Depth Resolution and Surface Sensitivity with the Multiple Detection System of a
HR‐SEM
Authors: Gernert Ulrich , Berger Dirk 
Published: December 2016

State‐of‐the‐art high‐resolution scanning electron microscopes (HR‐SEMs) attain a lateral resolution of about 1 nm. To obtain this high resolution even at low landing energies, a deceleration voltage is applied to the sample while the beam has a higher energy inside the electron column to minimize aberrations. The excellent lateral resolution is one key property of HR‐SEM images, but the quality of the obtained contrast and its relation to the sample properties have to be considered as well. It is obvious that an electric field above the specimen changes direction and energy of the detected electrons and thus the detected contrast. The purpose of this contribution is to investigate depth resolution and surface sensitivity obtained with the multiple detection system of the Hitachi SU8030 SEM. Fig. 1 illustrates a typical application requiring an excellent surface sensitivity to image the 50 nm thin wall of sheath forming bacteria at high lateral resolution.

As a test specimen to characterize the depth resolution we used glassy carbon, which was structured by an ion beam (FEI Helios NanoLab600). Three 5 µm × 15 µm trenches with a depth of 1 µm were milled and filled with platinum. On top of these structures and the contiguous glassy carbon, Pt‐layers with thicknesses of 12, 25 and 50 nm were deposited [Fig. 2]. For the evaluation of the depth information the following detectors were used: The standard inlens detector (upper), a second inlens detector (top) that is positioned above the upper detector, and a retractable photo diode backscattered electron detector (PDBSE). All the detectors were used both, in the normal imaging mode (specimen grounded) and in the deceleration mode (specimen applied with variable potential). The traditional chamber detector has been left out, because there is no possibility to control the ratio of the SE‐ and BSE‐signal with the Hitachi SU8030.

In normal mode, the upper detector receives a pure signal of secondary electrons [Fig. 2a]. The information depth depends on the accelerating voltage only. However, it is complicated to interpret the resulting contrast because topography, edge effects, charging, and potential differences affect the result. The top detector in normal mode [Fig. 2b,c] gives a high angle backscattered electron signal. Again, the information depth depends on the energy of the primary beam. As the amorphous test specimen did not produce channeling effects, the arising contrast is solely a result of the penetration depth of the primary electrons. If the annular control electrode located inside the objective lens is set to negative bias, the SE signal is filtered out and the upper detector also detects BSE. Because these are low angle BSE, the resulting contrast shows a mixture of topographic and depth information [Fig. 2d]. The PDBSE detector operated in the composition mode with the sum signal of four symmetric segments gives a pure BSE signal. Only the acceleration voltage is responsible for the information depth [Fig. 3a–d].

In the deceleration mode, the PDBSE and the upper detector deliver a proper depth information. However, the SE leaving the sample are accelerated towards the detector as well, and thus the result is a mixed SE‐BSE signal, and the depth information is superimposed by topographic, edge effect, potential, and material contrast. The top detector in the deceleration mode provides a pure SE signal with an excellent surface sensitivity, but no depth information at all.

Therefore, the most suitable way to get surface and depth information is using the PDBSE detector in composition mode with the sample set to ground potential. At 1 kV acceleration voltage there is no difference in signal intensity between the Pt‐area with 1 µm in thickness and the region with the 10 nm Pt‐layer on carbon. This means that the information depth is not more than about 10 nm [Fig. 3a]. At 3 kV the 50 nm Pt‐layer on carbon has the same contrast compared to bulk Pt, therefore the information depth is about 50 nm [Fig. 3b]. When using 4 and 5 kV there is a contrast visible even between the 1 µm thickness area of platinum and the 50 nm Pt‐coating. In this case, the depth of the volume of emitted BSE exceeds the 50 nm Pt‐layer [Fig. 3c,d].

In conclusion, it is recommended to use a low angle backscatter detector in composition mode and a grounded stage since other sample properties, like topography, edge effect, charging and potential do not affect the contrast and a pure and tunable depth information is obtained.