Abstract
In this paper, an analysis of the effects of pattern-ground-shield (PGS) and silicon-substrate on the performances of RF monolithic bifilar transformers are demonstrated. It was found that high-quality-factor and low-power-loss transformers can be obtained if the optimized PGS (OPGS) of polysilicon is adopted and the complementary metal–oxide–semiconductor (CMOS)-process compatible backside inductively-coupled-plasma (ICP) deep trench technology is used to selectively remove the silicon underneath the transformers completely. OPGS means that the redundant PGS of a traditional complete PGS (CPGS), which is right below the spiral metal lines of the transformer, is removed for the purpose of reducing the large parasitic capacitance. The results show that, if the OPGS was adopted and the backside ICP etching was done, a 253.6% (from 3.79 to 13.4) increase in Q-factor, a 14% (from 0.7 to 0.798) increase in magnetic-coupling factor (kIm), a 51.1% (from 0.55 to 0.831) increase in maximum available power gain (GAmax), and a 1.79 dB (from 2.597 to 0.807) reduction in minimum noise factor (NFmin) were achieved at 8 GHz for a bifilar transformer with an overall dimension of 230×215 µm2.
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