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This paper discusses the effect of dummy fills on the performance of spiral inductors. In Si-CMOS processes that use copper wire, metal dummy fills are required to prevent thickness fluctuation in the chemical mechanical polishing stage. Dummy fills have been thought to affect the wire capacitance; however, in high frequency, dummy fills also affect the wire resistance and the wire inductance due to the eddy current in dummy fills. This work evaluates the effect of the dummy fills by three-dimensional field solver and proposes a structure to suppress the effect of dummy fills. Experimental results show that the proposed method can decrease the effect of dummy fills on the Q-factor by 83% compared with the conventional dummy fill insertion.