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Numerical results on the evolution of thermal stresses in multilevel interconnects are presented. Two levels of aluminum lines with an aspect ratio of unity, aligned vertically or arranged in a staggered manner, are considered by recourse to the finite element analysis. The stresses are found to be significantly higher in the lower-level lines than in the upper-level lines, for both the aligned and staggered arrangements. The stress magnitudes are generally smaller in lines of staggered arrangement, compared to the case of aligned lines. Implications of the present findings are discussed, with directions of future studies highlighted.
The effects of gravity on the crystal nucleation of heavy metal fluoride fibers have been studied in preliminary experiments utilizing NASA's KC-135 reduced gravity aircraft and a microgravity sounding rocket flight. Commercially produced fibers were heated to the crystallization temperature in normal and reduced gravity. The fibers processed in normal gravity showed complete crystallization while the fibers processed in reduced gravity did not show signs of crystallization.
We discuss the magnetic properties of lead doped Bi-2223 bulk samples obtained through combined magnetic melt texturing and hot pressing (MMTHP). The ac complex susceptibility measurements are achieved over a broad ac field range (1 Oe < hac < 100 Oe) and show highly anisotropic properties. The intergranular coupling is improved in the direction perpendicular to the applied stress and magnetic field direction, and an intragranular loss peak is observed for the first time. A comparison is made with other bismuth-based compounds and it is shown that the MMTHP process shifts the ac irreversibility line (ac IL) toward higher fields. It is also shown that all the ac IL's for quasi 2D bismuth-based compounds show a nearly quadratic temperature dependence and deviate therefore strongly from the linear behavior observed in quasi 3D compounds and expected from a critical state model.
Two optical methods are presented for the mechanical characterization of thin films, namely real time holographic interferometry and a fringe projection method called “contouring.” These two methods are coupled to the interferometry by the phase measurements, thus allowing the displacement field to be measured at all points on the membrane. We discuss the solutions retained in terms of their precision and sensitivity. These methods are then applied to membrane bulging tests, a type of test that is widely used in micro-mechanical studies. The measurements are performed on silicon single crystal and the results are compared to the solutions calculated by finite element methods. In both cases, the good agreement between theory and experiments allows the experimental apparatus to be validated.
The work described in this paper is part of a systematic study of ohmic contact strategies for GaN-based semiconductors. Gold contacts exhibited ohmic behavior on p-GaN when annealed at high temperature. The specific contact resistivity (ρc) calculated from TLM measurements on Au/p-GaN contacts was 53 Ω · cm2 after annealing at 800 °C. Multilayer Au/Mg/Au/p-GaN contacts exhibited linear, ohmic current-voltage (I-V) behavior in the as-deposited condition with ρc = 214 Ω · cm2. The specific contact resistivity of the multilayer contact increased significantly after rapid thermal annealing (RTA) through 725 °C. Cross-sectional microstructural characterization of the Au/p-GaN contact system via high-resolution electron microscopy (HREM) revealed that interfacial secondary phase formation occurred during high-temperature treatments, which coincided with the improvement of contact performance. In the as-deposited multilayer Au/Mg/Au/p-GaN contact, the initial 32 nm Au layer was found to be continuous. However, Mg metal was found in direct contact with the GaN in many places in the sample after annealing at 725 °C for 15 s. The resultant increase in contact resistance is believed to be due to the barrier effect increased by the presence of the low work function Mg metal.
The intermixing and crystallization of amorphous Si/Ge multilayers (with individual layer thickness between 1.5 and 20 nm) and SiGe alloys produced by dc magnetron sputtering have been studied by cross-sectional transmission electron microscopy and x-ray diffraction. Measurement of the crystallization temperature as a function of the Si content showed that multilayers and alloys with equal composition crystallized at the same temperature. This implies that intermixing precedes crystallization in the multilayers. Close to the crystallization temperature, formation of Kirkendall voids was observed in the short-period Si/Ge multilayers. These voids were found at positions corresponding to the original Si layers, indicating that Si diffuses faster in amorphous Ge than Ge in amorphous Si. The Ge layers in short-period Si/Ge multilayers retained their amorphous state to much higher temperatures than thick amorphous Ge layers. This is shown to be due to inhibition of nucleation by the presence of the layer interfaces. A lower estimate for the Si diffusion constant in crystalline Ge is also determined.
Vickers microindentations obtained with loads between 0.05 N and 2 N were performed on crystalline (100) silicon. The residual stress field and the different structural states induced by loading were studied by mapping the indented zones by their micro-Raman response. A Raman signature of amorphous silicon is found in the center of the impression. The energy of the Γ25 zone center phonon is found to vary from 522 cm−1 when probing the silicon at a distance of 80 μm from the center of the indentation up to 527 cm−1 when probing the pileup region of the impression. When probing cracked zones in the vicinity of the pileup region, wave numbers as high as 536 cm−1 are measured. The stress components induced by a point indentation (1 N) have been calculated from analytical expressions given in the literature. For an average conversion factor of 3.2 cm−1/GPa, the residual local stresses after unloading are found of the same order of magnitude or even larger than the calculated stresses that are generated during loading. A tentative explanation is proposed. Finally a systematic laser-induced thermal treatment of the central area and of the pileup region of indentations was performed. It is shown that the amorphous silicon in the center can partly recrystallize but that the residual stress state in the pileup region cannot be completely relaxed by local laser heating.
Decaprismatic single crystals taken from a series of alloys of nominal compositions within Al65–77Co3–22Ni3–22 have been studied by means of x-ray diffraction techniques. The substitution of Co by Ni in increasing amounts changes the (pseudo)decagonal diffraction patterns drastically and indicates structural changes which range from a single-crystalline approximant via orientationally ordered nanodomain structures and quasiperiodic phases with different types of ordering phenomena, to a basic decagonal phase. A quantum phase diagram analysis shows a clear separation of the stability regions of the ternary systems described in this study and other decagonal phases.
Phase transformations of the Mo33Si66 powder mixture under different milling conditions have been systematically investigated by x-ray diffraction, and scanning and transmission electron microscopy. The effect of the milling conditions on the Mo/Si solid state reactions (SSR) has been examined in detail. The energy transfer from the milling tools to the powder under processing has been quantified by an already assessed collision model. It has been found that the higher energetic input favors the formation of the room temperature stable phase αMoSi2, while the lower energetic input promotes the formation of the metastable phase βMoSi2. In addition, if the energy transfer is high enough, the Mo/Si reaction proceeds in a form of self-propagating high temperature synthesis (SHS). Thermodynamics and kinetics aspects related to the different SSR's are discussed.
The effects of thermal cycling through the martensitic transformation have been studied in three Cu–Al–Ni–Mn–B high temperature shape memory alloys. An increase of the martensitic transformation temperatures with the number of cycles (up to ∼7 K after 60 cycles) has been generally observed by DSC measurements. The microstructure of these alloys is rather complicated, with the presence of big manganese or aluminum boride particles and small boron precipitates, as well as the formation of dislocations during thermal cycling. By means of aging experiments, it has been shown that the evolution of transformation temperatures during cycling is mainly due to the step-by-step aging in parent phase accompanying the thermal cycling, and that the dislocations formed during cycling have only a very small effect, at least up to 60 cycles.
The aging of the NC 19 Fe Nb alloy (Inconel 718), previously quenched from 990 °C, is characterized by a hardness peak at 650 °C, then a maximum in hardness at about 750 °C. Over this temperature, the hardness progressively decreases. In the 550–650 °C temperature range, TEM observations have revealed that β (Ni3Nb) precipitates are formed as long platelets parallel between them within the same grain, as well as extremely fine γ′[Ni3(Ti, Al)] particles responsible for the observed improvement in hardness. For a tempering temperature higher than 650 °C, a first hardening occurs after a 4 h treatment, which has been associated with the γ′ phase precipitation, with a more or less spherical shape. Beyond this time, a second hardening takes place linked to the γ″ phase precipitation (Ni3Nb, bct D022 structure), as thin platelet shaped, perfectly coherent with the matrix. The misfit between the γ and γ″ phases is about 3% in the 〈001〉γ″ direction and lower than 1% in the 〈100〉γ″ and 〈010〉γ″ directions. During a longer aging at 750 °C, the γ″ platelets progressively dissolve while β precipitates grow.
Fine-grained kappa carbide (Fe3AlCx) materials, containing 12.5 and 14% Al, and 3.5% C, were prepared by powder processing and hipping procedures. The creep behavior of the kappa materials was shown to be identical to that observed in superplastic iron carbide, and was shown to follow a grain boundary–diffusioncontrolled grain boundary sliding relation. The tensile fracture strains in kappa, however, were shown to be considerably less than in iron carbide with a maximum elongation of 92% noted. This difference is attributed to either a low stress intensity factor or to contamination of the powder surface in the kappa material. The compression creep strength, at a given strain rate, was shown to be about two times higher than the tension creep strength.
It is shown that Cu–Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400 °C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 15 to 40 at. %. The contacts exhibit a specific contact resistivity of 7 × 10−7 Ω cm2 on n-type GaAs with doping concentrations of 1 × 1017 cm−3. The contact resistivity is unaffected by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. Cross-sectional high-resolution transmission electron microscopy results show that the addition of Ge to Cu in this concentration range causes Cu to react only with Ge forming the ξ and ε1–Cu3Ge phases which correlate with the low contact resistivity. The ξ and ε1–Cu3Ge phases have a planar and structurally abrupt interface with the GaAs substrate without any interfacial transition layer. It is suggested that Ge is incorporated into the GaAs as an n-type impurity creating a highly doped n+-GaAs surface layer which is responsible for the ohmic behavior. n-channel GaAs metal-semiconductor field-effect transistors using ohmic contacts formed with the ξ and ε1–Cu3Ge phases demonstrate a higher transconductance compared to devices with AuGeNi contacts.
A liquid phase serves to relax stress concentrations caused by sliding at interfaces and grain boundaries in high-strain-rate superplasticity for aluminum matrix composites. However, the presence of a liquid phase does not always lead to high-strain-rate superplasticity because too much liquid causes decohesion at a liquid phase. The critical conditions of the optimum distribution, thickness, and volume in a liquid phase are discussed based on the observation results by differential scanning calorimetry and transmission electron microscopy. As a result, a very thin and discontinuous liquid phase is required both to assist relaxation of the stress concentrations and to limit decohesion at a liquid phase.
Laminated composites containing alternate layers of Si3N4 and TiN/Si3N4 materials were used as model material for investigating the crack behaviors and mechanical properties. Results indicated that both strength and toughness in laminated composites were higher than that of monolithic silicon nitride. The failure profiles were affected by the stored strain energy prior to failure and the stress gradient in each layer. Cracks deviated successively from one layer to the other due to periodic stress distribution. Samples with better strength and toughness also had a longer crack propagation path and higher amplitude of crack deviation. The periodic stress distribution in laminated composites was confirmed by the measurements of indentation crack length. Results also suggested a tensile stress in the Si3N4 layer and compressive stress in the TiN/Si3N4 layer, in directions normal to the free sample interface.
Ceramic-polymer composites with a 1–3 connectivity can be created via a novel process called dielectrophoretic assembly. The process involves an electric field which is applied to a suspension of ceramic particles in an uncured thermoset polymer matrix. Under appropriate conditions, the applied electric field acts to induce a spatial redistribution of the particles into a chained or fibril structure. It was shown previously that the electrorheological response and fibril microstructure are dependent on both the frequency and magnitude of the applied alternating electric field.3 This paper will show that the frequency dependence of the uncured thermoset polymer suspensions results from the complex electrical phenomena specific to each thermoset system. Specifically, it will be shown through low field dielectric measurements and high field current-voltage analysis that the dielectrophoretic effect can be limited by electrode polarization, ionic conductivity, and space charge relaxation. It is the frequency dependence of these limiting phenomena that gives rise to the observed frequency dependence in the dielectrophoretic force of attraction being utilized to drive particulate assembly.
The present study involved the fabrication of three-layered composites consisting of outer layers that contained Si3N4 and an inner layer that contained TiN in a Si3N4 matrix. Surface compressive stresses were developed upon cooling due to the relatively higher thermal expansion coefficient (CTE) in the inner layer. The flexural strength of layered Si3N4 composites was substantially greater than that of monolithic Si3N4. This was attributed to the surface compressive stress. The effects of TiN composition and inner layer thickness on the mechanical properties were investigated. Layered samples containing 20 vol.% TiN had lower flexural strength than Si3N4–10% TiN/Si3N4–Si3N4 due to the formation of microcracks in the inner layer. Crack behaviors in layered samples were affected by the residual stress, interface, and free sample surface. Both theoretical and experimental results indicated that the strength and toughness of layered composites were substantially greater than those of monolithic materials. The determination of fracture toughness in three-layered materials by the surface indentation technique should be done carefully due to the influence of residual stress.
A model of lead phosphate, which describes the rhombohedral-monoclinic phase transition, is used to form domain patterns in the annealing process. The obtained domain structures show W and W′ types of domain walls in agreement with the stress-free laws proposed in Sapriel's theory. The observed W domain walls are parallel to the ternary symmetry axis, while the W′ ones are tilted with respect to the same axis. The antiphase domain walls take no preferential orientations, and remain parallel to the ternary axis. The calculated density of the potential energy of the domain wall of type W is estimated to be Edw = 49 K/Å2 at T = 300 K.
Nanoparticles of yttria-doped tetragonal zirconia polycrystalline ceramics (Y-TZP) with an average crystallite size of less than 9 nm were prepared by a combustion synthesis process. Dense and fine-grained (<200 nm) Y-TZP ceramics were obtained by fast-firing using temperatures lower than 1400 °C and dwell times of less than 2 min. Impedance spectroscopy was employed to measure conductivities of oxygen vacancies in the grain and the grain boundary of the fine-grained Y-TZP. The relationships between the concentration of the oxygen vacancies in the grain boundary and measurable physical parameters were determined semiquantitatively. The oxygen vacancy concentrations and activation energies for the oxygen-ion conduction in the grain and the grain boundary of the fine-grained Y-TZP were found to be independent of the average grain size in the average grain-size range of 90–200 nm. These experimental results suggest that, in order to retain the abnormally high oxygen vacancy concentrations of the Y-TZP nanoparticles and thus enhance the oxygen-ion conductivity, it may be necessary to decrease the average grain size to approximately 10 nm.
A new methodology for mapping thermal diffusivity using a photothermal deflection method is introduced. Two case studies are made: fiber-reinforced composite structures and contact damage zones in alumina. In the former, characterization of thermal microstructural features is demonstrated; in the latter, microcrack density is quantified. Experimental data are analyzed and compared with literature results. Advantages and limitations of the technique are discussed.