Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Kaband

Transcription

1 Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Ka-band Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Kaband G. Carchon*, S. Brebels +, K. Vaesen +, P. Pieters +, W. De Raedt +, B. Nauwelaers*, and E. Beyne + *KULeuven, dep. ESAT-TELEMIC Kard. Mercierlaan 94, B-3001 Heverlee, Belgium Phone: Fax: geert.carchon@esat.kuleuven.ac.be + IMEC, div. MCP/HDIP Kapeldreef 75, B-3001 Heverlee, Belgium Abstract The successful design and measurement of high-performance coplanar waveguide (CPW) quadrature hybrid couplers integrated in MCM-D at Ku and Ka-band and Wilkinson power dividers at X-band are discussed in this work. The authors have investigated the feasibility of integrating 3-dB quadrature couplers in the MCM-D technology. Quadrature couplers based on two edge-coupled lines are not practical, as the required line-dimensions are too small. As an alternative, couplers using re-entrant sections and CPW Lange couplers have been realized. For the Lange coupler, the authors have obtained in the GHz band: a return loss and isolation better than 20 db, an amplitude balance below 0.13 db, and a phase balance of (90 +/- 1) deg. For the coupler using re-entrant sections, the researchers have obtained in the GHz band: a return loss better than 17 db, isolation better than 20 db, an amplitude balance below 0.25 db, and a phase-balance of (90 +/- 2.5) deg. A trade-off between these two design-approaches for integration in MCM-D is given. Distributed and lumped Wilkinson power-dividers at X-band have been designed using the in-house developped design library. An excellent agreement has been found up to 50 GHz. Key words: MCM-D, Integrated Passives, Quadrature Coupler, Power Divider, and Coplanar Waveguide. 1. Introduction The multilayer thin film multichip module technology (MCM- D) offers a very high reproducibility of very small line dimensions, and it is therefore a promising technology for the integration of RF and microwave circuits. The layer built-up of IMEC s MCM-D technology is given in Figure 1. It consists of a main copper (Cu) coplanar waveguide CPW-layer between two BCBlayers on a glass substrate. More information on the technology can be found in Reference 1. Various types of high-performance integrated passives (spiral inductors, TaN-resistors, Ta 2 O 5 -capacitors, among others) have already been demonstrated (Reference 1-3 ). Integrating these passives directly on the MCM-substrate provides a size and cost reduction and increases the packaging density. At the lower microwave frequencies, lumped elements (such as spiral inductors) are most often used. Transmission line effects are not of primary importance as the interconnect length is small compared to the wavelength. At the higher microwave frequencies, distributed elements are predominantly used. In this case, the distributed effect is used for the design of couplers and power dividers. International Microelectronics And Packaging Society 257

2 Intl. Journal of Microcircuits and Electronic Packaging 2 µ m Ti/Cu metal 3 µ m Ti/Cu/Ti metal 1 µ m top Al contact metal 1 µ m bottom Al contact metal TaN- resistor Ni/Au component layer 5 µ m BCB 5 µ m BCB Ta O capacitor µ m Glass substrate Figure 1. Layer built-up of IMEC s MCM-D technology. Quadrature couplers are an ideal component for use in balanced amplifiers and mixers and in the design of QPSK-modulators. In-phase power dividers are another basic building block in the design of high frequency circuits. Wilkinson power dividers are used for example in the design of QPSK-modulators. This paper discusses the design and measurement of 3-dB quadrature couplers based on re-entrant sections and Lange couplers. Wilkinson-based power-divider at X-band are designed using the in-house developped design library (Reference 3 ). An excellent agreement between measurements and simulations will be demonstrated. 2. Integration of Quadrature Couplers a) Architectures Different possibilities exist for the integration of 3-dB quadrature couplers in MCM-D technology. One option is to design a branchline coupler, which consists of four transmission lines with the correct phases and characteristic impedances at the design frequency. This approach is, however, more narrowband than the methods based on coupled lines. In Figure 2, the simplest coupling structure is depicted. It makes use of two edge-coupled strips in the same metal level. For a 3-dB coupler, this architecture generally requires very small line dimensions. A CPW Lange coupler (Figure 3) consists of four edge-coupled lines in the same metal level. Its main advantage is the larger line separation when compared with the previous structure. The coupler in Figure 4 makes use of re-entrant sections: it consists of two coupled strips in the same metal level, together with a floating potential strip. The floating potential patch increases the coupling in the odd mode, such that tight coupling can be achieved. Figure 3. Cross section of a CPW Lange coupler. Figure 4. Cross section of a coupler based on re-entrant sections. b) Method of Analysis The geometry of the coupled lines in the following sections is determined using a quasi-tem program (Reference 4 ) which accurately calculates the R, L, C, and G matrices of a multi-conductor transmission line in a multilayered substrate (the metalthickness is also properly accounted for). This approach has the advantage that one iteration in the determination of the linewidths and slots (necessary to obtain the correct even and odd-mode impedances) only requires a very short time-period (only a few seconds on a HP J2240 station). The verification of the couplers, including the effect of the feeding lines, can then be carried out with only one full electromagnetic simulation (for example using HP MOMENTUM). Both methods can account for a variation of the BCB-layer thickness. Planarization effects cannot be accounted for as the simultors assume that all dielectric layers are perfectly planar. The Lange couplers have been designed using the method of Paolino (Reference 5 ), in which the full capacitance matrix of the four-conductor system is calculated. The involved capacitances are schematically represented in Figure 5. The even and oddmode capacitances are then given by equations (1) and (2), as follows, C even = C1g + C4g (1) C odd = C1g + C4g + 2(C12 + C13 + C24 + C34) (2) Figure 2. Cross section of a coupler based on edge-coupled lines. When these capacitances are calculated with and without dielectrics, the even and odd mode characteristic impedance and effective dielectric constant can be obtained using the following well-known formulas (equation (3)), (3) 258 International Microelectronics And Packaging Society

3 Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Ka-band L4 C4g C24 C34 C12 C13 L2 L1 C2g C1g L3 C3g 3 db. This resulted in the dimensions w=56 µm, s=20 µm, s g =123 µm, and length=3040 µm. A picture of a realized CPW-Lange coupler is provided in Figure 6. The measurements are given in Figures 7-9. The authors have obtained in the GHz band: a return loss and isolation better than 20 db, an amplitude balance below 0.13 db, and a phase balance of (90 +/- 1) deg. Figure 5. Schematic representation of the capacitances in the four-conductor system. (the ground is drawn below the structure). C23 and C14 are neglected, as these lines are on the same potential. In this case, C is the capacitance with dielectrics, C 0 is the capacitance in the dielectric free medium, and c is the speed of light in vacuum. c) Measurements Couplers are four-port devices, the network analyzer, however, only has two ports. It is therefore necessary to alternatively terminate two ports at 50Ω while measuring between the two remaining ports. One possibility is to use four probes, two of which are terminated on a 50Ω load. In this case, only one on-wafer coupler is required but accurate measurements require an orthogonal-probe calibration technique (Reference 6 ). This is, however, not standardly available in commercial network analyzers. Another possibility is to use on-wafer 50Ω resistors. This has the drawback that more couplers are required (four to six depending on the symmetry of the coupler), but classical calibration methods (such as LRM or SOLT) can still be used. This method has been selected in this work. Accurate measurements also require that one compensates for the non-ideal 50Ω termination resistance (due to a variation in the sheet resistance or the inductive effect). This has been achieved using the method given in Reference 7. In this way, an accurate determination of the isolation, amplitude and phase balance becomes possible, essential parameters in the design of, for example, QPSK-modulators. Figure 6. CPW Lange coupler layout. Figure 7. Measured amplitude (x) and phase ( ) balance of the CPW Lange coupler. d) Architecture using Two Edge-Coupled Lines This method has the drawback that, for reasonable linewidths, a very small line separation is required to realize the required even and odd-mode impedances for a 3-dB coupler. In this technology, one may need a stripwidth (w) of 94 µm, a slot (s) of 2 µm, and a slot-to-ground (s g ) of 74 µm. Increasing the slot to reliable dimensions is possible, but then, the required stripwidth becomes unreasonable. This architecture is therefore only suited for the realization of loosely coupled couplers and has not been realized. e) CPW Lange Couplers The couplers dimensions were determined using the Quasi- TEM program. In a first stage, a 2.8 db coupler was targeted. When losses were included, the predicted coupling was about Figure 8. Measured return loss (x) and isolation ( ). International Microelectronics And Packaging Society 259

4 Intl. Journal of Microcircuits and Electronic Packaging Figure 11. Two realizations of a coupler using reentrantsections. the coupler on top, has the crossover in the middle, the coupler at the bottom, has the crossover at the end. Figure 9. Measured thru (x) and coupled ( ) port. The performance of this coupler can be very well predicted by 2.5-D simulators (such as HP MOMENTUM) as can be seen in Figure 10. As the losses in the even and odd mode are not too large, the amplitude balance is very well predicted. The predicted absolute value of the insertion loss is less accurate, due to the underestimated losses by the simulator. As the quadrature outputs are desired to be on the same side of the coupler, it was necessary to switch the location of the two strips. In one realization, this is achieved in the middle (leading to a more symmetric coupler), in the other realization, this is achieved at the end of the strips (less symmetry but the coupled lines are not disturbed in the coupling section). The measured performance of the two realizations was, however, nearly equal. A 2.8-dB coupler was first targeted using the Quasi-TEM program. It was expected that BCB-planarization effects would increase the coupling, while the larger losses in the odd mode would decrease the coupling. The dimensions are w=38 µm, s=40 µm, s g =58 µm, and length=1447 µm. The floating potential patch is extended 10 µm beyond the edge of the strips. The measured results are given in Figure 12 and Figure 13. The authors have obtained in the GHz band: return loss better than 17 db, isolation better than 20 db, amplitude balance below 0.25 db, and a phase-balance of (90 +/- 2.5) deg. Figure 10. Measured ( ) versus simulated (-) results for the CPW Lange coupler using a 2.5-D simulator. The Lange coupler is not prone to BCB-thickness variations. For example, when the thickness of each BCB-layer decreases by 0.5 µm, the predicted coupling only changes by 0.01 db. f) Couplers using Re-entrant Sections A picture of two different realizations of couplers using reentrant sections is given in Figure 11. The strips are located on the Cu-metallization, the floating potential patch is realized on the top metal level (see Figure 1 and Figure 4). Figure 12. Measured thru ( ) and coupled (x) port. 260 International Microelectronics And Packaging Society

5 Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Ka-band Figure 14. Measured profile of the surface of the quadrature coupler using re-entrant sections (units in µm). Figure 13. Measured isolation ( ) and return loss (-). A drawback is that the design is prone to BCB-thickness variations and planarization effects. For example, when the thickness of each BCB-layer decreases by 0.5 µm, the absolute value of the predicted coupling drops by 0.15 db. Placing the floating potential patch on the lower metal layer should lower the effect of BCB-planarization. It is hard to obtain accurate results for this kind of coupler using 2.5-D simulators. This is partly due to BCB-planarization effect, but also to the underestimated losses (nearly neglected in HP MOMENTUM). As the losses in the odd mode (highly confined field) are quite large compared to the losses in the even mode, the amplitude balance is not as well predicted as is the case for the Lange coupler. The measured coupling is smaller than the predicted value, due to the losses in the odd mode. g) BCB-planarization In Figure 14, the authors have depicted the measured surfaceprofile of the quadrature coupler using re-entrant sections. In this case, the researchers notice that the assumption of the perfectly flat BCB-layer is not completely fulfilled. The top metal, for example, shows a dip of approximately 1 µm in the middle of the coupler. Therefore, one can conclude that in this small region, the patch is only located 4 µm above the strips. On the one hand, this will change the coupling. On the other hand, the losses in the odd mode will increase as the field will be more confined to this region. A better configuration would therefore be, to place the patch on the lower metal: the dip and the accompanying disadvantages, then disappear. Also, in the region between the groundplane and the patch, a dip in the BCB occurs. The difference in height between the middle and the edges of the plot is due to the metal heigth of the patch. h) Discussion The performance of the Lange coupler can be very well predicted by 2.5-D simulators and is therefore an attractive way to design 3-dB quadrature couplers. Its lateral dimensions are, however, quite large, such that the design becomes prone to shielding effects. This also limits the high-frequency performance when the length becomes comparable to the total width of the coupler (the length is a quarter-wavelength at the design-frequency). An advantage is that the coupler is not sensitive to BCB-thickness variations and planarization effects. The coupler using re-entrant sections is better suited for high frequency operation as its lateral dimensions are smaller. It is less prone to shielding effects but due the highly confined field in the odd mode, it exhibits larger losses than the Lange coupler. The performance is also difficult to predict using commercial 2.5-D simulators. On the one hand, this is due to the sensitivity to BCB-thickness variations and BCB-planarization, on the other hand, it is due to the losses in the odd mode which are quite large and also determined by the BCB-planarization. 3. Design of Power Dividers Wilkinson power dividers have been designed and optimized using the in-house developped MCM-D library presented in Reference 3. A picture of a distributed and a lumped element realization are given in Figure 15. Figure 15. Layout of the Wilkinson power divider: distributed design (left) and lumped element design (right). International Microelectronics And Packaging Society 261

6 Intl. Journal of Microcircuits and Electronic Packaging The distributed design has an insertion loss of 0.42 db, an isolation loss better than 25 db in the GHz band, and a return loss better than 20 db in the GHz band. The circuit measures 1.9 mm x 2.65 mm. The lumped element design has an insertion loss of 0.44 db, an isolation better than 25 db in the GHz band, and a return loss better than 17 db in the GHz band. The circuit measures 2.60 mm x 1.47 mm. The lumped element design is smaller, but has a more narrow bandwidth compared to the distributed design. The measured and simulated results for the distributed design are given in Figures A very good correspondence can be observed up to 50 GHz. This indicates a high quality of the developed models, but also indicates the good repeatability that can be obtained between different processing runs. Figure 18. Measured (-) versus simulated (+) isolation of the Wilkinson power divider. 4. Conclusion and Future Directions Figure 16. Measured (-) versus simulated (+) return loss of the Wilkinson power divider. Figure 17. Measured (-) versus simulated (+) thru port of the Wilkinson power divider. In this paper, the authors have described the succesful integration of high-performance 3-dB quadrature couplers in MCM- D technology. The properties of different architectures have been investigated. Couplers using re-entrant sections and CPW Lange couplers have been realized. The couplers were measured by alternatively terminating two ports with an on-wafer termination resistor, and measuring between the two remaining ports. The effect of the non-ideal termination has been accounted for, to obtain accurate measurements. For the Lange coupler, the authors have obtained in the GHz band: return loss and isolation better than 20 db, an amplitude balance below 0.13 db, and a phase balance of < (90 +/- 1) deg. For the coupler using re-entrant sections, the authors have obtained in the GHz band: a return loss better than 17 db, isolation better than 20 db, an amplitude blance below 0.25 db, and a phase-balance of (90 +/- 2.5) deg. Measurement results for Wilkinson power dividers, designed using the in-house developed MCM-D design library, have been presented. An excellent agreement between measurement and simulation has been obtained up to 50 GHz which indicates the high quality of the developped models, but also indicates the good repeatability that can be obtained between different processing runs. The basic building blocks that have been designed in this work will be used to build complete microwave circuits such as low noise amplifiers, using Flip Chipped bare die transistors, and QPSK modulators. All passive elements will be integrated on the MCM-D substrate, while the active elements (transistors, diodes, ) will be mounted on the carrier MCM-D substrate. This has already been demonstrated for the lower GHz-range (Reference 8 ) and will be evaluated next for higher frequencies. 262 International Microelectronics And Packaging Society

7 Design of Microwave MCM-D CPW Quadrature Couplers and Power Dividers in X-, Ku- and Ka-band Acknowledgments Exhibition on High Density Interconnect and Systems Packaging, Denver, Colorado, pp , Geert Carchon was supported by a scholarship granted by the Flemish Institute for the Advancement of Scientific-Technological Research in Industry (IWT). The authors also acknowledge the support of the European Space Agency under contract number 13627/99/NL/FM(SC). References 1. G. Carchon, S. Brebels, W. De Raedt, and B. Nauwelaers, Accurate Measurement and Characterization up to 50 GHz of CPW-Based Integrated Passives in Microwave MCM-D, Proceedings of the Electronic Components and Technology Conference, ECTC 2000, Las Vegas, Nevada, pp , 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-D Integrated Passives up to 50 GHz, Proceedings of the International Conference and Exhibition on High Density Interconnect and Systems Packaging, Denver, Colorado, pp , 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 for Multilayer MCM-D Integrated Passives. Implementation in a Design Library offering Automated Layout, Proceedings of the International Conference and Exhibition on High Density Interconnect and Systems Packaging, Denver, Colorado, pp , P. Pieters, S. Brebels, E. Beyne, and R. P. Mertens, Generalized Analysis of Coupled Lines in Multilayer Microwave MCM-D Technology - Application: Integrated Lange Couplers, IEEE Transactions on Microwave Theory and Techniques, Vol. 47, pp , D. D. Paolino, Design More Accurate Interdigitated Couplers, Microwaves, Vol. 15, pp , S. Basu and L. Hayden, An SOLR Calibration for Accurate Measurement of Orthogonal On-Wafer DUTs, Proceedings of the IEEE MTT-S Digest, Denver, Colorado, pp , J. C. Rautio, Techniques for Correcting Scattering Parameter Data of an Imperfectly Terminated Multiport When Measured with a Two-port Network Analyzer, IEEE Transactions on Microwave Theory and Techniques, Vol. 31, pp , K. Vaesen, P. Pieters, G. Carchon, W. De Raedt, E. Beyne, A. Naem, and R. Kohlmann, Integrated Passives for a DECT VCO, Proceedings of the International Conference and About the authors Geert Carchon received the M.Sc. Degree in Electrical Engineering from the Katholieke Universiteit Leuven, Belgium in As a Research Assistant of the IWT, he is currently working towards a Ph.D. Degree at the K.U.Leuven in close cooperation with the High-Density Interconnect and Packaging group of IMEC, Leuven, Belgium. His main interests include the measurement, characterization and modeling of passive devices and the design of RF and microwave circuits (such as LNAs and modulators) in MMIC and multilayer MCM-D. Steven Brebels received the M.S Degree in Electrical Engineering from the Katholieke Universiteit Leuven, Leuven, Belgium, in 1994, and is currently working toward the Ph.D. Degree at IMEC, Leuven, Belgium. His work within the High-Density Interconnect and Packaging Group is directed to integrated microwave circuits and antennas in MCM- D. His research interests include microwave and millimeter-wave integrated circuits and active integrated antennas. Kristof Vaesen received a B.S. in 1996 from the KDG, Antwerp, Belgium, and a M.S. Degree in Electrical Engineering from the Katholieke Universiteit Leuven, Leuven, Belgium, in He joined the High Density Interconnect and Packaging group, IMEC, Leuven, Belgium, later in the same year. Since then, his main research activities are focussed towards the single package integration of RF front-ends. and the design of RF building blocks in the MCM-D technology. International Microelectronics And Packaging Society 263

8 Intl. Journal of Microcircuits and Electronic Packaging Philip Pieters received the B.S. in 1991 from the Industrial Science and Technology School of Oostende, Belgium. In 1994, he obtained the M.S. in Electrical Engineering from the Katholieke Universiteit Leuven, Leuven, Belgium. In the same year, he joined the High-Density Interconnect and Packaging group of IMEC, Leuven, Belgium, where he is preparing his Ph.D. on integrated passives in multilayer thin film MCM-D. He is currently with CS2, a semiconductor assembly and test foundry located in Zaventem, Belgium, where he is responsible for the high frequency development projects. Eric Beyne received the M.S. and Ph.D. Degrees in Electrical Engineering from the Katholieke Universiteit Leuven, Leuven, Belgium, in 1983, and 1990, 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 presently responsible for projects on multichip modules and advanced packaging at IMEC. Dr. Beyne is a member of the IMAPS-Benelux Committee. Walter De Raedt received the M.S. in Electrical Engineering at the Katholieke Universiteit Leuven, Leuven, Belgium, in Subsequently, he joined the ESAT laboratory as a research assistant and worked on direct write Electron Beam Technology. Since 1984, he is with IMEC where he started research on MMICs and submicron technologies for advanced HEMT devices. Since 1997, he joined the High Density Interconnect and Packaging group where he is working on integrated passives and interconnections for RF front-end systems. Bart Nauwelaers received the M.S. and Ph. D. Degrees in Electrical Engineering from the Katholieke Universiteit Leuven, Leuven, Belgium in 1981, and 1988, respectively. He also holds a Mastère Degree from ENST, Paris, France. Since 1981 he has been with the Department of Electrical Engineering (ESAT) of the K.U.Leuven, where he has been involved in research on microwave antennas, microwave integrated circuits and MMICs, and wireless communications. He teaches courses in microwave engineering, analog and digital communications, wireless communications, and design in electronics and telecommunications. 264 International Microelectronics And Packaging Society