Slow Wave Transmission Lines Used as Integrated Inductors with Improved Performance

Abstract

Slow wave transmission lines (SWTLs) are analyzed in literature as an interconnections having the ability to slow down the propagating wave and consequently saving area, shielding substrate and lowering losses. In this thesis, on the other hand, possible ways are investigated to exploit these improvements to enhance, optimize and tune the inductive parameters of SWTLs; effective inductance (Leff), quality factor (Q), self-resonance frequency (fsrf) and area efficiency. Integrated in low-cost silicon substrate, on-chip inductors are facing crucial challenges, like design flexibility, area efficiency, conductive and substrate losses. Early studies have identified many techniques to overcome these challenges, such as post processing, magnetic core filling, ground plane modification, and others, however each technique has its drawbacks at RF frequencies which are reflected on either performance or cost. The principle of operation of SWTLs is analyzed mathematically as a periodic structure by cascading multiple capacitive-loaded transmission line unit cells. Leff and Q are extracted from the ABCD parameters. It has been proved that the distributed nature of SWTLs provides an enhanced inductive performance. A parametric study on a coplanar stripline is introduced to explore and quantify the relation between eight spacial design variables related to the TL structure, and five inductive parameters. A parametric table, considered as a guideline to build SWTLs as inductors, have been created. To validate the idea practically, various configurations of slow-wave coplanar striplines are designed, fabricated and measured on CMOS 130nm process (silicon substrate). Optimized SWTLs are applied as inductors in a generic injection locked oscillator (ILO). Using SWTLs as inductors have provided additional degrees of freedom to lay out and tune Leff, Q and fsrf efficiently. The differential balanced structure of the designed SWTLs have provided an equal single-ended inductance to both sides of the circuit without using lengthy interconnections leading to 50% enhanced Q compared to conventional transmission lines. SWTLs as inductors inductors have shown a 75% and 60% enhanced Q compared to a single and double conventional spiral inductors. Additionally, an enhanced noise performance have been demonstrated in ILO using SWTLs as inductors compared to the conventional inductors.