Similar plasma-less or hybrid approaches, such as utilizing UV combined with ozone (O3) to cleave polymer backbones via ozonolysis, are heavily favored in advanced nodes to prevent plasma-induced dielectric damage .
In depth
Following the Upper OCL Coating Etch, this specific Ashing & Strip/Clean step is
strictly required to remove the remaining photoresist and post-etch fluorocarbon residues before proceeding to the Bond Pad Opening 2 lithography . Unlike front-end ashing steps that clean robust inorganic dielectrics, this step is uniquely challenging because it must aggressively clean deep pad excavations without degrading the adjacent, optically sensitive on-chip lens (OCL) planarization layers . If the crosslinked resist and polymer residues are not completely cleared, they will act as micromasks in the subsequent Upper Grid Seal Layer Etch, leading to blocked etch regions and uncontacted metal islands . Furthermore, incomplete residue removal severely compromises the structural reliability and adhesion of the final multi-layer pad connections . The physical mechanism of this step targets the highly crosslinked, C-F-rich dense network formed during the upstream fluorocarbon plasma etch . During the prior anisotropic reactive ion etch (RIE), the photoresist surface is subjected to high-energy ion bombardment and fluorocarbon passivation, converting the outer shell into a hardened, carbon-rich polymeric residue . Conventional high-temperature oxygen plasma ashing can exacerbate this issue by further crosslinking and hardening the resist, making it impervious to standard wet treatments . Therefore, an optimized process utilizes in-situ low-temperature plasma ashing to gently volatilize the bulk organic resist without excessively baking the hardened crust . This initial dry step transforms the bulk carbon chains into volatile CO, CO2, and H2O byproducts by bombarding the surface with reactive oxygen radicals (Engineering Practice). To clear the remaining stubborn fluorocarbon post-etch residues, the process relies on advanced structural modification paired with wet chemistry . Purely aqueous or conventional solvent cleans are completely insufficient for removing highly fluorinated sidewall polymers without unacceptably etching the underlying dielectric . Instead, ultraviolet (UV) irradiation can be integrated to induce photochemical chain scission within the polymer backbone, selectively breaking C-C and C-F bonds . This UV-induced modification decreases the crosslink density and introduces polar groups to the residue surface, drastically improving its wettability and solubility . Following modification, specifically formulated wet solvents can penetrate the loosened network and physically dissolve or lift off the residues without attacking the delicate OCL layer . Similar plasma-less or hybrid approaches, such as utilizing UV combined with ozone (O3) to cleave polymer backbones via ozonolysis, are heavily favored in advanced nodes to prevent plasma-induced dielectric damage . In the context of 40nm BSI CMOS Image Sensors, maintaining tight geometric control over the deep bonding pad trenches is critical to ensuring proper metal deposition and preventing electrical opens . Excessive plasma exposure or harsh wet etching could alter the trench sidewall profiles or cause volumetric shrinkage in adjacent flowable dielectric materials, leading to unintended asymmetric stress and structural tilting . By balancing low-temperature dry ashing with UV-assisted wet stripping, process engineers restore the delicate balance between reactive etching and surface passivation, ensuring a pristine surface for the subsequent grid seal layer opening .
Risks & Challenges
[High] Incomplete Fluorocarbon Residue Removal: The highly crosslinked, C-F-rich dense network generated during the previous OCL etch strongly resists penetration by conventional wet cleaning solvents . If the ashing temperature is too high, the photoresist shell undergoes severe thermochemical crosslinking, creating stubborn carbon-rich residues that act as micromasks in the next etch step and lead to blocked metal islands .
[Medium] Dielectric Surface Damage (Optical/Structural Degradation): Aggressive oxygen plasma ashing can chemically attack the underlying organic or hybrid OCL coatings, breaking carbon-based bonds and leaving a damaged, porous surface . This damage mechanism is analogous to the plasma-induced depletion of methyl groups in porous low-k dielectrics, which severely degrades material integrity and alters local interface properties .
[Medium] Uncontrolled Trench Sidewall Erosion: If the wet clean chemistry relies too heavily on aggressive aqueous acids to lift off polymers via dielectric under-etching, it will isotropically attack the exposed dielectric trench sidewalls . This lateral erosion uncontrollably enlarges the carefully defined trench spacing, which negatively impacts subsequent metal filling morphology and conductive trace interconnection reliability .
[Low] Structural Tilting due to Thermal Stress: Excessive thermal load during a poorly optimized, high-temperature ashing step can induce thermodynamic shrinkage and volumetric changes in the surrounding planarization dielectric layers . The resulting asymmetric shrinkage stress can lead to the deformation or tilting of nearby embedded structures, compromising structural integrity .