Wideband Tunable Radio Frequency Integrated Circuit Inductors Integrated With Domain-Patterned Permalloy

For wideband frequency-agile radio frequency (RF) integrated circuits, tunable components are essential. It has been a great challenge in RF electronics to utilize a magnetic tunable inductor for ultrawideband applications. This letter demonstrates an electrically tunable RF inductor encompassing a broad range of frequencies from 4 to 20 GHz. This is achieved by integrating domain-patterned Permalloy of various thicknesses, such as 20, 60, and 92 nm. There is a performance boost of the inductor, and a tunable inductor is achieved by applying dc current. Experimental results show that amongst all the Permalloy patterns, $11\,\mu\text{m}\times 13\,\mu\text{m}$ provides the highest inductance tunability of 15.8% at 20 GHz for 60 nm patterns, 10.8% at 15.1 GHz for 92 nm, and 4.3% at 15.7 GHz for 20 nm. This is the highest inductance tunability obtained so far for spiral inductors integrated with 60 nm Permalloy patterns using the dc bias method. The 15.8%, 10.8%, and 4.3% inductance tunability provides 8%, 5.7%, and 2.1% frequency tuning correspondingly.

I. Introduction

Tunable inductors occupying a smaller footprint area with better inductance and quality factor operating in a wide range of frequencies are essential to design better radio frequency integrated circuits (RFICs). Tunable inductors can be employed in tunable RF amplifiers, filters, reconfigurable matching networks, and voltage-controlled oscillators. In existing tunable RFICs, varactors (tunable capacitors) are usually used for tuning the frequency. The use of a tunable capacitor and inductor together provides greater variability in frequency ranges. Several tunable inductors with various geometries and tuning techniques have been developed. Among them, microelectromechanical system (MEMS) based inductors are explored with tuning methods, such as MEMS switches [Park 2004, Shirane 2012], coil coupling [Zine-El-Abidine 2005, Kim 2009], etc. Although these tuning methods achieved tunability to a certain extent, they have their own limitations. Tuning obtained using a MEMS switch provides a very high inductance tunability of 230% [Shirane 2012] but in discrete steps, and it depends on the number of switches. Most of the MEMS inductors provide a very high quality factor and tunability. However, they occupy a huge area, and their on-chip integration is difficult [Chen 2020]. Using ferromagnetic material as the core of a solenoidal inductor [Vroubel 2004, Wang 2017, Chen 2020] and integrating ferromagnetic patterns on the inductor [Salvia 2005, Zhang 2009, Rahman 2015, Wang 2016] is another approach that has been studied for tunability. The techniques used for tuning these ferromagnetic inductors are external dc bias and magnetic field biasing. Magnetic field biasing requires external magnets or heavy coils, so this tuning method is not possible for monolithic RFIC. The better method of tuning the inductor on the chip is by controlling the permeability of ferromagnetic material, and it can be done by varying dc current (dc bias method), which is compatible with complimentary metal–oxide–semiconductor (CMOS) RFICs. The most explored ferromagnetic material is Permalloy () because of its high and electrically tunable permeability, low coercivity, and no stress anisotropy [Wang 2016].