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3-D Integrated Inductors and Transformerson Liquid Crystal Polymer Substrate

Dae-Hee Weon, Linda P.B. Katehi, and Saeed Mohammadi

School of Electrical and Computer Engineering, Purdue University,West Lafayette, IN 47907-2035

Abstract - We have designed, fabricated, and testedinductors and transformers on LCP substrate whichexhibit high Q and high self-resonant frequencyusing a 3-D fabrication technology. A l.lnH inductorin this work achieves a quality factor Q of 76 at12GHz, with Q over 60 for frequencies from 8 to18GHz. A 1:1 ratio transformer with couplingfactors, k of higher than 0.6 and a self resonancefrequency of 23GHz is also fabricated and tested.The technology for these inductors and transformersis based on one step Cr/Au metal deposition andelectroplating.

Index Terms - High quality factor, high frequency,integrated inductor, LCP, stressed metal, threedimensional fabrication, transformer, flexibleelectronics.

I. INTRODUCTION

Deep submicron CMOS and SiGe BiCMOSprocesses have enhanced the performance of Si-based RFICs up to microwave frequencies. With Sitransistor high frequency performance approaching200GHz [1], inductors and transformers play avery important role in the performance achieved byRF circuits such as voltage controlled oscillators(VCOs), low-noise amplifiers (LNAs), filters,mixers, and power amplifiers (PAs) [1]-[4].Among inductor and transformer parameters thatlimit the performance of an RF integrated circuitsare quality factor (Q), self-resonance frequency(fsr) and distributed effects. Higher Q inductors andtransformers help minimizing RF power loss, RFnoise, phase noise and DC power consumption ofRFIC circuits.

Implementing RF circuits on integrated circuittechnology introduced integrated spiral inductorsthat are compatible with the IC technology [3][4].The drawback is that these inductors arecharacterized by relatively low quality factors [5].

This quality factor is determined by inductorgeometry, the type of interconnect metal (Al, Au orCu) [6], thickness of the metallization, verticaldistance between underpass/air-bridge to theinductor windings [7] and the dielectric loss of thesubstrate. Recent work has shown that removal ofthe substrate below the inductor structure canincrease the Q by two to three times [8]. Thissubstantial improvement in Q is due to thereduction of the substrate parasitic capacitance andparasitic loss due to the fringing electric fields inthe substrate.

There have been various reports on technologiesdeveloped for LCP-based passive components[9] [10], and micro-machining techniques for LCP[11] [12]. 2-D spiral inductors on LCP substratehave achieved high quality factor (Q - 80) but withlimited self-resonant frequency ( SRF - 11GHz) ormoderate quality factor (Q - 43) with high self-resonant frequency ( SRF - 29GHz).

In this work, we have designed, fabricated, andtested inductors and transformers on LCP substratewhich exhibit high quality factor and high self-resonant frequency using a three-dimensional (3-D) processing technologies[13][14]. LCP film usedfor this work is 50um thick Rogers R/flex'3000.Table 1 shows the properties ofLCP substrate usedin this work.

Table 1. Properties of Rogers R/flex83000 LCP.

Property R/flex 3000|

Dielectric Constant, 10GHz, 23 OC 2.9

Surface Resistivity 10o1 Mohm

Loss factor, tan6 - 0.004

Water Absorption (23 °C, 24hrs) 0.04 00

0-7803-9542-5/06/$20.00 ©2006 IEEE

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II. TECHNOLOGY

The inductors presented herein are based on arecently developed stressed metal technology [13]in which a Cr/Au metal layered-combination isdeposited to create a metal with a built-in stress.This built-in stress is used to create 3-D inductorsand transformers printed on LCP film. 3-Dinductors and transformers on LCP flexible filmare achieved by detaching the LCP from Sisubstrate after 3-D structure fabrication iscompleted. This technology allows us to fabricatehigh quality factor and high frequency inductorsand transformers on flexible substrates using a 3step process. These high performance componentswill find very interesting applications in low-costand high performance flexible RF electronics.

previously reported stressed metal technology [13](Fig. 1(c)). 4gm Au electroplating step is thenperformed to reduce metal loss and also to improvethe rigidity of the structure. Fig. 2(b) and (c) showthe fabricated one-turn and three-turn inductors atthis stage. Finally, 3-D inductors and transformerson flexible LCP substrate are achieved bydetaching the LCP film from Si substrate (Fig.1(d)). 3-D inductors and transformers on a flexibleLCP substrate are shown in Fig. 3.

Fig. 2. 3-D inductors and transformers patterns beforereleasing metal fingers (a). 3-D 1-turn (b) and 3-turn (c)inductor on LCP after Au electroplating.

3-D structure

3-D structure on LCP (d)

Fig. 1. Schematic cross-sectional views of thefabrication process flow for 3-D inductors andtransformers on LCP.

Fig. 1 shows the process sequence to fabricate the3D inductors and transformers on LCP film. First,polydimethylsiloxane (PDMS) is spin-coated on

the oxidized Si substrate and used as adhesivematerial between LCP film and Si substrate (Fig.1(a)). Then, metal layers of Cr and Au are

deposited and patterned on the top of sacrificiallayer (Fig. 1(b)). Fig. 2(a) shows patterns ofinductors and transformers printed on LCP for 3-Dinductors and transformers. 3-D structures are

achieved by releasing metal fingers using the

Fig. 3. High degree of flexibility of LCP with 3-Dinductors and transformers.

We have fabricated and characterized inductors onthe LCP substrate with one, two, and three turnsand metal line widths of 15um, 25 gim, 35 gim, and45 gim. We have also characterized 1:1 and 1:4turn transformers on LCP film.

III. RF CHARACTERIZATION

The S-parameters of the fabricated inductors weremeasured using 851 OXF network analyzercalibrated using SOLT up to 40 GHz. CascadeMicrotech coplanar GSG probes were used. Toperform accurate quality factor (Q) measurementswhen Q values are very high, a very thoroughcalibration and accurate de-embedding of padsparasitics are needed [15]. Herein, several

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inductors of the same geometry were measured toensure repeatability in the measurements. Thedeviation of the measurement values was within5%. Fig. 4 shows the measured quality factor, Q,and inductance value, L, extracted from themeasured S-parameters for one-turn and two-turninductors with a metal line width of 45umfabricated on the LCP using the presentedtechnology. As can be seen from this figure, forone-turn inductor, a peak Q of 76 is achieved at 12GHz. The self-resonant frequency of the inductoris over 40 GHz. For the two-turn inductor, amaximum Q of 48 and self-resonant frequency of33 GHz are achieved. Inductance values are 1. nHand 2. lnH, respectively.

20

15Ia)

0

< 100

5

o

100

80

060 0

U-

40 mCY

20

00 10 20 30 40

Frequency [GHz]

Fig. 4. Measured quality factor and inductance value of1-turn and 2-turn inductors with a metal line width of45um.

5

4

I

=2

10

r_

80

60o0

a20

0

0 10 20 30 40Frequency [GHzJ

Fig. 5. Measured quality factor and inductance value of1-turn inductors with different metal line width.

Fig. 5 shows the quality factor, Q, and inductancevalue, L, extracted from the measured S-parameters for one-turn inductors fabricated onLCP with different metal line width. As shown inthe figure, the inductor with 35tm width has ahigher quality factor and self-resonant frequencythan the inductor with a metal line width of 15gm.Inductance values of inductors are 1.2nH and1.4nH respectively. Table 2 summarizes theperformance of several inductors fabricated on theLCP using this technology. The data is extractedfrom the S-parameter measurement.

Table 2. Extracted parameters of 3-D inductorscalculated from S-parameters measurement. Inductors inthis table have 350gm diameter and 4.0gm electroplatedAu thickness.

# of Width L Quality SRFTurns [gm] [nH] Factor [GHz]

1 15 1.4 47@1OGHz >401 25 1.3 51@1OGHz >401 35 1.2 63@12GHz >40

|1 1 45 | 1.1 76w12GHz | >402 45 2.1 48 _8GHz 133

| 3 45 |3.6 |44g7GHz | 22

The following conclusions have been derived fromthe measured data:

* The inductance value increases with thenumber of turns. This is due to the high mutualinductance in the two-turn and three-turninductors.

* The self-resonant frequency decreases withincreasing the number of turns because of asignificant increase in the value of theinductance and increased distributed turn toturn capacitance.

* Increasing the width of the conductor increasesthe quality factor significantly since the qualityfactor Q at lower frequency is dominated bythe series ohmic resistance. The self-resonantfrequency seems to be a weak function of themetal line width.

Next, two transformers with different turn ratioswere measured. Fig. 6 shows typical curves ofcoupling coefficient, K, extracted from S-parameters for one to one turn and one to four

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turns transformers fabricated on the LCP. Asshown in this figure, coupling coefficient, K isabove 0.6 for one to one turns transformer andincreases as the frequency approaches the selfresonant frequency of 23GHz. For one to fourturns transformer, a lower coupling coefficient, Kis achieved which is attributed to the layout design(gap between the turns). Self resonant frequency isalso lower than that of one to one turn transformer.

The overall coupling coefficient of the transformerbecomes one at self-resonance frequency. This isdue to the fact that there is a parasitic capacitancecoupling in parallel with the mutual inductance. Atresonance frequency, this parasitic couplingresonates out the mutual inductance, resulting in adirect power coupling from primary to secondarywinding.

1

0.8

,0.6a,_

0

m 0.44-

0

0

O 0.2

0 5 10 15 20 25 30

Frequency [GHzJ

Fig. 6. Measured coupling coefficient (K) data for a 1:1and 1:4 turn 3-D transformer on LCP.

IV. CONCLUSION

Various three-dimensional inductors andtransformers were fabricated on a LCP (LiquidCrystal Polymer), Rogers R/flex63000 substrate.These inductors exhibited high self resonantfrequencies and very high quality factors comparedto their printed spiral counterparts. High-efficiencytransformers with coupling factors, k more than 0.6are achieved. The availability of high-Q and highself-resonant inductors and transformers on LCPwill open up new applications for RF andmicrowave integrated circuits on flexiblesubstrates.

ACKNOWLEDGEMENT

This work is supported by DARPA Technology forEfficient and Agile Microsystems (TEAM)DAAB07-02- 1-L430.

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