Ashing & Strip/Clean - 40nm

Ashing & Strip/Clean modifies fluoropolymer residues and removes remaining photoresist from the SiCN surface to enable subsequent deposition processes .

The Ashing & Strip/Clean step in the Via 2 (V2) module is a critical post-etch operation directly following the ILD 2-1 SiCN breakthrough etch

. The upstream reactive ion etch (RIE) utilizes fluorocarbon-based plasmas to selectively remove the SiCN barrier, which invariably leaves behind heavily crosslinked fluoropolymer residues and remaining photoresist or hardmask materials . This specific cleaning step is distinct from other routine strip operations in the flow because it must address complex residues that are directly in contact with the newly exposed Metal 2 copper and the highly sensitive porous low-k via sidewalls . Complete removal of these contaminants is essential to prepare a pristine, chemically stable surface for the subsequent Metal 3 trench photolithography and to prevent via-to-via leakage or contact resistance degradation . The fundamental removal mechanism for advanced nodes transitions away from traditional high-temperature oxygen plasma ashing, which severely damages porous low-k dielectrics by breaking Si-CH3 bonds and introducing polar groups . Instead, modern cleaning schemes employ a combination of structural modification and wet chemical dissolution . During the modification phase, ultraviolet (UV) irradiation is often applied to the fluorocarbon residues to induce photochemical chain scission . This photon energy breaks C-C and C-F bonds in the polymer backbone, significantly reducing the molecular weight, decreasing crosslink density, and altering surface energy . In some integration schemes, ozone (O3) is introduced alongside UV to selectively oxidize C=C unsaturated bonds via ozonolysis, further fragmenting the polymer network into soluble segments . Following this chemical activation, an all-wet cleaning solution containing specific organic solvents and reactive components penetrates, complex-binds, and dissolves the weakened polymer structure . The selection of an all-wet or UV-assisted wet cleaning method is driven by the thermodynamic and kinetic requirements of dissolving dense, plasma-modified residues without consuming the fragile dielectric framework . Standard organic solvents alone cannot dissolve unmodified, highly crosslinked CFx networks . By coupling the chemical solution with megasonic acoustic waves, cavitation and microjets are generated within the fluid, which significantly enhances mass transport into the high-aspect-ratio V2 structures and physically assists in interfacial delamination . Parameter control is highly critical: the UV dose must be precisely optimized, as insufficient exposure fails to adequately fragment the polymer, while an excessive dose can trigger secondary crosslinking or damage the low-k material . Furthermore, to counteract any unintended dielectric damage, post-clean recovery treatments utilizing UV and specialized recovery precursors can be used to chemically re-graft hydrophobic groups onto the low-k surface, restoring its intended dielectric constant and improving conditions for subsequent metallization . At the 40nm technology node, interconnect scaling enforces extremely tight critical dimension (CD) tolerances, rendering historical polymer removal strategies—such as relying on slight dielectric under-etching—entirely obsolete . The integration of highly porous, ultra-low-k materials drastically reduces the structure's tolerance to conventional plasma-induced thermal loads, which are known to cause severe resist hardening and carbon-rich residue formation . Consequently, the implementation of mild, plasma-less modification paired with cavitation-enhanced wet chemistries provides the exact chemical selectivity needed to clear via bottoms without compromising the structural integrity of the V2/M3 dual-damascene framework .

[High] Low-k Dielectric Damage (k-value degradation): Oxygen-based plasmas or overly aggressive wet chemistries strip organic methyl groups from the porous low-k framework, increasing the density of polar Si-OH groups and overall hydrophilicity . This plasma-induced bond scission raises the dielectric constant, leading to severe RC delay and interline insulation reliability issues such as non-planar top surfaces .
[High] Incomplete Fluorocarbon Residue Removal: If the UV modification dose is insufficient, the highly fluorinated and crosslinked CFx residues generated during the prior SiCN etch fail to undergo adequate chain scission . Conventional wet solvents cannot penetrate or dissolve this dense polymer network , leaving residues that block the underlying copper contact and cause catastrophic open circuits or high via resistance (Engineering Practice).
[Medium] Photoresist Hardening and Crust Formation: Exposing the remaining photoresist to high thermal loads during conventional plasma ashing drives thermochemical crosslinking reactions within the polymer . This forms an extremely hard, carbon-rich crust that vigorously resists subsequent wet stripping , necessitating carefully controlled low-temperature or in-situ plasma techniques to prevent defect clustering .
[Medium] Megasonic-Induced Dielectric Damage and Solvent Trapping: While megasonic waves enhance mass transport via cavitation and microjets , excessive acoustic power can mechanically fracture the fragile porous low-k structures (Engineering Practice). Furthermore, overly intense cavitation can drive organic solvents deeply into the low-k pores, requiring extended high-temperature, low-pressure baking to fully drive out the trapped solvent and restore the k-value .

Ashing & Strip/Clean modifies fluoropolymer residues and removes remaining photoresist from the SiCN surface to enable subsequent deposition processes .

The Ashing & Strip/Clean step in the Via 2 (V2) module is a critical post-etch operation directly following the ILD 2-1 SiCN breakthrough etch

[High] Low-k Dielectric Damage (k-value degradation): Oxygen-based plasmas or overly aggressive wet chemistries strip organic methyl groups from the porous low-k framework, increasing the density of polar Si-OH groups and overall hydrophilicity . This plasma-induced bond scission raises the dielectric constant, leading to severe RC delay and interline insulation reliability issues such as non-planar top surfaces .
[High] Incomplete Fluorocarbon Residue Removal: If the UV modification dose is insufficient, the highly fluorinated and crosslinked CFx residues generated during the prior SiCN etch fail to undergo adequate chain scission . Conventional wet solvents cannot penetrate or dissolve this dense polymer network , leaving residues that block the underlying copper contact and cause catastrophic open circuits or high via resistance (Engineering Practice).
[Medium] Photoresist Hardening and Crust Formation: Exposing the remaining photoresist to high thermal loads during conventional plasma ashing drives thermochemical crosslinking reactions within the polymer . This forms an extremely hard, carbon-rich crust that vigorously resists subsequent wet stripping , necessitating carefully controlled low-temperature or in-situ plasma techniques to prevent defect clustering .
[Medium] Megasonic-Induced Dielectric Damage and Solvent Trapping: While megasonic waves enhance mass transport via cavitation and microjets , excessive acoustic power can mechanically fracture the fragile porous low-k structures (Engineering Practice). Furthermore, overly intense cavitation can drive organic solvents deeply into the low-k pores, requiring extended high-temperature, low-pressure baking to fully drive out the trapped solvent and restore the k-value .

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