Using water structuring by ions to address pattern loading in wet HF-based oxide recess etch

In the FinFET fabrication flow, a gap fill oxide is used to fill the space between the Si fins that needs to be recessed to define the height of the fin trench. However, with the used flowable chemical vapor deposition (FCVD) oxide, the eth rate depends on fin spacing, a phenomenon called pattern loading. In this work, we used water structuring by dissolved ions to decrease the etch rate between fins and control pattern loading. We first showed, using ATR-FTIR (Attenuated Total Reflection – Fourier Transform Infra-Red), that structure making and breaking ions proposed by Marcus (2009) for bulk solutions, could be used to modulate water structuring in nanotrenches. Then, the Tetra-propyl Ammonium cation (TPA+) and the triply ionized Phosphoric acid anion (PO43−) were selected as structure making additives in HF 0.5%. Optimization of the etchant composition allowed to suppress pattern loading for a gap of 15 nm width using TPAH:HF mixtures, but not using HF:H3PO4 mixtures. The etch rate obtained with TPAH:HF at unity pattern loading and at room temperature was about 4.7 nm/min. It could be improved to about 7 nm/min by increasing the temperature. The inefficacy of the Phosphoric acid was ascribed to a pH effect, as the mono-ionized Phosphoric acid species formed in HF was classified as a borderline ion with little structuring capability by Marcus.