Characteristics of $0-\Omega$ Transmission Lines for Millimeter-Wave Circuits in $0.18\ \mu \mathrm{m}$ CMOS Technology

A 0-Ω transmission line ( 0-Ω TL) has been proposed as a decoupling capacitor in a high frequency band. 0-Ω TL is configured by combining three types of capacitors which are metal-insulator-metal (MIM) capacitors, metal-oxide-metal (MOM) capacitors, and MOS capacitors. By large capacitance of those capacitors, a 0-Ω TL has very low characteristic impedance. The optimum configuration of a 0-Ω TL in a 1P5M 0.18 μm CMOS technology was reported to be the configuration of MOM capacitors and MOS capacitors. However, it has not been evaluated by measurement. In this paper, the optimal configuration of 0-Ω TL is clarified by measurement. Moreover, we show that the variation of gate biases of MOS capacitors does not significantly affect the characteristics of a 0-Ω TL.

I. Introduction

Decoupling capacitors in integrated circuits are used to eliminate power supply noise and to prevent inter-circuit interference [1]. However, in a high-frequency band, self-resonance due to parasitic inductances of decoupling capacitors is a problem. Therefore, a transmission line () has been proposed as a decoupling capacitor in a high frequency band [2]–[5]. In general, a TL is configured by combining three types of capacitors which are MIM capacitors (MIMCAP), MOM capacitors (MOMCAP), and MOS capacitors (MOSCAP). The optimum configuration of a TL in a CMOS process was reported to be the configuration of MOMCAP and MOSCAP [6]. It is because the capacitance density of the TL with MIMCAP in this process is low and the distributed inductance is large. However, it has not been evaluated by measurement. Therefore, in order to clarify that the optimum configuration in the CMOS technology is the configuration of MOMCAP and MOSCAP by measurement. Moreover, we compare the input impedance of a TL and the decoupling frequency [7] with three configurations and three different lengths of a TL. Also, capacitance of MOSCAP is usually changed by the gate bias . It is not good for a TL to show the variation of due to the variation of . We clarify the influence of the variation of on of a TL. Fig. 1

Example of use as a decoupling capacitor for a transmission line.