Three-dimensional nanometric sub-surface imaging of a silicon flip-chip using the two-photon optical beam induced current method

Two- and three-dimensional sub-surface optical beam induced current imaging of a silicon flip-chip is described and is illustrated by results corresponding to 166 nm lateral resolution and an axial performance capable of localising feature depths to around 100 nm accuracy. The experimental results are compared with theoretically modelled performance based on analytic expressions for the system point spread functions valid for high numerical apertures, and are interpreted using numerical geometric ray tracing calculations. Examples of depth-resolved feature profiling are presented and include depth cross-sections through a matrix of tungsten vias and a depth-resolved image of part of a poly-silicon wire.