Study of the SiO2-to-Si3N4 etch selectivity mechanism in the presence of polymers in fluorocarbon plasmas

Abstract

The polymerization effects in 27/2 MHz capacitively coupled fluorocarbon plasmas during high selectively subquarter-micron contact hole etching, with the film structure of silicon dioxide (SiO2)/silicon nitride (Si3N4)/silicide (TiSix), are investigated. A high etch selectivity chemistry is first used to remove SiO2, then Si3N4 is removed with another chemistry with high selectivity of Si3N4-to-TiSix. To obtain good etch selectivity of SiO2-to-Si3N4, fluorocarbon plasmas containing a high carbon to fluorine (C/F) ratio is usually employed. Polymerization readily occurs in chemistries having a high C/F ratio. These unsaturated polymers are reactive and stick easily to contact hole sidewalls and bottoms, creating thick polymer films. While the chemistry used to etch Si3N4 has shown acceptable selectivity as measured on a patterned singular film, fluorine from the polymer will severely degrade the etch selectivity of Si3N4-to-TiSix on fully processed wafers. Process developments started with a design of experiment on real production wafers in order to understand the roles of fluorocarbon polymer during the Si3N4 etching. Scanning electron microscope analysis of this polymer film and its effects are included as well as the electrical parametric testing data of contact resistances. Different polymer removing methods to restore the Si3N4-to-TiSix etch selectivity back were initiated and will be briefly discussed. While this investigation pertains to contact etch, this phenomenologically observed result could be applied to any highly selective etching of oxide versus nitride.