Silicon Strain Research Articles

Strain effects and microstructural evolution in Ge–Si system materials prepared by ion implantation and by ultrahigh vacuum chemical vapor deposition

Appropriately utilizing some microstructures may be very helpful to acquire desirable Ge–Si system materials. In this work, the Ge–Si system materials have been prepared either by ion implantation or ultrahigh vacuum chemical vapor deposition (UHVCVD). The interesting microstructures including half-loop dislocations, SiGe nanoclusters, and dislocation dipoles have been found in these two kinds of Ge–Si system materials. It is demonstrated that the evident surface strain state and adequate surface layer quality have been realized by employing these microstructures. Compared with the dipole dislocations in the Ge–Si systems deposited by UHVCVD on the compliant silicon on insulator, the half-loop dislocations and the SiGe nanoclusters induced by Ge ion implantation and subsequent annealing can relax the SiGe layer more effectively and lead to relatively large strain in the surface silicon. It may provide some new approaches to the control of misfit strains for fabricating desirable Ge–Si system materials.

Atomistic Simulation of Boron Diffusion with Charged Defects and Diffusivity in Strained Si/SiGe

We discuss the boron diffusion in a biaxial tensile strained {001} Si and SiGe layer with kinetic Monte Carlo (KMC) method. We created a strain in silicon by adding a germanium mole fraction in silicon in order to perform a theoretical analysis. The generation of a strain in silicon influences in the diffusivity as well as the penetration profile during the implantation. The strain energy for the charged defects has been calculated from the ab-initio calculation while the diffusivity of boron was extracted from the Arrhenius formula. Hereby, the influence of the germanium content on the dopant diffusivity was estimated. Our KMC study revealed that the diffusion of the B atoms was retarded with increasing Germanium mole fraction in a strained silicon layer. Furthermore, we derived a functional dependence of the in-plane strain as well as the out-of-plane strain on the germanium mole fraction, which lies in the distribution of equivalent stresses along the Si/SiGe interface.

Atomistic Simulation of Boron Diffusion with Charged Defects and Diffusivity in Strained Si/SiGe

We discuss the boron diffusion in a biaxial tensile strained {001} Si and SiGe layer with kinetic Monte Carlo (KMC) method. We created a strain in silicon by adding a germanium mole fraction in silicon in order to perform a theoretical analysis. The generation of a strain in silicon influences in the diffusivity as well as the penetration profile during the implantation. The strain energy for the charged defects has been calculated from the ab-initio calculation while the diffusivity of boron was extracted from the Arrhenius formula. Hereby, the influence of the germanium content on the dopant diffusivity was estimated. Our KMC study revealed that the diffusion of the B atoms was retarded with increasing Germanium mole fraction in a strained silicon layer. Furthermore, we derived a functional dependence of the in-plane strain as well as the out-of-plane strain on the germanium mole fraction, which lies in the distribution of equivalent stresses along the Si/SiGe interface.

Increase of Carbon Substitutionality and Silicon Strain by Molecular Ion Implantation

We have shown that molecular carbon ion implantation instead of the atomic species enhances the substitutional fraction of carbon upon solid phase epitaxial regrowth. Ultra-high temperature annealing by a laser was used for the first time, to produce the regrowth.

Increase of Carbon Substitutionality and Silicon Strain by Molecular Ion Implantation

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Strain profile of (001) silicon implanted with nitrogen by plasma immersion

In this work, we investigate the strain and defect state of silicon implanted with nitrogen by plasma immersion ion implantation, with doses between 4.5×1016 and 8.7×1016 cm−2. For this purpose, we have used Auger electron spectroscopy, x-ray reflectivity, and high-resolution x-ray diffraction. Auger spectra showed that nitrogen concentration profiles broaden and shift deeper into the substrate as the dose increases. High oxygen concentration in the first 20 nm suggested the presence of an amorphous oxide layer at the sample surface, which was confirmed by x-ray reflectivity measurements. Reciprocal space maps revealed a tensile strain perpendicular to the surface, while no in-plane strain was detected. Since no significant diffuse scattering was found, randomly distributed point defects must be predominant in the strained region compared to large displacement field defects such as clusters and dislocations. ω∕2θ scans around (004) Bragg reflection were fitted using dynamical theory of x-ray diffraction. The strain profiles obtained from the best fits correlated well with nitrogen concentration depth profiles, signaling interstitial nitrogen as the main source of strain.

Kinetic Monte Carlo Modeling of Boron Diffusion in Strained Silicon

We discuss the boron diffusion in a biaxial tensile-strained {001} Si and SiGe layer using the kinetic Monte Carlo (KMC) method. We created strain in silicon by adding a germanium mole fraction to the silicon in order to perform a theoretical analysis. The generation of strain in silicon influences the diffusivity as well as the penetration profile during the implantation. The strain energy of the charged defects was calculated from ab initio calculation whereas the diffusivity of boron was extracted from the Arrhenius formula. Hereby, the influence of the germanium content on the dopant diffusivity was estimated. Our KMC study revealed that the diffusion of the B atoms was retarded with increasing germanium mole fraction in the strained silicon layer. Furthermore, we derived the functional dependence of the in-plane strain as well as the out-of-plane strain on the germanium mole fraction, which is based on the distribution of equivalent stress along the Si/SiGe interface.

Optimized FIB silicon samples suitable for lattice parameters measurements by convergent beam electron diffraction

The aim of this paper is to check the effect of artefacts introduced by focused ion beam (FIB) milling on the strain measurement by convergent beam electron diffraction (CBED). We show that on optimized silicon FIB samples, the strain measurement can be performed with a sensitivity of about 2.5 × 10 −4 which is very close to the theoretical one and we conclude that FIB preparation can be suitable for such measurements in microelectronic devices. To achieve this, we first used CBED and electron energy loss spectroscopy (EELS) which provide a procedure permitting an exact knowledge of the sample geometry, i.e. the thickness of both amorphous and crystalline layers. This procedure was used in order to measure the FIB-amorphized sidewall layer. It was found that if the FIB preparation is optimized one can reduce this amorphous layer down to around 7 nm on each side. Secondly different preparation techniques (cleavage, Tripod™ and FIB) permit to check if the surface damaged layer introduced by FIB influences the strain state of the sample. Finally, it was found that the damaged layer does not introduce measurable strain in pure silicon but reduces appreciably the quality of the CBED patterns.

Mechanical Stress Sensors for Copper Damascene Interconnects

AbstractWe propose embedded microsensors to investigate the mechanical stress in copper damascene lines in a standard CMOS microelectronic technology. Those sensors are based on silicon piezoresistive effect where strain in the active silicon is induced by orientated copper lines. The challenge is to correlate the electrical sensors signal directly to stress variation in lines.We have performed electrical measurements of the structures as a function of temperature. A coupled analytical and Finite Element thermomechanical Model of the structure was developed and a good agreement with measurements was obtained.

Micromachined pressure sensors for electromechanical characterization of carbon nanotubes

AbstractWe discuss the use of micromachined membrane‐based pressure sensors to investigate electromechanical properties of single‐walled carbon nanotubes. Among the basic transducer concept and the sensor design we mainly focus on the electromechanical transport through nanotubes. Advanced white light interferometer (WLI) based measurement techniques are used to extract the strain on nanotubes adhering to 100‐nm‐thick alumina (Al2O3) membranes. Finally, we present electromechanical measurements on strained single‐walled carbon nanotubes, and the physical nature of the piezoresistive gauge factor in carbon nanotubes and their contrast to classical silicon strain gauges is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanical Stress and Defect Formation in Device Processing

In this work we address the problem of estimating the mechanical stress and the related risk of crystal defect generation in a complex device process. The validity of numerical calculations of the mechanical stress developed in the device process flow is assessed by comparing the simulation results with the electrical measurements of test structures designed to monitor the dislocations formation and of other silicon strain sensitive structures. The results show that based upon numerical calculations it is possible to define mechanical stress criteria for preventing defect generation. By using this sort of criteria, potentially dangerous process variations can be easily identified. This method is quite general and can be applied to any device process flow. On the other hand, by comparing the electrical results of structures prone to defect generation and of stress-sensitive transistors with a non-critical geometry for defect generation, it is shown that after defect nucleation the elastic stress in non-critical structures and the defect- related failure fraction in critical structures may evolve into opposite directions. Therefore, suitable stress criteria are needed at all the levels in the process flow where crystal damage may cause defect nucleation. In this study it is also pointed out that a high temperature stress release annealing can be beneficial for stress reduction and defect prevention, but its position in the process flow is critical.

Maximizing uniaxial tensile strain in large-area silicon-on-insulator islands on compliant substrates

Recently we have demonstrated a process for generating uniaxial tensile strain in silicon. In this work, we generate uniaxially strained silicon and anisotropically strained silicon germanium on insulator with strain in both ⟨100⟩ and ⟨110⟩ in-plane directions. The strain is uniform over fairly large areas, and relaxed silicon-germanium alloy buffers are not used. The magnitude of uniaxial strain generated by the process is very dependent on the in-plane crystal direction, and can be modeled accurately using the known mechanical properties of silicon and germanium. A maximum uniaxial silicon strain of 1.0% in the ⟨100⟩ direction is achieved. Numerical simulations of the dynamic strain generation process are used to identify process windows for achieving maximum uniaxial silicon strain for different structural geometries.

Defect Engineering Considerations for Strained Silicon Substrates

Defect-engineering parameters influence the electrical performance of p-n junctions in strained silicon (SSi) substrates. The thin SSi layers were deposited on thin Strain Relaxed SiGe Buffers (SRBs), whereby strain relaxation is achieved by implementing a thin C-doped SiGe layer. Processing variables studied are the depth dC of this layer and the carbon concentration, determining the threading dislocations (TDs) density. As references, step-graded SRB material and standard Czochralski silicon was used. Combined current-voltage and capacitance-voltage measurements indicate that within the range studied (2-3x105 to 1-2x107 cm-2), the TDD has the strongest impact on the reverse current density and generation lifetime. Additional leakage current is created by defects associated with the carbon-rich layer, when present in the junction depletion region. The electrical activity of the TDs also depends on the well doping type (n- or p). The largest activity is observed in n+/p junctions, which are technologically the more relevant ones.

Mechanical properties of a micron-sized SCS film in a high-temperature environment

The effects of varying temperature on the mechanical properties of a micron-sized single-crystal silicon (SCS) film were investigated using an ‘on-chip’ tensile test. Young's modulus slightly decreased as the test temperature increased from room temperature (RT) to 500 °C. Furthermore, the fracture stress at 500 °C decreased to 1.84 GPa, which is about 32% of that at RT. The stress–strain relationship of the SCS film at 400 °C and 500 °C showed elastic–plastic deformation, and SEM images showed crossed slip lines on the side surface of the fractured specimen. From the measured stress–strain relationship, plastic strain in micron-sized silicon seems to have occurred at temperatures lower than the brittle-to-ductile transition temperature in bulk silicon.

Flexible substrate micro-crystalline silicon and gated amorphous silicon strain sensors

We present two different kinds of semiconductor strain sensors: ungated n+ micro-crystalline silicon (n+ /spl mu/C-Si), and gated hydrogenated amorphous silicon (a-Si:H). Both sensor types are fabricated on flexible polyimide substrates. The sensors were characterized with bending perpendicular, parallel, and at 45/spl deg/ with respect to the sensor bias direction, and for several bending diameters. Sensor size and power consumption are significantly reduced compared to metallic foil strain sensors. Small sensor size and ease of integration with a-Si:H thin-film transistors also allows arrays of strain sensors or combinations of strain sensors with varying geometric orientation to allow strain direction as well as magnitude to be unambiguously determined.

Electrical and Optical Characterization of Thin Semiconductor Layers for Advanced ULSI Devices

An overview is given of analytical techniques for the characterization of the electrical and transport parameters in thin (<1 µm) semiconductor layers. Some of these methods have been applied to the lifetime and diffusion length study in thin strain-relaxed buffer (SRB) layers of strained silicon (SSi) substrates, while a second group was dedicated to Silicon-on-Insulator (SOI) materials and devices. The employed techniques can be divided into two groups, whether a device structure (junction, MOS capacitor, MOSFET) is required or not. However, the MicroWave Absorption (MWA) technique can be used in both cases, making it a versatile tool to study both grown-in and processing-induced electrically active defects. The transport properties of SSi wafers are strongly determined by the density of threading and misfit dislocations, although the dependence of the recombination lifetime is weaker than expected from simple Shockley-Read-Hall (SRH) theory. This is related to the high injection regime typically employed, enabling the characterization of the 250-350 nm thick Si1-xGex layer only. At longer carrier decay times, multiple trapping events dominate that can be described by a stretched exponent approach, typical of disordered materials. For SOI substrates, transistor-based techniques will be demonstrated that enable to assess the generation or recombination lifetime in the thin silicon film (<100 nm). The lifetime can be severely degraded by irradiation or hot-carrier degradation. Finally, it will be shown that Generation-Recombination (GR) noise spectroscopy as a function of temperature allows identifying residual ion-implantation-damage related deep levels, which are otherwise hard to detect even by Deep Level Transient Spectroscopy (DLTS).

Strain and defect engineering in Si/Si 3N 4/Si by high temperature–pressure treatment

Strain and defects in silicon with buried nitride layer, prepared by annealing of nitrogen implanted Czochralski grown Si (Cz Si:N) under enhanced hydrostatic pressure (HP), were investigated. To produce Cz Si:N, silicon was implanted with N 2 + (atomic doses 1 × 10 17 cm −2 and 1 × 10 18 cm −2, energy 140 keV). The samples were next treated in Ar atmosphere for 1–10 h at up to 1400 K under HP ≤ 1.4 GPa and investigated by mass spectrometry, photoluminescence, X-ray, electrical and microhardness methods. Concentration profiles of nitrogen and of accumulated oxygen in Cz Si:N treated at ≤920 K were not affected markedly by HP . The strained nitride layers were formed in effect of the treatment at ≥1070 K. The treatments at ≥1270 K resulted in formation of the well-defined layered structures. Applied at 1400 K, HP assisted in a creation of the defect-free Si/Si 3N 4/Si structures.

Spatial Nonuniformity of Excitation–Contraction Coupling Causes Arrhythmogenic Ca 2+ Waves in Rat Cardiac Muscle

Ca2+ waves underlying triggered propagated contractions (TPCs) are initiated in damaged regions in cardiac muscle and cause arrhythmias. We studied Ca2+ waves underlying TPCs in rat cardiac trabeculae under experimental conditions that simulate the functional nonuniformity caused by local mechanical or ischemic local damage of myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small jet of solution with a composition that reduces excitation-contraction coupling (ECC) in myocytes within that segment. The jet solution contained either caffeine (5 mmol/L), 2,3-butanedione monoxime (BDM; 20 mmol/L), or low Ca2+ concentration ([Ca2+]; 0.2 mmol/L). Force was measured with a silicon strain gauge and sarcomere length with laser diffraction techniques in 15 trabeculae. Simultaneously, [Ca2+]i was measured locally using epifluorescence of Fura-2. The jet of solution was applied perpendicularly to a small muscle region (200 to 300 microm) at constant flow. When the jet contained caffeine, BDM, or low [Ca2+], during the stimulated twitch, muscle-twitch force decreased and the sarcomeres in the exposed segment were stretched by shortening normal regions outside the jet. Typical protocols for TPC induction (7.5 s-2.5 Hz stimulus trains at 23 degrees C; [Ca2+]o=2.0 mmol/L) reproducibly generated Ca2+ waves that arose from the border between shortening and stretched regions. Such Ca2+ waves started during force-relaxation of the last stimulated twitch of the train and propagated (0.2 to 2.8 mm/sec) into segments both inside and outside of the jet. Arrhythmias, in the form of nondriven rhythmic activity, were induced when the amplitude of the Ca2+-wave was increased by raising [Ca2+]o. Arrhythmias disappeared rapidly when uniformity of ECC throughout the muscle was restored by turning the jet off. These results show, for the first time, that nonuniform ECC can cause Ca2+ waves underlying TPCs and suggest that Ca2+ dissociated from myofilaments plays an important role in the initiation of Ca2+ waves.

On the beneficial impact of tensile-strained silicon substrates on the low-frequency noise of n-channel metal-oxide-semiconductor transistors

The low-frequency noise in n-channel metal-oxide-semiconductor field-effect transistors, fabricated on strained silicon (SSi) substrates has been investigated and compared with the results obtained on silicon reference wafers. The strained silicon was deposited on a thin strain-relaxed SiGe buffer layer. A 2-nm SiO2 layer was used as a gate dielectric. It is shown that a factor of 2–3 lower noise can be found in the SSi devices at a frequency f=10Hz, which appears to be correlated with the low-field mobility. This is interpreted in terms of the impact of the biaxial tensile strain on the gate oxide defectiveness.

Defect analysis of strained silicon on thin strain-relaxed buffer layers for high mobilitytransistors

The electrical activity of defects present in strained silicon (SSi) on thin strain-relaxedSi1−xGex buffer layers (SRBs) is evaluated using deep submicron CMOS compatiblen+/p andp+/n shallowjunctions. Strain relaxation has been achieved here by introducing a thin carbon-rich layer in an otherwiseuniform Si0.78Ge0.22 epitaxial layer, resulting in an SRB thickness of 248 or 348 nm andprocessing compatible threading dislocation densities in the range of a few106 cm−2. From a combination of electrical measurements (current– and capacitance–voltage;microwave absorption (MWA) recombination lifetime) and microscopic techniques(electron-beam-induced current; emission microscopy), it is concluded that generationcentres associated with the C layer can play an important role. Their electrical activityis shown to depend strongly on the relative position of the C-doped layer withrespect to the junction depth. The type of well dopant (implantation) also hasa strong impact on the electrical activity of the different defect types presentin the epitaxial layers. It is generally found that the 348 nm junctions show alower reverse current at practical operation temperatures and voltages, while thep+/n diodes exhibit a better performance compared with theirn+/p counterparts.