Recently developed scanning electron microscopes (SEM) are equipped by sophisticated detection systems, which offer very effective energy and angular separation of the signal electrons and extraordinary detection flexibility. The signal electrons can be collected by various types of detectors and character of the detected signal is possible to affect by many parameters (e. g. optical configuration of the column, detection geometry, presence of the specimen bias, etc.). Understanding of the detected signal origin and correct interpretation of the micrographs become very difficult, which hampers utilizing of full potential of modern SEMs.

Experiments have been performed with a novel Trinity detection system (Scios, FEI Comp.) consisting of three in-lens detectors:  the T1 and the T2 detectors located inside the final lens and the T3 detector situated inside the column just below the aperture strip (Fig. 1). The instrument is also equipped by a standard E-T detector (ETD) situated in conventional position. There is a possibility of simultaneous detection of all 4 images (i.e. T1, T2, T3 and ETD) and different type of information about the specimen can be achieved at the same time.

Oxide inclusions embedded in a conventional steel was used as an experimental material, which secures presence of the topographic, material and crystal orientation contrast in the micrographs. Moreover, the inclusions become charged by the electron beam irradiation and the influence of charging on the micrographs collected by the Trinity detectors can be observed.

There are many possibilities how to affect the detected signal origin. Fig. 2 demonstrates effect of the specimen bias on detected signal. The SEs are shared by the T3 and T2 detectors and are not detected by the ETD when the specimen bias of -4kV is applied. Strong collimation of the signal electrons towards the optical axis is evident. The high-angle BSEs are collimated towards the optical axis and the T1 detector shows topographical contrast.

Significant effect of a working distance (WD) on the signal collected by the Trinity detectors and the ETD is shown in Fig. 3. For a short WD, the T3 detector collects mainly the slow secondary electrons (SEs) and positive charging of the spinel inclusions is clearly visible. For a long WD, the electrons originally detected by the T3 detector are shifted towards the T2. The T1 detector collects the backscattered electrons (BSEs) and the channeling and topographical contrast are superimposed on the material (“Z”) contrast at short WD. Inversely, the material contrast intensifies with increasing WD. Obviously, increasing WD leads to less effective collimation of the slow signal electrons into the final lens (by the A-tube electrostatic field) and the ETD detection efficiency was improved.

Insight into an extraordinary detection flexibility of the Trinity system enables us more effective characterization of material microstructure. Accurate knowledge about the signal received at each detector and possibility of its modification can be successfully used for tuning of desired contrast or suppression of undesirable information.

The presentation is based on results obtained from pioneering project commissioned by the New Energy and industrial Technology Development Organization (NEDO).

Figures:

Fig. 1 Schematic sketch of the Trinity system.

Fig. 2 Oxide inclusions embedded in steel visualized by the Trinity detectors and the ETD: specimen bias 0 kV and –4 kV. (EL= 1 keV, WD = 1 mm, A-tube bias = 8 kV)

Fig. 3 Effect of the WD on the contrast in the images simultaneously obtained at 1 keV by the Trinity detectors and the ETD. (A-tube bias 8 kV, SB 0 kV)

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EMC Abstracts - https://emc-proceedings.com/abstract/benefits-of-angular-and-energy-separation-of-slow-signal-electrons-in-sem/