40nm BSI CMOS Image SensorPreview

ep is executed immediately after the ILD 3-1 SiCN breakthrough etch [P4]. Its primary function is to completely remove the remaining photoresist (PR), bottom anti-reflective coating (BARC), and complex post-etch residues (PER) without compromising the structural integrity of the dielectric [P3]. This prepares the pristine V3 cavity for the upcoming Metal 4 trench patterning and eventual dual-damascene metallization (Engineering Practice). Distinct from front-end contact cleans or tight-pitch V1/V2 ashing, V3 serves intermediate global routing and directly exposes the underlying Metal 3 copper pad (Engineering Practice). Exposing the copper renders the structure highly susceptible to uncontrolled oxidation and galvanic corrosion, demanding meticulously formulated stripping chemistries compared to earlier front-end polysilicon or contact cleans [P3]. The preceding fluorocarbon-based reactive ion etching (RIE) inherently generates a highly crosslinked, fluorine-rich polymeric network (CFx) on the dielectric sidewalls [P1]. Furthermore, high-energy ion bombardment during the SiCN breakthrough can physically dislodge Cu atoms, embedding them into this fluoropolymer matrix to form a chemically complex hardened crust [P3]. Historically, high-temperature oxygen plasma ashing was used to volatilize these organics, but excessive thermal loads cause the PR to crosslink and harden into intractable carbon-rich residues [P4]. Additionally, oxygen plasma induces severe damage to porous low-k dielectrics by depleting carbon-based groups, leading to structural collapse and severe dielectric constant degradation [P2]. Consequently, this back-end-of-line (BEOL) step utilizes highly controlled downstream or reducing plasmas (such as H2/He or heavily diluted O2) to convert organic polymers into volatile ash while preserving the dielectric framework [A1]. Following the dry plasma ash, a highly selective wet strip and clean is mandatory to eliminate inorganic byproducts and chemically modified polymer fragments [P1]. Diluted aqueous solutions, such as traditional HF-based cleans, are generally avoided because they isotropically etch the dielectric framework to lift off polymers, resulting in unacceptable critical dimension (CD) loss [P1]. Instead, formulated organic solvents containing specialized complexing agents are utilized to dissolve PR functional groups, such as esters and lactones, and to chelate the sputtered metallic species [P3]. To overcome mass transport limitations within the high-aspect-ratio V3 cavities, megasonic acoustic energy is coupled into the cleaning fluid [P2]. The megasonic waves induce acoustic cavitation and microstreaming, which mechanically weaken the interfacial adhesion between the highly fluorinated residues and the via sidewalls without inflicting mechanical damage on the delicate low-k structures [P3]. In advanced nodes like nanoscale, the narrowing process window necessitates novel physicochemical modifications prior to wet removal to ensure complete cleaning without damaging fragile materials [P2]. One mechanism involves exposing the post-etch wafer to nanoscale ultraviolet (UV) irradiation, which induces photochemical chain scission within the fluorocarbon polymer backbone [P1]. This photon-driven bond scission reduces the crosslink density and molecular weight of the residue, significantly enhancing solvent wetting and penetration during the subsequent wet clean [P1]. Alternatively, selective oxidation using controlled ozone (O3) exposure can cleave the polymer backbone via ozonolysis, rendering the hardened polymer shell soluble in organic solvents without relying on physically destructive plasma ion bombardment [P2].