Anti-Reflective Coatings in Multi-Layer Lithography - 2

Functional Mechanism and Technical Advantages of the Si-BARC Silicon-Based Anti-Reflection Coating

Si-BARC (Silicon-Based Anti-Reflection Coating) serves as the central functional layer in a three-layer architecture. It is an organic-inorganic siloxane hybrid film with a thickness controlled between 30 and 60 nm and a silicon content ranging from 5% to 40%. Compared to the pure absorption mechanism of SOC, Si-BARC employs a composite optical mechanism of interference extinction combined with weak absorption. This perfectly complements the strong absorption system of SOC, addressing the issue of insufficient reflection suppression in single-absorption systems under high NA and wide-angle incidence conditions.

Optically, Si-BARC strictly adheres to the 193 nm λ/4 thin-film interference criterion. By precisely controlling the film thickness and refractive index, it creates a 180^o phase difference between the reflected light at the “photoresist/Si-BARC” upper interface and the reflected light at the “Si-BARC/SOC” lower interface, achieving destructive interference and thereby realizing secondary optical attenuation. The SOC is responsible for the primary absorption of vertically incident light, while the Si-BARC handles the interferential cancellation of obliquely incident light. Together, they achieve ultra-low reflectivity across all angles and a wide process window, completely eliminating standing waves, pattern notches, and local scattering defects.

At the etching process level, the silicon component of Si-BARC is key to advanced pattern transfer. During oxygen plasma etching, a dense SiO₂ passivation layer spontaneously forms on the film surface, significantly reducing the etch rate and serving as a stable intermediate hard mask. The complete pattern transfer process consists of three steps: fluorine-based etching of Si-BARC using photoresist as a mask; oxygen-based etching of SOC using Si-BARC as a mask; and deep etching of the substrate using the thick carbon layer on SOC as a mask. This perfectly addresses the process challenge posed by ultra-thin photoresists that cannot withstand deep etching.

Mass Production Synthesis Formulation for Si-BARC Siloxane Monomers

The optical parameters, silicon content, and etch resistance of Si-BARC are entirely determined by the siloxane monomer formulation. Based on the sol-gel hydrolysis and polycondensation process, the industry has developed two mature synthesis formulations—a medium-silicon general-purpose type and a high-silicon hard mask type—to accommodate different process scenarios.

General-purpose Si-BARC with medium silicon content is formulated with a silicon content of 8% to 12%, focusing on optical matching and interlayer compatibility, and is used in 28/14 nm conventional logic processes. The molar ratio of monomers is 60% PTMS, 30% MTMS, and 10% GPTMS. PTMS introduces a phenyl-conjugated structure to enhance the n/k parameters and ensure interference extinction capability; MTMS provides a flexible methyl structure to improve resin solubility and reduce film-forming stress; GPTMS introduces epoxy-reactive groups to enable low-temperature self-crosslinking and reduce defects caused by small-molecule volatilization. When combined with β-naphthol as a light-absorbing copolymer monomer, the system consistently delivers high-quality optical performance with n=1.85~1.95 and k=0.28~0.35, featuring a wide process window and high yield stability.

The high-silicon hard mask model Si-BARC can achieve a silicon content of up to 40%, featuring an ultra-high etch selectivity ratio and is suitable for challenging processes such as 3D NAND and SAQP quadruple patterning. The monomer ratio is 45% DMDES, 35% PTMS, and 20% MTES. The bifunctional DMDES significantly increases silicon backbone density and enhances resistance to oxygen etching; PTMS provides aromatic light-absorbing structures, compensating for the relatively low k-value typical of high-silicon systems; MTES undergoes mild hydrolysis, resulting in a more uniform resin molecular weight distribution and excellent film thickness consistency. The system employs BHPF fluorene-based monomers in copolymerization, resulting in high film rigidity and minimal bake shrinkage, which effectively suppresses line wobble distortion. The etch selectivity ratio between Si-BARC and SOC can reach 1:18, with extremely low hard mask loss.

Both formulations follow a standardized synthesis process: low-temperature acid-catalyzed hydrolysis to produce silanol intermediates, temperature-elevated copolymerization with aromatic monomers, vacuum removal of alcohol byproducts, dilution with PGMEA solvent, and precision filtration, ultimately yielding photolithography-grade high-purity coating solutions.

The SOC+Si-BARC dual-layer architecture achieves optimal complementarity in material performance. SOC addresses the shortcomings of thin Si-BARC—namely, its inability to achieve planarization and insufficient light absorption—while Si-BARC compensates for the limitations of a single-layer SOC, such as the lack of interference control and etching selectivity. This composite system offers four major advantages: global planarization, low reflection at all angles, low-roughness imaging, and high selectivity in pattern transfer. It resolves the process challenges that traditional single-layer BARC cannot address, such as adapting to topographical variations, high-reflection substrates, and etching with high aspect ratios.

Currently, this three-layer lithography dual-layer anti-reflection system has become the dominant solution for advanced 193 nm immersion lithography. It is widely used in the mass production of logic chip FEOL gates, BEOL metal interconnects, high-density through-holes, and 3D NAND multi-layer stacked structures, providing a solid foundation of materials and processes to ensure nanoscale lithographic dimensional accuracy and process yield.

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