A review of the tunable microinductors

Dong-Ming Fang1,Hai-Xia Zhang

1, Norman C. Tien

2

1National Key Laboratory of Nano/Micro Fabrication Technology,

Institute of Microelectronics, Peking University, Beijing 100871, China 2Department of Electrical and Computer Engineering,

Case Western Reserve University, Cleveland, OH 44106, USA

Abstract: Radio frequency (RF) tunable inductors have a major role in current situation where

compact designs with high performance are demanded. The capability of the tunable inductors, to

tune the inductance with high or proper quality factor (Q-factor), has been a good advantage in the

tunable systems to the designers rather than to use the tunable capacitor which exhibit poor

reliability and require large die area. Tunable inductor could be a candidate to save die area and a

solution for designs of the tunable systems with large tuning range for portable communication

systems in the future. Therefore, a review of the tunable inductors from a device perspective is

provided. Based on their tuning mechanism, the tunable inductors can be classified into four

categories: discrete tuned, metal shielding tuned, magnetic core tuned and coil-coupled tuned. This

paper summarizes the major contributions to the tunable inductors and discusses the advantages

and disadvantages of these contributions. Some considerations and results on fabrication,

operation, comparison and application of the tunable inductors are presented.

Keywords: tunable inductor, tuning ratio, tuning range, inductance, quality factor, MEMS

1. Introduction

Radio frequency microelectromechanical system (RF MEMS) is a technology that enables the

batch fabrication of miniature mechanical structures, devices, and systems for microwave and

wireless communication applications.There are four basic RF MEMS components have been

reported so far: (i) the switch; (ii) the (tunable) capacitor; (iii) the (tunable) inductor and (iv) the

antenna. These four components can be used to realize high performance and digitally-controlled

components (e.g. RCL lumped-elements), circuits (e.g. attenuators, phase shifters, impedance

tuners, filters, oscillators) and subsystems (e.g. T/R modules and antenna arrays). By far, the most

common actuation mechanism is electrostatic, followed by piezoelectric, magnetic and

electrothermal. RF tunable capacitors and inductors can be used to form tunable filters, low-noise

voltage-controlled oscillators (VCOs), self-adjusting matching networks or power amplifiers. In

general, the tunable capacitors have better tuning range and quality factor（Q-factor） than tunable

inductors, however, the tunability of the tunable inductors provides additional functionality,

design flexibility and robustness, which make the tunable inductors promising applications in the

field of portable communication systems The tunable inductors can be divided into four categories:

discrete tuned (DT), metal shielding tuned (MST), magnetic core tuned (MCT) and coil-coupled

tuned (CCT). The discrete tuned inductor often uses microswitches [1-4] or microrelays [5, 6] to

increase or decrease the effective coil length of the inductor, but the combination of the

microswitches or micorelays will reduce the Q-factor of the inductor. The metal shielding tuned

inductor is realized using moveable metal structure with large range, resulting in the magnetic flux

of the inductor changed [7].The magnetic core tuned inductor is realized using solenoid inductor

imbedded with magnetic-core conductor whose permeability can be changed when applying

Corresponding author. Tel: +86 10 62752536-17, Fax: +86 10 62751789, E-mail: zhanghx@ime.pku.edu.cn

magnetic filed [8, 9]. The coil-coupled tuned inductor mainly adjusts its mutual inductance

between the primary coil and the secondary coil of the inductor [10-12]. Key features, measured

performance characteristics and various applications based on their performance of the reported

tunable inductors are summarized (and referenced) in table 1. Tuning types and configurations

vary widely. Tunable inductors with discrete tuned, metal shielding tuned, magnetic core tuned

and coil-coupled tuned have been reported are discussed further below.

Table 1 Tunable microinductors.

Author and year Tuning

type

Tuning ratio@

Frequency(GHz)

Q-factor@

Frequency(GHz)

Fabrication

process

Other

information

Applications

Zhou [13],1999 DT 129.2 1.7@0.5

3.3@1.6

SBM area

3150μm×930μm

, DV: 20V.

n/r

Park [1],2004 DT 2.87@2.4 4.8@1.5 CMOS area

206μm×217μm

driven voltage

1.8V

n/r

Balachandran[4],2004 DT 1.64@25 n/a MEMS DV:25-35V n/r

Mina[14], 2007 DT 1.66@5 45@5 post-CMOS-

compatiable

DV:40V n/r

Zekry [15],2007 DT 1.63@1.2 18.5@1.2 MEMS area

620μm×620μm

n/r

Okada [17],2006 MST A:2.13@1.8

B:2.11@2

A:33@1.8

B:50.1@2

MEMS area about

460μm×460μm

VCO:TR 146.5%

Sugawara[19],2004 MST 1.53@2.45 4.4@3.5 0.35μm

CMOS

area about

400μm×400μm

VCO:TR 123.7%

Ito [20],2005 MST 1.8@2.02 About 3@2.202 0.18μm

CMOS

area about

400μm×400μm

VCO:TR 72%

Vroubel [22],2004 MCT 6.66@0.1

1.54@1

1.25@2

Less than 2@？ n/r FM core area

about

200μm×50μm

n/r

Salvia [8],2005 MCT 1.17@5 5~10@over 5 BiCMOS

n/a n/r

Sarkar[25],2005 MCT 1.24@1~5

1.47@7.5

34@2.2

30@7.5

n/r

n/a n/r

Lubecke [26],2001 CCT 1.22@15GHz About

13@2GHz

MUMPS n/a n/r

Zine-El-Abidine[27],

2003

CCT 1.13@2~5 n/a MUMPS n/a n/r

Zine-El-Abidine[11,12],

2005

CCT 1.43@over 2 25@10 SBM SRF:35GHz n/r

Zine-El-Abidine[28],

2007

CCT 2.1@ n/a 5@3 MetalMUMPs n/a n/r

Fukushige [29],2003 CCT 1.03@2 Less than

2@0.05~16

MEMS n/a n/r

Chang[32],2006 CCT 1.46@0.5~5 15@3.5 MEMS DV:0~2V n/r

Dell [33],2002 CCT 3.33@0.001 n/a BM n/a n/r

Sugawara [16],2004 MST 2.11@2 50.1@2 MEMS n/a VCO:TR 146.5%

YANG [38],2006 CCT n/a n/a 0.18μm

CMOS

n/a VCO: TR 106.5%

Worapishet [39],2002 CCT n/a n/a BiCMOS n/a Filter: TR 171.4%

Lin [3],2005 DT 1.56@5.5 12@3.5 CMOS n/a LNA: TR 174.2%

Sugawara [41],2005 MST 1.4@2

3.5@2 0.18μm

CMOS

n/a LAN:

TR 188.2%

Tassetti[42],2004 CCT 2@1~5 n/a MEMS n/a phase shifter: 25°

at 5GHz and 48° at

8GHz

n/a: not applicable; n/r: not reported; SBM: surface and bulk micromachining; SRF:

self-resonance frequency; DV: driven voltage; TR: tuning range

2. Discrete tuned inductor

The electrical model of the discrete tuned inductor is usually described as shown in Fig.1.

When the switches at port two are open, the inductance is L1. When the switches at port two are

closed, the length of the coils at port two will be changed and the different mutual inductance will

affect the effective inductance Leq. Leq can have n2 discrete values depending on which coil at port

two is switched on. The effective inductance Leq and the effective resistance Req can be written as

2 2 2

1 2 2 22

(1 )n

n n neq

i n n

k LL L

R L

(1)

2 2

11 2 2 2

2

nn n n n

eq

i n n

k R L LR R

R L

(2)

where Ln is the inductance of the coil at port two, kn is the coupling coefficient, Rn is the series

resistance of the coil at port two and the contact resistance of the corresponding switch, δn is 1

when the switch is at the „on‟ status and 0 when it is at the „off‟ status. To achieve high Q-factor,

Rn must be much smaller than the reactance of the coil (Ln•ω ).

Fig.1 Electrical model of the discrete tunable inductor.

Zhou [13] fabricated a tunable inductor, as shown in Fig.2, with digitally controlled microrelays

by using combined surface and bulk micromachining technology. The microrelays used TaSi2/SiO2

layers as the bimorph cantilever beam, aluminum as sacrificial layer. The combined thermal and

electrostatic mechanism made the possibility of the gold-to-gold contacting. In order to reduce the

losses of the parasitic oxide capacitances and the eddy current loss in the substrate, the silicon

substrate under the inductor was etched. By using the planar spiral coil and four microrelays, the

inductances of the tunable inductor varied from 2.5nH to 324.8nH to obtain sixteen different

inductance values. The self-resonant frequencies were 1.9 GHz and 4.6 GHz, while the quality

factor (Q-factor) was 1.7 (f = 530 MHz) and 3.3 (f =1.6 GHz), respectively. The lowest thermal

power was 8mW and the electrostatic driven voltage was 20V. Because of the limitation of the

mechanical self-resonance, the highest operation frequency of the microrelays was 10 kHz, which

leaded the tunable inductor to be used in the portable mobile communication systems in the range

from 0.5GHz to 1.6GHz. Moreover, the whole device was 3150μm×930μm which is relatively

larger compared to other tunable inductors. Because the fabrication process combined surface and

bulk micromachining technology, seven masks was used, resulting in the complexity and

incompatibility with IC technology. Park [1] fabricated the stacked spiral inductor using standard

CMOS process and the inductor was connected with MOSFET switch. When the status of the

switch varied, the inductance of the inductor changed. The measured results showed that the

inductances changed form 8nH to 23nH at the frequency of 2.4GHz. The resonance frequency of

the proposed inductor decreased from 3.9 to 3.6 GHz for the three-stacked inductor and from 6.8

to 6.4 GHz for the two-stacked inductor. The stacked inductor had a comparable Q-factor 4.8 at

the frequency about 1.5 GHz. Compared to the traditional single-layer inductors, because the

structure of the inductor was stacked, fifty percent of the chip area was saved. However, because

of multi switches, which induced parasitic capacitances, resulting in the decrease of the

self-resonant frequency and thirty percent of the Q-factor. Balachandran et al. fabricated MEMS

tunable inductors using DC-contact switches [4]. In their fabrication process, PMMA was used as

sacrificial layer and resistive SiCr was used to provide DC-bias. Experimental results show that

the inductance value can varies from 0.34nH to 0.56nH at 25GHz, that is, the inductance ratio is

of 1.64. However, the measured insertion loss for the switch-actuated state had a difference of

0.3dB.

In 2007, Mina fabricated a novel switch controlled tunable inductor [14]. The switched tunable

inductor was fabricated by using a post-CMOS-compatiable process. In order to decrease the loss

of the Si substrate, the backside of the substrate was selectively etched under the device. As shown

in Fig.3, four discrete inductance values were obtained by using the contacts of the switch to

contact/leave the primary and the secondary coils of the inductor. The actuation voltage of the

switch was 40V. When the switch was off, the inductance was 1.01nH. Through the switch, the

maximum tuning range of the tunable inductor was 40% at 5GHz and the corresponding Q-factor

was 45. The authors pointed out that the Q-factor of the inductor didn‟t decrease obviously after

package. The key point of the structure was the switch. The switch used silver as the cantilever,

driven by electrostatic force, to realize the status of on and off. The authors said that compared to

other metal materials, silver has good electro and mechanical properties. For example, after

electroplated, silver film has high conductivity, low Young‟s module and small mechanical stress.

Zekry [15] fabricated a 2-bit digitally tunable inductor on a standard CMOS substrate by

using surface micromachined relays which were electrostatically actuated. The mechanism of

tuning was to use several microrelays to contact/leave the coil of the inductor, thus to change the

effective magnetic area of the inductor and the inductance values. Because there was an oxide

insulated layer between the substrate and planar spiral inductor and used a thick copper layer as

the spiral inductor and interconnects, for the 4.5-turns spiral inductor with 4-switching

microrelays, the maximum Q-factor was about 18.5 at 1.2GHz and the inductance values varied

from 2.29nH to 3.73nH resulting in 38.6% tuning range.

Fig.2 The unable inductor with digitally Fig.3 A novel switch controlled tunable

controlled microrelays [13]. By using the inductor [14]. Four discrete inductance values

planar spiral coil and four microrelays, were obtained by using the contacts of the

switch sixteen different inductance values to contact/leave the primary and the secondary

can be obtained. coils of the inductor.

3. Metal shielding tuned inductor

The metal shielding tuned inductor is tuned using a movable metal plate to shield the magnetic

flux and change the inductance of the inductor. Fig.4 is the schematic structure of the metal

shielding tuned inductor [16]. The inductance is changed using a moving metal plate, which is

moved by a MEMS actuator. The simplified equivalent circuit model [17] for the metal shielding

tuned inductor is shown in Fig.5.Ls and Rs are the series inductance and resistance of the spiral

inductor. Lmet and Rmet represent the equivalent inductance and resistance of the shielding metal

plate. Lsub and Rsub indicate the equivalent inductance and resistance of the substrate. Mmet is the

mutual inductance between the spiral metal and the shielding metal plate. Msub is the mutual

inductance between the spiral metal and the substrate. Yammouch et al. improved the physical

model by supplying two-port π equivalent circuit [18].

Fig.4 Schematic structure of the metal Fig.5 Simplified equivalent circuit model for

shielding tuned inductor [16]. the metal shielding tuned inductor [17].

Okada presented an on-chip high Q-factor tunable inductor embedded in wafer-level chip-scale

package (WL-CSP) [17]. The authors used a metal plate moved by a micromanipulator instead of

MEMS actuator (see Fig.6) because they thought that the MEMS actuator has not been

implemented up to date. The inductance values of the tunable inductor can be varied according to

the insertion with/without of the metal plate or the distance between the spiral inductor and the

metal plate. The width and the height of the metal plate were 600μm and 300μm, respectively.

They proposed two groups of tunable inductors. The resistivity of the substrate for Group A was

2~6Ωcm, while Group B was 1kΩcm.The measured inductance tuning range of Group A and

Group B were 53.4% and52.6%. The maximum Q-factor of Group A was 33 at 1.8GHz while

Group B of 50.1 at 2GHz. Though the parasitic capacitance may be appeared between the spiral

inductor and the metal plate, the effect of the parasitic capacitance on the self-resonance frequency

（SRF）was expected to be small in that structure because the inductance varied faster than the

parasitic capacitance.

Fig.6 Photograph of the metal shielding tuned inductor [17]. The metal plate

was moved by a micromanipulator instead of MEMS actuator.

Sugawara et al. fabricated a novel tunable inductor on Si CMOS chip [19]. The symmetrical

spiral inductor was fabricated by using AMS 0.35μm CMOS process (three metal layers). They

used a metal (copper) plate which could be moved by a MEMS actuator. When the movable metal

plate shielded the magnetic flux, the inductance value of the inductor varied continuously. At

2.45GHz, the inductance value varied from 5.81nH to 3.80nH, that is, the tuning ratio is about

1.35. The maximum Q-factor was about 4.4 at 3.5GHz. When this tunable inductor was applied to

a VCO, the tuning range of the VCO was extended into 123.7%.After a year, the authors‟ group

used the similar tunable inductors and some switched capacitors to form a novel wide-range

tunable VCO (0.18μm CMOS process)[20]. The tuning ratio of the tunable inductor was 1.8 at

2.02GHz and the corresponding Q-factor was about 3. The oscillation frequency could be changed

by three switches. The VCO could be tuned from 1.28GHz to 2.75GHz with the tuning range of

72%. The best performance of the phase noise was at 1.28GHz, from about -80dBc/Hz to

-160dBc/Hz with the increasing offset frequency from 10 kHz to 10MHz.

4. Magnetic core tuned inductor

Magnetic core tuned inductor is tuned based on the magneto impedance effect [21]. As the bias

dc magnetic field is applied along to the axis of the inductor body, the transverse permeability of

the soft magnetic layer can be changed to achieve the tunability of the inductance. In order to

design tunable microinductors for microwave communication systems, such as the frequency

range over 1 GHz, most magnetic materials become non-magnetic, with relative permeabilityμ

r=1. These materials are unsuitable for high frequency applications.

Vroubel [22] firstly fabricated tunable integrated RF inductor with magnetic ore. The tuning

effect of such magnetic RF inductor was achieved by the superposition of a dc current onto the

primary solenoid winding of a thin-film ferromagnetic (FM) core. The thin NiFe film

ferromagnetic core was implanted in the solenoid inductor as shown in Fig.7. By applying

different current through the FM core, the effective permeability of the FM core was changed. In

the range of inductance from 1nH to 150nH, the variation range of the inductance was 85%, 35%

and 20% at 0.1, 1 and 2 GHz, respectively. Although an 85% tuning range was demonstrated at

100 MHz, the Q-factor was limited and the magnitude of the dc current must be as high as 100mA

to achieve this. The Q-factors of all the inductors were less than 2. When the DC was 100mA, the

power was 0.015～0.3W. The authors thought the reason was because of high conductivity of the

FM core and the improper design of the magnetic core coil. They also pointed out that when the

conductivity of the FM core was less than 105S/m [23], the simulated permeability was more than

50 and the Q-factor of the tunable inductor would be over 15. However, applying dc control

current and RF current in the same inductance device may not be favorable in some applications

as the noise from the dc control current may feed into the working current path.

When the applied magnetic filed is parallel to the easy magnetizing axis of the NiFe permalloy,

the permeability of NiFe permalloy will be changed [24]. Salvia [8] used this effect to fabricate

on-chip tunable inductor (see Fig.8) with Ni80Fe20 laminations with BiCMOS process. In the

fabrication process, a conformal NiFe deposition followed by a photoresist lift-off results in a

permalloy film that wraped around three sides of the conductors. This design simplified the

fabrication in comparison to using the high-μ material as the inductor core.The permalloy

laminations can decrease the RF eddy current loss and ferromagnetic resonance (FMR) loss. The

measured results showed that the tuning range of the tunable inductor was 15% and the Q-factor

was between 5 and 11 over 5GHz. Sarkar et al. [25] also fabricated the tunable inductors by using

magnetic core of NiFe. In their work, the solenoid inductor and NiFe core are fabricated using

high aspect-ratio SU8 molding and electroplating on a Pyrex substrate serves as the assembly

incorporating signal routing. At high frequency, from 1 to 5GHz, the inductance varied from 5.5to

6.8nH, i.e., tuning ratio about 1.24. As frequency increased more than 5GHz, the insertion of the

magnetic core enhanced the inductance, tuning ratio achieved 1.47 at 7.5GHz and Q-factor was 30.

The maximum Q-factor was 34 at 2.2GHz.

Fig.7 Schematic of a biased solenoid tunable Fig.8 The photo of the tunable inductor

inductor with magnetic core [22]. The with patterned permalloy laminations [8].

magnetization is mainly oriented along

the easy axis of the FM film.

5. Coil-coupled tuned inductor

Coil-coupled tuned inductor is designed by controlling the magnetic coupling coefficient

between the two layers or two coils/loop of micoinductors. Fig.9 is the equivalent circuit model of

the coupled tuned inductors, where M is the mutual inductance between the two windings or

coils/loop, k is the coupling coefficient, Ls1 and Ls2 are the series inductances of the two coils,

respectively.

21 ss LLkM (3)

Fig.9 Transformer micro inductor/loop Fig.10 Tunable self-assembling inductor

equivalent circuit model. by means of an interlayer stress [26].

Lubecke [26] fabricated self assembling barrette-type tunable inductor (Fig.10) using

multi-user microelectromechanical process (MUMPS) . The inductor assembled by means of an

interlayer stress that causes portions of the inductor to bend away from the substrate in a

controllable manner and obtained high Q-factor and self-resonance frequency (SRF). The

measured Q-factor was above 13, SRF was greater than 15GHz and the tuning range is over 18%.

When the fabrication process was optimized, the simulated results showed that the performance of

the tunable inductor would be improved as the Q-factor was over 20 and the tuning range was

greater than 30%.

Zine-El-Abidine et al. fabricated the tunable inductors using the MUMPS process [27]. The

tunable inductor was formed of inner and outer inductors, a beam and the arrays of thermal

actuators. When the array was actuated, the beam buckled and lifted up the outer inductor. This

status called “ON” state, corresponding to the minimum inductance value. The measured

minimum and maximum inductance of the tunable inductor were 1.045nH and 1.185nH,

respectively, that is, the tuning ratio was 1.13. Zine-El-Abidine [11, 12] also fabricated the tunable

inductor using surface micromachining combined with bulk micromachining. When the thermal

bimorph were actuated, the outer coil (loop) would be moved from the inner coil, thus the mutual

inductance of the inductor was changed to realize the tuning. The authors fabricated three types of

tunable inductors with the pitch, between the outer coil and the inner coil, of 11, 21,31μm. Fig.11

is the tunable inductor with the pitch of 21μm. Its tuning range could reach 30% over 2GHz, while

the maximum Q-factor was 25 at 10GHz and SRF was 35GHz. Two years later, the authors

fabricated another tunable inductor using the commercial process MetalMUMPs (multi-user

microelectromechanical process) [28]. As shown in Fig.12, the inductor was formed by two

pre-bent beams. One beam end was anchored and the other beam end was attached to an array of

thermal flexure actuators. To reduce the required force for buckling, the beams were designed

such that they are curved at the rest state. In order to increase the Q-factor of the inductor, the

silicon underneath the inductor coil was selectively etched. The tuning ratio of the inductor was

2.1:1, that is, the tuning range of the tunable inductor was 52.4%. The maximum Q0factor was 5

at about 3GHz.

Fig.11 Tunable inductor using thermal Fig.12 Tunable inductor was formed by two

bimorph .When the thermal bimorph pre-bent beams. One beam end was anchored

were actuated, the outer coil (loop) would and the other beam end was attached to an

be moved from the inner coil [12]. array of thermal flexure actuators [28].

Fukushige [29] fabricated a new type of tunable inductor using MEMS technology. The inductor

was formed of spiral conical coil (see Fig.13). The height of the coil could be changed from zero

to several hundred micrometers. When the height of the coil was changed, the mutual inductance

of the inductor varied. The inductance value could be a few nano Henries. The inductor coil was

adopted a new MEMS material, Pd-based thin film metallic glass (TFMG) [30, 31]. The measured

and simulated results showed that the tunable inductor could be worked from 50MHz to 16GHz.

At 2 GHz, the inductance values varied from 3.64nH to 3.75nH which means that the tuning range

of the inductor is 3%. However, because the thin film metallic glass was used, the Q-factor of the

tunable inductor was less than two. The authors thought the low Q-factor dued to the high

resistivity of the TFMG (62μΩcm). Chang [32] fabricated the RF MEMS tunable inductor (see

Fig.14) using the bimorph effect of an amorphous silicon and aluminum structural layer. When

there was no applied voltage, the vertical height of the inductor was 450μm, the maximum

Q-factor was 15 at 3.5GHz and SRF was 7GHz. While 2V was applied to the inductor, the

inductance values varied from 5.6nH to 8.2nH which means the tuning range of the inductor was

31.7%.

Fig.13 The unable inductor, formed of spiral Fig.14 The fabricated inductor viewed at 75º tilt.

conical coil, could be changed from zero to The height of the outer turns can reach the

several hundred micrometers. In this photo , emaximum value of 450μm [32].

the height of the coil is 200μm [29].

Dell et al. [33] used silicon bulk micromachining technology to fabricate tunable spiral

inductors with the application of low DC bias voltages. With the bias voltages, the movement of

the spiral changed the geometrical structure of the inductor, resulting in a change of the inductance

value. However, if the spiral structure with suspended state was wanted, the intrinsic stress in the

spiral material should be tensile. Silicon nitride via PECVD (plasma enhanced chemical vapor

deposition) to form the spiral support material. The measured inductance values varied from 30 to

100nH at 1MHz. It was found that the inductance decreased as the bias voltage increased. The

inductance values of the tunable inductor at high frequencies were not mentioned in their work.

Hsu et al. presented a double layer spiral coils and studied their coupling effect [34]. Compared to

the initial inductance, the mutual inductance between the double coils decreased 33.5%.

6. Applications

Tunable inductors are widely used in wireless communication systems, such as

voltage-controlled oscillators (VCOs) [16, 20, 35-38], tunable filters [39, 40], tunable low-noise

amplifier (LNA) [3, 41], phase shifters [42], RF front-end [43], wireless sensor[44], especially

when a wide frequency tuning is desired. Tunable inductors would further benefit wireless

communication circuits where impedance matching and frequency tuning would increase the

flexibility and reliability of the system.

Sugawara [16] presented a tunable inductor using redistributed layers. A metal plate was used to

shield the magnetic flux of the inductor. The metal plate was moved by a MEMS actuator. The

inductance varied from 4.80nH to 2.27nH, that is, the tuning ratio was about 2.11. The maximum

value of Q-factor was 50.1at 2GHz.When the tunable inductor was applied to a VCO, the

oscillation frequency range of the VCO achieved 146.5%.YANG [38] presented tunable

inductance LC-tank VCOs implemented in 0.18μm CMOS technology. Two prototype LC-VCOs

utilized the tunable inductors to extend the operating frequencies at 2.85~3.12 GHz and 6.59~7.02

GHz, respectively. The 3GHz VCO using a symmetry transformer provided the tuning range of

2.85 to 3.12 GHz at 1V supply. The power consumption was 4.8mW while the measured phase

noise was -126 dBc/Hz at 1MHz. A small-area stacked transformer was employed in the 7GHz

VCO, which achieved a tuning range of 6.59 to 7.02 GHz and measured phase noise of -114

dBc/Hz at 1MHz , consuming 9mW from a 1.2V supply.

Worapishet [39] proposed an LC bandpass filter using an improved magnetically-coupled

tunable inductor, which was formed of two mutual coils. The best evaluated inductance of the

tunable inductor was 8nH with the mutual coupling coefficient, k=0.6. The simulated response at

the maximum Q value of ten of the LC bandpass filter could varied from 0.7GHz to 1.2GHz, more

than 50% frequency tuning. Moreover, the supplied voltage for the filter was 2V and the

consumption of the filter was less than 9mW of power. Georgescu [40] fabricated the tunable

coupled inductor using 0.18μm CMOS process with a low-resistivity expitaxial substrate. The

tunable inductor was used to form the LC tank in the design of a filter.When the inductance value

was 2nH, the simulated insertion loss in the passband was nearly 0dB.The noise figure and the

input third-order intercepting point (IIP3) were 23dB and -15dBm, respectively.

Lin et al. [3] used the variable planar spiral inductors with MOSFET switch to optimize the

CMOS wideband low-noise amplifiers (LNAs) and to implement low-phase noise VCOs. When

the maximum inductance of the variable inductor was 0.58 nH, a improvement of 74.2% (from 3.1

GHz to 5.4 GHz) in bandwidth of the CMOS wideband LNA were achieved. The measured noise

figures were2.2 dB at 1 GHz and 4.0 dB at 6 GHz. The measured input third-order intercepting

point (IIP3) was -3 dBm at 3 GHz. Sugawara [41] proposed a novel wide-tunable LNA using

on-chip tunable inductor with 0.18μm CMOS process. The tunable iuductor was realized using

metal to shield the spiral inductor. The minimum and maximum inductance values were 1.5 and

2.1nH, i.e., tuning ratio of 1.4. At the frequency of 1.9 GHz, the power gain (PG) of the LAN was

13.5 dB, the noise figure (NF) was 7.1 dB, and the IIP3 was -1.9 dBm. The LNA achieved PG of

14 dB and over 10 dB from 1.7 GHz to 3.2 GHz. The NF was not degraded by the shielding metal

plate. This tunable LNA is quite useful for multi-band RF communication system.

Fig.15 Photo of the tunable filter Fig.16 Photo of the LNA

with size of 650μm×650μm [40]. with size of 0.97mm×1.28mm [41].

Tassetti et al. designed and fabricated the tunable inductors [42]. They used two different

wafers, SOI wafer and glass wafer. SOI wafer was for mechanical parts while the glass wafer was

to build the microinductor, pads and electrodes. The inductance values were changed by

controlling magnetic coupling coefficient between the two different coils. The primary coil was

stationary and the secondary coil was patterned on a movable cantilever, which will bend when

there was electrostatic pressure produced by an electrostatic actuator. An anodic bonding step was

used to align the inductors and the electrodes. Thus, the electrodes were used to provide an

electrostatic actuation to change the gap between the two inductors. A high actuation voltage of

150 V was used. The measured results showed that the first prototypes present 50% inductance

variation and the second prototype variation ratio was expected to reach at least 4 over 1 to 10

GHz. Although the tuning range was close to 50%, the Q-factor was unacceptably low due to

strong interactions with silicon. The authors used the tunable inductors and tunable capacitors to

design and realize some tunable radio frequency basic functions, such as tunable impedance

(Fig.17) and tunable phase shifter (Fig.18). The simulated phase shift was of 25° at 5GHz and 48°

at 8GHz.

Fig.17 SEM photo of the tunable impedance Fig.18 SEM photo of the phase shifter

[42]. It was a series association of one tunable based on the simple π high –pass cell [42].

inductor and one basic metal-air-metal (MAM) It was composed of two parallel tunable

tunable capacitor. Either a capacitive behavior inductors and one metal-insulator-metal

or an inductive behavior was depended on (MIM) constant value capacitor.

the applied actuation voltages.

Sridhar et al. [44] developed new micromachined tunable inductors with folded flex-circuit

structures and apply it into a hydrogel-based wireless sensor (Fig.17) for biomedical applications.

The tunable inductor was formed of two parallel spiral coils with 5～10mm size, which were

connected and aligned to each other with air gap. The mutual inductance is positive when the

current flow in the two parallel conductors is in the same direction, while it is negative when the

current flow is in opposite direction. The response to the displacement of the coils of the tunable

inductor was 0.40nH/m. The authors though they should extend their efforts to adopt appropriate

response or other materials to form the simple fold-and-sandwich construction of the tunable

inductor.

7. Conclusions

A review of the tunable inductors, from the perspective of their tuning way (e.g. discrete tuned,

metal shielding tuned, magnetic core tuned and coil-coupled tuned) to their applications for

microwave communication systems (e.g. VCOs, filters, phase shifters) is presented. The intent of

this review paper is to provide perspectives to newcomers in the field, and empower potential

end-users with an overall device picture, current status, and a vision of their ultimate performance

capabilities. The difficulty for the device designers is to increase the tuning ratio of the tunable

inductors and reduce their inherently losses due to ohmic losses in the metal traces and due to

substrate resistance and eddy currents. The more efforts toward increasing the Q-factor at

microwave and millimeterwave frequencies for tunable inductors fabricated by standard silicon

technology should be devoted.

Acknowledgement

This work was supported by Fund of National Key Laboratory of Nano/Micro Fabrication

Technology and National Natural Science Foundation of China (NSFC, No. 60876080).

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