Parallel plate electron multiplier

Details Parallel plate electron multiplier Parallel plate electron multiplier

2000-03-28

2003-11-04

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Photosensitive

Secondary emitter type

C313S528000, C313S530000

Reexamination Certificate

active

06642637

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to electron multiplier devices and, in particular, to electron multiplier devices for detection of particles emitted from a surface as a result of an incident beam impacting the surface.
2. Related Art
Electron multipliers are useful tools for various applications, including the detection of photons, electrons, ions and heavy particles. Such detectors are utilized in various spectroscopic techniques, including Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and electron energy loss spectroscopy. Further, electron multipliers may be utilized for detection of secondary and back-scattered electrons in scanning electron microscopes, focused ion-beam tools, or e-beam lithography tools.
In general, electron multipliers have had two configurations, the channeltron multiplier or multi-channel plate multiplier.
FIG. 1
shows a parallel plate electron multiplier
100
as described in numerous publications, including S. Suzuki and T. Konno, “A Computer Simulation Study On the Detection Efficiencies of Parallel-Plate Electron Multipliers,” Sci. Instrum. 66 (6), p. 3483-87 (June, 1995); and L. P. Andersson, E. Grusell and S. Berg, “The Parallel-Plate Electron Multiplier,” J. Phys. E: Sci. Instrum., Vol. 12, p. 1015-22 (1979).
Electron multiplier
100
includes secondary emitting surfaces
101
and
102
, deposited on glass plates
111
and
112
, respectively, and separated by a spacing
104
. A voltage V
d
is applied along the length of electron multiplier
100
so that electrons entering at an open end
105
are accelerated along the length of electron multiplier
100
away from open end
105
. When the electron collides with one of secondary emitting surfaces
101
and
102
, multiple secondary electrons are emitted. The secondary electrons are then accelerated along electron multiplier
100
and themselves may collide with one of secondary emitting surfaces
101
and
102
. On each collision of an electron with sufficient kinetic energy with one of emitting surfaces
101
or
102
, further electrons are emitted. By repeated collisions of electrons with secondary emitting surfaces
101
and
102
, an output pulse containing a very large number of electrons is emitted from electron multiplier
100
.
The output pulse is received by collector
103
located on the side of electron multiplier
100
opposite from open end
105
. Typically, collector
103
is held at an elevated voltage from the voltage of that end of electron multiplier
100
. The output pulse is detected by detection circuitry
106
coupled to collector
103
. The gain of electron multiplier
100
depends on the voltage V
d
applied across electron multiplier
100
, the secondary emission properties of secondary emitting surfaces
101
and
102
, and the physical dimensions of electron multiplier
100
.
In some electron multipliers such as electron multiplier
100
, further voltages are applied to either end of one of secondary emitting surfaces
101
and
102
. In such cases, electric fields can be created that are not parallel with the length of electron multiplier
100
, thereby enhancing collisions with one of secondary emitting surfaces
101
or
102
. Further, collector
103
may be tilted (i.e., the collector surface may not be perpendicular to the surfaces of secondary emitters
101
and
102
) in order to further enhance collection of output pulses of electrons and to further supply a component of the electric field not parallel with electron multiplier
100
.
Some electron multipliers may be constructed from a glass tube coated with a secondary emitting surface. The resulting multiplier, in principle, operates as described above for parallel plate electron multiplier
100
except that, instead of parallel plate secondary emission surfaces, the secondary emission surface is cylindrical in shape. The tubular channeltron multiplier has the advantage that, because of its tubular nature, it can be shaped into loops and spirals that reduce its overall size without affecting the overall length of the multiplier.
However, each of these multipliers are difficult to use in certain environments. For example, in some instances, such as in lithography or in electron microscopes, it is difficult to detect reflected electrons that are close to an incident electron beam. In some applications, it is desirable to collect electrons from as close to the incident beam as possible. With electron multiplier
100
or the tubular channeltron multiplier, positioning of the opening surface can be difficult.
Therefore, there is a need for an electron multiplier that is easily constructed, of small size, and capable of monitoring the particles close to a beam incident on a surface that emanate from the surface.
According to the present invention, an electron multiplier capable of detecting particles such as, for example, ions, photons, or electrons, traveling close to an incident beam is presented. The electron multiplier includes a top plate and a bottom plate separated by a small gap. Each of the top plate and the bottom plate includes an access through which an incident beam can pass. The accesses of the top plate and the bottom plate are aligned so that the incident beam can pass through the electron multiplier. Particles traveling close to the incident beam, and in a direction opposite that of propagation of the incident beam, can enter the electron multiplier between the top plate and the bottom plate and thereby be detected.
The top and bottom plates each have a secondary electron emitting surface. The secondary electron emitting surface of each of the top and bottom plate emit electrons when the surface is impacted with a particle of sufficient energy. Further, each of the top and bottom plates are resistive so that a current can flow through them. Finally, in most embodiments, the top and bottom plates provide structural support for the secondary emitting surfaces.
In some embodiments, the top and bottom plates can be a single material, for example lead oxide glass, bismuth oxide glass, or iron borate glass. These materials are resistive, provide a secondary emitting surface, and provide structural support. In another embodiment, each of the top and bottom plates can include secondary electron emission layers, for example CVD diamond or an alkali halide, deposited on a resistive layer, for example a metal or low resistance semiconducting layer, deposited on a structural substrate, such as glass.
In one embodiment, the top plate and the bottom plate have an annular geometry. The top secondary emitting surface has an outside radius and an access with an inside radius, and the bottom secondary surface has an outside radius and an access with an inside radius. In another embodiment, the secondary electron emitting surface of the top and bottom plates have annular geometry. The access allows an incident beam to pass through the top plate, with the top secondary emitting surface, and the bottom plate, with the bottom secondary emitting surface, without impacting the electron multiplier.
The references to top and bottom or up and down in this disclosure is with reference to the direction of propagation of an incident beam. Bottom or down refers to a direction closest to a surface on which the incident beam is incident. Top or up refers to the opposite direction from bottom or down.
In most embodiments the outside radius of the top secondary emitting surface and the outside radius of the bottom secondary emitting surface are about the same. In some embodiments the inside radius of the access of the top is less than the inside radius of the access of the bottom secondary emitting surface so that particles (e.g., electrons, ions, or photons) are easily captured into the multiplier.
The access through the annular geometry, through which a beam can pass, has the inside radius of the top secondary emitting surface at the top plate and the inside radius of the bottom secondary emitting surface at the bottom plate of the electr

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