06457443.pdf

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 SubstrateSeonho Seok and Janggil Kim

Abstract— This 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 µmbetween 750-µm-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

AS THE need for distributed sensors for many applicationssuch as environment monitoring increases, multifunc-

tional and multisensing systems with communication capabil-ity are strongly demanded. These kinds of sensor systems haveto 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 transceiverand signal processing circuits [2]. In general, multilayer low-temperature cofired ceramic (LTCC)-based SIP technologyintegrating monolithic microwave integrated circuits (MMICs)and passive devices is a notable solution for millimeter-wave radio system integration due to its low loss, integrationcapability, similar coefficient of temperature expansion (CTE)value to MMICs, and cost effectiveness [3]. Recently, liquidcrystal polymer (LCP), as a new and advanced candidatefor 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 d’Electronique de Microelectronique et deNanotechnologie, Centre National de la Recherche Scientifique, Villeneuved’Ascq 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 newmaterials and the associated technology development dueto the high process temperature for laminating each layers,the high stress development of the multilayered ones, chipembedding for a compact SIP realization, etc. As an alternativeway to overcome these disadvantages, other polymer materialsare 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 RFSIP platform. The advantages of the proposed material are thelow temperature process (<150 °C), mechanical flexibility, lowcost, etc. In addition, its low dielectric constant provides someadvantages to an antenna implementation for SIP applications.Also, embedded passive components based on thin film tech-nology have been reported to benefit fine pitch interconnectand cost-effective solution [7].

In this paper, a wideband millimeter-wave filter fabricatedon a flexible PerMX polymer is presented. In Section II, thecharacterization of the PerMX polymer using a microstripline will be first assessed and the concept and design of theproposed filter will be described in Section III. Section IVdeals with the fabrication process of the filter based onPerMX lamination and gold electroplating. In Section V, thecharacterization result of the fabricated filter will be presented.

II. PerMX POLYMER

PerMX film-type polymer is very attractive for a millimeter-wave application because it has good electrical properties [8].Also, it can be processed on substrates such as Si and glassby lamination that can provide its multilayer structures forSIP-based millimeter-wave applications. First of all, differentlength microstrip lines are designed, fabricated, and measuredto find its suitability for millimerter-wave applications. Thewidth of the microstrip line is designed to be 85 µm on a50-µm-thick PerMX substrate with dielectric constant of 3.The lengths of each line are 1, 2, and 3 mm. Fig. 1 shows thefabricated microstrip lines and its S-parameter measurementresult. The microstrip line has an insertion loss of 0.5 dB/mmat 60 GHz while a return loss is better than 17 dB up to70 GHz.

III. CONCEPT AND DESIGN OF THE FILTER

A. Filter Concept

The concept of the proposed filter based on PerMX poly-mers is shown in Fig. 2. It consists of three PerMX layers

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SEOK AND KIM: DESIGN, FABRICATION, AND CHARACTERIZATION OF A WIDEBAND 60 GHz BANDPASS FILTER BASED 1385

(a)

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Fig. 1. RF characteristic of the PerMX polymer. (a) Fabricated microstrip lineon a 50-µm 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 thePerMX is 50 µm for the base layer and the substrate layerand 14 µm for the cover layer. The commercially availablePerMX films from Dupont Company are PerMX 3014, PerMX3020, and PerMX 3050 that have a thickness of 14, 20, and50 µm, respectively [8]. The filters have been implemented ina two-layered PerMX substrate or a three-layered PerMX oneincluding the cover. The covered PerMX can be considered asa polymer embedded filter chip.

B. Filter Design

A parallel-coupled, half-wavelength resonator filter shownin Fig. 3 was first designed following the standard designprocedure in [9]. The three-pole, 15% bandwidth, and 0.1 dBripple at midband f0 = 60 GHz were used to find g valuesfor a low-pass prototype. Even- and odd-mode characteristicimpedances of the coupled microstrip line resonators are foundand then the widths and the gaps of the coupled microstriplines that exhibit the desired even- and odd-mode impedancesare determined. Concerning the gap of the coupled resonator,it has a constraint of 7.5 µm due to technological issues. Thelength of the microstrip line is 750 µm corresponding to thequarter wavelength at the frequency of interest. Note that thedielectric constant of the PerMX material is 3 and the losstangent is 0.03.

Given the analytical dimensions, the HFSS model is setup to find the optimized dimensions of the filter having low

Probe access

Cover layer: PerMX, t=14 µm

Filter

Substrate layer: PerMX, t=50 µm

GNDBase layer: PerMX, t=50 µm

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 dimensionsare S1 = S4 = 7.5 µm, S2 = S3 = 20 µm, W1 = W4 =70 µm, and W2 = W3 = 80 µm. 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 63GHz varies from 3.9 dB for S1 of 5 µm to 4.5 dB for10 µm when S2 is assumed 20 µm and 3-dB bandwidthis from 30% for 15 µm S2 to 22% for 25 µm S2 at theassumption of S1 = 7.5 µm. In addition, the cover layer effecthas been investigated as a function of cover height as shown inFig. 5. The cover thickness is determined as the smallest one


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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.

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

among the commercially available films. The center frequencyof the filter decreases as the effective dielectric constant isproportional to the cover thickness.

IV. FABRICATIONS

The designed filter is fabricated using PerMX 3050 polymerfor substrate and base and PerMX 3014 for cover. Each PerMXlayer is named in Fig. 6(f). Gold metallization has been carriedout to have 2-µm-thick metal lines for the filter and groundplane. Fig. 6 shows the process flow of the filter. (a) PerMX

Si substrate

PerMXOmniCoat

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 µm) is laminated on the Si substrate coatedwith 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 µm) is laminated on the top ofthe ground plane. (d) Gold electroplating is carried out forthe filter and the PerMX film (t = 14 µm) is laminated. Itis patterned to make a via between the filter and the padaccess. (e) Gold electroplating is performed for the via andthe pad access. (f) The PerMX substrate is separated from theSi substrate by NH4F immersion.

The PerMX lamination process and its conditions are givenin Table I [10].

To investigate the filter characteristic itself, it is firstfabricated without the cover PerMX. The fabrication resultis shown in Fig. 7. The size of the implemented filter is5.4 mm (L) × 4.2 mm (W) including the probe pad and theground plane. The thickness of the fabricated PerMX substrateis 47 µm.

The actual dimensions of the fabricated filter are measuredusing a microscope: 5.4 µm for S1, 12.3 µm for S2, 72 µmfor W1, and 82 µm for W2. It can be said that the fabricationerrors on the metallization would be the main reason of thedisparity between the simulation and the measurements.

As shown in the aforementioned figures, there is a warpageat the PerMX filter chips. It is caused by the residual stress


A

(a) (b)

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

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ght (

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Fig. 8. Measured deflection of PerMX filter chips.

effect of the associated materials. Gold metallization on thethin PerMX polymer develops a tensile stress making sub-strate warpage. The deflection from five samples is measuredthrough A as indicated in Fig. 7(b). Fig. 8 shows the measureddeflection of the fabricated PerMX chips and the averagemaximum deflection is 215 µm.

The filter with the cover is implemented in wafer type,not in separated chips type as the previous one. It can beconsidered as a flexible substrate embedding a filter element.The fabrication result is shown in Fig. 9. The flexible PerMXsubstrate is successfully released as shown in Fig. 9(a) and itis bended as shown in Fig. 9(b).

V. CHARACTERIZATIONS AND DISCUSSION

The manufactured filters are characterized by the HP8510Cvector network analyzer and the ground-signal-ground(G-S-G) probe system. The filter without the cover having7.5 µm S1 and 20 µm S2 is first characterized. The measuredS-parameter is compared with the ADS and HFSS simulationresults 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 betterthan 10 dB including the CPW pads. It has a 3-dB bandwidthof 24% at the center frequency. The measurement has goodagreement with the simulation results.

The filter with the cover is then measured and comparedwith the measurement of the uncovered filter as shown inFig. 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.

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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 from4 to 3.8 dB after covering the filter. The 3-dB bandwidth isalso increased from 24% in the uncovered filter to 28% in thecovered filter.

The filter is also measured at flexible conditions as shownin Fig. 12. Three different radii of curvatures of 71.5, 25, and12.5 mm have been used to find the effect of the substrate


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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).

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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 filtercharacteristic because the gap change between each resonatorsof the filter due to the bending is not significant.

VI. CONCLUSION

The SIP approach with communication capability emergesas a notable solution to accomplish multifunctional sensorsystems. LTCC-based or LCP-based SIP technologies areregarded as one of the promising solutions for millimeter-waveradio system integration owing to the RF friendly materialcharacteristics, integration capability, similar CTE value toMMICs, 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 lowtemperature process (<150 °C). In RF aspect, microstriplines on PerMX showed promising performance in terms ofinsertion loss and return loss. Furthermore, a passive filterwas implemented on the PerMX polymer in a separate chipand in a flexible substrate. In particular, the flexible substrateis interesting due to its manufacturability and the capabilityembedding passive components for an RF SIP application. Theimplemented filter demonstrated competitive performance interms of insertion loss, return loss, and 3-dB bandwidth at60 GHz frequency and also demonstrated uniform charac-teristics at different curvatures. In conclusion, the proposedpolymer substrate can be a good solution for a flexibleminiaturized SIP employing embedded passives and embedded(or flip-chipped) thinned functional chips.

ACKNOWLEDGMENT

The authors would like to acknowledge the technical staffwith the Institute d’Electronique de Microelectronique et deNanotechnologie (IEMN), Villeneuve d’Ascq, France. Theywould also like to thank the CSAM Group, IEMN.

REFERENCES

[1] B. Lee, S. Seok, and K. Chun, “A study on wafer-level vacuumpackaging for MEMS devices,” J. Micromech. Microeng., vol. 13, no. 5,pp. 663–669, Sep. 2003.

[2] J. Miettinen, M. Mantysalo, K. Kaija, and E. O. Ristolainen, “Systemdesign issues for 3D system-in-package (SiP),” in Proc. IEEE Electron.Compon. Technol. Conf., Jun. 2004, pp. 610–615.

[3] Y. C. Lee and C. S. Park, “A fully embedded 60 GHz novel BPFfor LTCC system-in-package applications,” IEEE Trans. Adv. Packag.,vol. 29, no. 4, pp. 804–809, Nov. 2006.

[4] R. Bairavasubramanian, S. Pinel, J. Laskar, and J. Papapolymerou,“Compact 60-GHz bandpass filters and duplexers on liquid crystalpolymer technology,” IEEE Microw. Wireless Compon. Lett., vol. 16,no. 5, pp. 237–239, May 2006.

[5] M. F. Davis, S.-W. Yoon, S. Mandal, N. Bushyager, M. Maeng, K. Lim,S. Pinel, A. Sutono, J. Laskar, M. Tentzeris, T. Nonaka, V. Sundaram,F. Liu, and R. Tummala, “RF-microwave multi-band design solutionsfor multilayer organic system on package integrated passives,” in IEEEMTT-S Int. Microw. Symp. Dig., Jun. 2002, pp. 2217–2220.

[6] S. Seok, N. Rolland, and P.-A. Rolland, “Packaging methodology forRF devices using a BCB membrane transfer technique,” J. Micromech.Microeng., vol. 16, no. 11, pp. 2384–2388, Nov. 2006.

[7] L. Wang, W. Christiaens, S. Brebels, W. De Raedt, and J. Vanfleteren,“A novel approach to embed off-chip RF passives in PCB based onthin film technology,” in Proc. Electron. Syst.-Integr. Technol. Conf.,Sep. 2010, pp. 1–4.

[8] DuPont PerMX Series. (2010) [Online]. Avialable: http://www.microresist.de/produkte/dupont/pdf/permxseries.pdf

[9] J.-S. Hong and M. J. Lancaster, Microstrip Filters for RF/MicrowaveApplications. New York, USA: Wiley, 2001.


[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 theCenter for Advanced Transceiver Systems, SeoulNational University. In 2005, he joined the Insti-tute d’Electronique de Microelectronique et de Nan-otechnologie, Villeneuve d’Ascq, 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 withthe CSAM Group, IRCICA/Institute d’Electronique

de Microelectronique et de Nanotechnologie, Villeneuve d’Ascq, 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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