Thin Silicon Nitride Membrane Research Articles

Molecular transport of proteins through nanoporous membranes fabricated by interferometric lithography

Millimeter sized arrays of uniformly-distributed nanopores (180-220 nm) were created in thin (200 nm) silicon nitride membranes using interferometric lithography. Molecular transport properties of the fabricated devices were investigated experimentally and compared with those of state-of-the-art polycarbonate track etched membranes. Two similarly-sized proteins, bovine serum albumin (BSA) and bovine hemoglobin (BHb), were used as permeates in the transport experiments. Up to 40 fold higher pore fluxes were achieved with unmodified silicon nitride membranes relative to thicker commercial nanoporous membranes. Similarly, in mixed protein experiments, ∼5.0 and 1.9 fold higher BSA and BHb selectivities were obtained with fabricated thin membranes at pH 4.7 and 7.0, respectively, relative to the commercial nanoporous membranes.

High throughput fabrication of plasmonic nanostructures in nanofluidic pores for biosensing applications

One of the primary advantages of nanoscale sensors is that they often can provide conceptually new ways of performing sensing that are not feasible with their large-scale analogs. For example, the small size of nanoscale sensor elements, such as plasmonic metal nanoparticles, allows them to be combined with nanofluidic systems. Among the potential applications of such a combination is the efficient delivery of analyte to the sensor surface. With this in mind, in this work we look to address the challenge of creating and positioning nanoplasmonic sensor elements within nanofluidic pores. A scheme is presented that allows for the production of arrays of pores in a thin (220 nm) silicon nitride membrane with one plasmonic nanoparticle sensor element in each pore. The high throughput fabrication protocol is parallel and enables multiple sensor chips to be produced simultaneously, yet with accurate tuning of the dimension and shape of the nanoparticles. The presented system is shown to possess polarization-sensitive plasmonic resonances that can be tuned significantly in the visible wavelength range by just varying one process parameter. The thickness of the membrane could be optimized to minimize the influence of the optical membrane interference on the plasmonic readout. The sensitivity of the plasmon resonances to changes in refractive index, which forms the basis for using the system for biosensing, was found to be competitive with other nanoplasmonic sensors.

Porous TEM windows fabrication using CsCl self-assembly

Thin non-porous silicon nitride membrane has been used for transmission electron microscopy (TEM) “windows” due to its transparency to high-energy electrons. However, to completely eliminate the background scattering, a porous membrane is desirable. Here the authors report the fabrication of porous silicon nitride membranes by CsCl self-assembly to form hemispherical islands, followed by “image reversal” via drying etching and liftoff steps. Through controlling the evaporated CsCl film thickness, the exposure time of the film in air, and the relative humidity of air, the authors were able to fabricate porous silicon nitride membranes with pore diameter and interpore spacing of order 100 nm. The pore diameter could be further shrunk by coating a conformal film onto the porous membrane. In addition, conductive porous TEM windows can be achieved by simply skipping the final metal removal step. Such a porous membrane would also find applications in separation or filtration of macromolecules or nanoparticles.

Integrated nanopore sensing platform with sub-microsecond temporal resolution

Nanopore sensors have attracted considerable interest for high-throughput sensing of individual nucleic acids and proteins without the need for chemical labels or complex optics. A prevailing problem in nanopore applications is that the transport kinetics of single biomolecules are often faster than the measurement time resolution. Methods to slow down biomolecular transport can be troublesome and are at odds with the natural goal of high-throughput sensing. Here we introduce a low-noise measurement platform that integrates a complementary metal-oxide semiconductor (CMOS) preamplifier with solid-state nanopores in thin silicon nitride membranes. With this platform we achieved a signal-to-noise ratio exceeding five at a bandwidth of 1 MHz, which to our knowledge is the highest bandwidth nanopore recording to date. We demonstrate transient signals as brief as 1 μs from short DNA molecules as well as current signatures during molecular passage events that shed light on submolecular DNA configurations in small nanopores.

High-Bandwidth Solid-State Nanopore Sensors

Nanopores are attractive electrochemical sensors for their single-molecule sensitivity and simple experimental setup. Many of the electronic systems used for nanopore measurements are inherited from electrophysiology applications, and as a result they are primarily optimized for current levels of 50pA or less, at bandwidths below 10kHz. However, state-of-the-art solid-state nanopore sensors now commonly produce signals in the nanoampere range, for which common patch clamp amplifiers are no longer ideal. We present a new experimental platform which integrates a custom CMOS preamplifier with nanopores in thin silicon nitride membranes. The low parasitic capacitance of the integrated platform reduces noise at high frequencies, allowing nanopore measurements at finer temporal resolution than supported by popular instruments. We demonstrate the viability of this system with high-bandwidth electrical recordings of the passage of short DNA molecules through nanopores. This new platform will improve the signal-to-noise ratios of nanopore sensors, enabling further single-molecule studies of fast biophysical processes.

Capacitive Micromachined Ultrasonic Transducer Array with Pencil Beam Shape and Wide Range Beam Steering

Abstract A capacitive micromachined ultrasonic transducer (CMUT) array is designed as an alternative to conventional piezoelectric transducers. A thin silicon nitride membrane is suspended over a bottom electrode on a silicon wafer. In the immersion mode, the transducer cell shape and dimensions are optimized for an operating frequency of 10 MHz. We show that the proposed imager array can generate a pencil shape beam with a ∼1.5° half beam width, enhancing the detector resolution. A phased array technique is employed to excite multiple cells using time-delayed signals to steer the acoustic beam toward the object. This eliminates the need to mechanically move the detector, simplifying the transducer driving system. Moreover, unlike conventional transducers, the pencil beam can be effectively steered over a wide range of angles without producing grating lobes, which minimizes power loss in undesired directions. This can also improve the signal to noise ratio of the imager CMUT array.

Embedding a carbon nanotube across the diameter of a solid state nanopore

A fabrication method for positioning and embedding a single-walled carbon nanotube (SWNT) across the diameter of a solid state nanopore is presented. Chemical vapor deposition (CVD) is used to grow SWNTs over arrays of focused ion beam (FIB) milled pores in a thin silicon nitride membrane. This typically yields at least one pore whose diameter is centrally crossed by a SWNT. The final diameter of the FIB pore is adjusted to create a nanopore of any desired diameter by atomic layer deposition, simultaneously embedding and insulating the SWNT everywhere but in the region that crosses the diameter of the final nanopore, where it remains pristine and bare. This nanotube-articulated nanopore is an important step towards the realization of a new type of detector for biomolecule sensing and electronic characterization, including DNA sequencing.

Joule heating in nanowires

We study the effect of Joule heating from electric currents flowing through ferromagnetic nanowires on the temperature of the nanowires and on the temperature of the substrate on which the nanowires are grown. The spatial current density distribution, the associated heat generation, and diffusion of heat is simulated within the nanowire and the substrate. We study several different nanowire and constriction geometries as well as different substrates: (thin) silicon nitride membranes, (thick) silicon wafers, and (thick) diamond wafers. The spatially resolved increase in temperature as a function of time is computed. For effectively three-dimensional substrates (where the substrate thickness greatly exceeds the nanowire length), we identify three different regimes of heat propagation through the substrate: regime (i), where the nanowire temperature increases approximately logarithmically as a function of time. In this regime, the nanowire temperature is well-described analytically by You et al. [APL 89, 222513 (2006)]. We provide an analytical expression for the time t_c that marks the upper applicability limit of the You model. After t_c, the heat flow enters regime (ii), where the nanowire temperature stays constant while a hemispherical heat front carries the heat away from the wire and into the substrate. As the heat front reaches the boundary of the substrate, regime (iii) is entered where the nanowire and substrate temperature start to increase rapidly. For effectively two-dimensional substrates (where the nanowire length greatly exceeds the substrate thickness), there is only one regime in which the temperature increases logarithmically with time for large times, before the heat front reaches the substrate boundary. We provide an analytical expression, valid for all pulse durations, that allows one to accurately compute this temperature increase in the nanowire on thin substrates.

Design, performance and analysis of thermal lag of the UFS1 twin-calorimeter chip for fast scanning calorimetry using the Mettler-Toledo Flash DSC 1

This paper presents a new twin-membrane calorimeter chip for fast differential scanning calorimetry (DSC) with the Flash DSC 1 of Mettler-Toledo. The thin silicon nitride membranes enable scan rates in excess of 10 kK/s in heating and up to 4 kK/s in cooling for sample masses between 100 ng and 10 μg in the temperature range of −100 °C to 450 °C. The time constant for cooling is about 12 ms, the power resolution is typically 0.1–0.5 μW, the temperature accuracy of non-calibrated chips is typically better than ±5 K. The paper also shows measurements for the scan-rate dependent thermal lag of the device, showing an empty sensor thermal lag of about 0.2 ms, and a mass dependent thermal lag of about 0.3 ms/μg for Indium for a good thermal contact between Indium and membrane.

Carbon nanotube nanoelectronic devices compatible with transmission electronmicroscopy

We report on a novel method to fabricate carbon nanotube (CNT) nanoelectronic deviceson silicon nitride membrane grids that are compatible with high resolution transmissionelectron microscopy (HRTEM). Resist-based electron beam lithography is used to fabricateelectrodes on 50 nm thin silicon nitride membranes and focused-ion-beam milling is used tocut out a 200 nm gap across a gold electrode to produce the viewing window for HRTEM.Spin-coating and AC electrophoresis are used as methods to deposit small bundles ofcarbon nanotubes across the electrodes. We demonstrate the viability of this approach byperforming both electrical measurements and HRTEM imaging of solution-processed CNTsin a device.

Nanopore formation by low-energy focused electron beam machining

The fabrication of nanopores in thin silicon nitride and aluminum oxide membranes by watervapor assisted, low-energy (0.2–20 kV) electron beam machining using a scanning electronmicroscope (SEM) is described. Using this technique, pores with diameters ranging in size from < 5 to 20 nm are easily formed. The nanopores are characterized by SEM, transmission electronmicroscopy (TEM) and atomic force microscopy (AFM). The mechanism of etching isbriefly discussed.

The Nanoaquarium: A Platform for In Situ Transmission Electron Microscopy in Liquid Media

Transmission electron microscopes (TEMs) and scanning transmission electron microscopes (STEMs) are powerful tools for imaging on the nanoscale. These microscopes cannot be typically used to image processes taking place in liquid media because liquid simply evaporates in the high-vacuum environment of the microscope. In order to view a liquid sample, it is thus necessary to confine the liquid in a sealed vessel to prevent evaporation. Additionally, the liquid layer must be very thin to minimize electron scattering by the suspending medium. To address these issues, we have developed a flow cell with a height of tens of nanometers, sandwiched between two thin silicon nitride membranes. The cell is equipped with electrodes for actuation and sensing. The cell is thin enough to allow the transmission of electrons and the real-time imaging of nanoparticles suspended in liquid. This paper details the fabrication process, which relies on plasma-activated wafer bonding. Some of the advantages of our nanoaquarium include the thinnest observation chamber of any reported in situ TEM/STEM device, integrated electrodes for sensing and actuation, and wafer-scale processing that allows bulk device production. Device performance was demonstrated by STEM imaging of gold and polystyrene nanoparticles suspended in water with excellent resolution. Potential applications of the device include imaging of colloidal crystal formation, aggregation, nanowire growth, electrochemical deposition, and biological interactions.

TEM study of locally coated nanopore fabricated by ion-beam-induced deposition in a thin membrane

We studied the formation of locally coated sub-10-nm nanopores fabricated by ion-beam milling and ion-beam-induced deposition (IBID) in a thin silicon nitride membrane. Two typical precursor gases representing conductive ((CH3)3Pt(CpCH3), CPC for short) and insulating (tetra ethyl oxysilane, TEOS for short) material deposition are used. Three-dimensional electron tomography, EDX and EELS analysis are used to measure the changes in chemical composition and shape of the pores after their formation and at various stages of pore shrinkage. The formation and shrinkage are shown to be due to a shifting competition between IBID and material sputtering during ion-beam exposure. The chemical distribution at the rim of the nanopore is dependent on the precursor gases used: CPC forms a thin carbon layer with small embedded Pt particles at the top and inner surfaces of the nanopore, whereas TEOS forms SiOxCy with Ga particles dispersed at the rim of the nanopore.

Strain sensing with submicron Al–AlOx–Al tunnel junctions

We demonstrate a local strain sensing method for nanostructures based on metallic Al tunnel junctions with AlOx barriers. The junctions were fabricated on top of a thin silicon nitride membrane, which was actuated with an atomic force microscope tip attached to a stiff cantilever. A large relative change in the tunneling resistance in response to the applied strain (gauge factor) was observed up to a value of 37. This facilitates local static strain variation measurements down to ∼10−7. This type of strain sensor could have applications in nanoelectromechanical systems used in displacement, force, and mass sensing, for example.

Improved 10Be and 26Al-AMS with a 5 MV spectrometer

Detector and ion source changes have increased Be and Al count rates and reduced measurement background at SUERC. Low energy 16 MeV 26Al 3+ ions can be separated from interferences by adopting thin silicon nitride membrane detector windows. In contrast, a thick Havar detector window is used to preferentially slow boron ions for simplified 10Be vs. 10B separation without an additional gas cell.

Formation of individual protein channels in lipid bilayers suspended in nanopores

Free-standing lipid bilayers are formed in regularly arranged nanopores of 200, 400 and 800 nm in a 300 nm thin hydrophobic silicon nitride membrane separating two fluid compartments. The extraordinary stability of the lipid bilayers allows us to monitor channel formation of the model peptide melittin and α-hemolysin from Staphylococcus aureus using electrochemical impedance spectroscopy and chronoamperometry. We observed that melittin channel formation is voltage-dependent and transient, whereas transmembrane heptameric α-hemolysin channels in nano-BLMs persist for hours. The onset of α-hemolysin-mediated conduction depends on the applied protein concentration and strongly on the diameter of the nanopores. Heptameric channel formation from adsorbed α-hemolysin monomers needs more time in bilayers suspended in 200 nm pores compared to bilayers in pores of 400 and 800 nm diameters. Diffusion of sodium ions across α-hemolysin channels present in a sufficiently high number in the bilayers was quantitatively and specifically determined using ion selective electrodes. The results demonstrate that relatively small variations of nano-dimensions have a tremendous effect on observable dynamic biomolecular processes. Such nanopore chips are potentially useful as supports for stable lipid bilayers to establish functional assays of membrane proteins needed in basic research and drug discovery.

Ion Impact Detection and Micromapping With a SDRAM for IEEM Diagnostics and Applications

Ion electron emission microscopy (IEEM) can provide an alternative approach to microbeams for micrometric characterization of the sensitivity map to single event effects (SEE) of an electronic device. In IEEM technique, a broad (not focused) ion beam is sent onto the device under test (DUT). Secondary electrons emitted by the target surface during each ion impact are collected and focused by a system of electrostatic lenses and finally imaged by a high-rate and high-resolution position detector. We will report on the IEEM working at the SIRAD irradiation facility located at the 15 MV Tandem of INFN Legnaro National Laboratories. To estimate the IEEM resolution, a SDRAM is a good candidate to be used as a reference target, thanks to the micrometric feature size of the array of memory cells and the precise knowledge of their physical locations. Since an electronic device is not a good secondary electrons emitter, to ensure a copious and uniform emission of secondary electrons from the DUT, a very thin (100 nm) self-standing silicon nitride (Si3N4) membrane with a Au deposition (40 nm) is mounted on the top of the SDRAM. The Au/Si3N4 membrane and the underlying SDRAM are irradiated with a heavy ion beam. The physical map of ion impacts detected by the SDRAM is then compared with the one reconstructed by the IEEM in the same 500 ms time interval.

Ultra-dense hydrogen silsesquioxane (HSQ) structures on thin silicon nitride membranes

Ultra-dense nanometer-scale gratings (20 nm pitch) on thin silicon nitride (Si 3N 4) membrane substrates using hydrogen silsesquioxane (HSQ) resist have been fabricated. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were performed to evaluate the pattern quality of the HSQ gratings. The results are compared with HSQ gratings fabricated on silicon substrates.

Clusters of interacting single domain Co nanomagnets for multistate perpendicular magnetic media applications

In this work we develop prototype elements for multistate (beyond binary) perpendicular data storage using interacting nanomagnet clusters. This experimental work confirms earlier theoretical work that predicted multiple discrete values of stable remanent magnetization for such clusters. The fabrication scheme is based on ultrahigh resolution electron beam lithography performed on a thin suspended silicon nitride membrane to reduce the secondary backscattered electrons from the substrate. A Co nanomagnet cluster array is deposited into the nanotemplate via pulse-reverse electrodeposition to create nanomagnets with the favored uniaxial perpendicular anisotropy. Magnetic force microscopy (MFM) measurements show the perpendicular magnetization of individual Co nanomagnets and the combined multiconfiguration behavior of a nanomagnet cluster. In concept, the discrete values of net remanent magnetization of the cluster, which represent distinct information states, can be “programmed” by a uniform applied field.

Characterizing the Superconducting-to-Normal Transition in Mo/Au Transition-Edge Sensor Bilayers

We are developing arrays of Mo/Au bilayer transition-edge sensors (TES’s) for future X-ray astronomy missions such as NASA’s Constellation-X. The physical properties of the superconducting-to-normal transition in our TES bilayers, while often reproducible and characterized, are not well understood. The addition of normal metal features on top of the bilayer are found to change the shape and temperature of the transition, and they typically reduce the unexplained ‘excess’ noise. In order to understand and potentially optimize the properties of the transition, we have been studying the temperature, widths and current dependence of these transitions. We report on the characterization of devices both deposited on silicon substrates and suspended on thin silicon nitride membranes. This includes key device parameters such as the logarithmic resistance sensitivity with temperature α, and the logarithmic resistance sensitivity with current β, and their correlation with excess noise.