The preceding fluorocarbon-based plasma etch intentionally deposits highly fluorinated, crosslinked polymeric residues on the sidewalls to maintain etching anisotropy .
In depth
After the Optical Pad 2 Etch, the wafer surface is covered with degraded photoresist and tough fluorocarbon-based post-etch resi
dues . This Ashing & Strip/Clean step aims to completely eradicate these organic and organometallic byproducts before the subsequent Mid Vertical Grid Deposition . If left untreated, these residues can compromise the structural adhesion and electrical integrity of the grid materials . What distinguishes this specific step from other ashing steps in the flow is its location within the thick Lower Optical Clear Layer (OCL) and intermediate optical stack; it must selectively strip residues without degrading the optical transparency or dimensional stability of the surrounding isolation structures . Furthermore, any residual photoresist can be cleanly removed by a carefully tuned ash process or by dissolution with a specialized solvent, ensuring a pristine surface for the subsequent grid deposition . The preceding fluorocarbon-based plasma etch intentionally deposits highly fluorinated, crosslinked polymeric residues on the sidewalls to maintain etching anisotropy . Removing these crusts requires breaking down their robust chemical structure, as conventional organic solvents or purely aqueous solutions are fundamentally insufficient to dissolve the densely crosslinked network . Using a conventional high-temperature oxygen plasma ashing process can inadvertently cause further thermochemical crosslinking and hardening of the resist, transforming it into a stubbornly carbon-rich polymeric residue . To circumvent this, modern processes may employ an in-situ low-temperature oxygen-based plasma ashing to prevent thermal hardening, followed by a wet chemical stripping step to dissolve the loosened fragments . Alternatively, advanced non-plasma techniques utilize ultraviolet irradiation to induce photochemical chain scission of the C-C and C-F bonds within the polymer backbone, significantly reducing its molecular weight and crosslink density to enable subsequent dissolution . Following the initial structural modification of the residue, a wet cleaning formulation is deployed to selectively dissolve the remaining polymer fragments . These wet chemistries often incorporate multi-functional organic solvents and reactive components to penetrate the polymer matrix via interfacial delamination and dissolution mechanisms . To assist mass transport into deep, high-aspect-ratio pad openings, megasonic acoustic energy is frequently applied, generating cavitation bubbles that provide gentle mechanical fluid-dynamic force to detach weakened residues . The process parameters, such as solvent temperature, megasonic power, and exposure time, must be tightly balanced; excessively high thermal or acoustic energy can drive solvent ingress into adjacent porous dielectrics, thereby increasing the effective dielectric constant and causing interline capacitance issues . When metallic pads are exposed at the trench bottom, the cleaning solution may also integrate carefully selected oxidizing agents alongside primary alkylamine corrosion inhibitors to selectively dissolve post-etch residues without corroding the underlying conductive metal . At the 40nm node, the highly scaled physical dimensions of the optical grid and peripheral pads dictate extremely tight process windows for post-etch residue removal . Standard under-etching or aggressive aqueous cleans are no longer viable because they isotropically consume the surrounding dielectric layers, leading to unacceptable critical dimension loss . Furthermore, if the exposed interconnect pads suffer surface damage or metal redeposition during the clean, it can lead to large surface leakage currents and poor contact rectification characteristics, fundamentally degrading device reliability . Consequently, a hybrid approach combining mild structural modification and highly selective wet chemistry is strictly necessary to preserve the micro-lens optical paths and precision grid trenches inherent to advanced BSI CIS architectures .
Risks & Challenges
[High] Hardened Polymeric Residue: Utilizing conventional high-temperature oxygen ashing can cause the remaining photoresist to undergo severe thermochemical crosslinking, forming a carbon-rich hardened crust that strongly resists subsequent wet stripping .
[High] Incomplete Fluorocarbon Removal: Fluorocarbon plasmas used in the preceding etch deposit highly crosslinked CFx residues that are extremely difficult to dissolve; insufficient structural modification (e.g. , via inadequate UV dose or excessively mild plasma) leaves these dense networks intact, causing poor structural adhesion for subsequent depositions .
[Medium] Metal Pad Corrosion: When exposing underlying metallic pads, wet cleaning compositions containing strong oxidizers can chemically attack the bare metal surface if primary alkylamine corrosion inhibitors are not adequately balanced within the solution formulation .
[Medium] Dielectric and Optical Layer Degradation: Aggressive plasma ashing or prolonged exposure to polar organic solvents at elevated temperatures can induce bond scission or solvent absorption in porous isolation dielectrics, leading to an unwanted increase in the dielectric constant and potential device capacitance issues .
[Low] Micromasking from Residual Polymers: Residual polymers or incomplete clearing of hardened layers can act as localized micromasks during the clean, shielding underlying microscopic regions and leading to subsequent blocked deposition or localized contact failure .