Invited Paper PS2-TuA7
Plasma-induced Surface Roughening and Ripple Formation during Plasma Etching of Silicon

Tuesday, October 20, 2015, 4:20 pm, Room 210B

Atomic- or nanometer-scale surface roughness has become an important issue in the fabrication of nanoscale devices, because the roughness at feature sidewalls and bottom surfaces affects the variability in transistor performance. A better understanding of the mechanisms for the plasma-induced surface roughening is indispensable for suppressing the evolution of the roughness during plasma etching; moreover, the surface roughening through plasma exposure is positively employed in some cases, to obtain surface nanostructures such as nanopillars and nanocolumns. This paper presents a numerical and experimental study of surface roughening and ripple formation during Si etching in Cl-based plasmas, with emphasis on modeling, analysis, and control of the plasma-surface interactions concerned. A three-dimensional atomic-scale cellular model (ASCeM-3D) based on the Monte Carlo algorithm, which was developed to simulate plasma-surface interactions and the feature profile evolution during plasma etching, exhibited the nanoscale surface roughening and rippling in response to ion incidence angle onto substrate surfaces [1]: randomly roughened surfaces at normal incidence, and ripple structures or slit-like grooves perpendicular and parallel to the direction of ion incidence at oblique and grazing incidences, respectively. Such roughening and rippling of etched surfaces were found to be crucially affected by the ion scattering or reflection on microscopically roughened feature surfaces. Experiments of the surface roughening during Si etching in inductively coupled Cl₂ plasmas showed roughening and smoothing (or non-roughening) modes which occur depending on ion incident energy [2]. The analysis with the help of plasma diagnostics and the ASCeM-3D and classical molecular dynamics (MD) simulations [3] indicated that these two different modes of surface roughening correlate essentially to changes in the predominant ion flux from ions with high reflection probabilities to those with lower ones on surfaces on incidence at increased ion energy. The experiments further demonstrated that the pulse-biasing is effective for reducing the surface roughness during plasma etching, and the surface rippling with oblique and grazing ion incidences onto substrate surfaces was demonstrated using a sheath-control plate placed thereon.

[1] H. Tsuda et al., J. Vac. Sci. Technol. B 32, 031212 (2014).

[2] N. Nakazaki et al., J. Appl. Phys. 116, 223302 (2014).

[3] N. Nakazaki et al., Jpn. J. Appl. Phys. 53, 056201 (2014).