40nm BSI CMOS Image SensorPreview

a robust Ashing and Strip/Clean step is mandatory to prepare the surface for the subsequent Bond Pad Opening 3 photolithography [P3]. The preceding RIE step typically employs a fluorocarbon-heavy plasma chemistry to achieve anisotropic silicon etching by forming a protective polymeric film on the sidewalls [A1]. Consequently, this stripping step must eradicate the highly crosslinked, fluorine-rich polymeric residues and any hardened photoresist crust remaining after the deep via etch [P2]. Unlike front-end logic cleans or standard BEOL Ashing/Strip steps that primarily protect ultra-low-k dielectrics from plasma-induced capacitance shifts [P1], this specific packaging-level clean is distinguished by its need to handle heavily polymerized, thick fluorocarbon passivation residues generated during bulk structural silicon etching rather than microscopic feature definition (Engineering Practice). The removal of these post-etch residues relies on a sequenced combination of plasma ashing and targeted wet chemical dissolution [A2]. Oxygen-based plasma ashing is first utilized to chemically oxidize the bulk organic components, breaking them down into volatile species such as carbon monoxide and carbon dioxide [A1]. However, because conventional high-temperature oxygen ashing can induce further crosslinking and hardening of the outer photoresist shell, forming stubborn carbon-rich polymeric residues [P3], a carefully controlled thermal budget or an in-situ low-temperature plasma regime is often implemented to prevent defect generation [P3]. For the dense fluorocarbon networks that resist pure thermal oxidation, subsequent wet cleaning utilizes specific solvent mixtures that penetrate the loosened polymer structure [P2]. In advanced nodes, ultraviolet (UV) irradiation may be introduced prior to wet cleaning to induce photochemical chain scission of C-C and C-F bonds within the polymer backbone, drastically reducing the crosslink density and altering the surface energy to enhance solvent wetting and dissolution [P2]. The selection of cleaning chemistry and process parameters is driven by the need to balance aggressive residue removal with the structural integrity of the thinned silicon and underlying passivation films [A2]. Purely aqueous or simple organic solvent cleans are typically insufficient to dissolve crosslinked fluorinated polymers without prior structural modification or the addition of reactive components [P2]. Formulations may incorporate specialized multi-functional solvent blends, often assisted by megasonic acoustic energy, which generates cavitation and microjets to physically enhance mass transport into the deep backside vias and promote interfacial delamination of the residue [P4]. Furthermore, if the via etch approaches any metallic structures, the wet chemistry must carefully integrate corrosion inhibitors—such as primary alkylamines—that selectively adsorb onto metal surfaces while oxidizers dissolve the targeted residues [A2]. The process window requires strict optimization of temperature, megasonic power, and chemical concentration to maximize dissolution kinetics while preventing excessive solvent ingress or mechanical damage to adjacent fragile structures [P4].