External Compressive Stress Research Articles

Strain Engineering to Improve Data Retention Time in Nonvolatile Memory

Experimental data show that tensile stress improves and compressive stress degrades retention time for nonvolatile memory (NVM) devices. External mechanical tensile stress and compressive stress are introduced into the NVM floating-gate and nitride trap based memories via four-point wafer bending. The enhanced retention time under tensile stress results from stress-altered changes in the SiO2/Si barrier height and out-of-plane conductivity mass for floating-gate memories and from changes in the trap activation energy in nitride based memories

Low-temperature copper-induced lateral growth of polycrystalline germanium assisted by external compressive stress

Copper-induced lateral growth of polycrystalline germanium (poly-Ge) at temperatures as low as 150°C was enabled by the application of an external mechanical stress during the annealing step of sample processing. An equivalent compressive strain of 0.05% was externally applied at 150°C for 10h to a deposited amorphous Ge layer and crystalline growth rates of 2.5 and 1.8μm∕h were observed in directions parallel and perpendicular to the stress axis, respectively. These results were confirmed by scanning electron microscope and transmission electron microscopy (TEM) analyses. In addition, TEM and x-ray diffraction analyses indicate that a fraction of poly-Ge annealed in the presence of applied compressive stress possessed a tetragonal structure with space-group P43212. The presence of the tetragonal phase is hypothesized to be the primary mechanism responsible for the lateral growth of poly-Ge.

Magnetic field-induced reversible variant rearrangement in Fe–Pd single crystals

Variant rearrangements induced by external magnetic field in Fe–Pd FCT martensite transformed from FCC austenite single crystals have been investigated by means of optical microscopy. Fe–Pd single crystals in fully martensite state contain two large correspondence variants, and a kink due to {1 0 1} twinning is observed at the twinning boundary of the variants. The twinning boundary exhibits both large reversible and small irreversible movement by applying magnetic field as a result of variant rearrangement, and 0.49% of reversible strain is obtained. The variant rearrangement is not completed even though magnetic flux density is increased to maximum of an electromagnet used. The boundary also moves reversibly by application of external compression stress, however, the direction of the boundary movement is opposite with respect to the movement induced by magnetic field and the variant rearrangment is not completed. The reversible strain of 2.8% was also observed when the specimen was compressed.

Thin-film tunneling transistors on flexible plastic substrates based on stress-assisted lateral growth of polycrystalline germanium

Stress-assisted Cu-induced lateral growth of polycrystalline germanium (poly-Ge) at temperatures as low as 150 °C has been exploited to fabricate thin-film tunneling transistors on flexible plastic substrates. Applying external compressive stress during annealing, leads to the lateral growth of poly-Ge from Cu-seeded drain/source regions, progressing into the channel area. A potential barrier is formed midway in the channel where the two lateral growth frontiers, emanating from source and drain seeded areas, meet each other. As confirmed by electrical measurements, the barrier is controlled by the gate bias. An ON/OFF ratio of 104 has been measured for these transistors, which shows the potential of these devices for switching applications.

High mobility poly-Ge thin-film transistors fabricated on flexible plastic substrates at temperatures below 130°C

Depletion-mode poly-Ge thin-film transistors (TFTs) with an effective hole mobility of 110 cm2/Vs and an ON/OFF ratio of 104 have been fabricated on flexible polyethylene therephtalate (PET) substrates, taking advantage of a novel stress-assisted crystallization technique. Proper manipulation of an otherwise destructive mechanical stress leads to a drastic drop of crystallization temperature from 400°C to 130°C. External compressive stress is transferred to the Ge/PET interface by bending the flexible substrate inward, during the thermal post-treatment. Proper patterning of the a-Ge layer before thermomechanical post-treatment leads to a minimal crack density in the processed poly-Ge layer. Reduction in the crack density plays a crucial role in alleviating the stress-induced gate leakage current emanated from the crack traces propagating from the channel into the gate oxide.

電子部品における鉛フリーはんだのウィスカー発生・成長の防止に関する研究(オーガナイズドセッション,材料と組織)

A purpose of this research is to prevent the whisker being generated. To clarify the generation factor of the whisker, the generation examination of whisker was done with a designed examination treatment device. The result of obtaining is as follows; (l)The whisker was able to be generated by the examination treatment device proposed by this research. (2)It was observed that many whiskers had been generated from the location where plating had transformed plasticity by tightening the bolt, and it had not been generated from other locations by using SEM. (3) An external compression stress influences the growth of the whisker very greatly. One of the important point of the whisker prevention is not to cause the compression stress in the solder plating.

The origin of EL2 family evidenced by STM direct observations of individual photoquenching behaviors

Abundant point defects in low-temperature grown (LT-) GaAs, presumably As antisite defects, exhibit a photo-induced transformation at low temperatures with an excitation spectrum very close to that obtained for the macroscopic photoquenching effect of EL2 centers. Some of the defects do not exhibit the photo-induced transformation, disfavoring the hypothesis of variants of EL2-like centers differing in the atomic structures. The unquenchable EL2 centers are commonly located near the interface between the LT-GaAs epi-layer and the n-GaAs substrate. Separate macroscopic photoluminescence experiments showed that the photoquenching efficiency is strongly decreased by external compressive stress, which suggests that the absence of the phototransformation behavior in some centers was due to a local stress field induced by the lattice mismatch between the epi-layer and the substrate. This is shown by STM-electric field modulation spectroscopy that the epi-layer was locally strained to a different degree depending on the position from the interface. Therefore, we conclude that the EL2 variants are the result of the spatial variation of the internal stress environment that is felt by the EL2 centers in an identical atomic structure.

Martensitic transformation and magnetic-field-induced strain in Ni-Mn-Ga magnetic shape memory alloys

The present paper deals with the thermal- and stress-induced martensitic transformations, effects of bias magnetic field and external compressive stress on transformation- and magnetic-field-induced strain in Ni-Mn-Ga alloys. The effect of the external magnetic field and compressive stress on the reorientation of martensitic variants is discussed. It is found that the stress-induced spear-like martensitic crystal forms at the beginning of loading, at the temperature a little higher than Ms. The interfaces between the parent phase and the stress-induced martensite migrate reversibly under the increasing/decreasing stress, showing the characteristics of thermoelastic martensitic transformation. When the Ni 52 Mn 24 Ga 24 single crystal is cooled down to 200K, an intermartensitic transformation occurs, however there is no appearance of this case for the powder sample of the same alloy with average particle size of about 50 μm. About 1.8% magnetic-field-induced strain is obtained in the [100] direction of Ni 2 MnGa single crystal when a 1.5T pulse magnetic field is applied at the Ms temperature.

Residual Stress of Multilayer Ceramic Capacitors with Ni-Electrodes (Ni-MLCCs)

The residual stress of multilayer ceramic capacitors (MLCCs) has been studied. The capacitance decreased significantly under external compressive stress applied to MLCCs in the thickness direction, on the other hand, the capacitance increased under external stress in the width direction. These capacitance changes depended on the number of dielectric layers in MLCCs. The compressive residual stress at the surface of MLCCs has been shown by X-ray diffraction (XRD) analysis. The stress increased with the number of dielectric layers in MLCCs. Moreover the tensile stress was confirmed in the thickness direction of MLCCs by XRD analysis also. Therefore the dependence of electrical characteristics dependence on the number of dielectric layers, i.e., apparent dielectric constant, temperature dependence of capacitance, and aging deterioration can be explained by the residual stress.

Stress distribution on [100] Si wafer mapped by novel I-V analysis of MOS tunneling diodes

The current-voltage (I-V) characteristics of metal-oxide-semiconductor tunneling diodes distributed over a 3-in Si wafer were analyzed to investigate the stress distribution on the wafer. Generally, the substrate injection saturation current (J/sub sat/) decreases as the gate injection leakage current (J/sub g/) increases, the latter being dominated by oxide thickness via a trap related mechanism. A universal curve to fit all analyzed data was found and it is suggested that devices with extremely high (low) J/sub sat/ at a given J/sub g/ should be located in areas of the silicon lattice with relatively high external compressive (tensile) stress because of the stress-induced bandgap variation effect. The mapped locations of the highly stressed devices on a 3-in [100] Si wafer correspond to the patterns of slips caused by thermal stress during rapid thermal processing, as described in previous reports.

Low temperature stress-induced crystallization of germanium on plastic

Crystallization of a-Ge was performed on flexible plastic substrates at temperatures as low as 130 °C. Copper has been primarily used as the seed of crystallization, while an external stress was applied onto the flexible substrate to enhance the crystallization. The substrates used for this study were mainly 0.1 mm thick Polyethylene terephthalate (PET) films, which remain flexible during post treatment at low temperatures. Various samples were subject to tensile or compressive stress for the annealing step. It has been observed that only in the case of a compressive stress, crystallization becomes possible. A final electrical sheet resistance of less than 45 kΩ/□ is achieved after an annealing period of 3 h. XRD, SEM and Hall mobility measurement have been exploited to study the crystallization and explain the drastic drop of electrical resistance in samples subjected to external compressive stress. The 〈2 2 0〉 peak of Ge is discernible in the XRD spectra, confirming the crystallization. Also the study of surface morphology using SEM corroborates the crystalline nature of the annealed Ge film and the evolution of cracks. A carrier mobility of more than 110 cm 2/V s is observed for holes using Hall measurement. The shrinkage of PET during annealing induces compressive stress both in the substrate and the over-layer, in addition to the externally applied stress. This causes the progressive evolution of cracks in the Ge layer and a weak interfacial buckling of the Ge film.

A new fibre-bundle pull-out test to determine interface properties of a 2D-woven carbon/carbon composite

A newly developed fibre-bundle pull-out test is utilised to determine the interface parameters between fibre bundles and matrix. This test was applied to a bidirectionally layered carbon-fibre-reinforced carbon composite produced on the polymer route with the aid of 8H-satin weave prepreg. The interface between fibre and matrix and the matrix properties were varied by graphitisation (a final heat treatment) at three different temperatures. The new test method uses the superposition of external compressive stresses on the inner clamping stresses, which enables the separation of two important interface parameters, the coefficient of friction μ and the clamping stress q 0. The evaluation procedure takes into account the real geometry of the pulled-out bundles.

Effect of Mechanical Stress on Characteristics of Silicon Thermal Oxides

The characteristics of oxides grown on silicon wafers under external mechanical stress were studied. It was found that the SiO2 films grown under external compressive stress had a decreased etching rate. From capacitance–voltage (C–V) measurements, it was observed that a tensile sample had a reduced interface state density, while that of a compressive sample was increased.

Degradation in metal-oxide-semiconductor structure with ultrathin gate oxide due to external compressive stress

The effect of external stress on metal-oxide-semiconductor (MOS) structure with ultrathin gate oxide (∼1.5 nm) was studied. J–V characteristics of fresh and stressed samples revealed that the tensile stress had little effect on J–V curves, whereas the compressive stress obviously increased the leakage current by about several hundred in percentages with respect to the fresh sample, in both positive and negative gate biases. This increase in leakage current was suggested to be attributed to the increase of interface states and silicon bulk traps under external compressive stress in the MOS device with an inherent tensile stressed silicon. In addition, we also found that once the device was damaged by the previously applied compressive stress, the second applied compressive stress of the same magnitude would not create more damage unless the device was breakdown.

The effect of stress on the electrical properties of PZT thin films

We have investigated the effect of externally controlled bi-axial stress on the electrical properties of Pb(Zr, Ti)O3 (PZT) thin films. The PZT thin films exhibited a low residual tensile stress, i.e. 120 MPa. It is observed that the intrinsic residual stress of PZT films was independent of the stress state of the Pt bottom electrode, although the residual stress of the Pt bottom electrode is sensitively influenced by processing condition such as sputtering temperature. Under external compressive stress, the switching polarization or remanent polarization increased with the compressive stress. Coercive field also increased with the compressive stress. Upon applying a bi-axial stress of 300 MPa, the switching polarization increased to 40 μC/cm2 while the PZT films with the residual tensile stress had 32 μC/cm2. On the other hand, the polarization and coercive field was less dependent of the degree of the stress under the tensile stress. PZT thin films under the tensile stress exhibited slightly decreasing switching polarization and the coercive field with increasing the tensile stress.

The electrical properties and retention characteristics of strained PZT thin film capacitors

We have investigated the effect of externally controlled bi-axial stress on the electrical properties and retention characteristics of Pb(Zr0.52,Ti0.48)O3 (PZT) thin films. The PZT thin films exhibited a low residual tensile stress, i.e. 120 MPa. Under external compressive stress, the switching polarization or remanent polarization increased with the compressive stress. Coercive field also increased with the compressive stress. Upon applying a biaxial stress of 240 MPa, the switching polarization increased to 40 μC/cm2 while the PZT films with the residual tensile stress had 31 μC/cm2. On the other hand, PZT thin films under the tensile stress exhibited slightly decreasing switching polarization and the coercive field with increasing the tensile stress. Retention test revealed that both the external bi-axial stressed and unstressed PZT thin films showed the same polarization decay behavior (15% loss) within 1 second, whereas the mechanically stressed PZT thin films showed more suppressed polarization decay than unstressed one for a long period of retention time.

Interactive study of straight-sided buckling patterns in thin films under compressive stress

In situ atomic force microscopy observations have been carried out of thin films under external compressive stress. Straight-sided buckling patterns arise perpendicular to the compression axis which tend to attract one another during propagation a few hundred nanometers apart. The mechanisms whereby these debonding patterns interact have been investigated taking into account the elastic energy of both the film and the substrate. The equilibrium distance between two straight-sided wrinkles has been determined; good agreement has been obtained between the experimental results and the mechanics involved.

Giant field-induced reversible strain in magnetic shape memory NiMnGa alloy

A room temperature extensional strain of 5.1% was observed in martensitic Ni/sub 48/Mn/sub 31/Ga/sub 21/ alloy in the magnetic field of 480 kA/m. The magnitude of field-induced strain decreases with increasing external compressive stress applied in the direction of expansion. The compressive stress of about 3 MPa prevents the development of the substantial field-induced strain. Magnetization curves obtained by VSM exhibit an abrupt magnetization change and a transient hysteresis in the first quadrant. Large reversible field-induced strain and the abrupt magnetization change are due to the rearrangement or redistribution of martensitic twin variants by the applied magnetic field. It was confirmed by optical observation of movement and nucleation of martensitic twin boundaries.

Determination of x-ray elastic constants using an in situ pressing device

The experimental determination of x-ray elastic constants are performed by in situ measurements of the dependence of the strain state in selected crystallites for different applied external compressive stresses. The use of compressive applied stresses instead of tensile applied stresses is of interest for x-ray elastic constant determinations for materials which exhibit brittle crack-like behavior, which cannot be loaded to high tensile stresses in, for example, four-point bending devices. The x-ray elastic constants for {146} α–Al2O3 are determined with the pressing device and compared to calculated as well as experimentally determined values which were tested in tensile loading devices.

Buckling Analysis of Prestressed Rotating Circular Cylindrical Shells.

Buckling phenomenon of prestressed rotating circular cylindrical shells is investigated. As prestresses, the initial external pressure, axial compression stress and shear stress are considered. Buckling load is obtained through the frequency analysis of two traveling waves for a circumferential wave number, i.e., it is defined as the least value of preloads that make two frequencies of both traveling waves for all circumferential wave numbers be coincide. Frequency equation is derived on the basis of the first-order shear deformation theory of shells and numerical calculation method is formulated for four kinds of boundary conditions. Numerical results showed that for the cases of the initial external pressure and axial compression stress, the buckling load increases monotonously as the rotational speed increases, but for the case of the initial shear stress, buckling load shows the up-and-down inclination with rotational speed and it proved that there exists a rotational speed region in which buckling load becomes lower than that for non-rotational shells.