New bending mode in SAQP Si fins and its mitigation

Abstract

Self-Aligned Quadruple Patterning (SAQP) is used in advanced CMOS-FET technologies to enable the formation of arrays of fins at sub-40 nm pitches. As these technology nodes keep scaling down and the fins get taller for performance improvement, the patterns become denser and present very high aspect ratio (HAR). The scaling trends make the fin pattern more subject to mechanical instabilities during the processing steps. We describe in this paper a specific mechanical fin bending mode linked to the SAQP scheme targeting 24 nm pitch, the HAR and the formation and recess steps of the isolating oxide. We investigated the evolution of fin bending throughout the fin and isolation formation. We observed fins start bending at the FCVD deposition step exhibiting rotation angles up to 3 degrees. The bending was then worsened along the subsequent process steps causing mechanical failure after fin reveal where up to 7-8 of bending angles were observed. After having studied the influence of the key process steps, we can report a path to mitigate this phenomenon. Finally, we propose a simple mechanical analysis to validate quantitatively the root cause and extrapolate the mechanical robustness of this system for further scaling nodes where fin pitch is reduced.