Three-dimensional subsurface imaging through the back side of a silicon flip chip is reported with a diffraction-limited lateral resolution of 166nm and an axial performance capable of resolving features only 100nm deep. This performance was achieved by implementing sample-scanned two-photon optical beam induced current microscopy using a silicon solid immersion lens and a peak detection algorithm. The excitation source was a 1530nm erbium:fiber laser, and the lateral optical resolution obtained corresponds to 11% of the free-space wavelength.
We present experimental evidence for the resolution-enhancing effect of an annular pupil-plane aperture when performing nonlinear imaging in the vectorial-focusing regime through manipulation of the focal spot geometry. By acquiring two-photon optical beam-induced current images of a silicon integrated-circuit using solid-immersion-lens microscopy at 1550 nm we achieved 70 nm resolution. This result demonstrates a reduction in the minimum effective focal spot diameter of 36%. In addition, the annular-aperture-induced extension of the depth-of-focus causes an observable decrease in the depth contrast of the resulting image and we explain the origins of this using a simulation of the imaging process.
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