SUB-SURFACE OBJECT CHARACTERIZATION USING BACKSCATTERED ELECTRONS
Due to the increased 3-D complexity of microelectronic devices, one of the main goals of modern metrology is critical dimension measurements of sub-surface features, without using destructive techniques, i.e. techniques which can be employed in the production process. The information backscattered electrons (BSEs) can provide depends on the escape depth of BSEs from the material and can be in the range of microns. Understanding the collected BSE signal means understanding the mechanism of interaction between the incident electron beam and the material, the volume of interaction itself, and interaction between the BSE signal during the course of its exit from the sample.
By obtaining detailed information on the BSEs, we can extract the depth, thickness and lateral resolution of sub-surface objects. The first approach is by measuring the energy of electrons and constructing their spectrum which exhibits a peak close the primary energy of electrons. By increasing the depth of the sub-surface object, the peak position and intensity of the spectrum will shift since electrons will have a longer path in the sample, and thus lose more energy.
The second approach is by measuring the angular distribution of BSEs. As electrons reach greater depths inside the sample, the probability that they reach the surface is higher for shorter path lengths, thus the peak position of the angular distribution shifts toward smaller angles (closer to the incident beam direction). As such, measuring the shift in the peak position of angular distribution allows us also to measure depth.
The calculation of the peak position of the energetic spectrum obtained from simulations is shown in figure 1. There is approximately 8 eV peak shift for 1 nm depth. The results show that both depth and thickness of sub-surface objects can be measured simultaneously.
Figure 1: Simulation of BSEs spectrum for a gold wire embedded in silica at different depths.