Magnetic neutral loop discharge (NLD) plasmas for surface processing

A neutral loop discharge (NLD) is a plasma generated along a magnetic neutral (i.e. null field) loop at magnetic cusps by an externally applied radio frequency (RF) electric field. Due to partial electron cyclotron resonance and good electron confinement by the magnetic cusps, the NLD plasma can efficiently absorb power from the RF field. For applications to material surface processing, NLD plasmas have two important advantages. One is the high ionization rates, which allow the production of high-density plasmas with relatively low electron temperatures at low gas pressures. The other is the spatial and temporal controllability of magnetic field configuration, which allows optimization of plasma configuration during processing. For example, highly uniform plasma processing over a large wafer can be achieved with time-varying shape control of an NLD plasma. In this paper, some basic properties of NLD plasmas as well as several examples of their applications to plasma processing are reviewed. It is shown that, due to their high plasma densities and low collisionality, NLD plasmas are especially suited for highly anisotropic high-throughput etching processes such as those for fabrication of micro-electromechanical systems/nano-electromechanical systems. (topical review)