Chemical Vapor Deposited Silicon Nitride Research Articles

Power-efficient programmable integrated multiport photonic interferometer in CMOS-compatible silicon nitride

Silicon nitride (SiN x ) is an appealing waveguide material choice for large-scale, high-performance photonic integrated circuits (PICs) due to its low optical loss. However, SiN x PICs require high electric power to realize optical reconfiguration via the weak thermo-optic effect, which limits their scalability in terms of device density and chip power dissipation. We report a 6-mode programmable interferometer PIC operating at the wavelength of 1550 nm on a CMOS-compatible low-temperature inductance coupled plasma chemical vapor deposition (ICP-CVD) silicon nitride platform. By employing suspended thermo-optic phase shifters, the PIC achieves 2× improvement in compactness and 10× enhancement in power efficiency compared to conventional devices. Reconfigurable 6-dimensional linear transformations are demonstrated including cyclic transformations and arbitrary unitary matrices. This work demonstrates the feasibility of fabricating power-efficient large-scale reconfigurable PICs on the low-temperature ICP-CVD silicon nitride platform.

Permeation of oxygen and water into a plasma-enhanced chemical vapor deposited silicon nitride film as function of deposition pressure

In this work, we demonstrate that the permeability of a SiNx thin film (prepared by plasma-enhanced chemical vapor deposition) to water and oxygen is closely related to the deposition pressure. By dynamic secondary ion mass spectrometry, we confirmed that water penetration occurs into the SiNx film only in the oxidized layer. Furthermore, positron annihilation lifetime spectroscopy indicated that a SiNx film with a lower deposition pressure provides a smaller pore (free volume hole) radius, which is more effective in terms of blocking ambient molecular diffusion or penetration. The SiNx films were also applied as an encapsulation layer for organic light-emitting diodes; SiNx films with a lower deposition pressure exhibited higher encapsulation properties.

A Universal Approach to High‐Index‐Contrast Flexible Integrated Photonics

AbstractFlexible integrated photonics is an essential technology for emerging applications, including flexible optical interconnects, optogenetic stimulation, and implantable conformal sensing. Here, a novel and universal route for fabricating flexible photonic components with high‐refractive‐index contrast is reported. Central to such a unique method is the utilization of germanium oxide (GeO) as the sacrificial layer for releasing nanostructures from rigid substrates to flexible substrates. Various high‐quality inorganic optical materials can be grown directly on GeO by different thin‐film deposition methods due to its resistance to both high temperature and high‐power oxygen plasmas. In addition to the absence of restrictions on the material choices and integration processes for flexible photonic structures, the approach uses water as the etchant to remove the sacrificial layer, which has minimal impact on the optical performance of the photonic structures. Using this approach, a strain‐insensitive/sensitive microring resonator based on plasma‐enhanced chemical vapor deposited silicon nitride and reactive sputtered titanium oxide, respectively, is demonstrated, establishing the strategy as a facile and universal route for the fabrication of high‐index‐contrast flexible integrated photonic devices with various functionalities.

Long-Term Stability of Novel Crucible Systems for the Growth of Oxygen-Free Czochralski Silicon Crystals

The replacement of the silica glass crucible by oxygen-free crucible materials in silicon Czochralski (Cz) growth technology could be a key factor to obtaining Cz silicon, with extremely low oxygen contamination < 1 × 1017 at/cm3 required for power electronic applications. So far, isostatic pressed graphite or nitrogen-bonded silicon nitride (NSN) crucible material, in combination with a chemical vapor deposited silicon nitride (CVD-Si3N4) surface coating, could be identified as promising materials by first short-term experiments. However, for the evaluation of their potential for industrial scale Cz growth application, the knowledge about the long-term behavior of these crucible setups is mandatory. For that purpose, the different materials were brought in contact with silicon melt up to 60 h to investigate the infiltration and dissolution behavior. The chosen graphite, as well as the pore-sealed NSN material, revealed a subordinated infiltration-depth of ≤1 mm and dissolution of ≤275 µm by the silicon melt, so they basically fulfilled the general safety requirements for Cz application. Further, the highly pure and dense CVD Si3N4 crucible coating showed no measurable infiltration as well as minor dissolution of ≤50 µm and may further acts as a nucleation site for nitrogen-based precipitates. Consequently, these novel crucible systems have a high potential to withstand the stresses during industrial Cz growth considering that more research on the process side relating to the particle transport in the silicon melt is needed.

Determination of the nonlinear thermo-optic coefficient of silicon nitride and oxide using an effective index method.

There is little literature characterizing the temperature-dependent thermo-optic coefficient (TOC) for low pressure chemical vapor deposition (LPCVD) silicon nitride or plasma enhanced chemical vapor deposition (PECVD) silicon dioxide at temperatures above 300 K. In this study, we characterize these material TOC's from approximately 300-460 K, yielding values of (2.51 ± 0.08) · 10-5K-1 for silicon nitride and (5.67 ± 0.53) · 10-6K-1 for silicon oxide at room temperature (300 K). We use a simplified experimental setup and apply an analytical technique to account for thermal expansion during the extraction process. We also show that the waveguide geometry and method used to determine the resonant wavelength have a substantial impact on the precision of our results, a fact which can be used to improve the precision of numerous ring resonator index sensing experiments.

Selectively patterned Mg-doped GaN by SiNx-driven hydrogen injection

We demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

Effect of rapid thermal annealing on the mechanical stress and physico-chemical properties in plasma enhanced atomic layer deposited silicon nitride thin films

The present study reports the impact of process conditions and post annealing treatment on the stress and physico-chemical properties in different types of plasma enhanced atomic layer deposited (PEALD) SiNx films. More specifically, the SiNx films deposited at 500 °C were annealed subsequently at 1000 °C in N2 ambient for different times ranging between 30 and 6750 s and were subjected to thickness, mass, refractive index, chemical bonds and structure, H content, N/Si stoichiometry, morphology, roughness, and uniformity monitoring. Depending on the deposition process conditions, we confirm the possibility to form, at 500 °C with PEALD, some very compressive SiN films with reduced hydrogen content by controlling the plasma conditions along the deposition process. More interestingly, these compressive films with low hydrogen concentration exhibit high thermo-mechanical stability compared to hydrogenated PEALD and plasma enhanced chemical vapor deposition silicon nitride from this study or reported in the literature, films which typically tend to become tensile with a similar annealing sequence. These results confirm the importance of the plasma power to engineer the as-deposited silicon nitride stress but also highlight the role of hydrogen in the thermal instability of these layers, a low H concentration being needed for a better mechanical stability.

Estimation of the Young's Modulus of Nanometer-Thick Films Using Residual Stress-Driven Bilayer Cantilevers.

Precise prediction of mechanical behavior of thin films at the nanoscale requires techniques that consider size effects and fabrication-related issues. Here, we propose a test methodology to estimate the Young’s modulus of nanometer-thick films using micromachined bilayer cantilevers. The bilayer cantilevers which comprise a well-known reference layer and a tested film deflect due to the relief of the residual stresses generated during the fabrication process. The mechanical relationship between the measured residual stresses and the corresponding deflections was used to characterize the tested film. Residual stresses and deflections were related using analytical and finite element models that consider intrinsic stress gradients and the use of adherence layers. The proposed methodology was applied to low pressure chemical vapor deposited silicon nitride tested films with thicknesses ranging from 46 nm to 288 nm. The estimated Young’s modulus values varying between 213.9 GPa and 288.3 GPa were consistent with nanoindentation and alternative residual stress-driven techniques. In addition, the dependence of the results on the thickness and the intrinsic stress gradient of the materials was confirmed. The proposed methodology is simple and can be used to characterize diverse materials deposited under different fabrication conditions.

The Mechanism of Surface Passivation Degradation in SiO2/SiNx Stack Under Light and Elevated Temperature

In this article, a significant degradation of the surface passivation was observed in Czochralski silicon (Cz-Si) lifetime samples using effective lifetime measurements during 1 sun illuminated treatment at 130 °C–175 °C. The samples are passivated by plasma-enhanced chemical vapor deposited silicon nitride (SiNx) and thermally grown silicon oxide (SiO2). Samples showed strong variations in the kinetics of the surface passivation degradation depending on the passivation stacks, phosphorus diffusion, and bulk doping. Recovery of the surface passivation was only observed on samples passivated by a single silicon nitride layer. To further investigate the observed behavior, corona charging capacitance-voltage measurements were performed. It is shown that surface degradation arises from a combination of changes in interface defect density ( Dit ) and fixed charge density ( Qf ). We show that two separate degradation channels contribute to the degradation in surface passivation, where the fixed charge density continuously declines, while the interface defect density increases to a maximum and subsequently decreases. The decrease in interface defect density leads to a recovery of surface passivation.

Dispersion Optimization of Silicon Nitride Waveguides for Efficient Four-Wave Mixing

Silicon nitride waveguides have emerged as an excellent platform for photonic applications, including nonlinear optical signal processing, owing to their relatively high Kerr nonlinearity, negligible two photon absorption, and wide transparent bandwidth. In this paper, we propose an effective approach using 3D finite element method to optimize the dispersion characteristics of silicon nitride waveguides for four-wave mixing (FWM) applications. Numerical studies show that a flat and low dispersion profile can be achieved in a silicon nitride waveguide with the optimized dimensions. Near-zero dispersion of 1.16 ps/km/nm and 0.97 ps/km/nm at a wavelength of 1550 nm are obtained for plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD) silicon nitride waveguides, respectively. The fabricated micro-ring resonator with the optimized dimensions exhibits near-zero dispersion of −0.04 to −0.1 ps/m/nm over a wavelength range of 130 nm which agrees with the numerical simulation results. FWM results show that near-zero phase mismatch and high conversion efficiencies larger than −12 dB using a low pump power of 0.5 W in a 13-cm long silicon nitride waveguide are achieved.

Nanomechanical spectroscopy of ultrathin silicon nitride suspended membranes

Mechanical properties of a nanomechanical resonator significantly impact the performance of a resonant Nano-electromechanical system (NEMS) device. We study the mechanical properties of suspended membranes fabricated out of low-pressure chemical vapor deposited silicon nitride thin films. We fabricated doubly-clamped membranes of silicon nitride with thickness less than 50 nm and length varying from 5 to 60 μm. The elastic modulus and stress in the suspended membranes were measured using Atomic Force Microscope (AFM)-based nanomechanical spectroscopy. The elastic moduli of the suspended membranes are significantly higher than those of corresponding on-substrate thin films. We observed a reduction in net stress after the fabrication of suspended membrane, which is explained by estimating the thermal stress and intrinsic stress. We also use a mathematical model to study the stress and thickness-dependent elastic modulus of the ultrathin membranes. Lastly, we study the capillary force-gradient between the SiNxsuspended membrane-Si substrate that could collapse the suspended membrane.

Development of MEMS-based piezoresistive 3D stress/strain sensor using strain technology and smart temperature compensation

This paper presents the microfabrication and testing of a membrane-free eight-element single-polarity (n-type) sensing rosette integrated with strained silicon technology over (111) silicon plane to measure the full 3D stress/strain tensor with full temperature compensation. Such n-type piezoresistive (PR) sensor has low sensitivity to the out-of-plane components compared to the in-plane components. To improve the sensitivity of such sensors to the out-of-plane components, a strained silicon technique was integrated into the sensing rosette during the microfabrication process using a highly compressive film produced by plasma enhanced chemical vapor deposition silicon nitride. For experimental verification, a prototype device featuring the proposed sensing rosette was microfabricated using semiconductors fabrication processes. The experimental analysis applied both, in-plane and out-of-plane stresses at different temperatures over a range from −20 °С to 60 °С. In this work, a smart sensing calibration algorithm, utilizing machine learning, is employed to reduce the temperature impact on both sensitivity and resistance of PR coefficients during stress measurement. The developed sensor is capable of accurately extracting the applied stress/strain components with temperature compensation.

High-efficiency lithium niobate modulator for K band operation

This paper reports a hybrid silicon nitride–lithium niobate electro-optic Mach–Zehnder-interferometer modulator that demonstrates overall improvements in terms of half-wave voltage, optical insertion loss, extinction ratio, and operational bandwidth. The fabricated device exhibits a DC half-wave voltage of ∼1.3 V, a static extinction ratio of ∼27 dB, an on-chip optical loss of ∼1.53 dB, and a 3 dB electro-optic bandwidth of 29 GHz. In addition, this device operates beyond the 3 dB bandwidth, where a half-wave voltage of 3 V is extracted at 40 GHz when the device is biased at quadrature. The modulator is realized by strip-loading thin-film lithium niobate with low-pressure chemical vapor deposited silicon nitride; this enables reduced on-chip losses and allows for a lengthened 2.4 cm long interaction region that is specifically engineered for broadband performance.

Study of Silicon Nitride Inner Spacer Formation in Process of Gate-all-around Nano-Transistors.

Stacked SiGe/Si structures are widely used as the units for gate-all-around nanowire transistors (GAA NWTs) which are a promising candidate beyond fin field effective transistors (FinFETs) technologies in near future. These structures deal with a several challenges brought by the shrinking of device dimensions. The preparation of inner spacers is one of the most critical processes for GAA nano-scale transistors. This study focuses on two key processes: inner spacer film conformal deposition and accurate etching. The results show that low pressure chemical vapor deposition (LPCVD) silicon nitride has a good film filling effect; a precise and controllable silicon nitride inner spacer structure is prepared by using an inductively coupled plasma (ICP) tool and a new gas mixtures of CH2F2/CH4/O2/Ar. Silicon nitride inner spacer etch has a high etch selectivity ratio, exceeding 100:1 to Si and more than 30:1 to SiO2. High anisotropy with an excellent vertical/lateral etch ratio exceeding 80:1 is successfully demonstrated. It also provides a solution to the key process challenges of nano-transistors beyond 5 nm node.

Role of Capping Material and GaN Polarity on Mg Ion Implantation Activation

Ion implantation of magnesium for p‐type GaN presents many opportunities; however, activation has proven difficult due to the decomposition of GaN at relevant annealing temperatures. Herein, testing the efficacy of multiple in situ and ex situ caps based on aluminum nitride and silicon nitride for GaN protection during annealing is presented. Photoluminescence shows better activation for in situ metal organic chemical vapor deposition (MOCVD)‐grown aluminum nitride caps compared with ex situ sputtered aluminum nitride and the best performance by ex situ plasma‐enhanced chemical vapor deposition (PECVD) silicon nitride. Furthermore, only samples annealed at the highest temperatures tested show preferential growth of UV luminescence to yellow‐green luminescence reinforcing the need for better capping solutions and higher temperature annealing.

Highly sensitive CMUT-based humidity sensors built with nitride-to-oxide wafer bonding technology

In this paper, we present highly sensitive capacitive micromachined ultrasonic transducer (CMUT) based gravimetric humidity sensors that are fabricated with nitride-to-oxide wafer bonding technology. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the proposed process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. The proposed process provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Furthermore, additional benefits such as reduced fabrication complexity and more controllable membrane thickness are possible. Ten MHz CMUTs were designed, fabricated and functionalized by graphene oxide (GO) with three different concentrations. The device uniformities and resonance frequencies were characterized by a laser vibrometer and a vector network analyzer. Humidity sensing performance of the sensors was analyzed through the resonance shifts in a relative humidity (RH) range of 11.3%–93.6%. The CMUT coated with the highest concentration of GO (0.5 mg/ml) exhibited the highest sensitivity of up to 15.3 kHz/%RH. Decent repeatability and a short response/recovery time were achieved by all the CMUT sensors. This study demonstrates that the CMUT humidity sensors that are realized with nitride-to-oxide bonding technology can be an excellent candidate for humidity sensing due to their high sensitivity and CMOS-compatible fabrication process.

Hot wire chemical vapor deposition for silicon photonics: An emerging industrial application opportunity

In this work different silicon photonic devices, including straight waveguides, multi-mode interference devices and Mach-Zehnder interferometers, were fabricated and characterized on hot-wire chemical vapor deposition (HWCVD) silicon nitride (SiN) layers deposited at temperatures below 350 °C. These layers presented a hydrogen concentration of 13.1%, which is lower than that achieved with plasma enhanced chemical vapor deposition at these deposition temperatures. The lowest reported optical propagation losses of 6.1 dB/cm and 5.7 dB/cm, 1550 nm and 1310 respectively, for straight SiN waveguides prepared by HWCVD was measured. We demonstrated that silicon nitride SiN, prepared using HWCVD, is a viable material for silicon photonics fabrication.

Plasma-enhanced chemical vapor-deposited SiN and liquid-phase-deposited SiO2 stack double-layer anti-reflection films for multi-crystalline solar cells

We successfully fabricated a double-layer anti-reflection film for multi-crystalline silicon solar cells through liquid-phase deposition to deposit silicon dioxide (SiO2) film on a multi-crystalline silicon surface and plasma-enhanced chemical vapor deposition to deposit silicon nitride (SiN) film on a SiO2 film surface. The SiO2 film thicknesses were 10, 15, and 20 nm, and the SiN film thickness was 60 nm. The reflectance of the substrate markedly decreased as the double-layer film was deposited. Moreover, with different film thicknesses, the minimal values of the reflectance were 4.91%, 2.75%, and 2.83% at the wavelengths of 609, 597, and 634 nm. The average reflection reached 6.92% when the thicknesses of SiO2 and SiN films were 20 and 60 nm in the wavelength range of 400–1100 nm. The minority carrier lifetimes of the multi-crystalline silicon substrates were 7.44, 7.21, and 7.27 μs at different film thicknesses. The short circuit current density and efficiency of the solar cell reached 34.52 mA/cm2 and 16.56% when the thicknesses of the SiO2 and SiN films were 20 and 60 nm, respectively. Low reflectance and good cell performance indicated that the double-layer film composed of SiO2 and SiN films were suitable for multi-crystalline silicon solar cells as the anti-reflection film.

Ultra-thin alumina and silicon nitride MEMS fabricated membranes for the electron multiplication

In this paper we demonstrate the fabrication of large arrays of ultrathin freestanding membranes (tynodes) for application in a timed photon counter (TiPC), a novel photomultiplier for single electron detection. Low pressure chemical vapour deposited silicon nitride (SixNy) and atomic layer deposited alumina (Al2O3) with thicknesses down to only 5 nm are employed for the membrane fabrication. Detailed characterization of structural, mechanical and chemical properties of the utilized films is carried out for different process conditions and thicknesses. Furthermore, the performance of the tynodes is investigated in terms of secondary electron emission, a fundamental attribute that determines their applicability in TiPC. Studied features and presented fabrication methods may be of interest for other MEMS application of alumina and silicon nitride as well, in particular where strong ultra-thin membranes are required.

Spectra-Dependent Stability of the Passivation Quality of Al2O3/c-Si Interfaces

We examine the stability of the c-Si surface passivation quality by spatial atomic-layer-deposited aluminum oxide (Al2O3), plasma-enhanced chemical vapor deposited silicon nitride (SiN x ), and Al2O3/SiN x stacks under illumination with two different spectra. The Al2O3-passivated c-Si surfaces annealed at 350 °C show a weak degradation due to UV illumination, with surface recombination velocities (SRVs) of 122 cm/s after receiving a ultraviolet (UV) dose of 275 kWh/m2. Silicon samples passivated with Al2O3 layers that received a fast-firing step show an improvement due to UV illumination with a reduction of the SRVs initially from 14 to 5 cm/s for single Al2O3 layers. For the fired Al2O3 layers the negative fixed charge density increases from −6×1012 cm−2 up to −1.2×1013 cm−2 during UV illumination. We demonstrate that for the SiN x and the fired Al2O3 single layers, photons with energy greater than 3.4 eV are necessary to reduce the passivation quality. In contrast, low-temperature-annealed Al2O3 single layers and nonfired Al2O3/SiN x stacks showed a degradation already under illumination with a halogen lamp. Importantly, we observe a perfectly stable passivation on boron-diffused p + emitter for fired Al2O3/SiN x stacks featuring a stable saturation current density of 18 fA/cm2 for a p + sheet resistance of 90 Ω/sq.