Surface oxidation states of transition (Fe and Co) and noble (Pd and Pt) metals were tailored by controlled exposure to O2 plasmas, thereby enabling their removal by specific organic chemistries. Of all organic chemistries studied, formic acid was found to be the most effective in selectively removing the metal oxide layer in both the solution and vapor phase. The etch rates of Fe, Co, Pd, and Pt films, through an alternating plasma oxidation and formic acid vapor reaction process, were determined to be 4.2, 2.8, 1.2, and 0.5 nm/cycle, respectively. Oxidation by atomic oxygen was an isotropic process, leading to an isotropic etch profile by organic vapor. Oxidation by low energy and directional oxygen ions was an anisotropic process and thus results in an anisotropic etch profile by organic vapor. This is successfully demonstrated in the patterning of Co with a high selectivity over the TiN hardmask, while preserving the desired static magnetic characteristic of Co.

Building upon the depth and breadth of Harold Winters's work, this paper pays tribute to his pioneering contribution in the field of plasma etching of metals, and how that knowledge base helps guide the fundamental research in these areas. The fundamental understanding of the plasma–surface interactions during metal etch is key to achieve desirable etch efficacy and selectivity at the atomic scale. This paper presents a generalized methodology, combining thermodynamic assessment and kinetic verification of surface reactions, using copper, magnetic metals, and noble metals as examples, in an effort to demonstrate the applicability of this strategy in tailoring plasma–surface interactions at the atomic scale for a wide range of materials.

An organic chemical etch process based on tailoring the surface oxidation state was found to be effective in realizing directional etch of magnetic and noble metals for their integration and application in magnetoresistive random access memory devices. Using Pt, a noble metal, as a test case, plasma treatments with sulfur- and oxygen-based chemistries were able to oxidize Pt0+ to Pt2+ and Pt4+, which can be effectively removed by selected organic chemistries. The most effective control of the surface oxidation states of Pt was achieved with an O2 plasma, which was then applied with similar effectiveness to other transition and noble metals. By quantifying the reaction rate, the oxidation of transition metals (Fe and Co) was shown to follow an inverse log rate law, while that of noble metals (Pd and Pt) follows a parabolic rate law. This work highlights the importance of the surface oxidation states of magnetic and noble metals in enabling directional etch by organic chemistry.

An ion beam-assisted organic vapor etch process is demonstrated for patterning magnetic metal elements for potential applications in magnetoresistive random access memory devices. A thermodynamic analysis was performed to evaluate the feasibility of a chemical etch process, leading to the selection of acetylacetone (acac) and hexafluoroacetylacetone (hfac) chemistries. First, etching of cobalt and iron in acac and hfac solutions was studied, and it was determined that acac etches Co preferentially over Fe with a Co:Fe selectivity of ∼4, while hfac etches Fe preferentially over Co with an Fe:Co selectivity of ∼40. This motivates the use of acac and hfac to etch Co and Fe, respectively, but the etch rate was, in the gas phase, too small to be considered a viable process. An argon ion beam was employed in between organic vapor exposures and resulted in significant enhancement in the etch rates, suggesting an ion-enhanced chemical etching process is viable for the patterning of these magnetic metal elements.

A thermodynamic analysis of etch chemistries for Co, Fe, and Ni using a combination of hydrogen, oxygen, and halogen gases suggested that a single etchant does not work at 300 K; however, a sequential exposure to multiple etchants results in sufficiently high partial pressure of the reaction products for the process to be considered viable. This sequential dose utilized the two reactions, a surface halogenation followed by the secondary etchant exposure. (MX2 (c) + 3Y →MY(g) + 2XY(g), where M = Co, Fe, Ni; X = F, Cl, Br; Y = O, H) The volatilization reaction induced by sequential plasma exposure changed the equilibrium point, increasing the partial pressure of the etch product. Amongst all combinations, Cl2 or Br2 plasmas followed by H2 plasma were the most effective. From both the gas phase diagnostics and surface composition analysis, H2 plasma alone could not etch metallic Co, Fe, and Ni films but alternating doses of Cl2 and H2 plasmas resulted in more effective removal of chlorinated metals and increased the overall etch rate.