ABSTRACT Radiofrequency switches are critical components in wireless communication systems and consumer electronics. Emerging devices include switches based on microelectromechanical systems

and phase-change materials. However, these devices suffer from disadvantages such as large physical dimensions and high actuation voltages. Here we propose and demonstrate a nanoscale

radiofrequency switch based on a memristive device. The device can be programmed with a voltage as low as 0.4 V and has an ON/OFF conductance ratio up to 1012 with long state retention. We

measure the radiofrequency performance of the switch up to 110 GHz and demonstrate low insertion loss (0.3 dB at 40 GHz), high isolation (30 dB at 40 GHz), an average cutoff frequency of 35 

MEMRISTORS BASED ON TWO-DIMENSIONAL MATERIALS Article Open access 14 March 2024 MEMRISTIVE CIRCUITS BASED ON MULTILAYER HEXAGONAL BORON NITRIDE FOR MILLIMETRE-WAVE RADIOFREQUENCY

APPLICATIONS Article 01 July 2024 IMPERFECTION-ENABLED MEMRISTIVE SWITCHING IN VAN DER WAALS MATERIALS Article 17 July 2023 INTRODUCTION High-performance switches are critical components for

the radiofrequency (RF) circuits that are widely used in wireless communications and consumer electronics1,2,3,4. Intense research in emerging switch technologies has focused on achieving

levels of performance far beyond that available using CMOS (complementary metal oxide semiconductors) technology. Microelectromechanical system-based switches use a mechanical movement,

usually activated by a voltage/current, to control an ohmic or capacitive contact4,5. Although superior to gallium arsenide and silicon-based switches in terms of energy consumption and

transmission properties, microelectromechanical system switches suffer from issues such as speed, dielectric charging and contact interface degradation4,5. Other devices rely on

recrystallization and amorphorization of a phase-change material (for example, germanium telluride) or the metal–insulator phase transition in strongly correlated materials (such as vanadium

oxide, VO2) to change the resistance state6,7,8,9. However, germanium telluride-based switches are constrained by a large switching energy and a limited resistance modulation ratio6,7,8.

VO2 has an insulator-to-metal transition temperature of 67 °C (refs 10, 11) that limits the application of this device as a RF switch in a wider temperature range. Even worse is that the

insulator phase transition in VO2 is volatile, which means the device needs to be kept at above 67 °C to maintain a low-resistance state. Furthermore, most of the aforementioned emerging RF

switches are of too large physical size to permit large-scale integration with nanometre CMOS RF circuits. Memristive devices are two-terminal resistance switches whose internal states are

determined by the history of applied voltage and/or current12,13. Switching is typically related to the formation/rupture of one or more conductive filaments between two electrodes through

either electrochemical reactions or the migration of mobile ions that modulate the interfacial properties14,15,16. These devices have demonstrated a number of attractive features, such as

nonvolatility, low switching energy (∼pJ; ref. 17), fast switching speed (<ns; ref. 18), high endurance (1012 cycles)19, excellent scalability20,21,22 and CMOS compatibility25. As such,

memristive devices are promising candidates for universal memory23,24, reconfigurable bits25, nonvolatile logic26 and unconventional computing27. Here we report a nanoscale RF switch based

on a nonvolatile memristive device that consists of a pair of electrochemically asymmetric metal electrodes separated by a nanometre-scale air gap. Different from the aforementioned

applications, RF circuits require a switch having low ON-state resistance (_R_ON) and low OFF-state capacitance (_C_OFF) to achieve a high cutoff frequency (_f__c_=1/2π_R_ON_C_OFF) while

maintaining sufficient linearity to be used in practical wide-band systems. In the design of our memristive RF switches, we realized that the _R_ON_C_OFF product is proportional to the

product of the resistivity (_ρ_) of the conductive filament and the effective dielectric constant (_ɛ_eff) of the materials between the electrodes (_ρ·ɛ_eff). Consequently, we designed a

device on thermally grown silicon oxide with two metal electrodes that are separated by an air gap. The active metal we used is silver (Ag), which has the lowest bulk resistivity (_ρ_=1.59 ×

10−6 Ω cm) among all metals2, and the air gap on silicon oxide leads to a low effective dielectric constant (_ɛ_eff≈5.5) for the air–substrate interface in the active device region. The

ability for the switch to function within an air gap, rather than a gap filled with the Ag-doped GeSe2 film on which previous nanoionic switches have relied28, is enabled by the nanoscale

feature size of the demonstrated device. Our memristive RF switch can be programmed with a voltage as low as 0.4 V and has an ON/OFF conductance ratio up to 1012. We also demonstrate low

insertion loss (0.3 dB at 40 GHz), high isolation (30 dB at 40 GHz), a typical cutoff frequency of 35 THz and competitive linearity and power-handling capability. RESULTS DEVICE STRUCTURE

AND PROGRAMMING Figure 1 shows the image and geometry of a typical memristive RF switch. It was fabricated on an intrinsic silicon wafer (resistivity>10,000 Ω cm) with a 380-nm-thick

thermally grown silicon dioxide using photolithography, electron beam lithography (EBL) and tilted metal evaporation (see Methods, Supplementary Fig. 1 and Supplementary Note 1 for details).

We used gold (Au) with a thin titanium (Ti) adhesion layer as one terminal and Ag as the other. The terminals were separated by a 35-nm-wide air gap. Each terminal was connected to a much

thicker (>1 μm) copper/aluminium (Cu/Al) pad to minimize the contact resistance with the measurement probes. Additional ground pads and metallization were fabricated at the same time to

permit high-frequency characterization of the device within a well-defined coplanar waveguide (CPW) environment. The device exhibited nonvolatile bipolar resistance switching that crosses

zero in the current–voltage curve, a signature of memristors (Supplementary Fig. 2 and Supplementary Note 1). We used a voltage ramp (0 to 3 V or 0 to −0.4 V without return sweeps) to

programme the device to the ON or OFF state. A current compliance (_I_c) was set to minimize the possible damage to the device without affecting the nonvolatility (Fig. 2a). By applying a

maximum sweep voltage of 3 V to the Ag terminal relative to the Au/Ti terminal, a 110-nm-wide continuous Ag filament formed within the 35-nm gap (Fig. 2b). On the other hand, a −0.4-V bias

ruptured the filament, programming the device in the OFF state (Fig. 2c). Both the high and low-resistance states (OFF/ON) after programming were nonvolatile, as confirmed by probing with

lower voltages (±100 mV). In addition, the ON/OFF conductance ratio of the device was nearly 1012—7 orders of magnitude higher than that of a phase-change material switch29—indicating

extremely low current flow in the OFF state. We further discovered that the geometry of the continuous silver filament was dependent on the programing process (see Supplementary Fig. 3 and

Supplementary Note 2). Under otherwise identical conditions, a higher current led to a filament with larger diameter and hence lower resistance; however, this may also increase the energy

required for programming. The filament formation initiates from the silver side, consistent with the electrochemical reaction mechanism30,31,32. However, previously reported devices with a

similar structure but a larger gap size had considerably higher resistance because, rather than a continuous metal filament, isolated metal islands were formed between the electrodes28,31.

We used a narrow gap and achieved a continuous filament with low resistance using low programming voltages. DEVICE-RETENTION PROPERTIES We did not observe device failure from OFF to ON state

up to 200 °C. To evaluate the ON state retention of the memristive RF switches, we measured the failure time of ON-state devices at different temperatures. The device under test was

programmed to low resistance at room temperature and then mounted on a closed loop thermal chuck with an accuracy of ±0.5 °C. The resistance of each device was measured using a semiconductor

parameter analyser once every 5 minutes until the current dropped dramatically. Figure 3a shows the measured retention time for our devices at 85, 115, 150, 180 and 200 °C, which ranged

from 5 × 105 to ∼103 s. We found that the retention time (_τ_) followed an Arrhenius relation33 with respect to temperature (_T_), 1/_τ_∝exp(−_E_a/_kT_), where _E_a is the activation energy

for the Ag diffusion and _k_ the Boltzmann constant. By extending the fitted results to room temperature (20 °C), the expected retention time of our device was beyond 3.3 years (Fig. 3b).

The long retention time is likely due to the very low mobility of Ag atoms on the surface and inside of silicon oxide (4.5 × 10−13 cm2s−1) at room temperature34. It should be pointed out

that the retention measurement in this study was conducted in air without passivation or other hermetic protection. The ‘failed’ devices tested at or below 150 °C could still be SET to the

low-resistance state using a voltage sweep as described in Fig. 2a, indicating that the switching was nondestructive. The retention time could be further improved by introducing a hermetic

seal around the device. The _E_a extracted from Fig. 3b is 0.77 eV, which lies between the value reported for Ag diffusion on SiO2 surface (0.30–0.44 eV; refs 35, 36, 37) and that for Ag

diffusion inside SiO2 (∼1.24 eV; ref. 34). The intermediate activation energy suggests that part of the conductive Ag filament was embedded in the SiO2 substrate during the diffusion

process. This is consistent with previous observations that Ag diffusion occurs both on the SiO2 surface and inside of the SiO2 simultaneously38. TEMPERATURE AND HUMIDITY EFFECTS ON

PROGRAMMING VOLTAGES The switching behaviour is dependent on the ambient temperature. As shown in Fig. 4a, the SET voltage decreased with higher temperatures, while the RESET voltage was

insensitive to the ambient temperature. This implies that a higher temperature favours the formation of an Ag filament during the SET process39. The RESET process is dictated by Joule

heating and the ambient temperature does not significantly affect the fracturing of the filament. The temperature dependency suggests that a lower programming voltage is needed when the

device is used for RF circuits that operate at 85 °C or above. Furthermore, the programming voltage is also affected by the environment humidity (_H_r): the SET voltage decreased with higher

_H_r while the RESET voltage remained almost unchanged (Fig. 4b). The strong correlation between the SET voltage and the humidity could be explained both by the critical role of moisture in

the Ag ionization at the anode and by the migration of Ag ions during the SET process. At higher humidity levels, more water molecules are absorbed on the SiO2 surface. A dense

hydrogen-bond network is hence formed that provides necessary counter charges for Ag ionization40 and lowers the activation energy for Ag ion migration, leading to a smaller SET voltage. On

the other hand, the RESET process of our device originates from filament fracture due to strong Joule heating and is thus less influenced by moisture. It is worthwhile to point out that the

ambient humidity also affects the device endurance (see Supplementary Fig. 4 and Supplementary Note 3). RF PERFORMANCE MEASUREMENT AND MODELLING We measured the RF performance of our devices

up to 110 GHz using a vector network analyser (VNA). A multiline thru-reflect-line (TRL) calibration using a set of on-wafer standards (Supplementary Fig. 5 and Supplementary Note 4) was

used to place the measurement reference plane 5 μm away from the edge of the intrinsic device41,42. The transmission spectra of 33 devices were obtained. In a 50-Ω environment, the average

OFF-state isolation and ON-state insertion loss at 40 GHz were 30 and 0.33 dB, respectively (example curves and statistical metrics appear in Fig. 5). At higher frequencies, the OFF-state

measurement was limited by weak coupling to undesired modes supported by the intrinsic silicon substrate. This coupling could be eliminated if the switch were integrated in a thin-film

grounded CPW structure (Supplementary Fig. 6 and Supplementary Note 5). We analysed the small-signal performance of the nanoscale memristive switch using a simple physics-based

lumped-element equivalent circuit model shown in Fig. 6a. This equivalent circuit model was extracted for each device and used to compute the switch cutoff frequencies. Statistics for the

_R_ON, _C_OFF and _f_c are provided in Fig. 6b–d. Among the 33 measured devices, the _R_ON of the intrinsic switch has a mean value of 3.6 Ω and varies from 2.1 to 6.9 Ω. According to

electromagnetic simulations, the extracted range of ON-state resistance corresponds to nanofilaments with cross-sections varying from ∼5 × 10−12 to 80 × 10−12 cm2. The _C_OFF of ∼1.4 fF is

predominantly associated with the capacitance of the 35-nm gap, which depends on the effective dielectric constant of the substrate–air interface. This number is consistent with both

electromagnetic simulations and simple hand calculations (see Discussion and Supplementary Note 6). The average value of _f_c is 35 THz and can be further enhanced if an identical device

were fabricated at the top of a typical CMOS metal stack that has a much thicker SiO2 layer. Further enhancement of the small-signal performance could also be achieved by controlling the

filament geometry to reduce _R_ON. We also evaluated the linear performance of the switches using a VNA capable of standard two-tone distortion measurements. The third-order intermodulation

intercept in the ON- and OFF-states was evaluated using measurements of series and shunt switches to capture a realistic operating environment corresponding to a single-pole double throw

switch. In both cases, we found that the third-order intermodulation intercept was beyond the measurement limit of +45 dBm (Supplementary Figs 7 and 8 and Supplementary Note 7).

SELF-SWITCHING EVALUATION RF-induced resistance switching is another potential concern with a memristive switch due to the sub-1 V actuation voltage of these devices. We evaluated the

feasibility for self-switching under RF power by exciting both ON- and OFF-state devices and capturing scanning electron microscopy (SEM) photos as we gradually increased the RF power (Fig.

7a,b). A frequency of 5 GHz was selected for these measurements. We observed no evidence of self-switching in measurements of the OFF-state device up to the maximum available generator power

of +21 dBm. However, when measuring the ON-state device, we observed self-switching at a power level above +17 dBm. We also investigated the dependence of self-switching on the frequency of

excitation by measuring a variety of devices exited with RF energy at 10, 100 and 1,000 MHz. In all cases, we found that OFF state devices were not susceptible to self-switching. Moreover,

we observed little correlation between ON-state power-handling capability and RF excitation frequency. Instead, we observed a strong dependence between the ON-state resistance and the power

at which the device self-actuated (Fig. 7c). The inverse relationship between power handling and ON-state resistance is consistent with a Joule heating-based reset mechanism and indicates

that power handling can be improved by engineering nanofilaments with reduced ON-state resistance (see Supplementary Note 6 for further analysis). DISCUSSION To further understand the

potential of the memristive devices as RF switches, we verified the ON- and OFF-state parameters using a full-wave electromagnetic simulation carried out in ANSYS High-Frequency Structural

Simulator (ANSYS Inc., Canonsburg, Pennsylvania). A frequency range of 1–200 GHz was selected and the substrate was made sufficiently small to avoid substrate-moding effects. A wave-port

de-embedding was employed to set the reference plane of the simulation to that of the experimental measurement. The CPW feedline cross-section and silver electrodes were made identical to

the fabricated structures and a 35-nm gap was selected for the active memristor region. For ON-state simulations, a small rectangular silver filament was used to short the gap. The

conductivity of the silver was reduced by a factor of three from its bulk value to account for nanoscale effects43. The thickness and width of the filament were varied to study the impact of

the filament dimension on the ON-state resistance. The predicted OFF-state transmission appears in Fig. 8a. At 40 GHz, the predicted isolation is 31 dB, which is within 1 dB of the measured

value (30 dB) as shown in Fig. 5. Moreover, the predicted isolation remains better than 17 dB to 200 GHz. The OFF-state capacitance was calculated as _C_OFF=–imag{1/_Y_12}/_ω_. The result

is plotted in Fig. 8b. Over a broad range of frequencies, the predicted capacitance is 1.17 fF, which is consistent with both hand calculations and measurement. Moreover, the frequency

independence of the extracted capacitance validates our use of the simple lumped element equivalent circuit. The simulated transmission of the ON-state device is shown in Fig. 8c for

filaments with cross-sections of 110 nm × 70 nm and 50 nm × 10 nm. The average experimentally obtained insertion loss value (0.33 dB) corresponds to a nanofilament of cross-section 8 × 10−12

 cm2, which is significantly smaller than expected based on SEM imaging. This indicates that a significant component of the RF resistance may be related to contact resistance. The

corresponding ON-state resistance is plotted versus frequency in Fig. 8d. The memristive RF switch requires no power to maintain the ON or OFF state, potentially leading to power efficiency

advantages. The low programming voltage enables monolithic integration of memristive RF switches with low-power CMOS circuits to expand the functionality. The device fabrication process for

our memristive RF switches is also simpler as compared with other devices. Further scaling of the device, such as shrinking the gap size, will lead to optimized switching behaviour such as

longer endurance and lower power consumption44,45 with no degradation to the RF performance, provided the ON-state resistance is dominated by the filament and not other effects. It is

necessary to engineer the location and geometry of the metallic filaments for even lower ON-state resistance, to enhance the device endurance, and to reduce the energy required for each

switching event. Nonetheless, we believe that memristive RF switches will become one of the leading contenders in RF circuits for communication and consumer electronics. METHODS DEVICE

GEOMETRY AND FABRICATION The memristive RF switches were built on an intrinsic silicon wafer (resistivity>10,000 Ω cm) with a 380-nm-thick thermally grown oxide. The fabrication procedure

involved three major steps that defined the microscale contact pads, the CPW structure and the nanoscale gap. The microscale features were patterned by photolithography using 365-nm

wavelength ultraviolet radiation, and the nanogap was defined using EBL. The padframe and feedline structures consisted of a 10-nm-thick Ti adhesion layer and a 140-nm-thick gold layer, both

deposited in an electron beam evaporator. The distance between the central electrodes was 5 μm. The contact pads were made of 1-μm-thick Cu and 500-nm-thick Al that were deposited by RF

magnetron sputtering. Before fabricating the nanometre-scale gap, a rectangular window of 30 μm by 8 μm was opened by EBL in the centre area of the electrodes. The nanogap was created by

evaporating 70-nm-thick Ag with a tilted angle. For the 35-nm gap, the evaporation angle was 65° relative to the wafer surface. See Supplementary Note 1 for the detailed fabrication

procedure and schematics. DC PROGRAMMING The devices were programmed to the ON- or OFF-state with quasi-DC voltage sweeps using a Keithley 4200SC semiconductor parameter analyser. A current

compliance was set to protect the device, and to control the size and morphology of the metallic filament. During the programming, the inert metal electrode was grounded and the silver

electrode was biased. A positive voltage will SET the device to a low resistance by bridging the nanogap, while a negative voltage ruptures the filament and RESETs the device to the

high-resistance state. RF MEASUREMENTS Small-signal measurements were carried out ON-wafer using an Agilent N5247A VNA with a pair of N5260 frequency extender modules, permitting

measurements to be carried out from 0.01 to 110 GHz. To minimize substrate-moding when measuring the OFF-state-scattering parameters, the switches were first singulated to a size of ∼1 mm ×

1 mm × 0.55 mm (length × width × height) and then mounted using photo-resist on a rigid absorber composed of carbon-loaded epoxy. For small-signal measurements, the RF power was set to a

nominal level of −5 dBm. A multiline TRL calibration was carried out from 10 to 110 GHz using a set of standards fabricated on an identical substrate as the memristive switches (further

details are provided in the Supplementary Note 4). The multiline TRL calibration41,42 placed the measurement reference plane 5 μm from the edge of the intrinsic device. No additional

de-embedding was employed. A second calibration using the LRM+ algorithm carried out on a Cascade 104-783 calibration substrate was employed to measure the devices, referenced at the plane

of the bondpads so that a comparison of the ON-state device to a thru line could be made (Supplementary Fig. 9 and Supplementary Note 8). All calibrations were performed using Cascade WinCal

software (Cascade Microtech Inc., Beaverton, Oregon), before device measurement, allowing for data at the two reference planes to be acquired simultaneously. Scattering parameters were

collected for each device in both the ON- and OFF-states, and parameter extraction was carried out to determine the ON- and OFF-state equivalent circuit models. Two-tone linearity

measurements were carried out at 5 GHz with a 1-MHz-tone spacing and a swept power level ranging from −30 to +7 dBm. Power handling was evaluated in the ON- and OFF-states by driving the

memristor with a laboratory CW source (Agilent PSG8257D) and measuring the output power using a spectrum analyser (Agilent N9010A). The power was swept from −20 to +21 dBm, with steps of 1 

dBm for three different frequencies including 10 MHz, 100 MHz and 1 GHz. All cable losses were determined and de-embedded. The detailed evolution of one ON-state and one OFF-state device was

studied by incrementally capturing SEM images as the devices were gradually exposed to higher RF power levels. For this experiment the RF power swept from −3 to 21 dBm with steps of 4 dBm

and the frequency was set to 5 GHz. SEM photos were obtained at each step. ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Pi, S. _et al_. Nanoscale memristive radiofrequency switches.

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_Nanotechnology_ 22, 254027 (2011). Article  ADS  Google Scholar  Download references ACKNOWLEDGEMENTS This work is supported in part by the Defense Advanced Research Projects Agency (DARPA;

N66001-12-1-4217; N66001-11-1-4143), the National Science Foundation (NSF; ECCS-1253073) and the Air Force Office of Scientific Research (AFOSR; FA9550-12-1-0038). Part of the device

fabrication was conducted in the clean room of Center for Hierarchical Manufacturing (CHM), an NSF Nanoscale Science and Engineering Center (NSEC) located at the University of Massachusetts

Amherst. We thank Peng Lin for growing the thermal oxide on the intrinsic silicon wafers and preparing cross-sectional SEM imaging samples using a focused ion beam (FIB) microscope; Robert

Jackson, William Chappell and Roy Olsson for discussion; and J. Joshua Yang and Sigfrid Yngvesson for critical comments on the manuscript. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS *

Department of Electrical and Computer Engineering, Nanodevices and Integrated Systems Laboratory, University of Massachusetts, Amherst, 01003, Massachusetts, USA Shuang Pi & Qiangfei Xia

* Department of Electrical and Computer Engineering, Radio Frequency Nanoelectronics Group, University of Massachusetts, Amherst, 01003, Massachusetts, USA Mohammad Ghadiri-Sadrabadi & 

Joseph C. Bardin Authors * Shuang Pi View author publications You can also search for this author inPubMed Google Scholar * Mohammad Ghadiri-Sadrabadi View author publications You can also

search for this author inPubMed Google Scholar * Joseph C. Bardin View author publications You can also search for this author inPubMed Google Scholar * Qiangfei Xia View author publications

You can also search for this author inPubMed Google Scholar CONTRIBUTIONS Q.X. and J.C.B. conceived the idea. Q.X. and S.P. designed and fabricated the device, and did the DC measurements

and programming. J.C.B. and M.G.-S. measured and modelled the RF performance. Q.X. and J.C.B. wrote the main text, all authors wrote the Supplementary Information and commented on the entire

manuscript. CORRESPONDING AUTHOR Correspondence to Qiangfei Xia. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. SUPPLEMENTARY INFORMATION

SUPPLEMENTARY INFORMATION Supplementary Figures 1-9, Supplementary Notes 1-8 and Supplementary References (PDF 1217 kb) RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE

CITE THIS ARTICLE Pi, S., Ghadiri-Sadrabadi, M., Bardin, J. _et al._ Nanoscale memristive radiofrequency switches. _Nat Commun_ 6, 7519 (2015). https://doi.org/10.1038/ncomms8519 Download

citation * Received: 04 August 2014 * Accepted: 15 May 2015 * Published: 25 June 2015 * DOI: https://doi.org/10.1038/ncomms8519 SHARE THIS ARTICLE Anyone you share the following link with

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