This work demonstrated a generalizable approach for patterning metals on the nanometer scale. As the complexity of device structures keeps evolving, corresponding etching techniques need to be developed to accommodate the integration of new materials on ever-shrinking scales. The contrast in chemical reactivity is explored and leveraged as the source of etching selectivity in the illustrated atomic layer etching (ALE) process to meet the increasingly stringent dimension requirements on etch-resistant metals.

Thermodynamic assessments are performed on Ni to screen and predict the potential solution and gas-phase etchants. Organic etchants such as acetic acid, formic acid, acetylacetone, and hexafluoro-acetylacetone were screened to etch Ni and NiO in the solution phase. In concentrated solutions of formic acid, NiO was observed to etch selectively over metallic Ni at values up to 30. Such selectivity is further explored in the gas-phase via a sequential oxidation-etch process. Cyclic oxygen plasma and gas phase formic acid exposures resulted in an etch rateof 2.3 nm/cycle. A high (>100) gas-phase etch selectivity of NiO over Ni by this cyclic process is achieved, fulfilling the self-limiting requirement of an Atomic Layer Etching (ALE) process. Anisotropic etch profile for Ni thin film with SiO2 hard mask was achieved through this cyclic process and the final feature sidewall was measured to be close to 90�. This ALE approach is further facilitated by a prior RIE chemistry to realize highly directional removal at a high throughput. Density Functional Theory (DFT) simulation was conducted, and Ni thin film with an oxygen underlayer was calculated to be the configuration that enables favorable etching reaction with the presence of formic acid vapor.

A similar cyclic approach was applied to the patterning of Cu thin film and directional removal was also achieved. An average etch rate of 2.4 nm/cycle was determined experimentally. LMM Auger signal was used to confirm the change in Cu surface states from Cu dominant (as deposited) to CuO dominant (after oxidation) to Cu(OH)2 dominant (after etching). DFT based simulation was established with the most energetically stable Cu, CuO and Cu(OH)2 configurations. The most favorable etch product was determined to be Cu2(HCOO)4 dimer with a formic acid exposure partial pressure window of 150 Torr ~ 250 Torr. Experimental verification of the gas phase etching products was attempted, and Cu was detected in the etch product.

Manipulating atoms to form desired structures on an atomic scale has long been the ultimate goal of the semiconductor industry, as it would open up infinite possibilities for device innovation and manufacturing. The new ALE chemistry introduced in this work adds an essential piece of the puzzle to the bigger picture and brings the goal one step closer. It is believed that with the implementation and further optimization of the demonstrated ALE process, atomistic subtraction of metals would be eventually realized.