40nm BSI CMOS Image SensorPreview

ep immediately following MET0 Tungsten (W) Etch is critical for completely removing patterned photoresist, hardmask remnants, and highly reactive etch byproducts [A1]. Unlike subsequent Ashing & Strip/Clean steps that primarily target copper dual-damascene structures and porous low-k dielectrics [P4], this specific MET0 operation must manage dense, fine-pitch tungsten lines and barrier materials such as TiN or WN (Engineering Practice). The primary objective is to yield an ultra-clean metallic surface and a defect-free inter-line space, thereby preventing leakage currents and preparing the topography for the subsequent Pre-Metal Dielectric 4 (PMD4) deposition [A2]. Any residual organic or metallic contaminants left on the surface can act as interface states, fundamentally altering the local electrical properties and barrier heights of the metal-dielectric system [T1]. The physical and chemical mechanisms of this step typically involve a hybrid approach: dry structural modification followed by wet chemical dissolution [P1]. Traditional high-temperature oxygen plasma ashing can induce severe crosslinking in the photoresist, converting it into a hardened, carbon-rich polymeric shell that actively resists wet chemical penetration [P3]. To circumvent this, advanced processes utilize in-situ low-temperature oxygen plasma treatments to volatilize the bulk organics without baking the residue into an intractable state [P3]. Alternatively, ultraviolet (UV) irradiation (e.g. (Engineering Practice), at nanoscale) can be employed to drive photochemical chain scission within the fluorocarbon polymer backbone, reducing crosslink density and increasing the surface polarity of the residue [P1]. Following this structural relaxation, selective oxidation via ozone or highly formulated organic solvents attacks the unsaturated C=C bonds, swelling the polymer matrix and allowing the wet chemistry to fully detach the residue from the underlying substrate [P2]. Material selection for the wet stripping chemistry is heavily constrained by the necessity to dissolve post-etch residues without corroding the highly exposed tungsten lines [A2]. Conventional cleaning solutions, such as buffered fluorides or aggressive ammonia-peroxide mixtures, are prone to actively corroding metallic tungsten, which unacceptably increases interconnect resistance [A2]. Instead, modern formulations incorporate specific ammonium-salt oxidizers designed to selectively oxidize barrier residues (e.g. [A2], converting WN to soluble species) coupled with primary alkylamine corrosion inhibitors [A2]. These primary alkylamines operate via selective physical adsorption onto the elemental tungsten surfaces, passivating the metal against chemical attack while allowing the dissolution of non-metallic or nitrided byproducts [A2]. Process parameters such as fluid temperature and megasonic acoustic power are precisely tuned; elevated temperatures enhance dissolution kinetics, while megasonic cavitation provides the mechanical energy required to overcome interfacial adhesion forces in high-aspect-ratio spaces [P4]. At the 40nm technology node, the extreme proximity of the MET0 lines severely shrinks the tolerable process window for post-etch residue removal [P2]. If physical under-etching or aggressive isotropic wet stripping is applied, lateral CD loss of the tungsten lines can easily exceed the permissible electrical tolerance [A2]. Furthermore, the cleaning mechanics must ensure that highly volatile halogenated byproducts, such as Cl or F radicals trapped in the post-etch polymer shell, are completely complexed and washed away to prevent delayed corrosion of the metal interconnects [P3].