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1384 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 3, NO. 8, AUGUST 2013

Design, Fabrication, and Characterization of aWideband 60 GHz Bandpass Filter Based on a

Flexible PerMX Polymer Substrate

Seonho Seok and Janggil Kim

AbstractThis paper presents a wideband 60 GHz bandpassfilter fabricated on a flexible PerMX polymer substrate. Aconventional parallel-coupled half-wavelength resonator filter isselected as an embedded passive device. A narrow gap of 5 mbetween 750-m-long resonators is successfully fabricated thanksto a Si support substrate. Surface modification is used to releasethe flexible polymer substrate from the Si substrate after thefilter fabrication. A wideband filter is achieved through theoptimization of the narrow gaps between the adjacent resonators.The designed filters are implemented in two different types,

without a cover and with a cover. The filter without a covershows an insertion loss of 4 dB at the center frequency of63.5 GHz and a return loss of better than 10 dB includingtwo CPW pads, while the filter with a cover has an insertionloss of 3.8 dB at 59 GHz and a return loss of better than13 dB. In addition, the uncovered filter has a 3-dB bandwidthof 24% at 63.5 GHz, while the covered filter shows 28% at59 GHz.

Index Terms Bandpass filter, flexible, PerMX, polymer,wideband.

I. INTRODUCTION

A

S THE need for distributed sensors for many applications

such as environment monitoring increases, multifunc-

tional and multisensing systems with communication capabil-ity are strongly demanded. These kinds of sensor systems have

to be packaged to protect themselves [1] and to be assem-bled on a common platform, so-called system-in-package

(SIP), with other functional chips such as RF transceiver

and signal processing circuits [2]. In general, multilayer low-

temperature cofired ceramic (LTCC)-based SIP technology

integrating monolithic microwave integrated circuits (MMICs)

and passive devices is a notable solution for millimeter-

wave radio system integration due to its low loss, integration

capability, similar coefficient of temperature expansion (CTE)

value to MMICs, and cost effectiveness [3]. Recently, liquid

crystal polymer (LCP), as a new and advanced candidate

for RF substrate material, has attracted much attention overthe past years due to the unique combination of features

Manuscript received July 26, 2012; revised November 12, 2012; acceptedDecember 27, 2012. Date of publication February 7, 2013; date of currentversion July 31, 2013. Recommended for publication by Associate EditorD. G. Kam upon evaluation of reviewers comments.

The authors are with Institute dElectronique de Microelectronique et deNanotechnologie, Centre National de la Recherche Scientifique, VilleneuvedAscq 59652, France (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TCPMT.2013.2240040

and performance [4]. However, there are still needs for new

materials and the associated technology development due

to the high process temperature for laminating each layers,

the high stress development of the multilayered ones, chip

embedding for a compact SIP realization, etc. As an alternativeway to overcome these disadvantages, other polymer materials

are proposed as an RF substrate [5] or a packaging cap [6] dueto their excellent electrical properties and manufacturability.

Their multilayer lamination capability is useful to build an RF

SIP platform. The advantages of the proposed material are thelow temperature process (

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S EOK AND KIM : DES IGN, FABRICAT ION, AND C HARACT ER IZ AT ION OF A WIDE BAND 6 0 GHz BANDPASS FILT ER BASE D 1 385

(a)

0 10 20 30 40 50 60 70

-60

-50

-40

-30

-20

-10

0

-5

-4

-3

-2

-1

0

S21

(dB)

S11

(dB)

Frequency (GHz)

L = 1 mm

L = 2 mm

L = 3 mm

(b)

Fig. 1. RF characteristic of the PerMX polymer. (a) Fabricated microstrip lineon a 50-m PerMX polymer. (b) Measured S-parameters of the micropstriplines.

and three metal layers that are interconnected by two vias

(via1 and via2 as indicated in the figure). The thickness of the

PerMX is 50 m for the base layer and the substrate layer

and 14 m for the cover layer. The commercially availablePerMX films from Dupont Company are PerMX 3014, PerMX

3020, and PerMX 3050 that have a thickness of 14, 20, and50 m, respectively [8]. The filters have been implemented in

a two-layered PerMX substrate or a three-layered PerMX one

including the cover. The covered PerMX can be considered as

a polymer embedded filter chip.

B. Filter Design

A parallel-coupled, half-wavelength resonator filter shownin Fig. 3 was first designed following the standard design

procedure in [9]. The three-pole, 15% bandwidth, and 0.1 dB

ripple at midband f0 = 60 GHz were used to find g values

for a low-pass prototype. Even- and odd-mode characteristicimpedances of the coupled microstrip line resonators are found

and then the widths and the gaps of the coupled microstriplines that exhibit the desired even- and odd-mode impedances

are determined. Concerning the gap of the coupled resonator,it has a constraint of 7.5 m due to technological issues. The

length of the microstrip line is 750 m corresponding to the

quarter wavelength at the frequency of interest. Note that thedielectric constant of the PerMX material is 3 and the loss

tangent is 0.03.

Given the analytical dimensions, the HFSS model is set

up to find the optimized dimensions of the filter having low

Probe access

Cover layer

: PerMX, t=14 m

Filter

Substrate layer

: PerMX, t=50 m

GND

Base layer

: PerMX, t=50 m

Via2

Via1

Fig. 2. Concept of the filter based on PerMX polymers.

Z0

Z0

W1,S1 W2,S2 W3,S3

W4,S4

GND

Fig. 3. Parallel-coupled half-wave length resonator filter.

insertion loss and wide bandwidth. The optimized dimensions

are S1 = S4 = 7.5 m, S2 = S3 = 20 m, W1 = W4 =70 m, and W2 = W3 = 80 m. Through the optimization,

it is found that S1 and S2 are the critical parameters for lowinsertion loss and wide bandwidth of the filter, respectively.

The simulated results on these critical parameters are shownin Fig. 4. The insertion loss at the center frequency of 63

GHz varies from 3.9 dB for S1 of 5 m to 4.5 dB for

10 m when S2 is assumed 20 m and 3-dB bandwidthis from 30% for 15 m S2 to 22% for 25 m S2 at the

assumption of S1 = 7.5 m. In addition, the cover layer effect

has been investigated as a function of cover height as shown in

Fig. 5. The cover thickness is determined as the smallest one

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0 20 40 60 80 100

-100

-80

-60

-40

-20

0

-24

-20

-16

-12

-8

-4

0

S11

(dB)

S21

(dB)

Frequency (GHz)

S1=5 m, S

2=20 m

S1=7.5 m, S

2=20 m

S1=10 m, S

2=20 m

(a)

0 20 40 60 80 100

-100

-80

-60

-40

-20

0

-20

-16

-12

-8

-4

0

S11

(dB)

S21

(dB)

Frequency (GHz)

S1=7.5 m, S

2=15 m

S1=7.5 m, S

2=20 m

S1=7.5 m, S

2=25 m

(b)

Fig. 4. HFSS simulation results of the filter without the cover. (a) Filtercharacteristics as a function of S1. (b) Filter characteristics as a functionof S2.

0 20 40 60 80 100

-100

-80

-60

-40

-20

0

-24

-20

-16

-12

-8

-4

0

S11

(dB)

S21

(dB)

Frequency (GHz)

Cover height=14m

Cover height=28m

Cover height=50m

Fig. 5. HFSS simulation results of the filter with the cover.

among the commercially available films. The center frequency

of the filter decreases as the effective dielectric constant is

proportional to the cover thickness.

IV. FABRICATIONS

The designed filter is fabricated using PerMX 3050 polymerfor substrate and base and PerMX 3014 for cover. Each PerMX

layer is named in Fig. 6(f). Gold metallization has been carried

out to have 2-m-thick metal lines for the filter and ground

plane. Fig. 6 shows the process flow of the filter. (a) PerMX

Si substrate

PerMX

OmniCoat

Si substrate

PerMXGND

PerMX

Si substrate

PerMX

PerMX

PerMX

Si substrate

PerMX

Filter

PerMX

PerMX

Si substrate

PerMX

Pad Via

PerMX (Cover)

PerMX (Substrate)

PerMX (Base)

(a) (b)

(c) (d)

(e) (f)

Fig. 6. Filter fabrication process flow. (a) PerMX lamination. (b) Goldelectroplating for ground. (c) PerMX lamination. (d) Filter plating and PerMXpatterning. (e) Via and pad plating. (f) Separation of PerMX filter chip.

TABLE I

PerMX PROCESS CONDITIONS

Step Conditions

Lamination Hot roll @ 65 C

Soft bake 4 min @ 95 C

Expose 400 mJ

PEB 10 min @ 60 C

Develop PGMEA, 5 min

Hard bake 30 min @ 150 C

film (t = 50 m) is laminated on the Si substrate coated

with OmniCoat. It is used to modify the Si surface conditionfor easier release of PerMX substrate after the fabrication.(b) Gold electroplating is performed for the ground plane.

(c) PerMX film (t = 50 m) is laminated on the top of

the ground plane. (d) Gold electroplating is carried out for

the filter and the PerMX film (t = 14 m) is laminated. It

is patterned to make a via between the filter and the pad

access. (e) Gold electroplating is performed for the via andthe pad access. (f) The PerMX substrate is separated from the

Si substrate by NH4F immersion.The PerMX lamination process and its conditions are given

in Table I [10].

To investigate the filter characteristic itself, it is first

fabricated without the cover PerMX. The fabrication resultis shown in Fig. 7. The size of the implemented filter is

5.4 mm (L) 4.2 mm (W) including the probe pad and theground plane. The thickness of the fabricated PerMX substrate

is 47 m.The actual dimensions of the fabricated filter are measured

using a microscope: 5.4 m for S1, 12.3 m for S2, 72 m

for W1, and 82 m for W2. It can be said that the fabricationerrors on the metallization would be the main reason of the

disparity between the simulation and the measurements.

As shown in the aforementioned figures, there is a warpage

at the PerMX filter chips. It is caused by the residual stress

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A

(a) (b)

Fig. 7. Fabricated filter without the cover PerMX. (a) Frontside. (b) Backside.

0 1000 2000 3000 4000 5000

0

50

100

150

200

250

Height(m)

Scan length (m)

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

Fig. 8. Measured deflection of PerMX filter chips.

effect of the associated materials. Gold metallization on the

thin PerMX polymer develops a tensile stress making sub-

strate warpage. The deflection from five samples is measured

through A as indicated in Fig. 7(b). Fig. 8 shows the measured

deflection of the fabricated PerMX chips and the averagemaximum deflection is 215 m.

The filter with the cover is implemented in wafer type,not in separated chips type as the previous one. It can be

considered as a flexible substrate embedding a filter element.

The fabrication result is shown in Fig. 9. The flexible PerMX

substrate is successfully released as shown in Fig. 9(a) and it

is bended as shown in Fig. 9(b).

V. CHARACTERIZATIONS AND DISCUSSION

The manufactured filters are characterized by the HP8510C

vector network analyzer and the ground-signal-ground(G-S-G) probe system. The filter without the cover having

7.5 m S1 and 20 m S2 is first characterized. The measuredS-parameter is compared with the ADS and HFSS simulation

results as shown in Fig. 10. It has an insertion loss of 4 dB atthe center frequency of 63.5 GHz while its return loss is better

than 10 dB including the CPW pads. It has a 3-dB bandwidth

of 24% at the center frequency. The measurement has goodagreement with the simulation results.

The filter with the cover is then measured and compared

with the measurement of the uncovered filter as shown in

Fig. 11. The center frequency of the covered filter is shifted

(a)

(b)

Fig. 9. Fabricated flexible PerMX substrate embedding filters. (a) PerMXsubstrate after the separation of Si support wafer. (b) Bended PerMX flexiblesubstrate.

0 20 40 60 80 100

-120

-100

-80

-60

-40

-20

0

-20

-16

-12

-8

-4

0

S11

(dB)

S21

(dB)

Frequency (GHz)

ADS simulation

HFSS simulation

Measurement

Fig. 10. Characteristic of the filter without the cover.

to 59 GHz from the center frequency of 63.5 GHz ofthe uncovered filter while the insertion loss decreases from

4 to 3.8 dB after covering the filter. The 3-dB bandwidth is

also increased from 24% in the uncovered filter to 28% in thecovered filter.

The filter is also measured at flexible conditions as shown

in Fig. 12. Three different radii of curvatures of 71.5, 25, and

12.5 mm have been used to find the effect of the substrate

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0 20 40 60 80 100

-100

-80

-60

-40

-20

0

-20

-15

-10

-5

0

S11

(dB)

S21

(dB)

Frequency (GHz)

Without cover

With cover

HFSS simulation

Fig. 11. Characteristic of the filter without the cover comparing with HFSSsimulation and the filter with the cover.

Curved

chuck

Flexiblesubstrate

Fig. 12. Flexiblesubstrate on a curved chuck (radius of curvature=

71.5 mm).

0 10 20 30 40 50 60 70 80

-125

-100

-75

-50

-25

0

-20

-16

-12

-8

-4

0

S11

(dB)

radius of curvature = 71.5 mm

radius of curvature = 25 mm

radius of curvature = 12.5 mm

S21

(dB)

Frequency (GHz)

Fig. 13. Measurement results of the flexible substrate as a function of radiusof curvature.

bending on the filter performance. Fig. 13 shows the measure-

ment results and it does not show much difference at the filter

characteristic because the gap change between each resonators

of the filter due to the bending is not significant.

VI . CONCLUSION

The SIP approach with communication capability emerges

as a notable solution to accomplish multifunctional sensorsystems. LTCC-based or LCP-based SIP technologies are

regarded as one of the promising solutions for millimeter-wave

radio system integration owing to the RF friendly material

characteristics, integration capability, similar CTE value to

MMICs, and cost effectiveness although it has disadvantageof high dielectric constant and relatively high process temper-ature.

Unlike conventional approaches, polymers such as BCB,SU8, and PerMX can be strong candidates for the pur-

pose. In particular, PerMX has low residual stress and low

temperature process (

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[10] J. Kim, S. Seok, N. Rolland, and P. A. Rolland, Low-temperature,low-loss zero level packaging technique for RF applications using aphotopatternable dry film, J. Micromech. Microeng., vol. 22, no. 6,p. 065032, Jun. 2012.

Seonho Seok received the M.S. and Ph.D. degrees inelectrical engineering from Seoul National Univer-

sity, Seoul, Korea, in 1999 and 2004, respectively.He was a Post-Doctoral Researcher with the

Center for Advanced Transceiver Systems, SeoulNational University. In 2005, he joined the Insti-tute dElectronique de Microelectronique et de Nan-otechnologie, Villeneuve dAscq, France, as a Post-Doctoral Research Scholar, where he has been aCNRS Senior Researcher since 2007. His currentresearch interests include wafer bonding techniques,

wafer-level packaging of microelectromechanical system devices, and system-in-package.

Janggil Kim was born in 1977 in Korea. Hereceived the degree in mechanical engineering fromSeoul National University, Seoul, Korea, and thePh.D. degree, with research on development ofsoft-lithographic technology for micropatterning onnonplanar surfaces, from the University of Tokyo,Tokyo, Japan.

He was a Post-Doctoral Researcher with the Insti-tute of Industrial Science, University of Tokyo. Since2009, he has been a Post-Doctoral Researcher with

the CSAM Group, IRCICA/Institute dElectroniquede Microelectronique et de Nanotechnologie, Villeneuve dAscq, France.His current research interests include development of zero-level packagingtechnology for radio frequency-microelectromechanical system applicationsand realization of system-in-package.

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