| Summary: | Following the trend towards transistor miniaturization and power efficiency in Integrated Circuits (ICs), the concurrent miniaturization of possible defects challenges Failure Analysis (FA) engineers to accurately deprocess ICs with great precision before accessing the defects. As such, etch uniformity is crucial to avoid accidental removal of defects. However, in the context of this research, thickness differences between the passivation layer, consisting of silicon nitride stacked above silicon dioxide, above and between metal lines inhibit the pursuit for uniform etch profiles.
Hence, this research aims to develop a SF6/O2 etch recipe, using the Inductively Coupled Plasma Reactive Ion Etching (ICP RIE), with a high silicon nitride to silicon dioxide selectivity. By maximizing the etch selectivity, the objective is to overcome the thickness difference of the passivation layer (~400 nm) and minimize the extent of over etch as much as possible, whilst ensuring global uniformity.
Upon formulation of the SF6/O2 recipe, an investigation regarding the presence of microloading effects as well as development of techniques to overcome edge effects was carried out to address possible sources of etch non-uniformities. Following that, three different etching techniques were being evaluated using the Field Emission Scanning Electron Microscope (FESEM) and Focused Ion Beam-Scanning Electron Microscope (FIB-SEM).
By analysing the captured SEM images, the extent of over etch was measured and the technique involving a transition from low bias (silicon nitride removal) to high bias (silicon dioxide removal) yielded the best results due to an increase in etch selectivity from 3.5 to 8.7. The over etch was measured to be ~210 nm compared to a likely over etch of ~400 nm if an etch recipe with minimal selectivity was utilized.
However, microtrenching effects have led to a pronounced etch rate at the corners of trenches, leading to those regions experiencing an over etch of ~340 nm. As such, future works may look into promoting more isotropic etch profiles to reduce the presence of slanted etch profiles which often results in microtrenching. Furthermore, despite improvements in the extent of over etch, ~210 nm is still not ideal for Physical FA (PFA) studies and future works may explore adding N2 gas into the recipe to further promote the nitride to oxide selectivity.
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