Ultrathin extreme ultraviolet patterning stack using polymer brush as an adhesion promotion layer
Initial readiness of extreme ultraviolet (EUV) patterning has been demonstrated at the 7-nm device node with the focus now shifting to driving the "effective" k1 factor and enabling the second generation of EUV patterning. In current EUV lithography, photoresist thicknesses <30 nm are required to meet resolution targets and mitigate pattern collapse. Etch budgets necessitate the reduction of underlayer thickness as well. Typical spin-on underlayers show high defectivity when reducing thickness to match thinner resist. Inorganic deposited underlayers are lower in defectivity and can potentially enable ultrathin EUV patterning stacks. However, poor resist-inorganic underlayer adhesion severely limits their use. Existing adhesion promotion techniques are found to be either ineffective or negatively affect the etch budget. Using a grafted polymer brush adhesion layer, we demonstrate an ultrathin EUV patterning stack comprised of inorganic underlayer, polymer brush, and resist. We show printing of sub-36-nm pitch features with a good lithography process window and low defectivity on various inorganic substrates, with significant improvement over existing adhesion promotion techniques. We systematically study the effect of brush composition, molecular weight, and deposition time/temperature to optimize grafting and adhesion. We also show process feasibility and extendibility through pattern transfer from the resist into typical back end stacks.