Stereo Scan 440 - DCU

3-10 Chapter 3 Advanced Operation
FIgUre 3.5 BrightIDark Areas Due to Charging
Specimen charging of non or badly conducting specimens is dependent on the
charge equilibrium inside the sample. The secondary electron yield (S) is defmed
as the number of secondary electrons which leave the sample for one incoming
primary electron. The corresponding number for backscattered electrons is the
backscatter coefficient (B). If the total electron yield T=B+S equals I, the charge
balance is zero and therefore no charging occurs. If T increases above I, the
sample should charge up positively because more electrons leave the sample than
primary electrons enter it. If T falls below I, the sample will charge up
negatively. One of the most important parameters for changing the total electron
yield T is the primary beam energy. This is shown in Figure 3.7 which shows the
typical dependence of T on the primary beam energy Eo. If the primary beam =
Ec2' then no specimen charging occurs. Typically, Ec2 lies between 500e V and
2KeV.
Stereoscan 440 Operator Manual