40nm BSI CMOS Image SensorPreview

t damaging surrounding structures [P2]. Previous steps etched the bulk silicon and pad oxide, followed by CESL 1 Etch, making the CESL 2 Etch necessary to break through the next layer of the multi-tiered contact etch stop layer stack [P1]. CESL films, typically formed of amorphous hydrogenated silicon nitride deposited via PECVD, are widely used to induce targeted mechanical stress in device channels to enhance carrier mobility [P1]. In the bondpad region, sequentially etching these layered CESL films ensures controlled penetration into the underlying pre-metal dielectric (PMD) layers, paving the way for the subsequent PMD 1, 2, and 3 etch steps [A1]. The plasma-assisted etching of the SiNx CESL relies on the synergistic interaction between reactive radicals and ion bombardment [P3]. Using a fluorocarbon-based gas chemistry, fluorine radicals chemically react with the silicon in the SiNx film to form volatile byproducts [P4]. Concurrently, carbon from the plasma deposits a polymer layer on the surface, creating a dynamic mixing layer [P3]. The core mechanism for continuous etching involves tuning the ion energy to penetrate this C-rich polymer layer on the SiNx surface while maintaining a controlled etch rate [P3]. Unlike isotropic chemical downstream etching which relies entirely on neutral radicals and lacks ion bombardment [P4], the CESL 2 Etch utilizes anisotropic reactive ion etching (RIE) to maintain strict profile control over the vertical bondpad sidewalls (Engineering Practice). Fluorocarbon RIE is chosen over wet etching due to the need for high anisotropy and strict dimensional control at advanced technology nodes [P3]. The F-to-C ratio in the parent gas and the addition of diluents like H2 heavily influence the etch selectivity and polymer deposition rate [P3]. A lower F/C ratio promotes thicker polymer passivation, which can protect sidewalls but might induce an etch stop if the applied ion energy is insufficient [P3]. Modulating the bias power alters the ion energy distribution, which is critical for preferentially removing the bottom passivation layer while leaving the sidewall passivation intact, a mechanism fundamental to high-aspect-ratio patterning [A1]. Furthermore, precisely stopping the etch before over-penetrating the underlying oxide relies on the intrinsically different surface reaction kinetics of SiO2 and SiNx under fluorocarbon plasmas [P3]. What distinguishes CESL 2 Etch from CESL 1 Etch is its specific position within a dual-stress or stratified dielectric integration scheme [P1]. In advanced nodes, overlapping tensile and compressive CESL films are often used to independently modulate nMOS and pMOS channel mobilities, acting as separate strain sources [P2]. The CESL 2 Etch specifically targets the second film in this structural stack, which possesses a different intrinsic stress—ranging from -3.0 GPa compressive to +1.6 GPa tensile—and a distinct physical density [P2]. Because UV-cured tensile films exhibit different Si-N and Si-Si bond configurations compared to as-deposited compressive films [P1], the CESL 2 RIE plasma chemistry and bombardment energy must be distinctly tuned to achieve uniform removal without localized micro-masking (Engineering Practice).