As a rule, raster electron microscopes include one or several detectors in the specimen chamber for detecting secondary electrons and/or back-scattered electrons. These secondary and back-scattered electrons are released from the specimen by the primary electron beam. A different contrasting of the specimen image is generated depending upon whether the intensity of the back-scattered electrons or the intensity of the secondary electrons is the basis for the image generation.
An arrangement is described in the article of U. Golla et al entitled xe2x80x9cContrast in the transmission mode of a low-voltage scanning electron microscopexe2x80x9d, Journal of Microscopy, Volume 173 (1999), pages 219 to 225, with which the image generation is possible on the basis of electrons transmitted through the specimen utilizing a conventional raster electron microscope. The arrangement includes a cylindrically-shaped shielding unit which is accommodated on the specimen table and the specimen holder is accommodated in the interior of the shielding unit. The shielding unit ensures that no electrons can reach the detector which exit from the specimen on the incident side of the electron beam. The shielding unit extends directly from the lower poleshoe of the objective lens of the raster electron microscope. The contrasting, which is possible with this arrangement, corresponds to the brightness field contrast in transmission and makes possible only the detection of signals of the central region of the specimen because the region from which electrons exiting the specimen can reach the detector is delimited by the diaphragm diameter.
The generation of images in the dark field contrast in electron microscopy is, up to now, restricted to the so-called raster transmission electron microscope (STEM). These STEMs are transmission electron microscopes having, as a rule, a multi-stage condenser system between the electron source and the specimen and an imaging system with which the specimen is imaged magnified into a detection plane. The imaging system is, as a rule, likewise a multi-stage electron optical imaging system. In the STEM mode, the condenser system is so driven that it images the electron source demagnified on the specimen.
In transmission electron microscopes, the specimen plane is usually at the elevation of the poleshoe gap of a condenser-objective-single-field lens which operates as the last condenser lens as well as an objective lens. For this reason, with transmission electron microscopes, no investigations with inclined specimens are possible wherein the specimen surface is inclined to the optical axis of the incident primary electron beam, that is, a surface normal of the specimen lies at an angle to the optical axis of 30xc2x0 or more. Specimen inclinations of this kind are realizable up to now only in raster electron microscopes.
It is an object of the invention to provide a raster electron microscope with which the generation of specimen images in transmission is possible with different contrastings. It is another object to configure the invention in the form of an ancillary unit for raster electron microscopes wherein the ancillary unit is accommodated in the specimen chamber.
The raster electron microscope of the invention includes: an electron source for emitting electrons along a beam path toward a specimen; a specimen chamber mounted in the beam path; a detector mounted in the specimen chamber for detecting electrons; a specimen table mounted in the specimen chamber and having a specimen holder; a diaphragm system mounted on the specimen table and the diaphragm system being adjustable relative to the specimen holder; and, the diaphragm system including a diaphragm arranged between the specimen holder and the detector.
The raster electron microscope of the invention includes, as usual, a specimen chamber, and a detector for electrons as well as a specimen table having a specimen holder. The detector and specimen table are both mounted in the specimen chamber. A diaphragm system is accommodated on the specimen table and is adjustable relative to the specimen holder. The diaphragm system has a diaphragm between the specimen holder and the detector.
The contrasting of the recorded image can be changed with the aid of the diaphragm adjustable relative to the specimen holder. If the diaphragm is so designed and adjusted that the electrons are masked which run close to the optical axis of the raster electron microscope and are transmitted through the specimen, then only the electrons, which are scattered at larger angles, contribute to the image generation and the result corresponds to a dark-field contrast. This contrasting increases, for example, the contrast in specimens having Z-contrast or crystalline specimens having irregularities regarding the grating structure or specimens having phase transitions. If, on the other hand, the diaphragm is so configured and adjusted that just those electrons, which exit from the specimen close to the optical axis and only at a slight angle to the optical axis, transmit through the diaphragm and therefore contribute to the image generation, then this corresponds to the contrasting in a bright-field contrast.
Successive images of larger specimen regions or even of the entire specimen can be generated with the desired contrasting by displacing the specimen table including the shielding unit accommodated thereon, perpendicularly to the optical axis of the raster electron microscope and simultaneously shifting the diaphragm in the opposite direction.
The specimen holder is preferably mounted in a shielding unit and the diaphragm is accommodated on the base body at the side facing away from the specimen holder. The shielding unit ensures that only those electrons exiting the specimen can reach the detector which have passed the diaphragm opening. A corresponding shielding unit can be configured as a cylindrical tube impermeable to electrons. The cross-sectional form of the tube is of no consequence. The shielding body should have a ring on the side facing toward the objective. With this ring, the regions which follows directly upon the poleshoe of the objective is sealed against the exit of electrons. In this way, back-scattered electrons or secondary electrons, which exits from the specimen surface irradiated by the primary electron beam, cannot reach the detector.
The detector can be configured for the detection of secondary electrons and/or the detection of electrons which have an energy comparable to the energy of the primary electrons.
In a preferred embodiment, the detector includes a conversion element which emits secondary electrons having a yield of magnitude greater than 1 when irradiated with electrons. This conversion element is preferably mounted directly rearward of the diaphragm so that all electrons, which are transmitted through the diaphragm opening, are incident upon this conversion element.
The diaphragm system preferably has several diaphragms having different diaphragm openings which can be selectively introduced between the specimen holder and the detector. In this way, the nature of the contrasting can be easily changed. Especially preferable is a motor drive for the exchange of diaphragms so that no manual intervention in the specimen chamber is necessary. With adjustment paths of the drive for the diaphragm adjustment which are sufficiently long, these drives can, at the same time, be used for switching the diaphragms.
The diaphragms or one of the diaphragms should have a central region impermeable for electrons for the generation of dark-field contrast.
The specimen holder is preferably tiltable about an axis perpendicular to the optical axis of the raster electron microscope. Tilt angles of 30xc2x0 and more should be possible between the surface normal of the specimen holder and the optical axis.
The invention can be especially configured as an ancillary unit for a raster electron microscope. Such an ancillary unit includes: a cylindrical shielding unit which is accommodated on a specimen table; a specimen holder mounted in the interior of the shielding unit; and, a diaphragm holder having a diaphragm. The diaphragm holder closes off the shielding unit on the side facing toward the specimen table and is adjustable relative to the specimen holder. The generation of dark-field contrast images in transmission is possible in a simple manner with conventional raster electron microscopes with the aid of such an ancillary unit and the controls required for the adjusting drives of the diaphragm holder.
In an alternative ancillary unit, a shielding unit surrounds the electron detector and the inlet opening of the shielding unit is closed by a diaphragm which can be exchanged. A specimen holder is then accommodated at the shielding unit and is adjustable relative to the diaphragm or relative to the diaphragm holder.
In a further advantageous embodiment, an energy filter is provided between the specimen holder and the diaphragm system or between the diaphragm system and the detector. In this way, it is additionally achieved that only electrons, which are transmitted through the specimen, can reach the detector with a specific energy so that an additional contrasting possibility results. A corresponding energy filter can, in the simplest case, be configured as a counter field electrode. Alternatively, it is also possible to utilize more complex magnetic or electrostatic energy filters or spectrometers as they are known from the state of the art.