deposition programmes is required in order to form conductive region, accessing said predetermined deposition programme from said programme store, and configuring the signal characteristics in accordance with said predetermined deposition programme. The deposition programme may be provided by a feedback loop, in which case the feedback loop would feed deposition status information back to the bias signal generator.  
           [0032]    The method of the invention may also include the step of removing excess copper by re-sputtering.  
           [0033]    Advantageously, the plasma team is collimated so that the beam impacts the substrate in a direction substantially orthogonal to the substrate plane.  
           [0034]    According to another aspect of the present invention, there is provided a method for forming a conductive region on a first portion of a substrate, the method comprising: generating a substantially pure plasma beam including positively charged metallic ions from a cathode target; passing the beam towards the substrate through a plasma duct having a first bend with an angle of no less than 20° in a first plane and a second bend with an angle of no less than 20° in a second plane, thereby filtering unwanted particles from the beam; and exposing said first portion of the substrate to the filtered beam under a pulsed, modulated electrical bias.  
           [0035]    By using pulsed, modulated wave patterns to bias the substrate in the FCVA process to deposit copper particles, the number of copper deposition process stages is reduced (activation/electroless plating to form seed layer and electroplating processes are replaced by a single process).  
           [0036]    In one embodiment, the use of FVCA even removes the need for a final CMP stage altogether, by allowing resputtering to remove excess copper. This reduction in stages results in savings on equipment investment and more importantly provides a much more environmentally friendly process overall.  
           [0037]    While a separate barrier formation stage is necessary to prevent diffusion of copper into the underlying substrate, FCVA can be used as an alternative technique for depositing the barrier layer. To this end, a target material suitable for barrier formation (for example Ta, Ti or W (with and without nitrogen)) must be used.  
           [0038]    According to with a further aspect of the present invention, there is provided an apparatus for forming a conductive region on a first portion of a substrate, the apparatus comprising: a plasma generator, which generates a plasma beam including positively charged ions from a cathode target; a filter duct, which filters unwanted particles from the ion beam said duct having two bends such that there is no line-of-sight and no single bounce path through the duct; a beam steering arrangement, which steers the plasma beam through the duct and onto the substrate; and a beam biasing arrangement, for applying a pulsed, modulated electrical bias to the filtered plasma beam.  
           [0039]    The present invention is not limited to current trench and via standards, the FVCA process can be applied to the narrower target trench widths expected in next generation semiconductor applications. Nor is the invention limited to the deposition of copper, as mentioned earlier silver deposition may also be achieved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:  
         [0041]    [0041]FIG. 1A shows the prior art process of copper deposition;  
         [0042]    [0042]FIG. 1B shows stages of the process in FIG. 1A when applied to filling a via;  
         [0043]    [0043]FIGS. 2A and 2B show the deposition pattern resulting from conventional magnetron sputtering and ion assisted sputtering respectively;  
         [0044]    [0044]FIG. 3 shows a FCVA vapour deposition device for generating a filtered plasma beam;  
         [0045]    [0045]FIG. 4 shows an adapted FCVA vapour deposition device in accordance with the invention;  
         [0046]    [0046]FIG. 5 shows the components of a bias signal module for generating a pulsed, modulated bias signal;  
         [0047]    [0047]FIG. 6 illustrates a DC pulsed pattern with constant peal voltage;  
         [0048]    [0048]FIGS. 7A to  7 C illustrate modulation patterns corresponding to deposition programs in accordance with a further aspect of the present invention;  
         [0049]    [0049]FIG. 8 shows the deposition of copper over time in a high aspect ratio via with only a floating bias;  
         [0050]    [0050]FIG. 9 shows the effect of a substrate bias of 180 volts in accordance with the invention;  
         [0051]    [0051]FIG. 10 shows the effect of a substrate bias of 220 volts in accordance with the invention;  
         [0052]    [0052]FIG. 11A shows the laying down of a side wall seed layer coating using a first pulsed, modulated bias signal;  
         [0053]    [0053]FIG. 11B shows the filling of a trench using a second pulsed, modulated bias signal;  
         [0054]    [0054]FIG. 12 shows the direction of magnetic field lines in an uncollimated coil arrangement;  
         [0055]    [0055]FIG. 13 shows an unevenly filled via showing evidence of ‘shadowing’;  
         [0056]    [0056]FIG. 14 shows the direction of magnetic field lines in a collimated coil arrangement; and  
         [0057]    [0057]FIG. 15 snows an evenly filled via using the collimated coil arrangement of FIG. 14. 
     
    
     DETAILED DESCRIPTION  
       [0058]    Using positive ions generated from a cathodic arc source in a vacuum to deposit thin films is known for hard coatings such as ta-C and Al 2 O 3 .  
         [0059]    The cathodic arc is a form of electrical discharge in vacuum which is sustained in metal plasma created by the arc alone and does not require the addition of an inert gas. Currents used in cathodic arc systems are typically of the order of 100 amps, at around 28 volts. A large percentage of the metal vapour generated by the arc is ionised by the discharge and a fraction of the arc current escapes as a plasma beam that includes positively charged ions; this fraction is steered and optionally filtered to produce a coating on a distal substrate. The increased energy of these positive ions compared to the particles in previous deposition methods is thought to be a reason why arc evaporation techniques would deposit high density, high adhesion films. Deposition of thin films by filtered arc evaporation is described generally by P. J. Martin in Surface and Coatings Technology, Volumes 54155 (1992) pages 136-142, and further reviewed in Surface Engineering, Volume 9 (1993), no. 1, pages 51-57.  
         [0060]    FCVA improves upon other cathodic vacuum arc systems in addressing the requirements needed for industrial applications. For example FCVA can be made automatic and easily maintained and can produce a large coating area, free of blemishes.  
         [0061]    By filtering macroparticles from the plasma beam, FCVA thereby eliminates the undesirable side effects of the presence of macroparticles in the deposited coating. FCVA apparatus enables the production of coatings known in the art with improved coating properties due to the improved purity thereof.  
         [0062]    Deposition of copper or aluminium with FCVA requires careful arrangement of the deposition apparatus. The cathode target is arranged substantially horizontally, with the plasma beam of positive tons emitted substantially vertically therefrom. The cathode target is retained by a guide. This arrangement allows the cathode target to melt during preparation of the cathodic arc and therefore this arrangement is suitable for use with low melting point targets such as copper and aluminium.  
         [0063]    Prior to FCVA, cathodic arc sources were not suitable for use with low-melting point targets for various reasons. Firstly, the layout of the cathodic arc source forced the target to be held vertically or at an angle to the horizontal, so that if a low melting point target were used it would melt and flow away from the cathode station and drip on to the walls or the bottom of the source. Secondly, production of macroparticles from a molten cathode target is higher than from a solid cathode target. The prior art devices did not provide an efficient enough filter of macroparticles to enable acceptable quality coatings of low melting point metals to be obtained from a molten cathode.  
         [0064]    In FCVA, the anode is preferably water-cooled and the cathodic arc source used for long periods, ranging from a few minutes to several hours in duration. As a result coatings of low-melting point cathodes, and in particular copper cathodes, can be obtained using the apparatus.  
         [0065]    The FCVA device shown in FIG. 3 is more fully described in International patent application WO 96/26531. For present purposes the device can be described as follows.  
         [0066]    In FIG. 3, a filtered cathodic vacuum arc apparatus  300  generates pure plasmas to deposit dense and clean thin films on a substrate. The plasma is emitted from cathodic arc spots on the surface of a consumable cathode  302  and guided by a radial electric field (generated by a bias power supply  304 ) and a curvilinear axial magnetic field (generated by a magnetic coil  306 ) through a positively biased double bend toroidal duct  308  to the substrate in a coating vacuum chamber. Unwanted macroparticles emitted with the plasma from the cathode are effectively filtered out by a set of removable ceramic baffles  310 , a removable stainless steel bellows  312  and the double bend toroidal duct  308 .  
         [0067]    The clean plasma beam, virtually without macro-panicles at the exit of the duct  314 , can be scanned in one dimension by a beam scanning system  316 , 318 . This scanned beam, in combination with a rotating substrate holder in the coating vacuum chamber, can deposit a film over a large area with good uniformity, where the temperature of the substrate is, for example, room temperature. Alternatively, the beam scanning system  316 , 318  may be arranged to provide uniform scanning in two dimensions. The alternative scanning system may be provided in association with either a stationary holder or a rotating substrate holder. In the later case the substrate can be in excess of 30-40 cm in diameter. Ions of the deposited material with a desired energy can be extracted from the plasma beam by using a DC or RF bias on various types of substrates, such as metals, semiconductors, plastics, ceramics and glasses.  
         [0068]    [0068]FIG. 4 shows schematically how the plasma is generated from a target material disposed at a first end of a double bend filter duct  320 . Where features correspond to features of FIG. 3, identical reference signs are used.  
         [0069]    The cathodic target is surrounded by an anode coil  320 . The magnetic coil  306  along the length of the filter duct  308  is referred to as the filter coil  306  and serves in combination with the radial electric field to filter out unwanted contaminants, such as macroparticles and plasma particles having energies falling outside a predetermined energy range.  
         [0070]    Collective behaviour of the plasma emerging from the filter duct  308  is influenced by the magnetic field induced by three further coils: a scanning coil  318 ; a focussing coil  322 ; and an extension coil  324 .  
         [0071]    Thereafter, the magnetic field is controlled to direct a substantially fully ionised plasma onto a substrate  330 . Additional coils are provided to control the incident ions; the additional coils including a holder extension coil  326  and (optionally) a rear extension coil  328 . A bias signal module  340  applies a pulsed, modulated bias signal  350  to the substrate  330  or a substrate holder  332 .  
         [0072]    In the FCVA device discussed in WO 96126531, a bias is applied to the substrate holder  332  in order to dissipate electrostatic charge accruing on the substrate due to the deposition of positive ions. The bias there is neither pulsed nor modulated.  
         [0073]    In one aspect of the present invention, the bias signal module  340  includes a DC pulsed generator. In alternative embodiments, the bias signal  350  applied to the substrate is bipolar pulsed or RF modulated. As shown in FIG. 6, the bias signal  350  is typically a negative bias having a frequency  602  in a range from 20 to 200 kHz and a duty cycle of anywhere between 20 and 95%. The pulse duration  604  is therefore of the order of microseconds (μs). Peak voltage (−V)  606  has a constant value in this Figure but may be varied under the control of a programmable logic controller (PLC).  
         [0074]    [0074]FIG. 5 illustrates a bias signal module  340  for generating the pulsed, modulated bias signal  350 . A suitable pulse generator module  506  comprises a DC power supply  552  (with a peak voltage of the order of 1000V), a switching circuit  554  (typically an arrangement of transistors) and a PLC  550 . The power supply  552  outputs a DC signal to the output of the pulse generator  506  via the switching circuit  554 . The PLC  550  is coupled to both power supply  552  and switching circuit  554  and defines the frequency and duty cycle at which the switching circuit  554  is to switch the output signal. The PLC  550  also instructs the power supply  552  what value of the peak voltage to operate at. According to the output of the PLC  550 , the peak voltage (−V) generated by the power supply  552  can be made to vary over a predetermined number of frequency cycles (the peak voltage may be as little as 50 V or more).  
         [0075]    The PLC  550  can itself be controlled by a processing device  504 , for example a computer. The processing device  504  outputs a sequence of commands, each of which corresponds to a required power supply and switching circuit setting. The processing device  504  may be under the direct control of a user  508 , however it has been found advantageous to build a library  502  of appropriate programs of PLC commands. These programs result in modulated output signal patterns, for example the step pattern of FIG. 7A, the saw tooth pattern of FIG. 7B and the half sinusoidal pattern of FIG. 7C.  
         [0076]    The library  502  of programs may be stored in an electronic database and accessed by the processing device  504 . Any given program, “recipe” of patterns, when applied to the PLC  550  by the processing device  504 , achieves a specific deposition effect on the substrate: for example the formation of a thin copper film on the substrate or the filling of a high aspect ratio trench or via. Each program may be constructed as a number of tables of values of operating parameters, for instance peak voltage, duty cycle and frequency. For each operating parameter, me table contains required values of that parameter at a number of successive times: t 1 , t 2 , . . . t n . Each program (recipe) therefore defines the values of each parameter required at any given time during a predetermined deposition period, T. The choice of which program to execute may also be directly controlled by the user, the user choosing the program according to the desired deposition effect. The execution of programs may alternatively be automated.  
         [0077]    The programs stored in the library  502  correspond to different waveform patterns which in turn have corresponding effects upon characteristics of the incident plasma beam. Examples of plasma beam characteristics effected include:  
         [0078]    the ion energy upon impact upon either trench or substrate surface;  
         [0079]    the ratio of deposition and re-sputtering rate;  
         [0080]    the characteristic of trench filling (i.e. whether the arriving ions form an overhang or a conformal coating or indeed fill trenches from “bottom up”); and,  
         [0081]    the uniformity and thickness of the coating.  
         [0082]    As mentioned above, important operating parameters include voltage levels; waveform shape and frequency; duty cycle; and deposition time. The minimum voltage and the maximum voltage can both be important parameters, with a range of values being between ground and 1000 volts, more typically ranging between floating and 300 volts. A preferred embodiment of the present invention operates with voltages in the range 100V to 300V. Within any one waveform pattern there may be a plurality of different predetermined voltage set points, see for example the saw tooth pattern of FIG. 75.  
         [0083]    Duty cycle may vary from 20% to 95% but typically values in the range of between 70% and 90% are chosen. The preferred range for pulse frequency is between 20 KHz and 200 kHz, a frequency of 50 kHz to 100 kHz being typical. The frequency of the enveloping (or modulating) waveform ranges between 0.1 to 1000 Hz.  
         [0084]    [0084]FIGS. 8 through 10 illustrate the effect of an increasing holder biasing potential. In each Figure the effect of the biasing potential  350  is illustrated for a via formed on a substrate  330  held by the substrate holder  332 .  
         [0085]    [0085]FIGS. 8A to  8 C show the development over time of a copper layer in the via when the substrate holder is not biased and has a floating potential (FIG. 8A being the earliest). As may be appreciated, the neck of the via is almost pinched in the last Figure in the series. This is not desirable since this creates voids that contribute to overheating in electronic components. The coating in the remainder of the via is unlikely to have the thickness generally required in the industry.  
         [0086]    [0086]FIGS. 9A and 9B show the effect of a substrate bias of 180 volts (at a frequency of 50 kHz and with a duty cycle of 85%). Initially deposition effects dominate, and the neck of the via is noticeably pinched. The bias encourages the resputtering of the copper material. Resputtering may be considered as the non-adhesion of incoming material to the previously coated material. Resputtering is at its most efficient when the incoming materials are of the same species as the material on the substrate. Continuing exposure results in the distribution pattern shown in FIG. 9B. Here, sputtering and deposition effects combine to give a satisfactory neck profile and sufficiently thick sidewall and bottom filling.  
         [0087]    [0087]FIG. 10 shows the effect of re-sputtering with a higher biasing voltage again (220 volts). Again the frequency and duty cycle are 50 kHz and 85% as for the bias profile illustrated in FIG. 9. By using a yet higher biasing voltage, the deposition effects are relatively weaker than the resputtering and the neck of the via is scarcely pinched at all. Resputtering ensures that the required thickness of coating is applied on the sidewall and bottom of the via.  
         [0088]    The conditions in FIGS. 9 and 10 illustrate how the coating profile can be reshaped by adjusting the bias condition. Different surface conditions and/or via aspect ratios may need different biasing regimes. As a consequence, it is important to build a library of bias profiles appropriate to prevailing conditions.  
         [0089]    Using a library of appropriate modulation patterns, high aspect ratio trenches can either be coated with a side wall seed layer as shown in FIG. 11A or completely filled as shown in FIG. 11B. FIGS. 11A and 11B show pulses having the same frequency but operating over differing lengths of time. FIG. 11B shows the results of operation for a substantially longer time than FIG. 11A. The increased number of pulses is seen to encourage resputtering and therefore the filling of holes. The frequency of operation of the biasing voltage can itself be varied as can the peak voltage and the duty cycle.  
         [0090]    As the substrate area increases, the angle of incidence of the ion beam strays significantly from normal incidence. For example when disposing copper at the edge of a 30 cm diameter substrate the angle of incidence can be reduced to 75 degrees. Angle of incidence is predominantly defined by the direction of magnetic field lines.  
         [0091]    [0091]FIG. 12 illustrates now the magnetic filed behaves in a coil arrangement without compensation. The magnetic field lines in the coil arrangement begin to diverge at some distance from the substrate. The ions in the incident ion beam are substantially bound to the diverging magnetic field lines (trapped) and therefore move substantially parallel to the magnetic field lines.  
         [0092]    Since the plasma beam is generated by the FCVA device, the beam will be substantially wholly ionised. In the absence of neutral particles, substantially the whole beam is trapped by diverging magnetic field lines.  
         [0093]    The effect of a non-ideal incident angle on the filling of a via is illustrated in FIG. 13, where the right hand side of the via has been shadowed by the right hand nm of the via resulting in a thinner deposition of material on the shadowed side. The magnetic field lines are directed from top right to bottom left: the direction of incidence is substantially parallel to the fold lines.  
         [0094]    In order to ensure conformal deposition of copper, the magnetic field lines at the substrate surface are ideally normal to the plane of the substrate surface even at the outer edges of the substrate. Therefore, in another aspect of the present invention, collimator coils are added to guide the plasma beam. These coils, in particular the extension coil  324 , the holder extension coil  326  and the rear extension coil  328 , are illustrated in FIG. 4 and in more detail in FIG. 14. The extension coils are preferably water-cooled to ensure reliable operation.  
         [0095]    By controlling the current applied to the extension coil  324 , the holder extension coil  326  and the rear extension coil  328 , the ideal field profile can be more nearly approximated. The plasma beam, confined by the field lines, impacts on the substrate in a direction substantially orthogonal to the substrate plane: even high aspect ratio vias are filled or coated conformally with no appreciable ‘shadowing’ effect.  
         [0096]    In operation, the extension coil current is preset to induce a magnetic field directed from the exit of the FCVA source to the substrate. For the deposition of copper ions, this ensures that Cu ions have a narrow range of incidence angles at the substrate surface. The typical field strength ranges from 20 to 100 mT. Although the current in the extension coil is not generally adjusted during deposition, this current may be adjusted when required. The magnetic field due to the extension coils is typically independent of the electrical bias.  
         [0097]    As the plasma beam is fully ionised, the beam is trapped into the orthogonal incidence angle imposed by the magnetic field lines. The resulting beam is collimated and narrow, leading to a uniform coating on the side walls and across the surface of the substrate.  
         [0098]    [0098]FIG. 15 illustrates the coating formed using a normally incident collimated beam of ions. As required the coating is of equal thickness on either side of the via.  
         [0099]    The collimator coils and the bias voltage applied to me substrate cooperate to encourage resputtering of me copper already on the substrate. Sufficient resputtering removes any excess copper and thereby renders CMP  112  effectively redundant (see FIG. 1).  
         [0100]    FCVA technology can serve in place of yet another stage of the conventional process flow. Referring once more to FIG. 1A, the very first step in the process flow is the preparation of the substrate with the deposition of a barrier layer  102 . As discussed earlier, this layer is typically deposited with CVD or PVD. Using either the same (copper depositing) FCVA device or a second dedicated FCVA device with a target material of Ta, Ti or W (with and without nitrogen), FCVA technology could be used to replace the existing barrier layer deposition process (either CVD or PVD).  
         [0101]    As an alternative source of a modulated, pulsed bias signal, the bias signal module  340  (see FIG. 4) may include an RF generator. The bias signal is then RF modulated. The RF bias signal module  340  further comprises a processing device coupled to a database of deposition programs and a matching network. The programs in the RF database correspond to parameters necessary for RF operation, for example RF power settings and total required deposition time. The RF generator includes a PLC and the processing device sends required parameter settings to the PLC. Signals are generated by the RF generator in accordance to these parameter settings and the output is applied to the substrate via the matching network.