40nm BSI CMOS Image Sensor

The upstream Oxide Etch [WFR] step defines critical alignment markers by selectively removing the pad oxide layer using chemically reactive fluorocarbon plasma [A1].This high-energy etching process invariably leaves behind a combination of hardened photoresist (PR), bottom anti-reflective coating (BARC), and highly cross-linked fluorocarbon (CFx) post-etch residues on the wafer surface and trench sidewalls [P1, P3].These organic and polymeric residues must be entirely eliminated before the subsequent deposition of the SiO Hard Mask for the Frontside Deep Trench Isolation (F_DTI) module [P1].Any residual contamination at this stage will cause localized adhesion failures or act as masking defects during the hard mask deposition, fundamentally compromising the F_DTI integration [P2].Unlike later Back-End-Of-Line (BEOL) ash and strip steps that must carefully avoid damaging fragile, porous low-k dielectrics by minimizing plasma exposure [P2], this specific front-end step interacts primarily with robust thermal oxide and silicon substrates (Engineering Practice).Consequently, this step's position in the flow allows for the use of standard, higher-energy oxygen-based plasma ashing processes that would otherwise degrade sensitive BEOL structures [P1, P2].The physical and chemical mechanism of removal typically relies on a sequential dry-plasma and wet-chemical approach [P3].Initially, plasma ashing utilizes a localized plasma source to generate monatomic reactive species, primarily oxygen or fluorine radicals, which chemically combine with the bulk organic photoresist to form volatile byproducts (like CO2 and H2O) that are subsequently exhausted by a vacuum pump [A2].However, the preceding fluorocarbon RIE process typically induces the formation of a highly cross-linked, fluorine-rich dense polymeric network—often forming a crust on the PR and sidewalls—that resists pure oxygen oxidation [P1].To clear this robust composite shell, a subsequent wet stripping process is employed, utilizing multi-functional organic solvents to chemically penetrate the modified PR [P3].The solvents act by dissolving specific functional groups, such as ester and lactone functionalities present within the resist backbone [P3].To enhance the mass transport of these chemical agents into the patterned high-aspect-ratio alignment trenches, megasonic acoustic energy is often applied to the cleaning fluid; this generates acoustic cavitation and microjets that physically disrupt the residue-substrate interface and facilitate mechanical delamination [P3].Material and method selection for this step prioritizes maximizing residue removal efficiency without altering the critical dimensions of the alignment fiducials [P1].While advanced technology nodes increasingly adopt completely plasma-less or UV-assisted wet removal schemes to protect fragile low-k materials [P2], the front-end pad oxide and bare silicon can safely endure standard oxygen or forming-gas plasmas without suffering dielectric constant degradation (Engineering Practice).The interaction of process parameters dictates the overall efficacy of the strip: elevating the temperature of the wet solvent exponentially increases dissolution kinetics, while tuning the megasonic power directly modulates the cavitation intensity used to dislodge stubborn BARC or CFx crusts [P3].Proper optimization of the dry-to-wet ratio ensures that the plasma sufficiently breaks the chemical bonds of the fluorinated shell, allowing the wet chemistry to efficiently lift off the remaining fragments without requiring excessively long wet-exposure times that could otherwise cause subtle galvanic corrosion or oxide roughening [P1].

• [High] Incomplete Fluoropolymer Residue Removal: The fluorocarbon plasma used in the preceding oxide etch intentionally deposits a highly cross-linked CFx polymer on sidewalls to maintain etch anisotropy [P1].If the dry ashing plasma does not sufficiently break the C-C and C-F bonds of this dense network, or if the subsequent wet solvent fails to penetrate it, residual polymer will remain in the alignment marker trenches and impede the uniform adhesion of the next SiO hard mask layer [P2].- [Medium] Unintentional Substrate Oxidation: During the high-energy plasma ashing phase, the aggressive generation of monatomic oxygen reactive species can unintentionally oxidize the exposed bare silicon at the bottom of the alignment marker trenches [A2].Uncontrolled growth of this native-like oxide can subtly alter the optical step height and contrast of the fiducials, which degrades the interference signal required by the stepper tool for precise overlay in the subsequent F_DTI lithography [A3].- [Medium] Megasonic Cavitation Damage: The application of acoustic energy during the wet clean generates cavitation bubbles that collapse violently, creating high-velocity liquid microjets intended to dislodge residues [P3].If the megasonic power parameter is set excessively high, the intense mechanical shockwaves from transient cavitation collapse can fracture the sharp geometric corners of the patterned pad oxide or induce micro-pitting in the exposed silicon substrate [P3].