Stress Silicon Nitride Research Articles

Accurate and Precise Determination of Mechanical Properties of Silicon Nitride Beam Nanoelectromechanical Devices.

Accurate and precise determination of mechanical properties of nanoscale materials is mandatory since device performances of nanoelectromechanical systems (NEMS) are closely related to the flexural properties of the materials. In this study, the intrinsic mechanical properties of highly stressed silicon nitride (SiN) beams of varying lengths are investigated using two different techniques: Dynamic flexural measurement using optical interferometry and quasi-static flexural measurement using atomic force microscopy. The resonance frequencies of the doubly clamped, highly stressed beams are found to be inversely proportional to their length, which is not usually observed from a beam but is expected from a string-like structure. The mass density of the SiN beams can be precisely determined from the dynamic flexural measurements by using the values for internal stress and Young's modulus determined from the quasi-static measurements. As a result, the mass resolution of the SiN beam resonators was predicted to be a few attograms, which was found to be in excellent agreement with the experimental results. This work suggests that accurate and precise determination of mechanical properties can be achieved through combined flexural measurement techniques, which is a crucial key for designing practical NEMS applications such as biomolecular sensors and gas detectors.

Strained Pseudomorphic Ge1–xSnx Multiple Quantum Well Microdisk Using SiNy Stressor Layer

We demonstrate tensile-strained pseudomorphic Ge0.934Sn0.066/Ge quantum wells in a microdisk resonator using silicon nitride stressor layers. The hydrostatic and biaxial strain distributions are studied through finite element modeling, while confocal Raman spectroscopy shows local biaxial strain transfers as high as 1.1% at freestanding microdisk edges. These strains are sufficient to overcome the original compressive strain in Ge0.934Sn0.066 epitaxy and reach a direct band gap according to deformation potential theory. A red-shift in microdisk photoluminescence confirms the reduced band gap energies in response to tensile strain and suggests an average biaxial strain transfer of 0.55%.

Electron beam lithography patterned hydrogen silsesquioxane resist as a mandrel for self-aligned double patterning application

The semiconductor industry has developed various alternative approaches for obtaining high resolution patterns (sub-30 nm) using existing lithography techniques. Of the alternative approaches, self-aligned double patterning (SADP) is the approach which involves the fewest lithography steps. In SADP, a dummy pattern, also known as a mandrel, is first formed, followed by a conformal spacer material deposited on the patterned mandrel. Using dry reactive ion etching, spacer sidewalls are then formed. In the final step, the mandrel is removed by an etch process. The result is the double pattern of the spacer material. The advantage of this approach is that only one lithography step with the critical dimension of the double patterns is defined by the deposited spacer thickness. The disadvantage is the requirement of multiple, successive deposition and etch processes. In the present research, the authors demonstrate a reduction in required processing steps by using a novel mandrel/spacer approach for the SADP scheme. Using electron beam lithographically patterned hydrogen silsesquioxane resist and plasma enhanced chemical vapor deposition deposited low stress silicon nitride, the authors demonstrate a proof of concept of the SADP patterning scheme with reduced process steps.

Engineering Large In-Plane Tensile Strains in Ge Microdisks, Microrings and Racetrack Optical Cavities

Despite the fact that Ge is an indirect bandgap material, lasing has been made achievable due to the proximity of the direct band to the indirect in energy . Since the demonstrations of both optically [1] and electrically [2] pumped Ge lasers, there has been significant interest in techniques to lower the high lasing thresholds, which are due to the large carrier concentration required to fill the L valley before carriers can populate the direct band. Tensile strain has been shown to decrease the \U0001d6aa energy at a greater rate than the L, which leads to a transition to direct-bandgap [3], and would therefore allow a significant reduction in carrier injection for equivalent population inversion, while also reducing the free carrier absorption. This transition has generally been expected to occur between 1.6 and 2 % biaxial strain. In this work, high stress silicon nitride layers are used to transfer in-plane biaxial tensile strains > 2 % in Ge microcavities. A range of highly strained Ge on Si microcavities have been demonstrated, including Ge microdisks, microrings, and racetracks of varying undercut and diameter. The Ge layers are dry etched, before Tetramethylammonium hydroxide (TMAH) is used to wet etch the Si to provide an undercut. TMAH anisotropically etches Si, and leads to an undercutting profile which depends on the orientation of the waveguide segment, and therefore allows for the partially free standing structures to be engineered, in turn enabling high stress transfer from the silicon nitride stressors. It was found that Ge on Si racetrack structure can be fully undercut in straight sections, leaving supporting posts at the curved segments where the waveguide is aligned to <110> directions. The highest strained cavities demonstrated in this work are 4 µm diameter, highly undercut microdisks structures, which show in-plane biaxial tensile strains of ~ 2.3 % at the top surface, as measured by Raman spectroscopy. Photoluminescence up to 2.4 µm in wavelength is consistent with these strain levels. Equivalent structures with no undercut demonstrate biaxial strains as high as ~ 1 %, which would be suitable candidates for electrically injected devices. Finite element models (FEM) of the Ge disks are calibrated using the in-plane strain measured from Raman, and the disk curvature measured by transmission electron microscopy (TEM), allowing for an insight into the strain distribution within the disk, which is experimentally inaccessible by the techniques used here. It is found that while most of the top plane is biaxially strained, there is a radial uniaxial stress component around the disk edge, which induces εxy shear strains. This type of shear strain removes the glide reflection plane, and splits the L valleys. The L valleys that decrease in energy serve to counteract the effect of the biaxial strain, and reduces the carrier concentration at the \U0001d6aa, which would be detrimental for optical gain. The finite element model suggests that these regions of shear strain near the disk edge are predominantly between 0.3 and 0.5%. With 2 % biaxial strain, a 0.4% εxy shear strain results in a ~ 57 % reduction in carrier concentration at the \U0001d6aa. This work highlights that the optical overlap of the mode with shear strained regions has to be considered when calculating optical gain. Ge racetrack structures have the advantage that waveguide segments can be aligned to the correct crystallographic direction (<100>) to ensure that no εxyshear strains occur. However, reduced in-plane strains to ~ 1.2 % at the top surface suggest that disk structures are still preferable where high levels of strain are required. GeSn material systems have been recently shown to be direct bandgap [4], but still suffer from a lack of carriers at the \U0001d6aa band at room temperature, which could also be improved with moderate tensile strain. In such material systems, these racetrack geometries may be preferable. For instance, they can be engineered in such a way that they are undercut, but there is a Si post directly underneath the region where the optical mode will be supported. This allows the potential for electrical injection, and heat syncing, unlike in disks structures where the mode is supported away from the Si post. [1] J. Liu et al, Opt. Lett. 35, 679–81 (2010). [2] R. E. Camacho-Aguilera et al, Opt. Express 20, 11316–20 (2012). [3] D.J.Paul, Elec.Lett.45,582–584(2009). [4] S. Wirths, R. Geiger, et al, Nat. Photonics 9, 88–92 (2015). Figure 1

(Invited) Direct Band Gap Germanium

We show that direct band gap germanium can be obtained with external silicon nitride stressor layers. The cross-over from indirect to direct band gap is found to occur for an equivalent 1.67% biaxial tensile strain. Both whispering gallery modes and quasi-radial modes are observed with tensile-strained Ge microdisks. Quality factors up to 7100 have been measured around 2 µm wavelength. We demonstrate that circular Bragg reflectors can significantly enhance the quality factors of quasi-radial modes in strained germanium microdisks.

Wrinkled bilayer graphene with wafer scale mechanical strain

Wafer-scale strained bilayer graphene is demonstrated by employing a silicon nitride (Si3N4) stressor layer. Different magnitudes of compressive stress up to 840 MPa were engineered by adjusting the Si3N4 deposition recipes, and different strain conditions were analyzed using Raman spectroscopy. The strained graphene displayed significant G peak shifts and G peak splitting with 16.2 cm−1 and 23.0 cm−1 of the G band and two-dimensional band shift, which corresponds to 0.26% of strain. Raman mapping of large regions of the graphene films found that the largest shifts/splitting occurred near the bilayer regions of the graphene films. The significance of our approach lies in the fact that it can be performed in a conventional microfabrication process, i.e., the plasma enhanced chemical vapor deposition system, and thus easily implemented for large scale production.

Analysis of Ge micro-cavities with in-plane tensile strains above 2.

Ge on Si micro-disk, ring and racetrack cavities are fabricated and strained using silicon nitride stressor layers. Photoluminescence measurements demonstrate emission at wavelengths ≥ 2.3 μm, and the highest strained samples demonstrate in-plane, tensile strains of > 2 %, as measured by Raman spectroscopy. Strain analysis of the micro-disk structures demonstrate that shear strains are present in circular cavities, which can detrimentally effect the carrier concentration for direct band transitions. The advantages and disadvantages of each type of proposed cavity structure are discussed.

Direct Band Gap Germanium Microdisks Obtained with Silicon Nitride Stressor Layers

Germanium is an ideal candidate to achieve a monolithically integrated laser source on silicon. Unfortunately bulk germanium is an indirect band gap semiconductor. Here, we demonstrate that a thick germanium layer can be transformed from an indirect into a direct band gap semiconductor by using silicon nitride stressor layers. We achieve 1.75% (1.67%) biaxial tensile strain in 6 (9) μm diameter microdisks as measured from photoluminescence. The modeling of the photoluminescence amplitude vs temperature indicates that the zone-center Γ valley has the same energy as the L valley for a 9 μm diameter strained microdisk and is even less for the 6 μm diameter microdisk, thus demonstrating that a direct band gap is indeed obtained. We deduce that the crossover in germanium from indirect to direct gap occurs for a 1.67% ± 0.05% biaxial strain at room temperature, the value of this parameter varying between 1.55% and 2% in the literature.

Photonic Damascene process for integrated high-Q microresonator based nonlinear photonics

High confinement, integrated silicon nitride (SiN) waveguides have recently emerged as attractive platform for on-chip nonlinear optical devices. The fabrication of high-Q SiN microresonators with anomalous group velocity dispersion (GVD) has enabled broadband nonlinear optical frequency comb generation. Such frequency combs have been successfully applied in coherent communication and ultrashort pulse generation. However, the reliable fabrication of high confinement waveguides from stoichiometric, high stress SiN remains challenging. Here we present a novel photonic Damascene fabrication process enabling the use of substrate topography for stress control and thin film crack prevention. With close to unity sample yield we fabricate microresonators with $1.35\,\mu\mathrm{m}$ thick waveguides and optical Q factors of $3.7\times10^{6}$ and demonstrate single temporal dissipative Kerr soliton (DKS) based coherent optical frequency comb generation. Our newly developed process is interesting also for other material platforms, photonic integration and mid infrared Kerr comb generation.

PECVD silicon-rich nitride and low stress nitride films mechanical characterization using membrane point load deflection

An analysis of the mechanical properties of plasma enhanced chemical vapor (PECVD) silicon nitrides is presented, using micro fabricated silicon nitride membranes under point load deflection. The membranes are made of PECVD silicon-rich nitride and low stress nitride films. The mechanical performance of the bended membranes is examined both with analytical models and finite element simulation in order to extract the elastic modulus and residual stress values. The elastic modulus of low stress silicon nitride is calculated using stress free analytical models, while for silicon-rich silicon nitride and annealed low stress silicon nitride it is estimated with a pre-stressed model of point-load deflection. The effect of annealing both in nitrogen and hydrogen atmosphere is evaluated in terms of residual stress, refractive index and thickness variation. It is demonstrated that a hydrogen rich annealing atmosphere induces very little change in low stress silicon nitride. Nitrogen annealing effects are measured and shown to be much higher in silicon-rich nitride than in low stress silicon nitride. An estimate of PECVD silicon-rich nitride elastic modulus is obtained in the range between 240–320 GPa for deposited samples and 390 GPa for samples annealed in nitrogen atmosphere. PECVD low stress silicon nitride elastic modulus is estimated to be 88 GPa as deposited and 320 GPa after nitrogen annealing.

Ge microdisk with lithographically-tunable strain using CMOS-compatible process.

We present germanium microdisk optical resonators under a large biaxial tensile strain using a CMOS-compatible fabrication process. Biaxial tensile strain of ~0.7% is achieved by means of a stress concentration technique that allows the strain level to be customized by carefully selecting certain lithographic dimensions. The partial strain relaxation at the edges of a patterned germanium microdisk is compensated by depositing compressively stressed silicon nitride layer. Two-dimensional Raman spectroscopy measurements along with finite-element method simulations confirm a relatively homogeneous strain distribution within the final microdisk structure. Photoluminescence results show clear optical resonances due to whispering gallery modes which are in good agreement with finite-difference time-domain optical simulations. Our bandgap-customizable microdisks present a new route towards an efficient germanium light source for on-chip optical interconnects.

비정질 실리콘 희생층을 이용한 니켈산화막 볼로미터 제작

An infrared image sensor is a core device in a thermal imaging system. The fabrication method of a focal plane array (FPA) is a key technology for a high resolution infrared image sensor. Each pixels in the FPA have <TEX>$Si_3N_4/SiO_2$</TEX> membranes including legs to deposit bolometric materials and electrodes on Si readout circuits (ROIC). Instead of polyimide used to form a sacrificial layer, the feasibility of an amorphous silicon (<TEX>${\alpha}-Si$</TEX>) was verified experimentally in a <TEX>$8{\times}8$</TEX> micro-bolometer array with a <TEX>$50{\mu}m$</TEX> pitch. The elimination of the polyimide sacrificial layer hardened by a following plasma assisted deposition process is sometimes far from perfect, and thus requires longer plasma ashing times leading to the deformation of the membrane and leg. Since the amorphous Si could be removed in <TEX>$XeF_2$</TEX> gas at room temperature, however, the fabricated micro-bolomertic structure was not damaged seriously. A radio frequency (RF) sputtered nickel oxide film was grown on a <TEX>$Si_3N_4/SiO_2$</TEX> membrane fabricated using a low stress silicon nitride (LSSiN) technology with a LPCVD system. The deformation of the membrane was effectively reduced by a combining the <TEX>${\alpha}-Si$</TEX> and LSSiN process for a nickel oxide micro-bolometer.

Expanding the Ge emission wavelength to 2.25 μm with SixNy strain engineering

Photoluminescence up to 2.25μm wavelength is demonstrated from Ge nanopillars strained by silicon nitride stressor layers. Tensile biaxial equivalent strains of up to ~1.35% and ~0.9% are shown from 200×200nm, and 300×300nm square top Ge pillars respectively. Strain in the latter is confirmed by Raman spectroscopy, and supported by finite element modelling, which gives an insight into the strain distribution and its effect on the band structure, in pillar structures fully coated by silicon nitride stressor layers.

Extending the emission wavelength of Ge nanopillars to 2.25 μm using silicon nitride stressors.

The room temperature photoluminescence from Ge nanopillars has been extended from 1.6 μm to above 2.25 μm wavelength through the application of tensile stress from silicon nitride stressors deposited by inductively-coupled-plasma plasma-enhanced chemical-vapour-deposition. Photoluminescence measurements demonstrate biaxial equivalent tensile strains of up to ∼ 1.35% in square topped nanopillars with side lengths of 200 nm. Biaxial equivalent strains of 0.9% are observed in 300 nm square top pillars, confirmed by confocal Raman spectroscopy. Finite element modelling demonstrates that an all-around stressor layer is preferable to a top only stressor, as it increases the hydrostatic component of the strain, leading to an increased shift in the band-edge and improved uniformity over top-surface only stressors layers.

Tensile-strained germanium microdisk electroluminescence.

We report room temperature electroluminescence of tensile-strained germanium microdisks. The strain is transferred into the microdisks using silicon nitride stressors. Carrier injection is achieved with Schottky contacts on n-type doped germanium. We show that a biaxial tensile-strain up to 0.72% can be transferred by optimizing the carrier injection profile. The transferred strain is measured by the electroluminescence spectral red-shift and compared to finite element modeling. We discuss the impact of this strain level to achieve population inversion in germanium.

PECVD low stress silicon nitride analysis and optimization for the fabrication of CMUT devices

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed.The first option consists in a double layer of silicon nitride with a relatively high deposition rate of ~100 nm min−1 and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm−1, yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Second‐Order Optical Nonlinearity in Silicon Waveguides: Inhomogeneous Stress and Interfaces

The lack of a dipolar second‐order susceptibility (χ(2)) in silicon due to the centrosymmetry of its diamond lattice usually inhibits efficient second‐order nonlinear optical processes in the silicon bulk. Recently, the deposition of stressed silicon nitride layers and the corresponding inhomogeneous strain in silicon lead to the demonstration of second harmonic generation and electro‐optic modulation in strained silicon waveguides. However, the respective impact of the stress/strain gradient and the involved interfaces is not clear. Here, the influence of the stress and the stressing silicon nitride layer using second harmonic generation measurements in transmission is investigated. The results show that the enhancement of the second‐order nonlinearity arises from a constructive superposition of stress‐induced and interface‐related effects. Particularly, the stress gradient in silicon breaks the symmetry of the crystal lattice, while positive fixed charges at the silicon/silicon nitride interface are responsible for a pronounced electric‐field‐induced‐second harmonic (EFISH) contribution. These results demonstrate the impact of external factors for the creation of an effective χ(2) in materials and open new perspectives for the use of second‐order nonlinear optical processes in silicon photonics.

Highly-Doped, Highly-Strained Germanium and Schottky Electroluminescent Diodes

We demonstrate that room temperature electroluminescence can be obtained from n-doped germanium layers by using a Schottky contact. The electrical and optical properties of the Schottky device are improved by inserting a thin Al2O3 interfacial barrier. We also show that high active doping of Ge grown on Si can be obtained by molecular beam epitaxy by using a co-doping method with Sb and P. We finally discuss the transfer of tensile strain on microdisk resonators using silicon nitride stressor layers.

Highly-Doped, Highly-Strained Germanium and Schottky Electroluminescent Diodes

Achieving a monolithic germanium laser source represents a major challenge for silicon photonics. Two key parameters have an impact on the laser performances including threshold, output power, reliability... The first key parameter is the germanium doping. Modeling indicates that an active electron carrier concentration of 3 - 4 x 1019 cm-3 is requested to obtain a robust gain. We will show that such doping level can be achieved for germanium grown on silicon using a co-doping method followed by thermal anneal. The thermal annealing is necessary to minimize the non-radiative recombination processes. The second key parameter is the tensile strain present in germanium. The achievement of high or even record tensile strain values in small volumes is not sufficient for lasing. The homogeneity of the strain field within the germanium volume needs to be taken into account. We will show how it is possible to maximize the tensile strain homogeneity either for straight waveguides or germanium microdisks strained by a silicon nitride stressor. The trade-off between homogeneity and maximum strain value allows one to achieve a more robust modal gain. We will finally discuss the transfer of these concepts for electrical injection.

Dual mechanical behaviour of hydrogen in stressed silicon nitride thin films

In the present article, we report a study on the mechanical behaviour displayed by hydrogen atoms and pores in silicon nitride (SiN) films. A simple three-phase model is proposed to relate the physical properties (stiffness, film stress, mass density, etc.) of hydrogenated nanoporous SiN thin films to the volume fractions of hydrogen and pores. This model is then applied to experimental data extracted from films deposited by plasma enhanced chemical vapour deposition, where hydrogen content, stress, and mass densities range widely from 11% to 30%, −2.8 to 1.5 GPa, and 2.0 to 2.8 g/cm3, respectively. Starting from the conventional plotting of film's Young's modulus against film porosity, we first propose to correct the conventional calculation of porosity volume fraction with the hydrogen content, thus taking into account both hydrogen mass and concentration. The weight of this hydrogen-correction is found to evolve linearly with hydrogen concentration in tensile films (in accordance with a simple “mass correction” of the film density calculation), but a clear discontinuity is observed toward compressive stresses. Then, the effective volume occupied by hydrogen atoms is calculated taking account of the bond type (N-H or Si-H bonds), thus allowing a precise extraction of the hydrogen volume fraction. These calculations applied to tensile films show that both volume fractions of hydrogen and porosity are similar in magnitude and randomly distributed against Young's modulus. However, the expected linear dependence of the Young's modulus is clearly observed when both volume fractions are added. Finally, we show that the stiffer behaviour of compressive films cannot be only explained on the basis of this (hydrogen + porosity) volume fraction. Indeed this stiffness difference relies on a dual mechanical behaviour displayed by hydrogen atoms against the film stress state: while they participate to the stiffness in compressive films, hydrogen atoms mainly behave like pores in tensile films where they do not participate to the film stiffness.