A High-Performance Micromachined RF Monolithic Transformer With Optimized Pattern Ground Shields (OPGS) for UWB RFIC Applications

Abstract

In this brief, we demonstrate that high-quality-factor and low-power-loss transformers can be obtained if the optimized pattern ground shields (OPGS) of polysilicon is adopted and the CMOS processcompatible 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, 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 69.3% and a 253.6% increase in quality factor, a 10.5% and a 14% increase in magnetic-coupling factor (k Im ), a 17.2% and a 51.1% increase in maximum available power gain (G A max ), and a 0.682-and a 1.79-dB reduction in minimum noise factor (NF min ) were achieved at 5 and 8 GHz, respectively, for a bifilar transformer with an overall dimension of 230 × 215 µm 2 . Index Terms-Micromachined, optimized pattern ground shield (OPGS), power gain, quality-factor, radio frequency integrated circuits (RFICs), transformer.

A Survey on Microtransformers

“…These inductors are usually designed according to the classic modeling by Greenhouse [11], [85]- [87]. The spirals can be circular, square, elliptical or n-sided polygons, and the coils can be coplanar (interleaved or side by side) or placed in different layers (with or without magnetic layers between the coils) [6], [17]- [19], [22]- [25], [27], [31], [33]- [37], [39]. Due to their compact spiral coil assembly architecture, microtransformers having very small area and volume can be fabricated.…”

“…Publications reporting the development of microtransformers for communications are about 34% of the works analyzed [6], [16]- [19], [21], [23], [28], [32], [34]- [39], [48], [49], [51], [52], [55], [60], [61], [63], [68], [74], [75], [82], [83], [95], [103], [107], [110], [121]- [123], [136]. This includes applications like UHF and VHF circuits and microwave monolithic integrated circuits.…”

Integrated On-Chip Transformers: Recent Progress in the Design, Layout, Modeling and Fabrication

“…To be able to compare the different transformer designs and evaluate their performance, several metrics have been used. In its simplest form, a transformer can be considered as two inductors coupled together [30,31]; therefore, fundamental performance parameters of transformers are also similar to those of inductors which are primarily, the Q -factor ( Q ) and self-resonance frequency ( f SR ). Apart from these, there are other parameters which describe the performance of monolithic transformers, including their coupling factor ( k ), mutual inductance ( M ), insertion loss and power gain ( G ), area occupied, and transformer’s characteristic resistance ( TCR ).…”

“…On the other hand, substrate loss is likely to be one of the major loss mechanisms in inductive devices, and imposes a fundamental upper boundary on the Q -factor. Fabricating the transformer on a higher resistivity substrate, such as glass [34], or by shielding the conductors from the substrate have been shown to increase the Q -factor of on-chip transformers [31,35].…”

Design of Coil Length of On-Chip Transformer With High Turn Ratio and High Coupling Performance

“…The structure suffered from a low operating frequency since the use of a stack of two parallel coils increased the parasitic capacitance. To increase the operating frequency, the literature described a design by twining a half-coil at a single layer into full metal layers [3], [4]. Nonetheless, the turn ratio between the two coupling coils is still inferior(n = 1) in the cited works.…”