Thick Silicon Nitride Research Articles

Power Output Force Generation by a MEMS Phase Change Actuator

A microelectromechanical-systems-based phase change actuator has been developed and tested for high-speed mechanical power output and force generation. This actuator is well suited for a variety of advanced devices like tactile displays or micro fluidic systems. The device features two thin membranes that bound a cavity filled with working fluid. The working fluid boils at low temperature. Two sizes of actuator are tested, an actuator with membrane sidelengths of 5 mm and an actuator with top membrane sidelength of 10 mm. Two top membrane materials are explored consisting of 2 μm thick silicon and 300 nm thick silicon nitride. Heat addition is through the lower membrane which is fabricated with novel capillary structures designed to increase the efficiency of actuator operation. The actuator is shown to produce up to 2.6 mW of mechanical power output and generate an applied force of 43 mN. Operating speeds up to 100 Hz are demonstrated.

Fusion bonding of silicon nitride surfaces

While silicon nitride surfaces are widely used in many micro electrical mechanical system devices, e.g. for chemical passivation, electrical isolation or environmental protection, studies on fusion bonding of two silicon nitride surfaces (Si3N4–Si3N4 bonding) are very few and highly application specific. Often fusion bonding of silicon nitride surfaces to silicon or silicon dioxide to silicon surfaces is preferred, though Si3N4–Si3N4 bonding is indeed possible and practical for many devices as will be shown in this paper. We present an overview of existing knowledge on Si3N4–Si3N4 bonding and new results on bonding of thin and thick Si3N4 layers. The new results include high temperature bonding without any pretreatment, along with improved bonding ability achieved by thermal oxidation and chemical pretreatment. The bonded wafers include both unprocessed and processed wafers with a total silicon nitride thickness of up to 440 nm. Measurements of bonding strength, void characterization, oxidation rate and surface roughness are also presented. Bonding strengths for stoichiometric low pressure chemical vapor deposition Si3N4–Si3N4 direct fusion bonding in excess of 2 J cm−2 are found. The stoichiometry is verified indirectly through refractive index and intrinsic stress measurements. The importance of surface oxide in Si3N4–Si3N4 fusion bonding is investigated by x-ray photoelectron spectroscopy measurements.

Nanopore Analysis of Individual RNA/Antibiotic Complexes

Nanopores in thin solid-state membranes are used to rapidly analyze individual RNA/drug complexes. The interactions of a truncated A-site RNA model of the prokaryotic ribosome with aminoglycoside antibiotics are characterized by passing individual molecules through a 3-3.5 nm diameter pore fabricated in a 8-10 nm thick silicon nitride membrane. Complexes of the A-site RNA with aminoglycosides can be distinguished from unbound A-site based on the ion current signatures produced as they pass through the nanopores. Counting the fraction of free and drug-bound molecules affords label-free drug-RNA binding isotherms consistent with literature reports and with data generated using independent fluorescence-based assays. Our measurements are supported by molecular dynamics simulations, which illustrate the relationship between the ionic current and complexation of the A-site RNA with paramomycin, a prototypical aminoglycoside antibiotic.

In‐Situ Partial Sintering of Gold‐Nanoparticle Sheets for SERS Applications

A new, versatile substrate design for surface-enhanced Raman spectroscopy (SERS) is introduced that provides better illumination and collection efficiency than other solid substrates. It uses sheets of 5 nm diameter gold nanoparticles that are draped by drying-mediated self-assembly onto 100 nm thick silicon nitride membranes. During laser illumination, partial in-situ sintering of the nanoparticles into larger structures with tiny gaps (≈2 nm) greatly increases the SERS enhancement factor. The detection of 1 pM of p-mercaptoaniline and 1 fg of 2,4-dinitrotoluene is demonstrated. The use of self-assembled nanoparticle sheets furthermore makes it possible to perform SERS detection in situ on top of a probe solution droplet.

Self-aligned mask renewal for anisotropically etched circular micro- and nanostructures

The top–down fabrication of high aspect ratio circular micro- and nanostructures in silicon nitride is presented. A new method is introduced to increase the aspect ratio of anisotropically etched holes by a factor of more than two with respect to the results obtained from an established dry-etching process. The method is based on the renewal of an etching mask after a first etching step has been completed. Mask renewal is done by line-of-sight deposition of a masking layer on the surface of the sample, which is mounted at an angle with respect to the deposition direction. No additional alignment step is required. The proof of principle is performed for silicon nitride etching through a mask of titanium, but the method has great potential to be applicable to a wide variety of substrate–mask combinations and to find entrance into various engineering fields. Two specific applications are highlighted. Firstly, a thick silicon nitride hardmask is used for the fabrication of deeply etched photonic crystal holes in indium phosphide (InP). For holes of 280 nm diameter, a record aspect ratio of 20 and an overall selectivity of 28.5 between a positive-tone resist layer and InP are reported. Secondly, the use of perforated silicon nitride membranes for droplet formation for applications in food engineering or pharmaceutics is addressed. Preliminary results show a potential for the self-aligned mask renewal method to exceed state-of-the-art membrane quality in terms of pore size, aspect ratio and membrane stability.

Piezoelectric microspeakers built on various diaphragms

This paper describes and compares four different types of diaphragm-based piezoelectric microspeakers built on (1) a compressively stressed silicon nitride diaphragm, (2) a parylene diaphragm, (3) a dome-shaped silicon nitride diaphragm, and (4) a PZT bimorph diaphragm. The main innovation in the first device is the usage of a wrinkled diaphragm that supports a flat diaphragm, where piezoelectric actuation happens, and allows a large bending displacement. The second device is to exploit the very low elastic modulus of parylene (a polymer material), and is built on a 1.5 μm thick parylene diaphragm with electrodes and piezoelectric ZnO film. Also described in this paper is an acoustic transducer built on a 1.5 μm thick dome-shaped silicon nitride diaphragm (2 mm in radius, with a circular clamped boundary on a silicon substrate) with electrodes and piezoelectric ZnO film. The dome diaphragm is shown to effectively release residual stress through volumetric change of its shape. Finally, this paper describes a microspeaker (composed of 8 mm square PZT bimorph and bulk-micromachined silicon) that shows flat diaphragm displacement from DC to 8 kHz. A bimorph diaphragm is formed by gluing two 127 μm thick PZT sheets and attaching them to a micromachined silicon substrate.

Surface plasmon modes of a single silver nanorod: an electron energy loss study

We present an electron energy loss study using energy filtered TEM of spatially resolved surface plasmon excitations on a silver nanorod of aspect ratio 14.2 resting on a 30 nm thick silicon nitride membrane. Our results show that the excitation is quantized as resonant modes whose intensity maxima vary along the nanorod's length and whose wavelength becomes compressed towards the ends of the nanorod. Theoretical calculations modelling the surface plasmon response of the silver nanorod-silicon nitride system show the importance of including retardation and substrate effects in order to describe accurately the energy dispersion of the resonant modes.

Patterning of porous silicon nanostructures and eliminating microcracks on silicon nitride mask using metal assisted chemical etching

A method for selective formation of reproducible, high fidelity and controllable nano and micrometer size porous Si areas over n-type Si wafers is provided. A 400 nm thick Silicon Nitride layer was used as the mask layer while Platinum and Palladium nanoparticles were deposited over the unprotected areas to obtain porous areas through metal assisted chemical etching process. Nanoparticles were deposited by electroless plating solutions containing H 2PtCl 6 and PdCl 2. Good controls over pore size and depth were obtained with well defined and sharp edges of the patterned areas. The results were compared to porous structures obtained via electrochemical etching process, indicating the superiority of metal assisted etching in terms of its simplicity as well as the ability of Silicon Nitride layer acting as the mask layer.

Thermal Properties of Silicon Nitride Beams Below One Kelvin

We have investigated thermal properties of 1 μm thick silicon nitride beams of different lateral dimensions. We measured the thermal conductance by simultaneously employing a TES both as a heater and as a sensor. Based upon these measurements, we calculate the thermal conductivity of the beams. We utilize a boundary limited phonon transport model and assume a temperature independent phonon mean free path. We find that the thermal conductivity is determined by the fraction of diffusive reflection at surface. The following results are obtained from 0.30 K to 0.55 K: the volume heat capacity is 0.082T+0.502T3 J/m3-K . The width dependent phonon mean free path is 6.58 μm, 9.80 μm and 11.55 μm for 10 μm, 20 μm and 30 μm beams respectively at a 29% surface diffusive reflection.

Microelectromechanical system-based vacuum gauge for measuring pressure and outgassing rates in miniaturized vacuum microelectronic devices

Cr/Au meander-shaped resistors were fabricated on 0.2 μm thick square-shaped silicon nitride diaphragms with diaphragm dimensions ranging from 0.775 to 2.275 mm. The performance of these sensors was measured in a vacuum chamber as a function of resistor powers from 0.5 to 3 mW at pressures ranging from 2×10−4 to 760 Torr. The lengths of the meander-shaped resistors increase from 5.5 mm for the 0.775 mm diaphragm to 33.6 mm for the 2.275 mm diaphragm devices. It is shown that the pressure dependence of the devices is governed by the kinetic gas theory and that the devices can measure pressures from about 10 to 1×10−3 Torr. At a resistor power of 2 mW at 760 Torr, the 2.275 mm diaphragm device, which has the largest sensing area, exhibits the highest sensitivity.

Design and fabrication of a bidimentional microbolometer array for Terahertz detection characterized at different temperatures

We present the design, micromachining and characterization of a bidimentional bolometer array for radiation detection in the 0.7-1.5 THz frequency range. The detector is based on a boron doped amorphous silicon film (a-Si-B:H). The film optimized for sensitivity enhancement was obtained using 500 sccm diborane flow with 95 nm thickness. The sensing layer was deposited using plasma enhanced chemical vapor deposition (PECVD) technique at low frequency on a 0.45 μm thick silicon nitride membrane sustained by a micromachined frame in crystalline silicon. The design consists of four 5x5 bolometer arrays made by conventional lithography. The bolometer active area is 660 μm x 420 μm and the detector will operate as a focal plane array. The current–voltage characteristics present an ohmic behaviour; the temperature coefficient of resistance (TCR) was obtained by measuring the bolometer performance from room temperature down to liquid nitrogen temperature. The responsivity was measured under illumination from a black body radiating at 300, 500, 700, 900 and 1100 °C, obtaining a value of R =1.17 x 10−2 A/W with a dark current of 4.43 x 10−9 A

Microscale oscillating crack propagation in silicon nitride thin films

Controlled microscale (<50 μm) wavy cracks in silicon nitride thin films have been produced through a simple heating process. The crack paths were controlled by metal patterns under the silicon nitride thin films. Wavy crack characteristics were investigated by changing the metal, metal linewidths, metal thickness, and silicon nitride thickness. We discuss the differences in the characteristics and mechanisms of propagation of wavy cracks formed due to differential thermal expansion and those that result from thermal gradients.

Time-dependent simulations of a membrane-based nanocalorimeter

We present time-dependent numerical simulations of the thermal and electrical response of a membrane-based nanocalorimeter designed for general studies of heat capacity and latent heat of milligram to sub-microgram samples. The investigated device is based on freestanding, 150 nm thick silicon nitride membranes onto which thin film heaters and temperature sensors are fabricated. This design makes the thermal link small enough to allow both relaxation and ac steady-state methods to be used interchangeably. We compare simulations of the two-dimensional thermal behavior of the nanocalorimeter with the results of experiments. The simulations take current distribution, heat generation and heat flow into consideration, and shed light on the frequency dependent contribution of the membrane heat capacity in ac steady-state experiments. The simulations also illustrate where energy is stored, thus assisting further improvement of the device design.

Effect of interlayer trapping and detrapping on the determination of interface state densities on high-k dielectric stacks

The influence of the silicon nitride blocking layer thickness on the interface state densities (Dit) of HfO2/SiNx:H gate-stacks on n-type silicon have been analyzed. The blocking layer consisted of 3 to 7 nm thick silicon nitride films directly grown on the silicon substrates by electron-cyclotron-resonance assisted chemical-vapor-deposition. Afterwards, 12 nm thick hafnium oxide films were deposited by high-pressure reactive sputtering. Interface state densities were determined by deep-level transient spectroscopy (DLTS) and by the high and low frequency capacitance-voltage (HLCV) method. The HLCV measurements provide interface trap densities in the range of 1011 cm−2 eV−1 for all the samples. However, a significant increase in about two orders of magnitude was obtained by DLTS for the thinnest silicon nitride barrier layers. In this work we probe that this increase is an artifact due to the effect of traps located at the internal interface existing between the HfO2 and SiNx:H films. Because charge trapping and discharging are tunneling assisted, these traps are more easily charged or discharged as lower the distance from this interface to the substrate, that is, as thinner the SiNx:H blocking layer. The trapping/detrapping mechanisms increase the amplitude of the capacitance transient and, in consequence, the DLTS signal that have contributions not only from the insulator/substrate interface states but also from the HfO2/SiNx:H interlayer traps.

Theoretical and experimental investigation of optically driven nanoelectromechanical oscillators

The actuation of biologically functional micro- and nanomechanical structures using optical excitation is an emerging arena of research that couples the fields of optics, fluidics, electronics, and mechanics with potential for generating novel chemical and biological sensors. In our work, we fabricated nanomechanical structures from 200 and 250 nm thick silicon nitride and single crystal silicon layers with varying lengths and widths ranging from 4 to 12 μm and 200 nm to 1 μm, respectively. Using a modulated laser beam focused onto the device layer in close proximity to the clamped end of a cantilever beam, we concentrate and guide the impinging thermal energy along the device layer. Cantilever beams coupled to chains of thermally isolated links were used to experimentally investigate energy transport mechanisms in nanostructures. The nature of the excitation was studied through steady-periodic axisymmetric thermal analysis by considering a multilayered structure heated using a modulated laser source. Results were verified by finite element analysis, which was additionally implemented for the solution of steady-periodic and transient thermal, as well as steady thermoelastic problems. These theoretical investigations, coupled with our experimental results, reveal that the complex dynamics underpinning optical excitation mechanisms consists of two disparate spatial regimes. When the excitation source is focused in close proximity to the structure the response is primarily thermal. We show that as the source is placed farther from the clamped end of the structure, the thermal response progressively fades out, indicating the possibility of mechanical wave propagation. Understanding the excitation mechanisms may be useful for applications including compact integration of nanophotonic elements with functionalized nanomechanical sensors for ultrasensitive biochemical analysis.

Large-area nanoperforated SiN membranes for optical and mechanical filtering

Nanoperforated membranes can transmit light and modify either the spectral or the spatial properties of the beam passing through the membrane. Alternatively, the perforated membrane can be used as a precisely fabricated filter for nanoparticles, fluids or gases. We made membranes made on a silicon nitride layer deposited on silicon wafer with nanopatterning was realized by nanoimprint lithography. We present two components based on these nanopatterned membranes. Firstly, we demonstrate large-area nanoperforated membranes. The perforations through a 1500 nm thick free-standing silicon nitride (SiN) film have a diameter of 350 nm and a period of 700 nm. The area of the largest free-standing films was 25 mm 2. Secondly, we demonstrate small area membranous optical elements and a method to couple them to optical fibers. Our component uses an optical element processed on the SiN-membrane on silicon wafer. Light is launched and collected through the fiber glued in the trough-wafer via. This component demonstrates a novel and simple way to couple mass producible optical elements with optical fibers. Finally, we fabricate an optical filter utilizing guided-mode resonance on the SiN-membrane. The filter utilizes fiber coupling, and we demonstrate its use as a refractive index sensor.

Radiometry and metrology of a phase zone plate measured by extreme ultraviolet synchrotron radiation

The diffraction efficiency, focal length, and other radiometric and metrology properties of a phase zone plate were measured by using monochromatic synchrotron radiation in the 7-18.5 nm wavelength range. The zone plate was composed of molybdenum zones having a 4 mm outer diameter and 70 nm nominal thickness and supported on a 100 nm thick silicon nitride membrane. The diffraction efficiency was enhanced by the phase shift of the radiation passing through the zones. The measured first-order efficiency was in good agreement with the calculated efficiency. The properties of the zone plate, particularly the small variation of the efficiency with off-axis angle, make it suitable for use in a radiometer to accurately measure the absolutely calibrated extreme ultraviolet emission from the Sun.

Mechanical properties of PECVD thin ceramic films

Silicon dioxide (thickness 350 nm and 969 nm) and silicon nitride (thickness 218 nm) films deposited on silicon substrate using plasma enhanced chemical vapor deposition process were investigated using a Berkovich nanoindenter. The load-depth measurements revealed that the oxide films have lower modulus and hardness compared to the silicon substrate, where as the nitride film has a higher hardness and slightly lower modulus than the substrate. To delineate the substrate effect, a phenomenological model, that captures most of the ‘continuous stiffness measurement’ data, was proposed and then extended on both sides to determine the film and substrate properties. The modulus and hardness of the oxide film were around 53 GPa and 4–8 GPa where as those of the nitride film were around 150 GPa and 19 GPa, respectively. These values compare well with the measurements reported elsewhere in the literature.

MEMS-based microstructures for nanomechanical characterization of thin films

The measurement of mechanical properties of thin films is a major issue for the designof reliable microelectronic devices, microsensors or thin coatings. New simplemicrostructures actuated through the release of internally stressed long beamsmade of high temperature, low pressure chemical vapour deposition silicon nitridehave been developed to test under uniaxial tension submicron thin film materialspecimens. The relative displacement between a fixed and a moving cursor is used todetermine the strain applied to the specimen. The stress is inferred based onthe mismatch strain and Young’s modulus of the silicon nitride actuator beam.By multiplying the tensile test microstructures with different lengths, the fullstress–strain curve characterizing the thin material sample is generated fromwhich the elastic stiffness, yield strength, ductility and fracture stress can beextracted. The potential of the method is demonstrated through applicationson both brittle and ductile thin films. The Young’s modulus of 238 GPa for a373 nm thick silicon nitride film is extracted and size effects are observed for theyield strength of pure aluminium with a value of 220 and 550 MPa, respectively,for 373 and 205 nm thick films. An original variant of the procedure based onthis new test microstructure for measuring Young’s modulus is also presented.

The adhesion of SiN x thin layers on silica–acrylate coated polymer substrates

Plasma Enhanced Chemical Vapor Deposition (PECVD) was used to grow 200, 300 and 400 nm thick silicon nitride layers (SiN x ) on a high temperature aromatic polyester substrate spin coated with a silica–acrylate hybrid coating (hard coat). Layers deposited without oxygen plasma treatment remained attached to the substrate, while spontaneous buckle delamination of the layer deposited with oxygen plasma treatment was observed directly after layer deposition. This effect was investigated using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). SIMS analyses showed a considerable increase of silicon oxide by exposing the substrate to oxygen plasma treatment, while AFM showed an increased roughness. Bright-field transmission electron micrographs show the presence of a particulate SiO 2 layer. The oxygen plasma treatment thus removes the acrylate from the hard coat layer leaving behind the SiO 2 particles leading to lower adhesion and thus to spontaneous buckle delamination.