A Scalable Model of Shielded Capacitors Using Mirror Image Effects

Koji ISHIBASHI
Ivan Chee-Hong LAI
Kyoya TAKANO
Minoru FUJISHIMA

Publication
IEICE TRANSACTIONS on Electronics   Vol.E90-C    No.12    pp.2237-2244
Publication Date: 2007/12/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e90-c.12.2237
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Recent Technologies for Microwave and Millimeter-wave Passive Devices)
Category: 
Keyword: 
comb capacitor,  shield,  scalable model,  quality-factor,  CMOS,  

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Summary: 
Comb capacitors suitable for use in advanced complementary metal-oxide semiconductor (CMOS) technology nodes are frequently constructed from low metal layers located closely above the conductive silicon substrate. This results in high parasitic capacitances across the thin dielectric between the two layers. Therefore, a shield for reducing this parasitic capacitance is proposed in order to use the comb capacitor at high frequency. From electromagnetic (EM) simulation results using a 3D EM simulator, the quality factor (Q-factor) of the proposed shielded comb capacitor for the differential signal improved by 20% at 30-110 GHz compared to the unshielded capacitor. Consequently, a scalable model is proposed, which operates up to millimeter-wave frequencies. The results are verified by experimental data using fabricated comb capacitors from a 90 nm 1P9M CMOS process. Compared with the experimental results, the simulated common-mode and differential-mode S parameters of the model has a root-mean-square (r.m.s.) error of under 2.1%.