Suspended drums made of 2D materials hold potential for sensing applications. However, the industrialization of these applications is hindered by significant device-to-device variations presumably caused by non-uniform stress distributions induced by the fabrication process. Here, we introduce a methodology to determine the stress distribution from their mechanical resonance frequencies and corresponding mode shapes as measured by a laser Doppler vibrometer (LDV). To avoid limitations posed by the optical resolution of the LDV, we leverage a manufacturing process to create ultra-large graphene drums with diameters of up to 1000 μm. We solve the inverse problem of a Föppl–von Kármán plate model by an iterative procedure to obtain the stress distribution within the drums from the experimental data. Our results show that the generally used uniform pre-tension assumption overestimates the pre-stress value, exceeding the averaged stress obtained by more than 47%. Moreover, it is found that the reconstructed stress distributions are bi-axial, which likely originates from the transfer process. The introduced methodology allows one to estimate the tension distribution in drum resonators from their mechanical response and thereby paves the way for linking the used fabrication processes to the resulting device performance.

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The continuing drive to miniaturise electronic devices requires an understanding of how the materials in these devices behave under stress, particularly with respect to electromigration. In this study, we explore the relationship between the microstructure of copper (Cu) and electromigration by using an approach that combines in situ Scanning Electron Microscopy (SEM) with Electron Backscatter Diffraction (EBSD). This in-situ SEM-EBSD technique enables real-time observation and analysis of electromigration-induced microstructure changes. Our investigation provides detailed insights into the microstructure effect on electromigration. Specifically, samples annealed at 300 °C showed void formation after the electromigration test and higher Kernal average misorientation (KAM) values, indicating higher internal strains and an inhomogeneous microstructure. In contrast, samples annealed at 500 °C maintained lower KAM values with minimal changes in crystal orientation, highlighting a more stable and uniform electromigration-resistant microstructure. Our results demonstrate the critical role of microstructure in determining the electromigration resistance of copper interconnects. By optimizing the annealing temperature, the reliability of the copper microstructure can be significantly improved by reducing the dislocations and increasing grain size, thus extending its lifetime.

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Correction to: Microsystems & Nanoengineering https://doi.org/10.1038/s41378-024-00656-x published online 21 February 2024 After publication of this article¹, it was brought to our attention that two pressure values were not correctly copied from the submitted original work to the published version. Correction 1 (from PDF, Page 4 of 9): “These membranes show resonance frequencies above the audible range (f₀₁ > 20 kHz) at 1 × 10³ mbar by piezo-shaker actuation”. The described phrase needs to be changed reporting the right pressure value of 1 × 10⁻³ mbar. The new phrase will be: “These membranes show resonance frequencies above the audible range (f₀₁ > 20 kHz) at 1 × 10⁻³ mbar by piezo-shaker actuation”. Correction 2 (from PDF, Page 4 of 9): “Energy losses and dampening are minimized due to the low pressure of 1 × 10³ mbar”. Again, the described phrase needs to be changed reporting the right pressure value of 1 × 10⁻³ mbar. The new phrase will be: “Energy losses and dampening are minimized due to the low pressure of 1 × 10⁻³ mbar”.

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As a consequence of their high strength, small thickness, and high flexibility, ultrathin graphene membranes show great potential for pressure and sound sensing applications. This study investigates the performance of multi-layer graphene membranes for microphone applications in the presence of air-loading. Since microphones need a flatband response over the full audible bandwidth, they require a sufficiently high mechanical resonance frequency. Reducing membrane thickness facilitates meeting this bandwidth requirement, and therefore, also allows increasing compliance and sensitivity of the membranes. However, at atmospheric pressure, air-loading effects can increase the effective mass, and thus, reduce the bandwidth of graphene and other 2D material-based microphones. To assess the severity of this performance-limiting effect, we characterize the acoustic response of multi-layer graphene membranes with a thickness of 8 nm in the pressure range from 30 to 1000 mbar, in air and helium environments. A bandwidth reduction by a factor ∼ 2.8 × for membranes with a diameter of 500 μm is observed. These measurements show that air-loading effects, which are usually negligible in conventional microphones, can lead to a substantial bandwidth reduction in ultrathin graphene microphones. With analytical and finite element models, we further analyze the performance limits of graphene microphones in the presence of air-loading effects.

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The demand for accurate temperature sensing in extreme temperatures is increasing. Traditional silicon-based integrated temperature sensors usually cannot survive above 200 °C. Many researchers have started to focus on semiconductors with a large bandgap. Among them, silicon carbide (SiC) is the most promising one. Nevertheless, most reported SiC sensors are in the form of discrete components and are not compatible with integrated electronics. In this work, we demonstrate an open 4H-SiC CMOS technology, and the fabrication steps are detailed. The temperature sensing elements in this technology, including resistors based on different implanted layers and MOSFETs, are characterized up to 600 °C. At room temperature, the resistive-based elements demonstrate large negative temperature coefficients of resistance (TCRs). With increasing temperature, the TCR starts to decrease and even becomes positive. The TCR change is due to the interplay between increasing dopant ionization rate and decreasing mobility as a function of temperature. The resistance change with temperature fits well into the Steinhart-Hart model and second-order polynomial equation. The p-type diode-connected MOSFET has a sensitivity of 4.35 mV/°C with a good linearity. The nMOS-based sensor has a maximum sensitivity of -9.24 mV/°C but a compromised linearity. The characterization of these sensing elements provides important results for potential users who will work on SiC integrated temperature sensing with this technology.

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Silicon carbide (SiC) is recognized as an excellent material for microelectromechanical systems (MEMS), especially those operating in challenging environments, such as high temperature, high radiation, and corrosive environments. However, SiC bulk micromachining is still a challenge, which hinders the development of complex SiC MEMS. To address this problem, we present the use of a carbon nanotube (CNT) array coated with amorphous SiC (a-SiC) as an alternative composite material to enable high aspect ratio (HAR) surface micromachining. By using a prepatterned catalyst layer, a HAR CNT array can be grown as a structural template and then densified by uniformly filling the CNT bundle with LPCVD a-SiC. The electrical properties of the resulting SiC-CNT composite were characterized, and the results indicated that the electrical resistivity was dominated by the CNTs. To demonstrate the use of this composite in MEMS applications, a capacitive accelerometer was designed, fabricated, and measured. The fabrication results showed that the composite is fully compatible with the manufacturing of surface micromachining devices. The Young’s modulus of the composite was extracted from the measured spring constant, and the results show a great improvement in the mechanical properties of the CNTs after coating with a-SiC. The accelerometer was electrically characterized, and its functionality was confirmed using a mechanical shaker. (Figure presented.)

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Since the performance of micro-electro-mechanical system (MEMS)-based microphones is approaching fundamental physical, design, and material limits, it has become challenging to improve them. Several works have demonstrated graphene’s suitability as a microphone diaphragm. The potential for achieving smaller, more sensitive, and scalable on-chip MEMS microphones is yet to be determined. To address large graphene sizes, graphene-polymer heterostructures have been proposed, but they compromise performance due to added polymer mass and stiffness. This work demonstrates the first wafer-scale integrated MEMS condenser microphones with diameters of 2R = 220–320 μm, thickness of 7 nm multi-layer graphene, that is suspended over a back-plate with a residual gap of 5 μm. The microphones are manufactured with MEMS compatible wafer-scale technologies without any transfer steps or polymer layers that are more prone to contaminate and wrinkle the graphene. Different designs, all electrically integrated are fabricated and characterized allowing us to study the effects of the introduction of a back-plate for capacitive read-out. The devices show high mechanical compliances C_m = 0.081–1.07 μmPa⁻¹ (10–100 × higher than the silicon reported in the state-of-the-art diaphragms) and pull-in voltages in the range of 2–9.5 V. In addition, to validate the proof of concept, we have electrically characterized the graphene microphone when subjected to sound actuation. An estimated sensitivity of S_1kHz = 24.3–321 mV Pa⁻¹ for a V_bias = 1.5 V was determined, which is 1.9–25.5 × higher than of state-of-the-art microphone devices while having a ~9 × smaller area. (Figure presented.).

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As the dimensions of interconnects in integrated circuits continue to shrink, an urgent need arises to understand the physical mechanism associated with electromigration. Using x-ray nanodiffraction, we analyzed the stresses in Blech-structured pure Cu lines subjected to different electromigration conditions. The results suggest that the measured residual stresses in the early stages of electromigration are related to relaxation of stresses caused by thermal expansion mismatch, while a developing current-induced stress leads to reductions in the residual stress after longer test times. These findings not only validate the feasibility of measuring stress in copper lines using nanodiffraction but also highlight the need for a further understanding, particularly through in situ electromigration experiments with x-ray nanodiffraction analysis.

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In this paper, we present the surface modification of multilayer graphene electrodes with platinum (Pt) nanoparticles (NPs) using spark ablation. This method yields an individually selective local printing of NPs on an electrode surface at room temperature in a dry process. NP printing is performed as a post-process step to enhance the electrochemical characteristics of graphene electrodes. The NP-printed electrode shows significant improvements in impedance, charge storage capacity (CSC), and charge injection capacity (CIC), versus the equivalent electrodes without NPs. Specifically, electrodes with 40% NP surface density demonstrate 4.5 times lower impedance, 15 times higher CSC, and 4 times better CIC. Electrochemical stability, assessed via continuous cyclic voltammetry (CV) and voltage transient (VT) tests, indicated minimal deviations from the initial performance, while mechanical stability, assessed via ultrasonic vibration, is also improved after the NP printing. Importantly, NP surface densities up to 40% maintain the electrode optical transparency required for compatibility with optical imaging and optogenetics. These results demonstrate selective NP deposition and local modification of electrochemical properties in graphene electrodes for the first time, enabling the cohabitation of graphene electrodes with different electrochemical and optical characteristics on the same substrate for neural interfacing.@en

Silicon carbide (SiC) coated vertically aligned carbon nanotubes (VACNT) are attractive material for fabricating MEMS devices as an alternative for bulk micromachining of SiC. In order to examine the mechanical properties of SiC-CNT composites at high temperatures, we fabricated VACNT micro-pillars with different amounts of SiC coating and performed high-temperature micro-pillar compression on these samples. The indentation result shows that the coating can improve the elastic modulus up to three orders of magnitude. Samples were tested at room temperature, 300°C, 600°C, and 900°C under compressive load. No significant degradation of the mechanical properties was observed at elevated temperatures, demonstrating the harsh environment potential of this composite.@en

Synchronized rectifiers offer promising solutions for piezoelectric energy harvesting; however, achieving the promised energy extraction performance necessitates using either a bulky inductor or multiple large capacitors, which cannot be on-chip integrated and increase the system form factor. This article introduces a fully integrated sequenced synchronized switch harvesting on capacitors (3SHC) rectifier. The input piezoelectric transducer (PT) uses microelectromechanical system technology. The cantilever is equally split into multiple strongly coupled subcantilevers, with each cantilever treated as an individual PT connected to the proposed rectifier. The 3SHC rectifier cyclically operates multiple times to synchronously flip the voltage of each cantilever sequentially. With the proposed design, all the flying capacitors only need to match the capacitance of each subcantilever; hence, they can be fully integrated on-chip. The design is fabricated using standard 0.18 μ m CMOS technology. Measurement results show that the proposed 3SHC rectifier attains an 80% voltage flip efficiency and achieves a 730% power enhancement compared to a full-bridge rectifier.

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Due to the deficient passivation of the interface between silicon carbide and silicon dioxide, the defect-induced capture and release of trapped charges triggered by external Bias Temperature Stress (BTS) leads to parameter shifts and degraded device performance. This study models the trap-induced transient current in silicon carbide metal-oxide-semiconductor capacitors, providing insight into how capacitance and conductance change during C-V measurements under conditions of high temperature, varied frequency, and varied applied voltage.@en

Integrated circuits based on wide bandgap semiconductors are considered an attractive option for meeting the demand for high-temperature electronics. Here, we report an analog-to-digital converter fabricated in a silicon carbide complementary metal-oxide-semiconductor technology now available through Europractice. The MOSFET component in this technology was measured up to 500 °C, and the key parameters, such as threshold voltage, field-effect mobility, and channel-length modulation parameters, were extracted. A 4-bit flash data converter, consisting of 266 transistors, is implemented with this technology and demonstrates correct operation up to 400 °C. Finally, the gate oxide quality is investigated by time-dependent dielectric breakdown measurements at 500 °C. A field-acceleration factor of 4.4 dec/(MV/cm) is obtained by applying the E model.

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This study explores the application of a novel transfer-free method for the synthesis of multilayer Chemical Vapour Deposition (CVD) graphene directly on transparent sub-strates, specifically to create transparent Microelectrode Arrays (MEAs) for optogenetic studies. Traditional methods typically involve a graphene transfer step that can compromise the material's integrity and electrical properties. By eliminating this step, our approach simplifies the fabrication process. The developed MEAs were characterised by Raman spectroscopy, op-tical transmittance, and electrochemical impedance spectroscopy. We also assessed the stability and recording capabilities of the fabricated MEAs, alongside a comparative assessment with a commercial MEA. Turbostratic graphene grown directly on quartz and sapphire was successfully achieved. Our transfer-free MEAs exhibit promising signal detection capabilities, despite a relatively high baseline noise of ∼ 23μ V. and a significantly large impedance at 1 kHz (3.2 to 9.89 M Ω) surpassing values in other studies. The devices exhibited low stability after exposure to liquid media during the soaking and ageing tests, causing large variations in the electrochemical measurements post-exposure. This was due to the permeability of the encapsulation layer and the biodegradability of the molybdenum structures, which led to significant structural and chemical changes in the devices. While further work is required to prevent the failure mechanisms of the device, this study demonstrates the feasibility of transparent MEA fabrication by means of a transfer-free approach directly on quartz substrates.

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The fabrication and design of carbon-based hierarchical structures with tailored nano-architectures have attracted the enormous attention of the materials science community due to their exceptional chemical and physical properties. The collective control of nano-objects, in terms of their dimensionality, orientation and size, is of paramount importance to expand the implementation of carbon nanomaterials across a large variety of applications. In this context, porous anodic alumina (PAA) has become an attractive template where the pore morphologies can be straightforwardly modulated. The synthesis of diverse carbon nanomaterials can be performed using PAA templates, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanodiamonds, or can act as support for other carbon allotropes such as graphene and other carbon nanoforms. However, the successful growth of carbon nanomaterials within ordered PAA templates typically requires a series of stages involving the template fabrication, nanostructure growth and finally an etching or electrode metallization steps, which all encounter different challenges towards a nanodevice fabrication. The present review article describes the advantages and challenges associated with the fabrication of carbon materials in PAA based materials and aims to give a renewed momentum to this topic within the materials science community by providing an exhaustive overview of the current synthesis approaches and the most relevant applications based on PAA/Carbon nanostructures materials. Finally, the perspective and opportunities in the field are presented. @en

In this work, a highly linear temperature sensor based on a silicon carbide (SiC) p-n diode is presented. Under a constant current biasing, the diode has an excellent linear response to the temperature (from room temperature to 600°C). The best linearity (coefficient of determination ${R}^{{2}}$ = 99.98%) is achieved when the current density is 0.53 mA/cm2. The maximum sensitivity of the p-n diode is 3.04 mV/°C. The temperature sensor is fully compatible with Fraunhofer Institute (FHG) IISB's open SiC CMOS (complementary metal-oxide-semiconductor) technology, thus enabling the monolithic integration with SiC readout circuits for high-temperature applications. The sensor also features a simple fabrication process. To our knowledge, the presented device is the first SiC diode temperature sensor that does not require a mesa etch or backside contacts.

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This paper presents p-n diode temperature sensors and MOSFET temperature sensors in low-voltage silicon carbide (SiC) CMOS technology. The reported temperature sensors directly make use of the existing doping layers in the CMOS process, thus enabling the monolithic integration of the SiC temperature transducer and the SiC readout electronics. The temperature sensor is characterized from 25 to 200°C. The diode-based temperature sensor has a maximum sensitivity of 3.27 mV/°C and a maximum R 2 of 99.81%. The MOSFET-based temperature sensor achieved a maximal sensitivity of 16.5 mV/°C, however, with less linearity (R 2 max = 99.11%). This technology shows a unique potential for implementing harsh environment smart temperature sensors.@en

This paper presented a comprehensive experimental and simulation study for thermomigration (TM) accompanying electromigration (EM) at elevated current densities. Both Blech and standard wafer-level electromigration acceleration test (SWEAT)-like test structures, with aluminum (Al) as a carrier, were used for testing and analysis. In Part I of our study (Cui et al., 2023a), the experimental and numerical results with the current density of 1 MA/cm² were presented. We observed that Al stripes with a SWEAT structure did not show damage in the entire length, while Blech structures showed void and hillock formations only at the cathode and anode, respectively. The temperature gradient owing to Joule heating was neglected in our previous simulations, and the predicted results agreed well with the experimental observations. However, we have not theoretically verified the effect of the temperature gradient. In this paper, we first reported the new experimental data under the elevated current densities of 3 and 5 MA/cm². In both Blech and SWEAT structures, the spreading of voids in the middle region of conductors was observed. Moreover, in Blech structures, voiding in the middle region occurred after a period of time when voids/hillocks were formed at the cathode and anode, while the SWEAT structures did not show damage at the two ends. Next, based on the coupled 3D theory (Cui et al., 2023a), new analytical one-dimensional (1D) solutions were derived for the Blech and SWEAT structures in the un-passivated configuration considering TM. We found that TM played a significant role in the EM development in the middle of conductors under the elevated current density. The numerical results were in excellent agreement with the experimental data with the consideration of TM. We further established new EM failure's threshold criteria for the SWEAT structures in the form of the product of current density and square of conductor length. This is a major departure from the original Blech's theory in which only mechanical stress gradient was considered. We also studied the acceleration factor of the current density exponent and presented an insight into failure mechanisms associated with TM.

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Nitrogen dioxide (NO₂) is a potential hazard to human health at low concentrations, below one part per million (ppm). NO₂ can be monitored using gas sensors based on multi-layered graphene operating at ambient temperature. However, reliable detection of concentrations on the order of parts per million and lower is hindered by partial recovery and lack of reproducibility of the sensors after exposure. We show how to overcome these longstanding problems using ultraviolet (UV) light. When exposed to NO₂, the sensor response is enhanced by 290 % − 550 % under a 275 nm wavelength light emitting diode irradiation. Furthermore, the sensor's initial state is completely restored after exposure to the target gas. UV irradiation at 68 W/m2 reduces the NO₂ detection limit to 30 parts per billion (ppb) at room temperature. We investigated sensor performance optimization for UV irradiation with different power densities and target gases, such as carbon oxide and ammonia. Improved sensitivity, recovery, and reproducibility of UV-assisted graphene-based gas sensors make them suitable for widespread environmental applications.

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The continuous downscaling of microelectronics has introduced many reliability issues on interconnect. Electromigration and dewetting are major reliability concerns in high-temperature micro- and nanoscale devices. In this paper, the local dewetting of copper thin film during the electromigration test was first found and investigated. When the high current was applied, the dewetted copper forming around the edge was observed at the cathode of the conductor. Furthermore, the effect of temperature and conductor size on local dewetting was investigated. Our proposed mechanism for local dewetting is in good agreement with experimental findings.

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