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.
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 and 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) Antiprotonxe2x80x94Proton 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 xe2x80x9cinflectorxe2x80x9d, 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 xe2x80x9cdrift modexe2x80x9d 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 xe2x88x9296, 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.
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 filter described above can be used in four different modes.
a) Magnetic field only:
This mode can be used to transport low energy relatively high current beams around the filter curvature with minimal space charge blow up of the beam. This is especially useful when a high energy implanter with the capability of radio frequency acceleration, is to be operated at relatively low energy, e.g. with the rf accelerator turned off (in xe2x80x9cdrift modexe2x80x9d).
b) Electrostatic field only:
This mode is useful to provide relatively high energy dispersion as the beam is transported around the curvature of the filter, in order to provide good energy resolution of the beam delivered for implantation. An electrostatic field on its own provides approximately twice the energy resolving power of a magnetic field for the same path curvature through a filter. This is useful for energy filtering beams after rf acceleration. However, an electrostatic field may also be useful for removing from the beam ions having half the required charge state and a quarter of the required energy. When operating a beam of doubly charged ions, say doubly charged phosphorus (P2+), the ion source will normally also produce singly charged P2 ions. Such P2+ ions will decompose to P+ and P, each taking half the energy of the original P2+. Assuming the ions from the ion source are accelerated through a dc field V, the P2+ ions will be accelerated to an energy of 2V electron volts and the P2+ ions will be accelerated to an energy of V electron volts. After decomposition, the P30 ions thus have an energy of V/2 electron volts, one quarter of the energy of the desired P2+ ions. Ions with half the charge and a quarter of the energy cannot be separated from desired ions in a magnetic field alone, resulting in quarter energy pollution of the ion beam. In the field of mass spectrometry a corresponding effect produces so called xe2x80x9cAstonxe2x80x9d bands. An electric field sector filter can remove these Aston bands.
c) Electric field mode, with additional magnetic field having a selected polarity requiring either an increased or a decreased electric field to transport the beam around the curvature of the filter:
In this mode, the energy dispersion of the filter can be increased or decreased relative to that of the mode with electric field alone. This can be useful in producing beams for implantation having very high energy resolution, and can also be used to provide a predetermined energy dispersion in the beam travelling on from the filter, e.g. for counteracting energy dispersion in subsequent beam scanning arrangements. When the polarity of the magnetic field requires increased electric field in this mode, the magnetic field can be regarded as a bucking field. This will be referred to herein as magnetic bucking mode.
d) Adding electric field to the magnetic field mode in such a polarity as to oppose and increase the required magnetic field to transport the beam around the curvature of the filter:
In this mode, which will be referred to as electric bucking mode, the energy dispersion of the filter can be reduced below that of electric field alone mode. In fact, a combination of electric field and magnetic field can be set which can provide achromatic beam transport. This can be particularly useful in beam transport applications where it is required to eliminate beam spot broadening arising from energy spread in the beam. Also in this mode, the electric and magnetic fields can be adjusted to pass a multiply charged beam ion and reject all lower charge state ions of the same mass irrespective of the energy of the lower charge state ions.
The present invention is also concerned with a problem which can arise generally with scanned beam type ion implanters. These typically employ electrostatic or electromagnetic scanning fields which produce a degree of energy dispersion. The energy dispersion of the scanning arrangement can broaden the beam reaching the substrate for implantation.
The present invention also contemplates an ion implanter comprising an ion beam generator providing a beam of ions to be implanted in a substrate, a beam scanner to scan said beam at least in a scan plane to provide a scanned beam at said substrate, said scanner producing in said scanned beam a first amount of energy dispersion in a first sense in said plane, and a dispersion controller to produce in the beam before the beam is scanned by said scanner, a second amount of energy dispersion in a second sense in said plane opposite to said first sense. In this way, any energy dispersion introduced into the beam by the scanning arrangement can be counteracted and reduced by the dispersion controller.
In a practical arrangement, the dispersion controller may take the form of a cross field curved path charged particle filter of the kind disclosed generally above. As has been explained, the amount of dispersion applied by such a curved path cross field filter can be adjusted when operating the filter in the combined field mode. However, other forms of dispersion controller may also be contemplated, including the straight line Wien type filter. To provide controllable energy dispersion of an ion beam, without varying the beam direction leaving the dispersion controller, both magnetic and electric fields are required. In the above described curved cross field filter and also in the known Wien filter, the magnetic and electric fields are applied to the charged particle beam over the same region of the beam, so that the particles of the beam experience both fields simultaneously. However, a dispersion controller could apply the magnetic and electric fields to different spatial regions of the beam, so that beam particles experience the two fields in succession. This arrangement would have the disadvantage of increasing the beam path through the dispersion controller and in the case of an ion implanter, would tend to increase the overall size or xe2x80x9cfootprintxe2x80x9d of the ion implantation tool.
In an ion implanter, a typical beam scanner comprises a beam direction scanner to scan the direction of said beam at least in said scan plane, and a beam collimator to collimate the scanned beam direction in said plane to provide a collimated scanned beam at the substrate. Then, in a preferred embodiment of the present invention, each of said direction scanner and said collimator are arranged to produce a respective charge state and energy dependent dispersion in said first sense in said plane, to contribute to said first amount of energy dispersion. This is contrary to the normal arrangement of beam direction scanners and beam collimators in ion implanters with rf acceleration, which are usually arranged to provide energy dispersion in opposite directions to minimise the overall energy dispersion at the substrate. However, with the dispersion controller providing a cancelling dispersion of the beam before the beam direction scanner, the overall dispersion at the substrate can be reduced even when both the direction scanner and the collimator tend to produce energy dispersion in the same sense.
Arranging for both the beam direction scanner and the collimator to provide energy dispersion in the same sense can also have in itself advantages as will be explained later herein.
The invention also provides a method of filtering charged particles comprising using a combination of both electric and magnetic fields to bend a beam of charged particles so that desired particles follow a predetermined curved path. In another aspect, the invention provides a method of ion implantation comprising the steps of generating a beam of ions to be implanted in a substrate, scanning said beam at least in a scan plane to provide a scanned beam at said substrate, whereby said scanning produces in said scanned beam a first amount of energy dispersion in a first sense in said plane, and producing in the beam, before the beam is scanned, a second amount of energy dispersion in a second sense in said plane opposite to said first sense.