Practical Electron Microscopy and Database

Figure 4839a shows the schematic of low vacuum or environmental SEM systems. The primary electrons (PEs) from the electron gun enter the chamber from the high vacuum column via one or more pressure limiting aperture [1]. PEs scattered by gas molecules form a relatively delocalized electron ‘‘skirt’’ around the unscattered component of the electron beam [2-3].

Figure 4839a. Schematic of low vacuum or environmental SEM systems.

Figure 4839b gives the schematic of a low vacuum SEM chamber. The PEs and electrons ejected from a sample are amplified in a gas ionization cascade produced by placing a positively biased electrode Ve (typically 50 - 600 V) into the sample chamber [4].  The net electron current arriving at the electrode of the GSED (gaseous secondary electron detector) ring is given by [5]

              I_GSED = I_b + [δ(1-Ω)g_SE + ηg_BSE](1-s)I_PE ------------------------------------------------------ [4839]

              I_PE -- the primary beam current
              I_b -- a background offset generated by gas amplification of PEs and electrons ejected from the sample by skirt electrons
              s -- the fraction of PEs that form the skirt
              δ -- the secondary electron emission yield
              η -- the backscattered electron emission yield
              g_SE -- the secondary electron gas amplification factor
              g_BSE -- the backscattered electron gas amplification factor
              Ω -- the SE-ion recombination probability
              g -- the gas gain as a function of the electric field in the space between the sample and the electrode (detector)

Figure 4839b. Schematic of a low vacuum SEM chamber. BSE = backscattered electron, SE = secondary electron, ESE = environmental SE [6]

[1] G.D. Danilatos, Adv. Electron. Electron Phys. 71 (1988) 109.
[2] D.A. Moncrieff, P.R. Barker, V.N.E. Robinson, J. Phys. D 12 (1979) 481.
[3] A.N. Farley, J.S. Shah, J. Microsc. 158 (1990) 379.
[4] G.D. Danilatos, Adv. Electron. Electron Phys. 78 (1990) 1.
[5] M. Toth, M.R. Phillips, B.L. Thiel, A.M. Donald, J. Appl. Phys. 91 (2002) 4479.
[6] M. Toth, B.L. Thiel, A.M. Donald, Ultramicroscopy 94 (2003) 71–87.