The residue removal operates through a bipartite mechanism of plasma-induced oxidation followed by selective wet chemical dissolution .
In depth
Following the Oxide Grid Seal Layer Etch, the deep high-aspect-ratio trenches are coated with highly crosslinked fluorocarbon (CFx) polymers intentionally depos
ited for sidewall passivation, alongside hardened photoresist remnants . If left intact, these residues will prevent the conformal deposition of the Lower Vertical Grid Barrier layer, leading to adhesion failure or structural voids that severely compromise the pixel's optical isolation . Therefore, this specific Ashing & Strip/Clean step is engineered to completely eradicate complex post-etch residues while strictly preserving the anisotropic profile of the delicate oxide grid seal layer . Unlike routine planar cleans, this step must penetrate high-aspect-ratio optical isolation structures without inducing lateral critical dimension (CD) loss or corner rounding (Engineering Practice). The residue removal operates through a bipartite mechanism of plasma-induced oxidation followed by selective wet chemical dissolution . Initially, an oxygen-based plasma ashing step attacks the organic backbone of the resist and polymeric residues, converting carbonaceous materials into volatile CO and CO2 byproducts . To prevent heat-induced crosslinking and hardening of the photoresist—which can create intractable carbon-rich polymeric crusts—low-temperature ashing processes are preferred . Subsequently, wet cleaning addresses the highly fluorinated, crosslinked CFx networks that survive the plasma environment . These robust fluorocarbon polymers resist conventional organic solvents due to their dense C-F bonds . Introducing ultraviolet (UV) irradiation (e.g. (Engineering Practice), at nanoscale) prior to or during wet cleaning can induce photochemical chain scission in the polymer backbone, lowering its molecular weight and crosslink density . This structural loosening drastically enhances solvent wetting and subsequent dissolution of the modified residues . The selection of wet chemistries must balance aggressive organic residue removal with near-zero etch rates for the exposed silicon dioxide grid structures . Sulfuric Peroxide Mixture (SPM), combining H2SO4 and H2O2, is often employed because its strong oxidative potential efficiently decomposes residual organics while exhibiting virtually no chemical etching of SiO2, thereby preserving the grid's surface planarity and structural integrity . Conversely, alkaline cleans like APM (ammonia-peroxide mixture) are typically minimized in this step, as the hydroxyl ions (OH-) can chemically attack and roughen the SiO2 sidewalls . Furthermore, complex solvent formulations may incorporate specific chelating agents and organic buffers to selectively dissolve complex etch byproducts without corroding adjacent device structures, relying on a tightly controlled pH and oxidizer activity window . In 40nm BSI sensor geometries, the aspect ratios of the lower vertical grid trenches are extreme, exacerbating physical constraints during cleaning (Engineering Practice). Traditional high-energy oxygen descum processes risk inducing charging damage or modifying the dielectric constant of nearby isolation structures, necessitating the aforementioned milder modification-plus-wet-removal combinations . Furthermore, plasma non-uniformity and electron shading effects in these narrow trenches can cause differential charging between the top and bottom of the structure . This differential charging amplifies the need for highly controlled, optimized ashing parameters to prevent dielectric breakdown and tunneling currents during the plasma clean .
Risks & Challenges
[High] Incomplete Fluorocarbon Residue Removal: Highly crosslinked C-F networks from the preceding fluorocarbon etch resist conventional wet solvents, leaving polymeric residues that block subsequent barrier deposition . Without adequate chain-scission (e.g. (Engineering Practice), via UV modification) or sufficiently active chemistries, these residues remain anchored in the trench bottom .
[High] Photoresist Hardening and Carbonization: If the plasma ashing temperature is too high (e.g. , >275°C), the remaining photoresist undergoes thermochemical crosslinking . This reaction forms a carbon-rich crust that is virtually impervious to standard wet stripping, requiring optimized in-situ low-temperature ashing to prevent .
[Medium] Oxide Grid Sidewall Attack: Using alkaline APM or excessively aggressive fluoride-based chemistries can cause OH- or F- ions to chemically etch the SiO2 grid seal layer . This isotropic attack expands the trench critical dimension and roughens the sidewalls, which degrades the subsequent optical barrier's reflective properties .
[Medium] Plasma Charging Damage: High-density plasma ashing in narrow, high-aspect-ratio grid trenches can lead to electron shading, where angular disparities between ions and electrons cause differential charging . This localized charge buildup can drive large tunneling currents through adjacent thin dielectrics, causing structural degradation .