Nanometric three-dimensional sub-surface imaging of a silicon flip-chip

In an attempt to improve the resolution of optical microscopy of semiconductor devices, it is possible to use a Weierstrass solid immersion lens (SIL). A suitable SIL eliminates spherical aberration due to the refractive-index mismatch at the silicomair interface and increases the numerical aperture (NA) of the system, decreasing the focal spot size. An increase in the spatial sampling frequency also results because physical translations of the SIL and sample result in a smaller movement of the focused spot. It is known that in the lateral direction the reduction factor equals the square of the refractive index, 12.1 in silicon. In the axial direction the scaling factor is around 75. We report here 2D imaging with an improved resolution of 166nm and 3D sub-surface imaging with approximately 100nm resolution.