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A simple mathematical model based on atomic drift and diffusion is advanced to describe electromigration-induced stress in confined metal lines. Using a finite element approach, a MATLAB program was developed to simulate stress evolution in lines with variable geometry and microstructure under different boundary conditions. The simulation results show that the contact pads connected to the line end in the National Institute of Standards and Technology (NIST) test structure postpone the stress buildup in a microstructurally homogeneous line and the time to reach a certain stress is proportional to the pad sizes when the pads are much wider than the line. Subtractive defects are not the preferred failure sites when the defects fall on a wide polycrystalline line or a bamboo structure line. In a narrow polycrystalline conductor with a distribution of grain size, a subtractive defect may result in an abrupt change in the effective diffusion coefficient (blocking effect) along the line and quick buildup of the...

The microstructure of an integrated circuit interconnect material significantly affects the reliability of the device. Narrow lines with near bamboo structure exhibit an increased mean time to failure (MTF) and a concomitant increase in the dispersion ( σ ). A critical stress concentration site forms at the boundary between a single crystal segment and a polycrystalline segment of the line which leads to early failure. Simulation results for a 1000 μ m long aluminum line (a rigidly confined line with blocked ends is assumed) show that, if the grains are uniform in size and small relative to the line width, a missing metal defect located at the center does not produce a stress concentration site. This paper reports on properties of Al-1 wt% Y films deposited by DC magnetron sputtering. The addition of yttrium reduces both the mean grain size (299 nm) of aluminum films and the standard deviation of the grain sizes (89 nm). As-deposited films consist of α -aluminum supersaturated with yttrium and Al 4 Y phase. After annealing at 450°C for 30 min, β -Al 3 Y is precipitated, grain growth is not significant and the electrical resistance drops from 6.05 to 2.95 μ Ωcm.

Electromigration-induced stress in near bamboo structure interconnects has been simulated by the finite element method. The maximum stress at the juncture of polygranular clusters and bamboo segments increases when the stress profile interacts with the line end. The maximum stress at final steady state depends on the relative location of the polygranular cluster in the line under the constant source boundary condition. Under the blocking boundary condition, the steady state stress profile (if it exists) is determined only by current density and total line length and is independent of the microstructure of the line. Finally, the ratio of effective diffusivity of the cluster to that of bamboo segments affects the magnitude of maximum stress at quasi-steady state.

Microstructure plays a key role in reliability of interconnects. However, a quantitative correlation between microstructure and reliability of interconnects is lacking. This paper develops a statistical description of effective diffusivity of interconnects by considering grain boundaries, bulk lattice and line interface diffusion, and introduces a microstructure-based electromigration lifetime prediction model for normal failures of interconnects. Further, the probability of early failure of interconnects due to a severe microstructural inhomogeneity is calculated. Using the sub-population failure model, the mean time to failure of interconnects, with a range of microstructures from polycrystalline to hyper-bamboo, are simulated. The reported increase in lifetimes of wide polycrystalline lines with increasing w/d is mainly ascribed to reservoir effects of bonding pads in test structures. Reported lifetime prediction model may be used as guidance for circuit design and accelerated testing.

Final results obtained on the High Temperature Advanced Structural Composites Program at Rensselaer Polytechnic Institute are described in three volumes. Volume one contains the Executive Summary and reprints of papers on Intermetallic Compounds. Volume two contains reprints of papers on Ceramic Matrix Composites, Fiber Processing and Properties and on Interfaces. The third volume contains reprints of papers on Mechanics.... High temperature composites, Fibers, Matrices, Interfaces, Processing, Thermomechanical and Environmental behavior.

Final results obtained on the High Temperature Advanced Structural Composites Program at Rensselaer Polytechnic Institute are described in three volumes. Volume one contains the Executive Summary and reprints of papers on Intermetallic Compounds. Volume two contains reprints of papers on Ceramic Matrix Composites, Fiber Processing and Properties and on Interfaces. The third volume contains reprints of papers on Mechanics.... High temperature composites, Fibers, Matrices, Interfaces, Processing, Thermomechanical, Environmental behavior

Thin films of Al-1.27 wt %Y were deposited by dc magnetron sputtering. Adding yttrium to the aluminum drastically reduced the metal’s grain size and also improve the uniformity of grain size distribution. Upon annealing at a temperature of 450 °C for 30 min, grain growth was insignificant, while the electrical resistance dropped from 6.05 to 2.95 μΩ cm. The as deposited films consisted of Al4Y and α-Al supersaturated with yttrium. After annealing, β-Al3Y precipitated instead of α-Al3Y. The Al–Y films had much higher resistance to hillock formation than did Al-1 wt %Si films.

Graphite has a hexagonal close-packed crystal structure which is strong and stiff in the two directions of the basal plane and, in the third direction—perpendicular to the basal plane—is weak and compliant. High-performance carbon fibers must make use of the strong directions while suffering from the poor properties of the third. This paper describes, from fundamentals, the processes used to produce high-performance carbon fibers. The resulting fiber microstructures and the consequences of these structures on properties are presented.

The study of the deposition processes in pyrolytic graphite, made from the gas phase, has been largely empirical. The work presented attempts to show that the deposition is controlled by reactions in the gas phase. Experiments were performed at fast and slow rates of growth using natural, pyrolytic, and commercial graphites as substrates. The deposition processes occurring at various temperatures and pressures for all three substrates are described.

The kinetics of the reaction H + C3H8 = H2 + C3H7 have been studied in the range 95 to 170 °C by a method permitting the evaluation of the absolute value of the rate constant. This was found to be 2.2×10−10exp (−8.2×103/RT) cm3 molecule−1sec−1.

It is demonstrated that massive specimens of pyrolytic graphite heat-treated at 3600\ifmmode^\circ\else\textdegree\fi{}C may exhibit electrical characteristics comparable to those of the best single crystals presently available. Low-field magnetoresistance data taken perpendicularly to the $c$ direction of these specimens yield average carrier mobilities (1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ ${\mathrm{cm}}^{2}$/v-sec at 300\ifmmode^\circ\else\textdegree\fi{}K; up to 8\ifmmode\times\else\texttimes\fi{}${10}^{5}$ ${\mathrm{cm}}^{2}$/v-sec at 4.2\ifmmode^\circ\else\textdegree\fi{}K) in substantial agreement with Soule's and McClure's results for the layer planes of highly purified natural graphite. In conjunction with temperature dependence studies of the zero-field resistivity, it is found that the carrier density reproduces the theoretically predicted behavior down to 77\ifmmode^\circ\else\textdegree\fi{}K, but departs at lower temperatures. Plausible interpretations of this discrepancy and of the "knee" in the resistivity curve are briefly discussed.

Ceramic matrix composites have been produced by utilizing polymer pyrolysis as the processing technique. The precursor, polyvinylsilane, is a viscous, thermosetting polymer which yields a predominantly SiC ceramic material when pyrolyzed. This organometallic polymer in combination with SiC fibers and SiC whiskers was used to fabricate ceramic matrix composites. One of the major problems with a brittle/brittle composite system in which strong coupling exist between the fibers and matrix is the characteristic catastrophic failure that occurs once the strain to failure of one of the constituents is exceeded. This brittle behavior can be altered by the application of a suitable barrier layer between the fiber and matrix. Due to the success of a barrier layer between fiber and matrix in producing higher performance composites, multiple barrier layers were used to further improve the performance of the ceramic composite.

High modulus fibers are used mainly as fibrous reinforcement in resins, metals, or ceramics to provide strength and stiffness. It is instructive to consider what factors led to the development of composites. Structural designs are as likely to be limited by the stiffness as by the strength of the construction material. Hence, engineers have always desired suffer, stronger, less dense and lower cost structural materials. There are a number of materials in the upper center part of the periodic table, such as boron, carbon, silicon carbide and alumina, which all have significantly higher modulus/weight ratios than the common engineering metals. Theoretically, the high modulus would result in high strength if the materials were perfect. Unfortunately, most of the high modulus materials are covalently bonded and are brittle. Small flaws can produce catastrophic failure at drastically reduced stresses. (The flaws may be introduced during manufacture or during service.) Except in some unusual protected environments, primary structural elements can not be made from materials which fail catastrophically. Composite materials offer the potential of using these brittle materials in structures which will not fail catastrophically. The brittle materials are made into fibers to give redundancy and placed into a matrix. The matrix serves to transfer stress into and out of the fibers. The matrix and matrix/fiber interface must also serve to stop cracks which originate in the fibers from propagating through the solid. Hence, a localized impact may break a few fibers, but the crack can be stopped by a ductile matrix or fiber/matrix debond.

The axial properties of carbon fibers were found to correlate well with preferred orientations determined by either x-ray diffraction or optical techniques. High modulus carbon fibers were found to have significant gradients in preferred orientation. For HMS fiber, the modulus of the surface layers is about twice the average fibers modulus, while the interior modulus is only about one-half the average. This modulus gradient suggests that higher modulus fibers, with good strength, could be produced, if the modulus at the interior of these fibers could be increased. The gradient in preferred orientation produces high residual stresses in HMS fibers. While the high surface compressive stresses minimize the effect of surface flaws, the high axial tensile stresses in the interior may decrease strength by causing fracture to initiate at flaws in the interior rather than at the surface. Similarly, the high axial compressive stressed outer layers of a fiber may initiate buckling when the fiber is compressively loaded. Modifications of the residual stress pattern might allow increased tensile and/or compressive strengths to be obtained in high modulus carbon fibers. Many correlations between properties and fiber diameter, reported in the past, may be in error because it is very difficult to determine the fiber diameter precisely and accurately. (Author)

The activities associated with establishing a Center of Excellence in Rotary Wing Technology at Rensselaer Polytechnic Institute under a five year contract with the U.S. Army Research Office are reported here. These include measures taken to provide comprehensive and in-depth curricula at advanced levels in rotorcraft technology; attract and retain outstanding young people in these programs; perform basic research at the leading edge of technology in structures, structural dynamics, unsteady aerodynamics and aeroelasticity disciplines as applied to rotorcraft; developing useful new national research resources in these areas; and accomplishing technology transfer to the rotorcraft community beyond the Rensselaer campus. Descriptions, references and statistics are provided to allow assessment of the extent to which these goals have been realized. Keywords: Laminated plates, Composite materials, Helicopters, Rotorcraft, Aeronautical engineering education.

A workshop was conducted, dealing with the fundamentals of composite constituent materials and laminates as they influence applications to advanced rotorcraft developments. Included in the purview of the workshop were the theory, numerical analysis, fabrication, testing, and inspection of thermoset & thermoplastic resin matrix and metal matrix composites as they may have advantage for rotors, fixed airframe, drive system, controls, and engine 'cold section' components. Rotorcraft composite materials & structures; Rotary wing aircraft.