Cr and CrO_x etching using SF₆ and O₂ plasma

Chromium is a frequently encountered material in modern nanofabrication, directly as a functional material (e.g., photomask generation) or indirectly as a hard mask (e.g., to etch quartz). With the continuous downscaling of devices, the control of the feature size of patterned Cr and CrOx becomes increasingly important. Cr and CrOx etching is typically performed using chlorine-oxygen-based plasma chemistries, but the nanoscale imposes limitations. In this work, directional etching is demonstrated for the first time using fluorine-oxygen-based plasma. Two cases are studied to demonstrate the Cr etch performance: (i) a plasma mixture of SF6 + O2 and (ii) a switching SF6/O2 procedure in which the plasmas are used sequentially. The proposed mixture performs with Cr etch rates (ERs) up to 400 nm/min at 300 W platen power and is highest when the SF6/O2 gas ratio is ∼0.75%, i.e., almost pure O2 plasma. The profile shows reasonable directionality but the etch selectivity is low, less than 5 toward Si, due to the high generated self-bias of 420 V. The selectivity of the plasma mixture can be improved at a lower plasma power, but this is accompanied with considerable undercut. The etching of CrOx proceeds without the need for O2 in the feed, and, therefore, the ER can reach much higher values (beyond 2000 nm/min at 300 W). As the plasma mixture seems to be inadequate, a sequential process is studied with improved selectivity while preserving directionality. The high selectivity is achieved by using relatively low plasma power (to ensure a low self-bias) and the directionality is due to the time separation of the SF6 and O2 plasmas and a controlled directional removal of CrFx etch inhibiting species. Using such a switched procedure at 30 W plasma power, a selectivity beyond 20 with good profile directionality is achieved and having an etch rate of ∼1 nm per cycle (or 7 nm/min).