Apparatus and method relating to ion implantation and heat transfer

Abstract: Apparatus for presenting target elements (16) such as semi-conductor wafers to an ion beam (13) for ion implantation comprises a support rotor (19) for carrying the targets (16) on support bases (21) mounted on arms (22) extending radially from a core structure (20) of the rotor. The target elements (16) are rotated by the rotor (19) through the ion beam to produce scanning across the target elements. Each support base (21) is rotatable about the radial axis of its support arm. The rotation of the base (21) allows adjustment of the ion implantation angle. The implantation normally takes place with the target vertical and the axis of scanning rotation horizontal. Each support base (21) is also rotatable through 90° to allow loading and unloading of target elements while the support base is horizontal. Good thermal contact is made between the target element and the support base by an intervening sheet having upstanding flaps (61) which are urged outwardly into contact with the element (16) by the effect of centrifugal force.

Description: The present invention is concerned with apparatus and methods relating to ion implantation. In one aspect the present invention relates to apparatus for and a method of presenting target elements to an ion beam for ion implantation, and also relates to an overall apparatus for and method of implanting ions in a target element. In another aspect the invention also relates to an improved heat sink, and to a device for providing thermal contact between bodies, particularly in vacuum. In all aspects the invention relates in particular but not exclusively, to apparatus for and methods of implanting ions of a preselected species, into a target element, for example the controlled introduction of a species into the surface layer of another material. The technique is important in semi-conductor technology where it is used in the manufacture of integrated circuits and other devices, particularly by modifying the conductivity of semi-conductor material by introducing chemical impurities into the material. The general background to the use of ion implantation in the manufacture of large scale integrated circuit chips is set out in published Patent No. US-A-4578589, having the same inventor as the present application. An ion implantation apparatus normally consists of an ion source, an extraction system having an extraction electrode which accelerates the ions from the extraction slot in the ion source, an analysing magnet for the selection of the required ion species, acceleration stages before or after the analysing magnet, and a target region with means for producing relative scanning movement between the beam and the target, either by scanning the beam, or by moving the target. In high current ion implanters, beam scanning (that is to say electrostatic or magnetic movement of the beam) is not in general a viable technique for uniformly implanting silicon wafers, or other targets. Consequently, mechanical scanning of the target through the beam is normally the preferred technique. A number of different types of scanning are used, for example techniques which are known as linear mechanical scan, cylindrical carousel, Ferris wheel, racetrack carousel, and spinning disc. All these techniques have various drawbacks. The linear mechanical scan technique is slow. The cylindrical carousel and Ferris wheel have cosine scanning errors. The cylindrical carousel has an implant angle error. The linear mechanical scan, Ferris wheel and racetrack carousel systems all present difficulties in introducing coolant into the system. In general, the so-called spinning disc system represents the most suitable system for most semi-conductor requirements. The spinning disc has the advantages of a high scanning speed (by simple rotation, with no discontinuities in the motion), easy introduction of coolant (through a hollow central shaft), and in principle the system has no geometric scanning errors. In practice however problems do arise with scanning errors. Due to heating during ion implantation, it is necessary to provide good contact between a target element and a heat sink on which it is supported, and this is commonly provided by inclining the target element to a radial plane of the spinning disc, so that the target element is forced against an inclined heat sink surface, by effect of centrifugal force. This technique is known as centrifugal clamping of the target element, or wafer, against the heat sink, which commonly has an elastomer coating on its surface. Another factor is that there is increasingly a requirement for the ability to vary the implant angle. The requirements for variable implant angle and centrifugal clamping result in may systems using spinning disc scanning with the ion beam direction arranged at an angle to the spin axis of the disc. This causes an implant angle error, in excess of two degrees under certain conditions. This may lead to channelling inconsistencies during implantation across the wafer, and can make the arrangement unacceptable. Developments in semi-conductor technology have give rise to increasing demands upon ion implantation techniques. There is an increasing need for parallel scanning, where the implant angle is constant in magnitude and direction on the entire target surface. There is also an increasing requirement for different implant angles to be achieved and for a wider range of angles to be required. On occasions there may be a requirement to change the implant angle during implant. A number of implants may be required into the same wafer with different wafer orientations, for example for the implant of trench side walls. In Patent specification US-A-47330918 (Robinson et al), there is disclosed ion implantation apparatus including a mechanical scanning system which comprises a scan wheel assembly which includes a plurality of wafer mounting paddles. The scan wheel assembly is mounted for high velocity rotation about its central axis and is located within a vacuum chamber. Each mounting paddle is located on an individual heat sink assembly which includes spoke arm sections. The heat sink assemblies may be pivotally mounted for rotation under centrifugal force generated by rotation of the scan wheel assembly, and an adjustable stop screw is used to select the angle of the paddle and wafer relative to the path of the ion beam. Although provision is made for rotary movement of the individual wafer mounting paddles, this rotation takes place about an axis which is tangential to the main rotary axis of the spinning disc. The resulting mechanism is complex, and only allows a small degree of pivotal movement. It is one object of the present invention to provide ion implantation apparatus which allows variation of implant angle selectively without introducing scanning errors. Another requirement for an ion implanter in modern semi-conductor technology is for efficient, fast, automatic loading (preferably in vacuum), with no particulate generation. It is generally regarded that the preferred orientation of the target wafer during implantation is vertical or near vertical, for reasons of cleanliness. However it is easier to handle the target wafers during loading and unloading, in a horizonal or near horizonal orientation. This is probably also cleaner, because of the absence of wafer edge contact. It is another object of the present invention, in preferred embodiments, to provide an improved mechanism for loading and/or unloading target elements in an ion implantation apparatus. A final aspect of the invention is concerned with heat transfer. During implantation each wafer is mounted on a heat sink to disperse heat from the ion implantation. It is an object of the present invention, in preferred embodiments, to provide an improved heat sink which avoids distortion or stress on a target wafer, which reduces the creation of particulates by wafer edge contact during loading and unloading, and allows a cooling system which is compatible with change of implant angle through substantial angles, without introducing scan angle errors caused by inclining the spin axis to the beam direction. According to the present invention in a first aspect there is provided apparatus for presenting target elements to an ion beam for ion implantation comprising: a support rotor having a plurality of support bases for supporting target elements, the support bases being mounted on and distributed around the support rotor; main rotation means for rotating the support rotor about a main rotation axis with the target elements lying in planes transverse to the rotation axis through an ion beam to produce in operation scanning of the ion beam across the target elements; and means for angular positioning of each support base relative to the overall support rotor; characterised in that the angular positioning means comprises further rotation means arranged to rotate the support base about a transverse axis which is substantially radial to the main axis. The invention has particular application to apparatus for implanting ions in a target element. Such apparatus in accordance with the invention comprises, as a component assembly, a target presentation apparatus as set out in the previous paragraph, or as set out in succeeding paragraphs, together with means for producing an ion beam, and means for accelerating, analysing and focusing the ion beam, and directing the beam onto the target element. Most preferably the transverse axis of rotation of the support base may be radial to the axis of rotation of the rotor. By way of example the transverse axis of rotation of the support base could be inclined to a plane perpendicular to the axis of rotation of the rotor, at an angle of plus or minus 7°. In accordance with one preferred feature, the further rotation means for rotating the support base is arranged to provide a fine adjustment of the angle of the base relative to a plane perpendicular to the axis of rotation of the rotor, for adjusting the implant angle for ion implantation. The feature allows zero implant angle scanning with the target elements in an advantageous orientation perpendicular to the axis of rotation (preferably in a vertical or near vertical plane), together with selected variation of the implant angle without introducing implant angle errors. Preferably during ion implantation the ion beam is arranged parallel to or substantially parallel to the axis of rotation of the rotor, but in some cases the ion beam may be inclined to the axis of rotation at a substantial selected angle, for example up to plus or minus 30°, when parallel scanning is not an issue. In accordance with another preferred feature of the invention, the further rotation means for rotating the support base is arranged to provide for rotation of the base through at least plus or minus 90°, to allow the base to be brought to an orientation such as to lie in a plane containing the axis of rotation of the rotor, or approximately in such a plane, for loading and unloading of target elements to and from the support base. This feature is advantageous in allowing the support base to be rendered horizontal, or approximately horizonal, during loading and unloading of target elements to and from the support base. Considering now a preferred structure for the support rotor, it is preferred that the support rotor includes a central core structure and a plurality of support arms each extending outwardly from the core structure each support base being mounted on a support arm. Preferably each base is formed as a heat sink for a target element during ion implantation, the arms each having passageways for supplying coolant to, and for withdrawing coolant from, the region of the bases. Preferably there is provided means such as a gear drive for rotating each base relative to the arm on which it is mounted. In one preferred form, the junction between each base and its support arm is provided with a high conductivity, deformable medium, and the junction is so shaped that in operation the medium is compressed by centrifugal force to provide a void free, thermally conducting, medium between the base and the arm. In accordance with another preferred feature the junction between each base and its support arm is such as to allow easy rotational movement of the base relative to the arm when at rest, but such as to produce close metal to metal contact by the effect of centrifugal force during rotation of the rotor in operation. A preferred construction may be provided in which each base is mounted on its arm by a part of the arm extending into a bore in the base. Conveniently the high conductivity deformable medium may be vacuum grease. In one particularly convenient arrangement, each base is mounted on its arm by the arm being screw threaded into the base, preferably there being provided high vacuum grease in the screw thread junction to provide good thermal contact between the arm and the base. This feature gives the advantage that although the base may be rotated about the arm to give adjustment of the angle of the base while the rotor is stationary or moving slowly, the effect of centrifugal force during the spinning of the disc during ion implantation, will be to lock up the screw threaded junction in operation. This prevents unwanted movement during scanning, and, more important, gives good, virtually metal to metal, thermal contact between the base and the arm to assist dissipation of heat from a target mounted on the base. In a modification of this arrangement, the outer end of the arm is not threaded at all, but the support base is secured to the support arm by a thrust race. The outer surface of the end of the support arm in the bore in the base is a close sliding fit with the plain inner surface of the bore of the support base, and thermal contact is ensured by vacuum grease which is urged into contact with the surfaces by centrifugual force ensuring a void free interface medium. In another convenient arrangement, each base is mounted on its arm by a part of the arm extending into a bore in the base, the arm having walls which, at the portion within the bore, are deformable, to a degree such that in operation the effect of centrifugal force on the coolant in the arm produces a high hydrostatic pressure which expands the deformable wall outwardly to produce good thermal contact between the arm and the base. Again, in accordance with this feature, rotation of the base on the arm is possible when the arm is stationary, but is prevented during spinning of the rotor for scanning purposes. Also, the expansion of the deformable wall produces good thermal contact between the arm and the base and allows dissipation of heat produced during ion implantation. Turning now to the aspect of the invention concerned with loading and unloading of target elements, the apparatus may include a loading and/or unloading mechanism for loading and/or unloading target elements to and from the support bases, the mechanism comprising a moveable platform for supporting a target element, means moving the platform beween a first position in which a target element may be deposited on and/or removed from the platform by a feeding and/or collecting means, and a second position in which a target element may be lifted off and/or deposited on the platform by a support base of the rotor. Conveniently the platform includes a transfer aperture through which a support base may be moved upwardly to remove from the platform a target element resting on the platform over the aperture, and through which a support base may be moved downwardly to deposit a target element on the platform. Preferably the feeding and/or collecting means comprises means for presenting a cassette of target elements to the platform, and the platform includes a projecting portion for insertion into the cassette to remove a target element from, or deposit a target element into, the cassette. In accordance with one feature, the mechanism includes an orientation member for supporting a target element and for rotating the element to a selected orientation before transfer to a support base, and in which the platform includes a transfer aperture through which the orientation member may be moved upwardly to remove from the platform a target element resting on the platform over the aperture, and through which the orientation member may move downwardly to deposit a target element on the platform. In one arrangement, the apparatus includes drive means for effecting a loading sequence of movements in which: said platform moving means moves the platform to said first position; said feeding and/or collecting means deposits a target element on the platform; said platform moving means moves said platform to said second position; said orientation member moves upwardly lifts said target element from the platform, and rotates the target element to a selected orientation; said platform moving means moves said platform back to said first position; said orientation member deposits said target element on said platform by a downward movement; said platform moving means moves said platform back to said second position; and a support base lifts the target element from the platform by an upward movement produced by driven rotation of the rotor. In accordance with another feature there may be provided a corresponding sequence for unloading. Thus the apparatus may include drive means for effecting a sequence of movements in which: said platform moving means moves said platform to said second position; a support base deposits a target element on the platform by a downward movement produced by driven rotation of the rotor; said platform moving means moves said platform to said first position; said orientation member moves upwardly and lifts said target element from the platform; said platform moving means moves said platform back to said second position; said orientation member deposits said target element on said platform by a downward movement; said platform moving means moves said platform to said first position; and the said feeding and/or selecting means lifts the target element off the platform. It is prefered that throughout operation of the apparatus, the axis of rotation of the support rotor is horizontal or approximately horizontal. It is preferred that during ion implantation the orientation of each support base is vertical at the implant position, by that during loading and unloading, the orientation of each support base is horizontal or approximately horizontal. It is to be appreciated that features of the invention which have been set out with regard to apparatus, may also be provided with regard to a method according to the invention, and vice versa. In particular, there is provided in accordance with the invention a method of implanting ions in a target element, by directing an ion beam onto a target region, including arranging a plurality of target elements on support bases mounted on and distributed around a support rotor with the target elements lying in planes transverse to a main axis of rotation of the rotor; adjusting the angular position of each support base relative to the overall support rotor to provide adjustment of the implant angle for ion implantation; rotating the support rotor about the main axis so as to carry the target elements through the target region to produce fast scanning of the ion beam across the target elements; and producing slow scanning of the beam across a target element transverse to the fast scanning, by relative movement between the beam and the target; characterized in that the said step of adjusting the implant angle comprises rotating each support base about a transverse axis which is substantially radial to the main axis. In accordance with another aspect of the present invention, there is provided a method of implanting ions in a target element, by directing an ion beam onto a target region, including arranging a plurality of target elements on support bases mounted on and distributed around a support rotor with the target elements lying in planes transverse to a main axis of rotation of the rotor; rotating the support rotor about the main axis so as to carry the target elements through the target region to produce fast scanning of the ion beam across the target elements; and producing slow scanning of the beam across a target element transverse to the fast scanning, by relative movement between the beam and the target; characterized in that the step of arranging the target elements on the support bases includes the steps of loading a target element onto a support base lying in a plane containing the main axis of the rotor, or approximately in such a plane; and rotating the support base about a transverse axis which is substantially radial to the main axis, until the support base lies in a plane perpendicular to the main axis, or approximately in such a plane. Turning now to the aspect of the invention which is concerned with heat transfer, it may be arranged that each support base acts as a heat sink for its target element, the heat sink comprising a body of material for conducting heat away from the element, and a component adapted to be positioned between the body and the element and being responsive to centrifugal force to produce or enhance thermal contact between the element and the body. In one convenient form, the said component comprises a sheet of material fixed to the said body and having a multiplicity of moveable projections extending away from the body for making thermal contact with the said element, the moveable projections being such as to be moveable in operation away from the body towards the element in response to centrifugal force. In a particularly preferred combination, it may be arranged that each support base acts as a heat sink for its target element, the heat sink comprising a body of material for conducting heat away from the element during the rotary movement of the body and the element, a sheet of material for providing thermal contact between the element and the body, the sheet having one side secured to the body and having a multiplicity of flaps inclined outwardly away from the body for making thermal contact with the said element, the flaps being positioned in such a manner relative to the axis of rotation of the body during the said rotary movement, that centrifugal force acts to urge the flaps outwardly from the sheet during operation. In a preferred construction, each flap is constituted by an area of the sheet which has been partially separated from the remainder of the sheet, and is bent away from the remainder of the sheet. Preferably each flap is constituted by a partially separated area of the sheet which has been bent from the remainder of the sheet through an angle greater than 90°. Most preferably each flap is of rectangular shape, and is formed from an area of the sheet which has been separated from the remainder of the sheet along three sides of a rectangle, and has been bent away from the remainder of the sheet along the fourth unseparated side of the rectangle. Conveniently each sheet is formed of metal of high thermal conductivity, for example aluminum, such as aluminum baking foil. Conveniently the sheet is secured to the body by adhesive or by braze contact. In practice, a very large multiplicity of such flaps may be easily produced for example by stamping the flaps out of a plain sheet of aluminum. The flaps may for example be of diameter 1mm by 1mm, and may be closely spaced across the sheet. Thus excellent thermal contact is made between the sheet fastened to the heat sink, and the target element to be placed against the perforated sheet. In another construction, flaps are formed by folding a sheet of material concertina fashion. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side view of ion implantation apparatus embodying the invention; Figures 2A and 2B are, respectively, side and end views of target presenting apparatus embodying the invention, and as shown in Figure 1; Figures 3A and 3B are, respectively, side and end views of a heat sink support base, forming part of the target presentation apparatus of Figures 2A and 2B; Figures 4A and 4B are, respectively, diagrammatic side and end views of the apparatus shown in Figures 2A and 2B, but with the heat sink support bases rotated through 90° on supporting arms thereon, to present the support bases in a load/unload position; Figures 5A and 5B show, respectively, plan and side views of a loading and unloading mechanism for loading and unloading target elements onto support bases; Figures 6A and 6B show side views of a device for providing thermal contact between a target element and a heat sink support base embodying the invention, the Figures showing the device in an unloaded, and loaded, condition, respectively; Figure 7 is a diagrammatic plan view of the thermal contact device of Figures 6A and 6B; and Figures 8A and 8B show side views of an alternative device for providing thermal contact between a target element and a heat sink support base embodying the invention, the figures showing the device in an unloaded, and loaded, condition, respectively, and the figures corresponding generally to Figures 6A and 6B. Referring to Figure 1, an ion implantation apparatus comprises an ion source 11 and an extraction electrode 12 for producing an ion beam 13. An analysing magnet 14 isolates the species required to be implanted, and the ion beam passes through a post-analysis acceleration system 15 before being directed onto a target element 16. The ion source, extraction electrode, and anlysing magnet, are housed in a high voltage terminal indicated generally at 17. The target element 16 is typically a semi-conductor wafer for example a 200mm diameter circular wafer of silicon of thickness 0.75mm. The target element is one of a plurality of target elements presented to the ion beam by a target presentation apparatus indicated generally at 18, and shown in more detail in Figures 2A and 2B. The target presentation apparatus 18 comprises a support rotor 19 comprising a central core structure or hub 20, and ten support bases 21 arranged in a circular configuration around the hub 20, each support base 21 being mounted on a radial support arm 22. The wheel 19 is mounted for rotation on an axle 23 and rotated about an axis 41 by a spin motor 24, which in turn is mounted on a shaft 25. In other arrangements, the ten 200mm diameter heat sinks may be replaced by for example thirteen 150mm diameter, or seventeen 125mm diameter, or twenty 100mm diameter heat sinks. Each support base 21 consists of a cylindrical disc of material of a high thermal conductivity, such as aluminum and acts as a heat sink for the target element 16 during ion implantation. Passageways for coolant pass to and from the region of the heat sink support base 21 along the arm 22 and through the hub 20 and axle 23 and out of the apparatus along the shaft 25, as will be described further hereinafter. The wheel 19 is housed in a evacuated housing 26 (shown in Fig 2B). A seal 27 is provided between the shaft 25 and the vacuum housing 26, to allow movement of the shaft 25 along its length. In normal operation, the target elements 16 are presented to the ion beam 13 perpendicular thereto, or at a selected angle as will be described hereinafter, and the wheel 19 is rotated at high velocity, for example 600 to 1200 rpm by the motor 24. This produces scanning of the ion beam across the target elements in a direction substantially perpendicular to the axis of each arm 22. In addition, the shaft 25 is subjected to slow speed reciprocal motion along its length (that is to say upward and downward motion in Figures 2A and 2B). Together, the rotation of the wheel, and the reciprocating motion of the shaft 25, produce scanning of the ion beam across the surface of each target 16. There will now be described in more detail, with reference to Figures 3A and 3B, one form of the heat sink support base 21. Each heat sink support base 21 is formed from a solid cylindrical block 28. The block 28 has a screw threaded bore 29 aligned along a diameter of the block 28, terminating in a vent passage 30 which leads back to the outer circumference of the block 28. The support arm 22 has a corresponding threaded end 31, and the screw thread junction between the two is lubricated by and filled with a vacuum grease. This allows rotation of the support base 21 about the arm 22 when the wheel is stationary or moving slowly, but provides a good thermal contact between the heat sink 21 and the arm 22 during high speed rotation of the wheel 19. During this rotation considerable centrifugal forces act on the heat sink 21 (amounting to forces of the order of 500G) which give greatly improved, virtually metal to metal, contact. Throughout the screw thread, very good contact is made by the outer side of each thread on the block 28, with the inner side of each thread on the arm 22, with an extremely thin layer of grease across the interface to ensure contact. Cooling is carried out by coolant 33 being pumped outwardly along the arm 22 along a passageway 34, and being returned along a passageway 35, the passageways 34 and 35 being coaxial along the arm. Within the hub structure 20, corresponding coolant passageways 36 and 37 are provided. A further sleeve 38 is shown in Figures 3A and 3B, surrounding and forming part of the arm 22. This sleeve 38 is keyed to the heat sink 21 at its outer end, and at its inner end has a ring gear 39 mating with a rotary drive means indicated diagrammatically at 40. The purpose of this drive 40 is to produce rotary movement of the heat sink 21 about an axis 10 (shown in Fig 1) of the support arm 22. A first movement is a fine adjustment of, for example, plus or minus 7° on either side of a plane perpendicular to the axis of rotation of the wheel 19. This adjustment is provided to allow adjustment of the implant angle by the ion beam 13. A second movement provided by the drive means 40 comprises a rotation of the heat sink support base 21 through 90°, to an orientation in which the plane of the heat sink 21 contains the axis of rotation 41 of the rotor 19. The purpose of this orientation is for loading and unloading as will be described hereinafter. A wide variety of arrangements for rotation of the heat sink 21 can be provided in the hub structure 20, including centrifugally driven arrangements. There may be provided in accordance with the invention an alternative arrangement of fastening the heat sink 21 to the arm 22, to provide good thermal contact, yet allowing rotation of the heat sink 21. In this alternative arrangement (not shown), the outer end 31 of the arm 22 may be threaded for only a short portion at the entrance to the heat sink 21, or a thrust race may be provided close to the hub. This is followed by a smooth cylindrical surface which is a close rotational fit with a smooth bore in the block 28. Again the interface may be filled with vacuum grease but this is not essential. The wall of the end 31 of the arm 22 is in this case arranged to be a thin wall, between the passage 34, and the block 28, and is arranged to be deformable under the substantial hydrostatic pressure which builds up in the coolant 33 during high speed rotation of the heat sink 21. The inner wall 31 expands elastically under the influence of the high hydrostatic pressure, converting the easy sliding and rotation fit which is available when stationary, into intimate contact during high speed rotation. In another alternative (also not shown), the outer end 31 of the arm 22 is not threaded at all, but the support base 21 is secured to the support arm 22 by a thrust race. The outer surface of the end 31 of the support arm 22 is a close sliding fit with the plain inner surface of the bore 29 of the support base 21, and all the surfaces are rigid and non-deformable with centrifugal force. Thermal contact is ensured by vacuum grease which is urged into contact with the surfaces by centrifugal force ensuring a void free interface medium. This allows rotation while spinning. There will now be described with reference to Figures 4A, 4B, 5A and 5B, apparatus for, and a method of, loading and unloading target element wafers onto and from the wheel 19 of the target presenting apparatus. In normal ion implantation operation, the wheel 19 of Figures 1 to 2B is kept in a vertical, or approximately vertical plane, and the heat sinks 21 are kept substantially vertical, with variations from the verticality of the wheel 19 and heat sinks 21, chosen for particular implant requirements. For loading and unloading however, the heat sinks 21 are rotated through 90° so that they can be brought into a horizontal position in the 3 o'clock and 9 o'clock positions of the wheel. Target wafers can be loaded and unloaded in one or both of these positions. Referring to Figure 4B, twenty-five target wafers 16 are stacked vertically in a cassette 42 mounted in a load lock 43. Means are provided (not shown) for giving a vertical indexing movement of the cassette 42 to bring a selected target wafer 16 into alignment with a horizontally moveable platform 44 positioned inside the vacuum chamber 18, opposite a letter box gate valve 45 positioned between the vacuum chamber 18 and the load lock 43. The platform 44 is in the form of a thin flat blade, shown in more detail in Figure 5A and 5B. Also shown in Figure 4B is an orientation member 46, consisting of a pedestal having a vertical support shaft 47 and a horizontal portion 48. As shown in Figures 5A and 5B, the blade 44 has a left hand portion defining an aperture 49 between parallel sides 50, and a narrower projecting portion 51 defining an aperture 52. The pedestal 48 is able to pass through both apertures 49 and 52. The blade 44 is moveable between a first position indicated in full lines in Figures 5A and 5B, in which the blade 44 is moved towards the right, and a second position shown in dotted lines in Figure 5A in which the blade 44 is moved toward the left. When the blade is in the first position, towards the right, the projecting portion 51 of the blade 44 extends into the cassette 42, and the pedestal 48 is able to move upwardly and downwardly through the aperture 49 between the sides 50. When the blade 44 is in the second position, to the left in Figure 4A, the projecting portion 51 of the blade is withdrawn from the cassette, and the sides 50 are in register with the heat sinks 21 of the wheel 19. In this leftward position, the pedestal 48 is able to move upwardly and downwardly through the aperture 52, and the heat sinks 21 are able to rotate through the aperture 49 of the blade 44, between the sides 50. Extending inwardly from the sides 50 of the blade 44 are 4 wafer support fingers 53. Corresponding notches 54 are provided in each heat sink 21, allowing the heat sinks 21 to be rotated through the aperture 49 without contacting the support fingers 53. The operation of loading and unloading cassettes is as follows. The blade 44 passes through the letter box gate valve 45 to a position under a target wafer 16 to be loaded. The cassette 42 is indexed downwards to leave the target wafer 16 on the projecting portion 51 of the blade 44. At this time the blade is in the first position. The blade 44 then moves horizontally back through the letter box gate valve to the second position, and stops with the target wafer 16 over the orientation pedestal 46. The pedestal 46 passes upwardly through the aperture 51 and lifts the target wafer 16 off the blade 44. The target wafer is then rotated for orientation detection, and then rotated to the required position. While this is happening the blade 44 returns to the first position with the projecting portion 51 extending into the cassette 42. The cassette 42 is indexed downwardly to the next position to deposit a further target wafer on the platform 44. At the same time the pedestal 46 is lowered to deposit the oriented wafer onto the wafer support fingers 53 of the blade 44. The blade 44 is then moved back through the gate valve to the second position to the left in Figure 5A. The first selected target wafer 16 is now in a loading position in correlation with the rotating heat sinks 21, while the second mentioned target wafer is positioned above the orientation pedestal 47. The wheel 19 is indexed slowly in rotation to bring a heat sink 21 upwardly through the aperture 49 to pick a wafer 16 off the blade 44. At the same time the orientation pedestal 47 lifts a second target wafer from the blade 44, and the cycle is repeated to continue orienting and loading wafers. Wafers are unloaded by reversing the steps set out above, except that the orientation step can be omitted. In Figure 4B a single loading and unloading arrangement is provided, but if required loading and unloading can be effected at two stations, placed at the 3 o'clock and 9 o'clock positions of the wheel 19. In some methods of ion implantation it may be necessary to reorient the wafer between different stages of implanting. It can be done by unloading a wafer onto the blade, and lifting and reorienting the wafer by the pedestal 46, and returning the wafer to the heat sink, without the need to take the wafer back out of the vacuum chamber into the cassette. A number of advantages arise from the method of loading and unloading described. Horizonal loading is preferred because of the use of gravity which simplifies equipment. During ion implantation, a vertical position of the wafers is preferred for reasons of cleanliness. In previous machines it has been necessary to move the entire scanning wheel to a horizontal position, if horizontal loading is required. The present arrangement allows a simple rotation of the heat sink from the vertical to the horizontal position for loading purposes. Unlike known systems precise indexing of the wheel is not required; it can be loaded without stopping the wheel rotation provided that this is slow. The final wafer to heat sink motion does not need to be provided by the wafer handling mechanism. There will now be described a final aspect of the invention, concerned with improved heat transfer arrangements. Although it is possible to utilise features of the invention which have been described in embodiments so far, with conventional centrifugal clamping of the wafer onto a heat sink, the mechanisms described so far are particularly suitable for use where the target wafers are maintained in good thermal contact with the heat sink without the need for centrifugal clamping which can lead to implant angle errors under certain circumstances. One particularly advantageous way of achieving this will now be described with reference to Figures 6A, 6B and 7. A device for providing thermal contact between two bodies, for example between a heat sink 21 described in the previous text, and a target wafer 16, is shown in Figures 6A, 6B and 7. A sheet of aluminium 60 has stamped from it a series of flaps 61, each produced by cutting three sides 62, 63 and 64 of a rectangle. The flap 61 then remains attached to the sheet 60 by the fourth side 65 of the rectangle, which forms a hinge between the sheet 60 and the flap 61. In the stamping process, the flap 61 is pushed outwardly from the sheet, and in a further step, the flap 61 is then bent through an angle of more than 90°, to assume the folded position shown in Figure 6A. The final step of bending the flap 61 can be achieved for example by an air blast. By way of example, the aluminum sheet may have a thickness of 0.05mm, and each rectangular flap 61 may be a square of side lmm. The spacing of the flaps may be, as shown in Figure 7, equal to the dimensions of the flaps. In Figure 6A, the sheet 60 is shown secured by adhesive or braze 66 to a heat sink 21. In Figure 6B the same arrangement is shown, but with a target wafer 16 positioned on top of the heat sink 21, and lightly clamped thereto, by clamps not shown. The position of the flap 61 is shown in dotted lines, when the sandwich is at rest. When the wheel 19 is rotated at high speed, the effect of centrifugal force on the flaps 61 bends the flaps to the position shown in full lines in Figure 6B. As can be seen, the effect of this is to provide a large multiplicity of substantial contact areas 67 in which the flap 61 is forced into intimate contact with the wafer 16. This produces excellent thermal contact between the wafer 16 and the heat sink 21. The orientation of the flaps is chosen so that centrifugal force acting on the flaps tends to lift the flaps. The spacing between the wafer and the heat sink, and the thickness of the foil and the rotation speed of the disc are chosen so as not to allow the flaps to overbend. By way of example, with dimension illustrated, over 7000 contacts can be achieved on a 200mm diameter wafer. In Figures 8A and 8B there is shown an alternative form of device for providing thermal contact between a target element and a heat sink support base embodying the invention. Figures 8A and 8B correspond generally to Figures 6A and 6B, and show the device in an unloaded, and loaded, condition respectively. The device again consists of a sheet 60 of aluminium foil, but in this embodiment the foil is folded in a concertina arrangement, and is not cut to provide the flaps. In the unloaded position shown in Figure 8A, the sheet 60 provides projections 61 by virtue of the folding of the sheet. The sheet 60 is secured at line contacts 70 to the body 66 by braze contact. When loaded as shown in Figure 8B, the projections 61 are pressed sideways to form flaps as shown, by the effect of the target wafer 16. After the projections 61 have been deformed as shown in Figure 8B, the projections behave as flaps and operate in the same manner as has been described with reference to 6b, the thermal contact being made at the contact areas 67.

Claims: Apparatus for presenting target elements to an ion beam for ion implantation comprising: a support rotor (19) having a plurality of support bases (21) for supporting target elements (16), said support bases (21) being mounted on and distributed around said support rotor (19); main rotation means (24) for rotating the support rotor (19) about a main rotation axis (41) with the target elements (16) lying in planes transverse to the rotation axis (41) through an ion beam (13) to produce in operation scanning of the ion beam (13) across the target elements (16); and means (40) for angular positioning of each support base (21) relative to said overall support rotor (19); characterised in that said angular positioning means (40) comprises further rotation means (40) arranged to rotate the support base (21) about a transverse axis which is substantially radial to the main axis (41). Apparatus according to Claim 1 in which the transverse axis (10) of each support base (21) is radial to the main axis (41) of rotation of the support rotor (19). Apparatus according to Claim 1 or Claim 2 in which the said further rotation means (40) for rotating the support base (21) provides a fine adjustment of the angle of the support base (21) relative to a plane perpendicular to the main axis (41) of rotation of the support rotor (19), for adjusting the implant angle for ion implantation. Apparatus according to any preceding claim in which the said further rotation means (40) provides for rotation of the support base (21) so as to lie in a plane containing the main axis (41) of rotation of the support rotor (19), or approximately in such a plane, for loading and unloading of target elements (16) to and from the support base (21). Apparatus according to Claim 4 in which the said further rotation means (40) provides for rotation of the support base (21) through plus or minus at least 90°. Apparatus according to any preceding claim in which the support rotor (19) includes a central core structure (20) and a plurality of support arms (22) each extending outwardly from the core structure (20), each support base (21) being rotatably mounted on a support arm (22) for rotation about the said transverse axis (10), which lies substantially in the direction of the support arm (22). Apparatus according to Claim 6 in which there is provided in the junction between each support base (21) and its support arm (22) a thermally conducting, deformable medium, and the junction is so shaped that in operation the medium is compressed by centrifugal force to provide a void free, thermally conducting, medium between the base and the arm. Apparatus according to Claim 7 in which each of said support base (21) is mounted on its support arm (22) by a part of the support arm (22) extending into a bore (29) in the support base (21) with a junction between the support base (21) and the support arm (22) such as to allow easy rotational movement of the base relative to the arm when at rest, but such as to produce close metal contact by the effect of centrifugal force during rotation of the rotor in operation. Apparatus according to Claim 8 in which each support base (21) is mounted on its support arm (22) by the support arm being screw threaded into the support base (21). Apparatus according to any preceding claim when including the features of Claim 4, in which the apparatus includes a loading and/or unloading mechanism for loading and/or unloading target elements (16) to and from the support bases (21), the mechanism comprising a moveable platform (44) for supporting a target element (16), means moving the platform (44) between a first position in which a target element (16) may be deposited on and/or removed from the platform (44) by a feeding and/or collecting means (42), and a second position in which a target element (16) may be lifted off and/or deposited on the platform (44) by a support base (21) of the rotor (19). Apparatus according to Claim 10 in which the mechanism includes an orientation member (46) for supporting a target element (16) and for rotating the element to a selected orientation before transfer to a support base (21), and in which the platform (44) includes a transfer aperture through which the orientation member may be moved upwardly to remove from the platform (44) a target element resting on the platform over the aperture, and through which the orientation member (46) may move downwardly to deposit a target element (16) on the platform (44). Apparatus according to any preceding claim in which each support base (21) acts as a heat sink and there is provided a multiplicity of moveable projections (61) extending away from the support base (21) for making thermal contact with a target element (16) on the support base (21), the moveable projections (61) being such as to be moveable in operation away from the base (21) towards the element (16) in response to centrifugal force. Apparatus according to Claim 12 in which the moveable projections (61) extending outwardly from the support base (21) comprise a multiplicity of flaps (61) inclined outwardly away from the support base (21) for making thermal contact with the target element (16), the flaps being positioned in such a manner relative to the main rotation axis (41) that during rotation about the main rotation axis (41) centrifugal force acts to urge the flaps outwardly from the support base (21). Apparatus according to any preceding claim including means (25) for producing further, slow speed, scanning of the beam (13) across a target element (16) transverse to the first said scanning of the beam. A method of implanting ions in a target element, by directing an ion beam (13) onto a target region, including arranging a plurality of target elements (16) on support bases (21) mounted on and distributed around a support rotor (19) with the target elements lying in planes transverse to a main axis (41) of rotation of the rotor (19); adjusting the angular position of each support base (21) relative to the overall support rotor (19) to provide adjustment of the implant angle for ion implantation; rotating the support rotor (19) about the main axis (41) so as to carry the target elements (16) through the target region to produce fast scanning of the ion beam (13) across the target elements (16); and producing slow scanning of the beam (13) across a target element (16) transverse to the fast scanning, by relative movement between the beam (13) and the target (16); characterized in that the said step of adjusting the implant angle comprises rotating each support base (21) about a transverse axis (10) which is substantially radial to the main axis (41). A method of implanting ions in a target element, by directing an ion beam (13) onto a target region, including arranging a plurality of target elements (16) on support bases (21) mounted on and distributed around a support rotor (19) with the target elements lying in planes transverse to a main axis (41) of rotation of the rotor (19); rotating the support rotor (19) about the main axis (41) so as to carry the target elements (16) through the target region to produce fast scanning of the ion beam (13) across the target elements (16); and producing slow scanning of the beam (13) across a target element (16) transverse to the fast scanning, by relative movement between the beam (13) and the target (16); characterized in that the step of arranging the target elements (16) on the support bases (21) includes the steps of loading a target element onto a support base (21) lying in a plane containing the main axis (41) of the rotor (19), or approximately in such a plane; and rotating the support base (21) about a transverse axis (10) which is substantially radial to the main axis (41), until the support base (21) lies in a plane perpendicular to the main axis (41), or approximately in such a plane. A method according to claim 16, including, after ion implantation, the further step of rotating the support base (21) about the pivot axis (10) until the support base (21) lies in a plane containing the main axis (41) of the rotor (19), or approximately in such a plane, and unloading the target element from the support base (21).

Inventors: AITKEN DEREK, AITKEN, DEREK
Applicants: SUPERION LTD, SUPERION LIMITED
Application year: 1990
Main classification: H01L21
IPC codes: H01J37, H01L21