Simulation of the backscattered electron intensity of multi layer structure for the explanation of secondary electron contrast

The intensities of the secondary electrons (SE) and of the backscattered electrons (BSE) at energy 100 eV have been measured on a Ni/C/Ni/C/Ni/C/(Si substrate) multilayer structure by exciting it with primary electrons of 5, 2.5 and 1.25 keV energies. It has been found that both intensities similarly vary while thinning the specimen. The difference as small as 4 nm in the underlying layer thicknesses resulted in visible intensity change. Utilizing this intensity change, the thickness difference of neighboring regions could be revealed from the SE image. No simple phenomenological model was found to interpret the change of intensity, thus the intensity of the BSE electrons has been calculated by means of a newly developed Monte Carlo simulation. This code also considers the secondary electron generation and transport through the solid. The calculated and measured intensities agree well supporting the validity of the model.