Abstract

An n-well contrast enhancement process, which is used for backside navigation has been further developed to observe dopant regions on top-down prepared devices. Dopant regions for the following structures were delineated: ▪ p-active regions in n-wells; ▪ n-active regions in p-wells; ▪ n-well to p-well interface; ▪ p+/p interface. The enhancement process to obtain this contrast was done on a Credence OptiFIB using a coaxial FIB/optical column. Illumination of the sample is a critical component of the process. First, the high contrast level obtained at a p/n junction was only achieved by using light assisted FIB oxide deposition. Second, sample illumination was found to enhance contrast while imaging the region of interest using the focused ion beam. The contrast becomes permanent only after the oxide is deposited. Contrary to chemical etch techniques imaged in a SEM, this enhancement process as well as the imaging process were completed in-situ. Further, this process proved to be very reproducible. This dopant contrast enhancement process provides an alternative to wet chemical delineation methods for the verification of dopant photomask registration, for accomplishing reverse engineering and for observing missing implants.

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