To realize scaled, high-density and high- performance VLSI devices, such as memory, logic and imager devices, the isolation process is a key processing technique to reduce current leakage and/or optical cross-talk. One of the most commonly used techniques for isolation in high density integrated circuits is the local oxidation of silicon (LOCOS). LOCOS not only results in large encroachment of field oxide into the device active regions but also requires additional processing steps. Recently, enormous amount of research is being carried out to fill shallow trenches with polymers to prevent extra processing steps. The major challenge that industry is facing with this approach is voiding, delamination and cracks at the polymer- trench interface. Conventional techniques for characterization of voids and stresses in narrower trenches include wet chemical etching and electrical testing, which is spatially insensitive and it requires contact to the wafer. In the case of via chains, several metal levels must be fabricated before the electrical test can be completed.

To demonstrate the efficacy of SNFUH in detecting buried defects/ voids in such kind of shallow trenches, a model test sample was fabricated. Figure XXA shows schematic of series of isolated shallow trench structures having polymeric coating on top of them. Trenches are etched in SOG (spin-on-dielectric) with a 50 nm thin layer of LPCVD Si3N4 as a capping layer and then SiN is etched down in the trenches using the wet processing. Trenches were 1 !m deep. A 500 nm thick layer of polymer [Benzocyclobutene (BCB)] was spin-coated followed by thermal annealing for curing polymer. Some of the potential defect problems in such structures include the formation of voids, delamination and hardening at the interfaces.

Figure XXB depicts the conventional topography image, while Figure XXC is the corresponding (simultaneously recorded) spectacular SNFUH phase image. The typical 2 x 2 !m2 topography scan shows uniform and contiguous polymeric coating on SiN and inside the trenches. However, the corresponding SNFUH phase image shown in Figure XXC reveals phase contrast reminiscent of buried voiding at the interface of the SiN and polymer. The dark contrast in the phase image on polymer coated SiN lines corresponds to voids at polymer-SiN interface i.e. voiding underneath the contact and trench wall, which undergoes a distinct viscoelastic response. Interestingly, a hardening of the polymer in the trench and its sidewall is also evident in the phase image, which results from its thermal annealing and may be poor adhesion with SOG.

These images clearly demonstrate the reliability of this method not only as a microscopy technique but also as an extremely sensitive probe of the mechanical reliability of the interfaces.