Effect of ion energy and incident angle on the modification of a porous low dielectric-constant material in an O2 plasma ashing process

A porous silica film was treated in an O2 reactive ion etching (RIE) process, and changes in the film property with ion energy and incident angle were compared with those observed in a conventional ashing process. The production of hydroxyl (–OH) groups, which always occurs in a conventional ashing, was suppressed when the bias voltage was higher than a threshold value in an O2 RIE process. This means that the formation of an oxide-like dense layer at the film surface, which is known to inhibit the diffusion of oxygen radicals into the film, requires an ion energy above the threshold value. Conversely, the hydroxyl groups were produced even at bias voltages higher than the threshold value in an O2 RIE process, when the ion incident angle was higher than a threshold angle. The threshold angle increased with an increase in the bias voltage. Consequently, nearly vertical sidewalls can be fabricated without degradation in their dielectric properties due to the production of hydroxyl groups by employing high bias voltages in an O2 RIE process.