Low-k dielectric etch challenges at the 7 nm logic node and beyond: Continuous-wave versus quasiatomic layer plasma etching performance review

For logic nodes of 7 nm and beyond, back-end-of-line (BEOL) trench patterns have a critical pitch of less than 40 nm, directly affecting the plasma etch process window of the dual damascene etch process. Feature size dependent etch depth (reactive ion etch, RIE lag), hard mask selectivity, and ultra-low-k (ULK) damage have become significant challenges that must be overcome in order to meet target device performance. Recently, atomic layer etching has been used to widen the plasma etch process window in terms of selectivity and process control [S. Sherpa, P. L. F. Ventzek, and A. Ranjan, J. Vac. Sci. Technol. A 35, 05C310 (2017); T. Tsutsumi, H. Kondo, M. Hori, M. Zaitsu, A. Kobayashi, T. Nozawa, and N. Kobayashi, J. Vac. Sci. Technol. A 35, 01A103 (2017)]. In this work, the impact of a quasiatomic layer etch (QALE) process, a conventional continuous wave plasma, and a pulsed plasma process on ULK materials were investigated to determine the benefits of an ALE process approach for BEOL etching. Both blanket ULK film and patterned ULK samples were used for this study. The ULK etch damage from each process was characterized using Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy on three different ULK films. From patterned samples, it was determined that QALE could be used to successfully suppress RIE lag in low-k materials at advanced pitches, while keeping low-k damage to a minimum. In addition, the QALE technique showed improved hard mask selectivity and resulted in lower line edge pattern roughness. Based on this study, the authors concluded that QALE is a powerful plasma etch method to overcome BEOL etch challenges at advanced pitches.