Silicon wafers get fully covered by metal, then by an isolating layer (oxide, nitride). Lithography via masks defines areas where no metal routing should stay. Reactive ion etching removes the isolating layer and the metal below. At the end of this process, just when the routing connections are "cleared", i.e. cut off from residual metal, they can be charged by the impinging ions. If their metal area is large, enough charge can be caught to generate a voltage which causes a damaging break-through of the gate oxide. If a silicon junction is connected to such a metal wire, there's no such problem due to leakage.