Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-10-18
2003-10-28
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S296000, C250S298000
Reexamination Certificate
active
06639227
ABSTRACT:
FIELD OF THE INVENTION
The invention is concerned with charged particle filters and filtering and more particularly with ion implanters which may incorporate such filters and methods of ion implantation.
BACKGROUND OF THE INVENTION
Charged particle filters, sometimes referred to as particle separators, are known in the art for the purpose of separating out unwanted particles from wanted particles in a particle beam. A summary of known such filters or separators is given in Chapter 7, from page 121 of the text book, The Transport of Charged Particle Beams, by A. P. Banford, published by E & F N Spon Limited, London, 1966.
A sector magnetic field can be used to provide separation of charged particles by their momentum to charge ratio. Such devices are commonly used as mass analysers and in mass spectrometry, to separate particles by mass when the energy and charge of the particles is assumed to be the same.
An electric field on its own may also be used for particle separation, in which case the filter acts as an energy filter for particles of the same charge state. An example of a pure electrostatic filter, called an inflector, used in a high energy ion implanter, is disclosed in Production High Energy Ion Implanters Using Radio Frequency Acceleration by Glavish et al, Nuclear Instruments and Methods in Physics Research B21 (1987) 264-269.
Cross field devices are also known in which orthogonal electric and magnetic fields are provided simultaneously for the deflection of charged particles. See for example the above mentioned chapter in the book by A. P. Banford, and also pages 45 to 47 of Electron and Ion Optics by Miklos Szilagye published 1988 by Plenum Press. In particular, there is the well known Wien filter which is a straight path cross field filter, in which orthogonal electric and magnetic fields are directed perpendicular to a straight path through the filter. A Wien filter operates effectively as a pure velocity filter which is independent of the mass or charge of the particles in the beam.
Other cross field devices are described in
i) Trochoidal Electron Monochromator, by Stamatovlc et al, The Review of Scientif Instruments, Vol. 39, No. 11, pages 1752-1753, November 1968;
ii) A New Mass Spectrometer with Improved Focusing Properties, by Bleakney et al, Physical Review, Vol. 53, pages 521 to 529, April 1938;
iii) Antiproton—Proton Cross Sections at 133, 197, 265 and 333 MeV, by Coombes et al, Physical Review, Vol. 112, No. 4, pages 1303-1310, November 1958.
References i) and ii) contemplate providing trochoidal paths for charged particles passing through the devices so that the particle motion through the device can be in the electric field direction. Reference iii) appears to contemplate a velocity filter providing no deflection to required antiprotons.
Considering in particular the application of charged particle filters to ion implanters, ion implanters using radio frequency acceleration are known in, for example, U.S. Pat. No. 4,667,111, assigned to the Eaton Corporation, and also U.S. Pat. No. 6,423,976, assigned to Applied Materials, Inc. RF acceleration tends to produce a beam of ions having a band of energies, so that an energy filter is required if the beam to be implanted is to have high energy purity.
The use of a pure electrostatic energy filter or “inflector”, which bends the accelerated beam through a predetermined angle in order to provide the required energy dispersion at an exit aperture of the filter, can work satisfactorily at high energies but still has the following problems (see the Glavish et al reference above). It is impracticable to use such a filter when operating the ion implanter at high beam current and relatively lower implant energies. For example, if the ion implanter is operated in “drift mode” with the rf accelerator effectively turned off, an electric field is still required to bend the beam through the curved path of the energy filter. The presence of an electric field removes highly mobile electrons from the beam envelope so that the lower energy beam tends to blow up through space charge effects. Also, even at high energies using rf acceleration, the electrostatic inflector type energy filter cannot distinguish between particles of the same mass at different charge states and corresponding different energies. For example, the electrostatic inflector cannot distinguish between doubly charged phosphorus ions and singly charge phosphorus ions at half the energy of the doubly charged phosphorus ions. This can be important especially when multiply charged ions are used for higher energy applications.
The use of a pure magnetic sector filter as an energy filter for particles of the same charge to mass ratio is described in Purity of High Energy Beams in R. F. Linear Accelerator Based Implanters, by McIntyre et al, Ion Implantation Technology −96, pages 367-370. Such a magnetic sector filter can overcome the problems of transporting a low energy beam successfully through the filter. However, a magnetic sector filter has only approximately half the energy resolving power of an electrostatic inflector of the same path length. Furthermore, a magnetic field filter like an electric field filter will still pass particles of the same mass and different charge states at appropriate different energies. For example, a magnetic filter could not distinguish between doubly charged phosphorus and singly charged phosphorus at a quarter of the energy of the doubly charged phosphorus.
The use of straight path cross field filters (Wien filters) as velocity analysers in ion implanters has been proposed; see for example the book Ion Beams with Applications to Ion Implantation, by Wilson and Brewer, published 1973 by Robert E. Krieger Publishing Company, Inc., particularly pages 213-214, 431-435, 439-443 and 458-459. Wien filters have also been used as velocity filters in ion implanters sold prior to 1970 by Accelerations, Inc. and by High Voltage Engineering Corporation. However, a Wien filter is a pure velocity filter which is not sensitive to the charge state of the particles and so cannot distinguish between particles at different charge states and the same velocity. This is important in the case of scanning beam type implanters when the filtered beam is subsequently scanned either magnetically or electrostatically, since the ions at different charge states will be effected differently by the scanning field which can give rise to dose non-uniformities over the implanted wafers, and unwanted deviations in the implant angle into the wafer.
It is an object of embodiments of the present invention to provide a charged particle filter which can obviate some or all of the above problems with prior art filters.
More generally it is an object of the invention to provide a novel form of charged particle filter which may have applications not available from the prior art filters.
SUMMARY OF THE INVENTION
This invention provides a charged particle filter comprising a beam channel having a beam entrance and a beam exit, said beam channel defining, between said entrance and said exit, a predetermined curved path, in a plane of curvature, for a beam of charged particles transported through the filter, electrodes arranged for producing over said curved path an electric field in said plane substantially perpendicular to said path, and magnetic poles arranged for producing over said path magnetic field substantially normal to said plane.
The predetermined curved path normally has a constant radius of curvature and said electrodes are arranged to produce a radial electric field.
Such a curved path cross field filter has special attributes and advantages which will become apparent in the further discussion that follows and which are especially, though not exclusively, useful when the filter is used as an energy filter for an ion implanter, in particular following an rf accelerator. In particular, a curved path filter is sensitive to the charge state of beam particles of the same mass, so that particles of unwanted charge state can be filtered out.
The curved path fi
Glavish Hilton
Jen Causon Ko-Chuan
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