Analysis and optimum design of RF spiral inductors on silicon substrate

On-chip spiral inductors with high quality factors are very important in many RF circuits. In this paper, spiral inductors are analyzed to study the influence of substrate and spiral structure on an inductor´s performance in the multi-GHz frequency range. Firstly, the inductors with different substrate conductivity, thickness and permittivity of the dielectric layer are compared, and a general guide is given that the substrate of a good performance inductor should be light impurity, the dielectric layer should be thick, and use a low k material. And then, this paper presents a novel spiral structure with gradually reduced metal width and space from outside to inside, which can help to alleviate the problems of the magnetic field intensity of a spiral inductor increasing gradually from outside to inside. Compared to conventional inductor, the maximum Q of inductor using gradually changed structure increases by 13.66%, thus the results have corroborated the validity of the proposed method.