Ashing & Strip/Clean - 40nm

The selection of the cleaning methodology often involves pairing specialized solvent mixtures with physical or photochemical pretreatments to break down stubborn residues without attacking the underlying ILD .

Following the ILD 1-2 Oxide Etch, the MET2 trenches are structurally defined but their

surfaces remain heavily contaminated with unreacted photoresist (PR), bottom anti-reflective coating (BARC), and highly crosslinked fluorocarbon (CFx) polymers . This Ashing & Strip/Clean step is critically positioned to eradicate these residues entirely before the downstream deposition of the Ta-based liner and copper seed (Engineering Practice). Unlike general front-end sacrificial cleans, this specific step must operate within the chemically sensitive dual-damascene topography, ensuring the exposed porous low-k dielectric sidewalls remain pristine to guarantee optimal barrier adhesion and limit parasitic capacitance . If these dense etch residues are not thoroughly removed, the subsequent Ta liner will exhibit degraded step coverage, resulting in elevated contact resistance and a high probability of electromigration failure at the via-trench interface . The fundamental mechanism of residue removal in advanced BEOL integration has largely shifted away from pure oxygen plasma ashing, as high-energy reactive oxygen and fluorine radicals readily cleave the low-polarization Si-CH3 bonds within the low-k dielectric framework . This plasma-induced damage causes carbon depletion, pore collapse, and a transition to a hydrophilic state that drastically increases the dielectric constant (k-value) . To circumvent this, advanced strip processes rely on highly selective wet chemistries that remove the PR and BARC through solvent dissolution, penetration, and interfacial delamination . The core chemical action depends on the solvent's ability to complex and disrupt the inorganic/organic composite crust—often containing titanium from hard masks and fluorine from the etch plasma—while megasonic acoustic waves generate cavitation and microjets to overcome mass transport limitations in the high-aspect-ratio MET2 trenches . The selection of the cleaning methodology often involves pairing specialized solvent mixtures with physical or photochemical pretreatments to break down stubborn residues without attacking the underlying ILD . For example, ultraviolet (UV) irradiation at nanoscale can be introduced prior to wet cleaning to induce photochemical chain scission of C-C and C-F bonds within the dense fluoropolymer network . This targeted scission lowers the molecular weight and crosslink density of the residue, introducing polar groups that drastically improve the wetting and penetration of subsequent solvents like dimethyl sulfoxide (DMSO) and monoethanolamine (MEA) . Alternatively, trace typical metal ions can be introduced into water-soluble organic solvents to finely regulate interfacial reactivity, promoting the swelling and stripping of highly cross-linked fillers while actively suppressing chemical corrosion of the low-k material . Process parameters such as UV dose, megasonic power, and solvent temperature must be strictly optimized; excessive energy can drive solvent deep into the low-k nanopores, necessitating a subsequent low-pressure, high-temperature bake to drive out the trapped solvent and restore the dielectric constant . At the 40nm node, the reliance on highly porous low-k dielectrics (a-SiOxCyNz:H) to mitigate RC delay makes the system extraordinarily vulnerable to structural degradation . If the ashing or stripping process alters the dielectric such that its mass density falls below ~2.0 g/cm3 or its average pore size expands, the film loses its hermeticity, allowing rapid diffusion of moisture and wet chemicals . Therefore, this specific MET2 strip step is distinct from older-generation cleans because it must execute complete polymer removal while fundamentally maintaining the nanostructural integrity of the trench sidewall, thereby ensuring long-term time-dependent dielectric breakdown (TDDB) reliability .

[High] Low-k Dielectric Plasma Damage: High-energy ions, radicals, and ultraviolet photons from plasma treatments break low-polarization Si-CH3 bonds within the low-k framework . This results in carbon depletion, backbone densification, and the formation of polar Si-OH groups, which severely increase the dielectric constant and degrade interconnect RC performance .
[High] Incomplete Fluorocarbon Residue Removal: Post-etch fluorocarbon polymers form highly crosslinked, C-F rich networks that resist penetration by standard organic solvents . If structural modification (such as UV pretreatment) is insufficient, the residual polymer will block the trench sidewalls, preventing proper adhesion of the subsequent Ta-based liner and leading to eventual Cu diffusion .
[Medium] Moisture and Solvent Permeation: Aggressive wet cleaning conditions or high megasonic power can force solvents and water into the interconnected nanopores of the low-k dielectric . If the material's pore size approaches the scale of the solvent molecules and post-clean baking is inadequate, the retained moisture will chemically oxidize the downstream Ta/TaN barrier, leading to copper dewetting and degraded electromigration lifetimes .
[Low] Low-k Dielectric Chemical Corrosion: The use of strongly alkaline components or unoptimized chelating agents in the wet strip solution can chemically attack the porous SiCOH structure . Without the precise balance of protective additives or specific metal ions to regulate surface reactions, the solvent may induce lateral critical dimension (CD) enlargement, increasing intra-layer parasitic capacitance .

Following the ILD 1-2 Oxide Etch, the MET2 trenches are structurally defined but their

[High] Low-k Dielectric Plasma Damage: High-energy ions, radicals, and ultraviolet photons from plasma treatments break low-polarization Si-CH3 bonds within the low-k framework . This results in carbon depletion, backbone densification, and the formation of polar Si-OH groups, which severely increase the dielectric constant and degrade interconnect RC performance .
[High] Incomplete Fluorocarbon Residue Removal: Post-etch fluorocarbon polymers form highly crosslinked, C-F rich networks that resist penetration by standard organic solvents . If structural modification (such as UV pretreatment) is insufficient, the residual polymer will block the trench sidewalls, preventing proper adhesion of the subsequent Ta-based liner and leading to eventual Cu diffusion .
[Medium] Moisture and Solvent Permeation: Aggressive wet cleaning conditions or high megasonic power can force solvents and water into the interconnected nanopores of the low-k dielectric . If the material's pore size approaches the scale of the solvent molecules and post-clean baking is inadequate, the retained moisture will chemically oxidize the downstream Ta/TaN barrier, leading to copper dewetting and degraded electromigration lifetimes .
[Low] Low-k Dielectric Chemical Corrosion: The use of strongly alkaline components or unoptimized chelating agents in the wet strip solution can chemically attack the porous SiCOH structure . Without the precise balance of protective additives or specific metal ions to regulate surface reactions, the solvent may induce lateral critical dimension (CD) enlargement, increasing intra-layer parasitic capacitance .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges