Thick Silicon Nitride Research Articles

Nanomechanical thermometry for probing sub-nW thermal transport

Accurate local temperature measurement at micro and nanoscales requires thermometry with high resolution because of ultra-low thermal transport. Among the various methods for measuring temperature, optical techniques have shown the most precise temperature detection, with resolutions reaching (~10−9 K). In this work, we present a nanomechanical device with nano-Kelvin resolution (~10−9 K) at room temperature and 1 atm. The device uses a 20 nm thick silicon nitride (SiN) membrane, forming an air chamber as the sensing area. The presented device has a temperature sensing area >1 mm2 for micro/nanoscale objects with reduced target placement constraints as the target can be placed anywhere on the >1 mm2 sensing area. The temperature resolution of the SiN membrane device is determined by deflection at the center of the membrane. The temperature resolution is inversely proportional to the membrane’s stiffness, as detailed through analysis and measurements of stiffness and noise equivalent temperature (NET) in the pre-stressed SiN membrane. The achievable heat flow resolution of the membrane device is 100 pW, making it suitable for examining thermal transport on micro and nanoscales.

Metasurface-integrated elliptically polarized laser-pumped SERF magnetometers

The emergence of biomagnetism imaging has led to the development of ultrasensitive and compact spin-exchange relaxation-free (SERF) atomic magnetometers that promise high-resolution magnetocardiography (MCG) and magnetoencephalography (MEG). However, conventional optical components are not compatible with nanofabrication processes that enable the integration of atomic magnetometers on chips, especially for elliptically polarized laser-pumped SERF magnetometers with bulky optical systems. In this study, an elliptical-polarization pumping beam (at 795 nm) is achieved through a single-piece metasurface, which results in an SERF magnetometer with a high sensitivity reaching 10.61 fT/Hz1/2 by utilizing a 87Rb vapor cell with a 3 mm inner diameter. To achieve the optimum theoretical polarization, our design combines a computer-assisted optimization algorithm with an emerging metasurface design process. The metasurface is fabricated with 550 nm thick silicon-rich silicon nitride on a 2 × 2 cm2 SiO2 substrate and features a 22.17° ellipticity angle (a deviation from the target polarization of less than 2%) and more than 80% transmittance. This study provides a feasible approach for on-chip polarization control of future all-integrated atomic magnetometers, which will further pave the way for high-resolution biomagnetism imaging and portable atomic sensing applications.

Two-layered microwell-array device for preparation of single-neuron culture samples

When a single neuron is cultured in isolation from other neurons, its axon connects with its own dendrites to form a simple, independent network with no synaptic inputs from other neurons. This culture system enables detailed analysis of synaptic function and morphology change in neurites at the single-neuron level, which is useful for elucidating the pathogenesis of neurological diseases and for evaluating the efficacy of therapeutic drugs for them. However, there was previously no device technology capable of simultaneously forming multiple single-neuron samples while allowing co-culture with astrocytes, which is essential for culture of a single neuron isolated from other neurons. In this study, we propose a novel microwell-array device for preparing single-neuron samples. The device consists of an upper layer for cell seeding and a lower layer for cell culture. Each layer has 16 × 16 microwells, and the bottom of each well is made of a 1 μm thick silicon nitride membrane. The membrane of the upper well has one microhole for seeding a single neuron, and the lower membrane has multiple microholes for interaction between a single neuron and astrocytes which are co-cultured back-to-back on both sides of the membrane. When neurons are seeded into the upper well, only one of them passes through the microhole in the upper membrane and falls onto the lower membrane. We evaluated a seeding efficiency of single neurons by changing seeding hole diameter and seeding density. The results showed that the yield of more than 20% was obtained regardless of the seeding density when the seeding hole diameter was 13 μm. We also confirmed that single neurons seeded in this manner and co-cultured with astrocytes developed neurites and formed synapses. These results demonstrated the usefulness of this device for the preparation of single-neuron culture samples.

Ultra-broadband absorbance of nanometer-thin pyrolyzed-carbon film on silicon nitride membrane

Fifty percents absorption by thin film, with thickness is much smaller than the skin depth and optical thickness much smaller than the wavelength, is a well-known concept of classical electrodynamics. This is a valuable feature that has been numerously widely explored for metal films, while chemically inert nanomembranes are a real fabrication challenge. Here we report the 20 nm thin pyrolyzed carbon film (PyC) placed on 300 nm thick silicon nitride (Si3N4) membrane demonstrating an efficient broadband absorption in the terahertz and near infrared ranges. While the bare Si3N4 membrane is completely transparent in the THz range, the 20 nm thick PyC layer increases the absorption of the PyC coated Si3N4 membrane to 40%. The reflection and transmission spectra in the near infrared region reveal that the PyC film absorption persists to a level of at least 10% of the incident power. Such a broadband absorption of the PyC film opens new pathways toward broadband bolometric radiation detectors.

Damascene Process Development for Low-Loss Photonics Devices with Applications in Frequency Comb

Silicon nitride (SiN) is emerging as a material of choice for photonic integrated circuits (PICs) due to its ultralow optical losses, absence of two-photon absorption in telecommunication bands, strong Kerr nonlinearity and high-power handling capability. These properties make SiN particularly well-suited for applications such as delay lines, chip-scale frequency combs and narrow-linewidth lasers, especially when implemented with thick SiN waveguides, which is achieved through low-pressure chemical vapor deposition (LPCVD). However, a significant challenge arises when the LPCVD SiN film thickness exceeds 300 nm on an 8-inch wafer, as this can result in cracking due to high stress. In this work, we successfully develop a damascene process to fabricate 800 nm-thick SiN photonics devices on an 8-inch wafer in a pilot line, overcoming cracking challenges. The resulting 2 × 2 multimode interference (MMI) coupler exhibits low excess loss (−0.1 dB) and imbalance (0.06 dB) at the wavelength of 1310 nm. Furthermore, the dispersion-engineered SiN micro-ring resonator exhibits a quality (Q) factor exceeding 1 × 106, enabling the generation of optical frequency combs. Our demonstration of photonics devices utilizing the photonics damascene process sets the stage for high-volume manufacturing and widespread deployment.

Process Development of Low-loss Thick Silicon Nitride Waveguide on 8-inch Wafer

Process Development of Low-loss Thick Silicon Nitride Waveguide on 8-inch Wafer

Precisely controlled batch-fabrication of highly sensitive co-resonant cantilever sensors from silicon-nitride

Dynamic-mode cantilever sensors are based on the principle of a one-side clamped beam being excited to oscillate at or close to its resonance frequency. An external interaction on the cantilever alters its oscillatory state, and this change can be detected and used for quantification of the external influence (e.g. a force or mass load). A very promising approach to significantly improve sensitivity without modifying the established laser-based oscillation transduction is the co-resonant coupling of a micro- and a nanocantilever. Thereby, each resonator is optimized for a specific purpose, i.e. the microcantilever for reliable oscillation detection and the nanocantilever for highest sensitivity through low rigidity and mass. To achieve the co-resonant state, the eigenfrequencies of micro- and nanocantilever need to be adjusted so that they differ by less than approximately 20%. This can either be realized by mass deposition or trimming of the nanocantilever, or by choice of dimensions. While the former is a manual and error-prone process, the latter would enable reproducible batch fabrication of coupled systems with predefined eigenfrequency matching states and therefore sensor properties. However, the approach is very challenging as it requires a precisely controlled fabrication process. Here, for the first time, such a process for batch fabrication of inherently geometrically eigenfrequency matched co-resonant cantilever structures is presented and characterized. It is based on conventional microfabrication techniques and the structures are made from 1 µm thick low-stress silicon nitride. They comprise the microcantilever and high aspect ratio nanocantilever (width 2 µm, thickness about 100 nm, lengths up to 80 µm) which are successfully realized with only minimal bending. An average yield of % of intact complete sensor structures per wafer is achieved. Desired geometric dimensions can be realized within ±1% variation for length and width of the microcantilever and nanocantilever length, ±10% and ±20% for the nanocantilever width and thickness, respectively, resulting in an average variation of its eigenfrequency by 11%. Furthermore, the dynamic oscillation properties are verified by vibration experiments in a scanning electron microscope. The developed process allows for the first time the batch fabrication of co-resonantly coupled systems with predefined properties and controlled matching states. This is an important step and crucial foundation for a broader applicability of the co-resonant approach for sensitivity enhancement of dynamic-mode cantilever sensors.

Design, Fabrication, and Application of Micro-Structured Surfaces for Laser-Induced Breakdown Spectroscopic Analysis of Liquids: A Sample-Loading Target Development Studies

In this paper, design, fabrication, and application studies of a sample loading target patterned with periodical micro-structures were presented. Two different geometrical shapes; triangular prisms and cylinders of two different feature sizes; 5-µm, and 20-µm, were photo-lithographically patterned on Si-wafer substrates, and dry etched to 10 µm height. Followed by a 1-µm thick silicon nitride film coating over micro-patterned substrates, final products were obtained after dicing into one-inch size squares, each containing 36 patterned sample loading areas. Among the three different patterns studied; a geometric design with 20-µm diameter cylinders exhibited a more effective task in increasing the LIBS signal strength, compared to the other two patterns. The characterization of the surface morphology and the size-shape distribution of the micro-patterns were carried out through optical and scanning electron microscopic measurements. SEM images proved a more effective ablation occurring on triangular prism micro-structured surfaces that can be associated with an increased path length and enhanced absorption of the laser beam on the flat surfaces of triangular prism via multiple reflections. The results of structured surfaces were also compared with the ones from non-structured surfaces of 300 nm and 1000 nm thicknesses of silicon nitride-coating. The applications of micro-structured surfaces for heavy metals analysis were performed with Cr and Pb solutions via dried-droplet LIBS methodology. The enhancement factor of 4 for Pb(I) 405.8 nm, and 8 for Cr(I) 428.9 nm were observed from the 20-µm diameter cylinder, CYL-20, surfaces compared to 300 nm thick Si3N4 surfaces, respectively. This study of micro-structuring substrate surfaces with an emphasis on the signal enhancement effect is promising in terms of improving the capacity and limitations of the dried-droplet methodology by LIBS.

Registration of Functioning of a Single Horseradish Peroxidase Macromolecule with a Solid-State Nanopore

Currently, nanopore-based technology for the determination of the functional activity of single enzyme molecules continues its development. The use of natural nanopores for studying single enzyme molecules is known. At that, the approach utilizing artificial solid-state nanopores is also promising but still understudied. Herein, we demonstrate the use of a nanotechnology-based approach for the investigation of the enzymatic activity of a single molecule of horseradish peroxidase with a solid-state nanopore. The artificial 5 nm solid-state nanopore has been formed in a 40 nm thick silicon nitride structure. A single molecule of HRP has been entrapped into the nanopore. The activity of the horseradish peroxidase (HRP) enzyme molecule inserted in the nanopore has been monitored by recording the time dependence of the ion current through the nanopore in the course of the reaction of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) oxidation reaction. We have found that in the process of ABTS oxidation in the presence of 2.5 mM hydrogen peroxide, individual HRP enzyme molecules are able to retain activity for approximately 700 s before a decrease in the ion current through the nanopore, which can be explained by structural changes of the enzyme.

High Accuracy Protein Identification: Fusion of Solid-State Nanopore Sensing and Machine Learning.

Proteins are arguably one of the most important class of biomarkers for health diagnostic purposes. Label-free solid-state nanopore sensing is a versatile technique for sensing and analyzing biomolecules such as proteins at single-molecule level. While molecular-level information on size, shape, and charge of proteins can be assessed by nanopores, the identification of proteins with comparable sizes remains a challenge. Here, solid-state nanopore sensing is combined with machine learning to address this challenge. The translocations of four similarly sized proteins is assessed using amplifiers with bandwidths (BWs) of 100kHz and 10MHz, the highest bandwidth reported for protein sensing, using nanopores fabricated in <10nm thick silicon nitride membranes. F-values of up to 65.9% and 83.2% (without clustering of the protein signals) are achieved with 100kHz and 10MHz BW measurements, respectively, for identification of the four proteins. The accuracy of protein identification is further enhanced by classifying the signals into different clusters based on signal attributes, with F-value and specificity of up to 88.7% and 96.4%, respectively, for combinations of four proteins. The combined use of high bandwidth instruments, advanced clustering and machine learning methods allows label-free identification of proteins with high accuracy.

Thermal gradients integrated on-chip by passive radiative cooling of silicon nitride nanomechanical resonators

Radiative coupling of objects with outer space, through the atmospheric spectral transparency window, allows passive cooling below ambient temperature. Such passively created cold sinks can be used for converting ambient heat into other useful forms of energy. Using this approach, conversion of ambient heat to electricity has been demonstrated in macroscopic settings, where the radiatively cooled heat sink is a macroscopic object coupled with ambient heat through a heterogeneously attached, standalone, thermoelectric generator. In contrast, miniaturization and on-chip integration of such heat engines has not been demonstrated, although it could be useful in portable remote technological applications. Such integration would first require that both the radiatively cooled heat sink and the hot (ambient temperature) side of the engine are monolithically integrated on a single chip. This first step is the focus of our work: we demonstrate the creation of a local thermal gradient on-chip by radiative cooling of a 90 nm thick freestanding silicon nitride nanomechanical resonator integrated on a silicon substrate that remains at ambient temperature. The reduction in temperature of the thin film is inferred by tracking its mechanical resonance frequency, under high vacuum, using an optical fiber interferometer. Experiments were conducted on 15 different days during fall and summer months, under various solar illumination, resulting in successful radiative cooling of the membrane in each case. Maximum temperature drops of 9.3 K and 7.1 K are demonstrated during the day and night, respectively, in close correspondence with our heat transfer model. Future improvements to the experimental setup could improve the temperature reduction to 48 K for the same membrane, while emissivity engineering potentially yields a maximum theoretical cooling of 67 K with an ideal emitter.

Modelling a Human Blood-Brain Barrier Co-Culture Using an Ultrathin Silicon Nitride Membrane-Based Microfluidic Device.

Understanding the vesicular trafficking of receptors and receptor ligands in the brain capillary endothelium is essential for the development of the next generations of biologics targeting neurodegenerative diseases. Such complex biological questions are often approached by in vitro models in combination with various techniques. Here, we present the development of a stem cell-based human in vitro blood-brain barrier model composed of induced brain microvascular endothelial cells (iBMECs) on the modular µSiM (a microdevice featuring a silicon nitride membrane) platform. The µSiM was equipped with a 100 nm thick nanoporous silicon nitride membrane with glass-like imaging quality that allowed the use of high-resolution in situ imaging to study the intracellular trafficking. As a proof-of-concept experiment, we investigated the trafficking of two monoclonal antibodies (mAb): an anti-human transferrin receptor mAb (15G11) and an anti-basigin mAb (#52) using the µSiM-iBMEC-human astrocyte model. Our results demonstrated effective endothelial uptake of the selected antibodies; however, no significant transcytosis was observed when the barrier was tight. In contrast, when the iBMECs did not form a confluent barrier on the µSiM, the antibodies accumulated inside both the iBMECs and astrocytes, demonstrating that the cells have an active endocytic and subcellular sorting machinery and that the µSiM itself does not hinder antibody transport. In conclusion, our µSiM-iBMEC-human astrocyte model provides a tight barrier with endothelial-like cells, which can be used for high-resolution in situ imaging and for studying receptor-mediated transport and transcytosis in a physiological barrier.

Development of General-purpose Dielectric Constant Imaging Unit for SEM and Direct Observation of Samples in Aqueous Solution.

Electron microscopes can observe samples with a spatial resolution of 10 nm or higher; however, they cannot observe samples in solutions due to the vacuum conditions inside the sample chamber. Recently, we developed a scanning electron-assisted dielectric microscope (SE-ADM), based on scanning electron microscope, which enables the observation of various specimens in solution. Until now, the SE-ADM system used a custom-made SE-ADM stage with a built-in amplifier and could not be linked to the scanning electron microscopy (SEM) operation system. Therefore, it was necessary to manually acquire images from the SE-ADM system after setting the EB focus, astigmatism, and observation field-of-view from the SEM operating console. In this study, we developed a general-purpose dielectric constant imaging unit attached to commercially available SEMs. The new SE-ADM unit can be directly attached to the standard stage of an SEM, and the dielectric signal detected from this unit can be input to the external input terminal of the SEM, enabling simultaneous observation yielding SEM and SE-ADM images. Furthermore, 4.5 nm spatial resolution was achieved using a 10 nm thick silicon nitride film in the sample holder in the observation of aggregated PM2.5. We carried out the observation of cultured cells, PM2.5, and clay samples in solution.

Pulley Couplers for Broadband Microcomb Generation in Si3N4 Ring Resonators

Microresonator solitons enable high repetition-rate optical frequency combs. Their spectral span scales inversely with the strength of the resonator’s anomalous group velocity dispersion. In a thick (800 nm) silicon nitride platform, wide resonator waveguides (&gt;2 µm) with weak anomalous dispersion are especially promising for the generation of broadband spectra. As wider waveguides have a weaker evanescent field, the coupling strength to their bus waveguide is reduced. To address this challenge, alternative coupler designs, such as pulley couplers are required. Here, we investigate pulley couplers for wide waveguide specifically targeted at broadband soliton generation. We observe significant improvement of the coupling ideality compared to conventional coupler geometries and broadband four-wave mixing spectra are observed in 2.8 µm wide microresonators.

A Tall Gate Stem GaN HEMT With Improved Power Density and Efficiency at Ka-Band

In this letter, AlGaN/GaN HEMTs with tall-gate-stem structures were realized to improve the power performance of Ka-band devices, and a film thinning process is adopted in the fabrication process to reduce the parasitic capacitance caused by the thick silicon nitride film. According to the S-parameter measurement results, devices owning a tall-gate-stem structure and undergoing the film thinning process have higher cut-off frequency (fT) and maximum oscillation frequency (fmax) values with lower extracted parasitic capacitance. For the load-pull measurement result, the AlGaN/GaN HEMT with a tall gate stem has improved output power density (Pout) and power added efficiency (PAE) at Ka-band. The device with the elevated stem shows a steady-state current density of 883 mA/mm and a maximum transconductance of 323 mS/mm at 20 V bias, and it achieves fT of 39.5 GHz, fmax of 112.9 GHz with the maximum PAE of 24.6 % and the maximum Pout of 6.6 W/mm at 38 GHz.

Suppression of p-type parasitic channel formation at the interface between the aluminum nitride nucleation layer and the high-resistivity silicon substrate

In this study, GaN-based epitaxial structures were grown on high-resistivity silicon (HRSi) substrates by metalorganic chemical vapor deposition. The p-type parasitic channels generated at the interfaces of the aluminum nitride (AlN) nucleation layers and HRSi substrates were characterized by surface capacity measurement, backside secondary ion mass spectrometry, and spread resistance profiling. The 2-nm thick silicon nitride (SiNx) layer formed by nitridation on the HRSi substrates was used to suppress the Al diffusion from the 50-nm AlN nucleation layer and also prevented from generation of the p-type parasitic channels. The p-type parasitic channels induced insertion losses at high frequencies. Transmission line measurement was used to determine the insertion losses. The insertion loss of the GaN-based epitaxial structure with the optimized 25-nm AlN nucleation layer on the 2-nm SiNx layer was -2.72 dB/mm. It was only 0.04 dB/mm higher than that of the annealed HRSi substrate (-2.68 dB/mm) at 10 GHz. Obviously, the diffusion of Al from the optimized 25-nm thick AlN nucleation layer into the HRSi substrate can be alleviated by the SiNx layer, thereby reduced insertion loss.

Coupling strategy between high-index and mid-index micro-metric waveguides for O-band applications

The integration of fast and power efficient electro-absorption modulators on silicon is of utmost importance for a wide range of applications. To date, Franz-Keldysh modulators formed of bulk Ge or GeSi have been widely adopted due to the simplicity of integration required by the modulation scheme. Nevertheless, to obtain operation for a wider range of wavelengths (O to C band) a thick stack of Ge/GeSi layers forming quantum wells is required, leading to a dramatic increase in the complexity linked to sub-micron waveguide coupling. In this work, we present a proof-of-concept integration between micro-metric waveguides, through the butt-coupling of a {1.25},{upmu hbox {m}} thick N-rich silicon nitride (SiN) waveguide with a {1.25},{upmu hbox {m}} thick silicon waveguide for O-band operation. A numerical analysis is conducted for the design of the waveguide-to-waveguide interface, with the aim to minimize the power coupling loss and back-reflection levels. The theoretical results are compared to the measured data, demonstrating a coupling loss level of {0.5},hbox {dB} for TE and TM polarisation. Based on the SiN-SOI interconnection simulation strategy, the simulation results of a quantum-confined Stark effect (QCSE) stack waveguide coupled to a SiN waveguide are then presented.

Enhanced sensitivity of surface plasmon resonance biosensor for the selective detection of immunoglobin (IgG)

In this work, we propose a surface plasmon resonance (SPR) biosensor based on a hybrid structure of silicon nitride (Si3N4), molybdenum trioxide (MoO3), and graphene oxide (GO) in the Kretschmann configuration. To investigate the performance parameters of the SPR biosensor, we calculate, theoretically, the reflectance spectra of the proposed model and analysed it. The investigation of various performance parameters i.e., sensitivity, detection accuracy, and quality factor, of sensor configuration are theoretically done with the optimized thickness of silver (55 nm), silicon nitride (5 nm), molybdenum trioxide (10 nm), and graphene oxide (2.55 nm) respectively. The sensitivity, detection accuracy, and figure of merit for the proposed SPR structure are found at 301 deg./RIU, 4.01, and 133 RIU−1 respectively for the detection of IgG. Furthermore, the present SPR biosensor shows improved sensitivity as compared to the other reported works. The refractive index of the analyte changes from 1.33 to 1.37 due to adsorption of the analyte at the GO surface. The proposed sensor design can detect the small change in the refractive index of the sensing media doped with the analyte.

Highly efficient dual-level grating couplers for silicon nitride photonics

We propose and numerically demonstrate a versatile strategy that allows designing highly efficient dual-level grating couplers in different silicon nitride-based photonic platforms. The proposed technique, which can generally be applied to an arbitrary silicon nitride film thickness, is based on the simultaneous optimization of two grating coupler levels to obtain high directionality and grating-fibre mode matching at the same time. This is achieved thanks to the use of two different linear apodizations, with opposite signs, applied to the two grating levels, whose design parameters are determined by using a particle swarm optimization method. Numerical simulations were carried out considering different silicon nitride platforms with 150, 300, 400 and 500 nm thicknesses and initially employing silicon as the material for the top level grating coupler. The use of Si-rich silicon nitride with a refractive index in the range 2.7–3.3 for the top layer material enabled to obtain similar performance (coupling efficiency exceeding − 0.45 dB for the 400 nm thick silicon nitride platform) with relaxed fabrication tolerances. To the best of our knowledge, these numerical results represent the best performance ever reported in the literature for silicon nitride grating couplers without the use of any back-reflector.

Design and Optimization of a Compact Ultra-Broadband Polarization Beam Splitter for the SCL-Band Based on a Thick Silicon Nitride Platform

The polarization beam splitter is an essential photonic integrated circuit in applications where a high-performing on-chip polarization diversity scheme is required. The lower refractive index contrast of the silicon nitride material platform compared to silicon-on-insulator constitutes the separation of polarized light states a challenging task since for this purpose a large difference between the effective refractive indices of the fundamental TE and TM modes is highly desirable. In this paper, we present the design and optimization analysis of an ultra-broadband polarization beam splitter based on a thick silicon nitride platform through extensive 3D-FDTD simulations. The proposed device exploits two different Si3N4 thicknesses that enable the discrimination of the two polarizations at the proximity of an 800 nm thick slot and a 470 nm thick strip waveguide via directional coupling. The proposed two-stage PBS achieves higher than 30.6 dB polarization extinction ratio (PER) for both TE and TM polarizations across a 130 nm span at the SCL-band. The dimensions of the PBS are 94 × 14 μm2 and the insertion losses are calculated to be lower than 0.8 dB for both polarizations. The fabrication tolerance of the device is also discussed.