Patent Number: 050292498
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron microscope e.g, a scanning electron microscope or a transmission electron microscope. 2. Summary of the Prior Art An electron microscope normally comprises a specimen chamber in which the specimen to be analysed is located, and an electron column which generates a beam of electrons which are used to bombard the specimen. That electron column itself normally has two parts, an electron gun chamber containing an electron gun for generating the beam of electrons, and a casing containing an electron lens arrangement. That electron lens arrangement usually includes a plurality of condenser lenses arranged vertically inside the casing with the electron beam passing down a central axis of those condenser lenses. Alignment coils for aligning the electrons into a narrow beam are located between the condenser and the electron gun. Finally, in e.g, a scanning electron microscope, scanning coils (deflection coils) are located between the condenser lenses and the sample, which cause the beam to scan across that sample. Thus, an electron beam path is defined between the electron gun and the sample, passing through the lenses and coils. It is important that electrons following that path are not scattered, and for this reason the interior of the electron column has to be evacuated to a low pressure. The sample chamber must also be evacuated. The standard method of evacuating the interior of the electron column is to connect that column to a vacuum pump via a plurality of ducts which extend from the vacuum pump through the walls of the electron column. Example of such an arrangement is shown in Japanese patent application laid-open number 49-131376, in which there are three ducts extending from the electron column and one from the sample chamber which are interconnected at a diffusion pump. Similarly, in Japanese patent application laid-open number 55-136446, there are again a plurality of ducts leading from the electron column. However, the presence of such ducts causes a number of problems. The first problem is that, if a good vacuum is to be achieved within the electron column, it is necessary that the ducts be relatively large, so that they have satisfactory conductance. The size of the ducts is inconvenient, and limits the positioning of additional devices which may be heeded around the electron column, such as an X-ray system. A further problem associated with the presence of the ducts to the vacuum pump is that their presence means that apertures must be made in any magnetic shielding around the electron column. Stray magnetic fields may cause magnetic disturbance, and if this is allowed to affect the electron beam, the accuracy of the operation of the microscope may be compromised. For example, a magnetic disturbance due to e.g. noise may cause a ripple on the image produced by the electron microscope, thereby blurring that image. Therefore, in order to limit magnetic disturbance. it is known to place a shielding cylinder around the electron column, but it is necessary that apertures be made in that shielding to permit the ducting to the vacuum pump to emerge. Bearing in mind that that ducting needs to be large in order to achieve a good vacuum, there is thus a conflict between the need for a good vacuum and the need for good magnetic shielding. SUMMARY OF THE INVENTION The present invention therefore seeks to overcome, or at least ameliorate, these problems. The present invention proposes that the ducting to the electron column which has been used in the prior art be dispensed with, and that the evacuation path of the electron gun chamber and/or casing containing the electron lens assembly, be defined within the casing itself. At first sight, the beam path may be thought of as a suitable route for that evacuation path, but in fact that beam path will normally include a plurality of restricted apertures whose purpose is to collimate the beam, and therefore those small apertures would restrict the conductance of any evacuation path running along the beam path. Therefore, in the present invention, that evacuation path is off-axis relative to the beam. In the present invention, the casing containing the electron lens assembly may be made from a closed enclosure, together with the electron gun chamber and the sample chamber. Then, by connecting suitable evacuation means in the form of e.g., a vacuum pump to the sample chamber, the whole interior can be evacuated. Furthermore, since the ducting outside the electron column is dispensed with, it becomes possible for the magnetic shielding to be unbroken, except possibly where the electron microscope has an adjustable aperture (which is normally positioned between the condenser lenses and the deflection coils) since the control for that adjustable aperture has to project through the magnetic shielding. The magnetic shielding may simply extend along the casing, and be sealed to the outside of the sample chamber, but preferably there is a further shielding part which encloses the electron gun chamber. Of course, if the evacuation path is defined within the casing, its size needs to be sufficiently large to permit a high conductance. However, in practice it is found that suitable apertures between the electron gun chamber and the casing, adjacent the condenser lenses, and between the casing and the sample chamber permit a sufficiently high conductance to be achieved. Indeed, it is possible with the present invention to achieve a higher conductance than was normally achieved in standard arrangements using external ducting. Japanese patent application laid-open number 59-209045 apparently shows an arrangement in which there is no external ducting. However, the device shown in that disclosure is an electron analysing apparatus which involves the detection of Auger electrons, and therefore must operate under extremely high vacuum, much higher than is normally used in electron microscopes such as scanning electron microscopes or transmission electron microscopes. Therefore, one skilled in the art reading Japanese patent application laid-open number 59-209045 will immediately appreciate that what is illustrated is wholly schematic, and that additional ducting would be necessary in order to achieve the desired level of vacuum. Indeed, this can be seen from the fact that the arrangement is illustrated with the condenser lenses apparently directly contacting the vacuum, which would result in contamination and would not operate satisfactorily. In the present invention, the electron lenses should be sealed from the vacuum. To do this, a development of the present invention proposes that those electron lenses be mounted in sealed modules, and a plurality of such modules be positioned within the casing of the electron. A bore extends through the centre of those modules, to define the electron beam path, but by suitable design of the modules, the evacuation path may be between the outer wall of those modules and the inner wall of the casing. If the electron lens assembly is formed in this way, forming a plurality of modules, the modules may be spaced by suitable spacers. It is desirable that those spacers have apertures therein so that they may thus form part of the evacuation path. If the evacuation path extends around the condenser lenses in the way described above, then the main limit on conductance from the electron gun chamber to the specimen chamber is at the mounting of the electron gun chamber to the casing, and the mounting of the casing to the specimen chamber. At these points, a plurality of apertures are usually necessary, and, particularly between the electron gun chamber and the casing, this may be achieved by providing a tie member in the form of a plate with a plurality of apertures therein.