Sodium charge storage in ultrathin MnO2 films was studied using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements. The MnO2 films were fabricated by electrochemical oxidation of MnO films grown by atomic layer deposition (ALD). CV analysis confirmed that oxidation of MnO to MnO2 involved two moles of electrons per mole of Mn in the MnO ALD film. Scanning electron microscopy (SEM) images revealed that electrochemical oxidation of MnO led to the formation of MnO2 nanosheets. EQCM measurements suggested that Na+ cations participate in charge storage in MnO2. X-ray photoelectron spectroscopy (XPS) experiments measured sodium in MnO2 after both positive/anodic and negative/cathodic voltage sweeps. The stoichiometry was Na0.25MnO2 after negative/cathodic voltage sweeps. Approximately one-half of the sodium was removed after positive/anodic voltage sweeps. The areal capacitance increased progressively with initial MnO ALD film thickness. This increase in areal capacitance is consistent with bulk charge storage in MnO2 or higher surface area of MnO2 nanosheets resulting from larger MnO ALD film thicknesses. Experiments at varying scan rates indicated that charge storage in MnO2 originates from a combination of capacitive and diffusive processes. Bulk charge storage makes a significant contribution to total charge storage in the thicker MnO2 films.

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Thermal atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF has been a typical fluorination reactant. Various metal reactants have been used for ligand-exchange, such as Sn(acac)2, Al(CH3)3, AlCl(CH3)2, and SiCl4. This study explored TiCl4 as a new metal chloride reactant for ligand-exchange. Thermal HfO2 ALE using HF and TiCl4 as the reactants was studied using in situ quartz crystal microbalance (QCM) measurements from 200 to 300 °C. The HfO2 films were etched linearly versus the number of HF and TiCl4 reaction cycles. The sequential HF and TiCl4 reactions were also self-limiting versus reactant exposure. The QCM studies observed a mass change per cycle (MCPC) of −10.2 ng/(cm2 cycle) at 200 °C and −56.4 ng/(cm2 cycle) at 300 °C. These MCPCs correspond to HfO2 etch rates of 0.11 Å/cycle at 200 °C and 0.59 Å/cycle at 300 °C. To explore the selectivity of thermal ALE using HF and TiCl4 as the reactants, spectroscopic ellipsometry (SE) measurements were also employed to survey the etching of various materials. The SE results revealed that HfO2 and ZrO2 were etched by HF and TiCl4. In contrast, Al2O3, SiO2, Si3N4, and TiN were not etched by HF and TiCl4. The etching selectivity can be explained by the reaction thermochemistry and the stability and volatility of the possible etch products. Al2O3 can also serve as an etch stop for HfO2 ALE.

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A dramatic increase in the Al2O3 atomic layer etching (ALE) rate versus time was demonstrated using sequential, self-limiting exposures of hydrogen fluoride (HF) and trimethylaluminum (TMA) as the reactants with no purging. The normal purging expected to be required to prevent chemical vapor etching or chemical vapor deposition (CVD) is not necessary during the Al2O3 ALE. This purgeless, rapid atomic layer etching (R-ALE) was studied from 250 to 325 °C using various techniques. In situ quartz crystal microbalance (QCM) measurements monitored Al2O3 R-ALE at 300 °C. The Al2O3 R-ALE process produced linear etching versus number of R-ALE cycles. Each HF exposure fluorinates the Al2O3 substrate to produce an AlF3 surface layer. Each subsequent dose of TMA then undergoes a ligand-exchange transmetalation reaction with the AlF3 surface layer to yield volatile products. Using reactant partial pressures of HF = 320 mTorr and TMA = 160 mTorr, the fluorination and ligand-exchange reactions produced a mass change per cycle (MCPC) of −32.1 ng/(cm2 cycle) using sequential, 1 s exposures for both HF and TMA with no purging. This MCPC equates to a thickness loss of 0.99 Å/cycle or 0.49 Å/s. Comparison experiments using the same reactant exposures and purge times of 30 s yielded nearly identical MCPC values. These results indicate that the etch rates for Al2O3 R-ALE are much faster than for normal Al2O3 ALE because of shorter cycle times with no purging. Smaller MCPC values were also observed at lower reactant pressures for both Al2O3 R-ALE and Al2O3 ALE. The QCM studies showed that the Al2O3 R-ALE process was self-limiting versus reactant exposure. Ex situ spectroscopic ellipsometry and x-ray reflectivity (XRR) measurements revealed temperature-dependent etch rates from 0.02 Å/cycle at 270 °C to 1.12 Å/cycle at 325 °C. At lower temperatures, AlF3 growth was the dominant mechanism and led to an AlF3 atomic layer deposition (ALD) growth rate of 0.33 Å/cycle at 250 °C. The transition temperature between AlF3 growth and Al2O3 etching occurred at ∼270 °C. XRR scans showed that the Al2O3 ALD films were smoothed by Al2O3 R-ALE at temperatures ≥270 °C. Additionally, patterned wafers were used to compare Al2O3 R-ALE and normal Al2O3 ALE in high aspect ratio structures. Scanning electron microscope images revealed that the etching was uniform for both processes and yielded comparable etch rates per cycle in the high aspect ratio structures and on flat wafers. The HF and TMA precursors were also intentionally overlapped to explore the behavior when both precursors were present at the same time. Similar to ALD, where precursor overlap produces CVD, precursor overlap during Al2O3 ALE leads to AlF3 CVD. However, any AlF3 CVD growth that occurs during precursor overlap is removed by spontaneous AlF3 etching during the subsequent TMA exposure. This spontaneous AlF3 etching explains why no purging is necessary during R-ALE. R-ALE represents an important advancement in the field of thermal ALE by producing rapid etching speeds that will facilitate many ALE applications.