The effect of bias frequency on etch profiles in ultra-low electron temperature plasma

Achieving vertical etch profiles is essential for high-aspect-ratio (HAR) structures in semiconductor manufacturing. In our previous study, ideal vertical etch profiles were obtained using an ultra-low electron temperature (ULET, T_e < 1 eV) plasma, generated by inductively coupled plasma (ICP) with a negatively biased grid. However, when the bias power at 12.56 MHz was increased, the etch profiles became distorted, exhibiting undercut and bowing. This distortion is likely due to a significant portion of the bias power being consumed by electron heating, leading to an increase in electron temperature. In this study, etching behavior under a lower bias frequency of 2 MHz, which is known to induce less electron heating than 12.56 MHz, is investigated to mitigate profile distortions. At 2 MHz, the bias power is primarily used for ion acceleration toward the substrate. As a result, undercut is reduced by up to 50 % compared to 12.56 MHz under the same bias power, and the etch profile remains vertical without bowing. These findings highlight the critical role of bias frequency in controlling etching behavior and achieving ideal etch profiles in HAR structures.