Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2000-08-15
2003-04-08
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06545277
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to charged particle beam detection apparatus and in particular to a charged particle detector wherein a primary beam passes through the detector.
BACKGROUND OF THE INVENTION
For general background information reference is made to: “Sem Notes #1” (website: http://www.uga.edu/~caur/semnotel.htm); and “Scanning Electron Microscopy”, L. Reimer, Springer-Verlag, ISBN 3-540-13530-8 and ISBN 0-387-13530-8. For prior art particle detectors reference is made to: U.S. Pat. No. 4,149,074 Schliepe et al April 1979, U.S. Pat. No. 4,831,266 Frosien et al May 1989, U.S. Pat. No. 5,466,940 Litman et al November 1995.
A Scanning Electron Microscope (SEM) consists of inter alia, an electron source emitting a current of charged particles (the primary beam), an acceleration system for the charged particles, a steering and focusing system which brings the charged particles to a finely focused point on a sample and a scanning mechanism which causes the focal point to scan the sample in a controlled predefined manner. Charged particles which are created due to the interaction of the primary beam with the sample (secondary beam) scatter in the sample chamber. A detector collects these created charged particles (foremost secondary electrons or in short, SE's) utilizing an electromagnetic field. It should be noted that a multitude of particles are created in the above-mentioned interaction, such as inter alia, backscattered electrons (BSE's), Auger and X-Ray electrons, but are in following description not further discussed. In a widely common detection scheme, called in-lens detection, the secondary electrons (SE's) are pulled back into the microscope, in where an electron detector is placed somewhere close to the original path of the primary beam inside the microscope (in-lens detector). Another detection scheme wherein the detector is positioned somewhere in the chamber is herein not further discussed by virtue of its substantially different operation.
There follows now a description of prior art in-lens detection schemes.
In
FIG. 1
a prior art detector is shown, wherein scintillator
10
is positioned near or on light guide
11
and consists commonly of a phosphor or plastic or crystal scintillator. Light guide
11
is coupled to photomultiplier tube
12
. Primary particle beam
13
passes through the light guide
11
and scintillator
10
by means of primary beam shielding
14
. A protective sleeve
15
shields primary particle beam from the electrical field of the scintillator anode, biased at an electric potential of several kV's above V
COLUMN
. Column electric voltage potential (V
COLUMN
) is the electric potential of the beam transport tube or structure surrounding the detector along the primary beam path. Both primary beam shielding
14
and protective sleeve
15
are at V
COLUMN
electric potential. Thus, the primary beam does not experience any influence caused by electric fields originating from the significantly high electrical potential of the detector anode. After primary particle beam impinges on sample
16
, held at a defined electric potential V
SAMPLE
, charged particles
17
are accelerated into lens
110
, due to the electric potential difference between the sample and the column, and finally impinge on scintillator
10
. The energy needed to create a signal from the charged particles on the scintillation material is gained, in prior art, by the above-mentioned electric potential difference between the column and the sample.
FIG. 2
illustrates schematically the scintillator area in more detail. Scintillator
20
is attached to or forced on light guide
21
. Photons generated by the scintillator are directed through light guide
21
toward photomultiplier tube
22
that generates the electric signal representative of the number of charged particles impinging on it. Primary beam shielding
23
, through which the primary beam passes, is positioned inside a hole
24
in light guide
21
. A multitude of charged particles
25
after being generated by the interaction between the primary particle beam and the sample are accelerated towards scintillator
20
by the electric potential difference between the column and the sample.
Hole
24
is substantially responsible for creating an area
26
in which photons generated by impinging charged particles
27
are obstructed from reaching photomultiplier tube
22
. This area is therefore distinguished from the rest of the scintillator by its low efficiency properties, also commonly referred to as the “shadow” area of the primary beam shielding. Charged particles impinging onto the “shadow” area do contribute less to the generation of the image resulting from the transformation of photons to an electric signal by the photomultiplier tube, thus creating distinctive shadowing, detrimental to optimum detection performance.
There is accordingly a need in the art to provide for a system that substantially reduces or eliminates the shadowing effect of in-lens light guide scintillator detectors, and allow additional amplification of the signal without changing the general beam transport conditions, achieving improved and possibly uniform detection.
SUMMARY OF THE INVENTION
For purpose of clarity the acronyms SE and BSE are respectively used for secondary electron and backscattered electron in all discussions and claims below and more generally, relate to charged particles or electrons.
It should be noted that in the discussions below the phrase “in-lens detector” is generally applicable to any detector wherein the primary beam passes through.
Furthermore, it is appreciated that the reference to Scanning Electron Microscope (SEM), does not limit or confine the present invention in any way and is generally applicable to any charged particle beam detection apparatus.
In accordance with the preferred embodiment of the present invention, there is provided a high efficiency, enhanced detecting light guide of an in-lens detector for SEM, having a variable electric potential on the scintillator surface and having an open area. In the preferred embodiment, instead of being set at V
COLUMN
and open to the rest of the system, the scintillator surface is electrically isolated from its surroundings and biased at a higher electrical potential, bringing about impingement of electrons with greater energy. The higher energy results in more light being produced per electron and this leads to higher detection efficiency. The scintillator is shielded from the rest of the system by one, and preferably two grids. The external grid is set at V
COLUMN
and thus prohibits disturbance of the primary beam. The second, inner grid may be set at a low electric potential, which will inhibit some, or all, of the electrons from reaching the scintillator. This grid may be used as a high pass energy filter for the incoming electrons.
The electric field created between the grid and the scintillator is used to direct the electrons away from the “shadow” region (
26
of FIG.
2
). This is accomplished by creating an open area instead of the “shadow” region. The open area results in shaping the light guide and scintillator into fork-like parts, with the two sides of the fork-like parts protruding as prongs. A primary beam shielding is situated in the open area between the prongs of the fork-like part facilitating passage of primary particle beam. The open area covers substantially the same area where the shadow of the primary beam shielding causes low efficiency detection of impinging electrons.
In accordance with another embodiment there is provided a conductive grid or mesh-sheet fitted in the open area between the prongs of the fork-like part. The electric potential, shape and position of the conductive grid can be determined to achieve one out of many desired possible interactions with charged particles (inter alia, SE and BSE electrons), such as e.g. deflection and generation of SE's. Maintaining the conductive grid or mesh-sheet at a low enough electric potential below the kinetic energy of the fas
Kella Dror
Rechav Betsalel
Anderson Bruce
Applied Materials Inc.
Einschlag Michael B.
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