design and characterization of cpw feedthroughs in ... and characterization of cpw feedthroughs in...

Design and Characterization of CPW Feedthroughs in Multilayer Thin Film MCM-D

The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number 4, Fourth Quarter, 2000 (ISSN 1063-1674)

© International Microelectronics And Packaging Society 353

Design and Characterization of CPWFeedthroughs in Multilayer Thin Film MCM-DG. Carchon*, W. De Raedt+, B. Nauwelaers*, and E. Beyne+

*K.U.Leuven, dep. ESAT-TELEMICKard. Mercierlaan 94, B-3001Heverlee, BelgiumPhone: 32-16-281259Fax: 32-16-281501e-mail: [email protected]+IMEC, div. MCP/HDIPKapeldreef 75, B-3001Heverlee, Belgium

Abstract

In this publication, the researchers report on the design and characterization of CPW feedthroughs for RF and microwave applica-tions in multilayer thin film MCM-D. The feedthrough is based on an inverted multilayer microstrip line and two CPW-to-microstriptransitions. The bottom of the vertical metal wall is used as the ground-plane of the intrinsic feedthrough. Using 3-D simulations, itis shown that, for design and analysis purposes, the vertical metal wall can be replaced by a thin metal layer with only a very smallimpact on the performance. This equivalent structure can be more easily fabricated and measured. This allows for a faster designand characterization of the feedthrough. The transmission line properties (characteristic impedance and propagation constant) of theintrinsic feedthrough are extracted based on the measurement of two equivalent structures with different length. Two types of feedthroughshave been designed and realized. One design uses “all-pass” 50 ! lines and can be used up to at least 50 GHz. The other design isrealized on a different metal-layer and is based on a low-pass structure. It has a superior performance (insertion loss) up to 25 GHz.Measurements indicate that a low-loss (<0.5 dB), well matched feedthrough (return loss below -25 dB), can be realized up to at least50 GHz (intrinsic feedthrough length of 500 µm).

Key words:

MCM-D, Integrated Passives, Feedthrough, Shielding, and Co-planar Waveguide.

1. Introduction

The multilayer thin film multichip module technology (MCM-D) offers a very high reproducibility of very small dimensionsand is therefore promising for the low-cost integration of RF and

microwave circuits. IMECs MCM-D technology (Figure 1) con-sists of alternating thin layers of photosensitive benzo-cyclobutene(BCB) dielectric (CycloteneTM from Dow) and low loss coppermetallizations deposited on a borosilicate glass carrier substrate("r = 6.2, tan # ! 9.10-4). BCB has very low dielectric losses (tan #! 5.10-4), a low dielectric constant ("r = 2.65), and a low moistureabsorption. Various types of high frequency integrated passives(spiral inductors, TaN-resistors, Ta2O5-capacitors, ... ) can be in-tegrated1,2. Integrating these passives directly on the low costMCM-substrate gives a size and cost reduction and increases thepackaging density.

A coplanar waveguide topology is generally used in this tech-nology to realize the transmission lines and integrated passives.Compared to a microstrip-based approach, this has the advan-tage that the ground plane is directly accessible at the front sideof the wafer which makes it highly compatible with Flip Chip

© International Microelectronics And Packaging Society


Intl. Journal of Microcircuits and Electronic Packaging

354

mounting. Coplanar waveguides also have a very low dispersionand the presence of the groundplane results in a decreased cou-pling between neighboring lines. Thin dielectric layers can beused which reduces the size and the series inductance of the vias.The main CPW-lines are located on the 3 µm Cu-layer as thisoffers the lowest loss. The standard 50 !-line has the dimen-sions: width=77 µm, and slot=20 µm.

2 m top metalµ Ti/Cu5 m dielectricµ BCB5 m dielectricµ BCB3 m metalµ Ti/Cu/Ti

1 m µ bottom contact metal AlTaN resistor

Ni/Au component layer

700 m substrateµ Glass

1 m µ top contact metal Al

Ta capacitor materialTa O capacitor layer

2 5

Figure 1. Layer build-up of IMECs’ MCM-D technology.

The technology is highly suited for the Sytem-on-a-Package(SoP) approach, schematically represented in Figure 2. The SoP-approach assumes that it is not possible to integrate completefront-ends on a single chip as there will always be some subblocksthat are difficult to realize (such as filters, high-Q spirals, amongother components). This approach has the advantage that themost optimal technology (performance/cost) can be used to cre-ate each subblock. The passives are as much as possible inte-grated in the low-cost MCM-substrate, hereby, reducing the sizeand cost of the active chips.

Substrate

CPW-line Feedthrough

Metal Hood

FlippedBonded

BCB dielectric

CHIP

CPW-line Feedthrough

CHIP

Integratedpassive e.g. aspiral inductor

Figure 2. System on a package (SoP) concept.

Such an approach requires that a complete passive design-library is available to allow for an easy first-time right co-designbetween the active and the passive elements. This has recentlybeen presented in References2,3. Additional passive buildingblocks, such as quadrature couplers, have been presented in Ref-erence4. Some modules have been demonstrated in References5,6.

When different circuits are integrated in close proximity onthe MCM substrate, it may be necessary to provide some shield-ing, for example to prevent radiative coupling between the re-ceiver and the transmitter. It may also be required to protect thecircuit from the external environment. This is accomplished byplacing a metal box on top of the circuit. To connect the inside tothe outside world, a specially designed, impedance controlled,low-loss feedthrough is required. Results on ceramic feedthroughshave already been presented in References7-9. This paper de-

scribes how this component can be accurately designed and mod-eled in multilayer thin film MCM-D.

2. Equivalent Structure

A detailed view of the feedthrough structure is given in Fig-ure 3, it consists of a CPW input and output line that passesunderneath a grounded vertical metal wall. The length of thefeedthrough varies according to the thickness of the walls of themetal box (typical value 500 µm). For design and characteriza-tion purposes, it is rather cumbersome to work with a full metalwall. Thus, an equivalent structure (Figure 4) will be used, wherethe grounded metal wall is replaced by a thin (about 5 µm)grounded metal layer.

Figure 3. Layout of the feedthrough with a vertical metalwall.

Figure 4. Layout of the equivalent feedthrough structure:the metal wall is replaced by a thin metal strip.




This thin metal layer is directly available in the multilayerMCM-D process (top metal layer in Figure 1). This structurecan therefore be more easily fabricated, measured, and analyzed.This allows for a faster design and characterization of the feed-through structure.

The equivalent structure can only be used as long as its be-havior corresponds to that of the actual feedthrough (metal wallcase). This assumption has been verified using a commercial 3-D simulator (HP-HFSS) which accurately accounts for alldiscontinuities present in the feedthrough and is capable of simu-lating the effect of a metal wall. The simulated S-parameters forthe metal-wall and the metal-strip case are depicted in Figure 5.In these simulations, 250 µm long 50 ! feeding lines on themiddle metal layer have been assumed (Figure 1). The intrinsic500 µm long feedthrough is realized on the bottom metal layerusing a 15 µm wide strip. Replacing the metal-wall by a thinmetal strip only slightly influences the simulated S-parameters.Therefore, one can conclude that the extra capacitance-to-ground,induced by the presence of the metal wall, is sufficiently small.The equivalent structure can therefore be used to characterizethe feedthrough structure.

0 5 10 15 20 25 30-50

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0 5 10 15 20 25 30-0.20

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Figure 5. Simulated (using HP HFSS) return loss andinsertion loss for a feedthrough on the bottom metal layer(width 15 µm, length 500 µm), together with 250 µm 50 !!!!!CPW feeding lines on the middle metal layer: (-o-) met$$$$$lstrip case, (-V-) metal wall case.

3. Feedthrough Design

For design and modeling purposes, the feedthrough can bedivided into the following subsections (Figure 6):

· a CPW input and output line (subsection 1) on the middle(Cu) metal-layer,

· a via transition (if required) from the input/output line to thefeedthrough-layer (subsection 2),

· a step-in-width or a taper section beween the CPW input/output line and the feedthrough (subsection 3),

· the discontinuity at the edge of the metal wall (between sub-section 3 and subsection 4),

· the intrinsic feedthrough (subsection 4) where the signalpasses underneath the grounded metal-wall.

1 2 3 4 3 2 1

Metal 1

Metal 2

Metal 3

Figure 6. Layout of the feedthrough (only one half isdrawn) subdivided into different sections.

Models for the CPW input/output line, the via transition andthe taper are available in the design library2. The discontinuityat the edge of the metal wall can be assumed to be very small(this discontinuity also occurs at every bridge in a CPW basedtechnology and has a negligible effect on their performance).One can therefore further focus on the design and modeling ofthe intrinsic feedthrough.

3.1. Design of the Intrinsic FeedthroughAt first sight, there are several parameters that can be sepa-

rately optimized for the intrinsic feedthrough:· the feedthrough can be realized on the bottom or the middle

metal layer. This determines the distance between the feedthrough-strip and the grounded metal-strip (respectively 10 µm and 5 µm),

· the width of the feedthrough-strip, and· the slot of the CPW feedthrough: distance between the strip

and the CPW ground plane.However, for a high yield design, the mimimum slot-dimen-

sion is taken to be at least 20 µm. This implies that the CPWground plane is located much farther away from the feedthrough-strip than the overlaying grounded metal-strip (only 5 to 10 µm).The influence of the CPW ground-plane can therefore be ne-glected and the intrinsic feedthrough will behave as a multilayermicrostrip line.

The performance of the feedthrough can be very well pre-dicted by only considering the transmission line sections (feed-ing line, via section, and intrinsic “microstrip” feedthrough) while




356

the discontinuities (step-in-width and the discontinuity at theCPW-to-microstrip transition) are neglected. This is depicted inFigure 7 where full 3-D simulations (metal wall case) are com-pared with the simulation where only the transmission line sec-tions are taken into account. A good correspondence betweenthe simulated return losses and the insertion losses are obtained.

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Figure 7. 3-D metal-wall simulation (-V-) versus transmissionline model (-o-) for the feedthrough outlined in section 2. Anexcellent correspondence can be observed.

The parameters of the multilayer microstrip line can be deter-mined using an in-house developped quasi-TEM program ableto calculate the transmission line parameters of a multi-conduc-tor transmission line in a multilayer substrate. The thickness ofthe metal layers is also accurately taken into account10.

The charactersitic impedances, of the intrinsic feedthroughas a function of the selected metal-layer and the stripwidth, aresummarized in Table 1. From this data, one can see that a 50 !line can only be realized on the bottom metal layer. On the middlemetal layer, it would require a very small stripwidth. This wouldbecome difficult to process with sufficient accuracy and the asso-ciated conductor losses would also become excessively high. This

gives another reason to take a sufficiently large separation be-tween the feedthrough-strip and the CPW ground plane: a smallslot would result in an even lower Zc of the intrinsic feedthrough.

Table 1. Characteristic impedance of the intrinsic feedthroughas a function of the selected metal-layer and the stripwidth.

STRIPWIDTH

BOTTOM METALLAYER

MIDDLE METALLAYER

5 µM ZC=74 Ω ZC=60 Ω

10 µM ZC=60 Ω ZC=45 Ω

15 µM ZC=51 Ω ZC=36 Ω

20 µM ZC=44 Ω ZC=30 Ω

Based on the above parameters, it was decided to design afeedthrough on the bottom and the middle metal layer. This alsoillustrates the two design-approaches that can be followed.

The feedthrough on the bottom metal layer is based on an“all-pass” structure as it uses 50 ! lines for the feeding lines andthe intrinsic feedthrough section. It should therefore have a goodreturn loss up to very high frequencies. This design has, how-ever, two major drawbacks: a narrow strip (15 µm) is used andthe feedthrough is realized on the bottom, 2 µm thick Aluminalayer. This will lead to higher losses for the low-frequency appli-cations.

The second feedthrough is realized on the middle metal layer.It uses a 20 µm wide line. To increase the operating frequency,two 290 µm long standard high-impedance CPW lines (width:16 µm; slot: 50.5 µm; Zc=106 !) are added before and after theintrinsic feedthrough section. These high impedance lines com-pensate for the low impedance of the intrinsic feedthrough. Thetotal structure behaves as a low-pass filter. This design has animproved insertion loss due to the wider strip, the 3 µm Cu-metaland the absence of via sections.

The measured results for both designs will be provided insection 4.

3.2. Non-IdealitiesPredicting the performance of an actual feedthrough is com-

plicated by BCB-planarization effects: the surface profile of theBCB is not perfecly flat as assumed by electromagnetic simula-tors. This can be seen in Figure 8, where the surface profile of astandard 16 µm wide CPW line on the middle metal level is depic-ted (the ground-to-ground spacing is 117 µm). This complicatesthe accurate prediction of the exact distance between the intrin-sic feedthrough (middle or bottom metal layer) and the ground-plane (located on the top metal-layer). This distance, however,has a direct impact on the characteristic impedance of the intrin-sic feedthrough, and hence, the obtained return loss.

STRIPWIDTH

5 µm

10 µm

15 µm

20 µm




Figure 8. Non-ideal planarization of the BCB: measuredsurface profile of a 16 µm wide transmission line on themiddle metal level. the ground-to-ground distance is 117 µm.Units are in micron.

The effect of the BCB-planarization will be less pronouncedwhen the feedthrough is realized on the bottom metal-layer as athicker BCB-layer is being used.

4. Measurement and Characterization

4.1. Characterization of the IntrinsicFeedthrough

An illustration of a feedthrough realized on the bottom metallayer (stripwidth: 15 µm, length: 240 µm), is given in Figure 9.One clearly observes the transition from the main (middle) CPWlayer to the bottom metal layer and the tapered line on the bottomlayer.

Figure 9. Picture of a feedthrough (intrinsic length of 230µm) realized on the bottom metal layer. One clearly observesthe transition from the middle metal layer to the botton metallayer and a taper on the bottom layer.

To evaluate the performance of the quasi-TEM program, usedto predict the transmission line parameters of the intrincicfeedthrough, the researchers have fabricated and measured sev-eral feedthrough-pairs realized on the bottom and middle metallayer. One feedthrough has an intrinsic length of 240 µm, theother has an intrinsic length of 2070 µm.

Based on these measurements, it is possible to extract the trans-mission line properties of the intrinsic feedthrough using Refer-ence11. This method directly extracts the characteristic imped-ance and propagation constant while the influence of thediscontinuities at the edges of the feedthrough (e.g. an extra ca-pacitance-to-ground where the line goes underneath the overlay-ing ground-plane) are removed. The results for a feedthroughrealized on the middle metal layer (width of 20 µm) are given inFigure 10. The extracted characteristic impedance is about 25!, the effective dielectric constant is about 3 and the intrinsicfeedthrough shows an intrinsic loss of about -0.6 dB/mm @ 50 GHz. The difference between the measured (Zc=25!)and the simulated results (Zc=29 !) can be attributed to BCB-planarization effects.

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α (d

B/m

m)

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Figure 10. Extracted characteristic impedance, """""eff and losses(dB/mm) for a feedthrough on the middle metal layer. w=20µm, ground-to-ground spacing=117 µm.




358

4.2. Measurements of the FeedthroughsThe measured results for the two 500 µm long feedthroughs

are given in Figure 11. The feedthrough realized on the bottommetal layer can be used up to at least 45 GHz, having a returnloss superior to -30 dB and an associated insertion loss lowerthan –0.5 dB over the entire frequency band.

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Figure 11. Measured return loss and insertion loss for thetwo 500 µm long feedthroughs: (-%%%%%-) feedthrough reallizedon the middle metal layer, (-o-) bottom metal layer.

The feedthrough realized on the middle metal layer has a su-perior insertion loss up to 25 GHz and is therefore more suitedfor the low-frequency applications.

Naturally, the performance of the feedthrough can be furtherimproved using a thinner metal wall, which results in a shorterintrinsic feedthrough length.

5. Conclusion and Future Directions

In this paper, the authors have reported on the design, charac-terization and modeling of CPW feedthroughs implemented in amultilayer thin film MCM-D technology. Using 3-D simula-tions, the researchers have shown that the vertical wall may bereplaced by a thin metal layer with a very small impact on theperformance of the feedthrough. This equivalent structure canbe more easily realized and therefore allows for a faster designand characterization of the feedthrough. It is shown that thediscontinuities and the CPW-to-microstrip transition only havea small impact on the S-parameters such that a good approxima-tion of the feedthrough’s behavior can be obtained by a cascadeof three transmission line sections corresponding to the feedinglines, and the intrinsic microstrip-based feedthrough. The trans-mission line parameters of the intrinsic feedthrough can be accu-rately extracted based on the measurement of two equivalent struc-tures with different length.

Two feedthroughs have been designed using different metallayers and different design-approaches. The “all-pass” structurecan be used up to very high frequencies (at least 50 GHz), whereasthe “low-pass” design is limited up to 25 GHz. It has, however,the advantage of a superior insertion loss for the lower frequen-cies.

Measured results have been presented for both feedthroughs.They demonstrate that a low loss, well matched feedthrough, canbe realized up to at least 50 GHz (insertion loss below -0.5 dBand return loss below -25 dB for a typical length of 500 µm).

The feedthroughs will now be used to build complete micro-wave circuit-modules using bare-die active components and in-tegrated passives on the MCM-substrate. This will be achievedusing the custom design-library. Future work will also be di-rected towards the development of a chip-to-next level intercon-nection.

Acknowledgments

Geert Carchon was supported by a scholarship granted by theFlemish Institute for the Advancement of Scientific-TechnologicalResearch in Industry (IWT). The author also acknowledges thesupport of the European Space Agency under contract number13627/99/NL/FM(SC).

References

1. G. Carchon, S. Brebels, W. De Raedt, and B. Nauwelaers,“Accurate Measurement and Characterization up to 50 GHzof CPW-based Integrated Passives in Microwave MCM-D,”




Presented and Published at the Electronic Components andTechnology Conference, ECTC’ 2000, Las Vegas, Nevada,pp. 459-464, 2000.

2. G. Carchon, P. Pieters, K. Vaesen, S. Brebels, D. Schreurs, S.Vandenberghe, W. De Raedt, B. Nauwelaers, and E. Beyne,“Design-oriented Measurement-based Scaleable Models forMultilayer MCM-D Integrated Passives. Implementation in aDesign Library offering Automated Layout,” Presented andPublished at International Conference and Exhibition on HighDensity Interconnect and Systems Packaging, Denver, Colo-rado, pp. 196-201, 2000.

3. G. Carchon, S. Brebels, K. Vaesen, P. Pieters, D. Schreurs, S.Vandenberghe, W. De Raedt, B. Nauwelaers, and E. Beyne,“Accurate Measurement and Characterization of MCM-DIntegrated Passives up to 50 GHz,” Presented and Publishedat International Conference and Exhibition on High DensityInterconnect and Systems Packaging, Denver, Colorado, pp.307-312, 2000.

4. G. Carchon, S. Brebels, P. Pieters, K. Vaesen, D. Schreurs, S.Vandenberghe, W. De Raedt, B. Nauwelaers, and E. Beyne,“Design of Microwave MCM-D CPW Quadrature Couplersand Power Dividers in X-, Ku- and Ka-band,” Presented andPublished at International Conference and Exhibition on HighDensity Interconnect and Systems Packaging, Denver, Colo-rado, pp. 87-92, 2000.

5. K. Vaesen, P. Pieters, G. Carchon, W. De Raedt, E. Beyne, A.Naem, and R. Kohlmann, “Integrated Passives for a DECTVCO,” Presented and Published at International Conferenceand Exhibition on High Density Interconnect and SystemsPackaging, Denver, pp. 537-541, 2000.

6. K. Vaesen, S. Donnay, P. Pieters, G. Carchon, W. Diels, P.Wambacq, W. De Raedt, E. Beyne, M. Engels, and I. Bolsens,“Chip-Package Co-Design of a 4.7 GHz VCO,” Presentedand Published at International Conference and Exhibitionon High Density Interconnect and Systems Packaging, Den-ver, pp. 301-306, 2000.

7. M. P. Goetz and G. Mancias, “Measurement of a 24-GHzBroadband Multilayer Ceramic Feedthrough for MicrowavePackaging,” IEEE Microwave and Guided Wave Letters, Vol.2, pp. 171-173, 1992.

8. E. Holzman, R. Teti, B. Dufour, and S. Miller, “An HermeticCoplanar Waveguide-to-HDI Microstrip MicrowaveFeedthrough,” Presented at IEEE MTT-S Digest, pp. 103-106, 1998.

9. P. Monfraix, T. Adam, C. Devron, G. Naudy, B. Cogo, and J.L. Roux, “A New Concept of RF Feedthrough Applied toMultichip Modules for Space Equipment,” Presented at IEEEMTT-S Digest, Boston, Massachusetts, 2000.

10. P. Pieters, S. Brebels, E. Beyne, and R. P. Mertens, “Gener-alized Analysis of Coupled Lines in Multilayer MicrowaveMCM-D Technology - Application: Integrated Lange Cou-plers,” IEEE Transactions on Microwave Theory and Tech-niques, Vol. 47, pp. 1862-1872, 1999.

11. G. Carchon, B. Nauwelaers, W. De Raedt, D. Schreurs, andS. Vandenberghe, “Characterizing Differences Between Mea-

surement and Calibration Wafer in Probe-Tip Calibrations,”Electronics Letters, Vol. 35, pp. 1087-1088, 1999.

About the authors

Geert Carchon received the M.Sc.Degree in Electrical Engineering fromthe Katholieke Universiteit Leuven, Bel-gium in 1996. As a Research Assistantof the IWT, he is currently working to-wards a Ph.D. Degree at the K.U.Leuvenin close cooperation with the High-Den-sity Interconnect and Packaging groupof IMEC, Leuven, Belgium. His maininterests include the measurement, char-acterization and modeling of passive

devices and the design of RF and microwave circuits (LNAs,modulators, …) in MMIC and multilayer MCM-D.

Walter De Raedt received the M.S.Degree in Electrical Engineering at theKatholieke Universiteit Leuven, Leuven,Belgium, in 1981. Subsequently, hejoined the ESAT laboratory as a Re-search Assistant and worked on DirectWrite Electron Beam Technology. From1984, he is with IMEC where he startedresearch on MMICs and submicrontechnologies for advanced HEMT de-vices. Since 1997, he joined the High

Density Interconnect and Packaging group where he is workingon integrated passives and interconnections for RF front-end sys-tems.

Bart Nauwelaers received the M.S.and Ph. D. Degrees in Electrical Engi-neering from the Katholieke UniversiteitLeuven, Leuven, Belgium, in 1981, and1988, respectively. He also holds aMaster Degree from ENST, Paris,France. Since 1981, he has been withthe Department of Electrical Engineer-ing (ESAT) of the K.U.Leuven, wherehe has been involved in research on mi-crowave antennas, microwave integrated

circuits and MMICs, and wireless communications. He teachescourses on Microwave Engineering, Analog and Digital Com-munications, Wireless Communications, and Design in Electron-ics and Telecommunications.




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Eric Beyne received the M.S. andPh.D. Degrees in Electrical Engineer-ing from the Katholieke UniversiteitLeuven, Leuven, Belgium, in 1983, and1990, respectively. From 1983 to 1985,he was a Research Assistant at the K.U. Leuven. In 1986, he joined IMEC,where he worked towards his Ph.D.Degree on the interconnection of high-frequency digital circuits. He is pres-ently responsible for projects on

multichip modules and advanced packaging at IMEC. Dr. Beyneis a member of the IMAPS-Benelux Committee.

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