Abstract
We present the first known images acquired using near-field scanning optical microscopy (NSOM) through backside silicon on functional integrated circuit samples with higher resolution than conventional fault isolation (FI) tools. NSOM offers the possibility of substantially-improved lateral resolution independent of excitation wavelength. Current FI techniques have challenged the resolution limits of conventional optics technology, even in the best solid immersion lens (SIL) to date. This poses a problem for future process technology nodes. This resolution barrier is a by-product of the diffraction limit. In Fourier terms, a conventional lens filters out highfrequency information and thus limits the resolution. In NSOM, by placing a tip with an aperture in extreme proximity to the surface it is possible to capture the near-field light that contains high-frequency information, thereby circumventing the diffraction limit. The tangible benefit is that the resolution is substantially improved. We show that NSOM can be used in backside subsurface imaging of silicon, mirroring the paradigm used in typical optical FI. We present optical reflectance data through ~100 nm of remaining backside Si on functional 22 nm CMOS IC parts with lateral resolution approaching 100 nm. We then discuss potential methods for using NSOM in practical backside fault isolation applications and for improving signal-to-noise ratio (SNR).
Depolarization near-field scanning optical microscopy: influence of wavelength and tip shape on the lateral resolution