Optimizing Remote Plasma Sources for Selective Etching

Summary form only given. Remote plasma sources (RPS) are used in microelectronics fabrication to produce fluxes of radicals for etching and surface passivation in the absence of damage that may occur by charging and energetic ion bombardment. RPS reactors typically use flow distance and grids to reduce or eliminate charged particle fluxes from reaching the surface of the material being treated. Nitrogen trifluoride (NF 3 ) is frequently used in RPS for the ease with which F atoms are produced by dissociative attachment. RPS sustained only in NF 3 typically limits the types of reactive fluxes reaching the processing chamber to only F x and NF x atoms and molecules. RPS sustained in gas mixtures such as NF 3 /O 2 and NF 3 /H 2 , increases the variety of reactive species that can be produced, and so enables some customization of the fluxes to the substrate. For example, the use of NF 3 /O 2 mixtures increases the etch rate of Si 3 N 4 by production of NO which aids in the removal of the N atom from the surface. For certain applications it may be desirable to separately optimize the fluxes of different reactants. This separate optimization could be performed using pulsed power sources or multiple RPS. In this paper, we discuss results from a computational investigation of RPS sustained in Ar/NF 3 /O 2 mixture at pressures of 100s of mTorr using continuous-wave and pulsed power for downstream low-damage etching applications. Two modeling approaches were used - global modeling to investigate fundamental reaction mechanisms and scaling of RPS and 2-dimensional modeling to self-consistently address the spatial dynamics of flow through the system. Electron impact cross sections for NFx were provided by ab initio calculations. Results will be discussed for plasma properties and the etch characteristics in RPS systems, with comparison to experiments. In the plasma region, gas heating aids in the formation of NO molecules through endothermic reactions which may be important to selectivity. In the downstream region, the system transitions to an ion-ion plasma maintained by [F - ] ≈ [NO + ].