The strip/clean mechanism traditionally employs oxygen plasma ashing to volatilize organic components, but purely dry processes are often insufficient for removing the metal-rich resist crusts without causing severe collateral damage .
In depth
Following the anisotropic patterning of the W/TiN Light Shiel
d (LS) grid, the remaining photoresist, bottom anti-reflective coating (BARC), and post-etch polymeric residues must be completely removed to ensure optical clarity and electrical isolation . This specific Ashing & Strip/Clean step is critical because the preceding TiN etch, which utilizes halogen-based plasmas, forms a rigid, cross-linked "hard shell" on the photoresist containing titanium, tungsten, and halogen species . Failing to clear these hybrid inorganic-organic residues will impede the subsequent uniform deposition of the Optical Pad 1 layer, leading to void formation and optical scattering within the sub-micron pixel array . Furthermore, this step is distinct from typical front-end or via strip processes because it must operate directly on exposed refractory metals without causing barrier undercut or shifting the metal grid's dimensions (Engineering Practice). The strip/clean mechanism traditionally employs oxygen plasma ashing to volatilize organic components, but purely dry processes are often insufficient for removing the metal-rich resist crusts without causing severe collateral damage . Therefore, modern integration combines a carefully tuned plasma ash with an advanced all-wet chemical cleaning sequence . The wet cleaning phase operates via solvation, penetration, and interfacial delamination mechanisms, utilizing specific solvent mixtures to dissolve ester and lactone functional groups within the organic polymers . Simultaneously, reactive components in the solution chemically complex and mechanically disrupt the inorganic/organic composite crust . To protect the exposed tungsten grid during this aggressive clean, specialized inhibitors such as primary alkylamines are introduced, which selectively adsorb onto the metallic tungsten surfaces to form a passivating barrier . The selection of wet cleaning chemistries must carefully balance the complete dissolution of metallic residues with the preservation of the W/TiN stack . Conventional peroxide-based cleans indiscriminately oxidize the tungsten surface into soluble species, resulting in severe critical dimension loss . To mitigate this, advanced cleaning solutions often employ alternative oxidizers paired with long-chain alkylamine corrosion inhibitors that exploit the differential oxidation potentials between the residue and the bulk metal . Additionally, the process is frequently enhanced with megasonic agitation, which generates localized cavitation and microjets to improve the mass transport of chemical reactants deep into the high-aspect-ratio grid trenches . Adjusting the bath temperature and megasonic power directly controls the dissolution kinetics and cavitation intensity; higher temperatures accelerate residue removal but narrow the safe operating window before metal corrosion initiates . At the 40nm BSI node, the optical grid pitch is highly scaled, exacerbating the impact of any metal recession or sidewall roughening (Engineering Practice). The polycrystalline nature of the tungsten lines makes them particularly vulnerable to localized pitting if wet etchants attack the grain boundaries at a faster kinetic rate than the grain surfaces . Therefore, precisely controlled surface chemistry, potentially utilizing ligand-assisted passivation to suppress non-selective parasitic etching, is essential to maintain atomic-level smoothness on the metal grid prior to the optical pad deposition .
Risks & Challenges
[High] Tungsten Grid Corrosion and Pitting: Exposure to aggressive oxidizers during the wet clean without adequate inhibition leads to the uncontrolled oxidation of the W sidewalls . Because tungsten is polycrystalline, etchants can attack grain boundaries with faster kinetics, causing severe pitting, surface roughening, and overall loss of opacity in the Light Shield grid .
[High] Incomplete Removal of Titanium-Halogenated Crust: The plasma etch process embeds titanium and halogen species into the photoresist, creating a hardened crust that is highly resistant to standard organic solvents . Failure to chemically complex and dissolve this crust results in sub-micron residues that act as light scattering centers or cause voids during the subsequent Optical Pad 1 deposition .
[Medium] TiN Barrier Undercutting: The wet cleaning chemistry may exhibit excessive selectivity toward the TiN adhesion layer relative to the bulk W, driving an oxidation-complexation-dissolution mechanism at the metal-solution interface . If the pH and oxidizer concentration are poorly optimized, the TiN will isotropically recess beneath the tungsten, structurally weakening the high-aspect-ratio grid lines .
[Low] Megasonic-Induced Mechanical Damage: While megasonic energy is required to drive mass transport into tight grid trenches, excessive power generates violent cavitation events . The resulting microjets and localized pressure waves can mechanically fracture or topple the fragile, high-aspect-ratio W/TiN grid structures (Engineering Practice).