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Trilayer stacks with alternating etch selectivity were developed and extensively investigated
for high NA immersion lithography at 32nm node and beyond. The conveyance of pattern transfer
function from photoresist to Si-containing bottom anti-reflective coating (Si-BARC) and carbonrich
underlayer hard-mask (UL) elegantly solved the small etch budget issue for ultra-thin
photoresists in immersion lithography. However, due to the hybrid nature of Si-BARC, many
different behaviors were observed in comparison to conventional BARC. Lithographic
performance, stability, and reworkability were among the most challenging issues for trilayer
scheme.
Despite of the rapid improvement in lithographic performance and stability of trilayer
materials reported by several papers, the rework and cleaning of trilayer materials by wet chemistry
remained a challenging problem for manufacturability. The dual function requirement of reflection
control and pattern transfer (i.e. hard-masking) for spin-on Si-BARC mandates hybrid materials.
Si-BARC containing both organic moiety and inorganic backbone were extensively studied and
demonstrated excellent performance. However, the hybrid nature of Si-BARC necessitates the
revisit of different wet chemistries and process adjustment is essential to achieve desirable results.
In addition, the similarity in chemical structures between Si-BARC and low-κ dielectrics demands
subtle rework differentiation by wet chemistry from a chemistry point of view.
In our development, we strived to identify rework solutions for trilayer materials in both
front-end-of-line (FEOL) and back-end-of-line (BEOL) applications. Rework solutions including
diluted HF, Piranha, and low-κ compatible strippers were extensively investigated. The
optimization of solution mixture ratios and processing conditions was systematically studied.
Thorough defect inspection after rework was performed to ensure the readiness for
manufacturability. Extensive Piranha rework study on stack wafers and monitor wafers were
carried out and excellent results are reported.
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