Precise ion energy control with tailored waveform biasing for atomic scale processing

Anisotropic plasma-enhanced atomic layer etching (ALE) requires directional ions with a well-defined ion energy to remove materials in a highly selective and self-limiting fashion. In many plasma etching systems, the ion energy is controlled using radio-frequency (13.56 MHz) sinusoidal waveform biasing. However, this yields ions with a broad energy distribution, while also inducing electron heating mechanisms that can affect the ion flux. In this work, we report on precise ion energy control—independent of the ion flux—using low-frequency (LF: 100 kHz) tailored bias voltage waveforms in a commercial remote plasma reactor. A prototype LF bias generator has been used to apply tailored waveforms consisting of a positive voltage pulse and a negative linear voltage ramp. These waveforms yielded ions having narrow energy distributions (7 ± 1 eV full-width-at-half-maximum) measured on dielectric SiO₂ substrates for ion energies up to 200 eV in collisionless Ar plasmas. The mono-energetic ions were used to etch SiO₂ thin films by physical sputtering. In these sputter etch experiments, the ability to accurately control the ion energy in the <100 eV range is demonstrated to allow for a more precise determination of sputter thresholds, which serve as valuable input for the design of novel ALE chemistries. The feasibility of performing anisotropic plasma etching using LF tailored waveform biasing was established by etching a SiO₂ layer on a 3D trench nanostructure. The potential merits of this technique for the field of atomic scale processing are discussed.