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Nanotechnology-Inspired Multi-Layer Conductors for High Performance Passive Components

Arian Rahimi, Student Member, IEEE and Yong-Kyu ‘YK’ Yoon, Member, IEEE

 work, the RF ohmic loss reduction is achieved with a non- Abstract—In this report, a summary of research results and ferromagnetic conductor such as accompanied in close potential applications are provided on multi-layer conductors proximity by a ferromagnetic material such as Ni and NiFe with consisting of alternating nano-superlattice structures for use in negative magnetic permeability in the of interest. The microwave passive components. The proposed meta-conductors negative permeability ferromagnetic material will produce eddy are engineered to have properties in microwave that cannot be readily found in intrinsic materials in nature. Especially, currents in an opposite direction of those of copper with investigated are ferromagnetic materials that have extremely positive magnetic permeability, canceling out both eddy dynamic frequency-dependent properties in ranging currents from the two neighboring conductors, the so called from DC to K-band (18–26 GHz) such as Ni and NiFe. As results cancelling effect (ECC) [2]. Fig. 1 (a) shows the of this research, conductors with reduced ohmic resistance in K- ECC effect of the proposed multi-layer conductors with their band are realized and low loss transmission lines are current distribution. While the current amplitude drops demonstrated. Also, the proposed conductor architecture has been utilized in conjunction with a low loss glass interposer exponentially through the volume of a conductor, where so that RF passives with improved microwave performance are the thickness of the non-ferromagnetic conductor is greater than presented. As a new application to the multi-layer conductors, a the skin depth at a given frequency, the current amplitude will tunable conductor, the so-called effect be uniformly distributed in the superlattice conductor with thin transconductor (M-FET) with tunable ohmic resistance is Cu/NiFe layers, resulting in an increased effective skin depth presented with 700% frequency tunability at 600 Oe DC magnetic and therefore lowered ohmic resistance. Fig. 1 (b) and (c) show field. the micrographs of the fabricated and transmission Index Terms—Ferromagnetic materials, high-Q RF passives, lines as well as the cross section view of the multi-layer nano- nano-superlattice conductors, magnetically tunable conductors. superlattice conductors. More than 50% ohmic resistance reduction is achieved at the ECC frequency of 18-20 GHz using I. INTRODUCTION 10 pairs of Cu/NiFe conductors with a thickness of each Cu and New class of conductors are introduced which are NiFe layer of 150 nm and 30 nm, respectively (Fig. 1. (d)). specifically engineered to have dynamic properties in the A 1 1 microwave regime. As the operating frequency of circuits and

systems increases, the current is confined to the outmost area of Normalized Normalized Normalized

0 Density Current 0 Current -1.05-1 -0.7 -0.35 0 0.35 0.7 1.051 -4.2-4 -2.8 -1.4 0 1.4 2.8 4.24 the conductor based on the theory, which results in Position (µm) Position (µm) NiFe Cu the increase of frequency resistance. The current profile Cu: 150 nm Cu: 150 nm Cu: 600 nm can be altered by employing multiple nanoscale heterogeneous Ni: 25nm Glass Ni: 25 nm Ni: 100 nm layers. In this study, ferromagnetic materials are combined with (b) regular conductors to form nanotechnology-inspired multi- (a) 10 layer conductors, the so-called meta-conductors, with useful RF Ref. Cu 8 Multi-Layer properties such as low RF resistance and magnetic tunability 6

4 that are discussed in the remaining of this report. Resistance 2 drop due to

ECC effect Resistance, [Ω] R 0 II. CONDUCTOR ARCHITECTURES FOR RF LOSS REDUCTION 0 5 10 15 20 Frequency, f [GHz] The electrical conductivity of the transmission lines and RF (d) (c) passive components is limited by the inherent material Fig. 1. (a) The current distribution into the volume of the conductors with the resistance and the internally induced loss mechanism ECC effect, (b) the cross section view of the fabricated superlattice conductors, (c) the top view micrograph of the inductors, and (d) the measured ohmic dominated by the skin effect which causes the RF resistance to resistance of transmission lines made of reference solid Cu and 10 paired continuously grow as the frequency increases ultimately Cu/NiFe with the same total thicknesses. degrading the performance of overall RF systems [1]. In this Although the conductor loss reduction is experimentally

demonstrated and optimized for a single transmission line using magnetic field transconductor (M-FET). Since both Cu and multi-layer conductors, the research is further extended to Ni80Fe20 layers are selected a few times thinner than the skin utilize the proposed conductor architecture to improve the depth of current in frequencies close to ferromagnetic performance of other microwave and mm- passive resonance (fFMR), the Ni80Fe20 layers in proximity of the Cu components. For the first time, we report on in-substrate passive layers will absorb the electromagnetic energy by spin torque components using a high performance glass interposer and energy transfer when the DC magnetic field is perpendicular to through glass via (TGV) technology and a multi-layer the AC magnetic field of the electromagnetic wave, resulting in superlattice conductor architecture. Greatly reduced RF loss is the increased effective resistance of the total conductor. As the achieved using the combination of the low loss glass spin torque transfer mechanism occurs near the fFMR frequency, substrates and the superlattice conductors. Half mode substrate the realized M-FET conductors will show a frequency-selective integrated waveguide (HMSIW) and two-pole conductor behavior (Fig. 3 (b)). Also, the thickness ratio of the bandpass filters, embedded inside a glass interposer substrate, conductor architecture will behave like a effect in are used as test vehicles for the demonstration of insertion loss theory and change the resistance of the improvement in K-band. The utilized conductor is made of 10 conductor (Fig. 3 (e)). pairs of Cu/NiFe layers with each pair of 360 nm/30 nm, The unique property of the proposed conductors to have a respectively. Fig. 2 shows the measurement results of the tunable ohmic resistance (biased by an external magnetic field) implemented with a center frequency of 18 GHz for and also the dependence of the ohmic resistance of the line on the multi-layer conductor and the solid Cu conductor as the thickness of the ferromagnetic material will allow an reference. The devices made of the Cu/NiFe and solid Cu analogy with a MOSFET transistor in linear region except that conductors feature an insertion loss of 0.51 dB and 0.73 dB at M-FET is made of pure conductors (Fig. 3 (f)). The applications 18 GHz, respectively, showing a significant loss reduction of of the proposed M-FET include tunable transmission lines, 0.22 dB for the Cu/NiFe superlattice conductor as shown in Fig. resonators, , and microwave absorbers.  H  2 (b). Wgap Ws dM Frequency [GHz] dt  Frequency [GHz]  11 16 21 Signal Ground M 5 10 15 20 25 30 0 H ext Cu/NiFe W  0 -0.5 (360/30 nm) g H RF 0.22 dB Ref Cu 2um -5 -1 Ref Cu 2um - Sim (c) -1.5 -10 (a) HDC HRF Parameetrs [dB] Parameetrs -2 (b) - Simulation: Cu S -15 -2.5 8 25 (b) 180 nm/60 nm -3 7 -20 10 Layers Cu/NiFe (180 nm/60 nm) 10 Layers Cu/NiFe (360 nm/30 nm) 180 nm/30 nm 1 1 20 Parameetrs [dB] Parameetrs In-Plane In-Plane

- Measurement: Cu/NiFe 0.8 0.8 6 360 nm/30 nm Out-of-Plane Out-of-Plane

S S11 - Sim -25 0.6 0.6 Ref_Cu 0.4 0.4 5 0.2 S21 - Sim 0.2 15 -30 0 S11 - Measured 0 -0.2 -0.2 4

-0.4 S21 - Measured -0.4 (T) Magnetization Magnetization (T) Magnetization None 25 Oe 10 -35 -0.6 -0.6 (d) 3 -0.8 -0.8 (c) 100 Oe 150 Oe

-1 -1 [Ω] R Resistance, 2 250 Oe 400 Oe [Ω] R Resistance, (a) -40 -30 -20 -10 0 10 20 30 40 -40 -30 -20 -10 0 10 20 30 40 5 Frequency [GHz] Applied Field (Oe) 500 Oe 600 Oe Applied Field (Oe) 1 11 16 21 Cu Ref. 0 0 Fig.0 2. (a) The comparison of the simulation and measurement results of the 0 5 10 15 20 0 5 10 15 20 Cu/NiFe Frequency, f [GHz] (d) Frequency, f [GHz] (e) designed-0.5 resonator. The measurement(360/60 results nm) are performed for the devices using nano-machined Cu/NiFe conductorsRef Cu 2um while the simulations utilize a Regular MOSFET M-FET -1 reference solid , (b) Refthe Cu zoomed 2um - -in version of the insertion loss D E G S Sim H Field of-1.5 (a) where comparison of the measurement results of the devices made of Field Hdc Field ac Port Port Cu/NiFe and solid copper conductors is given, (c) magnetic characterizaion + Parameetrs [dB] Parameetrs -2 + - N N 1 2 results,S and (d) the SEM microraphs of the implimented passives. -2.5 P-Substrate (f) Ferromagnetic -3 Non-Ferromagnetic III. MAGNETIC FIELD EFFECT TRANSCONDUCTORS Fig. 3. (a) The proposed M-FET conductor architecture with AC and DC In this section, an with a magnetic field magnetic fields, (b) the demonstration of the precessing movement of the and magnetic spin torque transfer when RF magnetic field is dependent conductance change is presented, where the perpendicular to the DC magnetic field at resonance, (c) the SEM image of an conductor consists of multiple pairs of nanoscopic non- M-FET, (d) the ohmic resistance of the M-FET under different magnetic fields, ferromagnetic Cu and ferromagnetic Ni80Fe20 [3]. The (e) M-FET resistance dependence on the thickness of the NiFe layers, and (f) electrical conductance also shows its dependence on the the schematics comparing the semiconductor-based MOSFET and the proposed conductor-based M-FET. thickness ratio of Cu and Ni80Fe20 layers. Upon applying an external DC magnetic field in the same direction with the Arian Rahimi will pursue his future career as an R&D engineer in Intel electromagnetic (EM) wave propagation direction in a coplanar Corporation, Portland, OR, USA. The fellowship from IEEE MTT society was a great motivation for him to further investigate his research. Attending IMS waveguide (CPW), the magnetic moment of Ni80Fe20 is aligned 2016 gave him the opportunity to talk to industrial partners that inspired him to with the magnetic field direction of the EM wave, causing the apply his research outcomes and learnings in real life. EM energy to be transferred to the spin torque and dissipated as heat, resulting in the decrease of electrical conductance. With a REFERENCES magnetic field of 0 and 600 Oe, the ferromagnetic resonance [1] A. Rahimi, J. Wu, X. Cheng, and Y. K. Yoon, “Cylindrical radial frequency changes from 900 MHz to 7 GHz (more than 700 % superlattice conductors for low loss microwave components,” J. Appl. frequency tunability) and so is the maximum electrical Phys., vol. 117, no. 10, Mar. 2015, Art. no. 103911. resistance point. [2] A. Rahimi and Y.K. Yoon, “Study on Cu/Ni Nano Superlattice Conductors for Reduced RF Loss,” IEEE Microwave and Fig. 3. (a) and (b) show the operating theory of the proposed Components Letters, 26.4 (2016): 258-260.