Impact of reductive $N2∕H2$ plasma on porous low-dielectric constant SiCOH thin films

Porous low-dielectric constant (low-$κ$⁠) SiCOH thin films deposited using a plasma-enhanced chemical-vapor deposition have been comprehensively characterized before and after exposure to a reactive-ion-etch-type plasma of $N2$ and $H2$ chemistry. The low-$κ$ film studied in this work is a carbon-doped silicon oxide film with a dielectric constant $(κ)$ of 2.5. Studies show that a top dense layer is formed as a result of significant surface film densification after exposure to $N2∕H2$ plasma while the underlying bulk layer remains largely unchanged. The top dense layer is found to seal the porous bulk SiCOH film. SiCOH films experienced significant thickness reduction, $κ$ increase, and leakage current degradation after plasma exposure, accompanied by density increase, pore collapse, carbon depletion, and moisture content increase in the top dense layer. Both film densification and removal processes during $N2∕H2$ plasma treatment were found to play important roles in the thickness reduction and $κ$ increase of this porous low-$κ$ SiCOH film. A model based upon mutually limiting film densification and removal processes is proposed for the continuous thickness reduction during plasma exposure. A combination of surface film densification, thickness ratio increase of top dense layer to bulk layer, and moisture content increase results in the increase in $κ$ value of this SiCOH film.