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FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators
NPE-00032; No of Pages 5 Nanotechnology and Precision Engineering xxx (xxxx) xxx
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FlexMEMS-enabled hetero-integration for monolithic FBAR-aboveIC oscillators Chuanhai Gao, Menglun Zhang ⁎, Yuan Jiang State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Available online xxxx Keywords: FlexMEMS Hetero-integration Film bulk acoustic resonator System-on-chip Oscillator
a b s t r a c t In this work, a monolithic oscillator chip is heterogeneously integrated by a film bulk acoustic resonator (FBAR) and a complementary metal-oxide-semiconductor (CMOS) chip using FlexMEMS technology. In the 3D-stacked integrated chip, the thin-film FBAR sits directly over the CMOS chip, between which a 4 μm-thick SU-8 layer provides a robust adhesion and acoustic reflection cavity. The proposed system-on-chip (SoC) integration features a simple fabrication process, small size, and excellent performance. The oscillator outputs 2.024 GHz oscillations of −13.79 dBm and exhibits phase noises of −63, −120, and − 136 dBc/Hz at 1 kHz, 100 kHz, and far-from-carrier offset, respectively. FlexMEMS technology guarantees compact and accurate assembly, process compatibility, and high performance, thereby demonstrating its great potential in SoC hetero-integration applications. Copyright © 2019 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction High-performance electronic devices translate to increasing demands for integration, size, cost effectiveness, and reliability. Various attempts to meet the upward-trending requirements of ever-changing electronic systems have been made, and highly integrated electronic devices have evolved over the last few decades to achieve improved micro-fabrication technology, such as complementary metal-oxidesemiconductor (CMOS). The use of such technology renders electronic devices smaller, smarter, and more portable, all of which are desirable in internet-of-things (IoT) applications. However, process incompatibility may be observed when fabricating highly integrated chips. The thermal budget and other physically and chemically destructive incompatibilities may lead to degradation of device performance. Therefore, various solutions allowing selective integration of highperformance devices with improved functional systems for heterointegration have been proposed.1–4 In monolithic integration, all modules of the electronic system are integrated on a single chip to form a compact on-chip system. In a system-on-chip (SoC) system, short electrical inter-connections are created to realize high integration, which helps high-speed systems reduce electromagnetic interferences and parasitic. Oscillators function as clock references and are necessary components of most electronic systems, such as long-term evolution radio systems, radar electronics, and medical equipment. Oscillators provide clocking functions to RLC tank circuits, piezoelectric ceramics, such as piezoelectric ceramic transducer resonators, and quartz crystal ⁎ Corresponding author. E-mail address: [email protected] (M. Zhang).
resonators. Over the last several decades, quartz crystals have become the fundamental timing material of choice. However, given the advent of more-advanced communication networks and today's big data landscape, high-speed and high-precision clocks are urgently needed. Quartz crystal oscillators provide oscillation frequencies ranging from a few kilohertz to several tens of megahertz in their fundamental mode. They can also output odd-frequencies by using phase-locked loop and overtone technologies.5 Unfortunately, the clocking requirements of today's applications have extended beyond the capabilities of quartz crystal-based oscillators. Quartz crystals present a number of drawbacks, including an inability to integrate onto silicon wafers, nonindustry standard manufacturing and packaging processes, and sensitivity to radiated electromagnetic noise, heat, shock, and vibration.6 These issues can be addressed by MEMS-based resonators. Silicon micro-mechanical resonators and bulk acoustic wave or surface acoustic wave (BAW/SAW) resonators have recently been introduced to the market.7–10 MEMS-based resonators have much smaller volume and can be integrated into chip packages, leading to extreme PCB realestate reductions. Commercial MEMS-based oscillators can mostly overcome problems with aging and simplify the mechanism of temperature compensation.11 BAW oscillators have also been proven to exhibit excellent performance. Texas Instruments Inc., for example, has integrated BAW oscillators into its first crystal-less microcontrollers (MCUs).12 Thus, these controllers have a smaller form factor. BAWs operate at frequencies much higher than those of quartz crystals and provide higher beating precision and smaller phase jitter. Chip-packaged BAW oscillators also offer ultra-clean clock references and wake up faster than quartz crystals. Therefore, high-performance BAW/SAW oscillators are good candidates for high-performance high-speed systems. For example, they boost high-speed state machines to improve processing
https://doi.org/10.1016/j.npe.2019.08.002 2589-5540/Copyright © 2019 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
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efficiency or communication systems and reduce bit error rates. However, to date, most MEMS-IC chips are configured with a twodimensional side-by-side layout or system-in-package (SiP) form4; a feasible scheme for even higher-level integration is the SoC solution, which also allows potential superior performance. Inspired by the features of FlexMEMS technology, we demonstrate a monolithic film bulk acoustic resonator (FBAR) oscillator in this work. In our previous work, we applied this technology to mechanically flexible MEMS devices.13–15 Herein, FlexMEMS is used for the heterogeneous assembly of process-incompatible devices. We believe that FlexMEMS technology is a simple and reliable solution that could realize the extensive integration of high-performance heterogeneous parts. 2. Design and fabrication process 2.1. Analysis of oscillator circuit module Generally, a classic oscillator consists of a tank circuit and gain devices. Oscillator circuit systems must meet the Barkhausen criteria, i.e., the loop gain must be equal to or greater than one at the target frequency of oscillation, and the phase shift around the loop should be zero or any integer multiple of 2π. Fig. 1(a) shows a simplified oscillator circuit with a Colpitts topology. In the circuit diagram, the tank circuit is composed of two capacitors and an inductor for a certain resonant frequency; the bipolar junction transistor (BJT) works as an inverter and provides a necessary loop gain. The inverters can also be valves, field effect transistors (FETs), or op-amplifiers. Colpitts oscillators have good frequency stability and can work at gigahertz frequencies. On the basis of Fig. 1(a), we obtained a simplified single-ended Colpitts oscillator by introducing a piezoelectric FBAR for frequency selection and replacing the BJT with a metal-oxide-semiconductor FET (MOSFET), as shown in Fig. 1(b). Herein, capacitors C1 and C2 and FBAR form the tank network and feedback loop. Voltage applied to the C2 terminal is fed to the MOSFET through the feedback loop, while the current gained in the MOSFET is released back to sustain oscillation. This oscillator outputs via the C1 terminal. During circuit analysis, the FBAR is considered to be equivalent to its modified Butterworth–Van Dyke (mBVD) model, as shown in Fig. 1(b). In the mBVD equivalent circuit, Rm, Lm, and Cm represent the motional resistor, inductor, and capacitor, respectively, C0 is the static capacitor, and R0 and Rs are the parasitic resistors. The FBAR has two resonant frequencies of the zero phase, i.e., a series resonant frequency (denoted as fs) and a parallel resonant frequency (denoted as fp). As shown in Fig. 2, the FBAR reactance is inductive within its bandwidth from fs to fp and capacitive from fp to fs. The FBAR operates in inductive mode in an oscillator so that its bandwidth is the frequency selection range. A quality factor Q was used to characterize the ability of a resonator for energy reservation. Q also describes the efficiency of oscillation and
Fig. 2. Imaginary part of the electrical impedance of an FBAR as a function of frequency.
determines the maximum attainable stability of the FBAR resonator. As Q increases, the reactance slope becomes steeper, leading to morestable oscillation. However, an excessively large Q initiates strict oscillatory conditions. In this case, a more comprehensive characterization of the resonator in an oscillator is the product of Q and resonance frequency. In summary, an oscillator circuit places higher requirements on FBAR resonators. 2.2. Design of transferable FBAR film An FBAR resonator operates as a frequency-selecting component in an oscillator and must meet the oscillatory condition at its most basic form in the design phase. Here, we estimated the structural parameters of the FBAR by using Electronic Design Automation software and then established a Mason model of the FBAR. The Mason model is a onedimensional electromechanical model that corresponds to the FBAR stack structure. With the expected performances, the 2-GHz FBAR stack is determined with a 360 nm Mo top electrode, 700 nm AlN piezoelectric layer, and 480 nm Mo top electrode. The maximum total thickness of the substrate-free FBAR stack is 1.5 μm, which greatly reduces the volume occupied by the resonator and achieves higher heterogeneity. Stamp-like anchor structures are also designed to facilitate the transfer process, as disclosed in our previous work.13–15 2.3. FlexMEMS-enabled hetero-integration The oscillator chip was fabricated using the GlobalFoundries 0.13 μm CMOS process. The diced chip measures 1 mm2 and is 200 μm thick. The
Fig. 1. (a) Simplified diagram of a classic LC Colpitts oscillator. (b) The Colpitts oscillator circuit in this work; here, the FBAR resonator is equivalent to an mBVD model.
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
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Fig. 3. Diagram of the hetero-integration process.
transferable FBAR film was prepared via a standard MEMS process. A thin Au/Cr layer was also deposited to achieve an electrical connection. The FBAR film was fabricated on a silicon wafer. Fig. 3 shows the hetero-integration enabled with FlexMEMS technology. First, a 4 μm-thick photolithographic SU-8 (GM 1040, Gersteltec) layer is spin-coated on the CMOS chip. After exposure and development, the electrode pad windows and an air cavity are exposed. The air cavity is essential in this process because it provides high-efficiency acoustic reflection.16 The transfer printing process closely follows a short post-bake of 85 °C for 5 min. During the transfer process, the FBAR film is picked up from the source silicon wafer and then transferred onto the SU-8 layer using a pre-prepared viscoelastic PDMS stamp. Continuous attachment retention of 120 °C for 5 min is necessary before lifting the PDMS stamp. In this procedure, relatively stable adhesion is achieved between the CMOS chip and FBAR film after curing of the SU-8 layer. Fig. 4(a) shows an FBAR film transferred above the CMOS chip. The adhesion provided by the cured SU-8 is much more robust than that provided by van der Waals forces. Finally, Au wire bonding to achieve an electrical connection between the CMOS chip and FBAR film was completed, as shown in Fig. 4(b).
the simulated one, thus indicating the correct prediction of our Mason model. The results also reveal that the FBAR film shows no degradation in electrical performance before and after the transfer printing process, which is due to our incorporation of FlexMEMS technology. The transferred FBAR film exhibits fs and fp at 2001.5 and 2040.5 MHz, respectively. The calculated Q × f product is 3.17 × 1012 s−1, which meets the requirements of oscillation.
3. Results and discussion We measured the performance of the FBAR film by using a 150 μmpitch GSG RF probe before and after the transfer printing process with an Agilent EB8601 vector network analyzer. Fig. 5 shows the measured impedance–frequency curves and the simulated results of the Mason model. Fig. 5 reveals that the two measured curves agree well with
Fig. 5. Comparison of impedance curves. The simulation results of the Mason model and the measured results before and after the transfer printing process are plotted.
Fig. 4. (a) Transferred FBAR film above the CMOS chip. (b) Short Au wire bonding for electrical connection.
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
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noise could be achieved with further optimization in follow-up work, for example, by using voltage stabilizing circuits and a low-noise power supply. In this work, the total thickness of the SU-8 layer and FBAR film is only about 5.5 μm. The FlexMEMS-enabled integration produces an oscillator chip of a smaller size compared with that produced by side-by-side layout integration or an SiP-packaged FBAR on a hard Si substrate.17–19 The proposed approach also allows short electrical connections to reduce electromagnetic interferences and parasitic. Using our FlexMEMS technology, the FBAR film retains high electrical performance before and after transfer printing. Simple processes and a low processing temperature render monolithic heterointegration easy to fabricate and cost effective. In future work, we will optimize the long-term stability of the oscillators, including, but not limited to, the design of the packaging and temperaturecompensating layers. 4. Conclusion In this work, hetero-integration of an FBAR-above-IC oscillator chip using FlexMEMS technology is proposed. The MEMS-based resonator film is monolithically integrated with an oscillator CMOS chip. The simple integration process achieves high device performance and represents a low-cost, high-efficiency, and easy-to-fabricate method for hetero-integration. In the experimental results, the monolithic Colpitts oscillator exhibits a highly integrated 3D stack structure, and the total thickness of the above-IC SU-8 layer and FBAR film is only about 5.5 μm. The oscillation frequency and phase noise at far-from-carrier offset are 2.024 GHz and − 136 dBc/Hz, respectively. The demonstrations in this study confirm that FlexMEMS technology is a promising approach for hetero-integration fabrication involving thin-film devices. FlexMEMS technology could also be applied to a wide range of heterointegration of micro-systems, such as small-sized smart devices for IoT applications, portable monolithic wireless systems, and timing devices. Acknowledgments This work was supported by National High Technology Research and Development Program of China (863 Program) under Grant No. 2015AA042603, the 111 Project under Grant No. B07014, and Nanchang Institute for Microtechnology of Tianjin University. References
Fig. 6. (a) Under-test system with the oscillator chip on PCB. (b) Measured output spectrum. (c) Measured phase noise.
We measured the integrated oscillator chip on a designed PCB circuit, as shown in Fig. 6(a). The FBAR-above-IC oscillator operated in single-ended mode and was characterized by using a spectral analyzer. Fig. 6(b) and (c) show the output spectrum and phase noise, respectively. The under-test system generated a − 13.79 dBm output oscillation signal at 2.024 GHz with a 1.6 V and 10 mA DC power supply. Phase noises of −63, −120, and − 136 dBc/Hz were measured at 1 kHz, 100 kHz, and far-from-carrier offset, respectively. Lower-phase
1. Tanaka S. Heterogeneous integration by wafer-to-wafer transfer technology. In: Proceedings of 2014 4th IEEE International Workshop on Low Temperature Bonding for 3D Integration, LTB-3D 2014. IEEE; 2014:33. doi:10.1109/LTB-3D.2014.6886172. 2. Small M, Ruby R, Ortiz S, Parker R, Zhang F, Shi J, Otis B. Wafer-scale packaging for FBAR-based oscillators. In: 2011 Joint Conference of the IEEE International Frequency Control and the European Frequency and Time Forum (FCS) Proceedings 2011:1–4. doi:, https://doi.org/10.1109/fcs.2011.5977848. 3. Park K, Esashi M, Tanaka S. Wafer-level heterointegration process of SAW devices and LSI. Proceedings - IEEE Ultrasonics Symposium IEEE 2010:1486-9https://doi. org/10.1109/ULTSYM.2010.5935651. 4. Fischer AC, Forsberg F, Lapisa M, et al. Integrating MEMS and ICs. Microsyst Nanoeng 2015;1(March):1-16https://doi.org/10.1038/micronano.2015.5. 5. ElBarkouky M, Vandersteen G, Wambacq P, Rolain Y. A 7 GHz FBAR overtone-based oscillator. In: European Microwave Week 2009, EuMW 2009: Science, Progress and Quality at Radiofrequencies, Conference Proceedings - 39th European Microwave Conference, EuMC 2009. Vol 1. ; 2009:318–321. doi:, https://doi.org/10.1109/EUMC.2009. 5296327. 6. Rohde UL, Poddar AK. Electromagnetic interference and start-up dynamics in high frequency crystal oscillator circuits. In: 33rd IEEE Sarnoff Symposium 2010, Conference Proceedings. 2010. doi:10.1109/SARNOF.2010.5469744. 7. Sridaran S, Fouquet J, Parker R, Small M, Ortiz S, Bi F, Gilbert SR, Callaghan L, Ruby R. Low jitter FBAR based chip scale precision oscillator. IEEE International Ultrasonics Symposium, IUS 2014:85-8https://doi.org/10.1109/ULTSYM.2014.0022. 8. Tanaka S, Park K, Esashi M. Lithium-niobate-based surface acoustic wave oscillator directly integrated with CMOS sustaining amplifier. In: IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. Vol 59. IEEE; 2012:1800–1805. doi:, https:// doi.org/10.1109/TUFFC.2012.2384. 9. Tanaka S, Yoshida M, Hirano H, Esashi M. Lithium niobate SAW device heterotransferred onto silicon integrated circuit using elastic and sticky bumps. IEEE
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Menglun Zhang (1988- ), male, Ph.D., assistant professor in Tianjin University, research interest includes piezoelectric film MEMS devices, [email protected].
Yuan Jiang (1991- ), male, Ph.D., research interest includes flexible electronics and MEMS devices, [email protected]. cn.
Chuanhai Gao (1994- ), male, master student in Tianjin University, research interest includes flexible MEMS electronics, [email protected].
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
Contents lists available at ScienceDirect
Nanotechnology and Precision Engineering journal homepage: http://www.keaipublishing.com/en/journals/nanotechnologyand-precision-engineering/
FlexMEMS-enabled hetero-integration for monolithic FBAR-aboveIC oscillators Chuanhai Gao, Menglun Zhang ⁎, Yuan Jiang State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China
a r t i c l e
i n f o
Available online xxxx Keywords: FlexMEMS Hetero-integration Film bulk acoustic resonator System-on-chip Oscillator
a b s t r a c t In this work, a monolithic oscillator chip is heterogeneously integrated by a film bulk acoustic resonator (FBAR) and a complementary metal-oxide-semiconductor (CMOS) chip using FlexMEMS technology. In the 3D-stacked integrated chip, the thin-film FBAR sits directly over the CMOS chip, between which a 4 μm-thick SU-8 layer provides a robust adhesion and acoustic reflection cavity. The proposed system-on-chip (SoC) integration features a simple fabrication process, small size, and excellent performance. The oscillator outputs 2.024 GHz oscillations of −13.79 dBm and exhibits phase noises of −63, −120, and − 136 dBc/Hz at 1 kHz, 100 kHz, and far-from-carrier offset, respectively. FlexMEMS technology guarantees compact and accurate assembly, process compatibility, and high performance, thereby demonstrating its great potential in SoC hetero-integration applications. Copyright © 2019 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction High-performance electronic devices translate to increasing demands for integration, size, cost effectiveness, and reliability. Various attempts to meet the upward-trending requirements of ever-changing electronic systems have been made, and highly integrated electronic devices have evolved over the last few decades to achieve improved micro-fabrication technology, such as complementary metal-oxidesemiconductor (CMOS). The use of such technology renders electronic devices smaller, smarter, and more portable, all of which are desirable in internet-of-things (IoT) applications. However, process incompatibility may be observed when fabricating highly integrated chips. The thermal budget and other physically and chemically destructive incompatibilities may lead to degradation of device performance. Therefore, various solutions allowing selective integration of highperformance devices with improved functional systems for heterointegration have been proposed.1–4 In monolithic integration, all modules of the electronic system are integrated on a single chip to form a compact on-chip system. In a system-on-chip (SoC) system, short electrical inter-connections are created to realize high integration, which helps high-speed systems reduce electromagnetic interferences and parasitic. Oscillators function as clock references and are necessary components of most electronic systems, such as long-term evolution radio systems, radar electronics, and medical equipment. Oscillators provide clocking functions to RLC tank circuits, piezoelectric ceramics, such as piezoelectric ceramic transducer resonators, and quartz crystal ⁎ Corresponding author. E-mail address: [email protected] (M. Zhang).
resonators. Over the last several decades, quartz crystals have become the fundamental timing material of choice. However, given the advent of more-advanced communication networks and today's big data landscape, high-speed and high-precision clocks are urgently needed. Quartz crystal oscillators provide oscillation frequencies ranging from a few kilohertz to several tens of megahertz in their fundamental mode. They can also output odd-frequencies by using phase-locked loop and overtone technologies.5 Unfortunately, the clocking requirements of today's applications have extended beyond the capabilities of quartz crystal-based oscillators. Quartz crystals present a number of drawbacks, including an inability to integrate onto silicon wafers, nonindustry standard manufacturing and packaging processes, and sensitivity to radiated electromagnetic noise, heat, shock, and vibration.6 These issues can be addressed by MEMS-based resonators. Silicon micro-mechanical resonators and bulk acoustic wave or surface acoustic wave (BAW/SAW) resonators have recently been introduced to the market.7–10 MEMS-based resonators have much smaller volume and can be integrated into chip packages, leading to extreme PCB realestate reductions. Commercial MEMS-based oscillators can mostly overcome problems with aging and simplify the mechanism of temperature compensation.11 BAW oscillators have also been proven to exhibit excellent performance. Texas Instruments Inc., for example, has integrated BAW oscillators into its first crystal-less microcontrollers (MCUs).12 Thus, these controllers have a smaller form factor. BAWs operate at frequencies much higher than those of quartz crystals and provide higher beating precision and smaller phase jitter. Chip-packaged BAW oscillators also offer ultra-clean clock references and wake up faster than quartz crystals. Therefore, high-performance BAW/SAW oscillators are good candidates for high-performance high-speed systems. For example, they boost high-speed state machines to improve processing
https://doi.org/10.1016/j.npe.2019.08.002 2589-5540/Copyright © 2019 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
2
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efficiency or communication systems and reduce bit error rates. However, to date, most MEMS-IC chips are configured with a twodimensional side-by-side layout or system-in-package (SiP) form4; a feasible scheme for even higher-level integration is the SoC solution, which also allows potential superior performance. Inspired by the features of FlexMEMS technology, we demonstrate a monolithic film bulk acoustic resonator (FBAR) oscillator in this work. In our previous work, we applied this technology to mechanically flexible MEMS devices.13–15 Herein, FlexMEMS is used for the heterogeneous assembly of process-incompatible devices. We believe that FlexMEMS technology is a simple and reliable solution that could realize the extensive integration of high-performance heterogeneous parts. 2. Design and fabrication process 2.1. Analysis of oscillator circuit module Generally, a classic oscillator consists of a tank circuit and gain devices. Oscillator circuit systems must meet the Barkhausen criteria, i.e., the loop gain must be equal to or greater than one at the target frequency of oscillation, and the phase shift around the loop should be zero or any integer multiple of 2π. Fig. 1(a) shows a simplified oscillator circuit with a Colpitts topology. In the circuit diagram, the tank circuit is composed of two capacitors and an inductor for a certain resonant frequency; the bipolar junction transistor (BJT) works as an inverter and provides a necessary loop gain. The inverters can also be valves, field effect transistors (FETs), or op-amplifiers. Colpitts oscillators have good frequency stability and can work at gigahertz frequencies. On the basis of Fig. 1(a), we obtained a simplified single-ended Colpitts oscillator by introducing a piezoelectric FBAR for frequency selection and replacing the BJT with a metal-oxide-semiconductor FET (MOSFET), as shown in Fig. 1(b). Herein, capacitors C1 and C2 and FBAR form the tank network and feedback loop. Voltage applied to the C2 terminal is fed to the MOSFET through the feedback loop, while the current gained in the MOSFET is released back to sustain oscillation. This oscillator outputs via the C1 terminal. During circuit analysis, the FBAR is considered to be equivalent to its modified Butterworth–Van Dyke (mBVD) model, as shown in Fig. 1(b). In the mBVD equivalent circuit, Rm, Lm, and Cm represent the motional resistor, inductor, and capacitor, respectively, C0 is the static capacitor, and R0 and Rs are the parasitic resistors. The FBAR has two resonant frequencies of the zero phase, i.e., a series resonant frequency (denoted as fs) and a parallel resonant frequency (denoted as fp). As shown in Fig. 2, the FBAR reactance is inductive within its bandwidth from fs to fp and capacitive from fp to fs. The FBAR operates in inductive mode in an oscillator so that its bandwidth is the frequency selection range. A quality factor Q was used to characterize the ability of a resonator for energy reservation. Q also describes the efficiency of oscillation and
Fig. 2. Imaginary part of the electrical impedance of an FBAR as a function of frequency.
determines the maximum attainable stability of the FBAR resonator. As Q increases, the reactance slope becomes steeper, leading to morestable oscillation. However, an excessively large Q initiates strict oscillatory conditions. In this case, a more comprehensive characterization of the resonator in an oscillator is the product of Q and resonance frequency. In summary, an oscillator circuit places higher requirements on FBAR resonators. 2.2. Design of transferable FBAR film An FBAR resonator operates as a frequency-selecting component in an oscillator and must meet the oscillatory condition at its most basic form in the design phase. Here, we estimated the structural parameters of the FBAR by using Electronic Design Automation software and then established a Mason model of the FBAR. The Mason model is a onedimensional electromechanical model that corresponds to the FBAR stack structure. With the expected performances, the 2-GHz FBAR stack is determined with a 360 nm Mo top electrode, 700 nm AlN piezoelectric layer, and 480 nm Mo top electrode. The maximum total thickness of the substrate-free FBAR stack is 1.5 μm, which greatly reduces the volume occupied by the resonator and achieves higher heterogeneity. Stamp-like anchor structures are also designed to facilitate the transfer process, as disclosed in our previous work.13–15 2.3. FlexMEMS-enabled hetero-integration The oscillator chip was fabricated using the GlobalFoundries 0.13 μm CMOS process. The diced chip measures 1 mm2 and is 200 μm thick. The
Fig. 1. (a) Simplified diagram of a classic LC Colpitts oscillator. (b) The Colpitts oscillator circuit in this work; here, the FBAR resonator is equivalent to an mBVD model.
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
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Fig. 3. Diagram of the hetero-integration process.
transferable FBAR film was prepared via a standard MEMS process. A thin Au/Cr layer was also deposited to achieve an electrical connection. The FBAR film was fabricated on a silicon wafer. Fig. 3 shows the hetero-integration enabled with FlexMEMS technology. First, a 4 μm-thick photolithographic SU-8 (GM 1040, Gersteltec) layer is spin-coated on the CMOS chip. After exposure and development, the electrode pad windows and an air cavity are exposed. The air cavity is essential in this process because it provides high-efficiency acoustic reflection.16 The transfer printing process closely follows a short post-bake of 85 °C for 5 min. During the transfer process, the FBAR film is picked up from the source silicon wafer and then transferred onto the SU-8 layer using a pre-prepared viscoelastic PDMS stamp. Continuous attachment retention of 120 °C for 5 min is necessary before lifting the PDMS stamp. In this procedure, relatively stable adhesion is achieved between the CMOS chip and FBAR film after curing of the SU-8 layer. Fig. 4(a) shows an FBAR film transferred above the CMOS chip. The adhesion provided by the cured SU-8 is much more robust than that provided by van der Waals forces. Finally, Au wire bonding to achieve an electrical connection between the CMOS chip and FBAR film was completed, as shown in Fig. 4(b).
the simulated one, thus indicating the correct prediction of our Mason model. The results also reveal that the FBAR film shows no degradation in electrical performance before and after the transfer printing process, which is due to our incorporation of FlexMEMS technology. The transferred FBAR film exhibits fs and fp at 2001.5 and 2040.5 MHz, respectively. The calculated Q × f product is 3.17 × 1012 s−1, which meets the requirements of oscillation.
3. Results and discussion We measured the performance of the FBAR film by using a 150 μmpitch GSG RF probe before and after the transfer printing process with an Agilent EB8601 vector network analyzer. Fig. 5 shows the measured impedance–frequency curves and the simulated results of the Mason model. Fig. 5 reveals that the two measured curves agree well with
Fig. 5. Comparison of impedance curves. The simulation results of the Mason model and the measured results before and after the transfer printing process are plotted.
Fig. 4. (a) Transferred FBAR film above the CMOS chip. (b) Short Au wire bonding for electrical connection.
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002
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noise could be achieved with further optimization in follow-up work, for example, by using voltage stabilizing circuits and a low-noise power supply. In this work, the total thickness of the SU-8 layer and FBAR film is only about 5.5 μm. The FlexMEMS-enabled integration produces an oscillator chip of a smaller size compared with that produced by side-by-side layout integration or an SiP-packaged FBAR on a hard Si substrate.17–19 The proposed approach also allows short electrical connections to reduce electromagnetic interferences and parasitic. Using our FlexMEMS technology, the FBAR film retains high electrical performance before and after transfer printing. Simple processes and a low processing temperature render monolithic heterointegration easy to fabricate and cost effective. In future work, we will optimize the long-term stability of the oscillators, including, but not limited to, the design of the packaging and temperaturecompensating layers. 4. Conclusion In this work, hetero-integration of an FBAR-above-IC oscillator chip using FlexMEMS technology is proposed. The MEMS-based resonator film is monolithically integrated with an oscillator CMOS chip. The simple integration process achieves high device performance and represents a low-cost, high-efficiency, and easy-to-fabricate method for hetero-integration. In the experimental results, the monolithic Colpitts oscillator exhibits a highly integrated 3D stack structure, and the total thickness of the above-IC SU-8 layer and FBAR film is only about 5.5 μm. The oscillation frequency and phase noise at far-from-carrier offset are 2.024 GHz and − 136 dBc/Hz, respectively. The demonstrations in this study confirm that FlexMEMS technology is a promising approach for hetero-integration fabrication involving thin-film devices. FlexMEMS technology could also be applied to a wide range of heterointegration of micro-systems, such as small-sized smart devices for IoT applications, portable monolithic wireless systems, and timing devices. Acknowledgments This work was supported by National High Technology Research and Development Program of China (863 Program) under Grant No. 2015AA042603, the 111 Project under Grant No. B07014, and Nanchang Institute for Microtechnology of Tianjin University. References
Fig. 6. (a) Under-test system with the oscillator chip on PCB. (b) Measured output spectrum. (c) Measured phase noise.
We measured the integrated oscillator chip on a designed PCB circuit, as shown in Fig. 6(a). The FBAR-above-IC oscillator operated in single-ended mode and was characterized by using a spectral analyzer. Fig. 6(b) and (c) show the output spectrum and phase noise, respectively. The under-test system generated a − 13.79 dBm output oscillation signal at 2.024 GHz with a 1.6 V and 10 mA DC power supply. Phase noises of −63, −120, and − 136 dBc/Hz were measured at 1 kHz, 100 kHz, and far-from-carrier offset, respectively. Lower-phase
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Menglun Zhang (1988- ), male, Ph.D., assistant professor in Tianjin University, research interest includes piezoelectric film MEMS devices, [email protected].
Yuan Jiang (1991- ), male, Ph.D., research interest includes flexible electronics and MEMS devices, [email protected]. cn.
Chuanhai Gao (1994- ), male, master student in Tianjin University, research interest includes flexible MEMS electronics, [email protected].
Please cite this article as: C. Gao, M. Zhang and Y. Jiang, FlexMEMS-enabled hetero-integration for monolithic FBAR-above-IC oscillators, , https:// doi.org/10.1016/j.npe.2019.08.002