Detector for a scanning electron microscope with variable...

Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type

Reexamination Certificate

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C250S306000, C250S397000

Reexamination Certificate

active

06707041

ABSTRACT:

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH DEVELOPMENT
Not applicable.
BACKGROUND THE INVENTION
The invention relates to a detector for a scanning electron microscope, in particular for a scanning electron microscope with variable pressure, and a scanning electron microscope with such a detector. By “scanning electron microscope with variable pressure”, or HPSEM, is to be understood a scanning electron microscope with which operation is possible with gas in the sample chamber at a pressure of at least 0.1 Pa. In HPSEMs, there is usually used as detector a collector electrode with following operational amplifier, or a gas scintillation detector. The latter consists of a light guide with a following photomultiplier. In both cases, a secondary electron cascade in the gas is required. Arrangements which use a secondary electron cascade are described in, for example, U.S. Pat. Nos. 4,785,182, 5,396,067, 5,677,531, WO 99/27559, JP 2236939, JP 2276846, and JP 2273445, and also in the article by G. Danilatos in the journal Advances in Electronics and Electron Physics, Vol. 78, pp. 1-102, 1990. The following problems arise in connection with the secondary electron cascade:
1. The amplification factor and the secondary electron cascade are limited by flash-overs.
2. In the present HPSEMs with collector electrode, the final pressure limiting aperture is at a similar potential to the collector electrode, i.e., the electrode at the end of the secondary electron cascade. The secondary electron cascade therefore has to take place for the most part on the same path section in the gas along which the primary electron scattering takes place (in the reverse direction). The pressure and the gas section therefore cannot be freely chosen, but their product must be large enough in order to obtain a sufficient amplification factor of the secondary electron cascade, even when the pressure or the gas section otherwise often do not at all actually have to be so large. Correspondingly, under these conditions, an undesirably strong primary electron scattering has to be accepted. This disadvantage also occurs with detection in the beam guiding tube.
3. In HPSEMs with a collector electrode and a following operational amplifier, no high scanning speeds are possible. At low scanning speeds, not even normal scanning speeds are possible, such as are required for alignment. The reason for this is that the time constants of the operational amplifier are too large for these scanning speeds at too high an amplification factor of the operational amplifier.
4. In both HPSEMs with gas scintillation detectors and also HPSEMs with collector electrode, the efficiency of the detection system is not fully adequate. A worsened signal
oise ratio and a greater damage to the specimen by the beam are the consequences, due to which the carrying out of many tasks is frustrated.
SUMMARY OF THE INVENTION
The present invention has as its object to provide an improved detector for HPSEMs with which at least a part of the above-mentioned problems is eliminated. The object of the invention is furthermore to provide a HPSEM with such an improved detector. These objects are attained by a scanning electron microscope that operates with gas in a sample chamber having a beam guiding tube for primary electrons, a sample chamber, a sample holder arranged in the sample chamber, a final pressure limiting aperture through which the primary electrons enter the sample chamber, a first electrode at a positive potential with respect to the sample holder and the final pressure limiting aperture for acceleration of secondary electrons emergent from a sample received by the sample holder, the first electrode being arranged outside the beam guiding tube, and at least one second electrode comprising an end facing toward the sample holder that is at a smaller distance from the sample holder than the first electrode and is at a potential that is between the potential of the first electrode and the potential of the sample, or is at the potential of the sample. The second electrode surrounds the first electrode and is substantially in the form of a funnel having a funnel tip toward the sample.
These objects are also attained by a scanning electron microscope that operates with gas in a sample chamber, having a sample chamber, a sample holder arranged in the sample chamber, the sample holder having a sample potential, a final pressure limiting aperture through which the primary electrons enter the sample chamber, and an electrode arranged outside the beam guiding tube. The electrode is electrically poorly conducting and comprises at least two contacts, a first one of the at least two contacts having a first potential and a second one of the at least two contacts having a second potential. An end of the electrode facing toward the sample holder is at an electrical potential that is between a higher one of the first and second potentials and the sample potential, or at the sample potential. The contact with the higher one of the first and second potentials is at a positive potential with respect to the sample holder and the final pressure limiting aperture.
These objects are also attained by a detector for secondary electrons in a scanning electron microscope with high pressure in a sample chamber with the use of a secondary electron cascade. At least one electrode with low electrical conductivity is provided, which extends along an elongate interspace or elongate cavity. In an inlet-side region within or in front of the cavity or interspace, the at least one electrode can have a potential applied such that a high amplification for secondary electrons results, and an elongate volume region with a reduced amplification factor for secondary electrons adjoins this inlet-side region.
These objects are also attained by a detector for secondary electrons in a scanning electron microscope with high pressure in a sample chamber with the use of a secondary electron cascade. A plurality of electrodes are provided that extend along an elongate interspace or elongate cavity. In an inlet-side region within or in front of the cavity or interspace, the electrodes can have a potential applied such that a high amplification for secondary electrons results, and an elongate volume region with a reduced amplification factor for secondary electrons adjoins this inlet-side region. The application of potential to the electrodes in the elongate volume region takes place such that an adjacent electrical field counteracts a tenuation of the secondary electron cascade due to impacts in the gas and due to drifting of the secondary electrons to the walls, so that a high but uncritical ionization density remains sustained.
A scanning electron microscope according to the invention has, like the known HPSEMs, a beam guiding tube for the primary electrons, with a final pressure limiting aperture on the sample side through which the primary electrons enter the sample chamber; a sample chamber; a sample holder in the sample chamber; and a first electrode which is at a positive potential relative to the sample holder and the final pressure limiting aperture of the beam guiding tube. The potential difference between the sample and the first electrode serves to accelerate secondary electrons which are released by the primary electrons from the sample received in the sample holder, the known secondary electron cascade being formed by the impact of these accelerated secondary electrons with the surrounding gas molecules and leading to an amplification of the secondary electron current.
In the meaning of the present application, the region between the electron source and the final pressure limiting aperture is termed the beam guiding tube.
In a first embodiment of the invention, at least one second electrode is provided, the end of which facing the sample holder is spaced closer apart from the sample holder than is the first electrode. This second electrode is at a potential that is between the potential of the first electrode and the potential of the sample, or at the potential of the sample.

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