Characterization and analysis of patterned shields for millimeter-wave broadside-coupled balun in CMOS technology

Extensive studies on the performance of on-chip millimeter-wave broadside-coupled baluns with different patterned shields are carried out in this paper. Based on the advantage of multi-layer metals in the standard 0.18-μm RF CMOS process, the proposed Marchand balun takes the form of the broadside-coupled lines to enhance the coupling effect. In addition, to save the precious chip area, the proposed balun has been folded in the shape of a square geometry. The loss mechanism of the balun is analyzed and the losses of the balun in the conductive silicon substrate can be displayed with the aid of EM simulations. Different patterned shields are proposed and their influences on the balun device in terms of insertion loss, magnitude and phase balances are evaluated through EM simulations. According to the best of the authors´ knowledge, the patterned floating shield (PFS) is introduced for the first time to reduce the losses of the broadside-coupling Marchand balun in the CMOS technology. The results show that the PFS has the better effect than the patterned ground shield (PGS) for the proposed balun, the performance of the balun can be improved obviously with PFS in the operating frequency range. Besides, the influences of different PFS types, widths and spacing on the performance of balun have been compared and discussed.