A four element phased array antenna system monolitically implemented on silicon

by Ji, Taeksoo.

iii Steadily increasing need for wideband wireless communication services have promoted the development of wireless communication systems with higher data rates and increased functionality. Phased array antennas are well suited to satisfy the growing demand with its ability to increase channel capacity and steer multiple beams. Of the various types of antennas, microstrip antennas would be a good common element in constructing the array antenna due to their low cost, low weight, conformability, and easy integration into arrays or use with microwave integrated circuits. In this research work, a four element phased array antenna aimed for 15GHz has been monolithically implemented on silicon substrates using monolithic microwave integrated circuits (MMICs) technology. The array fabricated herein consists mainly of microstrip radiating patches and feed networks including coplanar waveguide (CPW) –to- Microstrip (MS) line transitions, phase shifters, Wilkinson power dividers, and DC blocking filters for CPW and MS lines. Each component of the fabricated array antenna was carefully designed for operational efficiency, and validated using a custom simulation tool. All circuits were realized on a high resistivity silicon (HRS) substrate surface-stabilized by polysilicon. This configuration achieved a significant reduction in RF losses by immobilizing the surface charges populated in the interface of SiO2/Si. The monolithic integration of the array antenna into silicon not only makes the whole circuitry compact, but also reduces the cost utilizing mature CMOS technology. A single microstrip patch showing a resonance frequency of 14.8GHz with a return loss (S11) of 21dB is connected to the feed networks based on CPW lines through iv a CPW-to-MS transition. This transition, as well as DC blocking filters for both CPW and MS lines exhibited the possibility for wideband applications by showing wide 3dB bandwidths of 168%, 123%, and 130%, respectively. Two types of phase shifter designs were constructed to compare performance: a microelectromechanical system (MEMS) phase shifter, and a ferroelectric phase shifter. Despite a high operating voltage of up to 300V, the ferroelectric phase shifter utilizing permittivity tunability of (Ba,Sr)TiO3 (BST) films was adopted as the phase shifting device in the array due to its high phase shift capability (~30o/dB), low leakage current level (~300nA at a bias voltage of 100V) and notably high operational reliability. The four element phased array antenna completely integrated on silicon showed a total scan capability of 10o measured at its resonance frequency, 14.85GHz with a return loss of 32dB. The phased array antenna presented herein will provide a basic view and understanding of the process of monolithic integration into silicon using MMIC technology. Improvements of antenna performance in terms of steering capability, side lobe level (SLL), half power beam width (HPBW) and bandwidth could be accomplished by further research on design modification as well as on process optimization for array antennas.