Impact of Floating Dummy Fill On Q Characteristics of Spiral Inductors

This paper describes the effect of floating dummy metal fill on the quality factor (Q) characteristics of spiral inductors. Fill shapes with minimum block width for a given fill density are shown to be optimal. Q of the inductor is shown to decrease significantly with the thickness of the fill block for a given density. Q degradation due to fill is shown to be more pronounced with lower substrate resistivities. Influence of fill is shown to increase with reducing inductance value or increased frequency of operation. A simplified on-chip spiral inductor model is used to explain the above findings. Finally, layout optimized spirals with variable width and spacing across turns are shown to be effective for Q improvement in spite of the presence of dummy metal fill.