N-wells voltage contrast imaging with a Focused Ion Beam

Abstract

Using a focused ion beam (FIB), secondary electron (SE) imaging of n-wells under oxide from the backside of thinned integrated circuits without electrical bias was accomplished. From the backside, the n-wells were initially observed at a remaining silicon thickness ∼4.5μm, which correlates to the actual implant depth where n and p carrier concentrations are equal. When the wells were FIB imaged, contrast appeared dark relative to the p substrate. During deposition of the oxide film, the n-well brightness changed from dark relative to the p-substrate, to bright. It appears that initially during this deposition step the interaction volume of the beam reached the silicon/oxide interface to create tunneling electrons. This phenomenon dominated the capacitive effect. Then as the film thickness increased the capacitive effect prevailed. The imaging structure is analogous to a Metal-Oxide-Semiconductor (MOS) capacitor. The n- and p-MOS capacitive properties yielded a permanent imaging contrast. At an optimized oxide thickness (130nm), the n-wells appear white relative to the p-substrate with a contrast up to 85% {(Ip-substrate − In-wells)/(Ipsubstrate + In-wells)}.