It has been found that a process-design interaction involving very large capacitor arrays is capable of causing voiding in well-formed copper trenches. This voiding is believed to be caused by the electrochemical dissolution of copper driven by the photovoltaic effect realized at p-n junctions. A series of experiments was performed to reproduce this voiding in a laboratory environment using water and exposure to light or dark. The experiments were performed on two different size capacitor arrays. In addition, similar experiments were performed on a large capacitor array that uses the opposite dopant isolation scheme. Conversely, copper dendrite growth was observed at the features of interest instead of dissolution, even under flowing de-ionized water (DI) conditions. The results are discussed in the context of known copper-water electrochemical equilibria and design implications are discussed.

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.