How the electron beam is specified by means of the graphical user interface (GUI) is described in Chapter 5. In this chapter it is only referred to the actual input files which are read by SESAME during program execution.
Figure 6.1 shows the main features of the electron beam. A beam divergence DIVERG and a spot-size SPSIZE may be specified (see Chapter 9.3). The spot size relates to the plane of focus, which may be specified using the ZFOCUS variable in the &EBEAM namelist (see Chapter 9.3).
Table 6.1: Variables needed for the specification of the electron beam
The electron density of the electron beam in the beam focus plane is Gaussian distributed over the spot size. The directions of the individual electrons are equally distributed in the cone stretched out over the maximum divergence. The electron beam may also show slight deviations in electron energy of the individual electrons. This can be specified by ESTD, the standard deviation of electron energy, in the &EBEAM namelist (see Chapter 9.3). The effective initial energy of individual electrons is also Gaussian distributed, with the standard deviation ESTD, around E0, the adjusted initial electron energy.
The default values for the beam divergence, the spot size, and the standard deviation in electron energy is zero. The default value for the plane of focus is the sample surface at the lateral beam position specified by the XESPOT (see Figure 6.4 and Chapter 9.6) variable if the beam is not scanned and is in midposition between the maximum and minimum z-coordinate of the sample surface in case of scanning.
The direction of the electron beam relative to the sample can be specified using the EBTILT and the EBROT variables (see Chapter 9.6 and Figures 6.2 and 6.3). The default value for both the tilt and the rotation of the sample is zero.
The total number of electrons which should be simulated must be specified ( Note: No default value exists!) by means of the NETRN variable in the &ETRNS namelist (see Chapter 9.2).
NETRN is a critical parameter for the simulation since a compromise between short simulation time and useful statistics of the output quantities must be achieved. For further information concerning the statistical variation in simulation data see Chapter 7.