The wet strip chemistry must provide high etching selectivity, completely dissolving post-etch polymeric and metal-containing residues without corroding the exposed interconnect materials like copper and tungsten .
In depth
In the 40nm BSI CMOS Image Sensor flow, the final Ashing & Strip/Clean step follow
s the Bond Pad Metal 7 Barrier Etch to comprehensively remove highly crosslinked photoresist and post-etch residues . During the preceding barrier etch, halogen-based plasmas interact with the Ta/TaN barrier and underlying copper to generate complex fluorocarbon polymers and organometallic byproducts . If left untreated, these residues form a highly resistive interfacial layer that severely degrades the specific contact resistance of the metal-semiconductor or metal-metal interfaces, ultimately impacting device power consumption and signal integrity . Unlike previous internal integration ashing steps (e.g. (Engineering Practice), steps #11, #17) which prepare intermediate via or trench structures, this final module step strictly targets the preparation of the uppermost terminal metallization for off-chip connectivity (Engineering Practice). Achieving a pristine pad surface here is the definitive prerequisite for reliable wire bonding or flip-chip packaging before the wafer proceeds to Final Inspection and Fab Out (Engineering Practice). The physical and chemical mechanisms of this clean rely on structurally modifying the robust residue network followed by targeted chemical dissolution . Conventional high-temperature oxygen plasma ashing tends to harden photoresist and induce thermochemical crosslinking, resulting in stubborn carbon-rich polymeric residues . To circumvent this, advanced strip processes often employ ultraviolet (UV) irradiation to induce photochemical scission of the C-C and C-F bonds within the polymer backbone . This chain scission reduces the molecular weight and crosslink density of the fluorocarbon crust, enhancing surface wettability and solvent penetration . Following modification, wet cleaning chemistries utilizing specific organic solvents, chelators, and mild oxidizers are deployed to chemically dissolve the fragmented polymers and complex the metallic residues . Acoustic agitation, such as megasonic energy, is concurrently applied to induce cavitation and microstreaming, providing mechanical energy that accelerates mass transport and physical delamination of the weakened residues from the pad sidewalls [P2, P3]. The selection of chemical formulations and process parameters represents a delicate balance between cleaning efficiency and material compatibility . The wet strip chemistry must provide high etching selectivity, completely dissolving post-etch polymeric and metal-containing residues without corroding the exposed interconnect materials like copper and tungsten . Formulations containing selective oxidizers, such as specific ammonium salts, can oxidize target residues while corrosion inhibitors like long-chain alkylamines selectively adsorb onto pristine metal surfaces to suppress unwanted etching . Parameter interactions play a critical role; for instance, increasing the solvent temperature or megasonic power accelerates dissolution kinetics but simultaneously increases the risk of solvent ingress into adjacent porous dielectrics or localized pitting on the metal pad . Conversely, an insufficient UV modification dose leaves the heavily crosslinked fluoropolymer intact, rendering the subsequent chemical dissolution ineffective . For the 40nm technology node, mitigating plasma-induced damage to the BEOL porous low-k dielectrics is a primary driving force behind the adoption of these specialized strip/clean sequences . Traditional oxygen or fluorine-based plasma stripping causes severe damage by breaking Si-CH3 bonds and extracting carbon from the low-k matrix, leading to moisture absorption and a deleterious increase in the dielectric constant . The combination of plasma-less modification, selective wet chemistries, and controlled acoustic physical assistance provides an integrated solution to thoroughly clean the bond pad topography while preserving the structural and electrical integrity of the surrounding advanced interconnect dielectrics [P2, P3].
Risks & Challenges
[High] Metal Interconnect Corrosion: Unoptimized wet chemistries, particularly those with excessive oxidizer concentrations or insufficient passivating inhibitors, can electrochemically attack the exposed copper or barrier metals at the pad bottom . This leads to localized pitting or structural thinning, which increases the contact resistance and degrades the mechanical reliability of the final package bond .
[Medium] Incomplete Fluorocarbon Residue Removal: An insufficient UV irradiation dose or low chemical solvent activity fails to achieve adequate photochemical chain scission in the highly crosslinked C-F polymer network generated during the preceding barrier etch . The remaining polymeric crust acts as an insulating barrier, drastically increasing the specific contact resistance of the bond pad interface .
[Medium] Low-k Dielectric Degradation: Over-exposure to highly polar organic solvents or aggressive cleaning chemistries can attack the adjacent porous low-k dielectrics, penetrating the pore structure and displacing hydrophobic functional groups . This chemical modification causes moisture absorption, which increases the dielectric constant and degrades inter-line capacitance and isolation reliability .
[Low] Post-Etch Metal Residue Redeposition: Halogen-containing etch byproducts, such as chlorine or fluorine reacting with sputtered aluminum or copper, form organometallic complexes that can precipitate back onto the structure if the wet clean lacks adequate chelating capacity . These redeposited metallic islands can act as micromasking defects or create localized leakage paths along the dielectric sidewalls .