HfO₂-Based Reconfigurable Radio Frequency Switches for All-Memristor Multistate Attenuator

Abstract

Reconfigurable radio frequency (RF) attenuators are critical passive components for 5G-Advanced and emerging 6G wireless systems. Conventional tunable attenuators rely on solid-state switches combined with fixed resistor networks, which suffer from unavoidable static power consumption and severe parasitic degradation at high frequencies. Here, we systematically demonstrate HfO₂-based non-volatile memristors as RF switches with tunable ON-state resistance (R_ON), enabling a switching-attenuation-integrated multistate attenuator. The fabricated Au/HfO₂/Ag devices exhibit stable bipolar resistive switching with an ON/OFF ratio exceeding 10⁹, reliable retention of 10⁵ s, and programmable R_ON continuously tuned from 5.8 Ω to 197.5 Ω. On-wafer RF characterizations from 10 MHz to 43.5 GHz reveal low insertion loss (−0.53 dB), high isolation (−26.8 dB), and clear scaling laws governing the effects of device geometry and R_ON on RF performance. Leveraging these unique characteristics, we propose a symmetric π-type programmable all-memristor attenuator architecture with a cascaded 2-unit configuration. The design achieves 12 discrete attenuation levels from 2 dB to 24 dB, a return loss better than 10 dB across the full band, and zero static power consumption without additional passive components or bias networks. This work establishes the fundamental material-device-RF performance relationship in HfO₂-based RF switches and provides a compact, low-power, and highly integrable solution for next-generation reconfigurable RF front-ends.

1. Introduction

The rapid advancement of 5G-Advanced and upcoming 6G wireless communication systems has imposed urgent demands for high-integration, low-power, wideband, and reconfigurable radio frequency (RF) front-ends [1,2,3,4,5]. As a core and indispensable passive component in RF front-end architectures, tunable attenuators fulfill critical functions including signal power leveling, receiver dynamic range optimization, and impedance mismatch compensation. These functions directly govern the signal integrity and overall energy efficiency of the entire system [6,7,8].

A π-type attenuator is a type of passive attenuator that uses a resistive network circuit (Figure 1a). Due to the fixed resistance, the π-type attenuator only outputs 1 attenuation state. To achieve tunable attenuators, the conventional methods adopt the cascading of solid-state switches (PIN diodes or GaAs field-effect transistors, FETs) with fixed resistor networks (Figure 1b), yet this architecture suffers from two inherent bottlenecks [9,10]. On one hand, to break through the limitation of fixed resistor networks, the program of solid-state switches requires continuous direct current (DC) bias to sustain their operating states. This leads to significant static power consumption, as well as severe insertion loss and linearity degradation at high frequencies [11]. On the other hand, the hybrid integration of discrete switches and resistive networks introduces pronounced parasitic effects, which not only hinder further device miniaturization but also incur additional signal loss in the gigahertz range [12]. These drawbacks render conventional solutions unable to satisfy the stringent requirements of next-generation RF systems for compact footprint, ultra-low power consumption, and wideband operation [13].

Conductive bridging random access memory, which achieves non-volatile resistive switching through the reversible formation and rupture of nanoscale conductive filaments (CF) in a simple metal-insulator-metal (MIM) structure, Refs. [14,15] has emerged as a promising alternative for next-generation RF devices due to the zero static power consumption [16,17,18,19]. Among various functional materials, HfO₂ stands out with its high tolerance to oxygen vacancies and excellent pathways for metal ion migration, which is a promising candidate to fabricate stable and repeatable non-volatile memristor with ultra-low multistate ON-state resistance (R_ON). The replacement of both fixed resistors and RF switches with HfO₂-based memristor in a π-type tunable attenuator shows the potential to overcome the inherent bottlenecks (Figure 1c,d). However, the utilization of reconfigurable R_ON ability in memristor for realizing switching-attenuation integrated multistate attenuators is still lacking.

In this article, we demonstrate a non-volatile HfO₂-based memristor as an RF switch with tunable R_ON, enabling an all-memristor attenuator with reconfigurable multistate attenuation. We first elucidate the electrochemical mechanism underlying CF formation, and establish the quantitative mapping between I_CC, R_ON, and RF insertion loss, as well as between device dimensions, OFF-state capacitance (C_OFF), and isolation. The fabricated memristor exhibits stable non-volatile bipolar resistive switching, with R_ON continuously tunable from 5.8 Ω to 197.5 Ω in Au/HfO₂/Ag and from 4.1 Ω to 22 Ω in Ag/HfO₂/Ag via I_CC modulation. We demonstrate the wideband RF performance of the device from 10 MHz to 43.5 GHz through on-wafer S-parameter measurements. We reveal the dependence of insertion loss (from −8.24 dB to −0.53 dB) on ON-state resistance (from 197.5 Ω to 5.8 Ω) and the OFF-state isolation (from −26.8 dB to −23.8 dB) on device geometry (4 μm² to 25 μm²). Finally, we demonstrate a reconfigurable attenuator based on our multi-state memristor with tunable low-resistance levels in a cascaded 2-unit configuration. The attenuator achieves 12 discrete attenuation levels from 2 dB to 24 dB and a return loss better than 10 dB across the full band. This work provides a systematic device physics framework for HfO₂-based RF switches, together with a facile, monolithically integrable solution enabling programmable multistate attenuators.

2. Materials and Methods

2.1. Device Fabrication and Characterization

Initially, the HfO₂ memristor and RF switches were fabricated on a high-resistance silicon (HR-Si) substrate via bilayer photolithography (AZ5214 and LOR10A photoresist). Memristors were patterned into crossbar array structures, while RF switches were configured in a ground-signal-ground (GSG) coplanar waveguide (CPW) layout. The bottom electrodes of Au (35 nm)/Cr (5 nm) and Ag (150 nm)/Cr (5 nm) were deposited by thermal evaporation. After bottom electrode deposition, the photoresist was removed in N-Methylpyrrolidone (NMP), followed by acetone and isopropanol rinses, and finally dried with nitrogen (N₂). Next, the HfO₂ switching layer was patterned by photolithography, followed by sputter deposition of a 30 nm HfO₂ film and lift-off in acetone. The deposition was performed using a HfO₂ target (99.99% purity) in a pure Ar atmosphere at a flow rate of 37 sccm. The RF power was set to 100 W, yielding a deposition rate of 0.067 nm/s over a duration of 450 s. Finally, the top electrode of Ag (150 nm) was deposited by thermal evaporation after which any residual photoresist was completely removed by sequential cleaning in NMP (30 min), acetone (10 min) and isopropanol (10 min), followed by N₂ drying. The thickness of the HfO₂ switching layer was determined by atomic force microscopy (AFM) (Bruker Dimension ICON, Bruker, Billerica, MA, USA). The structure of the RF switches was examined by scanning electron microscopy (SEM) (Zeiss Sigma 300, Zeiss, Jena, TH, Germany).

2.2. DC Measurement

The electrical characteristics of the devices were measured at room temperature using a Keithley 2600 semiconductor parameter analyzer (Keithley, Solon, OH, USA), including resistive switching, endurance, and retention characteristics. The temperature-dependent measurements were performed under vacuum conditions over a temperature range of 233 K to 358 K, with the chamber pressure maintained at 10⁻⁶ Torr. During the measurements, the voltage was applied to the top electrode, while the bottom electrode was grounded.

2.3. RF Measurement

The 100 μm-pitch GSG probes (DSK-F40-GSG-100, DesignCon Technology Co., Ltd., Shenzhen, China) and the Keysight N5234B PNA-L vector network analyzer (VNA) (Keithley, Solon, OH, USA) were used for on-wafer measurements from 10 MHz to 43.5 GHz. Before measurement, a two-port short-open-load-through (SOLT) calibration was performed using a commercial substrate to mitigate parasitic effects from the cables, probes, and VNA. Following calibration, the GSG probes were contacted to the pads of the device under test with a probe station, enabling direct characterization by the VNA.

3. Results and Discussion

3.1. DC Switching Characteristics of Au/HfO₂/Ag Memristors

Au/HfO₂/Ag crossbar memristors (Figure 1e) were implemented to obtain the ultra-low and multistate R_ON. The low resistance in conductive-bridge memristor makes it own the advantages for RF switch compared with oxygen-vacancy, and proton-based memristor [20]. HfO₂ was fabricated by magnetron sputtering to introduce sufficient defects, which are utilized as the active layer in memristors and RF switching devices. Due to the high tolerance for oxygen vacancies in the sputtered HfO₂ layer, excellent pathways were obtained for Ag ion migration. In addition, the contact quality at the HfO₂/Ag interface was also optimized by the bilayer-photoresist photolithography (BPP) process. This BPP process creates an atomically smooth, residue-free interface with uniform electric field distribution, enabling synchronous growth of multiple parallel and uniform Ag CFs. This controlled CF dynamics eliminates switching randomness and ensures consistent low-resistance state (LRS) values under identical programming conditions. Consequently, the device provides the controllability needed for accurate multi-state programming in RF attenuators [21].

Based on the above-mentioned fabrication process, systematic DC electrical characterizations were performed, revealing their intrinsic bipolar and non-volatile resistive switching characteristics. The devices can be SET from the high-resistance state (HRS) to the LRS via a positive voltage sweep, and RESET from the LRS back to the HRS via a negative voltage sweep. The inherent non-volatility of this resistive switching enables the devices to sustain their programmed R_ON persistently without continuous electrical biasing [22]. Figure 1f illustrates the current-voltage (I-V) characteristics of the 4 × 4 μm² Au/HfO₂/Ag device fabricated through the BPP process. We acquired this data via a sequential voltage sweep on the Ag top electrode over the range of 0 V to +3.0 V, back to 0 V, then to −1.5 V, and finally back to 0 V, while maintaining the Au bottom electrode at ground potential throughout the test. A compliance current (I_CC) of 1 mA was set for the SET process to avert permanent breakdown of the device. At a sweep voltage of ~2.2 V, the current increases sharply to the compliance value, indicating the SET transition from HRS to LRS via the formation of Ag CFs across the HfO₂ layer. When the voltage sweeps back from +3.0 V to 0 V, the device maintains the LRS, demonstrating its non-volatile characteristic. The RESET transition occurs at −1.1 V, where the current drops sharply as the CFs rupture via Joule heating, returning the device to the HRS. As shown in Figure 1g, the Au/HfO₂/Ag device achieves tightly concentrated R_ON (100–180 Ω) with small device-to-device variation. As a control experiment, we fabricated an Au/HfO₂/Ag memristor by the monolayer-photoresist photolithography (MPP) process. Compared with memristors via the BPP process, devices via the MPP process show extremely dispersed LRS resistance spanning ~80 Ω to 850 Ω (shown in Figure S1). Another control experiment adopts the atomic layer deposition (ALD) to fabricate dense HfO₂ in the Au/HfO₂/Ag memristors. The high R_ON of ~1200 Ω was obtained due to the difficulty of Ag ion migration in this dense HfO₂ film (Figure S2). These results demonstrate the successful optimization of our Au/HfO₂/Ag memristor.

As verified by the retention tests in Figure 1h, the device achieves a stable ON/OFF ratio of ~10⁹. Under a read voltage of +0.1 V at room temperature, both the LRS and HRS remained stable for 10⁵ s, which attests to the device’s excellent stability. Moreover, the endurance of the threshold-switching behavior has been proved by applying pulsed voltage stresses, showing no degradation of the response resistance after more than 1500 cycles, as shown in Figure 1i. The low, robust and nonvolatile ON state (LRS) resistance was obtained, which can be used for improving RF insertion loss. Furthermore, an additional endurance test for 10⁵ cycles under low switch ratio conditions (Figure S3) of the Au/HfO₂/Ag device indicated that it is suited for programmable attenuator applications requiring stable tuning. Figure 1j illustrates the dependence of the programming voltages on ambient temperature. The SET voltage shows a clear decreasing trend as the temperature increases from 233 K to 345 K, indicating that filament formation becomes easier at elevated temperatures due to thermally activated Ag ion migration. Ag ion diffusion is thermally dominated, which is driven by the concentration gradient (Figure S4). The dominant Ag ion drift is ensured after the competition between drift and diffusion, achieving non-volatile memory states due to the optimized defect density in sputtered HfO₂. In contrast, the RESET voltage remains relatively stable across the same temperature range, suggesting that the rupture of the CF is primarily governed by Joule heating and is insensitive to ambient temperature variations [23]. These observations align with the electrochemical mechanism of filament formation and thermal-assisted rupture in memristor devices.

In addition, given that Ag electrodes may promote more efficient CF formation and potentially yield lower on-state resistance, we also examined Ag/HfO₂/Ag devices for comparative evaluation. We characterized the DC switching performance of Ag/HfO₂/Ag devices with identical geometries and test conditions (Figure S5). The Ag/HfO₂/Ag device exhibited similar bipolar non-volatile resistive switching behavior and extremely low SET and RESET voltages. However, its retention and endurance characteristics were inferior to those of the Au/HfO₂/Ag device.

3.2. RF Characterization of HfO₂ Switches

Based on the ultra-low R_ON in Au/HfO₂/Ag, we further characterized the RF switching performance and its reconfigurability. Figure 2a,b represents 3D schematic illustrations of the fabricated Au/HfO₂/Ag vertical RF switch, which adopts a GSG CPW configuration for RF signal transmission and a vertical MIM stack for resistive switching. Figure 2c presents an SEM image of the fabricated RF switch, integrated with a GSG CPW for RF signal transmission. Figure S6 shows an AFM image and the corresponding thickness profile of a 4 × 4 μm² vertical Au/HfO₂/Ag sandwich structure fabricated on an HR-Si substrate, with the HfO₂ layer having a thickness of approximately 30 nm. The fabrication process flow of the HfO₂ RF switch is detailed in Figure S7.

The RF performance of the fabricated HfO₂-based resistive switches was characterized via on-wafer S-parameter measurements for both ON and OFF states using a VNA. A full on-wafer SOLT calibration was performed prior to testing to de-embed parasitic contributions from the test cables, RF probes, and probe station, enabling accurate extraction of the intrinsic RF characteristics of the devices. An equivalent lumped element circuit (Figure S8) was employed to extract the key performance parameters, namely R_ON and C_OFF.

For the Au/HfO₂/Ag switches, the ON-state RF performance and I_CC -dependent tunability of the HfO₂-based RF switches were evaluated. For the Au/HfO₂/Ag switches, the measured ON-state de-embedded S₂₁ under I_CC of 10 mA is shown in Figure 2d. The S₂₁ maintains a near-constant value of −0.53 dB across the frequency range of 10 MHz–43.5 GHz, exhibiting a low insertion loss characteristic for RF signal transmission. An R_ON of 5.8 Ω was extracted for the device from the measured S-parameters (see Note S1 for detailed calculations). The dependence of ON-state insertion loss on I_CC is presented in Figure 2e, where the S₂₁ insertion loss decreases from −8.24 dB to −0.53 dB as the I_CC increases from 0.1 mA to 10 mA. This trend is directly correlated to the linear scaling of R_ON with I_CC (Figure 2f), where R_ON decreases continuously from 197.5 Ω at 0.1 mA to 5.8 Ω at 10 mA, demonstrating defined tunability of the device’s conductive state via I_CC modulation, which is the core enabling technology for the monolithic integrated attenuator design in this work.

This R_ON–I_CC dependence can be understood from the conductive filament growth dynamics. During the SET process, the I_CC limits the total cross-sectional area or the number of parallel Ag filaments formed across the HfO₂ layer. A higher I_CC allows a thicker or more numerous filament bundle to stabilize before the current-limiting circuit engages, thereby reducing the overall R_ON. The quasi-linear scaling observed in Figure 2f is consistent with this picture, as the filament conductance scales approximately with its cross-sectional area. Consequently, the ON-state insertion loss, which is dominated by the ohmic loss of the series switch, can be quantitatively linked to R_ON through the relation IL (dB) ≈ −20 lg [1 + R_ON/(2Z₀)] for a well-matched 50 Ω system. This expression captures the measured trend from −8.24 dB at R_ON = 197.5 Ω to −0.53 dB at R_ON = 5.8 Ω, bridging the DC programming condition and the resulting RF transmission performance in a straightforward manner.

For comparison, the ON-state RF performance of Ag/HfO₂/Ag switches with identical test conditions is shown in Figure 2g–i. For the Ag/HfO₂/Ag device under 10 mA I_CC (Figure 2g), the ON-state de-embedded S₂₁ remains stable at −0.36 dB across 10MHz-43.5 GHz, with an extracted R_ON of 4.1 Ω, which is lower than that of the Au/HfO₂/Ag counterpart under the same operating conditions. The ON-state insertion loss of the Ag/HfO₂/Ag switches decreases with increasing I_CC, with the S₂₁ insertion loss reducing from −1.9 dB to −0.36 dB as the I_CC increases from 0.1 mA to 10 mA (Figure 2h). The R_ON of the Ag/HfO₂/Ag switches also scales linearly with I_CC, ranging from 22 Ω at 0.1 mA to 4.1 Ω at 10 mA (Figure 2i). Moreover, the different I_CC sweep curves for the two devices are shown in Figure S9.

Collectively, these results confirm that for a given I_CC, the Ag/HfO₂/Ag stack exhibits a lower R_ON than the Au/HfO₂/Ag stack. With two active Ag electrodes, Ag can be supplied from either side, depending on the bias polarity [24]. In this case, the low LRS resistance was obtained. This lower R_ON directly accounts for the reduced on-state insertion loss of the Ag/HfO₂/Ag device at the same operating frequency, which can be well explained by the series resistance loss model for RF transmission lines: a lower R_ON reduces the ohmic loss of the transmitted RF signal, thus improving the insertion loss performance [25,26]. This finding is also consistent with the widely accepted consensus in the field that the on-state insertion loss of RF switches is dominantly determined by their R_ON [18]. In addition, the Au/HfO₂/Ag stack structure offers a significantly wider R_ON tuning range and better long-term switching stability with the aid of the established Schottky barriers at the Au/HfO₂ interfaces. For the stability of intermediate resistance states, a higher I_CC produces thicker Ag filaments with reduced interfacial energy, making them thermodynamically more resistant to spontaneous rupture. As demonstrated by the retention measurements (Figure S10), all six intermediate resistance states used in our attenuator design (programmed at I_CC = 0.1 to 10 mA) exhibited negligible drift over 10⁵ s at room temperature, confirming their suitability for reliable reconfigurable attenuator operation. For reconfigurable attenuator applications, the wide-range, high-precision R_ON tuning capability of Au/HfO₂/Ag devices is a primary consideration, as it enables a wider attenuation tuning range and more discrete attenuation levels [8].

In addition, the measured OFF-state de-embedded forward transmission coefficient (S₂₁) of the 4 μm² device is shown in Figure 3a under the same test configuration as the ON-state characterization. From 10 MHz to 43.5 GHz, the S₂₁ increases from −75 dB to −26.8 dB. This reveals a frequency-dependent roll-off in isolation. This effect stems from intrinsic capacitive coupling in the MIM stack during the high-resistance OFF state. A C_OFF of 3.8 fF was extracted for this 4 μm² device from the measured S-parameters. The dependence of OFF-state isolation on device geometry is presented in Figure 3b, where the isolation at 43.5 GHz decreases monotonically from −26.8 dB to −23.8 dB as the device overlapped area increases from 4 μm² to 25 μm², demonstrating that larger device dimensions degrade OFF-state isolation. This trend is directly correlated to the linear scaling of C_OFF with the overlapped area (Figure 3c), where C_OFF increases from ~4 fF to ~7.2 fF across the same area range, consistent with the fundamental scaling behavior of parallel-plate capacitors.

For direct comparison, the OFF-state RF performance of Ag/HfO₂/Ag switches with identical device geometries is shown in Figure 3d–f. For the 4 μm² Ag/HfO₂/Ag device (Figure 3d), the OFF-state de-embedded S₂₁ increases from −75 dB at 10 MHz to −25.8 dB at 43.5 GHz, with an extracted C_OFF of 5.9 fF, which is higher than that of the Au/HfO₂/Ag counterpart with the same area. As with the Au/HfO₂/Ag devices, the OFF-state isolation of the Ag/HfO₂/Ag switches at 43.5 GHz degrades with increasing overlapped area, decreasing from −25.8 dB to −22.2 dB as the area increases from 4 μm² to 25 μm² (Figure 3e). The C_OFF of the Ag/HfO₂/Ag switches also scales linearly with the overlapped area, ranging from ~5 fF at 4 μm² to ~11.5 fF at 25 μm² (Figure 3f). Collectively, the isolation performance of RF switches in the OFF-state is primarily dictated by the C_OFF [27]. According to the parallel-plate capacitor theory, a larger electrode overlap area leads to a higher capacitance, thereby degrading the isolation of HfO₂-based RF switches [17,28,29]. These results confirm that for a given device area, the Ag/HfO₂/Ag stack exhibits a larger C_OFF than the Au/HfO₂/Ag stack, which directly explains the reduced OFF-state isolation of the Ag/HfO₂/Ag switches at the same operating frequency.

Based on the extracted R_ON and C_OFF values, we further calculated the cutoff frequency (f_c), a critical figure of merit for RF switches defined as f_c = 1/2πR_ONC_OFF, which represents the maximum operating frequency where the switch maintains acceptable ON/OFF performance. For the Au/HfO₂/Ag switch with R_ON = 5.8 Ω and C_OFF = 3.8 fF, the extrapolated f_c reaches ~7.2 THz. For the Ag/HfO₂/Ag counterpart with R_ON = 4.1 Ω and C_OFF = 5.9 fF, the f_c is ~6.6 THz. Both values are orders of magnitude higher than the measured upper frequency limit of 43.5 GHz in this work, demonstrating the exceptionally high-frequency potential of HfO₂-based memristor switches for millimeter-wave and even terahertz applications.

3.3. Tunable Attenuators for Signal Processing

Leveraging the non-volatile, wide-range R_ON tunability of the fabricated Au/HfO₂/Ag RF switches, we propose an all-memristor reconfigurable attenuator based on symmetric π-type topology, realizing monolithic integration of switching and attenuation functions without any additional passive components or bias networks. Figure 4a shows the architecture of the 2-unit cascaded attenuator. The attenuator adopts a path-switchable cascaded topology, consisting of 4 single-pole double-throw (SPDT) memristor switches and 2 identical memristor attenuation networks (MANs, Attenuation Units 1 and 2). The SPDT switches (optical image in Figure 4b) control the RF signal path. When an SPDT selects the through path (port 1–2), the RF signal bypasses the corresponding MAN with near-zero insertion loss. When it selects the attenuation path (port 1–3), the signal passes through the MAN to achieve the preset attenuation. The MAN unit is the symmetric π-type all-memristor topology shown in Figure 4c. Different from conventional π-type attenuators relying on fixed resistors, this topology is constructed entirely by three identical Au/HfO₂/Ag memristors: one memristor in the series arm (R_memristor,1) and two identical memristors in the shunt arms (R_memristor,2 and R_memristor,3). By precisely tuning the I_CC during the SET process, the R_ON of each memristor can be programmed to the target value, thus realizing accurate setting of the attenuation amount of the π-type network.

Based on the measured RF characteristics of the fabricated devices, we systematically quantified the attenuation performance of the proposed π-type architecture. According to microwave circuit theory, a quantitative relationship between the attenuation value and the resistance in the MAN is established (see Note S2 for details). Based on this, we can determine the four attainable attenuation levels from the memristor resistance values set via the I_CC. By setting the series memristor (R_memoristor,1) to the minimum programmable R_ON of 5.8 Ω and configuring the two parallel memristors (R_memoristor,2 and R_memoristor,3) to HRS, the desired 2 dB attenuation can be achieved. Detailed memristor configurations for another 3 attenuation levels are shown in Table S1. Thus, a single π-type MAN unit can achieve four discrete attenuation levels of 2 dB, 5 dB, 9 dB and 12 dB. Then, we further adopt a 2-unit cascaded architecture based on the proposed π-type unit. Benefiting from the consistent input/output impedance of each π-type unit (50 Ω), the total attenuation of the cascaded system realizes linear superposition of the attenuation of each individual unit. This is achieved without accumulating impedance mismatches or degrading high-frequency performance. The simulated RF responses are illustrated in Figure 4d,e, showing 12 different attenuation states with an attenuation tuning range of 2 dB to 24 dB across the full 10 MHz to 43.5 GHz band. The return loss is better than 10 dB for all attenuation states across the entire operating band, demonstrating excellent impedance matching. This path-switchable design enables the attenuator to achieve 12 discrete attenuation levels from 2 dB to 24 dB, only by tuning the resistance state of the memristors and the switching state of the SPDT switches, without hardware modification. It offers the advantages of a compact structure, zero static power consumption, broadband stability, and high reconfigurability.

Meanwhile, to minimize parasitic effects in practical circuits, several strategies were adopted. At the device level, the vertical MIM structure reduces parasitic series inductance compared to lateral designs, the optimized 4 μm² device area achieves a low OFF-state capacitance of 3.8 fF, and the high-resistivity silicon substrate (ρ > 10 kΩ·cm) suppresses substrate coupling losses. At the circuit level, the symmetric π-type topology inherently provides 50 Ω matching, while the GSG CPW layout with optimized dimensions ensures controlled impedance interconnections. All RF measurements were preceded by full on-wafer SOLT calibration to de-embed probe pad and cable parasitics.

Compared with conventional attenuator architectures, the proposed all-memristor π-type topology exhibits three distinct advantages for next-generation communication applications. First, the attenuator uses only identical two-terminal memristor switches. This eliminates the need for the separate switch matrices and resistor networks found in traditional designs. As a result, the device footprint is reduced, enabling ultra-high monolithic integration. Second, the zero static power consumption characteristic, derived from the non-volatile nature of memristor switches, is a critical improvement over conventional solid-state switch-based attenuators that require continuous DC bias. Third, the proposed architecture supports discrete attenuation tuning via I_CC regulation without hardware modification, exhibiting excellent reconfigurability.

For all-memristor reconfigurable attenuators, Au/HfO₂/Ag and Ag/HfO₂/Ag memristors with unique performance can enrich the design flexibility. We selected six core performance dimensions of RF switches, including insertion loss, isolation, retention, endurance, on-state resistance, and off-state capacitance, and performed a normalized comparison of the performance of Au/HfO₂/Ag and Ag/HfO₂/Ag devices through the radar plot (Figure 4f). The Au/HfO₂/Ag device demonstrates superior performance in terms of stability and durability, offering a stable and controllable programmable resistance state. The Ag/HfO₂/Ag device exhibits superior RF performance with ultra-low R_on and low insertion loss, desirable for RF transmission lines. Our work provides a reliable device foundation for the subsequent design and implementation of all-memristor reconfigurable attenuators. As summarized in Table S2, our Au/HfO₂/Ag and Ag/HfO₂/Ag memristor RF switches present clear advantages over both oxide-based and 2D material-based RF switches operating at frequencies below 120 GHz [8,17,19,27,30,31,32,33,34,35,36]. They achieve drastically higher switching ratios, better isolation, lower insertion loss and higher cutoff frequency than other oxide devices, while outperforming 2D devices in isolation and endurance with equally low insertion loss. Furthermore, Table S3 compares the performance of the all-memristor tunable attenuator with other reported tunable attenuators, Refs. [7,8,13,37,38,39] highlighting its significant advantages of zero static power consumption due to the non-volatile nature of HfO₂ switches, as well as higher integration density achieved with only 2 cascaded units.

4. Conclusions

In summary, we have systematically investigated HfO₂-based two-port RF switches designed for integrated attenuator applications. The Au/HfO₂/Ag devices demonstrate stable non-volatile bipolar resistive switching with a high ON/OFF ratio of ~10⁹ and reliable retention of 10⁵ s. A key finding is the quantitative relationship between I_CC, ON-state resistance, and RF insertion loss. The R_ON can be continuously tuned from 5.8 Ω to 197.5 Ω, providing the analog programmability essential for reconfigurable attenuation. On-wafer S-parameter measurements from 10 MHz to 43.5 GHz further reveal the scaling laws linking device geometry to OFF-state isolation and I_CC to ON-state insertion loss. We proposed a π-type cascaded attenuator architecture that achieves 12 discrete attenuation levels from 2 dB to 24 dB, enabled solely by I_CC tuning without additional passive components or static power consumption. These results not only provide a comprehensive understanding of the material-device-RF performance correlation in HfO₂-based memristor switches, but also present a simple, monolithically integrable pathway toward low-power, reconfigurable RF front-ends for next-generation communication systems.

Beyond insertion loss and isolation, practical RF front-end applications also require consideration of switching speed, linearity, and power handling. The intrinsic switching in HfO₂-based conductive bridge memristors is governed by nanosecond-scale Ag ion migration, suggesting inherently fast reconfiguration capability. Regarding linearity, the ON-state metallic filament behaves as a passive linear resistor under small-signal excitation, unlike semiconductor switches that introduce nonlinear capacitances. Future studies will systematically characterize these parameters under high-speed pulsed and large-signal conditions, and explore device engineering strategies, such as adjusting the HfO₂ thickness, to extend power handling toward transmitter-level requirements.