Reworkable spin-on trilayer materials: optimization of rework process and solutions for manufacturability

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 carbon-rich 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-kappa 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-kappa 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.