Thorsten Lill

Directional etch of magnetic and noble metals. II. Organic chemical vapor etch

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.

Directional etch of magnetic and noble metals. I. Role of surface oxidation states

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.

Plasma-assisted thermal atomic layer etching of Al2O3

In this paper, we report on plasma assisted thermal Atomic Layer Etching (ALE) of Al2O3. The surface was modified via a fluorine containing plasma without bias power. The removal was accomplished by a thermal reaction step using tin-(II) acetylacetonate Sn(acac)2. After a few cycles, material removal stopped and growth of a Sn-containing layer was observed. Insertion of a hydrogen plasma step was found to remove the Sn layer and a continuous material removal of 0.5 Å/cycle was measured. The results show that plasma assistance can be used to realize thermal ALE of Al2O3. Specifically, plasma can be used both in the fluorination step and to keep the surface free from contaminations.

Ion Beam Patterning of High-Density STT-RAM Devices

Dependence on ion beam energy, ion species, and incidence angles is investigated to reduce sidewall re-deposition on the magnetic tunnel junction barrier. Experimental and simulated etch data, for a representative spin-torque transfer random access memory structure with 40 nm critical dimension and 150 nm pitch, indicated a reduction in the sidewall re-deposition when operating at: high angle, high voltage, and with Xe as the source gas. The Monte Carlo binary collision model simulations showed re-deposition thickness reduced by ~75% with Xe versus Ar at 1 kV beam energy and 30° incidence angle.

Patterning in the era of atomic scale fidelity

Relentless scaling of advanced integrated devices drives feature dimensions towards values which can be expressed in small multiples of the lattice spacing of silicon. One of the consequences of dealing with features on such an atomic scale is that surface properties start to play an increasingly important role. To encompass both dimensional as well as compositional and structural control, we introduce the term “atomic scale fidelity.” In this paper, we will discuss the challenges as well as new solutions to achieve atomic scale fidelity for patterning etch processes. Fidelity of critical dimensions (CD) across the wafer is improved by means of the Hydra Uniformity System. Wafer, chip and feature level atomic scale fidelity such as etch rate uniformity, aspect ratio dependent etching (ARDE) /1/, selectivity and surface damage can be addressed with emerging atomic layer etching (ALE) approaches /2/.