Multi-directional scanning of movable member and ion beam monitoring arrangement therefor

FIELD OF THE INVENTION

P-0001[0001] This invention relates to a method and an apparatus for scanning of a movable member such as a semiconductor wafer holder in a plurality of different directions relative to an ion beam. The invention also relates to an ion beam monitoring arrangement for use with such an apparatus.

BACKGROUND OF THE INVENTION

P-0002[0002] In a typical ion implanter, a relatively small cross-section beam of dopant ions is scanned relative to a silicon wafer. This can be done in essentially one of three ways: scanning of the beam in two directions relative to a stationary wafer, scanning of the wafer in two directions relative to a stationary beam, or a hybrid technique wherein the beam is scanned in one direction whilst the wafer is mechanically scanned in a second, typically orthogonal direction.

P-0003[0003] Each technique has advantages and disadvantages. With smaller silicon wafers, the traditional approach was to mount a batch of wafers at the end of spokes on a rotating wheel. The wheel was then scanned to and fro to cause a fixed direction ion beam to impinge upon each wafer in turn.

P-0004[0004] For implantation into larger (300 mm) wafers, batch processing is currently not preferred. One reason for this is that the individual cost of each wafer introduces a significant financial risk should problems arise during implantation. Electrostatic or magnetic scanning of an ion beam in orthogonal directions relative to a stationary wafer tends to result in beams of poorer quality, and current single wafer scanning techniques tend to employ the hybrid mechanical/electrostatic scanning as outlined above. An arrangement suitable for achieving this is described in our commonly assigned U.S. Pat. No. 5,898,179, the contents of which are incorporated by reference in their entirety. Here, the ion beam is magnetically scanned in a first direction perpendicular to the beam line axis in the ion implanter, whilst the wafer is mechanically moved in a second, generally orthogonal axis.

P-0005[0005] There are, nevertheless, advantages (in terms of beam profile, beam stability and minimisation of the length of the beam line) in maintaining a static beam direction. This in turn requires dual direction scanning of the wafer. It is one object of the present invention to provide an arrangement which achieves this.

P-0006[0006] Determining the beam profile i.e., the ion density as a function of distance across the beam in a given direction) is generally desirable, but particularly when the beam is of fixed direction relative to the implantation chamber. This is because the speed of passage of the wafer across the beam is then slower than for hybrid scanning. For a reasonable throughput of wafers, therefore, it is necessary to minimise the raster pitch. It is then helpful to determine, for example, the beam profile (that is, the beam current intensity across the area of the beam) both prior to and during implantation. Profiling the beam prior to an implant allows the scanning of the wafer during implant to be controlled so as to ensure close uniformity across the wafer, rather than ‘stripes’ of lower or higher ion densities.

P-0007[0007] A number of different approaches to beam profiling are known in the art. For example, in our commonly assigned PCT Patent Application WO-A-00/05744, the signal output from the beam stop (located downstream of the batch processing wafer holder) is employed to obtain information on beam width, height and continuity during implantation. Such signal processing relies upon the gap between wafers on the rotary wafer holder and is accordingly not appropriate for single wafers.

P-0008[0008] Other beam profiling techniques include a travelling Faraday and a pair of Faradays held in a fixed position but spaced along the beam line as described in the above-referenced U.S. Pat. No. 5,898,179.

P-0009[0009] This invention also seeks to provide an improved ion beam profiling arrangement, therefore, particularly for use during set-up prior to implant.

SUMMARY OF THE INVENTION

P-0010[0010] One aspect of the present invention accordingly provides semiconductor processing apparatus which provides for movement of an elongate member, such as the arm of a substrate or wafer holder, in at least two generally orthogonal directions (so-called X-Y scanning). Scanning in a first direction is longitudinally through an aperture in a vacuum chamber wall. The elongate member is reciprocated, for example, by an elongate member driver such as a pair of linear motors. The elongate member and the driver are each preferably mounted on the carrier which in turn is driven in a second direction generally orthogonal to the first.

P-0011[0011] To achieve the longitudinal reciprocation of the elongate member, the carrier preferably includes a slide. The elongate member is supported relative to the slide which is in turn preferably cantilevered from a part of the carrier. It is to be understood that the term “cantilevered” refers not only to horizontal support, but also to vertical or other orientations of support.

P-0012[0012] In a particularly preferred feature of the invention, the elongate member is spaced from the slide by one or more gimballed bearings located towards a first end of the elongate member. These bearings provide cantilevered support for the elongate member as it reciprocates along the slide. Using this technique, a feedthrough for the elongate member into the vacuum chamber can be provided which acts as a vacuum seal but does not itself need to provide bearing support. The feedthrough is preferably compliant and another aspect of this invention provides for a plurality of elastomeric gaskets or the like to act as a vacuum seal and to permit compliance of the feedthrough relative to the carrier or vacuum chamber wall.

P-0013[0013] The feedthrough itself is also preferably a rotary feedthrough. This permits rotation of the elongate member about an axis parallel with the said longitudinal direction.

P-0014[0014] In still a further aspect of the invention, a Faraday is attached to the elongate member adjacent a substrate support. This allows beam profiling (the ion beam having a fixed direction relative to the vacuum chamber) to be carried out in the plane of the substrate to be implanted. Not only may beam profiling be carried out prior to implant, to allow (for example) the beam line to be “tuned”, but the presence of the Faraday adjacent the front face of the substrate support allows the beam to be profiled during a part of an implant cycle as well.

P-0015[0015] Where the elongate member is rotatable about its own axis, then a Faraday can instead or additionally be mounted adjacent the rear of the substrate support. Beam profiling can then be carried out with the substrate support reversed (that is, rotated through 180°. By coating the reverse side of the substrate support with a semiconductor material (e.g. silicon), beam profiling can be carried out without the need for a dummy wafer fitted to the ‘front’ of the substrate support. As an alternative, or additionally, the Faraday may be mounted so that its entrance is at 90° to the plane of the front and rear faces of the substrate support.

P-0016[0016] The invention, in a first aspect, accordingly provides semiconductor processing apparatus, comprising: a vacuum chamber having a chamber wall with an aperture therein; an elongate member extending through the aperture in the chamber wall and being movable in a longitudinal direction through the chamber wall; an elongate member driver arranged to cause reciprocation of the elongate member in the said longitudinal direction; a carrier external of the vacuum chamber, for supporting the elongate member and driver; and a carrier driver arranged to cause reciprocation of the carrier in a direction generally perpendicular to the direction of reciprocation of the movable elongate member.

P-0017[0017] According to a second aspect of the present invention, there is provided semiconductor processing apparatus, comprising: a vacuum chamber having a chamber wall; an elongate member extending horizontally through the chamber wall and being movable in a longitudinal direction through the chamber wall; an elongate member driver arranged to drive the elongate member in the said longitudinal direction; a carrier for supporting the elongate member and driver, the carrier being external of the vacuum chamber and providing cantilever support for an external end of the elongate member; and a feedthrough into the vacuum chamber, the feedthrough receiving the elongate member and including a vacuum seal for sealing against the elongate member.

P-0018[0018] In a further aspect, there is provided a method of mounting an elongate member for reciprocal movement into and out of a vacuum chamber of a semiconductor processing apparatus, the method comprising:

P-0019[0019] (a) supporting the elongate member relative to a carrier, the elongate member being supported by at least one load bearing device located toward a first end of the said elongate member, wherein the first end is external to the vacuum chamber; and (b) mounting the elongate member through a vacuum seal between the interior of the vacuum chamber and the exterior thereof; wherein the load presented by the said elongate member is substantially borne by the or each load bearing device such that the vacuum seal acts as a non-load bearing guide for the elongate member during reciprocal movement.

P-0020[0020] In still a further aspect there is provided a rotary and linear vacuum seal for feedthrough of an elongate member into a vacuum chamber of a semiconductor processing apparatus, the vacuum chamber having a chamber wall member, the vacuum seal comprising: an outer mounting fixed to the wall member and having a longitudinal axis extending in a direction through the chamber wall member; an inner bearing mounted radially inwardly of the outer mounting, the inner bearing being movable relative to the outer mounting, being sized to receive the elongate member therethrough, and likewise having a longitudinal axis extending in a direction through the chamber wall; and a plurality of compliant gaskets arranged between the inner bearing and the outer mounting, the compliant gaskets being axially spaced along the longitudinal axes of the inner bearing and outer mounting.

P-0021[0021] A further aspect of the present invention provides semiconductor processing apparatus, comprising: a vacuum chamber having a chamber wall with an aperture therein; a scanning arrangement including an elongate arm extending through the aperture in the chamber wall, and a substrate support attached to a first end of the elongate arm and located within the vacuum chamber, the substrate support comprising a front face adapted to receive a substrate to be processed, and a rear face opposed to the front face; scanning arrangement drive means for moving the beam scanning arrangement in a first direction generally longitudinally through the chamber wall, and in a second direction generally orthogonal to the said first direction; and a Faraday, mounted adjacent to and in fixed relation to the said substrate support.

P-0022[0022] In yet another aspect, there is provided a method of profiling an ion beam in a semiconductor processing apparatus, the apparatus comprising a vacuum chamber having a chamber wall with an aperture therein and a beam scanning arrangement including an elongate arm extending through the aperture in the chamber wall and a substrate support attached to a first end of the elongate arm and located within the vacuum chamber, the substrate support comprising a front face adapted to receive a substrate to be processed, and a rear face opposed to the front face; the method comprising: mounting a Faraday adjacent to and in fixed relation to the said substrate support; moving the beam scanning arrangement in one of a first direction generally longitudinally through the chamber wall, and a second direction generally orthogonal to the first direction, until the ion beam is generally aligned with the Faraday in that first or second direction respectively; scanning the beam scanning arrangement in the other of the said first and second directions such that the ion beam passes across the Faraday; obtaining a Faraday output signal as the beam scanning arrangement is scanned across the Faraday; and obtaining a profile of the ion beam in the said other of the first and second directions from the Faraday output signal.

P-0023[0023] The invention also extends to an ion implanter including the semiconductor processing apparatus and/or the vacuum feedthrough outlined above. It is to be understood, also, that each of the various aspects of the invention is by no means mutually exclusive and, indeed, the combination of various aspects of the invention provides benefits.

BRIEF DESCRIPTION OF THE DRAWINGS

P-0024[0024] The invention may be put into practice in a number of ways, and embodiments will now be described by way of example only and with reference to the accompanying drawings in which:

P-0025[0025]FIG. 1<i>a </i>shows a schematic side view of an ion implanter including a process chamber to which is mounted a substrate scanning arrangement including a scanning arm support structure in accordance with the present invention;

P-0026[0026]FIG. 1<i>b </i>shows a part section along the line AA of FIG. 1<i>a; </i>

P-0027[0027]FIG. 2 shows a more detailed third angle projection of the substrate scanning arrangement of FIGS. 1<i>a </i>and <b>1</b><i>b; </i>

P-0028[0028]FIG. 3 shows a side sectional view of the substrate scanning arrangement of FIG. 2;

P-0029[0029]FIG. 4 shows a close-up view of the region A of FIG. 3;

P-0030[0030]FIG. 5 shows a third angle projection of a substrate support attached to the substrate scanning arrangement of FIGS. <b>1</b> to <b>4</b>, and including a Faraday;

P-0031[0031]FIG. 6 shows a schematic view of a part of the Faraday of FIG. 5 as it traverses an ion beam;

P-0032[0032]FIG. 7 shows a schematic side view of the substrate support and Faraday of FIG. 5;

P-0033[0033]FIG. 8 shows a schematic front view of an alternative arrangement of a substrate support and Faraday; and

P-0034[0034]FIG. 9 shows a part-section through the Faraday of FIGS. 5 and 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

P-0035[0035] A schematic side view of an ion implanter is shown in FIG. 1<i>a</i>. A part sectional view along the line AA in FIG. 1<i>a </i>is shown in FIG. 1<i>b</i>. As best seen in FIG. 1<i>a</i>, the ion implanter includes an ion source <b>10</b> which is arranged to generate an ion beam <b>15</b>. The ion beam <b>15</b> is directed into a mass analyser <b>20</b> where ions of a desired mass/charge ratio are selected electromagnetically. Such techniques are well known to those skilled in the art and will not be detailed further. It should be noted that, for convenience, the mass analyser <b>20</b> has been illustrated in FIG. 1<i>a </i>as bending the beam of ions from the source <b>10</b> in the plane of the paper, which is a vertical plane in the context of other parts of the illustrated implanter. In practice, the analyser <b>20</b> is usually arranged to bend this ion beam in a horizontal plane.

P-0036[0036] The ion beam <b>15</b> exiting the mass analyser <b>20</b> may be subject to electrostatic acceleration or deceleration of the ions, depending upon the type of ions to be implanted and the desired implantation depth.

P-0037[0037] Downstream of the mass analyser is a process or vacuum chamber <b>40</b> containing a wafer <b>180</b> to be implanted, as may be seen in FIG. 1<i>b</i>. In the present embodiment, the wafer is a single wafer, for example 200 mm or 300 mm in diameter.

P-0038[0038] The ion beam which exits the mass analyser <b>20</b> generally has a beam width and height which is substantially smaller than the diameter of the wafer to be implanted. The scanning arrangement of FIGS. <b>1</b><i>a </i>and <b>1</b><i>b </i>(explained in detail below) allows for scanning of the wafer in multiple directions such that the ion beam may be maintained, during implant, along a fixed axis relative to the vacuum chamber <b>40</b>. Specifically, the wafer is mounted upon a substrate support which consists of a plate onto which the wafer is mounted within the vacuum chamber <b>40</b>, and an elongate arm <b>60</b> connected to the plate.

P-0039[0039] The elongate arm <b>60</b> extends out through the wall of the process chamber in a direction generally perpendicular with the direction of the ion beam. The arm passes through a slot <b>55</b> (FIG. 1<i>b</i>) in a rotor plate <b>50</b> which is mounted adjacent to a side wall of the process chamber <b>40</b>. The end of the scanning arm <b>60</b> is mounted through a sledge <b>70</b>. The scanning arm <b>60</b> is substantially fixed relative to the sledge <b>70</b> in the Y-direction as shown in FIGS. 1<i>a </i>and <b>1</b><i>b</i>, and the scanning plane may also be rotated in the direction R (FIG. 1<i>a</i>) as explained further below. The sledge <b>70</b> is movable in a reciprocating manner relative to the rotor plate <b>70</b> in the direction Y shown in FIGS. 1<i>a </i>and <b>1</b><i>b</i>. This permits movement, also in a reciprocating manner, of the substrate in the process chamber <b>40</b>.

P-0040[0040] To effect mechanical scanning in the orthogonal, X-direction (that is, into and out of the plane of the paper in FIG. 1<i>a </i>and left to right in FIG. 1<i>b</i>), the scanning arm <b>60</b> is mounted within a scanning arm support structure <b>30</b>. The scanning arm support structure <b>30</b> comprises a pair of linear motors <b>90</b>A, <b>90</b>B which are spaced from the longitudinal axis of the scanning arm <b>60</b> above and below it as viewed in FIG. 1<i>a</i>. Preferably, the motors are mounted around the longitudinal axis so as to cause the force to coincide with the centre of mass of the scanning arm support structure <b>30</b>. However, this is not essential and it will of course be understood that a single motor may instead be employed to reduce weight and/or cost.

P-0041[0041] The support structure <b>30</b> also includes a slide <b>100</b> which is mounted in fixed relation to the sledge <b>70</b>. Movement of the linear motors along tracks (not shown in FIGS. 1<i>a </i>or <b>1</b><i>b</i>) disposed from left to right in FIG. 1<i>b </i>causes the scanning arm <b>60</b> likewise to reciprocate from left to right as viewed in FIG. 1<i>b</i>; the scanning arm <b>60</b> reciprocates relative to the slide <b>100</b> upon a series of bearings.

P-0042[0042] With this arrangement, the substrate is movable in two orthogonal directions (X and Y) relative to the axis of the ion beam <b>15</b> such that the whole substrate can be passed across the fixed direction ion beam.

P-0043[0043] Sledge <b>70</b> in FIG. 1<i>a </i>is shown in a vertical position such that the surface of the wafer is perpendicular to the axis of the incident ion beam. However, it may be desirable to implant ions from the ion beam into the substrate at an angle. For this reason, the rotor plate <b>50</b> is rotatable about an axis defined through its centre, relative to the fixed wall of the vacuum chamber <b>40</b>. In other words, the rotor plate <b>50</b> is able to rotate in the direction of the arrows R shown in FIG. 1<i>a. </i>

P-0044[0044] Movement of the sledge <b>70</b> relative to the rotor plate <b>50</b> is facilitated with an air bearing between a surface of the rotor plate <b>50</b> and a surface of the sledge <b>70</b>. Movement of the rotor plate <b>50</b> relative to the process chamber <b>40</b> is likewise facilitated with an air bearing between a surface of the rotor <b>50</b> and a surface of a stator (not shown) which is mounted upon a flange extending radially from a wall of the process chamber <b>40</b> adjacent the aperture therethrough. Radial movement of the rotor plate is constrained by a series of guide wheels <b>80</b> arranged around the circumference of the rotor plate <b>50</b>. Unwanted axial movement of the rotor plate is prevented in use by the pressure differential between the two faces of the rotor plate; the outer face is at atmospheric pressure whereas the inner face is at a vacuum such that there is a significant force acting into the plane of the paper in FIG. 1<i>a </i>to maintain the rotor plate in position. The sledge <b>70</b> is likewise held against the rotor plate <b>50</b> by a pressure differential between an outer face of the sledge and an inner face of the sledge where it covers the aperture through the rotor plate and process chamber wall.

P-0045[0045] The details of the rotor plate <b>50</b> and its method of mounting relative to the stator on the process chamber wall (including the fluid bearing) are all described in detail in U.S. Pat. No. 5,898,179, the contents of which are incorporated herein in their entirety. The method of mounting of the sledge <b>70</b> for reciprocal movement in the Y-direction is likewise described in this patent. Details of a particularly suitable air bearing between the rotor plate and stator, and between the sledge and the rotor plate, which incorporates a porous graphite material and a differentially-pumped vacuum seal, are given in our copending U.S. application Ser. No. 09/527,029 (corresponding with published UK Patent Application No. GB-A-2,360,332), the contents of which are also incorporated herewith in their entirety. An annular piston member may be used to support the rotor plate <b>50</b> relative to the stator and thus to prevent “bowing” or “dishing” of the rotor plate <b>50</b>, and this is described in commonly assigned U.S. Pat. No. B1-6,271,530. The contents of this patent are incorporated by reference as well.

P-0046[0046] The scanning arm support structure will now be described in more detail with reference to FIGS. 2 and 3. FIG. 2 shows a more detailed preferred angle projection of the substrate scanning arrangement of FIGS. 1<i>a </i>and <b>1</b><i>b</i>, including the scanning arm support structure <b>30</b>. FIG. 3 shows a side sectional view of the features illustrated in FIG. 2.

P-0047[0047] As may be seen in FIGS. 2 and 3, the scanning arm support structure <b>30</b> is cantilevered from the sledge <b>70</b>. An air bearing <b>110</b> for the sledge <b>70</b> is seen (partly hidden) in FIG. 2. Further details of this differentially pumped air bearing <b>110</b> are set out in the above-referenced U.S. Pat. No. 5,898,179 and a more detailed description of this part of the substrate scanning arrangement will not be provided here.

P-0048[0048] The scanning arm <b>60</b> reciprocates in a horizontal plane and, since it has a non-trivial weight, there is a bending moment upon it. More particularly, when the scanning arm <b>60</b> is in a first, retracted position, as shown in FIG. 3, the weight of the scanning arm can be supported by the scanning arm support structure <b>30</b>, the sledge <b>70</b> and the rotor plate <b>50</b>. However, when it is in a generally extended position (i.e. with the scanning arm <b>60</b> moved to the right again as viewed in FIG. 3), the centre of gravity of the scanning arm <b>60</b> moves horizontally with respect to the chamber wall. Thus, there is a variation in loading for different extensions of the scanning arm. Furthermore, the scanning arm <b>60</b> is desirably rotatable about its own longitudinal axis S-S (FIG. 3) and this places yet further demands upon any feedthrough from atmosphere into vacuum as is necessary. It is also important that the surface of the scanning arm <b>60</b> does not ground upon the vacuum feedthrough as this causes wear. Furthermore, manufacturing such a cylindrical feedthrough to an appropriate tolerance, particularly in view of the variation in loading, is difficult.

P-0049[0049] To address these problems, a cantilevered support for the scanning arm <b>60</b> is instead employed. The scanning arm <b>60</b> is supported at an end <b>60</b>A distal from the vacuum chamber <b>40</b>, relative to the slide <b>100</b>, using a set of cantilever bearings <b>120</b>A, <b>120</b>B, <b>120</b>C and <b>120</b>D. With this arrangement, the scanning arm <b>60</b> may pass into the vacuum chamber <b>40</b> through a compliant vacuum feedthrough <b>130</b>. The feedthrough <b>130</b> is mounted within an aperture <b>140</b> in the sledge <b>70</b>. By mounting the distal end <b>60</b>A of the scanning arm <b>60</b> upon cantilever bearings <b>120</b>, the feedthrough <b>130</b> does not need to provide bearing support, and instead acts only as a vacuum-tight seal for the scanning arm <b>60</b>. The compliance of the feedthrough <b>130</b> accommodates any minor misalignment between the feedthrough and the cantilever bearings <b>120</b>.

P-0050[0050] The feedthrough also permits rotary motion of the scanning arm <b>60</b> about its own axis S-S. This is achieved by providing a motor for driving the arm <b>60</b> at the distal end <b>60</b>A. The purpose of providing for rotary motion of the arm <b>60</b> will be described below in connection with FIGS. <b>5</b> to <b>7</b>.

P-0051[0051] Further details of the compliant vacuum feedthrough <b>130</b> will be provided below in connection with FIG. 4.

P-0052[0052] To drive the scanning arm <b>60</b> backwards and forwards generally along the axis S-S, a pair of linear motors <b>90</b>A, <b>90</b>B are provided. As is best seen from FIG. 3, these linear motors are spaced equidistantly above and below the axis S-S respectively. The linear motors are connected to the end <b>60</b>A of the scanning arm <b>60</b> by a connecting bracket <b>150</b>. With such an arrangement, the direction of force upon the scanning arm by the linear motors is substantially along the axis S-S, minimising the risk of any bending moment which could occur with only a single, offset linear motor.

P-0053[0053] The scanning arm <b>60</b> is enclosed, on the atmospheric side of the sledge <b>70</b>, with an elastomeric gaiter <b>160</b>. The gaiter <b>160</b> is supplied with dry air and prevents atmospheric contaminants from being transferred into the vacuum chamber <b>40</b> as the arm moves from left to right.

P-0054[0054] The scanning arm support structure has a significant weight which means that the control of the linear motor which drives the scanning arm support structure <b>30</b> relative to the sledge <b>70</b> in the Y-direction can be difficult. To address this problem, vacuum piston counterbalances <b>170</b>, only one of which is visible in FIG. 2, are provided. The vacuum piston counterbalances <b>170</b> each have an axis generally parallel with the Y-direction. This arrangement is described in further detail in commonly assigned U.S. patent application Ser. No. 09/293,956 and in a Continuation-in-Part of that application filed on Sep. 20, 2001, the contents of each of which are incorporated by reference in their entirety. Published European Patent Application No. EP-A-1,047,102 corresponds with U.S. Ser. No. 09/293,956.

P-0055[0055] Because the end <b>60</b>A of the scanning arm <b>60</b> remains at atmospheric pressure, whilst the axially opposite end of the scanning arm <b>60</b> to which a substrate support <b>180</b> is attached is maintained in the vacuum chamber <b>40</b>, there is a significant force also along the axis S-S of the scanning arm <b>60</b> (in the direction from left to right in FIG. 3) in use. Again, therefore, scanning arm vacuum piston counterbalances <b>190</b> are mounted between the fixed and moving parts of the scanning arm support structure <b>30</b>, as may best be seen in FIG. 2.

P-0056[0056] The cantilever bearings <b>120</b> will now be described in more detail, together with their method of mounting relative to the slide <b>100</b>. Each cantilever bearing <b>120</b> comprises a bearing head and an elastomeric bearing support <b>210</b>. The elastomeric bearing supports <b>210</b> of the lower cantilever bearings <b>120</b>B, <b>120</b>D are mounted upon the lower linear motor <b>90</b>B. The upper elastomeric bearing supports <b>210</b> are mounted upon the scanning arm <b>60</b> at the distal end <b>60</b>A thereof; the scanning arm <b>60</b> is however constrained to move with the upper linear motor <b>90</b>A by means of the connecting bracket <b>150</b>.

P-0057[0057] The bearing heads <b>200</b> of the lower cantilever bearing <b>120</b>B, <b>120</b>D are mounted so as to bear against a lower surface <b>220</b> of the slide <b>100</b>. The bearing heads <b>200</b> of the upper cantilever bearings <b>120</b>A, <b>120</b>C bear against an upper surface <b>230</b> of the slide <b>100</b>.

P-0058[0058] The bearing heads <b>200</b> preferably include bearing pads formed from graphite or other porous material. As is explained in the above-referenced U.S. Ser. No. 09/527,029, the use of graphite provides for a generally uniform flow rate of air across the area of the bearing surface. This in turn permits a lower “ride height” to be achieved. The slide <b>100</b> is formed from or coated with a ceramic material such as alumina such that, even if the bearing head <b>200</b> touches the surface of the slide <b>100</b> during movement, there is minimal friction between the two bearing surfaces.

P-0059[0059] In operation of the cantilever bearings <b>120</b> the bearing heads <b>200</b> are extended towards the surfaces <b>220</b>, <b>230</b> of the slide <b>100</b> until they rest against it. This procedure is effected by the elastomeric bearing supports. It will be appreciated that the particular arrangement of the cantilever bearings <b>120</b> means that the bearing heads are gimballed and hence will self-level relative to the surfaces <b>220</b>, <b>230</b> of the slide <b>100</b>.

P-0060[0060] Once the bearing heads <b>200</b> have been tightened against the bearing surfaces <b>220</b>, <b>230</b> of the slide <b>100</b>, a supply of air or other fluid is provided for flow through the graphite of each bearing head <b>200</b>. Each bearing head <b>200</b> has a small plenum (not shown) adjacent to the graphite bearing surface and the plenum is fed with a compressed air supply via a tube (not shown). This tube passes from the plenum, through each elastomeric bearing support <b>210</b> and then out of the scanning arm support structure <b>230</b> to supply the compressed air along cable and pipe ducts <b>240</b>.

P-0061[0061] As the flow rate of air from the air supply is increased, the bearing heads <b>200</b> lift away from the surfaces <b>220</b>, <b>230</b> of the slide <b>100</b> and allow the scanning arm <b>60</b> to move relative to the slide. The slide is fixed at least in the X-direction relative to the scanning arm <b>60</b> as the arm moves; preferably the slide is mounted directly to the sledge <b>70</b>.

P-0062[0062] The cable and pipe ducts <b>240</b> (which carry the pipes to supply compressed air to the various air bearings) are flexible. This allows a first end of the duct <b>240</b> to be attached to a relatively fixed part of the substrate scanning arrangement, with the other end attached to a relatively moving part thereof. For example, the end <b>240</b>A of the cable and pipe duct <b>240</b> (seen best in FIG. 2) does not move relative to the scanning arm support structure <b>30</b>, whereas the other end <b>240</b>B moves in tandem with the scanning arm <b>60</b> as it reciprocates in the X-direction.

P-0063[0063] The support structure described above permits rapid, mechanical X-Y scanning. By way of example only, the scanning arm <b>60</b> may have a stroke of about 470 mm. The scan frequency in the longitudinal (X) direction may be around 1.5 Hz. A turnaround time at each end of the longitudinal scanning movement of 78 ms introduces accelerations and decelerations of the order of <b>4</b>G. The linear velocity during the main part of each stroke is about 2 m/sec. Each Y step (at the end of each longitudinal stroke) may be anywhere between zero and 30 cm and the acceleration here may be around <b>2</b>G.

P-0064[0064] A preferred embodiment of the compliant vacuum feedthrough <b>130</b> will now be described with reference to FIG. 4, which shows a close-up view of the region A of FIG. 3. The feedthrough <b>130</b> is generally cylindrical and in particular has a cylindrical bore <b>250</b> whose diameter is sized to receive the cylindrical scanning arm <b>60</b> through it. The vacuum feedthrough comprises an outer sheath indicated generally at <b>260</b> and an inner sheath indicated generally at <b>270</b> and which is radially inwardly of the outer sheath <b>260</b> but generally coaxial therewith. The outer sheath <b>260</b> is fixed to the sledge <b>70</b> (part of which is shown in section in FIG. 4) adjacent the aperture <b>140</b> in the sledge (FIG. 3). The inner sheath <b>270</b> is, by contrast, suspended from the fixed outer sheath by a plurality of annular membrane seals <b>280</b>. This arrangement allows the inner sheath <b>270</b> to float in the outer sheath. As explained in connection with FIGS. 2 and 3, this allows slight misalignment, particularly any slight angle between the axis S-S of the scanning arm <b>60</b> and the axis S′-S′ of the bore of the feedthrough <b>130</b> to be accommodated without the scanning arm <b>60</b> touching the inner bearing surface of the feedthrough <b>130</b>.

P-0065[0065] The compliant vacuum feedthrough <b>130</b> provides both an air bearing between the scanning arm <b>60</b> and the inner surface of the inner sheath <b>270</b>, and also a vacuum seal between the atmospheric pressure side of the feedthrough (on the left in FIG. 4) and the side of the feedthrough in vacuo (to the right in FIG. 4).

P-0066[0066] The air bearing part of the vacuum feedthrough <b>130</b> is indicated generally at <b>290</b> and is provided by a series of throughholes <b>300</b> formed radially through the inner sheath <b>270</b>. The inner sheath <b>270</b> is itself formed from an outer cylinder <b>310</b> and an inner cylinder <b>320</b>, the latter being an interference fit within the outer cylinder <b>310</b>. The inner cylinder <b>320</b> is formed from a porous material such as graphite. The throughholes <b>300</b> are formed right through the wall of the outer cylinder <b>310</b> and into, but not through, the wall of the inner cylinder <b>320</b>. Running in an axial direction through the wall of the outer cylinder <b>310</b> is a plenum which is closed at one end and opens into a connector <b>340</b> at the other end. A supply of compressed air is attached in use to the connector <b>340</b>.

P-0067[0067] For ease of manufacture, the throughholes <b>300</b> are machined right through the wall of the outer cylinder <b>310</b> of the inner sheath <b>270</b> (before the latter is suspended from the outer sheath <b>260</b>). The parts of the throughholes <b>300</b> which are radially outward of the plenum <b>330</b> are then blanked with grub screws or the like.

P-0068[0068] Because the scanning arm <b>60</b> and the inner diameter of the inner cylinder <b>320</b> of the inner sheath <b>270</b> are each cylindrical, and the air bearing <b>290</b> is circumferentially disposed within the inner sheath <b>270</b>, the provision of compressed air to the plenum <b>330</b> causes the scanning arm <b>60</b> to be centred relative to the inner diameter of the inner cylinder <b>320</b> in use.

P-0069[0069] Circumferentially disposed atmospheric pressure vents <b>355</b> allow high pressure gas between the bearing surfaces of the scanning arm <b>60</b> and inner cylinder <b>320</b> to escape to atmosphere. A first of the vents is located axially about halfway along the length of the air bearing <b>290</b>, and the second vent is located adjacent the innermost end of the air bearing <b>290</b>. This prevents pressures above atmospheric from arising in the vacuum feedthrough on the vacuum side of the air bearing <b>290</b>.

P-0070[0070] The second part of the vacuum feedthrough <b>130</b> is a differentially-pumped vacuum seal shown generally at <b>360</b>. This comprises a series of pumping rings <b>370</b>A, <b>370</b>B with pumping holes disposed radially therethrough. The membrane seals <b>280</b> are, as may be seen, spaced axially between the inner and outer sheaths. The regions <b>285</b><i>a</i>, <b>285</b><i>b </i>between adjacent membrane seals <b>280</b> form compliant vacuum chambers. The pumping rings <b>370</b>A, <b>370</b>B are connected to these plenum chambers. Pumping apparatus (flexible vacuum hoses and a rotary pump or the like, not shown) is attached to the outer sheath, which is fixed and is also on the atmospheric pressure side of the feedthrough, with compliant feedthrough to the inner sheath. Although only two pumping rings <b>370</b> are shown in FIG. 4, it will be appreciated that more pumping rings may be desirable depending upon the efficiency of the sealing arrangement and, in particular, the efficiency of vacuum pumps attached to the vacuum pipes (and thus to the pumping rings). Other factors, such as the fly height (that is, the gap between the outer bearing surface of the scanning arm <b>60</b> and the inner face of the inner cylinder <b>320</b>) will also affect the number of is differential stages in the differentially-pumped vacuum seal.

P-0071[0071] The principles of differentially-pumped vacuum seals are discussed in the above-referenced U.S. Ser. No. 09/527,029.

P-0072[0072] The right-hand side of the feedthrough <b>130</b>, as seen in FIG. 4, is at the reduced pressure of the vacuum chamber <b>40</b>. The left-hand side is at atmosphere. This makes no difference to the outer sheath <b>260</b> which is fixedly mounted to the sledge <b>70</b>. The inner sheath <b>270</b>, however, floats within the outer sheath <b>260</b> and the significant force (from left to right in FIG. 4) due to the pressure differential requires that the inner sheath <b>270</b> be axially supported to prevent the membrane seals <b>280</b> from shearing.

P-0073[0073] To provide such axial support, whilst still permitting compliance of the inner sheath <b>270</b> at least in the radial direction thereof, a thrust bearing assembly <b>390</b> is provided at the vacuum end of the feedthrough <b>130</b>. The thrust bearing assembly <b>390</b> comprises an annular reaction washer <b>400</b> which is screwed, rivetted or otherwise fixedly attached to the outer sheath <b>260</b> and is thus not movable. A pair of thrust washers <b>410</b>, <b>420</b> are provided axially inwardly of the reaction washer <b>400</b>. The first thrust washer <b>410</b> rests against a collar at the vacuum end of the inner sheath <b>270</b>.

P-0074[0074] The first thrust washer <b>410</b> has a pair of diametrically opposed thrust buttons <b>430</b><i>b </i>(only one of which is visible in FIG. 4), mounted upon a face of the first thrust washer. In use, the thrust buttons <b>430</b><i>b </i>of the first thrust washer bear against an opposing surface of the second thrust washer. Thus, the first thrust washer <b>410</b> is able to rock about a pivot provided by the two diametrically opposed thrust buttons <b>430</b><i>b </i>in the X-Y plane.

P-0075[0075] The second thrust washer <b>420</b> in turn has a pair of diametrically opposed thrust buttons <b>430</b><i>a </i>which bear against a face of the reaction washer <b>400</b>. The thrust buttons <b>430</b><i>a </i>are arranged orthogonally to the thrust buttons <b>430</b><i>b </i>of the first thrust washer <b>410</b>, and thus permit the second thrust washer to rock in the orthogonal (X-Z) plane.

P-0076[0076] By arranging the diametrically opposed thrust buttons <b>430</b><i>b </i>on the first thrust washer <b>420</b> orthogonally from the diametrically opposed thrust buttons <b>430</b><i>a </i>on the second thrust washer <b>420</b>, the inner sheath <b>270</b> is thus gimballed against the reaction washer <b>400</b> of the thrust bearing assembly <b>390</b> whilst the thrust bearing assembly <b>390</b> provides a reaction against the force due to atmospheric pressure. The force urges the first and second thrust washers against each other and against the reaction washer <b>400</b> so that they are held in place axially without requiring further fixing when the vacuum chamber is evacuated.

P-0077[0077] As an alternative to the thrust bearing assembly <b>390</b>, or in addition to it, the inner sheath <b>270</b> can be supported against the axial force from atmospheric pressure by, for example, piano wire attached between the atmospheric pressure end of the inner sheath <b>270</b> of the vacuum feedthrough and a fixed mounting point upon the sledge <b>70</b>, for example. Whilst this arrangement is simpler than the gimballed thrust bearing assembly <b>390</b>, it is potentially not as robust.

P-0078[0078] Turning now to FIG. 5, a third angle projection of the substrate support <b>180</b> and the end of the scanning arm <b>60</b> to which it is attached is shown. The substrate support <b>180</b> includes a chuck to hold electrostatically a semiconductor wafer of 300 mm diameter or the like. The chuck <b>440</b> holds the semiconductor wafer electrostatically in a manner well known in the art. Mounted adjacent to the chuck <b>440</b> is a first Faraday <b>450</b>, the details of which will be described in connection with FIG. 9 below. The first Faraday <b>450</b> is generally rectilinear and has a front face <b>455</b> which is generally parallel with and coplanar with the face of the chuck <b>440</b> onto which the wafer is mounted. A Faraday aperture <b>460</b> is formed within the front face <b>455</b> of the first Faraday <b>450</b>. Typically, the aperture has an area of about 1 cm^2.

P-0079[0079] The first Faraday <b>450</b> is used for beam profiling prior to implant of a wafer, that is, it is used to measure the current density across the incident ion beam in the X and Y-directions (the two Cartesian directions orthogonal to the direction of the ion beam). Such information is desirable to ensure accurate dosing of the wafer to be implanted, to avoid wafer charging damage during implant, and because the lateral position (center of gravity) of the beam defines the wafer angle alignment.

P-0080[0080] The ion beam is, as previously explained, held in a fixed direction during implantation. The direction and dimensions of the ion beam can, however, be “tuned” prior to implant by, for example, adjusting physical and electrical parameters of the ion source which generates the ion beam. To measure the current density of the ion beam prior to implant, using the arrangement of FIG. 5, the following procedure is followed. Firstly, a dummy wafer is mounted to the chuck <b>440</b>. This is typically carried out by extending the scanning arm <b>60</b> vertically along the sledge <b>70</b> (FIGS. <b>1</b>-<b>3</b>) to the extent of its travel in the Y-direction. The plane of the chuck <b>440</b> is rotated from the vertical to the horizontal by actuation of the rotary motor of the scanning arm <b>60</b>, so that the scanning arm is rotated about its own axis in the direction P shown in FIG. 5 until horizontal. The dummy wafer is loaded by a robot arm in a load lock which can be vented to atmosphere without needing to vent the vacuum chamber <b>40</b>.

P-0081[0081] Once the dummy wafer has been loaded, the substrate support <b>180</b> is rotated back so that the chuck <b>440</b> is in the vertical position (i.e., into the X-Y plane) and then the scanning arm support structure <b>30</b> is moved along the sledge <b>70</b> until the fixed direction ion beam is level with the aperture <b>460</b> in the Faraday, in the Y-direction. To profile the beam, the linear motors <b>90</b>A, <b>90</b>B are then actuated so that the Faraday <b>450</b> and the substrate support <b>180</b> move together in the X-direction.

P-0082[0082] In practice, the ion current density drops off slowly (i.e., not vertically) at the edges of the ion beam. It is important to know what the profile of the beam is, particularly in the Y-direction, since in use the wafer is typically scanned in a raster fashion across the beam. In other words, the scanning arm <b>60</b> is reciprocated from left to right whilst the scanning arm support structure <b>30</b> remains in a fixed position relative to the sledge <b>70</b>, until the whole of the substrate support has traversed the ion beam in the X-direction. The scanning arm support structure <b>30</b> is then moved vertically, that is, in the Y-direction by a distance related to the height of the beam in the Y-direction, whereupon the scanning arm <b>60</b> is moved back again from right to left with the scanning arm support structure <b>30</b> once more maintained stationary relative to the sledge <b>70</b>. By repeating this procedure, the whole of the wafer may be implanted. In order to ensure that stripes of higher or lower ion density are not created in the Y-direction, it is important to measure the beam profile prior to implant. Profile measurements are fed to a processor which controls the step size in the Y-direction so that the net implantation density of ions is maintained relatively constant across the wafer in that Y-direction.

P-0083[0083] The Faraday <b>450</b> is thus scanned across the ion beam with the dummy wafer in place, prior to implant, by moving the scanning arm support structure <b>30</b> relative to the sledge <b>70</b> so that the aperture <b>460</b> in the Faraday <b>450</b> moves across the ion beam in the Y-direction. The charge collected by the Faraday is measured as a function of distance (or time), and from this a profile of the ion beam in the Y-direction can be determined and used to set the parameters for the scanning of a wafer to be implanted.

P-0084[0084] Once the Y-direction profile has been obtained, the X-profile can also be obtained by maintaining the scanning arm support structure <b>30</b> at a fixed position relative to the sledge <b>70</b> and then extending the scanning arm <b>60</b> using the linear motors <b>90</b>A and <b>90</b>B which moves the aperture <b>460</b> of the Faraday <b>450</b> across the ion beam. This is shown schematically in FIG. 6. It will be noted that, typically, the area of the ion beam is larger than the aperture <b>460</b>; for low ion implantation energies (of order 1-5 kev) the ion beam has a relatively large area which decreases with increasing ion energy.

P-0085[0085] Whilst the profile of the ion beam in the Y-direction is of particular use to ensure correct dosing during implant, the profiles in the X and Y-directions are also useful for beam tuning prior to implant. If the measured profile in the X and Y-directions is considered by an operator (or by a processor suitably programmed) not to be optimised, then the beam line can be adjusted and the profiles re-measured using the techniques described above prior to implantation.

P-0086[0086] As an alternative to the procedure described above (which requires mounting and de-mounting of a dummy wafer), a dual Faraday arrangement may instead be employed and this will now be described by reference to FIG. 7 which shows a schematic sectional view along the line B-B of FIG. 5. The arrangement of FIG. 7 employs both a first Faraday <b>450</b> for use as described previously, and also a second Faraday <b>470</b> mounted diametrically opposite to the first Faraday <b>450</b>, also upon the scanning arm <b>60</b>. The second Faraday <b>470</b> includes its own aperture <b>480</b>. The second Faraday <b>470</b> and the aperture <b>480</b> therein face “backwards” when the chuck <b>440</b> faces towards the ion beam as in FIG. 5. However, by rotating the scanning arm <b>60</b> through 180° about its own axis P (FIG. 5), the second Faraday may instead then face towards the incident ion beam, as seen in FIG. 7, with the chuck <b>440</b> then facing backwards along with the first Faraday <b>450</b>.

P-0087[0087] The substrate support <b>180</b> has a body <b>490</b>, at least the rear of which may be coated with a semiconductor material to form a semiconductor layer <b>500</b>. The part of the scanning arm <b>60</b> adjacent to the substrate support <b>180</b> is likewise preferably coated with a semiconductor material. Other suitable materials which either do not sputter or sputter material that will not contaminate the beam line may be used to form the layer on the substrate support and/or scanning arm instead, such as graphite. With this arrangement, no dummy wafer is required as the layer <b>500</b> provides this function instead. Beam profiling can then be carried out using the second Faraday <b>470</b> in exactly the same manner as has been described above in connection with the first Faraday <b>450</b>.

P-0088[0088] In order to maintain the benefit of mounting the Faraday or Faradays upon the substrate support, it is desirable that the distance between the charge collection in the second Faraday and the longitudinal axis of the scanning arm <b>60</b> is the same as the distance between the point at which charge is collected in the first Faraday <b>450</b> and that longitudinal axis. Provided that this geometry is maintained, then the charge collector in the second Faraday <b>470</b> will lie in the same plane as the wafer to be implanted will lie when the substrate support <b>180</b> is rotated so that the chuck <b>440</b> faces forward towards the ion beam again.

P-0089[0089] Although two separate Faradays <b>450</b>, <b>470</b> are shown in FIG. 7, it is to be understood that a single physical structure incorporating apertures on opposing faces and a common (or abutting) dividing member can equally be employed.

P-0090[0090] Indeed, the possibility of cross contamination (that is, sputtering of previous beam species onto subsequent wafers) means that it may in fact be preferable to employ only a single Faraday on the reverse side of the substrate support <b>180</b>, that is, to have only the second Faraday <b>470</b> in FIG. 7, so that the aperture of that single Faraday is hidden from the beam during implantation.

P-0091[0091] When a Faraday is employed that faces away from the front face of the substrate support (e.g the Faraday <b>470</b> of FIG. 7), the front face of the substrate support will face backwards when that Faraday faces towards the beam. The front face then can become coated with contaminant material which arises from back-sputtering of ions from the beam stop downstream of the substrate support. To avoid this, it is desirable to include a shield that can be dropped down over the front face of the substrate support. Such a shield may either be mounted onto the scanning arm <b>60</b> or may be suspended for example from the chamber wall.

P-0092[0092]FIG. 8 shows an alternative arrangement of a substrate support or scanning arm Faraday. FIG. 8 shows a view looking along the beam line towards the Here, the Faraday <b>490</b> is mounted at an angle of 90° to the planes of the chuck <b>440</b> and rear face of the substrate support. In this case, when the aperture <b>460</b> faces towards the ion beam, the chuck <b>440</b> faces upwards and hence away from both the beam and any back sputtered material. Although a 90° angle between the plane of the Faraday aperture and the plane of the chuck is preferred, other angles such as about 120° may be employed (such that the chuck <b>440</b> faces slightly backwards from the incident ion beam).

P-0093[0093] A sectional view of a preferred embodiment of a Faraday <b>450</b> is shown in FIG. 9. The Faraday comprises a magnetic stainless steel housing <b>510</b> which is enclosed on three sides and which has the aperture <b>460</b> within the front face <b>455</b>. The edges of the front face <b>455</b> which define the aperture <b>460</b> are formed as a knife edge <b>520</b> for purposes which will be described below.

P-0094[0094] Within the housing <b>510</b> is an electrometer <b>530</b> connected to an outer stainless steel screen <b>540</b> and an inner graphite cup <b>550</b>. A pair of permanent magnets <b>560</b> are located between the inner walls of the housing <b>510</b> and the outer walls of the graphite cup <b>550</b>.

P-0095[0095] The purpose of the rotor plate <b>50</b> seen in particular in FIGS. 1<i>a </i>and <b>1</b><i>b </i>is to allow the Y-direction scanning to be carried out in a plane other than the vertical. The knife edge <b>520</b> of the Faraday <b>450</b> shown in FIG. 9 accommodates such high implant angles. It is particularly desirable that the ion beam profile is measured with the chuck and hence the Faraday at the angle of subsequent desired implant.

P-0096[0096] It will be understood that the arrangement of FIG. 9, described in relation to the first Faraday <b>450</b>, is equally applicable to the second Faraday <b>470</b> (FIG. 7). In particular, even if beam profiling is carried out with the rear of the substrate support <b>180</b> facing towards the ion beam as is shown in FIG. 7, the knife edge <b>520</b> is still desirable.

P-0097[0097] Moreover, although the use of a Faraday mounted upon the scanning arm and/or adjacent to the substrate support has been described in terms of beam profiling prior to implant, in the case where the aperture in the (or one of the) Faraday(s) faces forwards (i.e., in the same direction as the chuck), that Faraday can be used also for beam profiling during implantation as well. More particularly, when the Faraday is mounted close to the wafer on the chuck, so that the Faraday aperture is likewise close to the edge of the wafer and also faces towards the incident beam during implantation, it is possible to arrange for both the wafer and the Faraday to pass in front of the beam, at least over the part of the raster scan (in the Y direction) that is coincident with the Faraday aperture. Thus, a complete beam profile can be obtained at least once per total wafer scan (all X and all Y positions scanned). Indeed, by mounting two or more Faradays each facing forwards and each spaced in the Y direction, more than one beam profile per total wafer scan could be obtained.

P-0098[0098] Whilst various specific embodiments have been described, it is to be understood that these are for the purposes of illustration only and that various modifications may be made without departing from the scope of the invention which is to be determined in accordance with the accompanying claims. It is moreover to be appreciated that the various features of the present invention may be used together or separately.