plasma treatment apparatus according to the example of the invention is shown in fig1 . the apparatus comprises a chamber 2 in the form of a cylinder , the cylinder in this case being vertically oriented along its axis and with the diameter of the chamber being about 380 millimeters . a series of induction coils 3 are provided surrounding the upper part of the chamber , the coils forming part of an induction coupled plasma generator 4 . the position of the plasma generator 4 around the upper part of the chamber defines a plasma generation region s . a substrate support 6 is provided in a lower part of the chamber , this taking the form of a circular table arranged symmetrically about the cylinder axis and having an upper surface 7 upon which a substrate 8 is placed when in use . the substrate holder 6 is arranged to be moveable axially as indicated at 9 by the provision of a screw thread . the substrate support 6 is provided in the lower region of the chamber defining a substrate region generally indicated at 10 . an intermediate region 11 separates the upper plasma generation region 5 and the lower substrate region 10 . a frusto - conical guide 12 is positioned within this intermediate region 11 . this is arranged effectively as a funnel with the broader funnel “ mouth ” being provided at the top , adjacent the plasma generation region 5 , and facing towards it . as can be seen in fig1 , the guide 12 is arranged such that an aperture 13 is provided in the lower part of the guide . this aperture is approximately the same diameter as that of the substrate 8 ( which in this case is a circular substrate ). the upper mouth of the frusto - conical guide in turn has a diameter approximately equal to that of the chamber in the plasma generation region 5 . a gas inlet 14 is provided in the center at the top of the chamber 2 . one of more precursor gases ( from which the plasma is generated ) are supplied along a conduit 15 from a gas supply 16 , the conduit and gas supply forming part of a gas flow system for the apparatus . beneath the inlet 14 within the chamber , a deflector 17 is positioned centrally so as to deflect the gas ( es ) entering the chamber towards the chamber periphery . the chamber is exhausted through an outlet 18 positioned at one side of the chamber base . an exhaust conduit 19 and corresponding vacuum pumps ( turbo - molecular and rotary pumps ) 20 are provided to achieve this , these components also forming part of the gas flow system for the apparatus . optionally , a conductive mesh or magnet system may be provided as part of or adjacent to the guide 12 , these being indicated schematically at 25 . such devices may be used where it is desirable to terminate the plasma at the edge of the plasma generation region 5 , these devices therefore acting as a plasma terminating device . an electrical supply 31 is also provided so as to power such a device where necessary . an additional electrical supply 30 is connected to the substrate support 6 so as to provide an rf bias to the substrate 8 . this allows further control of the interaction of the species with the substrate as mentioned earlier . as indicated in fig1 , a further electrical supply 32 may also be connected to the guide 12 when the guide also comprises a heating device , so as to heat the guide during use to a temperature of between about 100 and 200 ° c . this could be an active heating system whereby external electrical power is used to heat the guide components as is done for the chamber . in one embodiment , the plasma is used to provide the heating for the guide 12 components . we have determined that , for a unit constructed from thin sheet metal , the plasma heating effect can be significant and rapid , achieving in excess of 300_c at 3 kw over a period of ten minutes or so . we have also determined that a unit designed for higher powers can exploit thermal expansion to control the temperature of the unit . if the guide section is constructed with an external diameter just less than that of the chamber 2 ( for example , 1 or 2 mm ), it is found that initially the temperature rise is relatively rapid given that the guide 12 has little or no thermal contact with the temperature controlled surface that is the chamber wall . as the unit heats up it will expand until it achieves a dimension sufficient to provide a good thermal contact with the wall of the chamber . experiment has shown this to be at a temperature in excess of 250_c . this provides a good thermal contact with the temperature - controlled wall , which then controls the temperature of the guide unit by providing a heat path to the heat sink that is the chamber wall . in this way a relatively simple construction is envisaged whereby initial heating may be performed during a heating stage with the wafer absent from the chamber but with the plasma running . once a reasonable temperature has been achieved then the process can be performed on the wafer . provided the guide section is of sufficient mass , the temperature decay will be minimal for periods of several tens of minutes as the wafer is loaded for the process stage . it should be noted that at low jcp power there might be insufficient heating to maintain a suitable temperature . we envisage the lighter material unit to be used in this instance . in this case , the unit is , in one embodiment , designed and implemented with a poor thermal contact to the temperature - controlled object ( chamber ). temperature rise would be rapid until it came into equilibrium with radiative and convective heat loss mechanisms . for such a unit running at high power , and for a more robust unit running at very high power , it is preferable that the guide incorporate additional cooling , which might take the form of a cooling fluid flowing in grooves machined into the under - side of the guide , fluid passages or channels , or fluid - containing pipes attached to the non - plasma facing surface of the guide by either welding or some form of compression device . the operation of the apparatus is now described with reference to fig2 and 3 from the point of view of an amount of gas flowing through the apparatus during use . it will be appreciated , that despite the steps described below , the method is effectively steady state during operation . prior to these steps , the substrate is loaded into the chamber , the chamber is evacuated and the gas flow at the predetermined pressure is established . referring to fig2 , the gas in question firstly leaves the gas supply 16 and passes down the conduit 15 before entering the chamber via the inlet 14 at step 100 . the gas flow is indicated schematically by the arrows 60 in fig3 . in the present example the gas is sulphur hexafluoride ( sf 6 ). fig4 shows , on the left , a low power version of the guide ( including a support framework 70 ) and , on the right a medium power version . at step 101 the gas entering the chamber is deflected outwards towards the side walls of the chamber by the deflector 17 . a plasma is present within the chamber due to the stimulation of the gas within the region 5 by the plasma generator 4 . in the system illustrated in fig1 , the maximum energy is imparted within a roughly toroidal region indicated at 35 . it will be noted that the gas is deflected into this region by the deflector 17 . at step 102 , the gas approaches and enters the toroidal region 35 . here the energy provided by the plasma generator 4 is at a maximum and this readily causes the newly arrived gas to dissociate into plasma species . various highly reactive species are formed as a result within the gaseous phase and subsequently , these species are carried downwards by the gas flow until they are redirected by the guide 12 at step 103 . at this stage , if a plasma termination device 25 is present , any charged species are either deflected or neutralised at the plasma termination device 25 , incorporated into the guide 12 . at step 104 , the gas exits the aperture 13 and impinges upon the upper surface of the substrate 8 . interaction of the substrate surface with the species contained within the gas then occurs at this stage ( step 105 ), so as to produce etching , deposition or modification , depending upon the parameters and gases used . if an electrical bias is applied to the surface of the substrate during this process then this bias has a short - range effect and modifies the energy of the charged species striking the surface . the deposition or etching effect is therefore influenced accordingly . at step 106 the gas then flows over the outer edge of the substrate and downwards to be exhausted from the chamber via the outlet 18 , exhaust conduit 19 and vacuum pumps 20 . in order to test the efficacy of the apparatus and method described above , experiments were performed with a pure sf 6 , silicon etching process on blank 100 millimeter diameter silicon wafers , these each acting as the substrate 8 . a plasma source with a diameter of 380 millimeters was used ( this being the diameter of the chamber 2 ). a 100 millimeter electrostatic wafer holder was used as a substrate support 6 . an induction coupled plasma generator was used with a power of 5 kw operating at 2 mhz . in addition , an rf potential was applied to the substrate via the substrate support , this having a power of 25 watts and operating at 13 . 56 mhz . the sf 6 pressure in the chamber was 35 millitorr ( 4 . 66 pascals ) with a flow rate of 500 sccm ( standard cubic centimeters per minute , as is well known within the art ). experiments were performed in identical apparatus with and without the use of the frusto - conical guide 12 . in the apparatus without the guide , the gas flow was not re - directed onto the substrate surface , and a significant proportion of the species generated in the plasma generating region would pass by the substrate holder and not interact with the substrate . in the apparatus fitted with the guide , all of the species generated in the plasma generation region and contained within the gas flow would be re - directed onto the substrate surface . the use of the guide 12 doubled the observed etch rate in these experiments . the etch uniformity was also investigated in the apparatus with the guide and found to be ± 2 . 3 %. and well within the requirements of industrial practice . the effectiveness of the grid ( plasma termination device 25 ) in the mitigation of the ionic content of the plasma in contact with the wafer being processed has been demonstrated during development work . if ions are present , they show their presence by accelerated erosion of the material used to provide the pattern to be etched on the wafer , the mask . measuring the rate of erosion of the mask relative to the material it is intended to etch ( e . g . silicon ) will give a measure of the control over the ionic composition of the plasma with and without the grid mounted as shown in fig1 ( item 25 ). in this particular case the grid was mounted so that it was in electrical contact with the guide and thus at the same electrical potential as the guide . a higher ionic content will erode the mask material more quickly thereby reducing the selectivity of the etch process , measured as the ratio of the etch rate of the experiments indicate that without the grid , the selectivity ( si : mask ( photoresist )) was 58 : 1 , whereas with the grid this figure increased to 107 : 1 , an improvement of about 1 . 9 indicating that the mask lasts 1 . 9 times longer with the grid in place than without , and hence that the ion content of the plasma has been significantly reduced . uniformity of process is an important issue to be addressed . for a substrate wafer of a diameter larger than the throat of the guide it is possible for the neutral density to drop as a result of the expansion of process gases downstream of the guide throat . as the material flows down through the guide from the plasma source it is compressed during its transit through the gap between the guide throat and the table . once past this point the material expands as both radius increases and in the axial direction away from the table surface . this is illustrated at 71 in fig5 . there is as associated reduction in the density of reactive material in contact with the edge of a wafer located at a radius greater than that of the throat , which produces non - uniformity in the process . if the gap behind the guide is filled so that the underside of the guide is parallel with the surface of the table then the axial expansion is prevented ( fig6 ). to compensate for the expansion of gases as radius increases the underside of the guide is , in one embodiment , sloped such that the gap diminishes as radius increases ( see fig7 ). this has the effect of recompressing the gas flow as it travels outwards across the wafer and table toward the periphery of the chamber and thence to the pumping port . simulations demonstrate that the neutral density can be sustained across the table surface . they show that a flat underside to the guide has the effect of changing the rate of decrease of the density of material in close contact with the wafer beyond the throat dimension ( 0 . 0675 m in this example ). this can be seen by comparing curve d with curve c in fig8 . there is also an associated increase in the overall density because of the restrictive effect such a design will have on the flow of gas . this factor is increased with the sloping underside ( b ) and is further exacerbated when the flat surface area of the table is increased ( a ). the optimum position will be that which balances the benefit of compression against the limitations imposed upon the system pumping speed , which will also diminish as the gap is reduced . these experiments therefore indicate that the apparatus and method according to the present invention provides a significant improvement in the plasma treatment of substrates .