Silicon Plane Research Articles

Low-speed fracture instabilities in a brittle crystal

Multiscale models predict detailed features of surfaces left by crack propagation and rationalize the occurrence of fracture instabilities in a technologically important material, silicon. As a crack propagates along the most stable cleavage plane in silicon at relatively low speeds (800 metres per second), an instability suddenly appears. The authors find that beyond the very tip of the crack, when fracture speed is slow enough, bonds are broken one atomic layer below the fracture plane leading to a systematic downward deflection of the crack. Conversely, deflecting of fracture on another cleavage plane of silicon occur when the fracture speed is very high. Preliminary simulations reveal that similar instabilities could occur in diamond and silicon carbide. When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack1. The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen2. The way the crack propagates, however, is entirely determined by atomic-scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front1,3. The physical interplay of multiple length scales makes brittle fracture a complex ‘multi-scale’ phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of microdefects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known1, and significant advances have been made recently in understanding their origin4,5. Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack-tip reconstruction that makes low-speed crack propagation unstable on the (111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto (111) planes, as previously observed experimentally6,7.

Atomic structure of the interface between SrTiO3 thin films and Si(001) substrates

The structure of the SrTiO3/Si interface is determined by high-angle annular dark field imaging in combination with a recently developed technique based on aberration-corrected high-resolution transmission electron microscopy. At the interface, a monolayer of SrO faces the terminating plane of silicon. In this monolayer, the strontium atoms lie above the face-center of four silicon atoms in the terminating plane, and the oxygen atoms are located directly above the terminating silicon atoms. This structure, which is the dominant type of interface structure observed in this system, agrees with one of the interface structures predicted by first-principles calculations.

A planar micro-flame ionization detector with an integrated guard electrode

The flame ionization detector (FID) quantifies small concentrations of organic compounds by flame ionization of hydrocarbons and measurement of the resulting ion current. The ion current represents the number of carbon atoms in the sample gas. The miniaturization of the FID by MEMS technology (µFID) is expected to increase its use, because of reduced oxyhydrogen consumption. This loosens safety precautions and makes portable applications possible. In contrast to a former µFID design, the current planar µFID is designed to prevent environmental air from entering the system and deteriorating the measurement signal. The oxyhydrogen flame burns in the silicon plane of an almost completely encapsulating glass–silicon–glass sandwich. Only a small opening remains for removal of the exhaust gas from the system. In between the detector electrodes, a guard electrode is integrated to intercept and by-pass leak currents past the picoammeter, which then only measures the ion current. Due to the design of the guard electrode, small leak currents are still measured by the picoammeter. Yet, these leak currents can be corrected for to obtain the ion current. Measurements of the ion current as a function of the applied voltage and the sample gas flow show expected FID behaviour.

Research on Nano-Cutting Processes Based on Parallel Molecular Dynamics

To investigate the effect of tool geometry on single-crystal silicon nano-cutting, parallel molecular dynamics (MD) simulations are carried out with different tool rake angles. In this study, a parallel arithmetic based on mechanism of spatial decomposition together with MD is applied to simulate nano-cutting processes of single-crystal silicon (100) plane by using a single-crystal diamond tool. The simulation results show that tool rake angle has great effects on cutting forces and subsurface stress, and the effect of tool rake angle variation on work-piece potential energy is not evident while cutting single-crystal Silicon (100) plane. Moreover, the analysis of cutting forces and potential energy show that there is not evident dislocation in the nano-cutting.

Dimensionality Dependence of Optical Properties and Quantum Confinement Effects of Hydrogenated Silicon Nanostructures

The excited state properties of linear, planar, and spherical hydrogenated silicon nanostructures are studied systematically with use of a time-dependent Hartree-Fock (TDHF) approach with a semiempirical Hamiltonian. The calculated optical gaps decrease significantly from linear, planar, to spherical silicon structures, showing that the optical gap is dimensionality dependent and hence it can be varied by solely managing the shape of the nanostructures. Remarkably, the calculated exciton sizes of the lowest dipole-allowed excited states for both silicon chains and planes are approximately 26 A, revealing that the quantum confinement effect should be significantly enhanced when the sizes of silicon nanostructures are smaller than this value but not dependent on the dimensionality. A similar trend is also observed for hydrogenated silicon spherical clusters.

Influence of oxygen on the elastic properties of nanocrystalline diamond films studied by laser-induced surface acoustic waves

Laser-induced surface acoustic waves (SAWs) were used to study the influence of oxygen on the elastic properties of nanocrystalline diamond films. A series of samples was grown by chemical vapor deposition (CVD) on the (1 0 0) plane of p-type silicon for 3 h under different O 2/(CH 4 + H 2) flow ratios from 0% to 8%. The elastic properties of these nanocrystalline diamond films were determined from the SAW dispersion curves. The maximum frequency realized was about 310 MHz in wideband SAW experiments and about 700 MHz using a narrowband mask method with several higher harmonics. It was found that the densities of all samples were surprisingly high, approaching the ideal value of diamond, whereas the Young’s modulus increased from 700 to 950 GPa with the addition of oxygen. It is concluded that oxygen has a significant positive effect on the elastic properties of nanocrystalline diamond.

Study on anisotropic silicon etching characteristics in various surfactant-added tetramethyl ammonium hydroxide water solutions

Three ion-typed surfactants, including anionic sodium dihexyl sulfosuccinate (SDSS), cationic ammonium salt of poly(ethylene glycol) (ASPEG) and non-ionic poly(ethylene glycol) (PEG), were added to 10 wt% tetramethyl ammonium hydroxide water (TMAHW) solutions to evaluate the silicon anisotropic etching properties of the (1 0 0) silicon plane without agitation and no isopropyl alcohol (IPA) additive. The results indicate that the wetting capacity of the etchants cause the efficacies of the etchants on the roughness reduction to follow the order cationic ASPEG, non-ionic PEG, pure solution and anionic SDSS in TMAHW solutions, especially at high etching temperatures. Moreover, the chemical activities of the etchants cause the efficacies of the etchants on the etching rates to follow the order anionic SDSS, pure solution, cationic ASPEG and non-ionic PEG in TMAHW solutions at a given etching temperature. The cationic ASPEG has a reasonable etching rate of 0.7 µm min−1 and the lowest surface roughness of 4 nm in TMAHW solutions at an etching temperature of 100 °C. ASPEG and PEG in TMAHW solutions markedly affect aluminum passivation. The undercutting of the convex corners in PEG-added TMAHW solutions can be drastically reduced without using corner compensation; the undercutting ratio obtained using a PEG surfactant is about 45% lower than that obtained in pure TMAHW solution. This finding reveals that non-ionic PEG should be added to TMAHW solutions when accurate profiles are required without extremely deep etching. This study also demonstrated that non-ionic PEG is more appropriate than IPA for anisotropic silicon TMAHW etching.

512 TBハイブリットMD法によるシリコン(110)面き裂先端場の評価(OS-3 分子動力学法,研究発表講演)

In order to understand the atomic level crack propagation of silicon, we have proposed the hybridized molecular dynamics(TB-MD), being coupled with the linearly scaled tight-binding method and the ordinary MD with the empirical Tersoff potential. Double count of energy, caused by the so-called ghost forces in the patched region between the two, has been modified and the exacter total energy description has been given. In this paper, using the proposed hybridized computational method, we discuss the directionality propagations of the silicon (110) plane, which has been observed in the micro testings.

Effects of various ion-typed surfactants on silicon anisotropic etching properties in KOH and TMAH solutions

Three ion-typed surfactants, including anionic SDSS, cationic ASPEG and non-ionic PEG, which are powerful wetting agents in electroforming, were added to 30 wt.% KOH and 10 wt.% TMAH solutions to evaluate the silicon anisotropic etching properties of the (1 0 0) silicon plane without agitation and no IPA additive. The results indicate that the surfactant ion-types are not the main determinants of the silicon anisotropic etching properties in KOH and TMAH solutions. The wetting capacity of the etchants causes the efficacies of the etchants on the roughness to follow the order anionic SDSS, cationic ASPEG, non-ionic PEG and pure solution in KOH solutions, and the order cationic ASPEG, non-ionic PEG, pure solution and anionic SDSS in TMAH solutions, especially at higher etching temperatures. Moreover, the chemical activities of etchants differ so that the etching rates follow the order anionic SDSS, pure solution, non-ionic PEG and cationic ASPEG in KOH solutions, and the order anionic SDSS, pure solution, cationic ASPEG and non-ionic PEG in TMAH solutions at a given etching temperature. Anionic SDSS has the highest etching rate of 5.4 μm/min and the lowest surface roughness of 7.5 nm, which are about 1.69 times higher and 7.87 times lower, respectively, than those obtained in pure KOH solution. The cationic ASPEG has a reasonable etching rate of 0.7 μm/min and the lowest surface roughness of 4 nm in TMAH solutions for etching temperature of 100 °C. Furthermore, the surfactants used here were demonstrated to allow the utilization of usual mask materials and anionic SDSS can even increase the selectivity of silicon dissolution toward silicon dioxide in KOH solutions. A drastic reduction of the undercutting of the convex corners is obtained in TMAH solutions with non-ionic PEG surfactant. This finding reveals that the addition of non-ionic PEG to TMAH solutions is ideal when accurate profiles are required without extremely deep etching.

Effects of mechanical agitation and surfactant additive on silicon anisotropic etching in alkaline KOH solution

Agitation is a key method that significantly affects silicon wet anisotropic etching quality including the etching rate and surface roughness. This study introduced the ultrasonic agitation to improve the roughness quality of etched (1 0 0) silicon plane in 30 wt.% KOH solution. These etching characteristics have been compared with those using magnetic stirring and no agitation. In ultrasonic agitation condition, the etching rate increases linearly with agitating power, but the surface roughness worsens. Although the etching rate always exceeds 1.2 μm/min and the average roughness, R a is below 15 nm, the membrane microstructures are damaged easily by ultrasonic agitation. Moreover, the KOH solution with added anionic surfactant, dihexyl ester of sodium sulfosuccinic acid, is also used to evaluate the etching properties of (1 0 0) silicon under without agitation. Owing to increasing hydrophilic ability between the hydrogen bubble and silicon surface and the effect of silicon wettability, the etching properties are promoted drastically. Experimental results show that the average roughness, R a can be reduced to 7.5 nm in aqueous KOH with anionic surfactant, which is about eight times better than achieved by the pure KOH solution without agitation for etching temperature of 100 °C. Meanwhile, the etching rate can be enhanced to 4.9 μm/min, which is 1.44 times better than that is obtainable in a pure KOH solution with ultrasonic agitation. The etching rate of the KOH solution with added surfactant is about twice that in the KOH solution with isopropyl alcohol (IPA) for etching temperature of 80 °C. Furthermore, this study also illustrates the reflectivity of etched surface for visible wavelength and the fabrication of thin film microstructures.

Development and its Applications of Diamond Array Tool using Silicon Mold (1st Report)

In this study, we proposed a fabrication process of diamond array tool for micro-to-nano machining. The diamond array tools are fabricated by depositing polycrystalline diamond using CVD on silicon molds which are made by anisotropic wet etching of the single crystal silicon. It has a characteristic which can control a tip shape of cutting edge with different silicon planes. Moreover, the sizes and the arrangements of cutting edge are decided minutely by changing mask pattern on silicon substrate. The diamond array tools are expected to have a possibility of industrial applications as a micro-to-nano machining tool.

EXTREMELY SHARP {111}-BOUND, SINGLE-CRYSTALLINE SILICON NANO TIPS

This paper presents (111) nano tips made using (111) single crystalline silicon. Many research efforts have reported on the fabrication methods of micro and nano tips in silicon. Most of the developed fabrication methods are based on (100) or (110) silicon. However, (111) silicon will give the sharpest {111}-bounded nano tips, and in addition, (111) silicon has the best mechanical properties compared with other types of silicon. In this paper, we use (111) silicon wafers to fabricate two type of nano tip. One type is the sharpest nano tip that can be fabricated by using only {111} silicon planes. The other type is a high aspect ratio nano tip, which is composed of one {111} plane and two vertical passivation planes. In the first case, the tip height is 5 μm and the total cone angle of the tip is 54.7°, which represents the sharpest angle for a nano tip bounded only by {111} planes on silicon. In the second case, the nano tip height is 15 μm, and the aspect ratio is greater than 3:1. The cone angle of the nano tip is only 19.5°. The tip radius is less then 10 nm, which is of the sharpest {111}-bound, single-crystalline-silicon nano tips.

Periodicity of linear and nonlinear surface acoustic wave parameters in the (111) plane of cubic crystals

Previous numerical and experimental studies of surface acoustic waves (SAWs) in the (111) plane of cubic crystals have shown that linear properties, including the wave speed and the direction of power flow, exhibit sixfold symmetry. However, recent measurements by Lomonosov and Hess of finite-amplitude SAW pulses propagating in opposite directions in the (111) plane of crystalline silicon have demonstrated that the same periodicity does not hold for the nonlinear distortion. Calculations based on a model for nonlinear SAWs in anisotropic media [Hamilton et al., J. Acoust. Soc. Am. 105, 639–651 (1999)] indicate that complex-valued nonlinearity matrix elements, which describe the coupling between harmonics, have sixfold symmetry in magnitude but only threefold symmetry in phase. As a result, pulses traveling in opposite directions exhibit different nonlinear phase shifting between harmonics that give rise to the distinctly different types of observed distortion. Threefold symmetry of nonlinearity is also predicted for other cubic crystals besides silicon. Additional computations show that the complex-valued eigenvalues and eigenvectors of the linearized equations (physically corresponding to the depth decay coefficients and component amplitudes, respectively) also have sixfold symmetry in magnitude but only threefold symmetry in phase. [Discussions with A. P. Mayer are gratefully acknowledged.]

A combined scanning tunnelling microscope and x-rayinterferometer

A monolithic x-ray interferometer made from silicon and a scanning tunnellingmicroscope have been combined and used to calibrate grating structures withperiodicities of 100 nm or less. The x-ray interferometer is used as atranslation stage which moves in discrete steps of 0.192 nm, the latticespacing of the silicon (220) planes. Hence, movements are traceable to thedefinition of the metre and the nonlinearity associated with the opticalinterferometers used to measure displacement in more conventional metrologicalscanning probe microscopes (MSPMs) removed.

Elastic properties of textured diamond and silicon

We have used invariance properties of fourth-rank tensors within the Voigt–Reuss–Hill averaging procedure, to calculate the elastic stiffness and compliance of cubic polycrystalline materials exhibiting texture. The computation is extended to the bulk and shear modulus as well as to Young’s modulus, volume compressibility and Poisson’s ratios. Numerical values and polar plots are given for all texture directions in the plane (11̄0) of diamond and silicon.

Etch rates of (100), (111) and (110) single-crystal silicon in TMAH measured in situ by laser reflectance interferometry

Laser reflectance interferometry has been used to measure in situ etch rates of (100), (110) and (111) single-crystal silicon planes in 5–35 wt.% tetramethyl ammonium hydroxide (TMAH) solutions. The laser reflectance method was seen to give very accurate etch rates for samples that developed a surface roughness lower than 100 nm during the experiments. The etch rate of (100) and (111) was found to decrease with increasing etchant concentration, while the highest and lowest etch rates of (110) were observed for the 25 and 15 wt.% solutions, respectively. Activation energies were calculated as 0.54–0.59 eV for the 20–35 wt.% solutions, about 0.54 eV for the 25–35 wt.% solutions and 0.50–0.54 eV for the 25–35 wt.% solutions for the (100), (111) and (110) crystal orientation, respectively.

Experimental study about nano-deformation field near quasi-cleavage crack tip

Using the nano-moire method, we measure the near tip nanoscopic deformation on the {111} plane of single crystal silicon with a loaded quasi-cleavage crack running in the [110] direction. The measured strain distribution ahead of the crack tip agrees with the linear elastic fracture mechanics prediction up to 10 nm from the crack tip. Dislocations of Peierls type are detected and they extend from the crack tip over a length of hundreds of Burgers vectors.

The ZEUS microvertex detector

A new vertex detector for the ZEUS experiment at HERA will be installed during the 1999–2000 shutdown, for the high-luminosity runs of HERA. It will allow to reconstruct secondary vertex tracks, coming from the decay of long-lived particles with a lifetime of about 10 −12 s, and improve the global momentum resolution of the tracking system. The interaction region will be surrounded with single-sided silicon strip detectors, with capacitive charge division: three double layers in the central region (600 detectors), and 4 “wheels” in the forward region (112 silicon planes). Due to the high number of readout channels, 512 readout strips per silicon plane in the barrel region and 480 in the forward part, and the large coverage of the vertex detector (almost 1 m long), the front-end electronics has to be placed on top of the detectors and has to be radiation tolerant since doses up to 2 kGy are expected near the interaction region. The HELIX chip has been chosen as analog chip with a low-noise, charge-sensitive amplifier/shaper. The chip integrates 128 read out channels which are sampled in an analog pipeline with the HERA bunch crossing frequency of 10.4 MHz. A review of the status of the project is presented. A test program is underway in order to gain experience in the understanding of the detectors and their performance characteristics (i.e. detection efficiency, noise, large angle track position resolution, irradiation studies).

Leaky pseudo surface wave on the water–Si(110) interface

The present letter demonstrates that a leaky pseudo surface wave (LPSW) should exist on the interface of water and a cubic crystal silicon (110) plane. The phase velocity distribution of the LPSW on the above interface has been theoretically predicted by the reflection coefficient analysis and experimentally measured by using acoustic spectro-microscopy (ASM). The measured phase velocity distribution of the LPSW is in good agreement with the predicted one.

Blue photoluminescence from thermally grown SiO2 on micromachined arrays of silicon planes

SiO 2 was thermally grown on arrays of silicon planes obtained from 〈110〉 substrates by means of micromachining. Blue photoluminescence was observed under pulsed-laser excitation from SiO2 grown on these planes. Experiments revealed that this emission was not affected by the Si/SiO2 interface properties or the silicon thickness, whereas its intensity and spectral features depended on the oxide thickness. Moreover, no detectable luminescence was observed from the oxide grown on unpatterned regions, where a smaller amount of oxide was excited by the laser beam. The photoluminescence disappeared when the oxide was removed.