UCLA

As demand for computing power and memory continues to increase, continued scaling requires the integration of Ru as a replacement for Cu in the lowest level of metal wiring, and CoFeB for enabling magnetoresistive memory cells. To etch these materials, a systematic approach was taken. In this work, thermochemical analyses were performed to assess the feasibility of each chemistry and guide the experimental work. A combination of modelling, diagnostics, and sample characterization was used to determine the etch mechanisms, enabling selectivity and anisotropy.A cyclic chemical process with sequential exposure to N2, H2, and O2 plasmas was developed to circumvent the isotropic etch profile formed by O2 plasma without loss of selectivity. RuN was formed in N2 plasma to act as a passivation layer against O2 plasma etching of Ru. A hydrogen plasma was able to reduce RuN to Ru, with low energy hydrogen ions enhancing the reduction. The cyclic process yielded an etch rate of 3.6 nm per cycle with ions present, but did not etch when ions were excluded. The ion-dependent etch was confirmed by etching nano-scale patterned ruthenium samples. Oxygen plasma produced an isotropic etch profile with an etch anisotropy of 4 and a selectivity of 27 with respect to the hard mask, SiO2. The N2/O2 cyclic process increased the anisotropy to 10, but the etch profile was tapered due to passivation by RuN. The N2/H2/O2 cyclic process maintained an anisotropy of 10 while achieving a more vertical etched profile and a selectivity of 16. On nanoscale features, the cyclic process was anisotropic, less dependent on aspect ratio, and maintained a high selectivity versus the hard mask.To remove CoFeB chemically while circumventing redeposition of less volatile reaction products, atomic layer etching (ALE) of CoFeB was performed using a two-step process. Oxygen plasma was used to increase the oxidation state of the metal, enabling reactions with organic acid vapor to form volatile products. The use of O2 plasma was found to cause the formation of Co3O4, a mixed valence oxide that led to lower etch rate of Co by preventing the formation of Co (II) products. Formic acid or hexafluoroacetylacetone (hfac) were both shown to be able to etch the oxidized CoFeB surface in a controlled manner. The process using formic acid a had a lower etch rate, 0.26 nm per cycle, and organic residue remained on the surface, but the selectivity vs. W was effectively infinite. Etching with hfac had a higher etch rate of 0.55 nm per cycle without organic residue on the surface, but the selectivity vs. W was 6 and the surface was enriched with Co.This work utilized a systematic approach involving thermochemical analysis followed by experimental verification to develop cyclic etching processes for Ru and CoFeB. Surface modification steps were used to induce anisotropy in Ru or control the removal of CoFeB, allowing these materials to be integrated into next-generation devices where control of etch results is critical.