Design of miniaturized RF SAW duplexer package

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ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 7, july 2004

Design of Miniaturized RF SAW Duplexer Package Hao Dong, Student Member, IEEE, Thomas X. Wu, Senior Member, IEEE, Kamran S. Cheema, Benjamin P. Abbott, Member, IEEE, Craig A. Finch, Member, IEEE, and Hanna Foo Abstract—This paper provides a comprehensive methodology for accurate analysis and design of miniaturized radio frequency (RF) surface acoustic wave (SAW) duplexer package. Full-wave analysis based on the three dimensional (3-D) finite element method (FEM) is applied to get the package model. The die model is obtained by combining the parasitics and acoustics models. The modeling of bonding wire is also discussed. The models of package, die, and bonding wires are assembled together to get the total response. Based on this methodology, several novel ideas are proposed to significantly improve the isolation. Simulation and measurement results are compared, and excellent agreement is found. The technique developed in this paper reduces the design cycle time greatly and can be applied to various RF SAW device packages.

I. Introduction typical analog or digital mobile phone system consists of a super-heterodyne radio, which communicates between the phone and the base station at two different frequencies. The signal is transmitted from the phone to the base station within the transmitter channel (Tx) with the center frequency fT and from a base station to the phone within the receiver channel (Rx) with the center frequency fR . A full duplex radio system such as code division multiple access (CDMA) IS95 or IS98 requires a device that separates the transmitting signals from the receiving signals and permits transmission and reception simultaneously. The enabling device is known as a duplexer [1]–[3]. In the past, the ceramic resonators dominated the duplexer market, but recent advances in the surface acoustic wave (SAW) technology have enabled a smaller and lighter duplexer for the mobile phone market. Because the main function of the duplexer is to isolate the Tx and Rx, smaller physical size of the package makes it very difficult to implement isolation due to the electrical coupling between the transmitter and the receiver portion of the duplexer. The SAW duplexers are implemented using ladder filters. This technique must be used because it handles power better than other types of radio frequency (RF) SAW filters [1], [4], [5]. Ladder filters also provide a high degree

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Manuscript received November 7, 2003; accepted March 16, 2004. H. Dong and T. X. Wu are with the Department of Electrical and Computer Engineering, University of Central Florida, Orlando, FL 32816 (e-mail: [email protected]). K. S. Cheema, B. P. Abbott, C. A. Finch, and H. Foo are with Sawtek, Inc., Orlando, FL 32703.

Fig. 1. Schematic of SAW duplexer.

of attenuation near the passband. The duplexer is a threeport device, i.e., the receiver output, the transmitter input, and the third port connected to the antenna as shown in Fig. 1. Most critical electrical requirements for a duplexer are very low loss in the transmitter and receiver filter paths with rejection approaching 60 dB of each filter. Transmitter to receiver isolation in the transmission band is one of the most critical requirements. Because the duplexer is a front-end component in a full-duplex radio, the insertion loss for the receiver band sets the noise figure; furthermore, it degrades the sensitivity of the receiver. However, the insertion loss of the transmitter band has direct impact on the battery life of the radio. Poor isolation of the transmitting signal causes the leakage of signal in the receiver chain of the radio, hence posing a jamming threat to the receiver portion of the radio. The packaging requirements of SAW duplexers are unique compared to the other electronic components. Because a wave travels on the surface of the substrate, any physical contact can severely degrade the electrical performance of a SAW device. Typically, hermetically sealed packages are used to package SAW devices. Ceramic packages with appropriate cavities are mostly used due to small size, frequency stability, and easier handling in assembly [6]. Duplexer package, however, consists of a phasematching network making it more challenging for electrical characterization. This paper focuses on techniques of RF SAW filter and duplexer package design. The paper is organized as follows. In Section II, the full-wave analysis of the duplexer package is introduced. Ansoft high frequency structure simulator (HFSS) (http://www.ansoft.com/products/hf/hfss), based on the finite element method (FEM), is used to simulate

c 2004 IEEE 0885–3010/$20.00

dong et al.: analysis and design of miniaturized duplexer package

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Fig. 2. 3.8 × 3.8 mm2 KPCS duplexer package. (a) 3-D view of the package. (b) Detailed view of the package.

the SAW duplexer package. In Section III, the die model is obtained by combining the acoustics model and parasitics model generated from Agilent Momentum (http://eesof.tm.agilent.com). In Section IV, the bonding wire modeling is investigated. The total response of the package with die is discussed in Section V. Based on this methodology, in Section VI, investigation on the improvement of the isolation for the duplexer package is described. Several novel ideas are proposed to significantly improve the isolation. In Section VII, based on our investigation, a Korea personal communication system (KPCS) duplexer package is designed. Simulation and measurement results for the KPCS duplexer are compared and excellent agreement is found. Conclusions are given in Section VIII. II. Full-Wave Analysis of Duplexer Package Effects of package parasitics on the performance of SAW filters have been studied by many researchers. Several papers [3], [7]–[9] describe the equivalent circuit with parasitics. The finite difference and current simulation methods are used for calculating electrical parasitic parameters of SAW packages in [10]. These methods use measurements to fit the equivalent circuit model by minimizing the disagreement of measurements and simulations. The problems of these models are the necessity of measurements and the large effort to determine the relevant parameters. However, the range of validity of such models is limited. Furthermore, these models are static without relevant dependencies [11]. Recently, electromagnetic simulation tools were successfully used in package modeling [11]–[14]. For duplexers, a key specification is the rejection that has to reach 60 dB levels. To solve this challenging problem, we use the full-wave analysis tool, Ansoft HFSS, to simulate

the duplexer package. The model of the duplexer package is created using the 3-D modeler tool, which can be created in AutoCAD (Autodesk, San Rafael, CA) and imported into HFSS. After creating the dimensioned model in 3-D modeler, the boundary conditions and excitations should be carefully considered to get good results [12], [13]. Here we consider the modeling of 3.8 × 3.8 mm2 KPCS duplexer package that has the transmission center frequency fT = 1765 MHz and receiver center frequency fR = 1855 MHz. This is the ceramic surface mounted device (SMD) package. Fig. 2 shows the 3-D structure of this package. The ceramic layers, lid, and inner ground plane are displayed as wireframe in 3-D view to allow insight into the connecting structure. For the same reason, the ceramic layers are displayed as wireframe in the detailed view. The solder pads are on the bottom side of the first ceramic layer. The fourth ceramic layer comprises bonding pads for the bonding wire and a cavity, wherein the chip is mounted. The electrical connections between the bonding pads and solder pads are achieved through external castellations. The meander delay line inside the package gives the phase shift at the front of receiver channel. Two ladder filters are located inside the package. A cascaded T-network is used for transmitter path and a Pinetwork is used to implement the receiver path of the duplexer [15]. The die is mounted using conductive epoxy to ensure ground on the backside of the die. The connections between the die and package are made using gold bonding wires. The schematic of the duplexer assembly is shown in Fig. 3. For the duplexer package simulation, eight ports should be defined for the inner bonding pads that can be connected with die through bonding wires. The other three ports should be defined for the external signal pads. Fig. 4

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Fig. 3. Schematic of the duplexer assembly.

(b) Fig. 4. Port definition for 3.8 × 3.8 mm2 KPCS duplexer package.

shows the port definition for a 3.8 × 3.8 mm2 KPCS duplexer package. After finishing the duplexer package simulation, 11 by 11 S-matrix can be obtained. All the useful information for the package is included in the S-parameters. From port 1 to port 5, we can find the effect of meander delay line. Fig. 5 shows the magnitude and phase for the meander line from 1.65 to 1.95 GHz. Other scattering parameters also can be found but are neglected here because of the space limitation.

Fig. 5. Magnitude and phase for the effect of meander delay line from 1.65 to 1.95 GHz. (a) Magnitude of S15 . (b) Phase of S15 .

IV. Bonding Wire Modeling

III. Extraction of the Die Model

Bonding wires are extensively used in integrated circuit packaging and circuit design in RF applications. The RF designers usually use approximate analytical formulae for straight wires to estimate bonding wire inductance L and mutual inductance M as shown below [18], [19]. µ0 · l 2l · ln − 0.75 , (1) L≈ 2π r µ0 · l 2l D M≈ · ln −1+ , (2) 2π D l

The die model is comprised of die parasitics and acoustics models. We use Agilent Momentum to generate the die parasitics model. The acoustics model is obtained using the coupling-of-modes (COM) techniques [16], [17]. Fig. 6 shows the schematic diagram of the die with port assignments. Based on the die results, we can investigate the electrical response of transmitter and receiver path of the duplexer die and the effect of the phase shift in Agilent advanced design system (ADS). Fig. 7 shows a schematic of the connection for the die simulation and the electrical response of each path with an ideal 90◦ phase shift applied to the input of the receiver path. In Fig. 7(b), the solid line is for Tx. The dashed line is for Rx.

where µ0 is the permeability in free space, l is the wire length, r is the radius of the wire, and D is the distance between two wires. For the bonding wire with 2-mm long and 1-mil diameter, the formula can yield 2 nH for the inductance. So usually we can estimate the inductor value of the bonding wire by 1 nH/mm. As the circuits become more complex, the package and bonding wires become more complex as well. For the high frequency, the parasitics caused by bonding wires, mainly inductance and capacitance, can no longer be ignored and require careful modeling. The inductance of the bonding wire is shape dependent [20]. The general trend is that the larger curvature a wire has, the smaller its inductance.


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Fig. 8. The dimension of the bonding wire with the unit in mils and the port assignments.

Fig. 6. Schematic of the die with port assignments.

Fig. 9. Inductance for the bonding wire versus the frequency.

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The reason that curved wires have smaller inductance is due to the mutual inductance cancellation of the different segments of a single wire [21]. To get the accurate bonding wire model, we can calculate the S-parameters of the bonding wire in HFSS. Fig. 8 shows the dimension of the bonding wire and port assignments. Two lumped gap source ports are defined at both terminators of the bonding wire. After we get the S-parameters, the inductance can be calculated using the microwave network theory. The bonding wire can be considered as a series RL model. Therefore, for this two-port network, we have: R + jωL = Z0

(1 + S11 )(1 + S22 ) − S12 S21 . (1 − S11 )(1 + S22 ) + S12 S21

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From (3), the inductance can be obtained as shown in Fig. 9. From Fig. 9, we can find that the inductance almost keeps constant when the frequency changes from 1.65 to 1.95 GHz. In this case, we can use the constant inductor value for the single bonding wire instead of using the Sparameters. (b)

V. Total Response of the Package With Die

Fig. 7. Schematic of the connection and electrical response of the die with an ideal 90◦ phase shift applied to the input of the receiver path. (a) Schematic of the connection for the die simulation. (b) Simulated electrical response of the die.

After getting the S-parameters of the package, die, and bonding wires, we can assemble the results in Agilent ADS (http://eesof.tm.agilent.com). Fig. 10 shows the schematics of the assembled duplexer. In the bonding scheme, if the bonding wires are close to each other and are connected to the same bonding pad and metal pad in the die

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Fig. 10. Schematics of the assembled duplexer.

layout, the equivalent inductor is used as Leq71 and Leq72 in Fig. 10. If the bonding wires are close to each other and are connected to different bonding pad or a metal pad in the die layout, the mutual inductance is considered. The mutual inductive coupling coefficient is selected according to the distance. In Fig. 10, L23 is the mutual inductor between L2 and L3 . In the other cases, the mutual couplings are ignored. From our results, this approximation is reasonable and acceptable. Fig. 11 shows the simulated total frequency response of the KPCS duplexer. From Fig. 11, the insertion loss is less than 3 dB from 1.75 to 1.78 GHz for the Tx band and less than 1.8 dB from 1.84 to 1.87 GHz for the Rx band. But the isolation in the Tx band is about −52 dB from 1.7 to 1.75 GHz and about −55 dB from 1.75 to 1.78 GHz. In order to reach −60 dB, the redesign of the package, die, bonding wire scheme should be considered.

structure shown in Fig. 2, the chip is mounted on the inner ground plane, which is a big metal piece. If we cut the center part of the inner ground plane, part of the fields above the inner ground plane will be absorbed by dielectric material. The isolation then can be improved. We cut different shapes to investigate the effect and finally we find cutting the “x” shape from the center of inner ground plane can give us better isolation. Fig. 12 shows the inner view of the package with the modified inner ground plane. The isolation can be improved from 1.73 to 1.76 GHz at the Tx band, and 6.3 dB improvement is achieved at 1.75 GHz as shown in Fig. 13. The solid line shows the new results with the modified inner ground plane. The dashed line shows the results shown in Fig. 11. At the same time, the performance in passband can stay the same.

VI. Novel Methods to Improve the Isolation

Note that the rejection is also dependent on the bonding wire location due to the self and mutual inductances of the bonding wires. The inductive couplings between the bonding wires generate parasitic effects in the stopband [22]. After combining the results of the package and die in ADS, we can change the positions of bonding wires to investigate the influence of the bonding wires. For the bonding wire scheme shown in Fig. 14, the bonding wire positions are changed for the Rx ground, and two more bonding wires are added compared with the bonding wire scheme shown in Fig. 3. The inductance to ground for the Rx channel is reduced. The isolation can be improved from 1.65 to 1.8 GHz and 4.6 dB improvement is achieved at 1.75 GHz as shown in Fig. 15. The solid line shows the new results with the new bonding wire scheme shown in Fig. 14. The dashed line shows the simulation results shown in Fig. 11.

As mentioned above, small insertion loss and high isolation in stopband are the most important specifications in the SAW filter and duplexer package design. With higher frequency and smaller size of the duplexer package, we should have good physical understanding of each part of the package. We easily can analyze the influence of each part based on the methodology developed in this paper. For the KPCS duplexer package, we have found several novel methods to improve the isolation at the Tx stopband. A. Cut on Inner Ground Plane Good isolation is achieved by reducing the coupling between transmitter and receiver channels. For the package

B. New Bonding Wire Scheme


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Fig. 12. Inner view of the package with the modified inner ground plane.

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(c) Fig. 11. Simulated total frequency response of the KPCS duplexer. (a) Simulated frequency response in the Tx channel. (b) Simulated frequency response in the Rx channel. (c) Simulated isolation between Tx and Rx channels.

C. Adding Vias Between the Ground Bonding Pad and Inner Ground Plane The via has big influence on the package performance. Two vias are added between the inner Rx ground bonding pad and the inner ground plane as shown in Fig. 16 to reduce the inductance from the Rx ground bonding pad to the ground. The isolation can be improved from 1.74 to 1.79 GHz at the Tx band, and 6 dB improvement is achieved at 1.765 GHz as shown in Fig. 17. The solid line shows the new results with two more vias shown in Fig. 16. The dashed line shows the results shown in Fig. 11.

(b) Fig. 13. Comparison of the previous simulation results shown in Fig. 11 and the new results with modified inner ground plane. (a) Isolation between Tx and Rx channels. (b) Frequency response in the Tx channel. The solid line shows the new results with the modified inner ground plane. The dashed line shows the results shown in Fig. 11.

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Fig. 17. Isolation between the Tx and Rx channels. The solid line shows the new results with two more vias shown in Fig. 16. The dashed line shows the results shown in Fig. 11.

Fig. 14. New bonding wire scheme. The bonding wire positions are changed for Rx ground, and two more bonding wires are added.

Fig. 18. Twelve vias are added between the inner ground plane and the PCB ground.

D. Adding Vias Between the Inner Ground Plane and PCB Ground Fig. 15. Isolation between the Tx and Rx channels. The solid line shows the new results with the new bonding wire scheme shown in Fig. 14. The dashed line shows the simulation results shown in Fig. 11.

Fig. 16. Two vias are added between inner bonding pad and inner ground plane to reduce the inductance from the Rx ground bonding pad to the ground.

Next, we add twelve ground vias between the inner ground plane and printed circuit board (PCB) ground. These vias are distributed around the meander delay line and form a gridded ground wall to stop the electromagnetic coupling between the meander delay line and the other components [23]. This wall also reduces the impedance between the inner ground plane and PCB ground. Fig. 18 shows this new structure. Fig. 19 shows the comparison of results. The solid line shows the new results with twelve ground vias between the inner ground plane and PCB ground. The dashed line shows the results shown in Fig. 11. The isolation can be improved from 1.65 to 1.8 GHz, and 4.7 dB improvement is found at 1.75 GHz. The attenuation for the Tx channel is improved from 1.65 to 1.71 GHz and from 1.84 to 1.95 GHz.

VII. Experiment Results Based on our investigation of improving the isolation, the KPCS duplexer package is designed using the new


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fore, the empty fixture has excellent isolation among three ports. Fig. 21 compares the new simulated response of the duplexer with the de-embedded measurement of the duplexer. The previous simulated response shown in Fig. 11 is also put in Fig. 21 to see the improvement clearly. The solid line shows the measurement results on fixture. The long dash line shows the new simulation results. The long dot-dash line shows the previous simulated results shown in Fig. 11. From Fig. 21, we find the simulation can match the measurement results very well. Although we focus on investigating the methods to improve the isolation, we find from Fig. 21 that the passband performance is also improved. This shows the effectiveness of our methodology and the novel ideas we proposed here. There still are differences between the simulation and measurement results. This may come from the modeling assumptions. For example, the rectangular shape is used in our model for the cross section of the meander delay line that is not an ideal shape for the real package. We use the rectangular castellation instead of the curved one in our model to reduce the computation time. Due to the complex structure of the duplexer, we model the package, die, and bonding wires separately, then we combine these models together to obtain the total response. The couplings among these parts are ignored. Although we make these modeling assumptions, the results are good enough to analyze and design the duplexer package.

Fig. 19. Twelve ground vias are added between inner ground plane and PCB ground. (a) Isolation between the Tx and Rx channels. (b) Frequency response in the Tx channel.

VIII. Conclusions

Fig. 20. The package ready in the fixture for measurement.

bonding scheme and adding twelve ground vias between the inner ground plane and PCB ground. After fabrication, we measured the new KPCS duplexer package in the lab using a 2-port vector network analyzer (HP-8753, Hewlett-Packard, Palo Alto, CA). As apparent from the construction of the package, it is difficult to measure all possible port combination. Only the transmission parameters are extracted in the lab, then port extensions are applied to de-embed the line delay of the PCB. Fig. 20 shows the package ready in the fixture for measurement. Periodic holes are made in the PCB of fixture to reduce the coupling among the feeding transmission lines. There-

In this paper, a comprehensive methodology for analysis and design of the SAW filter and duplexer package is established. Ansoft HFSS is successfully used for the characterization of the RF SAW duplexer package. The die parasitics model is extracted by Agilent Momentum. Then the die model is obtained by combining the die parasitics and acoustics models. We also have discussed the bonding wire modeling method. After that, all the results are assembled in ADS to get the total response. Using this design method, the isolation of the KPCS duplexer package is significantly improved by redesigning the inner ground plane, bonding wire scheme, and ground via. Based on our investigation, a KPCS duplexer package is designed using the new bonding scheme and adding twelve ground vias between the inner ground plane and PCB ground. The excellent transmitter to receiver isolation in the transmission band is achieved. Simulation and measurement results are compared, and excellent agreement is found. Although we focus on investigating the methods to improve the isolation, the passband performance is also improved. The technique developed in this paper reduces the design cycle time greatly and can be applied to various RF SAW device packages.

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Acknowledgment The authors would like to thank Dr. Albert Gu for discussions; Shawn Hester and Chad Thompson for technical support; and Prof. Donald Malocha for suggestions and encouragements.

References

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(c) Fig. 21. Simulation and measurement results for new KPCS duplexer package. (a) Frequency response in the Tx channel. (b) Frequency response in the Rx channel. (c) Isolation between the Tx and Rx channels. The solid line shows the measurement results on fixture. The long dashed line shows the new simulation results. The long dotdashed line shows the previous simulated results shown in Fig. 11.

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Hao Dong (S’02) was born in Xi’an, Shanxi, P.R. China in 1969. He received the B.S. degree and the M.S. degree in automatic control from Northwestern Polytechnical University, Xi’an, Shanxi, P.R. China, in 1991 and 1994, respectively. He received the Ph.D. degree in industrial automation from Zhejiang University, Hangzhou, Zhejiang, P.R. China, in 1997. He is currently pursuing his Ph.D. degree in electrical engineering at the University of Central Florida, Orlando, FL, under the supervision of Prof. Thomas X. Wu. His current research includes electronic packaging and metamaterials.

Thomas X. Wu (S’97–M’99–SM’02) received the B.S.E.E. and M.S.E.E. degrees from the University of Science and Technology of China (USTC), Hefei, Anhui, in 1988 and 1991, and the M.S. and Ph.D. degrees in electrical engineering from the University of Pennsylvania, Philadelphia, PA, in 1997 and 1999. He was awarded the Prize of the President of the Chinese Academy of Sciences in 1991. After that, he was with the faculty of the Department of Electrical Engineering and Information Science at USTC as assistant and lecturer from 1991 to 1995. In the fall of 1999, he joined the School of Electrical Engineering and Computer Science (SEECS), University of Central Florida (UCF), Orlando, FL, as an assistant professor. Dr. Wu’s current research interests and projects include electronic packaging of RF SAW devices, complex media, liquid crystal devices, electrical machinery, magnetics and EMC/EMI in power electronics, chaotic electromagnetics, millimeter wave circuits, and CMOS/BiCMOS RFICs. He is now chairman of IEEE Orlando section and chairman of IEEE AP and MTT joint chapter. Recently, he was listed in Who’s Who in Science and Engineering, Who’s Who in America, and Who’s Who in the World.

857 Kamran S. Cheema received his B.S. and M.S. degree from the University of Central Florida, Orlando, FL, in 1998 and 2002, respectively. He joined Sawtek, Inc., Orlando, FL, as a SAW filter design engineer in 1998. While working at Sawtek, he pursued his graduate degree and researched electromagnetic (EM) modeling for SAW duplexers. He is currently manager of test and product engineering team at Sawtek.

Benjamin P. Abbott (S’84–M’86–S’88– M’89) was born in North Hampton, MA, on February 3, 1962. He received the B.S.E. degree, M.S.E. degree, and Ph.D. degree at the University of Central Florida, Orlando, FL, in 1984, 1986, and 1989, respectively. His research efforts have included the analysis and design of SAW IF filters, radio frequency filters and duplexers. He currently holds the position of research and development manager at Sawtek, Inc., Orlando, FL.

Craig A. Finch (M’00) graduated from the University of Illinois Urbana-Champaign, in 1997 with the B.S. degree in electrical engineering. He received the M.S. degree from the University of Central Florida, Orlando, FL, in 2001. He was employed by Sawtek, Inc., Orlando, FL, from 1997 to 2003, working on SAW filter design and package modeling. Currently he is a graduate student at the University of Central Florida. His research interests include mathematical modeling and computer simulation.

Hanna Foo was born in Malacca, Malaysia, on July 28, 1977. She received the B.S. and M.E. degrees in electrical engineering from the University of Wisconsin-Madison, Madison, WI, in 1999 and 2001, respectively. She is currently a design engineer for Sawtek, Inc., in Orlando, FL.