the wet stripping composition must incorporate highly selective inhibitors to prevent the oxidative corrosion of any exposed underlying interconnect metals while still effectively complexing and dissolving the etch byproducts .
In depth
The Ashing & Strip/Clean step following the ILD 3-2 Oxide Etch is a c
ritical surface preparation phase in the bond pad module of the 40nm BSI CMOS image sensor flow . During the upstream reactive ion etching sequence that opens the deep bond pad via through multiple dielectric tiers, fluorocarbon (CFx) polymers are intentionally deposited on the etched sidewalls to ensure etching anisotropy and protect the dielectric from lateral degradation . Consequently, the resulting cavity is coated with hardened photoresist, organometallic byproducts, and dense fluorocarbon polymeric residues . Before the subsequent deposition of a Ta-based bottom barrier and Aluminum metallization, all polymeric masks and residues must be completely eradicated to guarantee pristine electrical contact and prevent barrier layer delamination . Unlike intermediate BEOL cleans (such as steps 11 or 17), this specific step must navigate a highly challenging, multi-layer, high-aspect-ratio topography without inducing dimensional loss in the thick ILD stack (Engineering Practice). The physical and chemical removal mechanism typically involves a hybrid approach of plasma ashing followed by wet chemical stripping . In the plasma ashing phase, reactive radicals oxidize the bulk organic photoresist into volatile compounds like carbon dioxide and water vapor . However, the fluorocarbon residues generated by the prior dielectric etch form a highly crosslinked, carbon- and fluorine-rich network that resists conventional dissolution . To prevent these residues from hardening into an impenetrable shell, low-temperature plasma processing is heavily favored, as high thermal loads during ashing can induce severe thermochemical crosslinking within the polymer backbone . Following the dry ash, a wet cleaning process is deployed to penetrate and dissolve the remaining inorganic-organic composite residues . Organic solvents containing specific reactive components dissolve functional groups such as esters and lactones within the polymer fragments, while the application of megasonic acoustic waves induces cavitation, enhancing mass transport of the chemistry deep into the high-aspect-ratio bond pad trenches and mechanically disrupting interfacial adhesion . Material and method selection for this step is strictly governed by the delicate balance between residue removal efficiency and the structural preservation of the exposed low-k dielectric layers . Traditional oxygen plasma ashing, while highly effective at resist stripping, aggressively attacks the Si-CH3 bonds in carbon-doped oxide (SiOC) low-k films, converting them to polar Si-OH groups and leading to severe moisture absorption and dielectric constant (k-value) degradation . To circumvent this, advanced integration schemes may utilize H2O-based plasma chemistries that offer superior critical dimension (CD) control and minimize the damaged dielectric thickness . Alternatively, nanoscale ultraviolet (UV) irradiation can be employed as a pretreatment to induce photochemical chain scission of C-C and C-F bonds within the fluoropolymer backbone . This UV modification significantly reduces the polymer's molecular weight and crosslink density, altering its surface energy so that subsequent wet solvents (e.g. , DMSO and monoethanolamine) can effectively wet and dissolve the residues without requiring damaging ion bombardment . Furthermore, the wet stripping composition must incorporate highly selective inhibitors to prevent the oxidative corrosion of any exposed underlying interconnect metals while still effectively complexing and dissolving the etch byproducts .
Risks & Challenges
[High] Low-k Dielectric Degradation (k-value Shift): Oxygen plasma radicals or highly polar wet solvents can penetrate the porous low-k dielectric sidewalls during the strip process, breaking hydrophobic Si-CH3 bonds and replacing them with polar groups . This chemical modification causes the dielectric to absorb ambient moisture, drastically increasing the wire-to-wire capacitance (RC delay) and potentially inducing leakage currents .
[High] Incomplete Fluorocarbon Residue Removal: If the ashing temperature is too high, the photoresist and etch byproducts undergo thermochemical crosslinking, transforming into a hardened, insoluble carbon-rich polymeric shell . If the subsequent wet chemistry or UV modification dose is insufficient to break these dense C-F networks, residual polymer will remain at the via bottom, leading to unacceptably high contact resistance or structural delamination of the subsequent Ta barrier .
[Medium] Critical Dimension (CD) Loss / Trench Bowing: Aggressive plasma ashing inevitably creates a thin layer of damaged, carbon-depleted oxide on the ILD sidewalls . If the subsequent wet cleaning chemistry contains etchants that readily dissolve this modified oxide, it will isotropically remove the damaged layer, resulting in unintended CD enlargement and bowing of the bond pad profile .
[Low] Metal Interconnect Corrosion: If the wet stripping composition relies on excessively potent oxidizers or lacks properly balanced long-chain alkylamine corrosion inhibitors, the chemical solution may electrochemically or directly attack the metallic structures exposed at the bottom of the trench, leading to voiding and reliability failures .