The impact of an aluminum top layer on inductors integrated in an advanced CMOS copper backend

The performance of inductors realized in various metal layer combinations available in an advanced CMOS process with a mixed copper-aluminum backend is compared. Due to the skin effect, the gain in performance is generally smaller than anticipated from the reduction in dc resistance. We show that beyond a certain frequency, adding an aluminum layer on top of a copper inductor stack actually degrades the inductor performance because the skin resistance of the top metal is enhanced.