Directional Nanoscale Silicon Etching using SF₆ and O₂ Plasma

Plasma etching is important to realize fine geometry for the fabrication of silicon nanostructures. The Bosch process is probably the most popular technique in the nanofabrication industries today. However, the high roughness with finite sidewall scallop size and hard to remove fluorocarbon residue on the sidewalls of etched structures make the process less favorable for nanoscale engineering. This PhD project focuses on the development of a reliable and stable technology for nanoscale silicon plasma etching which addresses both issues. The specific contributions of this thesis are summarized below.
First, a fluorocarbon-free directional silicon etching procedure called CORE (stands for Clear, Oxidize, Remove, and Etch) has been developed in which a switching sequence of SF₆ and O₂ is  operated at room temperature. Compared to other etch processes, the CORE sequence has the advantage of precise profile control, high selectivity, small sidewall roughness and flexible process programming. It also shows an excellent performance in nanoscale structures with an accurate and controllable etch rate between 1 and 50nm/min (and SiO₂-selectivity of ca. 35) using the etch-tool in the RIE-mode. By adding the ICP source (DRIE-mode), a directional etch rate up to 1μm/min and selectivity >200 for SiO2 is possible.
Second, the formation of black silicon (BSi) in SF₆/O₂ plasma has been investigated based on the CORE sequence. By manipulating its parameters and utilizing the self-limiting property of the oxidation step, the CORE sequence can easily be modified to create either BSi-full or BSi-free surfaces independent of the aspect ratio of the etch features. The latter distinguishes the BSi formation clearly from other directional processes, thus provides a versatile tool for creating BSi anywhere at anytime, as it is called  ‘BSi on Demand’.
Third, the performance of the CORE sequence has been demonstrated to fabricate ultra-high aspect ratio (HAR) nanofeatures. The effect of different CORE parameters on the etch rate and profile are investigated and optimized with respect to low mask undercut and high directionality (vertical) etch. The nanopillar arrays (200nm diameter, 400nm pitch and 60nm diameter, 500nm pitch) have smooth straight sidewalls with aspect ratios beyond 55 for gaps and up to 200 for pillars. Due to the very mild plasma condition (less than 40W RIE power), the mask selectivity can be tuned above 500.
Finally, a procedure for Cr etching using SF₆ and O₂ plasma has been demonstrated for the first time. The etch mechanism is explained by considering the formation of volatile chromyl fluoride in which the Cr is first reacting with oxygen radicals and the formed CrOx subsequently reacts with fluorine radicals into CrO₂F₂. For the mixed mode, the proposed etch procedure performs at 300W plasma power with etch rates of Cr up to 150nm/min for a SF₆/O₂ gas ratio below 1%. For the switched mode, the etch rate is around 7nm/min with high selectivity with respect to silicon (> 20) and better profile control.