Modeling, Design, Fabrication and Demonstration of RF Front-End 3D IPAC Module with Ultra-thin Glass Substrates for LTE Applications

Junki Min, Zihan Wu, Markondeya Raj Pulugurtha, Vanessa Smet, Venky Sundaram and

Rao Tummala 3D System Packaging Research Center

Georgia Institute of Technology Atlanta GA USA

e-mail: junki.min@ece.gatech.edu

Arjun Ravindran and Christian Hoffmann RF Module Team TDK-Epcos Inc.

Maitland FL USA e-mail: arjun.ravindran@epcos.com

This paper demonstrates, for the first time, an integrated radio frequency (RF) front-end module (FEM) with precision matching circuits in ultra-miniaturized glass substrates for LTE applications. Through full-wave electromagnetic (EM) simulations, electrical performance of these glass-based long term evolution (LTE) packages is compared with traditional RF modules with surface mount devices (SMDs), and organic laminates with embedded passives and actives. RF front-end modules with 3D or double-side thinfilm passive components on glass-based substrates are fabricated and characterized to correlate their performance with EM simulations.

Keywords-component; RF module; Glass substrate; TPV; Embedded passives; Double-side assembly

I. INTRODUCTION Today’s RF modules are predominantly single or

multichip modules in organic laminates or low temperature co-fired ceramic (LTCC) substrates [1]. Organic substrates that are 0.3-0.5 mm thick, with up to two-metal layers on each side of the core are the preferred platform for today’s modules. The need for form-factor reduction in radio sub-systems in both z and x-y direction has led to the evolution of embedded die-package architectures with dies facing up or down. This also reduces insertion loss and improves signal integrity by minimizing package parasitics. Embedded wafer-level ball grid array (eWLB) approaches based on fan-out wafer-level package (FO-WLP) have achieved improved miniaturization and performance.

Georgia Tech PRC and its partners are pioneering an alternative concept to achieve the same goals of miniaturization and performance and therefore propose to lower the cost by panel-based processes using three dimensional integrated passive devices (3D IPDs) concept, as the next stage of evolution, beyond LTCC and organic multi-chip modules (MCMs) and embedded modules. The three dimensional integrated passive and active component (3D IPAC) or 3D IPD RF module starts with an ultra-thin substrate (30-100 microns) made of glass, with ultra-low electrical loss and ultra-short through-package vias for double-sided assembly of active and passive components separated by only about 50 μm in interconnect length. Actives and passives are assembled on both sides of the thin

glass using ultra-short and low-temperature interconnections. The module also integrates thermal and shielding functions with innovative materials and structures.

Glass as a core substrate provides several benefits such as: ultra-low loss similar to ceramics; ultra-high precision circuits similar to silicon; excellent thermal stability for 1μm layer-to-layer registration; larger panel processing for low cost; lower warpage than organic packages; ultra-thin (30-100μm) and ultra-smooth surfaces (1-2nm roughness); high density through Cu vias for improved heat transfer; ultra-short interconnections between actives and passives enabled by though vias and double-side component assembly [3][4].

To reduce the size of RF modules, the RF passives research begins with miniaturizing passives with thin-films technologies. This paper demonstrates a 3D IPAC-based RF Front-end module with embedded matching circuits on ultra-thin glass substrates for LTE applications. The cross section of 3D IPAC module is illustrated in Fig. 1. In this study, matching networks are integrated directly onto ultra-thin glass substrates for minimizing the routing issues and associated parastics. Modeling of various values of thinfilm passives is also performed to optimized better RF performance.

II. DESIGN OF RF MODULE The RF front-end module consists of a switch, surface-

mount surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters with embedded matching networks [4].

Figure 1. Cross-section view of FEM 3D IPAC module.

2016 IEEE 66th Electronic Components and Technology Conference

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DOI 10.1109/ECTC.2016.310

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The design and layout was addressed based on the size of each RF component and its performance through EM simulation with embedded passives. All embedded passives, optimized by HFSS and SONNET EM simulators, are included in the RF front-end module. To measure the switch modules with many ports, termination pads for each probing port are used to reduce the impedance mismatch. The materials and stack up for 3D IPAC module are shown in Table I.

TABLE I. MATERIALS FOR MODULE STACK UP

The RF functional front-end module (FEM) with single-side components consists of a switch, surface-mount type low-band filters on the bottom side and high-band filters on the top side with embedded matching networks. The layouts for RF FEM module with single-side assembly are shown in Fig. 2.

Figure 2. Functional module design with embedded passives: (a) low-band filter, (b) high-band filter, and (c) RF switch.

Figure 3. Simulation results of low-band filter w/wo embedded matching circuits: (a) insertion loss, (b) isolation, (c) return loss, and (d) Impedance in smith chart.

Figure 4. Simulation results of high-band filter w/wo embedded matching circuits: (a) insertion loss, (b) isolation, (c) return loss, and (d) Impedance in smith chart.

In the low-band operation, the filter has to reject the high-band, whereas in the high band operation, it rejects the low-band. In order to improve RF performance, the functional modules of SAW and BAW filters include embedded inductors as a matching network. All embedded passives such as inductors were simulated by using 2.5D and 3D EM simulators and then the functional and full-

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chain modules with matching networks were simulated through ADS circuit simulator. Return loss and flatness in passband are improved with design optimization of the matching networks, as illustrated in Fig. 3 (low-band filter), Fig. 4 (high-band filter) and Fig. 5 (RF switch).

Figure 5. Simulation results of RF switch w/wo embedded matching circuits: (a) insertion loss, (b) isolation, (c) return loss, and (d) Impedance in smith chart.

Figure 6. Design of RF front-end module for single-side with LGA.

The RF diversity module consists of a switch, surface-mount SAW and BAW filters with embedded matching networks. The layouts for RF full-chain diversity module with single-side assembly are shown in Fig 6. The fabricated test vehicle is assembled with RF switch and

filters. After the electrical characterization, the designs will be further optimized for a double-side four-metal layer test vehicle with TPVs.

III. FABRICATION OF ULTRA-THIN GLASS SUBSTRATE

A. Glass Substrate

Figure 7. Fabrication process flows of the glass substrate with TPV.

The 4ML samples were fabricated using 100 um ultra-thin glass cores [5][6]. Low-loss dielectric films were laminated onto bare glass. The panels were laser-drilled to form 100 micron diameter TPVs in glass. After that, the TPV-drilled panels were coated with electroless copper, followed by lithography and metallization of inner layers. The targeted copper thickness for inner metal layers is 8 um. After plating was completed for the inner layers, outer dielectric layers were laminated and the samples were laser-drilled to form blind vias. The drilled samples went through the identical semi-additive process as the inner layers. Electroless and electrolytic plating were completed, followed by solder mask lamination and ENIG plating for surface finish. RF functional modules with embedded matching networks were also fabricated with two metal

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layers on 100 um thick glass substrates. Fabrication process flows of the glass substrate with TPV are shown in Fig. 7.

B. TPV Double-via approach for TPV fabrication in glass has

been developed and adopted for 4-metal layer module fabrication. The idea of double-via is to firstly drill through bare glass panel to form through glass vias. During the dielectric layer (primer) lamination and curing process, the polymer flows into the glass vias and fills them completely. After second time TPV drilling at the same glass via locations, smaller vias are formed inside the original pre-drilled glass vias, creating the double-vias. Since there is still certain amount of polymer coating on the TPV sidewall, standard SAP process can be easily applied for TPV metallization. The fabricated test vehicles with embedded passives of low band filter, high band filter and RF switch are illustrated in Fig. 8.

Figure 8. Fabricated test vehicle with embedded passives: (a) low-band filter, (b) high-band filter, and (c) RF switch.

IV. SINGLE SIDE ASSEMBLY RF switch and SAW filters are assembled on the top side

of the glass substrate while the bottom side is assembled directly onto PCB with LGA. Assembly was performed using solder reflow with a flip-chip bonder. The assembly results of the 2ML functional modules are shown in Fig. 9.

Although the functional modules can be measured directly by probing the surface, they were designed to be measured through RF evaluation board with surface mount connectors (SMA connectors). The board-level assembly was achieved using land grid array (LGA). The solder was placed onto the backside of the glass package and then attached to the evaluation board and reflowed. The results of the fabricated and assembled 4ML full-chain module with TPV integration are shown in Fig. 10.

Figure 9. Fabricated and assembled test vehicle with embedded passives: (a) low-band filter, (b) high-band filter, and (c) RF FEM module.

Figure 10. Demonstration of full-chain RF LTE module on glass: (a) cross-sectional view of full-chain module, (b) high-band SAW filter assembly on glass, and (c) full-chain module mounted on evaluation board.

V. MEASURED PERFORMANCE OF THE MODULE Following fabrication and component assembly, the low-

band and high-band filters were characterized using a Vector Network Analyzer (VNA) with RF probes. Two types of surface-mounted SAW and BAW type filters with embedded matching network are characterized. Prototypes of RF front-end module with component assembly are measured. Initial measurements showed good correlation

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between simulation and measurements, indicating good performance of the embedded matching network.

A. Low-band and High-band Filter

Following fabrication and component assembly, the low-band and high-band filters were characterized using a Vector Network Analyzer (VNA) with RF probes.

Figure 11. Measurement results of low-band filter: (a) insertion loss, (b) isolation, and (c) return loss.

Figure 12. Measurement results of high-band filter: (a) insertion loss, (b) isolation, (c) return loss.

The measurement results for two types of surface-mounted SAW and BAW type filters with embedded matching network are shown in Fig. 11 and Fig. 12. The measurement of the low-band filter showed a very good correlation between simulations and measurements, indicating good performance of the embedded matching network. The high-band filter measurement deviated from the simulation results, which is attributed to the filter-ground inductance implementation. The lower return loss than anticipated from simulations for high-band filters is also attributed to the variation in the filter ground inductance implementation.

B. RF Front-End Module This RF front-end module consists of an RF switch, and

SAW and BAW surface-mount filters. Matching networks were integrated for all the components and were embedded as thinfilm passives. The test-vehicle is assembled with switches and filters and characterized. In order to measure the switch module without any reflection from the open ports of the switch, SMA-type 50 ohm terminations are connected during the measurement. Testing of RF front-end module showed good yield with continuous end-to-end connection, thus, demonstrating basic technology building blocks. Good match between designed and fabricated filters confirmed the performance benefits with 3D IPAC LTE module along with miniaturization.

VI. SUMMARY OF PROTOTYPE RESULTS An ultra-thin RF front-end module that offers high-

performance and low cost for RF matching circuits is presented. This approach involves an ultra-thin 3D glass package with through vias. Circuit modeling is employed to determine the passive component design and to improve the RF characteristics. Simulation results are also correlated with measurement results to determine possible EM coupling in a 3D IPAC package.

ACKNOWLEDGMENT The authors would like to thank Chris White and Jason

Bishop for help with fabrication and assembly and Stacy Dudley and Dr. Xiaomin Yang in TDK-Epcos Inc. for help with RF module with RF switch measurements. Additionally, the authors would like to thank the industry sponsors of the consortia program at GT-PRC for their technical guidance and support.

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[2] B. C. Ham et al., “A GPS/BT/WiFi triple-mode RF FEM using Siand LTCC-based embedded technologies,” in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2012, pp. 1–3

[3] V. Sukumaran, et al., “Design, Fabrication and Characterization of Low-Cost Glass Interposers with Fine-Pitch Through-Package Vias,” ECTC 2011,61st, pp.583-588

[4] Y. Sato, et al., “Ultra-miniaturized and surface-mountable glass-based 3D IPAC packages for RF modules,” ECTC 2013,63rd, pp.1656-1661

[5] V. Sridharan, et. al, “Design and Fabrication of Bandpass Filters in Glass Interposer with Through-Package-Vias (TPV),” in Electronic Components and Technology Conference (ECTC), 2010

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