The nature of particle generation in vacuum process tools

Particle levels in a variety of VLSI process tools are shown to have relatively constant baselines punctuated by short bursts. That these excursions decay rapidly while the baseline remains constant suggests that a strong feedback mechanism exists to hold particle levels in equilibrium. It is shown that this equilibrium is a function of process and wafer conditions and can be disrupted, for example, by running monitor wafers instead of product wafers. This implies that process monitoring methodologies using monitor wafers must be used with caution. The bursts are shown to have a strong potential yield effect when multiple process steps are considered. This effect is strongest when products are early in their life cycle or relatively sensitive to defects.>