High-Performance Slow-Wave Transmission Lines With Optimized Slot-Type Floating Shields

Abstract

A novel slow-wave transmission line with optimized slot-type floating shields in advanced CMOS technology is presented. Periodical slot-type floating shields are inserted beneath the transmission line to provide substrate shielding and to shorten the electromagnetic (EM) propagation wavelength. This is the first study that demonstrates how the wavelength, attenuation loss, and characteristic impedance can be adjusted by changing the strip length (SL), strip spacing (SS), and metal layer position of the slottype floating shields. Wavelength shortening needs to be achieved with a tradeoff between slow-wave effect and attenuation loss. The slot-type floating shields with different SLs, SSs and metal layer positions are analyzed. It is concluded that minimum SL provides the most optimal result. A design guideline can be established to enable circuit designers to reach the most appropriate slot-type floating shields for optimal circuit performance. Transmission line test structures were fabricated by using 45-nm CMOS process technology. Both measurement and EM waves simulation were performed up to 50 GHz. Transmission lines are frequently used at a length of half-or quarter-wavelength. With a shortened wavelength, a saving in silicon area of more than 67% can be achieved by using optimized slot-type floating shields. Experimental results demonstrated a higher effective relative permittivity value, which is improved by a factor of more than 9, and a better quality factor, which is improved by a factor of more than 6, as compared to conventional transmission lines.

A unified model for on-chip CPWs with various types of ground shields

“…capacitance of CPW with floating shield increases with frequency obviously in contrast with the trend of classical CPW without shield. This phenomenon can be also observed in the experimental data of [4] but can't be explained by conventional model.…”

Slow Electromagnetic Waves: Theory and New Applications

“…A slow-wave coplanar waveguide transmission lines, combining the advantages of both coplanar transmission lines and microstrip lines, is reported [11] to create high-performance passive components for millimeter wave integrating circuits. To achieve a slow wave, periodical slottype floating shields are used to enable a reduction in a chip area while still maintaining high performance.…”

A K-band power amplifier in 0.18 μm CMOS process with slow-wave structure

“…With the increase of operation frequency, how to build a high performance matching network with less attenuation loss, small area and shield between signal line and underlying substrate is the primary problem to be solved. [5][6][7] The patterned ground shield (PGS) is commonly used for substrate shielding. However, when the operation frequency increases, especially beyond 20 GHz, the quality factor with PGS degraded further.…”