This invention relates to magnetic immersion lenses.
In particular, the invention relates to a magnetic immersion lens of the type comprising inner and outer pole-pieces arranged symmetrically about a longitudinal axis of the lens, the inner pole-piece having a through-bore. In use, a specimen is positioned in front of the tip end of the inner pole-piece at a suitable working distance, typically about 2 mm. An example of an inverted magnetic immersion lens is shown in FIG. 1.
In one application, the magnetic immersion lens forms part of an electron microscope, such as a scanning electron microscope. In this case, the magnetic imaging field of the lens constrains a beam of primary electrons to follow an axial trajectory along the through-bore and focuses the beam onto the specimen. Secondary electrons emitted from the specimen surface are guided into the through-bore by the magnetic imaging field and are detected by means of a secondary electron detection arrangement located within the through-bore. Due to the limited availability of space within the through-bore the detection arrangement needs to be of compact design, and this presents a significant technical problem.
Furthermore, a detection arrangement using electrostatic deflection fields may distort the magnetic imaging field of the lens causing, inter alia, a misalignment of the primary electron beam and a consequent deterioration in the electron optical properties of the microscope. This problem is particularly acute in the case of low voltage microscopy, particularly when the energy of the primary electrons is 1 keV or less.
A known, within-the-lens, secondary electron detection arrangement has the form of a Wein energy filter and is described by M. Sato, H. Todokoro, K. Kageyamaxe2x80x94xe2x80x9cA snorkel type conical objective lens with E cross B fields for detecting secondary electronsxe2x80x9d SPIE, Vol 2014, Charged-Particle Optics (1993).
This detection arrangement has electrostatic deflectors for producing an electrostatic field. in a plane perpendicular to the axis of the primary electron beam which is used for the extraction of secondary electrons. The detection arrangement also has coils for producing a magnetic field perpendicular to, and in the same plane as, the electrostatic field. The strength and direction of the magnetic field are such that it compensates for the force exerted on the primary electron beam by the electrostatic field. Thus, there is no appreciable misalignment of the primary electron beam, even at energies as low as 1 keV. Nevertheless, the detection arrangement has a complex structure and is difficult to adjust to obtain the optimum result.
According to the present invention there is provided a magnetic immersion lens comprising inner and outer pole-pieces arranged symmetrically about a longitudinal axis of the lens, the inner pole-piece having a through-bore and the lens producing a magnetic imaging field for directing, along said through-bore, charged particles emitted from a specimen positioned in front of the inner pole-piece, and an axialiv-symmetric, charged-particle detection arrangement located within the through-bore for detecting charged particles being directed along the through-bore by the magnetic imaging field, the charged-particle detection arrangement comprising repeller means for deflecting said charged particles away from the longitudinal axis and detector means for detecting charged particles deflected by the repeller means.