Fundamental Improvement to the Efficiency of On-Chip mmWave Phased Arrays Using MEMS Suspension

A major shortcoming of current silicon-based millimeter-wave (mmWave) on-chip antennas is the low radiation efficiency (<; 50%). In this letter, we present a novel on-chip antenna array with the radiation efficiency >80% at 60 GHz. The proposed array addresses the issues of on-chip air-suspension using micro electro mechanical systems (MEMS) processes, thus elevating the hovering radiating elements on a substrate at the height of 35 μm (0.007 λ
₀
). The micofabricated SU-8 posts serve as the vertical support, ensuring mechanical stability and monolithic integration. The simulated -10 dB fractional bandwidth of this array is 5.5% or 3.4 GHz at 60 GHz. Further, the designed array is capable of scanning of ±55° in the E plane and ±58° in the H-plane. The noncontact backscattering method is proposed and employed to derive the radiation efficiency from measured radar cross-section (RCS) data. RCS measurements are performed using a precision six-axis robotic arm with the spatial repeatability of 20 μm (λ
₀
/250). The measured radiation efficiency is 82% at 60 GHz. The measured peak gain of 9 × 9 array is 23.3 dBi versus simulated 23.6 dBi. Fundamental enhancement in suspended array efficiency is achieved by eliminating unwanted radiation in the silicon substrate. Compared to other on-chip integration approaches, the proposed array can be monolithically integrated on a silicon substrate while maintaining high radiation efficiency.