40nm BSI CMOS Image SensorPreview

CN Etch that exposes the underlying Metal 1 (M1) copper [P3]. During the preceding fluorocarbon-based plasma etch, highly crosslinked fluoropolymer residues are intentionally deposited on the dielectric sidewalls to ensure etching anisotropy and profile control [P1]. However, these post-plasma etch residues, along with any remaining photoresist and sputtered copper species from the exposed M1, must be completely removed to ensure a pristine interface for the subsequent Via 1 metallization [P1]. Failure to remove these residues interferes with subsequent contact formation and degrades the electrical integrity of the metal-to-metal via connection [A2]. What distinguishes this specific step from subsequent higher-level ashing steps is its direct exposure to the tightest-pitch M1 interconnects, demanding an exceptionally delicate balance between complete residue removal and the absolute prevention of copper oxidation [P4]. The fundamental physical and chemical mechanisms of this cleaning step rely on breaking down the heavily crosslinked, carbon-fluorine rich dense network of the post-etch residues [P1]. Conventional high-temperature oxygen plasma ashing can cause severe damage to porous low-k dielectrics by stripping functional groups, leading to moisture uptake, an increased dielectric constant, and structural damage such as non-planar top surfaces under hardmask edges [P2]. To mitigate this dielectric degradation, advanced strip processes often employ a combination of gentle structural modification followed by selective wet chemistry [P1]. For example, applying specific ultraviolet photon energy can induce photochemical chain scission in the polymer backbone, reducing crosslink density and introducing polar groups that enhance subsequent solvent wetting [P1]. Alternatively, all-wet processes utilizing organic solvents combined with reactive components and megasonic agitation rely on selective solvation and acoustic cavitation to disrupt and delaminate the hardened composite shell of the residues [P3]. The selection of the wet stripping formulation is strictly governed by the thermodynamic requirement to achieve high etching selectivity between the post-etch residues and the exposed copper and dielectric materials [A2]. The cleaning composition typically incorporates a precise aqueous mixture of organic solvents, active oxidizers, corrosion inhibitors, and metal chelating agents [A3]. While aggressive chemistries containing hydroxylamine or ammonia-peroxide are highly effective at dissolving carbonaceous and metallic residues, they carry a high risk of corroding exposed metal lines such as copper or tungsten [A2]. Therefore, formulations often include primary alkylamines that selectively adsorb onto and passivate the metallic surfaces, creating a physical and kinetic barrier against undesired oxidation [A3]. The interaction of process parameters, such as chemical temperature and megasonic power, dictates the mass transport rates; higher temperatures accelerate dissolution kinetics but simultaneously increase the risk of the solvent penetrating the pores of the low-k dielectric [P3]. At the 40nm technology node, the shrinking interconnect dimensions and the integration of highly porous low-k dielectrics profoundly shrink the available process window for post-etch residue removal [P2]. As the half-pitch decreases, removing polymeric residues via dielectric under-etching is no longer viable without causing unacceptable critical dimension losses [P2]. Furthermore, plasma processing used to open bottom anti-reflective coatings and metal hardmasks generates complex crusts containing fluorine and metallic species, necessitating highly specialized, multi-functional cleaning combinations rather than simple single-solvent approaches [P3]. Consequently, the industry transitions toward sophisticated modify-and-dissolve sequences that preserve both the geometrical profile and the electrical characteristics of the advanced interconnect system [P2].