US 3689766 A
An electromagnetic separator adapted for ion implantation on an industrial production scale has its beam current stabilized and a mechanism within the target chamber for automatically moving targets through the ion beam according to a predetermined scanning pattern.
Description (OCR text may contain errors)
United States Patent Freeman Sept. 5, 1972  APPARATUS FOR BOMBARDING A 2,348,031 5/ I944 Raschman ..250/49.5 TARGET WITH IONS 3,326,176 6/1967 Sibley ..118/49.l X 72 Inventor: James Han-y Freeman, Abingdon 3,547,074 12/1970 HllSChfCld ..1 18/495 X Engkmd 3,434,894 3/1969 Gale 148/ l 87 ,131, l 64 t t l. ..25 49.5  Assignee: United Kingdom Atomic Energy 3 300 4/ 9 Je a 0/ R Authori Lo don E 1 nd 3,235,727 2/1966 Shaprro ..250/49.5 PE n a 3,206,336 9/1965 Hora ..l48/l.5  Filed: Aug. 21, 1970 3,358,239 12/1967 Franke et al ..250/49.5 TE
 Appl. No.: 65,941
' Primary Examiner-William F. Lmdqulst A d  Foreign Application Priority Data nomey Larson Taylor & Hm S Sept. 5, 1969 Great Britain ..44,17l/69  ABSTRACT An electromagnetic separator adapted for ion implan- 4 on an has us  Field of 'swc't.....1256/49.5 R, 49.3 TE, 49.5 T, current stabilized and a mechanism within the target 50 5 p 4 5 5; 2 21 chamber for automatically moving targets through the 118/491, 49,5 ion beam according to a predetermined scanning pattern.
 References Cited 13 Cl 8 Drawing figures UNITED STATES PATENTS 2,257,774 10/1941 Von Ardenne ..250/49.5
EHT POWER SUPPLY .PATENTEDSEP 5|972 3 689 766 SHEET 1 BF 6 I BACKGROUND OF THE INVENTION The invention relates to apparatus for bombarding a target with ions, for example to implant ions into the target.
SUMMARY OF THE INVENTION The invention provides apparatus for bombarding a target with ions, which apparatus comprises an ion beam source, a magnet for deflecting the ion beam towards a window, means for modulating the energy of the ion beam so that the deflection produced by the magnet varies in correspondence with the modulation to thereby cause the deflected ion beam to sweep back and forth over the window, detector means for detecting the ion beam current passing through the window, ion beam control means responsive to the output of the detector means for controlling the ion beam energy modulating means so as to tend to maintain constant the intensity of the ion beam current passing through the window, target holder means for supporting a target behind the window, and drive means for imparting controlled movement to the target holder to scan the target through the ion beam according to a predetermined scan pattern. I
For certain applications, especially for example where the ions comprise boron ions which can be readily separated from other ions, the window can be as wide as the target to be bombarded and the means for stabilizing the ion beam control the ion beam to provide a substantially uniform ion beam intensity over the area of the window.
In this case, in a preferred arrangement according to the invention the target holder locates the target hehind the window with the width of the target in register with the width of ion beam passing through the window, and the said drive means move the supported target lengthwise past the window at controlled speed.
Preferably the target holder is adapted to carry a plurality of targets in a row and, when moved as aforesaid, moves the targets in succession at controlled speed past the window.
Preferably the said drive means reciprocates the target holder, whereby the targets are moved repeatedly past the window, the total ion dose being determined by the number of reciprocations.
Preferably the target holder is adapted to carry a plurality of rows of targets and the drive means includes means for indexing the target holder to bring another row of targets into register with the window.
Preferably the target holder comprises a cylinder with a plurality of racks arranged around the periphery, each rack being adapted to carry a row of targets extending parallel to the axis of the cylinder.
Where a narrower window is required for separating the wanted ions from unwanted ions, an alternative arrangement is preferred wherein the target holder comprises a plate capable of supporting one large target or a plurality of smaller targets, and a support for the plate, which support is moved by the said drive means.
With this arrangement, the drive means preferably comprise an X-Y drive.
Preferably the said support is adapted to release a plate it is carrying automatically at the end of a complete scanning operation and to return to the starting position for a further scanning operation.
Preferably means are provided for mounting a plurality of plates loaded with targets at a loading station having means for automatically loading a plate onto the said support when it returns to the starting position. perpendicularly ions In a preferred arrangement according to.the invention, the ion beam is produced by an ion beam separa tor in which the magnet is an electromagnet. In ion beam separators, a beam containing ions of a plurality of isotopes is caused to pass through a strong magnetic field which extends'substantially perpendicularly to the ion beam path. The magnetic field deflects the ions into a curved path, the magnitude of the deflection being dependent, inter alia, upon the mass and the energy of the ions. Thus, ions of different isotopes may be separated into diverging beams, and, by appropriate geometrical arrangement of the components of the separator and location of the window, ions of a selected isotope may be focused upon the target. In this case, it is important that the sweep imparted to the ion beam should not be such as to cause unwanted ions of a mass different from that selected for bombarding the target to pass through the window.
Preferably the means for modulating the energy of the ion beam comprises an electrical source connected to superimpose an alternating voltage upon a high direct voltage supplied to the accelerating electrode of the ion beam source.
BRIEF DESCRIPTION OF THE DRAWINGS the apparatus,
FIG. 6 is a diagrammatic perspective view, partly cut away, of part of modified apparatus, and
FIGS. 7 and 8 are like views of a detail of the part of the apparatus shown in FIG. 5, illustrating different stages in the operation thereof.
DESCRIPTION OF PREFERRED EMBODIMENTS In these examples, the apparatus comprises an electromagnetic ion beam separator of the type described in US. Patent Application Ser. Nos. 3,959 and 3,958 both filed on Jan. 19, 1970. FIG. 1 illustrates the separator diagrammatically and shows an accelerating electrode 11 of an ion source which is as described in Patent Application Ser. No. 3,958 mentioned above.
A diverging beam 12 of ions emerges from the ion source through the accelerating electrode 11 and passes between pole pieces, of which one pole piece 13 is shown, of an electromagnet. The magnetic field between the pole pieces is perpendicular to the direction of travel of the ion beam and perpendicular to the plane in which the ion beam is to be deflected. That is, as seen in FIG. 1, the magnetic field lines between the pole pieces are perpendicular to the plane of the paper.
The ion beam is deflected by the magnetic field towards a window 14 provided by an aperture in a plate 15. Ions in the beam passing through the window 14 impinge upon a target 16. A detector probe 17 projects a small amount into the path of the ion beam passing through the window 14 and intercepts a small fraction of the ion beam for providing an indication of the ion current passing through the window 14. A high, stabilized, direct voltage applied to the accelerating electrode 11 is provided by a stabilized E.I-I.T. extra high tension) power supply indicated at 18. Superimposed upon this direct voltage applied to the accelerating electrode 11 is an alternating voltage generated by a sweep voltage generator 19. The amplitude of the sweep voltage is controlled by a sweep control indicated at 21 which, in turn, is controlled by an electrical'signal indicative of the ion beam current passing through the window 14 and derived from the probe 17 via amplifier 22.
FIG. '1 illustrates diagrammatically the separation of two ions of differing mass. The solid lines indicate the ion beam of ions of one mass with the sweep effect indicated by arrow 23. The dotted lines indicate the ion beam path of the ions of the other mass with the sweep effect indicated by arrow 24. It will be appreciated that the deflections indicated on the scale of FIG. 1 are greatly exaggerated. However, the amplitude of the sweep should be such that unwanted ions do not pass through the window at any position of the sweep.
In operation, the apparatus is set up so that the window l4 intercepts the desired ions and the direct voltage applied to the accelerating electrodel 1 is adjusted, as a coarse setting, to give the desired ion beam intensity at the target 16. A fine setting of this intensity is achieved by adjustment of the sweep voltage amplitude. Once the ion beam intensity through the window 14 has been set up, feedback from the probe 17 operates via the sweep control 21 to control the amplitude of the sweep in such a way as to tend to reduce variations in the ion beam intensity.
It is an important feature of the arrangement that the sweep effect is generated by the magnetic field simply by varying the energy of the ion beam.
Although the magnetic field provides the principal focusing of the emergent ion beams, provision is required for fine focusing of the desired ions onto the target.
This is provided, together with the window 14, by a lens assembly shown in FIGS. 2, 3 and 4.
Referring to FIG. 2, a vacuum enclosure is provided by an electrically insulating glass cylinder 23 clamped at each end respectively to flanges 24 and 25 with interposing sealing rings 26, 27. I
The flange 24 is sealingly coupled to the vacuum system of the electromagnetic separator. The flange 25 is sealingly coupled to an evacuated target chamber hereinafter more fullydescribed with reference to FIG. 5. 1 t
Threeapertured metal plates comprising respectively a mass-defining slit 28, a suppressor electrode 31,
and a probe 29 are mounted so as to be electrically insulated from one another upon a tubular support 32 within the glass cylinder 23. v
The tubular support 32 in addition to supporting the three metal plates 28, 29 and 31 also supports a further earth electrode plate 34 and, in between thisearth electrode plate 34 and the suppressor electrode plate 31, a beam stop device 35.
The fine focusing of the ion beam is produced in the lens assembly by an electrostatic field generated between the earth electrode plate 34 and an electrode plate 36 spaced from the earth electrode plate 34 and mounted upon the flange 25. In this example, the electrodeplate 36 is held at a positive potential of 140 kV. It will be appreciated that both the the earth electrode plate 34 and the electrode plate 36 are centrally apertured to permit passage of the ion beam therethroughf Adjustment of the focusing effect-is produced by adjusting the separation of the earth electrode plate 34 and the electrode plate 36. For this, the support tube 32 is telescopically mounted upon a tube 33 fixed to the flange 24. A rigid tube 37, fixed at 38 to the tubular support 32, extends through a slot 39 in the tube 33 to a drive mechanism 41 (see FIG. 4) adapted to move the tube 37 back or forth longitudinally with appropriate rotation of a drive pinion 42.
The inside of the tube 37 provides a convenient passage for the cable for making the various electrical connections to the components carried on the tubular.
As may be seen from FIG. 2, the wires in the cable fan out to a connector 43 from which connections to the various components are made.
Mounted upon the tubular support 32 and encompassing the region of the electrostatic lens is an X-ray shield 44.
The beam stopping device 35 comprises a metal plate 45 mounted within and electrically insulated from afaraday cup 46 which encompasses the metal plate 45 except for the aperture facing the direction of the incident ion beam and through which the ions pass onto the beam stopping plate 45. The beam stopping device 35 as a whole is rotatable-about a pivot 47. Rotation of the device 35 into and out of the beam stopping position is effected by an actuating solenoid 48 via coupling 49 and rotatable rod 51.
The mass-defining slit 28 has an area of l in (6.4 cm and, in operation, the electromagnetic separator is set up so that the ions of the desired mass focus upon the mass-defining slit and are swept over an area of approximately l% in. X 1% in. (3.2 cm. X 3.2 cm.).
The probe plate 29 has its aperture arranged so that a small known area of the plate intercepts the ion beam passing through the mass-defining slit 28. The suppressor electrode 31 is connected to a positive electrical potential in order to attract and remove secondary electrons that may be produced by the bombardment by the ion beam of the probe 29 or the plate 28 with the mass-defining slit.
The faraday cup 46 is similarly held at a positive potential to perform a similar function in relation to the beam stopping plate 45 when this is in the beam stopping position.
It will be appreciated that it is important to prevent secondary electrons from penetrating back into the separator as they may be accelerated to high energies, travelling in the opposite direction to the positive ion beam, and produce X-radiation at some unspecified location in the electromagnetic separator.
It will be appreciated that the mass-defining slit provided by the plate 28 as described with reference to FIG. 2 comprises the window 14 as described with reference to FIG. 1, and the probe plate 29 as described with reference to FIG. 2 comprises'the probe 17 as described with reference to FIG. 1.
The ion beam passing through the mass-defining slit is focused by the electrostatic lens onto a target mounted within the target chamber coupled to the flange 25.
FIG. 5 is a view of one form of target chamber from behind looking towards the oncoming ion beam.
In this example, provision is made for moving targets systematically so that they may be scanned through the ion beam in such a way as to achieve a uniform dose of ions applied to the target. I
One example of the mechanism for moving the targets is shown in FIG. 5,'in which outer cylinder 51a comprises the vacuum enclosure of the target chamber. This mechanism is particularly appropriate where it is necessary to use a narrow mass-defining slit for selecting the desired ions.
The mechanism is intended for automatic processing of a large number of slices of substrate, for example for the manufacture of semiconductors by implanting doped ions into substrate slices.
The substrate slices are carried on rectangular supporting plates (not shown) approximately 5 in. X 4 in. (12.7 cm. X 10.2 cm). Generally, several semiconductor slices will be mounted on each single support plate, but where a particularly large substrate surface is to be implanted, a support plate may support but a single semiconductor slice.
The rectangular support plates are provided with a pair of laterally projecting ears at the opposed top edges.
A batch of plates carrying substrate slices is supported within the target chamber upon two spaced horizontal chain runs not shown), the ears of the support plates resting on the chains. The chains are driven forward in steps synchronized with the drive to the remainder of the mechanism to bring the plates forward in succession to a loading station located at the top left-hand corner of the mechanism as seen in FIG. 5.
At this loading station, a plate is loaded onto a carrier 52 in a manner described hereinafter.
The carrier 52 is mounted on an X-Y drive mechanism. The Y movement is provided by movement of the carrier 52 itself upon supporting rods 53, 54. The X movement is provided by lateral movement of the rods 53 and 54 themselves upon horizontal shafts 55, 56.
Y drive is derived from a stepping motor which drives shaft 57 coupled to the carriage 52 via a wire and pulley drive mechanism (not shown). The X drive, also derived from a stepping motor, is applied to a tube 59 supported for lateral movement upon a spider 58 carried by bearings 61 upon shafts 62.
The operation of the mechanism will be described starting from the moment when the carriage 52 is at the top left-hand comer of the framework as seen in FIG. 5 and about to start to scan the plate, which has been automatically loaded onto the carrier 52, through the ion beam. The X drive carries the carrier 52 across'at a uniform speed to the right-hand side of the framework and one step of Y drive is applied to make an incremental movement of the carriage 52 downwards. The X drive then drives the carriage back across to the lefthand side as seen in FIG. 5 and a further incremental Y step downwards occurs before X drive back to the right-hand side. This sequence is repeated until the whole area of the plate mounted on the carrier 52 has been scanned through the ion beam. The arrangement is such that the plate is clear of the ion beam before reaching either end of the X scan so that there is no dwell effect at the ends of the traverse. I
At the completion of the scanning operation, the carriage 52 is moved downwards in the bottom left-hand corner as seen in FIG. 5 by the Y drive, whereupon two downwardly projecting sliders 63, 64 engage upon fixed studs 112 (See FIGS. 7 and 8) in the framework. Referring to FIGS. 7 and 8, the upper ends of these sliders 63 and 64 have inclined surfaces (surface 63a being shown in FIGS. 7 and 8) which engage with a ears one of which, denoted 111, is shown in FIGS. 7 and 8) of the plate mounted on the carrier 52, as the carriage 52 continues to move downwards, and the sliders 63 and 64 lift the plates 110 by their ears 11 1 out of U- shaped slots 52a in the carriage 52 in which they are resting. As the ears 111 clear the U-shaped slots 52a, the plates 110 slide away from the carriage 52 on the inclined surfaces of the sliders 63 and 64 onto a fixed inclined trackway 113 appropriately located adjacent the framework.
The Y drive then reverses and drives the empty carriage 52 up towards the top left-hand corner (as seen in FIG. 5). During this Y travel, a rack (not shown) on the side of the carriage 52 is knocked into an operative position by a cam so that the rack engages gear wheel 65. The rotation of gear wheel 65 is transmitted via gear wheel 66 to fingers 67 and 68. The length of the rack is such that one complete revolution is imparted to these fingers'67 and 68 which are so positioned as to engage the top of the plate which has just been released from the carriage 52 and move it along the trackway into a storage position. When the rack on the carriage 52 has completed its actuation of gear wheel 65, it is moved by a further cam surface into an inoperative position.
As the plate is released from the carriage 52 in the unloading operation, the chains carrying the plates awaiting exposure to the ion beam are driven to move the leading plate up to a loading station on tracks close to the starting position at the top left-hand comer of the mechanism as seen in FIG. 5.
As the carriage 52 is moved upwards by the Y drive into the loading position, a rack 69 on the carriage engages with a pinion 71 and causes two fingers 72 and 73 to rotate. These fingers engage behind the top of the plate which has moved into the loading station and push the plate forward so that it drops onto the carriage 52 with its ears engaging in the U-shaped recesses provided on the carriage 52. The Y drive then reverses and moves the carriage 52 down until the rack 69 is clear of the pinion 71 and carriage 52 is in the starting position for the X-Y scan. The return movement of the rack 69 over the pinion 71 causes a harmless reverse revolution of the fingers 72 and 73.
It will be seen that, with the apparatus of this example, a high uniformity and reproducibility of doping of the target substrates is achieved by the combination of two important features, namely, beam sweeping with feedback stabilization to give constant ion beam intensity and X-Y scanning of the substrates in front of the stationary ion beam. The mechanical system for scanning the targets through the beam is completely separate fromthe beam control equipment so that any coupling between the X and Y scanning frequencies and modulation of the ion beam (arising for example from ripple on the ion source supplies) is avoided. This is important because, if any such coupling does occur,
standing waves will result and give non-uniform doping.
Further, the beam shape is unimportant since summation of repeated implants leads to smoothing out of non-uniformities.
To avoid faulty doping of targets, mechanism is provided for actuating the beam stop device 35 automatically in the event that the ion beam should go out of control. Provision is made for stopping the X-Y drive mechanism when the beam stop device 35 is actuated and for starting the X-Y drive mechanism again when the beam stop device 35 is removed after the correct ion beam has been restored. In this example, to simplify the control mechanism, the X-Y driveis not stopped until the end of the X scan in which the beam stop device 35 is actuated. However, provision may be made for both X and Y drives to stop and start automatically simultaneously with corresponding operation of the beam stop device 35.
The arrangement ofthis example has a number of advantages for the ion implantation of substrates. In particular, a large number of target substrates can be processed in a single run so that the through-put of the apparatus is up to industrial production quantities whilst, at the same time, the problems of uniformity of dose and beam heating of the substrates have been solved.
The use of a large plate for supporting the substrate targets reduces the beam heating effect without reducing implantation rate because individual slices can cool during periods out of the beam. The large plate also permits implantation of any size sample up to the plate dimensions. This is important for example when doping large area (e.g., 3 in. diameter) nuclear detectors.
Automatic programme control of the X-Y movements allows (a) repeated scans if required for very high doping levels, or, for example, for successive implantation with different dopants, and (b) control over speed of scanning and number of X scans per unit distance of Y movement allows through-put of the target chamber to be adjusted. For example, the throughput can be increased if the loss of uniformity is acceptable. Automatic programme control of the X-Y movement further permits control over the size of the scanning area so that individual small samples can be rapidly implanted for experimental requirements.
By use of an electrically isolated target chamber, the ion beam can be accelerated up to 180 keV total) or decelerated to give precise energy control of the implantation over a wide range. If necessary, the target chamber voltage can be programmed to give an implanted profile control without the need to adjust the electromagnet of the separator. For this, however, it is important that there is no coupling between the slow scan frequency and the fast frequency of the voltage programme. V
This facility for programming the target chamber voltage has imposed special requirements upon the design of the lens assembly (a) to minimize loss of beam angular definition, and (b) to ensure that in spite of large voltage changes all 'ofthe beam transmitted through the beam defining and monitoring slits passes through a l in. X l in. aperture in the target chamber.
FIG. 6 illustrates an alternative-form of mechanism for moving targets past the window. This mechanism is particularly appropriate for special implantations where separation of the desired ions from unwanted ions is comparatively easy and a wider mass-defining slit is acceptable.
Thus, for example, boron ions and, to a somewhat lesser extent, phosphorus ions are quite widely separated in mass from impurity ions commonly produced by ion sources in association with boron or phosphorus ions. For implantation of such ions it is consequently possible to arrange for the mass-defining slit to be as wide as the width of the target substrate to be implanted.
Referring to FIG. 6, a target chamber comprises a cylindrical enclosure 81 with a pipe 82 for connection to a vacuum pump. A tubular extension 83 is adapted for connection to the vacuum system of the electromagnetic separator and houses a lens assembly, which may be similar to that shown in FIG. 2, but with a wider window or mass-defining slit.
Extending along the axis of the target chamber enclosure 81 is a lead screw 84, driven by a stepping motor 85. The drive program'forthe stepping motor is controlled by an electronic programmer 86.
Within the target chamber enclosure 81 is a multisided target holder 87 of generally cylindrical form. The target holder 87 is co-a'xial with the enclosure 81 and is driven by the lead screw 84. Rotation of the target holder 87 is normally resisted by a torque resisting tube 88, to which the target holder 87 is keyed. The target holder 87 is, however, free to slide up and down as indicated by arrow B.
Each side of the multi-sided target holder 87 is in the form of a rack adapted to support a row of targets which, in this example, are'two inch diameter wafers 89. For simplicity of drawing, only one rack is shown filled with wafers.
The arrangement is such that one row of targets supported on the holder 87 is in register with the window. The ion beam from the separator is illustrated diagrammatically at 91. Arrows C indicate the sweep, which is large enough, in this example, for a substantially uniform intensity ion beam to pass through the window and fall upon the full 2 inch width of'the wafer immediately behind the window. ion implantation to a uniform dose of the whole row of eight wafers can thus be secured by a simple Y-scan. In practice the holder 87 is reciprocated several times in one angular position so that the implantation dose is built up with greater net uniformity and also to avoid undue heating of the wafers. Dwell effects are avoided by arranging the 9 holder 87 to carry the wafers clear of the ion beam before reversal.
After the desired implantation of one row of wafers has been carried out, the holder 87 is indexed by one step angularly to bring the next row of wafers into register with the ion beam.
The indexing mechanism comprises fingers 92 and 93 with co-operating cam surfaces 94, 95 fixed to the target chamber enclosure 81.
During reciprocation of the holder 87 for an implanting operation, the stepping motor 85 is programmed to reverse before an indexing finger 92 or 93 engages the cam surface 94 or 95. At the end of an implantation operation, the holder 87 is driven up or down (as the case may be) until a finger 92 or 93 engages the cam surface 94 or 95. Means, not shown, is provided for releasing the holder for rotation at this moment and the effect of the cam surface upon the finger is to rotate the holder 87 until the next adjacent fingeris positively stopped against shoulder 96 or 97 at the high end of the cam surfaces 94 and 95 With this particular indexing arrangement, the fingers 92 at one end have to be slightly-displaced relative to those 93) at the other end. The cam surfaces 94 and 95 have to be correspondingly relatively displaced. Further, each successive indexing step has to be effected at opposite ends of travel of the holder 87, that is, the holder must execute an odd number of reciprocations between each indexing operation. It will be appreciated that other techniques for indexing may be employed if desired, but that described has the virtue of mechanical simplicity.
Assuming an ion beam current of 1 mA, corresponding to approximately X ions/sec, a two inch wafer will receive a very heavy dose of 5, X 10 ions/cm in about 25 seconds exposure.
Assuming maximum utilization of current, a row of eight wafers will thus take about 4 to 5 minutes to implant up to this high dose. A typical scanning rate would be 5 inches in 1 second leading to several reciprocations required to implant each row.
The invention is not restricted to the details of the foregoing example. For example, provision may be made for heating the samples by radiation using large area heaters facing the sample plates. Alternatively, the samples may be cooled by positioning in front of them large plates cooled with liquid nitrogen.
1. Apparatus for bombarding a target with ions, which apparatus comprises an ion beam source, a window, a magnet for deflecting the ion beam towards the window, means for modulating the energy of the ion beam so that the deflection produced by the magnet varies in correspondence with the modulation to thereby cause the deflection ion beam to sweep back and forth over the window, detector means for detecting the ion beam current passing through the window, ion beam control means responsive to the output of the detector means for controlling the ion beam energy modulating means so as to tend to maintain constant the intensity of the ion beam current passing through the window,
target holder means forsupporting a target behind the window, and drive means for imparting controlled movement to the target holder means to scan the target through the ion beam according to a predetermined sc attern.
5? pparatus as claimed in claim 1, wherein the window is at least as wide as the target to be bombarded and the ion beam control means control the ion beam to provide a substantially uniform ion beam intensity over the area of the window. I
3. Apparatus as claimed in claim 2, wherein the target holder locates the target behind the window with the width of the target in register with the width of ion beam passing through the window, and the said drive means move the supported target lengthwise past the window at controlled speed.
4. Apparatus as claimed in claim 3, wherein the target holder carries a plurality of targets in a row and, when moved as aforesaid, moves the targets in succession at controlled speed past the window.
5. Apparatus as claimed in claim 4, wherein the said drive means reciprocates the target holder, whereby the targets are moved repeatedly past the window, the total ion dose being determined by the number of reciprocations.
6. Apparatus as claimed in claim 5, wherein the target holder is adapted to carry a plurality of rows of targets and the drive means includes means for indexing the target holder to bring another row of targets into register with the window.
7. Apparatus as claimed in claim 6, wherein the target holder comprises a cylinder with a plurality of racks arranged around the periphery, each rack carrying a row of targets extending parallel to the axis of the cylinder.
8. Apparatus as claimed in claim 1, wherein the target holder comprises a plate for supporting one large target ora plurality of smaller targets, and a support for the plate, which support is moved by the said drive means.
9. Apparatus as claimed in claim 8, wherein the drive means comprise an X-Y drive.
10. Apparatus as claimed in claim 9, wherein the said support is adapted to release a plate it is carrying automatically at the end of a complete scanning operation and to return to the starting position for a further scanning operation.
11. Apparatus as claimed in claim 10, wherein means are provided for mounting a plurality of plates loaded with targets at a loading station having means for automatically loading a plate onto the said support when it returns to the starting position.
12. Apparatus as claimed in claim 1, wherein the ion beam is produced by an ion beam separator in which the magnet is an electromagnet.
13. Apparatus as claimed in claim 1, wherein the means for modulating the energy of the ion beam comprises an electrical source connected to superimpose an alternating voltage upon a high direct voltage supplied to the accelerating electrode of the ion beam source.