Non-volatile RF and mm-wave Based on Monolayer hBN

Myungsoo Kim1, Emiliano Pallecchi2, Ruijing Ge1, Xiaohan Wu1, Vanessa Avramovic2, Etienne Okada2, Jack C. Lee1, Henri Happy2, Deji Akinwande1 1Microelectronics Research Center, The University of Texas at Austin, Austin, TX 78758. Email: [email protected] 2IEMN, University of Lille, CNRS UMR8520, Avenue Poincaré, CS-60069, 59652 Villeneuve d'Ascq, France

Abstract—Non-volatile radio-frequency (RF) switches based switching time (<15 ns), and small footprint. Furthermore, the on hexagonal boron nitride (hBN) are realized for the first time switching voltage of ~1 V is compatible with many with low insertion loss (” 0.2 dB) and high isolation (• 15 dB) semiconductor technologies for practical integration. up to 110 GHz. Crystalline hBN enables the thinnest RF II. DEVICE STRUCTURE device with a single monolayer (~0.33 nm) as the memory layer owing to its robust layered structure. It affords ~20 dBm A. Fabrication power handling, 10 dB higher compared to MoS2 switches due An atomic-resolution tunneling electron microscopy (TEM) to its wider bandgap (~6 eV). Importantly, operating image (Fig. 2) shows the representative honeycomb structure frequencies cover the RF, 5G, and mm-wave bands, making of hBN with a lattice constant of ~0.25 nm, indicating the this a promising low-power switch for diverse communication growth of high-quality monolayer hBN. The inset is an image and connectivity front-end systems. Compared to other switch of CVD-grown hBN transferred onto Si/SiO2 4-inch wafer. technologies based on MEMS, , and phase-change Raman spectroscopy (Fig. 3) was used to characterize the hBN memory (PCM), hBN switches offer a promising combination sheet on Si/SiO2 substrate showing the expected spectrum. Fig. of non-volatility, nanosecond switching, power handling, high 4a shows the key steps for the fabrication of hBN RF switches. figure-of-merit cutoff frequency (43 THz), and heater-less First, a ground-signal-ground (GSG) pad and bottom electrode ambient integration. Our pioneering work suggests that (BE) was patterned by electron beam lithography (EBL) on a atomically-thin nanomaterials can be good device candidates cleaned diamond substrate. Then 100 nm-thick gold film was for 5G and beyond. deposited by electron beam evaporation. After the monolayer hBN was transferred onto the BE using a wet transfer method, I. INTRODUCTION 200 nm-thick gold top electrode (TE) was prepared using the In recent years there has been increasing interest in low- same fabrication process as BE. Fig. 4b represents a top-view power RF switches particularly for mobile communication optical image of a fabricated hBN RF switch with Au systems and reconfigurable platforms [1] with multiple-input- electrodes. The dashed circle in Fig. 4b marks the area where multiple-output. In the ideal limit, an RF switch will consume the vertical metal-insulator-metal (MIM) structure is located zero DC power, and finite power only during a switching event. (see Fig. 4c for zoomed-in SEM image). Fig. 4d represents an To realize this ideal switch, memory devices functioning as illustrated side view of the vertical heterostructure of RF switch non-volatile switches at GHz frequencies have been under on a diamond substrate. Since the overlap area of the device is continuous development [2-4]. The basic requirement of nanoscale, it is vulnerable to Joule heating failure. Therefore, memory device as RF switch is low ON-state resistance (RON < we used the diamond substrate for efficient heat dissipation. 5 Ÿ), and low OFF-state capacitance (COFF) in order to achieve B. Characterization a high figure-of-merit cutoff frequency (Fco=1/2ʌRONCOFF) in the THz range. Fig. 5 shows a representative I-V curve of Au/monolayer Towards this end, we previously reported memory effect in hBN/Au memory device. Originally, the device is at high resistance state (HRS), before a positive bias is applied to MoS2 monolayers, which afforded outstanding non-volatile RF switch the device to low resistance state (LRS). The voltage switches with Fco ~ 70 THz [5]. However, the power handling was quite low (~10 dBm), limiting its applications. In this work, sweeps from 0 to 1.5 V and back to 0 V, then reverse bias is we report the first demonstration of monolayer hBN as a non- applied from 0 to -1 V. At about 1.5 V, the current abruptly volatile RF switch that can afford 10x more power handling increases, indicating a transition (SET process) from HRS to LRS. While at about -1V, the device changes from LRS to HRS than MoS2 switches owing to its larger bandgap and thermomechanical stability. OFF-state power, limited by self- (RESET process). Fig. 6 represents the ON and OFF-state DC switching is ~20 dBm while ON-state power is about 30 dBm. current measured at 0.1 V showing stable retention over a week In contrast to other RF switch technologies, this all solid-state at room temperature. On-going measurements indicate a switch enables heater-less and simple planar integration retention over a month (not shown). Low compliance current compared to phase-change [3, 6] and MEMS [7] switches, was used for the retention time measurements. In addition to respectively. Moreover, hBN switches afford higher operating DC operation, 15 ns pulse is sufficient to switch the states (Fig. frequencies (>100 GHz) and figure of merit (Fig. 1), faster 7). The I-V curves before and after applying a pulse present the switching from OFF-state to ON-state.

978-1-7281-4032-2/19/$31.00 ©2019 IEEE 9.5 .1 IEDM19-194 Authorized licensed use limited to: University of Texas at Austin. Downloaded on May 11,2020 at 16:38:13 UTC from IEEE Xplore. Restrictions apply. III. RF SWITCH C. Power Handling The RF switches were characterized using two on-wafer Power measurements for the hBN switch at 40 GHz are high-frequency probe stations. Both setups are equipped with a presented in Fig. 12a, showing that POUT increases linearly with source monitoring unit from Keysight for IV measurement and PIN with negligible compression up to 30 dBm in the ON-state. DC switching of devices. The first is a 110 GHz probe station Fig. 12b plots the power handling results in the OFF-state, with a vector network analyzer (VNA) for measuring the S- which features a linear profile up to 19 dBm when the isolation parameters after proper calibration. The second probe station is suddenly vanishes. This indicates ‘self-switching’ of the device a non-linear VNA setup dedicated to power measurements at from the OFF to the ON state. The self-switching is non- 40 GHz, with a maximum CW power of 30 dBm at the probe volatile, i.e. the switch does not return to the HRS with power tips. The insertion losses measured for the same hBN switch in off. We verified this by re-measuring the device. Moreover, the both setups are consistent and within 0.1 dB. hBN device is not damaged, as it can be again switched from the ON to the OFF state with a normal DC sweep. A. Frequency Performance Insertion loss and isolation of a representative monolayer IV. BENCHMARKING hBN RF switch with sub-micron dimensions are shown in Fig. Table 1 benchmarks the hBN device to other switch 8. The device showed a relatively flat insertion loss, less than technologies. The hBN switch, though at a nascent stage, 0.2 dB, till 110 GHz (Fig. 8a). The extracted RON is ~ 1.6 Ÿ. affords higher power among solid-state switches, and shows The isolation performance was similarly wideband with 20 dB higher Fco than PCM, memristive, and MEMS switches with or higher at 100 GHz and below, and ~15dB at 110 GHz (Fig. shorter switching time and ambient integration. Compared to 8b). The extracted COFF is ~ 2.3 fF. The corresponding Fco ~ our prior MoS2 switch [5], the hBN RF switch offers 10 dB 43 THz is more than 50% higher than non-volatile phase- more power though with lower Fco owing to the larger size. change, and volatile MEMS switches [3, 7]. Fig. 8c is the Further size scaling of hBN switch will lead to even higher Fco. equivalent lumped circuit model of the hBN RF switch V. CONCLUSION consisting of an RON and CON in the ON-state, and COFF in the OFF-state. The other RLC components in the model are the In summary, non-volatile RF switches based on monolayer interconnect parasitics which are de-embedded with on-chip hBN have been demonstrated for the first time featuring higher THRU and OPEN devices. power among solid-state switches, and nanosecond switching. The hBN switch is quite versatile and can be used in tunable Operating frequencies exceed 100 GHz covering many small-range varistor or attenuator applications as shown in Fig. practical applications from IoT to 5G to mm-wave systems. 9a, where the insertion loss and RON is dependent on the Importantly, higher frequencies and isolation can be achieved compliant current (Fig. 9b) at which the device switches to the with size scaling, and higher power by using bilayer hBN. ON-state. Remarkably, a (slight) upswing is seen in the insertion loss (Fig. 8a and 9a), which is attributed to the ACKNOWLEDGMENT extreme thinness of monolayer hBN resulting in an appreciable The authors acknowledge GrollTex, Inc. for wafer-scale hBN samples and materials characterization. This work is C that produces a high frequency zero, therefore, mitigating ON funded in part by a PECASE award (D.A), and NSF ECCS- R and reducing the insertion loss. This favorable effect is not ON 1809017 grant. observed in other switch technologies, illustrated in Fig. 9c. B. Area Scaling REFERENCES In conventional transistor, PCM, and MEMS switches, RON [1] R. H. Olsson, K. Bunch, and C. Gordon, "Reconfigurable Electronics for is inversely, and COFF is directly proportional to area, Adaptive RF Systems," in IEEE Compound Semiconductor Integrated respectively, resulting in an area scaling that trades insertion Circuit Symposium (CSICS), pp. 1-4, 2016. [2] S. Pi, M. Ghadiri-Sadrabadi, J. C. Bardin, and Q. Xia, "Nanoscale loss against isolation. In switches based on atomically-thin memristive radiofrequency switches," Nature communications, vol. 6, p. materials, RON is ~area-independent, and COFF remains 7519, 06/25/2015. proportional to area [4]. Therefore, higher isolation and [3] A. Léon, B. Reig, V. Puyal, E. Perret, P. Ferrari, and F. Podevin, "High frequency can be achieved without increasing loss. Fig. 10a,b Performance and Low Energy Consumption in Phase Change Material shows the simulated dependence of isolation vs. MIM overlap RF Switches," in 48th European Conference (EuMC), pp. 491-494, 2018. area, providing design guidelines to meet application-specific [4] M. Kim, R. Ge, X. Wu, X. Lan, J. Tice, J. C. Lee, and D. Akinwande, isolation requirements. The extracted COFF (Fig. 10c) shows the "Zero-static power radio-frequency switches based on MoS2 expected parallel-plate dependence for large sizes but saturates atomristors," Nature Communications, vol. 9, p. 2524, 2018/06/28. [5] R. Ge, X. Wu, M. Kim, P. Chen, J. Shi, J. Choi, X. Li, Y. Zhang, M. (~1 fF) for small dimensions. The latter is not desirable since it Chiang, J. C. Lee, and D. Akinwande, "Atomristors: Memory Effect in limits the design space. With further analysis, the COFF Atomically-thin Sheets and Record RF Switches," in 2018 IEEE IEDM, saturation can be explained by the device structure (Fig. 11) pp. 22.6.1-22.6.4. where the overlap area can be scaled lithographically but the [6] M. Field, C. Hillman, P. Stupar, J. Hacker, Z. Griffith, and K.-J. Lee, Vanadium dioxide phase change switches vol. 9479: SPIE, 2015. sidewall capacitance remains invariant with a fixed COFF. This [7] C.D. Patel, and G.M. Rebeiz, “A compact RF MEMS metal-contact switch indicates that a completely planar trench-like device structure and switching networks,” IEEE Microwave and Components is more favorable than mesa structures used in this initial work, Letters, 22(12), pp.642-644. an important insight that should guide future research.

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Fig. 1. Comparison of various RF switches plotted as a Fig. 2. TEM image of CVD-grown hBN. Fig. 3. Raman spectrum of CVD-grown function of RON vs. COFF. The contour color mapping The inset is an image of hBN transferred monolayer hBN showing the expected represents the cut-off frequency. hBN, though, nascent onto Si/SiO2 4-inch wafer. characteristics. is already competitive or superior to other switches.

Fig. 4. (a) Key fabrication process steps of hBN RF switch. (b) Top-view optical image of a fabricated hBN RF switch with Au GSG electrodes. The dashed circle in (b) marks the area where the vertical MIM switch device is located. (c) Representative zoomed-in top view SEM (false-

color) image of the circled region in (b) with an overlap area of ͲǤͷ ൈ ͲǤͷ ߤ݉ଶ. (d) Illustrated side view of the RF switch device on a diamond substrate. The diamond substrate is employed for efficient heat dissipation arising from Joule heating or high-power RF.

Fig. 5. Typical I-V resistive switching behavior Fig. 6. Retention time of selected hBN Fig. 7. Pulse switching behavior of hBN

of monolayer hBN device. switchmemory device. Current levels are memory device. The I-V curves before and measured at a low voltage = 0.1 V. after pulse demonstrate 15 ns switching.

Fig. 8. Measured (de-embedded) S-parameters S21 data in both (a) the ON (Insertion Loss), and (b) OFF (Isolation) states of RF switch of Fig. ଶ 5. Device size is ͲǤͷ ൈ ͲǤͳߤ݉ . The extracted RON and COFF are 1.6 ȳ and 2.3 fF, respectively. The corresponding cutoff frequency, FCO is 43 THz. (c) Equivalent lumped element circuit model consisting of intrinsic device (RON, CON, COFF) and interconnect parasitics.

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Fig. 9. (a) The ON-state insertion loss can be tuned by the DC complia nce current. The upswing is attributed to the ON-state capacitance, a unique feature arising from the atomically-thin layer resulting in appreciated CON. (b) The ON-state resistance features an exponential dependence on the DC compliance current. (c) The insertion loss trends for various RF switch technologies.

Fig. 10. CST 3D electromagnetic simulation results of (a) OPEN and OFF-state S21 (Isolation) with various overlap area. Extracted (b) OFF- state isolation, and (c) OFF-state capacitance dependency on overlap area, useful as design guideline. The parallel plate model is a good fit for large sizes, however, at small sizes side-wall fringe capacitance dominates (see Fig. 11) and leads to a constant capacitance.

Fig. 11. COFF model for (a) large, and (b) small overlap Fig. 12. Power handling in the (a) ON, and (b) OFF states measured at 40 GHz for hBN areas. The side-wall fields dominate for small areas. RF switch of Fig. 5. OFF-state measurement shows self-switching at ~20 dBm. Device Non- Control Cutoff Operating Switching Dimension Max Power Reference Technology volatility Voltage frequency Environment time (single device W x L) (dBm) This Work hBN Switch Yes ~1 - 3V 43 THz Ambient condition < 15 ns 0 5 um x 0.1 um 20 Ge et al, IEDM, 2018 MoS2 Switch Yes ~0.5 – 1.5V 70 THz Ambient condition < 15 ns 0.15 um x 0.2 um 10 Kim et al, Nat. Comm, 2018 GeTe Phase- Leon et al., EuMC, 2018 Yes 3.5 V 22 THz Heater needed < 0.5 us 19 um x 1 um Not reported change Switch Memristive Pi et al., Nat. Commun., 2015 Yes 3V 35.2 THz Ambient condition Not reported 0 11 um x 0.035 um 17 switch Field et al., Proc. SPIE 9479, VO2 Phase- No ~2 V 45 THz Heater needed 2 us 2 um x 10 um 17 2015 change Switch Patel et al., IEEE Trans. MEMS Switch No 80~90 V 12.4 THz Hermetic packaging ~5 us 250 um x 250 um >25 Microw. Theory Tech, 2012 Table 1. Comparison of this work with other representative RF switch papers. hBN RF switch features the best combination of cutoff frequency and power handling.

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