Electric lamp and discharge devices – With positive or negative ion acceleration
Reexamination Certificate
2001-02-15
2003-02-04
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
C313S231010
Reexamination Certificate
active
06515408
ABSTRACT:
The present invention is concerned with neutralising space charge in ion beams travelling through regions of applied magnetic field, and in particular, although not exclusively, with neutralising space charge in an ion beam as it travels through the flight tube of an analysing magnet.
An analysing magnet generates a substantially uniform magnetic field in its flight tube, causing an ion travelling through the flight tube to follow a curved path in a plane perpendicular to the direction of the magnetic field. The radius of the curved path is given by:
r=mv/qB=
(2
Em
)
½
/qB
where v,E,m and q respectively are the velocity, kinetic energy, mass and charge of the ion, and B is the magnitude of the magnetic flux density in the flight tube. The analysing magnet can therefore be used to resolve spatially (in a dispersion plane perpendicular to the magnetic field in the flight tube), ions in a beam according to their energy, mass and charge.
In ion implanters, an analysing magnet is used in conjunction with a selection slit to select ions of the required species from an incident beam for implantation in a target semiconductor substrate. Typically, the incident beam will comprise ions having substantially the same energy, and the magnet is arranged to focus those ions having the desired mass/charge ratio at the selection slit so that only they pass through the slit and go on to impinge on the target.
In spectrometry applications, analysing magnets are used to resolve ions in a beam according to their mass, energy and charge for separate detection.
Ideally, for both ion implantation and spectrometry applications, ions in the beam entering the flight tube having the same energy, charge and mass should all be focused by the analysing magnet onto a common line, perpendicular to the dispersion plane, as they exit.
However, in the absence of any neutralising effect, a beam containing only ions of a particular polarity will experience space charge effects. The mutual repulsion of the ions in the beam tends to cause the beam to diverge or “blow up”. This mutual repulsion means that the position at which an ion exits the magnet is no longer solely determined by its incident velocity, mass and charge, and the applied magnetic field.
In ion implantation applications, although an incident ion may have the desired mass/charge ratio, because of space charge effects it may not be focused on the selection slit and so may not reach the target. This reduces the ion implantation current reaching the target from a given source and increases the process time required to achieve a desired implantation dose. In addition, space charge effects may result in an increased number of incident beam ions hitting the sides of the flight tube. This further reduces the implantation current reaching the target and can result in contamination of the target by particles sputtered off the flight tube.
Similarly, in spectrometry applications, beam “blow up” inhibits the spatial resolution of different ions and reduces signal intensity.
In applications where a scanning magnet is used to deflect an ion beam (for example to scan the beam across a target) beam blow up inside the magnet is also undesirable. It reduces beam intensity and control accuracy.
The effect of beam space charge is especially severe for relatively low energy beams (e.g. 1-2 keV)since for the same beam current there is a higher density of ions in a low energy beam.
In regions of zero electric field, such as the flight tube of an analysing or scanning magnet, self neutralisation of ion beams tends to occur through the production of electrons and positive ions resulting from collisions between beam ions and atoms of residual gas in the vacuum chamber through which the beam is passing. However, this self neutralisation may be insufficient adequately to reduce beam blow up. This is particularly true for relatively low energy ion beams, as, at low energies, the cross sections for electron production during interaction between the beam ions and residual gas atoms are extremely small.
Also, some self neutralisation may occur as a result of local electron production from the beam striking the inside of the flight tube along the entire beam path through the magnet. Again however, this may be insufficient to reduce beam blow up to acceptably low levels, and in any case is to be avoided as it results in beam loss and unwanted particle generation.
In regions of zero electric field and zero magnetic field (i.e. “drift space”) a known technique to neutralise space charge is to flood the region through which the ion beam travels with low energy (typically a few eV) electrons or ions, produced, for example, in a plasma chamber adjacent to the beam flight path. In this drift space the electrons or ions are mobile and can move along and across the beam to minimise beam potential.
In regions of applied magnetic field however, the magnetic field severely limits the mobility of these electrons or ions. In applied fields of sufficient magnitude to deflect beams of ions with energies in excess of, say, a few keV, low energy charged particles, and electrons in particular (with their small mass,) will follow paths having circular projections of very small radii on a plane perpendicular to the direction of applied field. In effect, the electrons are restricted just to following the magnetic field lines. They have substantially zero mobility perpendicular to the applied field, which in the case of analysing magnets or scanning magnets means that the electrons have substantially zero mobility along the beam axis.
In regions where the electric field is nominally zero, such as inside the earthed flight tube of an analysing magnet, there may in fact be a small amount of electron motion perpendicular to the direction of applied magnetic field owing to the presence of various small electric fields such as those resulting from the beam itself. This motion is known as E×B motion. However, in such nominally E-field-free regions, electron mobility along the beam is, in general, very restricted.
Thus it is not possible to neutralise beam space charge inside regions of applied field by introducing low energy charged particles to adjacent drift space regions as the charged particles will not be able to migrate into the beam.
According to a first aspect of the present invention there is provided ion beam apparatus comprising:
a analysing magnet including a flight tube for receiving and conveying through the magnet a beam of ions, the magnet being operable to generate a substantially uniform magnetic field in the flight tube to deflect beam ions according to their mass/charge ratio in a dispersion plane perpendicular to the direction of said uniform magnetic field; and
a thermionic electron source inside the flight tube, arranged adjacent and outside a nominal cross section of a beam of ions travelling through the magnet and extending along a nominal flight path of a beam travelling through the magnet,
the thermionic electron source being further arranged such that the projection of the thermionic electron source on the dispersion plane and the nominal projection on the dispersion plane of an ion beam travelling through the magnet overlap at a plurality of positions along the nominal flight path of the beam.
Thus, magnetic flux generated by the magnet may link the thermionic electron source and a beam of ions travelling through the magnet at a plurality of positions along the flight path, and electrons may be emitted thermionically from the source into the beam at these positions.
This enables space charge to be neutralised at a plurality of positions along the beam in spite of low electron mobility along the beam, and so reduces “blow up”, keeping the beam tight. The thermionic electron source is positioned outside the nominal beam envelope to reduce beam contamination resulting from sputtering off the source and the associated erosion of the source.
In the presence of the magnetic field generated by the analysing magnet, electrons emitted from the thermionic electron
Armour David
England Jonathan Gerald
Holmes Andrew
Moffatt Stephen
Van Den Berg Jaap
Applied Materials Inc.
Patel Vip
Tennant Boult Wade
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