Thin Silicon Nitride Membrane Research Articles

Development of hard X-ray photoelectron spectroscopy in liquid cells using optimized microfabricated silicon nitride membranes.

We present first hard X-ray photoelectron spectroscopy (HAXPES) results of aqueous salt solutions and dispersions of gold nanoparticles in liquid cells equipped with specially designed microfabricated thin silicon nitride membranes, with thickness in the 15-25 nm range, mounted in a high-vacuum-compatible environment. The experiments have been performed at the HAXPES endstation of the GALAXIES beamline at the SOLEIL synchrotron radiation facility. The low-stress membranes are fabricated from 100 mm silicon wafers using standard lithography techniques. Platinum alignment marks are added to the chips hosting the membranes to facilitate the positioning of the X-ray beam on the membrane by detecting the corresponding photoemission lines. Two types of liquid cells have been used, a static one built on an Omicron-type sample holder with the liquid confined in the cell container, and a circulating liquid cell, in which the liquid can flow in order to mitigate the effects due to beam damage. We demonstrate that the membranes are mechanically robust and able to withstand 1 bar pressure difference between the liquid inside the cell and vacuum, and the intense synchrotron radiation beam during data acquisition. This opens up new opportunities for spectroscopic studies of liquids.

TEM sample preparation of lithographically patterned permalloy nanostructures on silicon nitride membranes.

We have prepared ferromagnetic nanostructures intended for the investigation of high-frequency magnetization dynamics in permalloy (Py) nanodisks using Lorentz transmission electron microscopy (LTEM) and electron holography. Py nanodisks were fabricated on thin silicon nitride (SiN) membranes using three different fabrication methods: lift-off, ion beam etching (IBE), and stencil lithography. They were further analyzed using different instruments, including scanning electron microscopy, LTEM, and electron holography. A bilayer of positive PMMA resist was utilized in the first fabrication method to form an undercut structure that guarantees a clean lift-off procedure. The second approach used dry etching with an Ar beam to etch a thin Py film, while an electron-beam-patterned negative resist mask kept the desired structure. In the third process, nanostencils (shadow masks) with submicrometer apertures were milled on SiN membranes using a focused ion beam. Furthermore, we have developed a new TEM sample preparation method, where we fabricated Py nanostructures on a bulk substrate with a SiN buffer layer and etched the substrate to create a thin SiN membrane under the Py nanostructure. Finally, we observed the vortex dynamics of the Py nanodisk under magnetic fields using LTEM and off-axis electron holography. A correlation between preparation methods and the properties of the Py nanostructures was made.

Impact of supporting nanometric membranes on the thermo-optical dynamics of individual plasmonic nanodisks.

The thermal dynamics and transient optical response of individual gold nanodisks supported on thin silicon nitride membranes were investigated using optical time-resolved pump-probe spectroscopy and finite-element modeling. The effect of reducing the membrane thickness from 50 nm to 15 nm on the nanodisk thermal dynamics was explored. A significant deceleration of the nanodisk cooling kinetics was observed, and linked to a quasi-two-dimensional heat diffusion process within the 15 nm thick membrane, without detectable modification of its thermal conductivity. Systematic measurements involving different optical probe wavelengths additionally revealed the contribution of indirect membrane heating to the measured time-resolved signals, an effect particularly pronounced in the spectral range where direct optical heating of the nanodisk induces minimal ultrafast modifications of its extinction cross-section.

Laser patterning captured in real-time: surface modifications of multilayer thin-films under nanosecond laser heating.

Controlled wrinkle formation has attracted intensive research interest as a means to modify surface properties. However, most of the currently explored methods rely on mechanically stretching polymer substrates with hard surface coatings, and the application of these methods to different materials is limited. Here, for the first time, we demonstrate laser-assisted periodic wrinkle formation on silicon nitride (SiN) membranes that are coated with titanium (Ti)/nickel (Ni) multilayers. Corrugated surface formation mechanism, as well as grain nucleation and growth, are studied using ultrafast transmission electron microscopy (UTEM). Periodic wrinkling and patterning of multilayers on thin SiN membranes are induced by a nanosecond pulsed laser, and the deformation of the film is captured by single-shot imaging. The investigated structures revealed periodic wrinkling of the membrane and corrugated surface formation on both sides of the film. The findings of this work could allow laser-irradiation-based fast fabrication of corrugated films with tailored properties.

(Invited) Operando Approaches for Observing Reactions at Buried Electrochemical Interfaces

The performance of materials for electrochemical energy storage and conversion depends critically on reactions taking place at the interfaces between these materials and their operating environment: from the reversible incorporation of ions at electrode−electrolyte interfaces in rechargeable batteries, to the conversion of the waste carbon dioxide into liquid fuels. As the reaction conditions change so these interfaces change, with often dramatic effects on their structure and chemical state. Understanding the evolution of these interfaces under realistic conditions is critical to the selection and design of improved electrochemical materials. However, it is extremely challenging to extract this information during operation, due to interference from the bulk phases either side of these buried interfaces, which disturb most interface-sensitive probes.1,2 I will give an overview of our recent efforts to develop operando approaches for resolving the structural and chemical evolution of electrochemical interfaces.3-8 This includes enclosed reaction cells sealed with X-ray/electron-transparent membranes such as thin (<100 nm) silicon nitride or graphene membranes that remain impermeable liquids. We thereby use soft X-ray absorption spectroscopy (sXAS) to observe solid-electrolyte interphase (SEI) formation on Li-ion battery anodes,4 and changes in the chemical state of transition metal electrocatalysts under electrochemical control. I will further discuss our recent progress in depositing thin lithium layers onto solid electrolytes,5,6 and using hard X-ray Photoelectron Spectroscopy (XPS) to study electrode-electrolyte interfaces in all-solid-state batteries.7,8 The development of these interface-sensitive operando approaches is expected to provide valuable tools for revealing a wide range of interfacial reactions. Collaboration across European and international partners has been critical to this work, and I will highlight how European funding has helped make this possible and the benefits associated with this. References Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.Weatherup et al. Top. Catal. 2018, 61, 2085.Velasco-Velez et al. Angew. Chemie 2015, 54, 14554.Swallow et al. Nature Commun. 2022, 13, 6070Weatherup et al. J. Phys. Chem. B 2018, 122, 737.Khatun et al. Electrochimica Acta 2022, 431, 141145Narayanan et al. Nature Commun. 2022, 13, 7237Gibson et al. Faraday Discuss., 2022 , 236, 267

Polymer Brushes on Silica Nanostructures Prepared by Aminopropylsilatrane Click Chemistry: Superior Antifouling and Biofunctionality.

In nanobiotechnology, the importance of controlling interactions between biological molecules and surfaces is paramount. In recent years, many devices based on nanostructured silicon materials have been presented, such as nanopores and nanochannels. However, there is still a clear lack of simple, reliable, and efficient protocols for preventing and controlling biomolecule adsorption in such structures. In this work, we show a simple method for passivation or selective biofunctionalization of silica, without the need for polymerization reactions or vapor-phase deposition. The surface is simply exposed stepwise to three different chemicals over the course of ∼1 h. First, the use of aminopropylsilatrane is used to create a monolayer of amines, yielding more uniform layers than conventional silanization protocols. Second, a cross-linker layer and click chemistry are used to make the surface reactive toward thiols. In the third step, thick and dense poly(ethylene glycol) brushes are prepared by a grafting-to approach. The modified surfaces are shown to be superior to existing options for silica modification, exhibiting ultralow fouling (a few ng/cm2) after exposure to crude serum. In addition, by including a fraction of biotinylated polymer end groups, the surface can be functionalized further. We show that avidin can be detected label-free from a serum solution with a selectivity (compared to nonspecific binding) of more than 98% without the need for a reference channel. Furthermore, we show that our method can passivate the interior of 150 nm × 100 nm nanochannels in silica, showing complete elimination of adsorption of a sticky fluorescent protein. Additionally, our method is shown to be compatible with modifications of solid-state nanopores in 20 nm thin silicon nitride membranes and reduces the noise in the ion current. We consider these findings highly important for the broad field of nanobiotechnology, and we believe that our method will be very useful for a great variety of surface-based sensors and analytical devices.

Investigating the Potential of Thin Silicon Nitride Membranes in Fiber-Based Photoacoustic Sensing.

The detection of methane, a strong greenhouse gas, has increased in importance due to rising emissions, which partly originate from unreported and undetected leaks in oil and gas fields. The gas emitted by these leaks could be detected using an optical fiber-based photoacoustic sensor called PAS-WRAP. Here, we investigate the potential of silicon-based membranes as more sensitive microphones in the PAS-WRAP concept. Toward this goal, we built a setup with which the frequency response of the membranes was interrogated by an optical fiber. Multiple mounting mechanisms were tested by adapting commercial interferometry systems (OP1550, ZonaSens, Optics11 B.V.) to our case. Finally, methane detection was attempted using a silicon nitride membrane as a sensor. Our findings show a quality factor of 2.4 at 46 kHz and 33.6 at 168 kHz for a thin silicon nitride membrane. This membrane had a frequency response with a signal-to-background ratio of 1 ± 0.7 at 44 kHz when tested in a vacuum chamber with 4% methane at 0.94 bar. The signal-to-background ratio was not significant for methane detection; however, we believe that the methods and experimental procedures that we used in this work can provide a useful reference for future research into gas trace detection with optical fiber-based photoacoustic spectroscopy.

Refractive axicon for X-ray microscopy applications: design, optimization, and experiment.

In a full-field transmission X-ray microscopy (TXM) setup, a condenser X-ray optical element is used to illuminate the sample by condensing the X-ray beam delivered by the synchrotron storage ring. On-going and future upgrades of synchrotron facilities to diffraction-limited storage rings will pose new challenges to these TXM setups, such as much smaller X-ray beams on the condenser. Here, we demonstrate that a refractive axicon can be used as an X-ray beam shaper to match the ring-shaped aperture of the condenser. Aiming at more efficient use of the incoming X-ray intensity, we explore several axicon designs both analytically and with numerical simulations. The axicons were produced by two-photon polymerization 3D printing on thin silicon nitride membrane substrates. The first characterization of the axicon was carried out at the TOMCAT beamline of the Swiss Light Source (Switzerland).

Multi‐Scale Lipid Membrane Flow by Electron Beam‐Induced Electrowetting (Adv. Mater. Interfaces 18/2021)

Lipid Membrane Flow Control In article number 2100257, Hiroki Miyazako, Kunihiko Mabuchi, and Takayuki Hoshino propose a new method for lipid membrane flow control in supported lipid bilayers (SLBs) using an electron beam (EB). An EB is indirectly irradiated onto SLBs which are formed on a thin silicon nitride membrane, and then a focused electric field around the beam spot causes membrane flow.

The influence of chromatic aberration on the dose-limited spatial resolution of transmission electron microscopy

The effect of chromatic aberration (CC) on the spatial resolution in transmission electron microscopy (TEM) was studied in thick specimens in which the sample becomes the limiting factor in the resolution. The sample influences the energy spread of the electron beam, allows only a limited electron dose, and modulates electron scattering events. The experimental set-up consisted of a thin silicon nitride membrane and a silicon wedge containing gold nanoparticles. The resolution was measured as a function of electron dose and sample thickness for different sample configurations and for different microscopy modalities including regular TEM, energy filtered TEM (EFTEM) and CC-corrected TEM. Comparison with an analytical model aided the understanding of the experimental data applied over varied conditions. The general trend for all microscopy modalities was a transition from a noise-limited resolution at low electron dose to a CC-limited resolution at high-dose in the absence of beam blurring. EFTEM required an accurate energy slit offset and an optimal energy spread to energy-slit width ratio to surpass regular TEM. The key advantage of CC correction appeared to be the best possible resolution for larger sample thickness at low electron dose outperforming EFTEM by about fifty percent. Several hypothetical sample configurations relevant to liquid phase electron microscopy were evaluated as well to demonstrate the capabilities of the analytical model and to determine the most optimal microscopy modality for this type of experiment. The analytical model included an automated optimization of the EFTEM settings and may aid in optimizing the sample-limited resolution for experimental analysis and planning.

Characterization of a split-ring-resonator-loaded transmission line at terahertz frequencies.

Recently we demonstrated the fabrication and testing of a variety of RF-engineered passive transmission-line-based components designed for operation at terahertz frequencies and fabricated on thin (1 μm) silicon-nitride membranes. In this work we measure the transmission response of a coplanar-strip transmission line loaded with split-ring resonators up to 2.5 THz. We observe three dominate modes within the measured frequency range; the predicted LC resonator mode at 0.510 THz, a higher-order LC resonator mode at 1.03 THz, and a higher-order dipole mode at 1.85 THz. The LC resonator mode is investigated using a modified version of the standard lumped element model which incorporates the transmission line between adjacent meta-atoms using ABCD matrices.

Field-emission electron gun for a MEMS electron microscope

This article presents a field-emission electron gun intended for use in a MEMS (microelectromechanical system) electron microscope. Its fabrication process follows the technology of a miniature device under development built from silicon electrodes and glass spacers. The electron gun contains a silicon cathode with a single very sharp protrusion and a bundle of disordered CNTs deposited on its end (called a sharp silicon/CNT cathode). It was tested in diode and triode configurations. For the diode configuration, a low threshold voltage <1000 V and a high emission current that reached 90 µA were obtained. After 30 min of operation at 900 V, the emission current decreased to 1.6 µA and was stable for at least 40 min, with RMS fluctuation in the anode current lower than 10%. The electron beam spot of the source was observed on the phosphor screen. In the diode configuration, the spot size was the same as the emission area (~10 µm), which is a satisfactory result. In the triode configuration, an extraction electrode (gate) control function was reported. The gate limited the emission current and elongated the lifetime of the gun when the current limit was set. Moreover, the electron beam current fluctuations at the anode could be reduced to ~1% by using a feedback loop circuit that controls the gate voltage, regulating the anode current. The developed sharp silicon/CNT cathodes were used to test the MEMS electron source demonstrator, a key component of the MEMS electron microscope, operating under atmospheric pressure conditions. Cathodoluminescence of the phosphor layer (ZnS:Ag) deposited on the thin silicon nitride membrane (anode) was observed.

A Micromachined Picocalorimeter Sensor for Liquid Samples with Application to Chemical Reactions and Biochemistry.

Calorimetry has long been used to probe the physical state of a system by measuring the heat exchanged with the environment as a result of chemical reactions or phase transitions. Application of calorimetry to microscale biological samples, however, is hampered by insufficient sensitivity and the difficulty of handling liquid samples at this scale. Here, a micromachined calorimeter sensor that is capable of resolving picowatt levels of power is described. The sensor consists of low‐noise thermopiles on a thin silicon nitride membrane that allow direct differential temperature measurements between a sample and four coplanar references, which significantly reduces thermal drift. The partial pressure of water in the ambient around the sample is maintained at saturation level using a small hydrogel‐lined enclosure. The materials used in the sensor and its geometry are optimized to minimize the noise equivalent power generated by the sensor in response to the temperature field that develops around a typical sample. The experimental response of the sensor is characterized as a function of thermopile dimensions and sample volume, and its capability is demonstrated by measuring the heat dissipated during an enzymatically catalyzed biochemical reaction in a microliter‐sized liquid droplet. The sensor offers particular promise for quantitative measurements on biological systems.

Assessment of 1/f noise associated with nanopores fabricated through chemically tuned controlled dielectric breakdown.

Recently, we developed a fabrication method-chemically-tuned controlled dielectric breakdown (CT-CDB)-that produces nanopores (through thin silicon nitride membranes) surpassing legacy drawbacks associated with solid-state nanopores (SSNs). However, the noise characteristics of CT-CDB nanopores are largely unexplored. In this work, we investigated the 1/f noise of CT-CDB nanopores of varying solution pH, electrolyte type, electrolyte concentration, applied voltage, and pore diameter. Our findings indicate that the bulk Hooge parameter (αb ) is about an order of magnitude greater than SSNs fabricated by transmission electron microscopy (TEM) while the surface Hooge parameter (αs ) is ∼3 order magnitude greater. Theαs of CT-CDB nanopores was ∼5 orders of magnitude greater than theirαb , which suggests that the surface contribution plays a dominant role in 1/f noise. Experiments with DNA exhibited increasing capture rates with pH up to pH ∼8 followed by a drop at pH ∼9 perhaps due to the onset of electroosmotic force acting against the electrophoretic force. The1/f noise was also measured for several electrolytes and LiCl was found to outperform NaCl, KCl, RbCl, and CsCl. The 1/f noise was found to increase with the increasing electrolyte concentration and pore diameter. Taken together, the findings of this work suggest the pH approximate 7-8 range to be optimal for DNA sensing with CT-CDB nanopores.

Stamping Nanoparticles onto the Electrode for Rapid Electrochemical Analysis in Microfluidics.

Electrochemical analysis is an efficient way to study various materials. However, nanoparticles are challenging due to the difficulty in fabricating a uniform electrode containing nanoparticles. We developed novel approaches to incorporate nanoparticles as a working electrode (WE) in a three-electrode microfluidic electrochemical cell. Specifically, conductive epoxy was used as a medium for direct application of nanoparticles onto the electrode surface. Three approaches in this work were illustrated, including sequence stamping, mix stamping, and droplet stamping. Shadow masking was used to form the conductive structure in the WE surface on a thin silicon nitride (SiN) membrane. Two types of nanomaterials, namely cerium oxide (CeO2) and graphite, were chosen as representative nanoparticles. The as-fabricated electrodes with attached particles were characterized using atomic force microscopy (AFM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Electrochemical analysis was performed to verify the feasibility of these nanoparticles as electrodes. Nanomaterials can be quickly assessed for their electrochemical properties using these new electrode fabrication methods in a microfluidic cell, offering a passport for rapid nanomaterial electrochemical analysis in the future.

Temperature dependence of the anomalous Nernst coefficient for Ni80Fe20 determined with metallic nonlocal spin valves

The anomalous Nernst effect, which generates an out-of-plane charge voltage in response to a thermal gradient perpendicular to the magnetization of a ferromagnet, can play a significant role in many spintronic devices where large thermal gradients exist. Since they typically include features deep within the submicron regime, nonlocal spin valves can be made very sensitive to this effect by lowering the substrate thermal conductance. Here, we use nonlocal spin valves suspended on thin silicon nitride membranes to determine the temperature dependence of the anomalous Nernst coefficient of 35 nm thick permalloy (Ni80Fe20) from 78 K to 300 K. In a device with a simple ferromagnet geometry, the transverse Seebeck coefficient shows a weak temperature dependence, with values at all T near 2.5 μV/K. Assuming previously measured values of the Seebeck coefficient for permalloy, which has a near-linear dependence on T, leads to a low temperature upturn in the anomalous Nernst coefficient RN. We also show that the temperature dependence of this coefficient is different when a constricted nanowire is used as the ferromagnetic detector element.

Achromatic Talbot lithography with nano-ring masks for high-throughput periodic patterning

Periodic patterning is important for various scientific and technological applications, especially in the nanoscale. Achromatic Talbot lithography (ATL) utilizing extreme ultraviolet (EUV) wavelengths, notably 13.5 nm, is a powerful lithographic technique enabling high-resolution and high-throughput nanopatterning over large areas. Improving the resolution and the throughput of the technique requires elaborate designs based on simulations and nanofabrication of transmission diffraction gratings on thin silicon nitride membranes. Our simulations point to the fact that compared to conventional ATL masks with hole arrays, masks consisting of annular rings and intersecting annular rings show increased performance in terms of throughput. A set of masks with uncrossed and crossed annular rings have been nanofabricated and exposed with spatially coherent synchrotron EUV light and the experimental results confirm our theoretical predictions that masks with annular rings and crossed rings yield dot arrays with improved throughput. The presented technique may enable applications in science and technology where large-area and periodic nanopatterning is needed.

Immobilization of Bioengineered Portal Protein Within a Solid-State Nanopore for Molecular Sensing

Site-specific, reproducible, and uniform-orientation protein immobilization on an inorganic surface while preserving protein conformation and activity is of wide interest for applications in biosensing, protein microarrays, and enzymology, among others. Successful immobilization requires understanding of material's surface chemistry, protein properties, and nature of their interaction to eliminate non-specific binding. Portal protein is a naturally occurring pore (biological nanopore) and part of the bacteriophage packaging machine that pumps the viral genome inside its capsid. In this work, we have explored different approaches to immobilize G20c portal protein from double-stranded bacteriophage G20c into a thin (∼30 nm) silicon nitride (SiNx) membrane embedded in a silicon chip. Desired orientation of the immobilized protein is achieved by maintaining a voltage bias across the membrane to electrokinetically drive a single protein into the synthetic nanopore. The nanopore geometry dictates a predominant favorable protein orientation while the portal preserves its conformation, as indicated by ion current measurements through the “hybrid nanopore”. Application of nanopores (both synthetic and biological) as label-free, single-molecule biosensors for electrical and/or optical probing of structural features in biomolecules have been widely explored. Confirmed by our single-molecule electrical sensing results, our hybrid nanopore system provides mechanically robust and chemically compatible synthetic protein framework, superior to its natural counterparts such as organic membranes (lipid bilayer, for example), and exploits tunable and engineerable characteristics of thermostable G20c portal protein rendering an active, high-resolution biomolecule sensing platform. We demonstrate here through chemical functionalization of synthetic nanopores and/or engineering G20c portal protein assemblies, a protein nanopore chemically linked to a synthetic nanopore, rendering considerably improved protein stability, sensing lifetime, and signal-to-noise ratio compared to our previous hybrid system. This development will be widely applicable to coupled nanopore sensor arrangements such as electro-optical and electro-pressure systems.

Preliminary research on imaging in MEMS electron microscope

The purpose of this article is to present preliminary results of research on imaging in a newly designed and micro-electro-mechanical system (MEMS) transmission electron microscope being developed. The main element of the MEMS microscope is an electron gun with an anode, which contains a very thin silicon nitride membrane; it separates the high-vacuum chamber of the microdevice from a sample that is located outside an electron optics column, on the anode in the air. The parallel electron beam that passes the membrane illuminates the studied sample. Thus, a new detection system, located outside the MEMS electron microscope structure, has to be developed. In this paper, the results of the computer simulations and the measurements of transmissivity and a scattering angle of the electron beam passing through a very thin silicon nitride membrane are presented. The influence of these parameters on the image generated on the scintillator crystal screen, placed above the anode with a sample, is considered. We also present an image of a copper wire placed over the thin silicon nitride membrane that was produced using the designed detection system.

Demonstration of a low-distortion terahertz system-on-chip using a CPS waveguide on a thin membrane substrate.

Distortion-free transmission of THz-bandwidth pulses over centimeter-scale distances is desirable for future THz system-on-chip (TSoC) applications. In this work we achieve this by utilizing a coplanar strip (CPS) transmission line fabricated on a thin (1 µm) silicon nitride membrane. To generate and detect the THz-bandwidth pulses we use a well-known lift-off technique to construct thousands of small (20 µm × 40 µm) thin-film LTG-GaAs photoconductive devices from a small (approx. 4 mm × 4 mm) substrate. The devices are then bonded to the CPS transmission line on the thin silicon nitride membrane, DC biased and optically pumped by a sub-picosecond laser. We demonstrate the generation and detection of a pulses containing frequencies up to 1.5 THz after propagating for 10 mm.