Taeseung Kim

Viable chemical approach for patterning nanoscale magnetoresistive random access memory

A reactive ion etching process with alternating Cl2 and H2 exposures has been shown to chemically etch CoFe film that is an integral component in magnetoresistive random access memory (MRAM). Starting with systematic thermodynamic calculations assessing various chemistries and reaction pathways leading to the highest possible vapor pressure of the etch products reactions, the potential chemical combinations were verified by etch rate investigation and surface chemistry analysis in plasma treated CoFe films. An ∼20% enhancement in etch rate was observed with the alternating use of Cl2 and H2 plasmas, in comparison with the use of only Cl2 plasma. This chemical combination was effective in removing metal chloride layers, thus maintaining the desired magnetic properties of the CoFe films. Scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy showed visually and spectroscopically that the metal chloride layers generated by Cl2 plasma were eliminated with H2 plasma to yield a clean et...

Predicting synergy in atomic layer etching

Atomic layer etching (ALE) is a multistep process used today in manufacturing for removing ultrathin layers of material. In this article, the authors report on ALE of Si, Ge, C, W, GaN, and SiO2 using a directional (anisotropic) plasma-enhanced approach. The authors analyze these systems by defining an “ALE synergy” parameter which quantifies the degree to which a process approaches the ideal ALE regime. This parameter is inspired by the ion-neutral synergy concept introduced in the 1979 paper by Coburn and Winters [J. Appl. Phys. 50, 5 (1979)]. ALE synergy is related to the energetics of underlying surface interactions and is understood in terms of energy criteria for the energy barriers involved in the reactions. Synergistic behavior is observed for all of the systems studied, with each exhibiting behavior unique to the reactant–material combination. By systematically studying atomic layer etching of a group of materials, the authors show that ALE synergy scales with the surface binding energy of the bu...

Thermodynamic assessment and experimental verification of reactive ion etching of 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.

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.

Ion beam assisted organic chemical vapor etch of magnetic thin films

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.

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.

In-situ etch rate study of HfxLayOz in Cl2/BCl3 plasmas using the quartz crystal microbalance

The etch rate of HfxLayOz films in Cl2/BCl3 plasmas was measured in-situ in an inductively coupled plasma reactor using a quartz crystal microbalance and corroborated by cross-sectional SEM measurements. The etch rate depended on the ion energy as well as the plasma chemistry. In contrast to other Hf-based ternary oxides, the etch rate of HfxLayOz films was higher in Cl2 than in BCl3. In the etching of Hf0.25La0.12O0.63, Hf appeared to be preferentially removed in Cl2 plasmas, per surface compositional analysis by x-ray photoelectron spectroscopy and the detection of HfCl3 generation in mass spectroscopy. These findings were consistent with the higher etch rate of Hf0.25La0.12O0.63 than that of La2O3.

Mechanistic study of atomic layer deposition of AlxSiyO thin film via in-situ FTIR spectroscopy

A study of surface reaction mechanism on atomic layer deposition (ALD) of aluminum silicate (AlxSiyO) was conducted with trimethylaluminum (TMA) and tetraethoxysilane (TEOS) as precursors and H2O as the oxidant. In-situ Fourier transform infrared spectroscopy (FTIR) was utilized to elucidate the underlying surface mechanism that enables the deposition of AlxSiyO by ALD. In-situ FTIR study revealed that ineffective hydroxylation of the surface ethoxy (–OCH2CH3) groups prohibits ALD of SiO2 by TEOS/H2O. In contrast, effective desorption of the surface ethoxy group was observed in TEOS/H2O/TMA/H2O chemistry. The presence of Al-OH* group in vicinity of partially hydroxylated ethoxy (–OCH2CH3) group was found to propagate disproportionation reaction, which results in ALD of AlxSiyO. The maximum thickness from incorporation of SiOx from alternating exposures of TEOS/H2O chemistry in AlxSiyO was found to be ∼2 A, confirmed by high resolution transmission electron microscopy measurements.