2003 Volume 6 Issue 6 Pages 473-480
This paper describes a study to improve the quality factor (Q-factor) of a silicon-on-chip spiral inductor by determining the most appropriate shape and structure of inductor wires and a ground shield. Electro-magnetic field simulation is carried out under the condition that the spiral inductor is formed on a P-substrate with an electric conductivity of 10 [Ω-1 m-1] in assuming the use of a conventional CMOS process. After assessing the simulation results, we propose a new, ring-type structure for the wires and a comb-shaped ground shield that is placed under the inductor wires only. This comb-like shape reduces the stray capacitance between the shield and the inductor wires. As the Q-factor value of a spiral inductor depends on the distance between the ground shield and the inductor wires, as well as on the resistivity of the ground shield material, those influences are also clarified. As a result, we obtain 15% to 23% increases in the Q-factor values in the frequency range of 2 to 3.5 GHz for a 7 nH inductor. We also identify a material possessing the appropriate resistivity to serve as a ground shield.
