1ST-WAFER EFFECT IN REMOTE PLASMA PROCESSING - THE STRIPPING OF PHOTORESIST, SILICON-NITRIDE, AND POLYSILICON

We have identified a first-wafer effect for photoresist ashing and sil icon nitride-polysilicon stripping in remote plasma reactors. The firs t-wafer effect consists of the first wafer etching differently from th e subsequent wafers in a lot. For photoresist ashing, the first wafer ashes faster than subsequent wafers. For silicon nitride and polysilic on stripping, first wafers show higher etch rates of silicon nitride a nd polysilicon, while silicon dioxide first wafers etch faster for the polysilicon strip process, and slower for the silicon nitride strip p rocess. We have modeled the first-wafer effect for photoresist ashing. We found an inverse relationship between the percentage change in the time to clear the photoresist from the wafer and the time delay betwe en processing sequential wafers. We have included this first-wafer eff ect in the on-line statistical process control strategy for the photor esist asher in our laboratory. Examination of this first-wafer effect suggests that it may be caused by the generation of species in the dis charge in the first few seconds of operation that alter the reactivity of the chamber walls. While these species are quick to adsorb on the walls, they only desorb slowly. Pumping on the chamber in the absence of a microwave discharge returns the chamber to its original state.