A physical approach to metal insulator metal (MIM) tuning capacitor modeling

Abstract

Due to the extensive use of capacitors both as tuning and RF bypass elements in monolithic integrated circuit (MMIC) design, it becomes increasingly important to accurately predict the performance of such structures in terms of their losses and behavior as lumped elements. Many attempts have been proposed to predict shunt capacitor MIM behavior. However, past models are valid for 20 GHz or less and do not directly correspond to the physical structure of the element. The research presented here attempts to establish a model in which parameters are taken solely from the physical structure of the device or from material properties. Three shunt capacitors of values 0.024 pF, 0.18 pF, and 0.90 pF have been fabricated on 50 [u]m GaAs. S-parameter data has been measured from 0.5 GHz to 50 GHz and compared against the predicted response of each model. The research reports an evolution of six models, each attempting to incorporate more and more physical features into the mode. The final model, Model 6, has been shown to offer best accuracy of the six developmental models presented in this research. Model 6 incorporates actual physical dimensions into the model including physical gaps and approximating the capacitance with a parallel transmission line model. The latter addition proved to be the difference in accurately predicting the shunt capacitor behavior.