GB2270413A

GB2270413A - A heavy ion accelerator

Info

Publication number: GB2270413A
Application number: GB9218918A
Authority: GB
Inventors: James Harry Freeman
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-09-07
Filing date: 1992-09-07
Publication date: 1994-03-09
Anticipated expiration: 2012-09-07
Also published as: GB2270413B; GB9218918D0

Abstract

A heavy ion accelerator incorporates a charge exchange cell 9 (fig. 3), 12 (fig. 4) which is negatively biased and is located between a source of positive ions 8 (fig. 3), 11 (fig. 4) and a target chamber 10 (fig. 3), 14 (fig. 4). Both the ion source and the target chamber are maintained at earth potential. The positive ion beam, thus generated, accelerates due to the voltage V4 and is converted to negative ions, as it passes through lithium vapour within the change exchanger cell, and is further accelerated due to the fall in potential on reaching the target. Therefore, since there are two stages of acceleration, V4 need only be raised to a potential which is half that which would be required to obtain the bombardment energy necessary. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO APPARATUS FOR BOMBARDING A TARGET WITH IONS This invention relates to apparatus for bombarding a target with ions, for example to implant ions into the target.

A known form of heavy ion accelerator, such as is commonly used for ion implantation, comprises a source of positive ions, an ion beam analyser and a target chamber. Since the target chamber is normally at earth potential, the ion source, and in many instances the ion beam analyser also, are at elevated potentials in order to provide the required degree of ion acceleration.

This form of accelerator has some disadvantages, one of which is the substantial inconvenience and the relatively high cost of operating the ion source, and in many instances the ion beam analyser also, at an elevated voltage, Another is the requirement for X ray shielding when such apparatus is operated at the high voltages which are commonly required for some semiconductor implantation applications. Another is the well established and deleterious effect of the surface charging which may result when certain types of semiconductor devices or other target materials are bombarded with energetic beams of positive ions.

According to the present invention there is provided a heavy ion accelerator which incorporates a charge exchange cell which is negatively biased and which is located between the primary source of positive ions and the target chamber. This layout has the important advantage that it allows both the ion source and the target chamber to be maintained at earth potential. Moreover, in such an apparatus the charge exchange cell, and in some instances the ion beam analyser also, are raised to a potential which is only one half that which would be required to obtain the same bombardment energy in a more conventional accelerator. This is because the process of charge exchange which follows the first stage of acceleration results in a fraction of the positive ions becoming negatively charged.These negative ions are then further accelerated to a final energy which corresponds to twice the potential of the negatively biased charge exchanger as they are transported into the earthed target stage of the apparatus. In addition, and in contrast with the more conventional positive ion accelerator described above, the subsequent ion bombardment is effected with a beam of negative ions.

A specific proposed embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 illustrates the layout of a relatively conventional and simple heavy ion accelerator in which the ion source and its associated power supplies 1 are maintained at a positive potential V1 in order to provide the required degree of ion acceleration, while the ion beam analyser 2 and the target chamber 3 are both at earth potential.

Figure 2 illustrates the layout of a more typical heavy ion accelerator such as is used commonly for the implantation of semiconductor wafers and in which the energy range is extended by operating the ion source 4 at a positive potential V2 with respect to the analyser 5, and by additionally raising the entire ion beam production and analysis element 6 of the apparatus to a further positive potential V3. In consequence the ion beam is accelerated with a total energy V1 + V2 into the earthed target chamber.

Figure 3 illustrates the layout of one embodiment of the present invention in which the ion source 8 and the target chamber 10 are both at earth potential and in which the charge exchange and beam analysis element 9 of the apparatus is raised to a negative potential which is only half the required final energy of the ion beam.

Figure 4 illustrates the layout of a simpler embodiment of the present invention which is appropriate to a somewhat more restricted mass and/or energy regime of ion bombardment and, in which the ion beam analyser 13, the ion source 11 and the target chamber 14 are all at earth potential. In this embodiment only the ion beam acceleration and charge exchange stage of the apparatus 12 needs to be raised to a negative potential which is half the required final energy of the ion beam.

Referring to the schematic illustration of the embodiment of the present invention in Figure 3, the source of positive ions is similar to that commonly used in the conventional accelerators illustrated in Figures 1 & 2, except that it is operated at earth potential. The positive ions are accelerated by the negative potential V4 into the element 9 of the apararatus where a fraction become negatively charged as they pass through the lithium vapour of a charge exchange cell which is similar in its mode of operation to those which have been used elsewhere for the production of negative ion beams. The resulting spectrum of negative and positive ions leaving the cell will then pass into a mass analyser where the required beam of negative ions will be selected for transmission into the earthed target chamber 10.At this point in the apparatus the negative ions will be subject to a second stage of acceleration and will reach the target with a final energy of twice the potential V4.

In other embodiments of the invention: - The lithium vapour in the charge exchange cell may be replaced by that of an alkaline earth or a different alkali metal; - The process of charge exchange in the embodiment described above may follow rather than precede the stage of mass analysis.

- An apparatus which is significantly simpler may be employed for a substantial number of implantation applications which involve the use of a somewhat more restricted mass and/or energy regime of ion bombardment. In this accelerator, which is illustrated in Figure 4, the ion beam analyser 13; the ion source 11, and the target chamber 14 are all at earth potential, and only the beam acceleration and charge exchange stage 12 of the apparatus, is raised to a potential V4 corresponding to half the final beam energy.

Advantages of the present apparatus for bombarding a target with ions include: (a) an accelerator layout in which the ion source and the target chamber, and in some instances the ion beam analyser, are at earth potential, (b) a reduction by a factor of two in the accelerating potential required in order to achieve a specified ion bombardment energy, (c) a reduction in the requirement for X ray shielding, (d) a reduction in the surface charging problem, as compared with that encountered with the more conventional use of positive ions for ion implantation.

Claims

IMPROVEMENTS IN OR RELATING TO APPARATUS FOR BOMBARDING A TARGET WITH IONS 1. A heavy ion accelerator incorporating a charge exchange cell which is negatively biased and which is located between the primary source of positive ions and the target chamber; such that both the ion source and the target chamber may be maintained at earth potential and that the charge exchange cell, and in some instances the ion beam analyser, are raised to a potential which is only half that which would be required to obtain the same bombardment energy in a more conventional accelerator, and such that the ion bombardment is effected with a beam of negative ions.
2. An apparatus as claimed in claim 1 in which the lithium vapour in the charge exchange cell may be replaced by the vapour of an alkaline earth or a different alkali metal.
3. An apparatus as claimed in claim 1 in which the process of charge exchange follows, rather than precedes the stage of mass analysis.
4. An apparatus as claimed in claim 1 in which the ion beam analyser 13, the ion source 11, and the target chamber 14 are all at earth potential, and only the beam acceleration and the charge exchange stage 12 is raised to a potential corrresponding to half the final beam energy.
5. An apparatus for bombarding a target with ions substantially as herein-before described with reference to Figures 1-4 of the accompanying drawing.