Etch considerations for directed self-assembly patterning using capacitively coupled plasma

Abstract

Alternative patterning strategies are pursued to push the device feature size below the physical limit of optical lithography as the semiconductor manufacturing industry is preparing for production at sub-10 nm technology node. Extreme ultraviolet (EUV) lithography, 193 nm immersion augmented with multiple patterning schemes (“self-aligned double patterning,” “self-aligned quadruple patterning”) and “directed self-assembly (DSA)” are being evaluated as alternatives to meet rising demands of aggressive patterning. EUV lithography reduces the number of processing steps, but it is yet to achieve full maturity in terms of resist materials, throughput, and manufacturability. DSA when augmented with 193 nm immersion guide prepatterns can aid in reducing the pitch of final structures. There is no infrastructure upgrade cost involved as the key processing steps of DSA are conducted in existing wafer track systems. The authors have successfully demonstrated DSA pattern transfer into metal hard masks for the back end of the line application and nonmetal hard masks for the front end of the line applications. However, DSA comes with its own challenges posed in the form of polymer-to-polymer selectivity, mask budget, post-lithography defects, mask shape, critical dimension control, and line edge roughness (LER). The authors address the challenge of selectivity and roughness correction by using spatially uniform low-density plasma obtained in dual (low and high) frequency midgap capacitively coupled plasma etcher. A parametric study of an O2/Ar gas chemistry based plasma etch of widely studied poly(styrene-block-methyl methacrylate) (PS-b-PMMA) films is used to describe how plasma parameters impact PMMA removal selective to PS and LER during plasma etch pattern transfer using DSA lithography. The effects of etchant gas concentration and deposition gas addition for preferential passivation of the PS mask are investigated during PMMA etch. Their results indicate modulation of ion energy through bias power adjustments can be used to improve selectivity. Zero bias power optimal; however, roughness degrades at this condition necessitating inclusion of other solutions. Controlled addition of hydrocarbon gas enhances the selectivity further. Low frequency peak-to-peak voltage and high frequency power most strongly correlate with LER; hydrocarbon addition has little effect. Relative balance between various fluxes and ion energy is needed to obtain the maximum reduction in roughness with the required selectivity.