Versatile Monochromator Module for XHR SEM

Abstract

Low voltage SEM image resolution is predominantly limited by chromatic aberration. In order to strongly reduce this effect, FEI has incorporated a monochromator in its latest XHR SEM system (called the Magellan). This enables the reduction of energy spread to 0.15 eV FWHM. To illustrate the potential of a monochromator for low voltage SEM, figure 1 shows the probe size as a function of energy spread, for accelerating voltages U = 0.5 and 1 kV. Sub-nm resolution Magellan images at low voltage have revealed unprecedented information, such as fine surface details and nanoparticle distributions, without any sample preparation [1].An electrostatic Schottky-FEG module has been developed that can provide a monochromatized beam for XHR imaging, and which can also provide large probe currents for analysis. The three modes of operation of this module are shown in figure 2. The extractor contains a first aperture plane with two apertures, defining an axial and offaxial beam. An electrode below the extractor serves as gun lens. This electrode is segmented, so it can also be used for deflection and stigmation. The second aperture plane has an axial aperture and a small, off-axial slit [2]. The latter is used to monochromatize the off-axial beam. A deflector below the second aperture plane is used to direct either the axial or off-axial beam into the column, where a final beam limiting aperture is used to determine the probe current. Unwanted electrons are blocked.In the first mode of operation, the axial beam is used with gun lens off. The axial aperture in the second aperture plane limits the beam current. In the second mode of operation, the gun lens is turned on to deliver more beam current, which can go up to 22 nA. The third mode (called UC mode) uses the off-axial monochromatized beam. Voltages on the gun lens segments are used to position, focus and stigmate the off-axial beam on the energy selecting slit. The beam is dispersed due to the off-axial traversal through the strong gun lens. The spherical aberration of the gun lens results in coma in the off-axial beam. However, the effect of this coma on the energy spread is negligible for useful probe currents, i.e. < 100 pA. For larger probe currents, the loss of brightness becomes more important than the reduction in energy spread.

Microstructure of Bentonite in Iron Ore Green Pellets

“…In this instrument, high resolution imaging by the through-the-lens detector is partly achieved by the application of a strong immersion magnetic field onto the sample to reduce focal length and thus chromatic aberration (Young et al, 2008). In addition, the column is provided with a monochromator-like device, which reduces the energy spread of the electrons emitted by the Schottky source to <0.2 eV (Henstra et al, 2009). In addition to the magnetic immersion field and monochromatic beam, deceleration was used to reduce the landing energy of the electrons onto the samples by applying a bias voltage to the sample holder.…”

A Novel Monochromator with Double Cylindrical Lenses

“…This MC is lateral dispersion-free, (x|κ) =0, at the exit plane Z 4 , as shown in Figure 3. Therefore, this MC is better than single-stage MCs [2,3,9]. It can prevent rainbow illumination, which can cause limitations in the TEM operation conditions.…”

“…With regard to the corrected ranks of aberrations, a MC with CLs can be classified into the same category as MCs with multiple Wien filters [11][12][13]. This is worse than sophisticated MCs [1,10] but better than single-stage MCs [2,3,9]. With the loss of its simplicity, this MC can be improved further by canceling aberrations with additional correctors.…”

A new monochromator with multiple offset cylindrical lenses

“…3. The use of the central part of the beams from the electron source In previous MCs using a modified condenser lens, beams from the off-axis region of the electron emitter are used [23,24]. By using the offset CLs on the emitter axis, this MC enables the use of the central part of the beams from the emitter.…”

“…As applications of MCs to SEM after Troyon's proposal [5], estimations of the probe sizes with a reduction of the chromatic terms and an improvement in the size in the low-energy region have been discussed based on D50 methods [22]. Later, an MC with a modified condenser lens was proposed and commercialized [23,24]. It also includes two off-axis apertures and a deflector.…”