D. A. Lilienfeld

Low-temperature ion beam mixing of medium-Z metals

Abstract Low-temperature ion beam mixing of metals is generally ascribed to a combination of ballistic and thermal spike mechanisms. For heavy ions incident on sufficiently heavy metals the mixing rate varies much faster than linearly with the nuclear energy deposition density, ϵ, possibly quadratically as predicted by a model based on interdiffusion within overlapping thermal spikes. At lower energy densities the mixing rate still exceeds ballistic estimates significantly, but does not vary faster than linearly. Comparisons of results for medium- Z systems ranging from Pd-Ni to Al-Ti in this regime show that the same fundamental mixing mechanism must still be involved - a mechanism that is strongly sensitive to chemical driving forces, i.e., some type of “diffusion”. The evidence for a thermal spike mechanism is considered.

Material and Reliability Considerations for Anisotropically Conductive Adhesive Based Interconnects

An adhesive based interconnect design in which the contact force between a bumped die and substrate is maintained by the shrinkage stress of the adhesive was investigated for use in high performance applications. In general, heating will reduce the contact force because of the differential thermal expansions and load relaxation in the adhesive. Special experimental techniques have been developed to measure the relevant materials properties. The potential for optimization in terms of materials selection is discussed.

Phase Formation and Phase Stability in the Al-Ti Thin Film System

This study focuses on the sequential formation of aluminide phases during annealing of titanium and aluminum thin film bilayers. The formation of titanium-rich intermetallic phases at higher annealing temperatures is emphasized. Using Rutherford Backscattering Spectrometry (RBS) analysis, and x-ray diffraction, phases formed as a function of temperature have been identified. The phases Al 3 Ti through Ti 3 Al were observed over the temperature range 450–750°C, where reaction with the SiO 2 substrate occurred. All phases were present as discreet layers within the samples with several layered phases coexisting at the higher temperatures.

Thermal Stress Induced Void Formation in Narrow Passivated Cu Lines

0.75–3 μm wide copper lines encased in a thin adhesion layer of Al or Cr were passivated at 300°C and annealed at 400°C. The passivation was then removed and the lines examined in a scanning electron microscope. The development of thermal stress induced voiding was found to depend primarily on aging conditions and adhesion.

Radiation Induced Grain Growth in Transition Metals

Nanocrystalline self supporting thin films of Ni, Co, Cr, V, and Ti were irradiated with 600 keV Xe ions at liquid nitrogen temperature and the resulting grain growth was measured by transmission electron microscopy. Average grain sizes were seen to increase from about 10 nm to 18–35 nm, depending on the metal. Systematic variations did not support current thermal spike models.

Ion Beam Mixing of High-T c Superconductor Components.

The design of the necessary multilayer structures for producing superconducting thin films by ion beam mixing methods requires, among others, the knowledge of the individual (binary) mixing rates. In order to measure these, various combinations of Y, Ba, Cu, and Bi were irradiated with 600 keV Xe-ions at 80K and 300K. The systems exhibited a wide range of mixing behaviors which are also of fundamental interest. Ba and Cu readily formed the BaCu phase, and further mixing with Cu progressed only via binary collision mechanisms. At 80K Cu and Y were rapidly mixed in any ratio by thermal spikes, whereas a Cu rich sample rapidly formed the Cu 6 Y phase at 300K. Ba could not be mixed into Y or a Y-Cu mixture. Finally, irradiation of polycrystalline layers of Cu and Bi apparently lead to rapid motion of Bi along grainboundaries at both temperatures.

A Damage Integral Based Analysis and Simulation of the Thermal Fatigue of Diebonds

Microelectronic packages undergo substantial thermal excursions during processing and often in service. Differences in the thermal expansion of the various components may therefore lead to fatigue of the interconnects. A computer simulation has been developed which is based on a damage integral approach for the modeling of damage and failure in the various bonding layers found throughout such a package. This code should apply equally well to solder die bonds and polymeric or epoxy adhesive layers, assuming that the corresponding crack growth parameters and constitutive relations are known. Using literature values for these properties, predictions have been compared to experimental thermal cycling data for solder die bonds. Consequences for the extrapolation of accelerated test results to service conditions are discussed.

Ion Induced Quasicrystalline Transformation in the Al-Cr-Er System

The Aluminum-rich end of the Al-Cr-Er ternary system was investigated in an effort to find new quasicrystalline alloys. Composition, dose dependence and substrate temperatures during ion irradiation were varied. A quasicrystal alloy with the Al-Cr intensity pattern was found at low Er content. Another phase was observed that did not index to any known equilibrium structure. This new phase has d spacings and diffraction line intensities which are similar to those of the Al-Li-Cu quasicrystalline phase.