Mechanism of Plasma Charging Damage III

In modern VLSI manufacturing, extremely tight control on every production step is required. Plasma processing steps are no exceptions. Plasma processing tool has been developed at great expense to achieve excellent uniformity across the wafer. Gone are old tools such as barrel etcher with poor plasma uniformity. Yet, in modern IC manufacturing, plasma charging damage is more prevailing. The reason for this increase in incidences of damage is due mostly to the continued scaling down of gate-oxide thickness. Thinner gate-oxide requires a lower voltage to support Fowler-Nordheim (FN) tunneling and therefore can be damaged with a less severe plasma non-uniformity. On the other hand, if one were to calculate the plasma potential variation across the wafer based on the measured ion current or etch-rate in most modern plasma processing tools, one would have found that the voltage variations are often lower than what is needed to cause damage. Clearly, the actual floating potential variation across the wafer must be higher than what is suggested from these measurements. It is, of course, quite possible that the measurement method is inadequate and that the real variation in plasma potential is larger. This possibility will be explored in the next chapter. In this chapter, the focus is on some of the mechanisms with which charging damage can arise even with perfectly uniform plasma.