IT-4-O-1411 Influence of the work-function changes on the contrast of images in SEM.

In the present contribution the role of the local change of the work function on some contrasts in SEM is suggested and illustrated. When electrons, BSEs or SEs, escape from the sample they are partly refracted at the sample/vacuum interface. The refraction effect is given by [1]: √E_S sin β= √E_k sin α (1) with E_S: inner kinetic energy (referred to the bottom of the conduction band); β: inner incident angle to the normal; E_k: kinetic energy into the vacuum (referred to the vacuum level); α: emission angle into the vacuum. The relationship between E_S and E_k obeys to E_S = E_k+E_F+φ (2a) for metals of  work function φ and  Fermi energy E_F and to E_S = E_k+χ (2b) for semiconductors or insulators of affinity χ. In addition, the transmission probability of the escaping electrons, T(α), differs from 100%. Then a local change of φ or χ with the crystalline orientation or oxidation or contamination will change the SE or BSE yields δ or η.

Fig. 1a shows the calculated change of T(α) when χ changes from 4.05 eV (Si; Ge; SiC) to 4.55 eV for SEs of an initial inner kinetic energy of E_S= 5.05 eV. Fig. 1b shows the corresponding distortion of the spectral distribution of the emitted SEs, ∂δ/∂Ek, when χ changes by steps of 0.2 eV and the corresponding change in δ, from 100% to 69%, is indicated in caption. Performed for the calculated dependence of few-layer graphene on SiC [1] this type of evaluation applies also  to contamination effects on synthetic diamond:Fig. 2a.

Fig. 2b shows the contrast of dendritic SiGe crystals embedded in a SiGe amorphous matrix[2]. Such a contrast may be explained from the local change of χ between the two crystalline forms of SiGe.

The same analysis applies to the angular selective detection of the BSEs where the refraction effects increase with the detection angle α –Eq. (1)-.This point should be considered for the interpretation of Fig. 3 for two different Fe grains, A and B [3].

The same analysis may be transposed to the reflectivity of Very Low Energy Electrons, R(α)=1-T(α) [4] , a reflectivity changing rapidly when the incident beam energy, E°, varies from 1 to 10 eV: a point fairly illustrated by Frank et al.[5].

In conclusion, some material and crystalline contrasts reported in SEM using SE or BSE detection as well as in LVSEM may be explained from the local change of the work function or the electron affinity.

1. J. Cazaux Appl. Phys. Lett. 98 (2011) p 013109 1

2. M. Itakura, N. Kuwano, K. Sato and S. Tachibana, Journal of Electron Microscopy 5 (2010) pS165

3. J. Cazaux , N. Kuwano and K. Sato, Ultramicroscopy, 135 (2013) 43

4. J. Cazaux J. Appl. Phys. 111 (2012) DOI: .1063/1.3691956

5. L. Frank, S. Mikmekova, M. Hovarka, Z Pokorna and I. Müllerova; Proceed. 15 th European Microscopy Congress, Manchester, (2012) PS2.2