Electron detectors

Details Electron detectors Electron detectors

2000-04-17

2003-05-27

Nguyen, Kiet T. (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

With detector

C250S310000

Reexamination Certificate

active

06570163

ABSTRACT:

The present invention relates to electron detectors, and is concerned particularly, although not exclusively, with electron detectors for use in electron microscopes.
It is known to provide a focused electron beam, referred to as an electron probe, to impact onto a specimen under observation. The probe is scanned over a viewfield of adjustable dimensions and signal electrons, emitted from the specimen and detected, are fed to a CRT monitor scanned synchronously with the electron probe so that an image is formed on the monitor screen, with the magnification given by ratio of sizes of both of the scanned fields.
From all possible signals excited by an electron probe, so-called Auger electrons are of particular importance. They are released from atoms which are originally ionised in an inner shell by an electron from the electron probe; the vacancy is then filled by another electron from some other outer shell of the same or even different atom, and the energy difference is transmitted to one more electron which leaves the atom with an energy characteristic to it. Consequently, when analysing the energies of the signal electrons, one can recognise the surface elemental composition. By means of a finely focused electron probe, this can be done even at high spatial resolution. The energy analysis is made by an electron spectrometer, i.e. some special configuration of electrostatic or magnetic fields or combination of fields that ensures spatial dispersion of the electron trajectories according to their energy so that, by screening with suitable apertures, only a narrow energy window can be filtered, or alternatively, parallel detection of all emitted electrons is also possible.
Among possible spectrometer configurations utilising serial detection, the so-called Cylindrical Mirror Analyser (CMA) is popular and often used. It consists of two coaxial cylinders, the outer of which is negatively biased so that electrons outgoing from a specified entry point on the cylinder axis through a suitable slit in the inner cylinder wall are back reflected through a second slit and a final small aperture, again on the cylinder axis at an exit point, such that only those electrons which fall inside an energy window are passed.
In order to perform Scanning Auger Microscopy (SAM), i.e. to achieve elemental surface analysis with a high spatial resolution, an electron gun is co-axially positioned inside the inner cylinder of the CMA and is used for illuminating the specimen, placed on the device axis perpendicularly or inclined to it at the entry point, and has to be a high-quality electron gun with a very finely focused electron probe and with facilities to scan the probe within the scanned field. Such a sophisticated gun, the scanning column, is currently available equipped with a field-emission single-crystal tip cathode and one or two electron lenses. It is of importance that the scanning column has to fit inside a very limited place surrounded by the spectrometer cylinder so that no mechanical actuators, lightpipes or similar connections can be led to it. All connections to the outside have to be exclusively electric and the scanning column is not allowed to produce any electromagnetic fields in its vicinity, which would interfere with the spectrometer action.
The Auger electron is recognised according to a fixed value of its energy so that it can be detected only when it does not suffer from any further scattering event causing an energy change. It means that only Auger electrons released within a few topmost atomic layers at the surface of a specimen can contribute to the signal and the elemental composition of only such an ultrathin surface layer can be reliably mapped. Nevertheless, a typical electron probe will generally penetrate much deeper into the specimen and a good number of excited electrons are reflected backwards from the surface with various energies, mostly sufficient to excite additional Auger electrons. In frequent cases, the specimen is heterogeneous in its depth so that the back-reflected electrons vary locally, together with the quantity of the additional Auger electrons, and the in-depth specimen heterogeneity projects itself spuriously into the surface elemental mapping. In order to suppress this signal contribution, some other depth sensitive signal has to be available. A second most important problem inherent to Auger mappings is the contribution of the surface topography to the elemental mapping. Again, the mapping should be correlated with another microscopic signal showing the surface relief.
There is currently a general trend in electron microscopy to lower the energies of electrons in the electron probe. The reasons for this include achievement of higher secondary electron signal (which has its maximum somewhere between a few hundred eV and a few keV); achievement of reduced charging of non-conductive specimens (owing to the total yield of the emitted electrons approaching unity, so that only a small proportion of them is dissipated inside the specimen); and better resolution of tiny relief protrusions and ridges (owing to smaller interaction volume and shorter penetration depth of the electron probe). Nevertheless, it is not possible to operate known probes below a few hundreds of eV because of some principal obstacles, which include deteriorated extraction of electrons from the gun cathode and consequently lower electron probe current, longer electron wavelength and higher relative fluctuations in energy of the electron probe (which causes larger spotsize due to increasing diffraction and chromatic aberrations) and more pronounced influence of spurious ac electromagnetic fields distorting the electron probe geometry. So called low-voltage microscopes working down to some 200 to 500 eV energy of the electron probe are currently highly attractive and well marketed. Nevertheless, it is well known that below such an energy range, at tens and units of eV, many new extremely interesting contrasts appear which visualise the surface crystallographic structure, energy band structure above the vacuum level, the potential barrier shape and its changes etc.
The only previously known way to realise very low energy microscopy, in the range of tens and units of eV, is to use a cathode lens. The cathode lens is the crucial component of an emission electron microscope (EEM) in which the specimen itself emits the electrons and after necessary acceleration they pass a projection electron-optical system forming the magnified image of the emitting surface. In principle, the cathode lens is an electrostatic lens consisting of two electrodes: the cathode, the specimen surface itself, and a suitably shaped anode with a central opening. The axial uniform electrostatic field between them, mostly produced by a high negative bias of the specimen/cathode, acts to accelerate the emitted electrons. The non-uniform part of the field, penetrating through the anode opening, forms a diverging lens, which is combined with some additional converging lens, the EEM objective lens. It is has been accepted for a long time that such a combination has very low aberration coefficients so that even at very low energies, a broad beam of the emitted electrons can be collimated into the imaging bundle.
Some attempts have been made during the last thirty years or so to utilise the cathode lens also in the reverse direction for deceleration of the electron probe immediately above the specimen surface. Nevertheless, none of these attempts has achieved significant success and no scanned very low energy pictures have been published. An important exception is the so called Low Energy Electron Microscope (LEEM) invented 35 years ago and successfully realised in the eighties [
E. Bauer, Rep. Progr. Phys.
57(1994),895]. It is not a scanning device but an EEM with the specimen emission excited by the impact of a coherent planar electron wave. The cathode lens is passed twice, first by the electron wave being decelerated and then by the emitted electrons in the opposite direction. The LEEM practice revealed the

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