Next Generation High-Q Compact Size IPD Diplexer for RF Frond End SiP

Transcription

1 2017 IEEE 67th Electronic Components and Technology Conference Next Generation High-Q Compact Size IPD Diplexer for RF Frond End SiP Sheng-Chi Hsieh, Pao-Nan Lee, Hsu-Chiang Shih, Chen-Chao Wang, Teck Chong Lee Advanced Semiconductor Engineering Inc., Kaohsiung 811, Taiwan Abstract It is demonstrated that the high quality factor with multi-layers of thick metal spiral inductors and low harmonics power level can be achieved by using a mature glass substrate technology. The thicker copper metal stacking for inductor is greater than 30 μm. The quality factor of inductors can achieve in this study. In addition, the compact size of RF diplexer with thicker copper layer applied on WiFi SiP module is also demonstrated. The wifi module thickness should be below 1.1mm within 10x5mm area on coreless substrate. In order to reduce component size, the diplexer uses 2 layers of thicker metal as high-q spiral inductor. After design optimization, the diplexer size can be reduced from 1.6mmx0.8mm to 1mmx0.5mm and cost can be saved. It is very critical and beneficial with compact size diplexer for the WiFi SiP module of current and next generation s commercial mobile system application. Keywords- IPD; SiP; Wireless module; glass wafer; inductor I. INTRODUCTION Nowadays, a novel of packaging for Wi-Fi and bluetooth modules miniaturization is trending to combine with complex functionality elements by System-in- Package (SiP). Commonly, discrete components such as band pass filters, multiplexers, couplers and SAW components are the ceramics, and are often located external the ASIC packages. These kinds of components were occupied a big footprint for mounting and increases the layout challenge for wireless systems, especially in compact size of modules. Glass IPD has attracted much attention in the last few years, because its combination with ASIC IC to further reduce whole packages size and enhance functionality [1-5]. The glass IPD has been used to reach the size shrinkage in WiFi and bluetooth modules of mobile device utilizations. Currently, some of passive devices are manufactured on high resistivity silicon substrate. The problem with this is that the RF performance is restricted by conducting surface under high frequencies especially in harmonics distortion issues because of parasitic surface conduction (PSC) at the silicon substrate and silicon deoxide interface [6]. Glass wafer is an outstanding candidate to improve harmonics due to being a non-conductive material. In this paper, IPD is fabricated by a multi-layers processing technology and a high-effective of metal plating to form the high-conductivity copper for each metal layers on glass substrate. The structures contain the following features: 1) MIM (metal-insulator-metal) capacitor with high-dk dielectric layer to increase capacitance density; 2) thick plated copper can larger then 10-μm to decrease the connection resistance; 2) Two or three of metal layers stacking can achieve μm copper thickness to realize very high-q inductor; 4) Glass substrate to improve the substrate loss and reduce thickness to less than 150um. In this study, the RF diplexer employed on Wi-Fi module is designed with miniature size of 1000 μm by 500 μm. The diplexer high is less than 200um with glass thickness of 180um. The IPD contains four metal layers; two are thin AlCu layers and the others are thick Cu metal layers. The capacitor is composed by MIM configuration with a high-dk dielectric layer between the two AlCu layers. 3D-EM HFSS software is used to evaluate and simulate the RF performance of RF device. The RF performance measurement results show the insertion loss of RF diplexer on 2.4GHz-band and 5GHz-band are smaller than 0.4 and 0.8 db. This result shows a good agreement with simulation results. The rejection bands on 2.4 GHz and 5 GHz frequency also meet the WiFi SiP module requirement. Although the LTCC passive devices are more employed on wireless systems, they are hard to integrate with an ASIC IC by wire bonding and Flip chip bonding for the thin module. Normally, the LTCC diplexer thickness is greater than 300um. This paper also compares the functionality and performance of WiFi module between LTCC and IPD diplexers. The validation conducted over TX-EVM and RX-sensitivity on 2.4GHz/5GHz channel. Comparing the WiFi module s EVM and sensitivity for 2.4 GHz-band and 5 GHz-band between both LTCC and RF diplexers testing, the Pout linearity and loss of IPD design is superior to the LTCC design at 2.4GHz. Based on this result, the IPD diplexer can provide a better electrical performance and is a significant promise for RF frond end SiP applications /17 $ IEEE DOI /ECTC

2 Figure 1. Two and three metal stacking structure of proposed IPD 4-metal layer structure 5-meatl layer structure II. PLANAR HGIH QUALITY INDUCTOR The thick copper layer by metal stacking technology is shown in Figure 1. 1 st AlCu layer, high-dk layer and 2 nd AlCu layers are sputtered on glass substrate after RCA cleaning, respectively. The process method is compatible with photo lithography and dry etching to form the MIM capacitor with expected diameters. Next, the negative passivation of polymer layer is used to protect the MIM capacitors. Then, the 15μm thick 3 rd metal layer is plated on top of P1 polymer layer. After that, 4 th metal layer and Metal-5(M5) are also fabricated by electroplating. P2 and P3 are continued to cover M4 and M5 for protection and isolation. The two and three copper layers stacking for inductor are about μm as shown in Figure 2. Figure 3 demonstrates high quality factor for 3 nh spiral inductor by 800 μm internal diameter. At 1GHz, it exceeds 60 with a peak value of 100. III. IPD DIPLEXER DESIGN A RF diplexer is used to RF frond-end that implements two of frequencies band coexistence. The RF diplexer includes of a low band (LB) path which passes 2.4 GHz signals, a high band (HB) path which passes GHz signals, and an antenna port which received the coexistent WiFi signal from the 2.4 GHz and 5 GHz frequency ports. In this situation, the RF diplexer includes of a 2.4 GHz low pass filter and 5 GHz band pass filter connecting with a common port. Ideally, all the 2.4 GHz signal power on LB port is passed through to the common port and all the 5 GHz signal power on HB port is passed through to common port. The wireless signals from WiFi transmitter are frequencies separated as co-existing 2.4/5 GHz bands; therefore none of the 2.4 GH signal is isolated with 5 GHz band. Consequently, the signals at 2.4/5 GHz can coexist on common port without interfering with both frequencies bands. The schematic circuits of proposed IPD diplexer are shown in Figure 4. Couples of L-C resonator are to form the transmission zero and pass band. Thus, simple design should to meet the matching required, the lowpass filter and band pass filter of the diplexer can be independently designed. Figure 3. Quality factor of 2D spiral inductor with thick copper. Figure 2. SEM cross-section of thick metal 2 thick metal stacking 3 thick metal stacking. 1420

3 Figure 4. Two schematic circuits of proposed IPD diplexer for low band, high band. Figure 6. Comparison of diplexer dimension between 3- metal layers and 4-mateal layers IPD structure Figure 5. Outline of LTCC diplexer. Figure 5 is shown the outline of LTCC diplexer, the 4- pads on the component are LB port for WiFi 2.4GHz signal path, HB port for WiFi 5GHz signal path, a ANT (common) port for WiFi signal input/output, and a ground port connected to system ground, respectively. Dependent on LTCC standard pin construction and dimensions, the device area of IPD diplexer should match with a dimensions of 1000 μm by 500 μm. After design optimization, we choice the 4-metal layer structure to design IPD diplexer which it can has a 60% reduction then 3-metal layers of IPD diplexer as shown in figure 6. The dimensions and pin construction of IPD diplexer are compatible with the LTCC design as shown in Figure 7. The bonding pad pitch in the short side is 0.45um and the length is 1mm for the long side and 0.55mm for the short side. In this paper, 4- metal layers and MIM capacitor to act L/C tanks for diplexer design. Two thin metal layers, M1 and M2 with a High-Dk layer form the MIM capacitors. Two thick metal layers, M3 and M4 to form the inductors and signal interconnectors. Finally, the 15 um thick of Cu/Ni/Sn/Ag pad are fabricated by electroplating on top of M4, form the solder pad for SMT assembly. The device area is 1.0mm by 0.5mm and photographs as shown in figure 8. Based on EM-HFSS simulator, analyzing the RF performance results can be shown in Figure 9. A summary result between both diplexers is shown in table I. For 2.4 GHz, the loss and 2 nd harmonic rejection of IPD diplexer are the similar to LTCC solution; however the 3 rd harmonic rejection of IPD is better than LTCC component. Although the loss is similar to LTCC component, but the IPD diplexer has superior performance than LTCC component for second and third harmonics rejections. Figure 7. Bottom view of IPD diplexer Figure 8. Photograph of IPD Diplexer bottom-view side-view Figure 9. Simulation of S-Parameter 1421

4 Table I. Summary result for LTCC and IPD diplexers Low Band Freq. (MHz) LTCC IPD Insertion Loss(dB) Attenuation Low Band Freq. (MHz) LTCC IPD Insertion Loss(dB) Attenuation Figure 12. Connection of the setup used to measure harmonic level on IPD diplexer Figure 10. Optical photography of DUT RF probing Figure nd /3 rd harmonic level measurement results. Figure 11. DUT RF probing S-Parameter measurement results. IV. HARMONIC MEASUREMENT RF diplexer not only gives a low insertion loss, but also restricts harmonic interference in WiFi system for 2.4GHz/5GHz co-existence application. For example, critical performances are provided to high-order signal modulation levels not to degenerate the RX demodulation. Furthermore, wireless system is also must to restrict the harmonics interference due to the nonlinear problems in the passive circuits and the active devices. In this case, the Spec. for 2 nd harmonic is smaller than -85 dbm at input power of 30 dbm. The harmonic distortion of the IPD diplexer is measured. The test vehicle of the DUT diplexer is a 50Ω CPW feeding in order to RF probing as shown in Figure 10. Figure 11 shows the measurement of S-Parameter on 2.4GHz and 5GHz. As a result, the insertion loss has a good correlation with the simulation, the return loss also can below 10dB. Figure 12 shows the connection of the setup used to measure harmonic level on IPD diplexer. The signal generator provides the signal source and power amplifier enhances the drive signal as the input to the device-undertest (DUT). A low pass filter and a high pass filter are adopted to minimize the system harmonics and offer a high dynamic of power range for harmonics measurement at a desired frequency (2.4GHz). Then, power amplitude level of the harmonic is obtained by the spectrum analyzer. Figure 13 shows the 2 nd and 3 rd harmonic level measurement results of the IPD diplexer. The harmonic level measurement is obtained from the input power sweep from +5 to +30 dbm at 2.4GHz. As shown in the results, the harmonic levels of the IPD diplexer barely changes as the input power levels increase +5 to +30 dbm. The 2 nd and 3 rd harmonic values of IPD diplexer are lower than the specification. V. RF FROND-END ANALYSIS A RF diplexer acts like a Antenna switch when applied on wireless module by separating the signals to 2.4/5GHz channels. The system diagrams of WiFi function block is shown in the Figure 14. The RF front-end devices contain three major passive components which are Bandpass filter, Balun and Diplexer. In the traditional WiFi module, these devices are implemented as SMD which had occupied 1422

5 large footprint and thicker die thickness on the substrate. Figure 15 shows a cross-section view of WiFi module. The module dimension is 10 mm by 5mm with SMT assembly process and the module s height is about 1.1mm with LGA type. The thickest LTCC components on WiFi module include diplexer, band pass filter and balun with 0.33mm, 0.4mm and 0.55mm thickness. In this paper, the LTCC diplexer is replaced by IPD diplexer with about 0.2mm height as mentioned above. In addition, if the balun and band pass filter are also can replace by IPD solution or a combined IPD device into one device for multi-function, then the height of WiFi module can be further down to be below 0.8mm. This is significant improvement as it meets the market trend of thin SiP modules applied on mobile devices. Figure 16 shows the ADS schematic circuit and EM- HFSS model from common port to 2.4 GHz and 5 GHz channel. For 2.4 GHz path, the RF signal passes through the diplexer, band-pass filter and 2.4GHz balun. The 5 GHz RF signal goes through the diplexer and 5 GHz balun. All of module profile, RF path, connection and substrate loss have to consider by EM-HFSS simulation Figure 17 shows the S-parameter of RF frond-end for WiFi module. Figure 16. Wireless module EM simulation ADS circuit model EM-HFSS model. Figure 17. RF frond-end performance for WiFi 2.4 GHz, 5GHz, and ANT ports. Figure 14. Wireless module function block. Figure 15. Cross-section profile structure of wireless module. Table II. Comparison of RF performance for WiFi module Low Band Freq. (MHz) LTCC IPD Insertion Loss(dB) VSWR Attenuation High Band Freq. (MHz) LTCC IPD Insertion Loss(dB) VSWR Attenuation A compared performance between both solutions is shown in table II. The overall loss with IPD solution is about 1.6dB at 2.4 GHz and 2.1dB at 5 GHz. Most of which exceed the results of LTCC solution. In the antenna port, the voltage standing wave ratio (VSWR) is about 1.2 and 1.6 for 2.4 GHz and 5 GHz. Based on this result, IPD diplexer is not only has a compatible RF performance with LTCC diplexer for WiFi module but also can provide a benefit at miniature profile. VI. WIRELESS MODULE FUNCTION TEST In order to measure the wireless module performance, RF diplexer is assembled on the wireless module, thus the bottom side is assembled directly onto test board (EVB) 1423

6 with LGA. The function test of WiFi module on EVB is shown in Figure 18. Figure 19 is shown the function test results of TX-EVM and RX-Sensitivity by using IPD and LTCC diplexer, respectively. For TX-EVM and RXsensitivity measurement between LTCC and IPD diplexer, the wireless module with IPD has 1-5 db better than on LTCC diplexer, especially in low band. From this result, IPD diplexer can provide a better functionality in wireless module applications. Fig. 18 Optical photography of wireless module on test board VII. CONCLUSION An IPD diplexer with miniature dimensions has been demonstrated and employed on WiFi/Bluetooth module. The RF performance of IPD diplexer is better than LTCC solution, especially for second and third harmonic attenuation.the validation conducted over TX-EVM and RX-sensitivity on 2.4GHz/5GHz channel. Comparing the WiFi s channel EVM and sensitivity for 2.4 GHz and 5 GHz between LTCC and IPD diplexers, the Pout linearity and loss of IPD solution is superior to the LTCC solution at 2.4GHz. For the future RF SiP module application, the IPD diplexer not reaches the performance compared to LTCC solution on WiFi/Bluetooth module, but also has a miniature profile to manufacture than current LTCC solution. Furthermore, by integrating other passive devices, such as band pass filter and balun on the same glass substrate can further decrease the height of WiFI SiP modules for mobile device utilizations. ACKNOWLEDGMENT The authors would like to thank Aaron Hung for the assistance on wireless module measurement. REFERENCES [1] Z. Wu, J. Min, M. Kim, M. R. Pulugurtha, V. Sundaram, R. R. Tummala, " Design and Demonstration of Ultra-Thin Glass 3D IPD Diplexers," in Proc. ECTC, Las Vegas, USA, pp , June [2] S. Gandhi, P. M. Raj, B. C. Chou, P. Chakraborti, M. S. Kim, S. Sitaraman, H. Sharma, V. Sundaram, R. Tummala, " Ultra-Thin and Ultra-Small 3D Double-Side Glass Power Modules with Advanced Inductors and Capacitors," in Proc. ECTC, San Diego, USA, pp , May [3] T. C. Tang, K. H. Lin, Design of Antenna on Glass Integrated Passive Device for WLAN Applications, IEEE Antennas and Wireless Propagation Letters, vol. 12, pp , September [4] S. Hong, S. H. Kang, Y. Kim, C. W. Jung, Transparent and Flexible Antenna for Wearable Glasses Applications, IEEE Transactions on Antennas and Propagation, vol. 64, pp , July [5] J. Min, Z. Wu, M. R. Pulugurtha,V. Smet, V. Sundaram, A. Ravindran, C. Hoffmann, R. Tummala, " Modeling Design Fabrication and Demonstration of RF Front-End Module with Ultra-Thin Glass Substrate for LTE Applications," in Proc. ECTC, Las Vegas, USA, pp , June [6] C. R. Neve, D. Lederer, G. Pailloncy, D. C. Kerr, J. M. Gering, T. G. McKay, M. S. Carroll, and J.-P. Raskin, Impact of Si Substrate Resistivity on the Non-Linear Behavior of RF CPW Transmission line, in Proc. EuMIC, Amsterdam, Netherlands, pp , Oct Figure 19. Function test results for wireless module 1424