High performances of shielded LTCC vertical transitions ... - IEEE Xplore

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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005

High Performances of Shielded LTCC Vertical Transitions From DC up to 50 GHz Rosine Valois, Dominique Baillargeat, Member, IEEE, Serge Verdeyme, Member, IEEE, Markku Lahti, and Tuomo Jaakola

Abstract—This paper reports on research on two generic shielded vertical transitions in low-temperature cofired ceramic technology. These interconnections are simulated and optimized by three-dimensional elecctromagnetic simulations. The first circuit, a coplanar waveguide (CPW) or microstrip-to-stripline transition, presents great experimental performances from dc up to 50 GHz, and the second, a CPW-to-waveguide transition, is defined for -band applications. Index Terms—Interconnections, low-temperature ceramic (LTCC), waveguides, wide band.

cofired

I. INTRODUCTION

T

HE development of low-cost modules for millimeter-wave applications sets many challenges for packaging and interconnections. One of the most promising solutions to realize such modules is the low-temperature cofired ceramic (LTCC) technology. This process allows the conception of highly integrated three-dimensional (3-D) modules with a great flexibility for the designers. In the literature, we could find a growing interest for the LTCC multilayer package, and particularly for transmissionline structures. Some interconnections are presented in earlier papers [1]–[5]. These papers deal with vertical transitions; these interconnections allow to route an RF signal from the top surface to an inner layer or to the bottom surface and, therefore, allow the integration of passive and active components such as millimeter-wave monolithic integrated circuits (MMICs) mounted on top. Within the framework of the European project Low-Cost Millimeter-Wave T/R Module for Telecommunication Applications (LOTTO), our paper first reports on a generic wide-band topology such as an LTCC vertical microstrip (MS) or coplanar waveguide (CPW) to stripline (SL) transition [6]. Our objective differentiates ourselves from earlier papers [1]–[5]: it is to design transitions with a return loss below 10 dB from dc up to 50 GHz and with a very simple topology to allow its use in many applications. Thus, we have focused our attention on the Manuscript received September 30, 2004; revised December 15, 2004. This work was supported in part by the Packaging and Interconnection Development for European Applications under the European Project Low-Cost MillimeterWave T/R Module for Telecommuncations Applications. R. Valois, D. Baillargeat, and S. Verdeyme are with the Institut de Recherche en Communications Optiques et Microondes–Unité Mixte de Recherche Centre National de la Recherche Scientifique 6615, University of Limoges, 87060 Limoges, France (e-mail: [email protected]; [email protected]; [email protected]). M. Lahti and T. Jaakola are with VTT Electronics, 90571 Oulu, Finland (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TMTT.2005.848832

packaging of the modules and on keeping a good adaptation. We will present the differences made by our work compared with the other papers. Secondly, we have modified this first interconnection, and worked on a CPW-to-SL-to-waveguide transition. In this case, the transition is defined for -band applications. Some waveguide transitions are presented in [7] and [8]. Our studies are distinct from these papers due to the excitation waveguide method. As we will show in this paper, a short-circuited SL allows waveguide excitation. This paper is organized as follow. First we describe the LTCC process on Ferro A6-S developed by VTT Electronics, Oulu, Finland. Some theoretical studies carried out at the Institut de Recherche en Communications Optiques et Microondes (IRCOM) Laboratory, Limoges, France, are then described. The transitions are designed and optimized applying 3-D electromagnetic (EM) simulation tools. Parametric studies are presented and discussed. Finally, the measurement results of the manufactured test structures are then presented, and as will be shown, they present good performances. II. LTCC PROCESS The conductor lines were screen-printed on a Ferro A6-S tape system using a CN33-398 Ag conductor and CN33-343 Ag via fill paste. The MS line, presented in Fig. 1, was printed using a 400-mesh screen, and SLs and all ground layers were printed with a 325-mesh screen. Typical widths of the MS and SL were 500 and 150 m, respectively. The diameter of via-holes was 150 m. The layers were laminated at a pressure of 3000 psi and temperature of 70 C for 10 min. The peak temperature of the co-firing process was 850 C and the duration was approximately 16 h. III. EM SIMULATION TOOLS The design of the transitions is simulated and optimized by our in-house EM software. This software, based on the finiteelement method coupled with a Padé approximation [9], solves Maxwell equations in the frequency domain. It can be applied to the study of complex 3-D structures composed of linear and isotropic or anisotropic media. This analysis method has a great accuracy and presents an attractive advantage in terms of computation time: in brief, instead of performing one EM computation at each frequency point, only a single EM computation is necessary to analyze the electrical parameters in the whole frequency band. Consequently, the electric behavior of the module can be determined faster.

0018-9480/$20.00 © 2005 IEEE

VALOIS et al.: HIGH PERFORMANCES OF SHIELDED LTCC VERTICAL TRANSITIONS FROM DC UP TO 50 GHz

Fig. 4. Top view. (a) MS–SL–MS (b) CPW–SL–CPW transitions. Fig. 1.

Fig. 2.

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transitions

with

probe

pads.

MS line.

(a) MS-to-SL transition. (b) Zoom-in on the parameters to optimize.

Fig. 3. MS-to-SL transition with the ground plane invisible in order to see the vias. Vias A: vias around the circular aperture. Vias B: vias near the SL.

IV. VERTICAL TRANSITION A. Description of the Test Structures Our study reports on a vertical MS-to-SL transition. Figs. 2 and 3 depict this interconnection. The LTCC packages are designed and optimized with EM simulations, using eight-layer Ferro A6-S tape systems. This substrate has a relative dielectric constant equal to 5.9 and a fired layer thickness of 99 m. A metallization layer, located on the fifth LTCC layer from the bottom surface, is used as ground plane for the MS and SL. A circular aperture through this ground plane allows connecting MS and SL signal lines by a center via. The diameter of the aperture (parameter in Fig. 2(b), m) is optimized according to the diameter of the signal via in order to obtain an optimum coaxial effect. In this case, the diameter of the via-hole was 150 m, which permits the reduction of the dimensions of the whole aperture.

As we can see in Figs. 2 and 3, some additional vias are introduced in the structure. The locations of the vias are optimized by EM simulations taking into account EM-field distributions. These vias first permit the connection of the middle ground plane to the bottom. Some of them are located around the hole through the metallization layer (vias on Fig. 3) and then reinforce the coaxial effect by confining EM energy around the signal via. Some other vias are used to shield the SL (vias on Fig. 3). The use of the vias also allows avoiding the excitation of parasitic modes that can appear at high frequencies due to the dimensions of the whole structure that is comparable to the wavelength. This distinguishes our study from [1]–[5] and allows us to widen the frequency band. Furthermore, as shown in Fig. 2, the MS width is not the same all along the length. The SL and MS characteristic impedances are 50 . Due to the aperture through the MS ground plane, we widened the MS (parameter in Fig. 2) above this aperture in order to keep a good adaptation. The MS width and length in Fig. 2 are also parameters that are optimized by EM simulations. This study allows also us to wide the frequency band compared with the other papers. Moreover, from the interconnection presented in Fig. 2, two other configurations, illustrated in Fig. 4, are studied, i.e., (a) back-to-back MS–SL–MS transitions with probe pads and (b) back-to-back CPW–SL–CPW transitions. In (b), where almost all the surface of the LTCC module is metallized, many parasitic modes appear between the top and inner metallization layers and, thus, in order to avoid their excitation, other vias are added between these two layers. B. EM Simulations and Optimizations The whole transition is optimized by EM simulations. Fig. 5 presents simulated results of the back-to-back structure with MS–SL–MS transitions, and CPW–SL–CPW transitions. We can see that parasitic modes, due to the enclosure dimensions, appear after 46 GHz in both cases. Fig. 6 shows the importance of two parameters presented in Figs. 2 and 3 for the optimization of the transition behavior. These parameters are the MS width (see Fig. 2) and the vias on both sides of the SL (see Fig. 3). We have chosen to present only parametric results concerning the MS–SL–MS transition. The optimized response, presented by curve 1 in Fig. 5, is taken as reference curve 1 in Fig. 6. Concerning the parameter , we have established that it must be higher than parameter (see Fig. 2), and we show on curve 2 in Fig. 6 that for a 20% decrease of the parameter from its optimized value, the transition response is really degraded. Curve 3 considers the transition without vias . We can notice that parasitic modes as shown appear at lower frequencies when we omit vias by -field distributions in Fig. 6. Of course, these parasitic

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Fig. 5. Simulated losses).


[

S -parameters ]

of the back-to-back structure (without

Fig. 7. Measured [S ]-parameters of the: (a) back-to-back structure. (b) MS–SL–MS transitions with probe pads. (c) CPW–SL–CPW transitions.

Fig. 6. Simulated [S ]-parameters of the back-to-back structure, and E -field distribution at two frequencies where parasitic modes appear on curve 3.

modes have a negative impact on the electric performances of the transition. C. Measurements The LTCC modules are realized by VTT Electronics using the technology process described in Section II. The transitions are tested using a cascade probe station and an HP 8510 C vector network analyzer. Fig. 7 presents the measured -parameters. In Fig. 7(a) and (b), we present MS–SL–MS transitions with probe pads [see Fig. 4(a)], and in Fig. 7(c), we present CPW–SL–CPW transitions [see Fig. 4(b)]. The transitions shown in Fig. 7(a) and (b) are not very different: (b) presents the results for the dimensions of the transition described in Fig. 2 and, in (a), the MS length (parameter in Fig. 2) is 200 m shorter, and the distance between the vias near the SL (vias in Fig. 3) is slightly longer than in (b). Globally, according to these different results, the return loss is below 10 dB from dc to 50 GHz. In Fig. 7(a) and (b), the measured insertion losses are better than 2 dB up to 35 GHz and better than 3 dB up to 49 GHz. In Fig. 7(c), the measured insertion loss is lower than 1 dB up to 44 GHz and 2 dB up to 47 GHz.

D. Comparison Between Simulations and Measurements Figs. 5 and 7 present simulated and measured -parameters. In MS–SL–MS transitions, parasitic modes appear at a slightly higher frequency in the measured results than in the simulated ones. One explanation could be that the experimental dielectric constant of the LTCC substrate decreases with the frequency (this phenomenon is not taken into account by EM simulations), and also could be caused by the dispersion due to the technology. We can also notice that for CPW–SL–CPW transitions, insertion loss is 1 dB better than for the MS–SL–MS transitions. It can be explained by a slight mismatch caused by the pads for the MS–SL–MS, and for the CPW–SL–CPW transition, the EM-field distributions imposed by the CPW is more consistent with the coaxial effect compared to those imposed by the MS. E. Measurements With an Other LTCC Substrate We have also tested this transition with another LTCC substrate: Dupont 951 A2. This substrate has a relative permittivity of 7.8 and a fired layer thickness of 130 m. The topology of the transition previously describes is unchanged; dimensions are adjusted only according to the characteristics of the substrate. -parameters of Fig. 8 presents the measured MS-to-SL-to-MS transition with probe pads in Dupont 951 A2. We could see that the return losses are around 12 dB from dc is better than up to 43 GHz. The transmission parameter 2 dB up to 34 GHz, and 3 dB up to 42 GHz.


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Fig. 8. Measured [S ]-parameters of the back-to-back MS-to-SL-to-MS transition with probe pads in Dupont 951 A2.

Fig. 10.

Cross-sectional view of the back-to-back SL-to-waveguide transition.

Fig. 11.

E -field distribution.

Fig. 9. Top view of the back-to-back SL-to-waveguide transition (the CPW is not represented).

Thus, in these conditions, measurements show that the LTCC substrate Ferro A6-S allows to obtain better performances for a widest band that the LTCC substrate Dupont 951 A2. V. WAVEGUIDE TRANSITION A. Description of the Test Structures Using the MS-to-SL transition previously described, we have studied a new interconnection shown in Figs. 9 and 10: a CPW-to-SL-to-waveguide transition. This new transition is defined for -band applications. Our objective is to obtain a transition with a satisfying electrical behavior in order to design a 40-GHz narrow bandpass waveguide filter with the SL used as waveguide excitation. This waveguide filter could be easily bounded on a substrate carrier for planar integration or easily connected to other components (MMICs, filter, etc.) inside the same LTCC block to conceive highly integrated modules for future applications. This package is also designed with 3-D EM simulations. The CPW-to-SL transition is unchanged (topology and dimensions are the same). The SL then permits to excite the funof the waveguide, as shown damental propagating mode in Figs. 9 and 10. As we can see in Fig. 10, the waveguide is defined between the middle and bottom ground planes. Several vias (vias in Fig. 9) placed between these two ground planes allow defining the width of the waveguide. The distance between these vias is

minimum, taking into account the restrictions of the manufacat the upper frequency turing. This distance is smaller than and, hence, the electrical behavior of the vias is comparable to the electric behavior of a perfect electric wall, as shown in Fig. 11. The optimization of the topology and the waveguide realization are also easy compared with [7]. In Fig. 10, we could see on the SL extremity a via called “via .” It is connected to the bottom ground in order to short circuit the SL and, thus, to create an EM loop, used as waveguide excitation. As we can see in Fig. 12, the -field distribution is maximum around the vias. B. Theoretical Analyses by EM Simulations According to the design rules, the whole transition is designed by EM simulations. Fig. 13 presents simulated results of the back-to-back CPW-to-SL-to-waveguide transition. We can see that the cutoff frequency of the waveguide is approximately 32 GHz, and the return loss is below 10 dB from approximately 34 to 45 GHz. This interconnection will be used to design a narrow bandpass filter at 40 GHz. Thus, the theoretical behavior shown in Fig. 13 is sufficient. In Fig. 14, we show that the SL length in the waveguide is an important parameter. The optimized response presented in

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S

Fig. 15. Simulated [ ]-parameters of the back-to-back CPW-to-SL-towaveguide transition with a different position of the via .

Fig. 12.

D

H -field distribution. S

Fig. 16. Measured [ ]-parameters of the back-to-back CPW-to-SL-towaveguide transition in order to see the cutoff frequency of the waveguide.

S

Fig. 13. Simulated [ ]-parameters of the back-to-back CPW-to-SL-towaveguide transition (without losses).

S

Fig. 14. Simulated [ ]-parameters of the back-to-back CPW-to-SL-towaveguide transition with different SL lengths.

Fig. 13 is taken as reference curve 1 in Fig. 14. Curves 2 and 3 represent the responses for two other SL lengths: on curve 2, the SL is 150 m shorter than for optimized response, and on curve 3, it is 150 m longer. The waveguide length is constant for the three cases. As we can see in Fig. 14, for a slightly different SL length in the waveguide, the transition response is degraded in the frequencies around 40 GHz. The position of the via on the extremity of the SL is another important parameter, as shown in Fig. 15. The optimized

S

Fig. 17. Measured [ ]-parameters of the back-to-back CPW-to-SL-towaveguide transition. The waveguide in the (b) is 5 mm longer than waveguide in (a).

response presented in Fig. 12 is taken as reference curve 1 in Fig. 15. Curve 2 represents the response for another position of the via : via is moved away 300 m of the SL extremity. The SL length and waveguide are unchanged. We can see that, for this position, the response is degraded. C. Measurements The LTCC modules are realized by VTT Electronics by applying the process described in Section II. The modules are then


tested using a cascade probe station and an HP 8510C vector network analyzer. -parameters of the Figs. 16 and 17 present the measured back-to-back CPW-to-SL-to waveguide transition. As shown in Fig. 16, the cutoff frequency of the waveguide is approximately 30 GHz, and the behavior below this frequency is identical to a classical waveguide. We could also notice that there are no parasitic modes. In Fig. 17, the results are presented for two different lengths of the waveguide: in Fig. 17(b), the waveguide is 5 mm longer than the waveguide in Fig. 17(a). The CPW-to-SL transition is unchanged in these two cases, the only difference is due to the waveguide losses. We can notice that, at 40 GHz, the insertion loss is around, in Fig. 17(a), 0.9 dB, and in Fig. 17(b), 1.7 dB. Thus, the estimated losses per length unit of the waveguide are 0.16 dB/mm around 40 GHz; these performances can be sufficient for filtering application. In Fig. 17(a), the return loss is below 10 dB from approximately 33 to 41 GHz, and in Fig. 17(b), from approximately 34 to 46 GHz. Thus, the waveguide presents excellent performances in the -band.

VI. CONCLUSION Two generic shielded LTCC vertical transitions realized on Ferro A6-S have been presented. The first consists of a vertical MS or CPW-to-SL transition. We have shown that this interconnection could offer great performances from dc up to 50 GHz. The second consists of a CPW-to-SL-to-waveguide transition. This interconnection has presented great performances around 40 GHz and is, therefore, suitable to design 3-D LTCC -band components such as filters.

ACKNOWLEDGMENT The authors acknowledge the partnership between the Institut de Recherche en Communications Optiques et Microondes (IRCOM) and VTT Electronics, which was made possible thanks to the European project LOTTO. The authors further acknowledge Institut du développement et des Ressources en Informatique scientifique (IDRIS), Orsay, France, for providing with computing tools.

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[5] A. Panther, C. Glaser, M. G. Stubbs, and J. S. Wight, “Vertical transitions in low temperature co-fired ceramics for LMDS applications,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, May 2001, pp. 1907–1910. [6] R. Valois, D. Baillargeat, S. Verdeyme, M. Lahti, and T. Jaakola, “High performances of shielded LTCC transitions from DC up to 50 GHz,” in Proc. 34th Eur. Microwave Conf., Amsterdam, The Netherlands, Oct. 11–15, 2004, pp. 537–539. [7] H. Uchimura, T. Takenoshita, and M. Fujii, “Development of a laminated waveguide,” IEEE Trans. Microw. Theory Tech., vol. 46, no. 12, pp. 2438–2443, Dec. 1998. [8] Y. Huang, K. Wu, and M. Ehlert, “An integrated LTCC laminated waveguide-to-microstrip line T-junction,” IEEE Microw. Wireless Compon. Lett., vol. 13, no. 8, pp. 338–339, Aug. 2003. [9] B. Thon, D. Bariant, S. Bila, D. Baillargeat, M. Aubourg, S. Verdeyme, P. Guillon, F. Thevenon, M. Rochette, J. Puech, L. Lapierre, and J. Sombrin, “Coupled Padé approximation-finite element method applied to microwave devise design,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, Jun. 2002, pp. 1889–1892.

Rosine Valois was born in Limoges, France, in May 1980. She received the Master’s degree in high-frequency and optical telecommunication from the University of Limoges, Limoges, France, in 2002, and is currently working toward the Ph.D. degree at the University of Limoges. She is currently with the Microwave Circuits and Devices Team of the Institut de Recherche en Communications Optiques et Microondes (IRCOM), University of Limoges. Her research interests are dedicated to millimeter-wave packaging as filters and interconnections based on LTCC technology.

Dominique Baillargeat (M’04) was born in Le Blanc, France, in 1967. He received the Ph.D. degree from the Institut de Recherche en Communications Optiques et Microondes (IRCOM), University of Limoges, Limoges, France, in 1995. From 1995 to 2005, he was an Associate Professor with the Microwave Circuits and Devices Team, IRCOM Laboratory. He is currently a Professor with IRCOM. His fields of research concern the development of methods of design for microwave devices. These methods include computer-aided design (CAD) techniques based on hybrid approach coupling EM, circuits and thermal analysis, synthesis and EM optimization techniques, etc. She is mainly dedicated to the packaging of millimeter-wave and opto-electronics modules and to the design of millimeter original filters based on new topologies, concepts (electromagnetic bandgap (EBG), etc.) and/or technologies (silicon, LTCC, etc.).

REFERENCES [1] F. J. Schmückle, A. Jentzsch, W. Heinrich, J. Butz, and M. Spinnler, “LTCC as MCM substrate: Design of strip-line structures and flip-chip interconnects,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, Jun. 2001, pp. 1903–1906. [2] J. Heyen, A. Gordiyenko, P. Heide, and A. F. Jacob, “Vertical feedthroughs for millimeter-wave LTCC modules,” in Proc. 33rd Eur. Microwave Conf., Munich, Germany, Oct. 6–10, 2003, pp. 411–414. [3] A. Ziroff, M. Nalezinski, and W. Menzel, “A novel approach for packaging using a PBG structure for shielding and package mode suppression,” in Proc. 33rd Eur. Microwave Conf., Munich, Germany, Oct. 6–10, 2003, pp. 419–422. [4] W. Simon, R. Kulke, A. Wien, I. Wolff, S. Baker, R. Powell, and M. Harrison, “Design of passive components for -band communication modules in LTCC environment,” in IMAPS Symp., Oct. 1999, pp. 183–188.

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Serge Verdeyme (M’99) was born in Meilhards, France, in June 1963. He received the Doctorat degree from the University of Limoges, Limoges, France, in 1989. He is currently Professor with the Institut de Recherche en Communications Optiques et Microondes (IRCOM), University of Limoges, and Head of the Microwave Circuits and Devices Team. His main area of interest concerns the design and optimization of microwave devices.

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Markku Lahti received the Master’s degree from the University of Oulu, Oulu, Finland, in 1993. His research with the Microelectronis Laboratory, University of Oulu, was the development of printing plates for the gravure offset printing. Following this, he was a Research Scientist, until in 2001, when he joined VTT Electronics, Oulu, Finland. His current main interests are the manufacturing, interconnection, and packaging issues related to multiplayer ceramics boards.

Tuomo Jaakola received the Master’s degree in technical physics and Licenciate of Technology (Lic. Tech.) degree in electrical engineering from the University of Oulu, Oulu, Finland, in 1980 and 2002, respectively. Upon graduation, he was a Research Scientist with the Microelectronics Laboratory, University of Oulu. His main interests were within the field of electronic ceramic fabrication processes. In 1987, he joined VTT Electronics, Oulu, Finland, where he is currently a Senior Research Scientist. His current interests are in the field of advanced printed circuit boards, LTCC boards, bare chip interconnection methods, and reliability of electronic assemblies.