Following Moore’s Law, feature dimensions will soon reach dimensions on an atomic scale. For the most advanced structures, conventional plasma etch processes are unable to meet the requirement of atomic scale fidelity. The breakthrough that is needed can be found in atomic layer etching or ALE, where greater control can be achieved by separating out the reaction steps. In this paper, we study selective, directional ALE of silicon using plasma assisted chlorine adsorption, specifically selectivities to bulk silicon oxide as well as thin gate oxide. Possible selectivity mechanisms will be discussed.

JSS FOCUS ISSUE ON ATOMIC LAYER ETCHING AND CLEANING Highly Selective Directional Atomic Layer Etching of Silicon

In this article, we give an overview about the chemical and physical processes that play a role in etching at lower wafer temperatures. Conventionally, plasma etching processes rely on the formation of radicals, which readily chemisorb at the surface. Molecules adsorb via physisorption at low temperatures, but they lack enough energy to overcome the energy barrier for a chemical reaction. The density of radicals in a typical plasma used in semiconductor manufacturing is one to two orders of magnitude lower than the concentration of the neutrals. Physisorption of neutrals at low temperatures, therefore, increases the neutral concentration on the surface meaningfully and contributes to etching if they are chemically activated. The transport of neutrals in high aspect ratio features is enhanced at low temperatures because physisorbed species are mobile. The temperature window of low temperature etching is bracketed at the low end by condensation including capillary effects and diminished physisorption at the high end. The useful temperature window is chemistry dependent. Besides illuminating the fundamental effects, which make low temperature processing unique, this article illustrates its utility for semiconductor etching applications.

Dry etching in the presence of physisorption of neutrals at lower temperatures

In order to etch CoFeB and MgO constituting the magnetic tunnel junction (MTJ) layer of magnetic random access memory, rf-biased reactive ion beam etching (RIBE) with a H2:NH3 gas mixture was introduced and the etching characteristics were investigated. The H2:NH3 gas mixture of 8:1 was used for rf-biased RIBE because the etch selectivity is increased with H2 percentage in the H2:NH3 gas mixture while the etch rates of MTJ materials are the highest between 2:1 and 1:1 ratio of H2:NH3 due to the synergy effect of H2 and NH3 in the etching of MTJ materials. When a high rf power was applied to the substrate (high rf-biasing) during RIBE, even though etch rates of MTJ materials were increased, the etch selectivities of MTJ materials over hard mask materials were decreased possibly due to the high physical sputtering effect. However, when small rf-biasing was added to the substrate during RIBE, improved etch characteristics such as higher etch selectivity of MTJ over the top electrode (TE) material, improved etch anisotropy of the MTJ pattern masked with TE, and a thinner sidewall residue on the MTJ pattern could be observed. The improvement in etch characteristics by the addition of small rf-biasing during RIBE is believed to be related to the increased chemical etching effect through H2/NH3 gas dissociation and the increased H/NH ratio in the plasma generated just above the substrate without increasing the physical sputtering effect. Specifically, for the substrate located on the insulating substrate holder, the charging of the substrate could be removed without using an electron emitting neutralizer by supplying electrons from the plasma generated by rf-biasing during RIBE.

Study on etch characteristics of magnetic tunnel junction materials using rf-biased H2/NH3 reactive ion beam

Radical flux control in reactive ion beam etching (RIBE) by dual exhaust system

Precise and selective removal of silicon nitride (SiNx) over silicon oxide (SiOy) in a oxide/nitride stack is crucial for a current three dimensional NOT-AND type flash memory fabrication process. In this study, fast and selective isotropic etching of SiNx over SiOy has been investigated using a ClF3/H2 remote plasma in an inductively coupled plasma system. The SiNx etch rate over 80 nm/min with the etch selectivity (SiNx over SiOy) of ~ 130 was observed under a ClF3 remote plasma at a room temperature. Furthermore, the addition of H2 to the ClF3 resulted in an increase of etching selectivity over 200 while lowering the etch rate of both oxide and nitride due to the reduction of F radicals in the plasma. The time dependent-etch characteristics of ClF3, ClF3 & H2 remote plasma showed little loading effect during the etching of silicon nitride on oxide/nitride stack wafer with similar etch rate with that of blank nitride wafer. 

https://www.nature.com/articles/s41598-022-09252-3.pdf

Sebastian Petrov

Papers

Radical flux control in reactive ion beam etching (RIBE) by dual exhaust system

Radical flux control in reactive ion beam etching (RIBE) by dual exhaust system

Selective etching of silicon nitride over silicon oxide using ClF3/H2 remote plasma