MODELING CHEMICAL DOWNSTREAM ETCH SYSTEMS FOR NF3 O-2 MIXTURES/

Analysis of chemical downstream etch processes investigates ion and ra dical generation in a plasma source, the transport of species through a transport tube to the process chamber, and resulting etch behavior i n the downstream reactor. Downstream etching of silicon dioxide or pol ysilicon material on a wafer results from chemical etching by F atoms, while ion flux to the wafer must be minimal to avoid ion-induced devi ce damage. In the plasma source, results for NF3 chemistry show that F is the dominant neutral species, while NF2+ and F- are the dominant i ons. Comparison of species concentrations in a plasma source with mass spectrometry data available in the literature shows good quantitative agreement. Inclusion of a quartz-etch mechanism within the plasma sou rce reproduces observed erosion rates of quartz applicators used in th ese systems. Ions persist a small distance beyond electrons in the tra nsport tube region due to negative ion content, but the ion density is reduced to very low levels after traversing the length of the transpo rt tube. The addition of oxygen generally reduces the F-atom concentra tion downstream and changes the ion composition. Analysis of the react ing flow in the downstream chamber shows the role of species diffusion in determining the etch uniformity and species concentration profiles . Predicted downstream etch rates agree well with measured data over a wide range of process conditions.