Free-standing Silicon Nitride Membrane Research Articles

Focused helium ion beam nanofabrication by near-surface swelling

The focused helium ion beam microscope is a versatile imaging and nanofabrication instrument enabling direct-write lithography with sub-10 nm resolution. Subsurface damage and swelling of substrates due to helium ion implantation is generally unwanted. However, these effects can also be leveraged for specific nanofabrication tasks. To explore this, we investigate focused helium ion beam induced swelling of bulk crystalline silicon and free-standing amorphous silicon nitride membranes using various irradiation strategies. We show that the creation of near-surface voids due to helium ion implantation can be used to induce surface nanostructure and create subsurface nanochannels. By tailoring the ion dose and beam energy, the size and depth of the swollen features can be controlled. Swelling heights of several hundred nanometers are demonstrated, and for the embedded nanochannels, void internal diameters down to 30 nm are shown. Potential applications include the engineering of texturized substrates and the prototyping of on-chip nanofluidic transport devices.

Laser machining of free-standing silicon nitride membranes

Patterning free-standing Silicon Nitride membranes has enabled the development of various SiN-based devices such as quantum resonators, biosensors, temperature detectors, and plasmonic devices. Patterning SiN membranes usually involves expensive and time-consuming cleanroom techniques that limit flexibility in the development of new SiN membrane designs. While ultrafast laser machining has been proposed to create micron to submicron features in these membranes, laser machining of large area features has not been reported as it often results in membrane failure due to stress concentration during laser machining. We show that continuous laser machining can be used to create a crack and to extract the fracture toughness of the membrane (1.9 ± 0.3 MPa.m1/2). To successfully patterned SiN membranes without failure, a low stress laser machining method is developed, where stress concentration can be minimized by adjusting the laser drilling sequence, a result confirmed by finite element simulations. Complex shapes can thus be machined over a large area with a very high success rate. Laser parameters are optimized to improve the quality of the laser-cut edges without the formation of laser-induced periodic surface structure. Transmission Electron Microscopy and Electron Energy Loss Spectroscopy indicate a modification of the Si/N ratio over a small region next to the laser machined features. These results demonstrate that low stress ultrafast laser machining can be used to create patterns in SiN membranes quickly and economically, enabling the rapid development of new membrane designs.

Micro-Kelvin Resolution at Room Temperature Using Nanomechanical Thermometry.

Ultrahigh sensitivity temperature measurement is becoming increasingly relevant for different scientific and technological fields from fundamental physics to high-precision engineering applications. Here, we demonstrate the use of a nanomechanical resonator—free standing silicon nitride membranes with thicknesses in the nanoscale—for room temperature thermometry reaching an unprecedented resolution of 15 μK. These devices were characterized by using an interferometric system at high vacuum, where there are only two possible mechanisms for heat transfer: thermal conductivity and radiation. While the expected behavior should be to decrease the frequency of the mechanical resonance due to the thermoelastic effect, we observe that the nanomechanical response can be both positive and negative depending on the thermal flux: a heat point source always shifts the mechanical resonance to lower frequencies, while a distributed heat source shifts the resonance to higher frequencies.

Radiative Heat Transfer in Freestanding Silicon Nitride Membranes

Freestanding silicon nitride ($\mathrm{Si}\mathrm{N}$) mechanical resonators are of central interest in applications such as temperature and mass sensing, and for fundamental optomechanical research. Understanding thermal coupling between a membrane resonator and its environment is required to predict thermal noise, frequency noise, and sensor responses to temperature changes. In this work, we provide closed-form derivations of intrinsic thermal-coupling quantities in freestanding thin films---namely, total thermal conductance with the surroundings, thermal response time, and the relative contribution of thermal radiation. Our model is valid for any freestanding thin film anchored on all sides when small temperature differences between the film and its environment are considered. We particularly emphasize the specific case of $\mathrm{Si}\mathrm{N}$, for which spectral emissivity is thoroughly investigated as a function of thickness and temperature. We find that radiative heat exchanges can play a non-negligible role, and can even dominate thermal coupling for membranes of sizes commonly used in optomechanics experiments. We find that our model is in agreement with preliminary experimental results on radiative heat transfer between a ceramic heater and a $3\ifmmode\times\else\texttimes\fi{}3\phantom{\rule{0.2em}{0ex}}{\mathrm{mm}}^{2}$ membrane in a high vacuum.

Enhanced Reduction of Thermal Conductivity in Amorphous Silicon Nitride-Containing Phononic Crystals Fabricated Using Directed Self-Assembly of Block Copolymers.

Studies have demonstrated that the thermal conductivity (κ) of crystalline semiconductor materials can be reduced by phonon scattering in periodic nanostructures formed using templates fabricated from self-assembled block copolymers (BCPs). Compared to crystalline materials, the heat transport mechanisms in amorphous inorganic materials differ significantly and have been explored far less extensively. However, thermal management of amorphous inorganic solids is crucial for a broad range of semiconductor devices. Here we present the fabrication of freestanding amorphous silicon nitride (SiNx) membranes for studying κ in an amorphous solid. To form a periodic nanostructure, directed self-assembly of cylinder-forming BCPs is used to pattern in the SiNx highly ordered, hexagonally close packed nanopores with pitch and neck width down to 37.5 and 12 nm, respectively. The κ of the nanoporous SiNx membranes is 60% smaller than the classically predicted value based on just the membrane porosity. In comparison, holes with much larger neck widths and pitches patterned by e-beam lithography lead to only a slight reduction in κ, which is closer to the classical porosity-based prediction. These results demonstrate that κ of amorphous SiNx can be reduced by introducing periodic nanostructures that behave as a phononic crystal, where the relationship between the smallest dimension of the nanostructure and the length scale of the mean-free paths of the dominant, heat-carrying phonons is critical. Additionally, changing the orientation of the hexagonal array of nanopores relative to the primary direction of heat flow has a smaller impact on amorphous SiNx than was previously observed in silicon.

Detection of Tubulin and TAU Proteins Aggregations using Solid-State Nanopore and Atomic Force Microscopy (AFM)

Aggregations of tau and tubulin proteins are one of the leading causes of many neurodegenerative diseases, such as Pick's, Alzheimer's, and Parkinson's. In this work, protein aggregations are analyzed in salt solution using a solid-state nanopore sensing system. A varieties of fabrication techniques, such as photolithography, chemical vapor deposition, focused ion beam milling and ion beam sculpting are used to fabricate 8 - 30 nm nanopores in free-standing silicon nitride membrane on silicon. The nanopore is placed in between two polydimethylsiloxane chambers with microfluidic channels filled with 1 M KCl solution, and a pair of silver chloride electrodes are embedded in the channels. After a stable ionic current is established by applying bias voltage across the electrodes, charged protein molecules are added to the top chamber (ground), and driven to the bottom chamber by the applied electric field. The passing protein molecules partially block the ion flow in the nanopore and increase the pore resistivity, as a result the ionic current drops. The amplitude of the current drops is proportional to the size of the protein molecules. In this work, the aggregations of tau and tubulin proteins are detected at bias voltages ranges from 60 - 210 mV and pH ranges from 7.4 - 12.0. Monomer proteins were measured with 8 - 10 nm nanopores, aggregated proteins were measured using 15 - 30 nm nanopores. Finally, the amplitude of current drops for monomer and aggregated proteins are compared. Our results show that the tau and tubulin proteins are aggregated to dimer, trimer and multimers. The nanopore results are verified by AFM scanning of dried protein samples on mica surface. The AFM results are found to be consistent with the nanopore measurements.

Low-Temperature Vapor-Solid Growth of ZnO Nanowhiskers for Electron Field Emission

One-dimensional zinc oxide nanostructures have aroused interest from scientists and engineers for electron field emission applications because of their experimentally accessible high aspect ratio in combination with their low work function. A comprehensive study of the vapor-solid growth of zinc oxide (ZnO) nanowhiskers by utilizing zinc acetylacetonate hydrate and oxygen at low temperature (580 °C) is reported herein. The nanowhiskers morphology was investigated by varying different growth parameters, such as temperature, substrate type and position, gas flow, precursor amount, and growth time. According to the obtained parameter dependences, the process was optimized to achieve homogenous crystalline nanowhiskers with high aspect ratios and clearly defined surface facets and tips. We show electron field emission measurements from tailor-made ZnO nanowhiskers grown on n-doped silicon, titanium thin films, and free-standing silicon nitride membranes, revealing field emission turn-on fields significantly lower compared to a perfect flat ZnO thin film. Especially the latter devices—ZnO nanowhiskers on a free-standing membrane—might pave the way into a novel nanomembrane detector unit in proteomics, which can significantly extend the mass range of current time-of-flight mass spectrometers.

How tiny patterned membranes open the Cosmos to scientists

Layering metals in unique patterns reduces the conductance of freestanding silicon nitride membranes, opening new opportunities for improving low temperature sensors.

Electric Field and Ionic Strength Dependent Translocation of Tau Protein Through Solid-State Nanopore

Aggregation of tau protein is one of the leading causes of neurodegenerative diseases like Alzheimer's, Parkinson's, Pick's etc. In this report, nanopores are fabricated in free-standing silicon nitride membrane on silicon substrate by ion beam sculpting method to the desired sizes, ∼10-25 nm diameters. The nanopore chip is placed in between two chambers made of polydimethylsiloxane. The chambers are filled with potassium chloride (KCl) solution. A stable ionic current is established by applying a biased voltage on a pair of silver chloride electrodes across the nanopore membrane. When charged tau proteins are put into one of the chambers, the applied electric field will drive these molecules to the other chamber through the nanopore, as a result of the increased resistivity by the passing molecules, the ionic current drops. Tau proteins are characterized by measuring the parameters of current drop events produced by their translocation through a solid-state nanopore: the time duration, the current drop amplitudes and the integrated area which are directly proportional to the excluded volume of tau protein. The nanopore experiments were performed under applied voltages from 60-210 mV at pH 4.0 and 12.0 with 0.1 M, 0.5 M, 1 M, 1.5 M, and 2 M KCl. Our data show that at pH 7.4 and 12.0 the current drop and time duration amplitudes both increase with increased applied voltages. The current amplitude is found to be linearly increased with salt concentration at both pHs. The time duration increases at pH 7.4 and decreases at 12.0 with increased salt concentrations. Furthermore, our results show that tau protein is aggregated to multimers when tau with heparin (tau : heparin = 1:4) are incubated at 50 oC in 1 M KCl at pH 12.0 for 12 days.

Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles

We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100 nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. We discover that a key effect to explain the coupling in these systems is the extinction cross section enhancement due to the excitation of the LSPR at selected wavelengths. In order to validate this coupling, we have developed a fixed wavelength interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs onto the membrane, discerning in between single particles and dimers by the mechanical frequency shift. We have also introduced three-axis mechanical scanners with nanometer-scale resolution in our experimental setup to selectively study single nanoparticles or small clusters. Whereas the single particles are polarization-insensitive, the gold dimers have a clearly defined polarization angle dependency as expected by theory. Finally, we found an unexpected long-distance (∼200 nm) coupling of the LSPR of separated AuNPs coming out from the guided light by the silicon nitride membrane.

Bipolar Electrochemistry on a Nanopore-Supported Platinum Nanoparticle Electrode.

In this Technical Note, we describe a method to fabricate nanopore-supported Pt nanoparticle electrodes and their use in bipolar electrochemistry. A Pt nanoparticle is deposited on the orifice of a solid-state nanopore inside a focused-ion beam (FIB) system. Complete blockage of the nanopore with Pt metal forms a closed bipolar nanoparticle electrode whose size and shape can be tunable in one simple step. Nanoparticle electrodes and their arrays can be prepared on different substrates such as the tip of a glass pipet, a double-barrel pipet, and a freestanding silicon nitride membrane. Steady-state voltammetry can be performed on such nanoparticle electrodes via bipolar electrochemistry. Moreover, an array of Pt nanoparticles can be used for fluorescence-enabled electrochemical microscopy. Future use of highly advanced FIB systems may allow nanoparticles of <10 nm to be fabricated which may enable coupled electrochemical reactions of single redox molecules. Pipette-supported single particle electrodes may also find useful applications in high resolution imaging with nanoscale scanning electrochemical microscopy (SECM) and neurochemical analysis inside single cells.

Magnetoresistance (MR) of twisted bilayer graphene on electron transparent substrate

We studied the magnetoresistance (MR) of twisted bilayer graphene (tBLG) on electron transparent substrate. Samples of tBLG were assembled on free-standing silicon nitride (SiNx) membranes (<100nm thick) by transferring chemical vapor deposition (CVD)-grown single layer graphene (SLG) twice; this allowed the measurement of the angle of rotation between the two layers, the twist angle, by electron diffraction using a transmission electron microscope (TEM). To compare with the previous reports on tBLG, we performed Raman spectroscopy on our samples. We measured the MR of tBLG for two different twist angles: 2° (small) and 18° (large). The MR showed superposition of two Shubnikov de Haas (SdH) oscillations for both angles. An analysis of the oscillation peaks by Landau fan diagrams showed difference as twist angle. While the large twist angle (18°) sample had two anomalous π Berry’s phases, the small twist angle (2°) sample had conventional 2π and anomalous π Berry’s phase depending on carrier density.

Emissivity of freestanding membranes with thin metal coatings

Freestanding silicon nitride membranes with thicknesses down to a few tens of nanometers find use as TEM windows or soft X-ray spectral purity filters. As the thickness of a membrane decreases, emissivity vanishes, which limits radiative heat emission and resistance to heat loads. We show that thin metal layers with thicknesses in the order of 1 nm enhance the emissivity of thin membranes by two to three orders of magnitude close to the theoretical limit of 0.5. This considerably increases thermal load capacity of membranes in vacuum environments. Our experimental results are in line with classical theory in which we adapt thickness dependent scattering terms in the Drude and Lorentz oscillators.

A Mask-Free Passivation Process for Low Noise Nanopore Devices.

Solid-state nanopores have been studied widely for the label-free analysis of single biomolecules. The translocation of charged biomolecules through a solid-state nanopore is driven by the applied voltage across a thin membrane. The ionic current changes in response to the translocation of DNA through the nanopore. Solid-state nanopores have many advantages over biological nanopores, such as α-hemolysin and MspA, but the high DNA translocation velocity and the inherent noise in solid-state nanopores have hindered its applications to more precise measurements, such as DNA sequencing. This paper reports a simple and reproducible way of passivating the surface of a nanopore device using an insulating layer, photodefinable PDMS (P-PDMS), to reduce noise and enhance the accuracy of the electrical measurements. This new approach does not require a separate photo-mask or sophisticated micro-alignment equipment to pattern the insulating layer. The pit structure on the back side of the support chip serves as a mask, enabling mask-free photolithography, and the insulating layer only on top of the free-standing silicon nitride membrane can be irradiated selectively by UV and removed by subsequent development in toluene. The resulting nanopore device with a small free standing silicon nitride membrane surrounded by a thick insulating layer showed improved noise characteristics. The root-mean-square noise of the ionic current was reduced to 3.8 pA from 90.8 pA by the formation of a micron-thick insulating layer. The overall performance of the nanopores with an insulating layer was improved significantly when tested with the double-stranded DNA (λ-DNA).

Low aspect ratio micropores for single-particle and single-cell analysis.

This paper describes microparticle and bacterial translocation studies using low aspect ratio solid-state micropores. Micropores, 5 μm in diameter, were fabricated in 200 nm thick free-standing silicon nitride membranes, resulting in pores with an extremely low aspect ratio, nominally 0.04. For microparticle translocation experiments, sulfonated polystyrene microparticles and magnetic microbeads in size range of 1-4 μm were used. Using the microparticle translocation characteristics, we find that particle translocations result in a change only in the pore's geometrical resistance while the access resistance remains constant. Furthermore, we demonstrate the ability of our micropore to probe high-resolution shape information of translocating analytes using concatenated magnetic microspheres. Distinct current drop peaks were observed for each microsphere of the multibead architecture. For bacterial translocation experiments, nonflagellated Escherichia coli (strain HCB 5) and wild type flagellated Salmonella typhimurium (strain SJW1103) were used. Distinct current signatures for the two bacteria were obtained and this difference in translocation behavior was attributed to different surface protein distributions on the bacteria. Our findings may help in developing low aspect ratio pores for high-resolution microparticle characterization and single-cell analysis.

Electroless plating of thin gold films directly onto silicon nitride thin films and into micropores.

A method to directly electrolessly plate silicon-rich silicon nitride with thin gold films was developed and characterized. Films with thicknesses <100 nm were grown at 3 and 10 °C between 0.5 and 3 h, with mean grain sizes between ∼20 and 30 nm. The method is compatible with plating free-standing ultrathin silicon nitride membranes, and we successfully plated the interior walls of micropore arrays in 200 nm thick silicon nitride membranes. The method is thus amenable to coating planar, curved, and line-of-sight-obscured silicon nitride surfaces.

Rapid manufacturing of low-noise membranes for nanopore sensors by trans-chip illumination lithography

In recent years, the concept of nanopore sensing has matured from a proof-of-principle method to a widespread, versatile technique for the study of biomolecular properties and interactions. While traditional nanopore devices based on a nanopore in a single layer membrane supported on a silicon chip can be rapidly fabricated using standard microfabrication methods, chips with additional insulating layers beyond the membrane region can provide significantly lower noise levels, but at the expense of requiring more costly and time-consuming fabrication steps. Here we present a novel fabrication protocol that overcomes this issue by enabling rapid and reproducible manufacturing of low-noise membranes for nanopore experiments. The fabrication protocol, termed trans-chip illumination lithography, is based on illuminating a membrane-containing wafer from its backside such that a photoresist (applied on the wafer’s top side) is exposed exclusively in the membrane regions. Trans-chip illumination lithography permits the local modification of membrane regions and hence the fabrication of nanopore chips containing locally patterned insulating layers. This is achieved while maintaining a well-defined area containing a single thin membrane for nanopore drilling. The trans-chip illumination lithography method achieves this without relying on separate masks, thereby eliminating time-consuming alignment steps as well as the need for a mask aligner. Using the presented approach, we demonstrate rapid and reproducible fabrication of nanopore chips that contain small (12 μm × 12 μm) free-standing silicon nitride membranes surrounded by insulating layers. The electrical noise characteristics of these nanopore chips are shown to be superior to those of simpler designs without insulating layers and comparable in quality to more complex designs that are more challenging to fabricate.

Coexistence of guided mode resonance and extraordinary optical transmission in metal/dielectric/metal photonic crystal slab

This study examines the optical properties of metal-dielectric-metal (MDM) photonic crystal slabs (PCS) which consist of a 100-nm freestanding silicon nitride membrane that is sandwiched on either side by a 20-nm-thick gold film with an array of hexagonal tapered holes. It shows the extraordinary optical transmission phenomenon that is generated from the coupling of internal surface plasmon polaritons that are excited by dual gold layers. In addition, the presence of transmission dips can be attributed to the guided mode resonance of silicon nitride based PCS. Numerical calculation of photonic band structure confirms this attribution. This coexistence of extraordinary optical transmission and the guided mode resonance in MDM PCS may contribute to the development of multifunctional optical devices.

Acoustic vibrations in free‐standing single‐, bi‐layer nanomembranes and patterned wires

AbstractBrillouin light scattering investigations of acoustic vibrations in freestanding membranes, soft‐hard bi‐layers and patterned wires are described. Studies of longitudinal and transverse standing wave resonances, the wave vector dependence of surface and guided waves as well as their hybridization with flexural and dilational waves are presented. Size effects and the role of reduced dimensions on the acoustic behavior are investigated in soft rectangular polymeric wires on freestanding silicon nitride membranes. The mode frequencies and amplitudes are analyzed in terms of surface Green's functions and the xyz algorithm.

Waveguide-plasmon resonances in gold-capped silicon-nitride disk photonic crystal slabs

Waveguide-plasmon resonances of gold-capped silicon-nitride (SiN) disk photonic crystal slabs (PCSs) were investigated experimentally and numerically. The waveguide couplings were supported by the freestanding SiN membrane and the SiN-disk PCS. A tunable waveguide-plasmon resonance was modified by the SiN-disk height. When the SiN-disk height increased, the waveguide-plasmon resonances were enhanced and were blueshifted in optical-transmission spectra. The SiN-disk PCS was also fabricated to distinguish the presence of the waveguide-plasmon resonance in the gold-capped SiN-disk PCS from the guided resonance in the SiN-disk PCS. The finite-element method was used to calculate the optical transmittance of the gold disk array-coated SiN membranes, the gold-capped SiN-disk PCSs, and the SiN-disk PCSs. They were all in good agreement with the experiments.