An area efficient high turn ratio monolithic transformer for silicon RFIC

Abstract

A novel way of manufacturing an on-chip transformer that produces high inductance ratio (L Sec /L Pri > 30) with excellent area efficiency is presented. This technique uses an electrical all-round coupling effect of a conductor A (Primary Coil), having large effective width, and a densely routed conductor B (Secondary Coil). Thus, a high turn ratio monolithic transformer, using minimum die size, is realizable on silicon. The coil having the dense routing can also be doubled up as a monolithic RF choke on silicon. In this work, area efficiency is compared between various type of existing transformer structures (i.e. Interleaved and Stacked transformer), based on unit inductance. The method presented is fully compatible to all the foundry standard CMOS processes.

A Survey on Microtransformers

“…An advantage of multilayer over coplanar devices is that they can reach higher L-values, up to 500 µH [31]. The performances of multilayer and coplanar microtransformers are comparable, with coupling coefficients higher than 70%, according to the articles that reported this parameter for multilayer devices [24], [26], [28], [32]- [38]. The quality factors of multilayer microtransformers can be higher than 10, as stated by some authors [39]- [41].…”

“…Microtransformers using geometries different from those already discussed have been reported. Some of the examples are: e-shaped, fractal, polygonal and meander [16], [21], [28], [40], [59], [121]- [127]. Most of these singular microtransformers have coplanar or multi-layer structures and present 1:1 turn ratios.…”

“…Most of these singular microtransformers have coplanar or multi-layer structures and present 1:1 turn ratios. The most noticeable exception is a monolithic device with polygonal geometry, developed by Lim et al [28], with a 1:5.8 effective turn ratio. For these unusual geometries, coupling coefficients between 0.45 and 0.92 and quality factors up to 16 have been reported.…”

“…Since 2010 the wire bonding technique has been added to the list, although one of the first devices built with this technology appeared in an application bulletin of the Burr-Brown Corporation in 1994 [114]. One of the most versatile processes for building microtransformers with any structure and geometry, is microfabrication [2], [5], [6], [17], [22], [25], [28], [40], [49]- [51], [54], [58], [62], [63], [70], [71], [80], [81], [84], [89], [90], [92], [105], [107]- [109], [122], [128]- [130]. Of course, any of the other fabrication approaches may involve microfabrication in some of the processing steps.…”

“…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.…”