INFLUENCE OF THE NATURE OF THE MASK ON POLYSILICON GATE PATTERNING INHIGH-DENSITY PLASMAS

High density plasma etching processes of polysilicon gates on thin gat e oxide (4.5 nm) have been studied for sub-quarter micron device fabri cation. The influence of the mask material on the etching performance has been investigated using either a photoresist mask or an oxide hard mask. Trenching phenomena can be observed at the edges of the gates w ith both types of mask. When using a photoresist mask, severe defects are formed in the gate oxide near the polysilicon gate, showing that t he gate oxide has been preferentially etched during the process. We sh ow that these defects can be attributed to the trenching induced by th e main etching step of the process, which is transferred into the gate oxide before the overetch starts. The transfer of the trenching effec t depends strongly on the polysilicon-to-oxide selectivity which is sh own to be dependent on the presence of carbon in the process chamber. When replacing the photoresist mask by an oxide hard mask the polysili con-to-oxide selectivity can be improved by a factor of greater than t hree. Therefore, the use of an oxide hard mask results in a larger pro cess window without creating undesirable defects in the active areas o f the devices.