Thermal atomic layer etching of silicon nitride using an oxidation and “conversion etch” mechanism

Abstract

Thermal atomic layer etching (ALE) of silicon nitride was achieved using sequential exposures of oxygen (O2) or ozone (O3), hydrofluoric acid (HF), and trimethylaluminum [TMA, Al(CH3)3]. Thermal Si3N4 ALE will be useful to etch Si3N4 in semiconductor, optoelectronic, and MEMS devices. Thermal Si3N4 ALE was performed with Si3N4 thin films deposited on silicon wafers using low pressure chemical vapor deposition. In situ spectroscopic ellipsometry (SE) was employed to monitor the changes in the Si3N4 film thickness as well as the SiO2 layer thickness. The SE results at 290 °C yielded an Si3N4 etch rate of 0.25 Å/cycle with an O2-HF-TMA reactant sequence using partial pressures of 250, 0.65, and 1.2 Torr for O2, HF, and TMA, respectively. The O2, HF, and TMA reactants were held statically at the indicated partial pressures for 10, 5, and 5 s, respectively. Larger etch rates were observed using O3 instead of O2 as the oxidant. A higher Si3N4 etch rate of 0.47 Å/cycle was measured at 290 °C using an O3-HF-TMA reactant sequence at the same partial pressures and static exposure times as the O2-HF-TMA sequence. The Si3N4 etch rate was observed to decrease at lower temperatures. An Si3N4 etch rate of 0.07 Å/cycle was measured at the lowest temperature of 210 °C using an O3-HF-TMA reactant sequence. The Si3N4 surface roughness was reduced after Si3N4 ALE. The SiO2 layer on Si3N4 could be removed using sequential HF and TMA exposures. These sequential HF and TMA exposures could also very slowly etch the Si3N4 substrate. The Si3N4 etch rate was dependent on the reaction sequence. When an O3-TMA-HF sequence was employed with reactant partial pressures of 250, 0.65, and 1.2 Torr for O3, HF, and TMA, respectively, the Si3N4 etch rate was 0.20 Å/cycle at 290 °C. Thermal Si3N4 ALE adds to the growing list of materials that can be etched with atomic layer control using thermal chemistry.

Spontaneous Etching of Metal Fluorides Using Ligand-Exchange Reactions: Landscape Revealed by Mass Spectrometry

“…Many materials can be etched using thermal ALE . These materials include metal oxides such as Al 2 O 3 , ,, HfO 2 , , ZnO, Ga 2 O 3 , and WO 3 , metal nitrides such as AlN, TiN, and GaN, semiconductor materials such as Si, SiO 2 , and Si 3 N 4 , and metals such as W, Cu, and Co . Thermal ALE will be critical to provide atomic layer precision for etching to fabricate advanced three-dimensional semiconductor devices …”

Thermal Atomic Layer Etching of Nickel Using Sequential Chlorination and Ligand-Addition Reactions

“…One possible thermal ALE mechanism involves fluorination and ligand-exchange reactions. ,, This mechanism has been used to etch a variety of metal oxides such as Al 2 O 3 , HfO 2 , and ZrO 2 . ,− Other thermal ALE pathways are possible using conversion reactions that convert the initial material to a different material . These conversion reactions have facilitated the etching of various materials such as SiO 2 and WO 3 . , Oxidation reactions, sometimes together with conversion reactions, have also been employed for the etching of a variety of materials such as W, TiN, Si, Si 3 N 4 , and SiGe. − Many thermal ALE mechanisms have been established over the last 5 years …”

Molecular Mechanism of Thermal Dry Etching of Iron in a Two-Step Atomic Layer Etching Process: Chlorination Followed by Exposure to Acetylacetone

“…The majority of current ALE approaches to fabricate thin films of magnetic metals are based on a two-step scheme, where the first step is kinetically controlled formation of metal oxides, , nitrides, , or chlorides. − The second step is then based on a selective self-limiting reaction of the produced activated layer with the co-reactant, often delivering an organic ligand that can form a volatile metal-containing compound. If the metal-containing products are volatile and thermally stable, they can be removed by simply controlling the process temperature.…”

“…1,2 The thermal ALE process also eliminates the need for additional stimuli, such as plasma or ion sputtering, that are often responsible for altering the properties of the resulting surfaces. 3 The majority of current ALE approaches to fabricate thin films of magnetic metals are based on a two-step scheme, where the first step is kinetically controlled formation of metal oxides, 4,5 nitrides, 6,7 or chlorides. 8−12 The second step is then based on a selective self-limiting reaction of the produced activated layer with the co-reactant, often delivering an organic ligand that can form a volatile metal-containing compound.…”