Atomic Layer Etching - Chemistry
One method to approach atomic scale precision is by optimizing the plasma chemistry. As in well studied processes such as the wet etching of silicon oxide by dilute hydrofluoric acid (dHF), the idea is to optimize the fluxes of species participating in the etching process of material A while no reacton takes place with Material B.
The most studied system for plasma etch may be selective silicon oxide etching. Typically mixtures of flurocarbon (FC) gases with Argon are employed, where the Ar ions provide a basic mean to sputter away the oxide material, whereas the FC gas is optimized to passivate surfaces. This is schematically shown here:
Schematic picture of selective oxide etch process
The FC gas produces a plasma deposit on the exposed surfaces, which is able to reduce the sputter rate provided by the Ar bombardment. In case of an oxide substrate however, O from the surface reacts with the plasma-deposited FC layer by formation of COF2, thinning the FC layer substantialy and allowing a higher sputter rate at given ion energy. As seen in the schematic picture, in theory a process of "infinite selectivity" can be obtained in such fashion.
Patterning issues like etch stop have lead to operation of oxide etch processes in the "selective etch" window as suggested above. However, by redesign of the FC precursor, species fluxes can be optimized. A result of this is shown here:
Due to redesign of the FC molecule, a much more selective process could be obtained by optimization of the FC species. A selectivity increase from about 15 to about 60 was achieved by an optimized process.
Another process that is of critical importance in semiconductor processing is polysilicon etching stopping on thin silicon oxide. In a similar fashion, by optimizing the fluxes of the species participating in the etch process, a highly selective process was found:
Optimization of the process pressure, RF and bias power as well as HBr and O2 fluxes resulted in the ability to pattern 100nm tall gates with straight profile while landing on a 4A thin oxide.
Another process that has gained much attention lately is the Spacer etch. Here, a Silicon Nitride material is etched selective to Silicon as well as silicon oxide. By redesigning the FC gas for nitride etch, optimizing the plasma parameters, a highly selective process was developed:
in this example, stopping within a few monolayers of Si was observed, as shown in the Figure.