Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1999-09-27
2002-10-15
Anderson, Bruce (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S292000
Reexamination Certificate
active
06465792
ABSTRACT:
TECHNOLOGICAL FIELD AND PRIOR ART
The invention relates to the field of electrode architectures that permit the generation of a multi-polar field, or permit the filtration or the deflection or the focusing of charged particles. More particularly, the invention relates to micro-devices that integrate assemblies of micro-electrodes, also for the purpose of generating a multi-polar field in particular for the filtration or deflection or focusing of charged particles.
The invention finds application notably in the field of mass spectrometry: in effect, the assemblies of electrodes according to the invention can be used in mass spectrometers. The invention therefore also relates to the field of mass spectrometry.
Mass spectrometry is an analysis technique widely used in laboratories and in industry. Using mass spectrometry the nature of constituents of a gas can be determined with a sensitivity better than ppm. To do this, the gas to be analyzed must be at low pressure, in general less than 10
−4
mbar. This is a limitation for applications where the pressure is greater (10
−2
mbar) and for which it is then necessary to add pumps and supplementary circuits so as to reduce the pressure in the area where the mass spectrometer is. The creation of a spectrometer of small size (≈1 cm
3
) would allow one to work under a lower level of vacuum.
In a general way, a mass spectrometer comprises three distinct parts, as illustrated in FIG.
1
: an ionization chamber
2
, a separator
4
(filter) and an ion detector
6
. Many separators are of the quadrupolar type. The theory of uni-polar and quadrupolar mass spectrometers etc is for example described in the book “Techniques de l'Ingénieur”, volume P3, P 2615, p. 1-39. The mass filter
4
is the place where, through the set of electromagnetic forces, the ions of different masses are separated. In a quadrupole (an accepted term meaning “a mass analyzer fitted with a quadrupolar type filter”), a high frequency electric field is generated between 4 parallel bars
8
,
10
,
12
,
14
such as those shown in FIG.
1
. It is assumed that the ions move along the mean direction OZ parallel to the bars.
In a general way, a quadrupolar electric field is such that its amplitude is a linear function of the co-ordinates. The electrical potential is therefore a quadratic form of the co-ordinates. It can be written in the form V=(&phgr;/r
2
)×(x
2
−y
2
) where r is the distance between the axis OZ and the bars (r is also called the throat radius of the quadrupole) and &phgr; a constant value of the potential. In order to obtain such a potential distribution, two opposite bars are polarized to +V, while the two others are polarized to −V.
In the case of a quadrupole being used as a filter or a means of focusing or of deflection, the potential V in addition comprises a time dependent component (term in cos &ohgr;t) which is used to make the charged particles oscillate. The lines of equipotential, which, at a given moment, correspond to this distribution of potentials (as a function of the potentials applied to the bars) are hyperbolae in the plane XOY. In the ideal case, the cross sections of the bars have this same hyperbolic form. A quadrupolar system with bars
16
,
18
,
20
,
22
of hyperbolic section is illustrated in FIG.
2
and is, for example, described in U.S. Pat. No. 5,373,157.
In the majority of cases, and in order only to use machining that is easy to carry out, the bars have circular cross sections osculating to hyperbolae at their peak; such cross sections
26
,
28
,
30
,
32
are also shown in
FIG. 2. A
degradation of resolution results from passing from the hyperbola to the circle.
If one wishes to reduce the size of a quadrupolar spectrometer to about 1 cm
3
, all the dimensions of the filter are affected by this, in particular the radius of the bars, their distance to the center and, of course, their length.
Document WO-96/31901 describes a miniaturized quadrupolar mass spectrometer. This device uses cylindrical beams made from metal coated optical fibers. In such a device, the insulators situated between the cylindrical bars can lead to charge effects which are prejudicial to the operation of the device.
The patent U.S. Pat. No. 5,401,962 describes a miniature quadrupole. In this document, the miniaturization or the reduction in size, originates from the assembly of a plurality of quadrupoles in parallel. This device lacks resolution. Furthermore, its production, even when it uses cylindrical electrodes is rather time consuming.
Document U.S. Pat. No. 4,994,336 describes a control plate for a lithography device. This control plate essentially comprises a semi-conductor substrate in which a window or opening is made to allow the passage of beams of particles. Deflection elements allow the beams to be deflected.
This document also describes methods of producing such a plate. In these methods, the deflection elements are obtained by etching a layer to produce depressions in it that have the shape of deflection elements. The deflection elements are then created along a direction perpendicular to the plane of these layers.
Finally, in this document, the field generated is a uniform field in all directions and in all the space between the flat electrodes. This field is not multi-polar.
DESCRIPTION OF THE INVENTION
The problem posed therefore is that of producing components, notably for their application to mass spectrometry, that allow among other things the miniaturization of the spectrometers.
In particular, the problem is posed of creating assemblies of electrodes which would enable easy production of micro mass spectrometers.
More precisely, a subject of the invention is a micro-device for generating a multi-polar transverse field, comprising n conductive longitudinal micro-beams of polygonal cross section, arranged around a longitudinal axis.
Advantageously, the field is constant along the longitudinal axis.
By a multi-polar field one understands any electrical field, even a mono-polar one. Such a field is not uniform since it is multi-polar or mono-polar.
Consequently, according to the invention, the creation of electric fields on a sub-millimeter scale, and which are derived from potentials which are for example, of the quadrupolar, hexapolar or octopolar type or more generally are N-polar (N>1) can be carried out with electrode structures (also called lenses) in which the electrodes have a cross section of polygonal shape. Such electrodes are compatible with production using micro-electronic or micro-technology techniques and therefore permit the manufacture of miniaturized mass spectrometers.
Another subject of the invention is a micro-device for the filtration or the deflection or the focusing of charged particles, comprising n conductive longitudinal micro-beams, with polygonal cross section, and arranged around a longitudinal axis of propagation of the charged particles.
Another subject of the invention is a micro-device for the filtration, or the deflection or the focusing of charged particles that comprises a micro-device to generate a multi-polar transverse field such as that already described above.
For this, the invention specifies an electrode structure for a micro-device for filtration, or deflection or focusing, which is compatible with production by the techniques of micro-electronics or micro-technology. Electric fields can also be generated for a micro-device for filtration, or deflection or focusing on a sub-millimeter scale.
At present, the electrodes used in a quadrupolar type device, or more generally an N-polar device, have sizes which cannot be reduced below certain dimensions, which limits the lower size of the device. In particular, document U.S. Pat. No. 5,401,962 mentions cylindrical electrodes of a diameter between 0.5 mm and 1 mm, and of a length between about 1 cm and 2 cm. The electrode structure according to the invention allows sub-millimeter devices to be produced (for example with bars whose thickness is a few hundreds of &mgr;m) which are able t
Anderson Bruce
Commissariat a l'Energie Antomique
Krebs Robert E.
Thelen Reid & Priest LLP
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