A MEMS-Based Evaluation of the Mechanical Properties of Metallic Thin Films

Characterizing the mechanical properties of metal thin films is critical for the design and fabrication of metal microelectromechanical systems and integrated circuit devices. This paper focuses on wafer-level determination of the mechanical behavior of sputtered aluminum and nickel thin films, using a variety of measurement techniques. Elastic moduli have been determined in devices fabricated with standard micromachining techniques using bulge testing of square diaphragms and lateral resonator structures. We find a Young's modulus of ~70 GPa for Al and ~200 GPa for Ni, in agreement with data for the bulk metals. Using pressurize/depressurize cycles, the load-deflection curves of the membranes have also been determined, and in conjunction with finite element simulations, were used to determine the yield strength and fracture strength of these films. Residual stresses in the films have also been investigated using wafer curvature, bulge testing, and X-ray diffraction. The merits of each measurement technique are discussed.     

Dynamic analysis of polygonal Mindlin plates on two-parameter foundations using classical plate theory and an advanced BEM

Forced vibrations of moderately thick plates on two-parameter, Pasternak-type foundations are considered. Influence of plate shear and rotatory inertia are taken into account according to Mindlin. Excitations are of the force as well as of the support motion type. Formulation is in the frequency domain. An analogy to thin plates without foundations is given. This analogy to classical plate theory is complete in the case of polygonal plan-forms and hinged support conditions. In that case the higher order Mindlin-problem is reduced to two (second order) Helmholtz-Klein- Gordon boundary value problems. An advanced BEM using Green's functions of rectangular domains is applied to the latter, thereby satisfying boundary conditions exactly as far as possible. This problem oriented strategy provides the frequency response functions for the deflection of the undamped Mindlin plate with high numerical accuracy. Structural damping is built in subsequently, and Fast Fourier Transform is applied for calculation of the transient response.Part of the paper has been presented at the IUTAM-Symposium Advanced BEM, San Antonio, Texas, 1987. Another part has been presented at the 6th Int. Conf. Numerical Methods for Geomechanics, Innsbruck, Austria 1988     

Microstructural Characterization of Cofired Tungsten-Metallized High-Alumina Electronic Substrates

Microstructural characterization of a high-Al₂O₃ substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al₂O₃ grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO₂ and reduced rutile (TiO_{2- x} ). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al₂O₃ grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl₂Si₂O₈), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α-cristobalite (SiO₂), and α-cordierite (Mg₂Al₄Si₅O₁₈). In addition, small rutile particles precipitated within the Al₂O₃ grains.     

The Calcia–Zirconia Phase Diagram Revisited: Stability of the Ordered Phases φ1 and φ2

Single crystals of CaO-stabilized ZrO₂ containing between 15.3 and 18.9 mol% CaO were heat-treated for 5000 h at 1200°C to study the stability of the ordered defect-fluorite phase, CaZr₄O₉ (φ₁). Subsequent TEM analysis of the equiaxed φ₁ domains in samples richer than approximately 18 mol% in CaO showed a random distribution of φ₁ variants, with no preferred interfacial habit planes. A critical review of the literature, combined with the new data, supports the Stubican-Hellmann-Hannon version of the phase diagram in the region 15 to 26 mol% CaO and 1000° to 1400°C and strongly suggests that φ₁ is a stable phase in the ZrO₂-CaO systems.     

Repeated Price Search

Consumers check few sites in online purchases. Previous research and experiments we perform demonstrate that consumers can not calculate the optimal strategy for price search. They use heuristics whose performance is better than random and less than optimal. To investigate online price search performance we survey student online textbook purchases. Students achieve good performance because they start with a good strategy and online market organization of marketplace and meta-search sites. An important factor is that algorithms at sites searched perform calculations that reduce the computational complexity of the search.     

Surface hardening of Ti alloys by gas-phase nitridation: Kinetic control of the nitrogen surface activity

A new concept for surface hardening of titanium alloys has been developed and successfully applied to Ti-6Al-4V alloys: gas-phase nitridation under kinetic control of the nitrogen activity. This method avoids the formation of detrimental second-phase nitrides by nitriding under a very low nitrogen activity, combined with rapid diffusion of nitrogen into the specimen. The surface hardness of the Ti-6Al-4V alloy was increased by a factor of 2 to ≈ 12 GPa, with only modest attenuation of ductility. We have realized conditions for generating case depths of ≈ 25 μm in reasonable nitridation times. The nitrogen activity in the gas phase is generated by heating a powder pack of Cr and Cr₂N. A closed two-zone system allows the powder pack and the specimen to be at different temperatures, optimizing both the nitrogen partial pressure and the nitrogen diffusion into the specimen. This low-cost, conformal nitridation process generates a smoothly graded nitrogen concentration profile and can be applied to finished Ti alloy components.