40nm BSI CMOS Image SensorPreview

iers to connect external terminals to internal routing [A2]. The alternating etch sequence—proceeding through ILD 5-1 SiCN, ILD 4-2 Oxide, ILD 4-1 SiCN, and finally ILD 3-2 Oxide—indicates a step-by-step unlanding of a deeply buried pad structure [P2]. Unlike early front-end oxide etches or shallow pad oxide clearing steps, this step must selectively remove a specific, deeply embedded bulk oxide layer (ILD 3-2) while preserving the geometric integrity of the already-opened upper tiers [P2]. Following this step, the structure undergoes ashing, cleaning, and barrier deposition to prepare for Metal 7 Aluminum deposition, making complete and precise oxide removal critical to minimize subsequent contact resistance [T3]. The removal of the ILD 3-2 oxide relies primarily on ion-enhanced chemical reactions within a fluorocarbon-based plasma [P1]. During the process, energetic ions impinge vertically on the oxide surface, breaking strong Si–O bonds and generating highly reactive surface sites [P1]. This localized ion bombardment activates the surface, dramatically increasing the probability of reactions between neutral fluorine radicals and the oxide to produce volatile SiFx and COx byproducts [P1, P3]. Simultaneously, carbon-rich fluorocarbon radicals (such as CFx) deposit a protective polymer film on the exposed sidewalls [P3]. The process relies on a dynamic balance: ion-driven physical sputtering removes the polymer at the trench bottom to allow continuous chemical etching, while the lack of ion bombardment on the vertical sidewalls allows the polymer to accumulate and suppress lateral etching [P2]. High-density plasma systems are typically selected for deep dielectric etching to provide the high ion flux necessary for acceptable etch rates [P3]. However, the high degree of molecular dissociation in these high-density plasmas can reduce etch selectivity, necessitating the careful selection of fluorocarbon gases and the precise addition of polymerizing agents to maintain the etch-passivation balance [P3]. Adjusting the gas mixture, pressure, and bias power directly influences the ion angular distribution, which must be tightly controlled to prevent shadowing effects that limit the achievable aspect ratio in deep trenches [P2]. This careful parameter tuning ensures the etch stops cleanly at the designated depth without punching through underlying barrier layers or damaging adjacent critical structures [P2].