Patent Number: 
Section: description

Examples of the present invention will now be described with reference to the accompanying drawing which is a schematic illustration of an ion source for positive ions with an extraction arrangement incorporating an example of the present invention. Referring to the FIGURE, an ion source comprises an arc chamber 10 which may be configured as a Bernas source, or any other known ion source. A feed gas, here shown as BF3 is supplied to the arc chamber along a pipe 11 and the arc chamber operates to create a plasma within the arc chamber in which are formed the ions B+and BF2+, for example. These positive ions are then extracted from the arc chamber via an exit aperture 12 to form a beam. In the described example, the source is providing a beam of boron ions (or BF2 ions) which may be useful for implanting a silicon substrate with boron. However, other ions may be produced where required, such as As+, P+or Ar+, for example. The ions are extracted from the arc chamber 10 by the electric fields produced by electrodes 13 and 14. To extract positive ions from the source, the arc chamber 10 and in particular the front face 15 containing the exit aperture 12 is held at a positive potential relative to the electrodes 13 and 14. In practice, the electrode 14 may form the entrance aperture to a mass analysing magnet and will usually be at ground potential. The mass analysing magnet and a subsequent mass selection slit are used to select from the ions drawn from the arc chamber 10 ions of the precise mass required for implanting. For example, the mass analyser and mass selection slit may select only B+ions for onward transmission to the semiconductor substrate target. Neither the mass analyser magnet nor the subsequent elements of the beam path to the substrate target are illustrated in the drawing. These components may be typical of those customarily used in this art. In order to ensure that ions entering the mass analyser magnet, passing through the electrode 14, have a well defined energy, the potential of the front face 15 of the arc chamber 10 is controlled by a power supply 16. In the illustrated example, power supply 16 applies a potential of about 2 kV or less to the front face, so that the energy of the beam in the mass analyser will be correspondingly 2 keV or less depending on the potential applied. The electrode 13 constitutes an electron suppression electrode and is set by a power supply 17 at a negative potential relative to the electrode 14 so as to prevent electrons in the beam downstream of the electrode 14, in the direction of the arrow 18 from being drawn out of the beam by the positive potential on the arc chamber 10. In this way, space charge neutralisation within the analyser magnet, downstream of the electrode 14, is largely maintained. In the described example, the potential on the suppression electrode 13 is about xe2x88x92200 volts, but suppression potentials of several kilovolts below the ground potential of the analyser magnet and electrode 14 may be used. As can be seen in the drawing, a substantial electric field exists between the exit aperture 12 of the arc chamber and the suppression electrode 13, and also, though to a lesser extent, between the suppression electrode 13 and the ground electrode 14 at the entrance to the mass analyser. In these regions, any electrons in the beam have very short residence times due to their small mass and high mobility. As a result, space charge neutralisation by electrons in the beam in these regions is ineffective. In the described example, a source of argon gas is supplied along a pipe 20 to expand in a chamber 21. The sudden expansion of the argon gas in the chamber 21, causes clusters of argon atoms to condense together, producing clusters each of at least 100 atoms and in appropriate conditions, up to 1000 atoms or more. Within the chamber 21, a heated cathode, 22 emits electrons, which arc accelerated through a grid electrode 23. The cathode 22 is biased relative to the grid 23 by a power source 24 to produce electrons of low energy (below about 50 eV). The resulting xe2x80x9csprayxe2x80x9d of low energy electrons passing through the grid 23 ionises argon clusters within the chamber 21, forming negatively charged cluster ions. The cluster ions in the chamber 21 diffuse from the chamber through an aperture 25 immediately adjacent the electron suppression electrode 13. The resulting flood of negatively charged cluster ions emerging from the aperture 25 assists in neutralising the space charge of the portion of the ion beam between the exit aperture 12 of the arc chamber 10 and the suppression electrode 13. As explained previously, for total space charge neutralisation, the density in a particular region of the ion beam of positive ions should equal the density of negative cluster ions (Nb=Nc). Further, Nb=Jb/evb=Nc=Jc/evc, where Jb is the current density resulting from the positive beam ions, vb is the velocity of those ions in the beam, Jc is the current density in the beam of negative cluster ions, and vc is their velocity. If at a particular location in the beam, the energy of the required positive ions is equal to the energy of the negative cluster ions, mb vb2=mc vc2, where mb is the mass of the positive ions and mc is the mass of the cluster ions. From the above, Jb/Jc=vb/vc=(mc/mb). Cluster ions comprising between 200 and 300 argon atoms are typically formed in the chamber 21. However, taking a minimum of 100 atoms in a cluster ion, mcxe2x89xa74000 a.u. If the positive ion in the ion beam is B+(mass≈10.8), mc/mb≈400 and Jb/Jc≈20. Thus, for full space charge neutralisation in the region of the beam where both the positive ions and the negative cluster ions have the same energy, the current of cluster ions in the beam, accelerated by the electric field towards the exit aperture 12 of the arc chamber 10, must be about one-twentieth of the beam current of boron ions from the arc chamber. For a typical boron beam current of 5 mA, this implies a cluster ion current of 0.25 mA. In the Figure, a power supply 26 is illustrated connected to apply a negative bias to the cluster ion source relative to the electron suppression electrode 13. In fact, the cluster ion source may be held at the same potential as the suppression electrode 13, relying on any residual positive charge in the ion beam to draw cluster ions from the aperture 25 and into the beam. However, a small negative bias may additionally be applied to the cluster ion source to control the flow of cluster ions. In the illustrated example, the cluster ion source is shown delivering cluster ions on the upstream side of the suppression electrode. Since the potential difference between the arc chamber 10 and the suppression electrode 13 is likely to be greater than that between the suppression electrode 13 and the grounded electrode 14, the problem of space charge suppression in this region is most severe, especially if it is desired to extract relatively high currents at low energy from the arc chamber 10. However, cluster ions may also be delivered on the other side of the suppression electrode 13 to neutralise space charge of the ion beam in the region between the suppression electrode 13 and the grounded electrode 14. The example of the invention described above refers to a positive ion beam comprising boron ions. However, the invention is applicable equally to other desired positive ion beams. The invention may also be applied to beams of negative ions, in which case positive cluster ions are introduced. Positive cluster ions may be formed in the chamber 21 by spraying the condensed clusters of atoms with electrons of higher energy. Further, the above examples describe using argon cluster ions. Other gases may be used which can be made to produce large clusters of atoms. For ion implantation purposes, the cluster ions should be of a species which can be tolerated in the implantation process. Further, for minimum mobility of the cluster ion in the electric field regions of the ion beam, relatively heavy atoms are preferred such as xenon. Also, the above described example refers to neutralising the ion beam at the point of extraction from the arc chamber of the ion source. Examples of the invention may be used also at other regions of a beam where an external electric field renders the lifetime of any electrons in the beam extremely short so that space charge neutralisation becomes a problem. For example, negative cluster ion neutralisation may be employed in a region where the ion beam is accelerated, and more particularly decelerated, by means of an electric field prior to impact on a target. Also cluster ions may be used to provide beam space charge neutralisation in a region where a beam is scanned transversely. In this case, the cluster ion neutralisation process described may be useful even if the beam scanning is conducted by magnetic fields, rather than electric fields. In such regions, self neutralisation of the ion beam, by the creation of electrons through collisions with residual gas atoms, may be insufficient to maintain adequate control of the beam potential. The beam may be scanned too rapidly to allow sufficient numbers of electrons to accumulate in the beam to provide adequate neutralisation. Flooding the scanning region with massive negative cluster ions should substantially improve beam neutralisation. Ions of opposite polarity may also be used for neutralising an ion beam at other locations along the beam between the ion beam source and the ion beam process target. For example, such opposite polarity ions may be injected into the ion beam containing volume of a magnet used for mass analysis (or energy analysis) of the process ion beam. Improved beam neutralisation and control may then be achieved in the magnet, especially at low beam energies and high beam currents. Opposite polarity ions may also be used to neutralise an ion beam in so called drift regions of no electric or magnetic field. Examples of drift regions in an ion implanter are between the ion source extraction optics and the entrance to the mass analysis magnet, between the mass analysis magnet and the mass resolving system, and between a post mass selection acceleration (or deceleration) stage and a substrate neutralisation system. The invention is not restricted to the use of cluster ions for neutralising the beam. Some improvement in beam control may be achieved with ions of the second polarity (negative ions for a positive ion beam). Even He has a mass 4 which is about 104 times the rest mass of an electron (xcx9c5.5xc3x9710xe2x88x924 a.u.), so that the current density of Hexe2x88x92for the same neutralising effect is only one hundredth that for electrons. Generally, the second polarity ions should be of a species which will not have substantial deleterious effects in the process. More massive ions and cluster ions are preferred, especially for neutralising in regions of applied electric field.