fluorine-radical-based dry etching can cause localized fluorination on the exposed dielectric sidewalls, forming fluorosilicate glass (FSG) structures that effectively lower the local dielectric constant and improve resistive-capacitive (RC) delay performance .
In depth
The ILD 6-1 Etch is the concluding
dry etch step in the multi-stage dielectric patterning sequence for the Direct Bond Interconnect (DBI) module . Proceeding through ILD 6-4 to 6-2, the structure's upper trench and via geometries are defined, while this final step typically completes the via opening and breaks through the bottom etch-stop layer to expose the underlying metal pad or contact . Precise pattern transfer at this stage is critical because the subsequent steps involve ashing, cleaning, and the deposition of a Ta-based liner and Cu seed . By ensuring a fully cleared and well-profiled contact opening, this step guarantees optimal geometric conformity for the subsequent barrier and seed layers, minimizing contact resistance and ensuring reliable Cu electroplating . The physical mechanism of ILD etching relies on the synergistic effect of chemical reactions driven by neutral radicals and physical ion bombardment provided by high-density fluorocarbon plasmas . Fluorine atoms and CFx radicals generated in the plasma chemically react with the silicon-based dielectric surface to form volatile byproducts such as SiF4, which are continually pumped out of the chamber . Simultaneously, directional ion bombardment accelerates bond breaking and promotes the desorption of these reaction products, maintaining anisotropic profiles . However, the process must strictly manage differential charging effects on the insulating sidewalls . As ions enter high-aspect-ratio features, charge accumulation on the mask and dielectric surfaces can establish lateral electric fields . These fields deflect incoming ions toward the sidewalls, potentially damaging the protective passivation layer and causing profile bowing . To counter this, carbon-rich species in the plasma deposit a protective polymeric passivation layer on the sidewalls during the etch, balancing anisotropy and minimizing lateral erosion . Fluorocarbon gas chemistries (e.g. , CF4, CHF3) are selected because they offer highly tunable etch selectivities among SiO2, low-k dielectrics, and silicon nitride etch-stop layers . Modulating the hydrogen and carbon ratios in the feed gas alters the balance between etching and polymer deposition; for instance, polymerizing gases help build up sidewall passivation to maintain feature critical dimensions (CD) . Furthermore, fluorine-radical-based dry etching can cause localized fluorination on the exposed dielectric sidewalls, forming fluorosilicate glass (FSG) structures that effectively lower the local dielectric constant and improve resistive-capacitive (RC) delay performance . Process parameters such as bias power directly control the kinetic energy of the bombarding ions . Higher bias power enhances vertical anisotropy but exacerbates mask erosion and introduces potential plasma-induced damage at the trench bottom . At the 40nm technology node, stringent overlay and CD margins necessitate advanced interconnect alignment techniques, such as the use of self-aligned vias and multiple etch-stop layers . Because classical multi-layer masks introduce significant process complexity and edge roughness, precise control of the etch chemistry is required to terminate etching perfectly at the underlying interface without over-etching into adjacent structures . Furthermore, mitigating subthreshold leakage and static power issues in scaled devices requires that backend thermal budgets and plasma damage be kept strictly in check, preventing degradation of underlying active devices .
Risks & Challenges
[High] Profile Bowing and CD Distortion: Caused by differential charging effects on the insulating sidewalls during high-density plasma etching . Accumulated charges create a lateral electric field that deflects incoming ions, causing them to strike the sidewalls at grazing angles, eroding the passivation layer and producing a bowed profile .
[Medium] Incomplete Etch-Stop Breakthrough (Open Circuit): Occurs if the etch selectivity to the underlying etch-stop layer or complementary dielectric fill is excessively high, or if ion energy is insufficient to break through the interfacial layer . This leaves residual dielectric material at the via bottom, preventing the subsequent liner and metal from making direct electrical contact with the underlying pad .
[Medium] Dielectric Reliability Degradation due to Fluorination: While controlled fluorination can improve RC performance by forming FSG, excessive fluorine incorporation can weaken the silicon-oxygen network . This leads to increased material hygroscopicity, which can absorb moisture during subsequent wet clean or CMP steps, ultimately raising the dielectric constant and causing reliability failures .
[Low] Contact Resistance Variation from Plasma Damage: High-energy ion bombardment during the over-etch phase can induce surface damage or modify the stoichiometry of the exposed underlying metal pad . Although subsequent wet cleans mitigate this, persistent nonvolatile polymeric residues or amorphized layers can severely increase the interface resistance of the final interconnect .