As conventional materials in CMOS manufacturing, Si as a gate material and SiO2 as a gate dielectric, approach their performance limit, the search for new materials becomes key point. Patterning of the new stacks containing these materials require both new plasma etch chemistries and new approaches.
We propose a BCl3/N2 based plasma mixture for the advanced gate patterning (in this case pure Ge gates and TaN metal gates). There are three reasons to select this combination:
a) The gas mixture generates Cl* species able to etch a diversity of materials, b) it is selective towards Si due to formation of passivating Si-B bonds and c) it improves profile control possibly by formation of a passivating BN-like film on feature side walls. It was found that BCl3 in presence of N2 results in a film deposition if no bias is applied to the substrate (i.e. there is no ion bombardment). The film is hexagonal BN-like since the characteristic peaks corresponding to the in-plane B-N and out-of-plane B-N-B bonds were found in FTIR spectra. The composition of the film surface as found by XPS is B, N and O (as no O2 is present in the plasma it may be a result of oxidation in the atmosphere), the amount of Cl is approx. 1%. The film is soluble in water that makes its removal easy. The deposition rate can be as high as 300 nm/min depending on plasma power, pressure, flow rates and BCl3 to N2 ratio.
We propose to use the BCl3/N2 mixture to etch materials too sensitive to Cl-based plasma. Pure BCl3 plasma might distort gate profiles, as materials are etched in a lateral direction as well, this is the case, e.g. for pure Ge gates. Addition of small amount of nitrogen (5% to 10%) to the BCl3 plasma preserves the vertical profile, apparently by the formation of a passivating BN-like layer on the vertical surfaces where there is no ion bombardment. Too high nitrogen concentration results in positively sloped gate profile or even in the etch stop that could be attributed to the too high deposition rate that exceeds the etch rate. All experiments have been performed in Lam Versys 2300 etch chamber.