P. Børgesen

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.

Shape of a nonaxisymmetric liquid solder drop constrained by parallel plates

A differential equation describing the shape of a nonaxisymmetric solder-gas interface in cylindrical coordinates is developed using the Euler-Lagrange method. This equation is solved numerically using the method of finite differences for the geometry of a liquid drop constrained by two parallel circular plates that are separated vertically at a fixed height and displaced in the horizontal direction. For passive alignment applications, the shapes obtained can be used to calculate the restoring force produced by a drop constrained by circular plates when displaced from equilibrium. >

Stress and current induced voiding in passivated metal lines

Thermal stress and current induced damage processes may well determine the ultimate limit for the achievable line widths in microelectronic circuits. According to our current understanding, thermal stresses cause void nucleation during cool-down from high temperature process steps. The voids may continue to grow during subsequent storage. The application of an electrical current may lead to further void growth, as well as migration and coalescence of voids. A recently developed model provides the framework for improved reliability assessments, including the effects of statistics, as well as for the development of remedies.

Low temperature ion beam mixing of bilayers and multilayers in the Ti-Cu system

The mixing rate for Ti-Cu bilayers irradiated by 600 keV Xe ions near liquidnitrogen temperature was found to be a factor of 1.6 smaller than estimated on the basis of a cylindrical thermal spike model. At room temperature radiation enhanced diffusion contributes measurably to the mixing. As was found previously for Fe-Ti and Ni-Ti samples, mixing was found to be considerably faster at the Cu-Ti interfaces of a multilayer sample.

Mechanical design considerations for area array solder joints

Concurrent engineering often requires mechanical reliability to be traded off against a number of other criteria. Rather than simple optimization, mechanical design usually relies, explicitly or implicitly, on the assessment of the relative merits of a number of alternatives. However, the evaluation of a large number of designs by finite element and damage integral methods would, at best, be extremely cumbersome. The present work describes design tools based on elastic stress analysis for rapid assessment of area array assembly designs in terms of mechanical reliability. Examples in the discussion illustrate various means of improving the reliability of such assemblies. >

Room temperature ion beam mixing of aluminum with titanium hydride

Abstract Electrolytical charging with hydrogen has recently been found to inhibit the subsequent ion-irradiation-induced mixing of Fe, Co, Ni or Pd into Ti by factors of 7, 2.2, 2 and 1–1.5. It has been suggested that the effect depends on the stability of the hydrogen-vacancy complexes in these metals. An important consequence of this dependence would be that AlTi samples should behave like FeTi in this respect. In reasonable agreement with this we find that charging an AlTi bilayer sample with hydrogen reduces the ion beam mixing rate due to 600 keV Xe ions by a factor of 4.

Stress Induced Void Nucleation in Narrow Aluminum Alloy Lines

The stress induced failure of passivated aluminum based metallizations is the combined result of the stress induced void nucleation and growth of voids. The present work investigates the dependence of the void nucleation on aging temperature and cooling rate. Narrow Al, Al(0.2%Cu) and Al(2%Si) lines of different widths were fabricated by lift-off and passivated with various thicknesses of Si 3 N 4 . After a 400°C anneal, the specimens were cooled directly to the selected aging temperature and aged for up to 1100 hours. Void nucleation was found to occur predominantly during the initial cool-down. The rate of nucleation varied in quite different ways with the combination of aging temperature and cooling rate for the different alloys, which is consistent with a grain boundary sliding controlled nucleation mechanism.

Predicting and Comparing Electromigration Failure for Different Test Structures

Electromigration and stress migration are important reliability concerns in the semiconductor industry. Relative and absolute assessments of lifetimes generally rely on the accelerated testing of ‘standard’ test structures. However, the sensitivity to various important electromigration phenomena is found to depend strongly on the type of test structure. Two common structures for electromigration testing are (i) a long line with large pads at either end and (ii) a more realistic interconnect line with W-studs at the line end. The effects of (a) different flux divergences at the line end, (b) interfacial diffusion, and (c) Cu-depletion generally result in different lifetimes for the two types of test structures under the same testing conditions. In this paper, we compare void growth rates and mean times to failure for both types of test structure taking these effects into account. Our results are used to explain differences in MTF values reported in the literature for different test structures.

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.

The Effect of Thermally Induced Stresses on Electromigration Lifetime Of Near-Bamboo Interconnects

Thermal stresses may affect significantly electromigration induced open failures in nearbamboo interconnect structures ending in W-vias. Based on a physical model, we investigate the thermal stress effects on void nucleation, on early failures due to grain boundary diffusion, and on longer term bulk diffusion dominated failures. It is shown that thermal stress is all important as far as early failures are concerned. In particular, during accelerated tests and service, the early failures, when occur, arise due to stress and current directed grain boundary diffusion along grain clusters. In the accelerated test, bulk diffusion dominated void growth at vias determines the long term failures while during service bulk diffusion contribution is usually negligibly small within time frames of practical interest. Because the dominant failure mechanisms change, the extrapolation from the accelerated tests to the service conditions is not straight-forward.