Paper PS-TuP29
The Role of Charge Exchange Collisions in Selective Etching of Si

Tuesday, October 31, 2017, 6:30 pm, Room Central Hall

Continuous shrinkage of transistors in sub-7nm technological nodes requires new integration and etching challenges. As the direct scaling of the FinFET to 5nm is extremely difficult, the use of a 3D integration scheme is considered to be a prospective way toward the next technological node. In addition to generic process requirements (high anisotropy, minimum CD loading and high selectivity to the mask films), conductor etching in such structures is more complicated due to different positions of the stopping layers. Features with deep etch targets may face underetching, while features with short targeted depths may have severe notching due to excessive positive charge of the stopping layer. Fast neutral beam processing in ion-assisted reactive etching is one of the ways for the induced charge mitigation and to eliminate this unwanted effect.

We report a study of Si etching in an HBr/Ar surface-wave Radial Line Slot Antenna plasma. The ability of RLSA^TM plasma etchers to operate in a very wide range of the pressures allows the etch process well above 100mT. Process operation in this high pressure range and large Ar⁺-Ar charge-exchange cross sections (~3x10¹⁵ cm^-2 at 100’s eV) result in numerous charge exchange collisions in the plasma sheath, leading to a significant presence of the fast neutrals.

A simple model for the ion and neutral energy distributions in an HBr/Ar plasma discharge has shown a significant input of the neutral ion beam. With the exception of the results for the hydrogen and bromine ions, the calculated fast neutral fluxes for Ar can be up to 70% of the total ion flux with the energies comparable to the ions. Reducing operating pressures below 50mT with increase in the plasma density provides etching mostly caused by ions. This correlates well with the experimental etch profiles suffered from notching and bowing.