Diffusional processes, either solid- or liquid-state, are essential in promoting reliable metallurgical interconnections used in many electronic packaging applications. The same phenomena often degrade the integrity and reliability of the interconnections as well. In this talk, both examples are presented to illustrate how the diffusional processes are important in many interconnection technologies. Thermocompression bonding, eutectic bonding, soldering and liquid-phase bonding are chosen for this purpose. Several issues related to soldering, such as dissolution, intermetallic formation and microstructure of solder joints, will also be discussed in terms of diffusional processes.

Epoxy resins of novolac type were molded to real copper and nickel plated copper framesin plant conditions. Three resins, formulated with different amount of tetrabromobisphenol A flame retardant, were used. The samples were stored at 200°C and then mechanically fractured at given time intervals. The surfaces obtained in the fracture have been analyzed by ESCA and Ga+ excited static and imaging SIMS. The kinetics of migration of the flame retardant to the resinmetal interfacial zone have been followed. The process was found to be diffusion controlled. Evidence of a redox reaction between the copper oxides found on the surface of the copper frame and the tetrabromobisphenol A was obtained. A copper diffusion through the Ni plating was observed in the case of the nickel plated frames. This phenomenon is most important at the point on the frame where the bromine concentration is higher.

Bonding phenomena with clean and smooth surfaces at room temperature (293 K) in ultrahigh vacuum has been studied. The metal surfaces, finished to required smoothness in nm level, were bombarded by Ar ions to remove surface contaminants. Then, two cleaned surfaces were put together. Interaction of silver and copper was examined by SEM, AES, and TEM. In order to observe the fundamental phenomena, bonding was attempted between a silver disk with a flat surface and a copper sphere 80 pm in diameter. A bonding area of about 40 μm2 was confirmed under slight load at 293 K. Strong bonds were achieved and the fracture took place almost completely in silver bulk. Furthermore, it was found that the bonding area obtained by the process was much larger than the one derived from conventional plastic theory.

Phase formation of NiAl3 and Ni2Al3 were studied individually in lateral diffusion couples A and B and simultaneously in sample C. The lateral growth of certain phases was controlled by varying the thickness ratios of Al to Ni at the source and thin film area. Parabolic dependence of the phase width on annealing time was found for NiAl3 and Ni2Al3 in the one-phase and two-phase growth at 425 °C.

Room temperature deformation data of leadless solder joints are reported. The joints were sheared under cyclic, displacement controlled loading at frequencies between 0.001 and 0.01 Hz. A microplastic model was utilized to simulate the stress-strain loops, which demonstrated a pronounced Bauschinger effect. The implications of microplasticity on fatigue life of solder joints are discussed. This phenomenon must be taken into account in an accurate prediction of solder deformation at low strain ranges.

The wetting of metal surfaces by molten solder is usually considered to be driven solely by an interfacial energy imbalance. The effect of chemical reactions on the wetting process is neglected, although the growth of an intermetallic layer in the wetted interface is commonly observed. In this work, the energy release during the incremental advance of a spreading solder droplet due to the interfacial energy imbalance and the formation of the intermetallic layer is calculated. The free energy of formation, ΔG, of the intermetallic layer is shown to be an important driving force for solder wetting.

This approach to wetting has been applied to three systems, Cu-Sn, Cu-Sb and Cu-Cd. Liquid Sn, Sb and Cd react with solid Cu to form Cu6Sn5 (η), Cu2Sb (γ) and CuCd3 (ε), respectively. The free energy of formation, ΔG, for these intermetallic compounds is unknown experimentally, but can be calculated from the phase diagrams and other solution data using classical thermodynamics. These thermochemical calculations yield ΔG(η) = 465–3.09T for Cu-Sn, ΔG(γ) = −2500+0.54T for Cu-Sb and ΔG(ε) = −825+0.44T for Cu-Cd (cal/mole). These relations were evaluated at the respective melting temperatures and compared with the interfacial energy exchange. In all three cases the ΔG contribution was approximately two orders of magnitude larger than the interfacial energy exchange making it the dominant driving force for wetting kinetics.

The microstructure of a Cu3Au v. Pb-5wt%Sn diffusion couple was examined in the as-dipped and annealed conditions. The as-dipped couple showed no reaction between the solder and the base metal. The couple developed intermediate layers of two intermetallic phases upon annealing for 289 hours at 169°C. These phases were identified as phi and eta’ which are contained in the Au-Cu-Sn ternary system. No phases containing significant amounts of Pb formed, indicating that even at low concentrations in a Pb-Sn solder, Sn is the active chemical specie in the solder for the Au-Cu-Pb-Sn system. The microhardness of the eta’ phase was found to be equal to that of Cu3Sn, which forms in the absence of Au in the Cu base metal. However, the the phi phase was harder, and therefore the presence of Au in the base metal may adversely affect the thermal fatigue resistance of the solder joint.

Transmission electron microscope observations of the structures of single-phase metallic thin films have been made. Films prepared by sputtering and by thermal evaporation have been used and the structures have been evaluated quantitatively as a function of film thickness. It is found that the grain size in all films increases linearly with the film thickness, indicating that grain growth occurs during film deposition.

Numerical techniques are used to calculate the stress concentrations arising near the interface of a single-crystalline film which is bonded to a stiff substrate. The film has cubic elastic symmetry, and it is characterized by the anisotropic constants A and H which show the deviation of the material frcm elastic isotropy. The normal to the film-substrate interface is taken to be along the 100, 111 or 110 directions. The inhomogeneous stresses near the free edge and the uniform stresses far from the free edge are calculated, and the effects of cubic elastic anisotropy and of film growth direction are established for material parameters typical of metallic and semiconducting films.

Conventional interconnect and switching technology is rapidly becoming a critical issue in the realization of systems using high speed silicon and GaAs based technologies. In recent years clock speeds and on-chip density for VLSI/VHSIC technology has made packaging these high speed chips extremely difficult. A strong case can be made for using optical interconnects for on-chip/on-wafer, chip-to-chip and board-to-board high speed communications. GaAs Integrated Optoelectronic Circuits (IOC's) are being developed in a number of laboratories for performing Input/Output functions at all levels. In this paper integrated optoelectronic materials, electronics and optoelectronic devices are presented. IOC's are examined from the standpoint of what it takes to fabricate the devices and what performance can be expected.

A number of promising tedcniques used to intercmnect two or more optical fibers and optical fibers with guided wave optical devices are reviewed. Such waveguide-to-waveguide interconnections impose severe optical and mechanical design constraints. components used in single mode infrared optical systems use fibers of five to ten microns in core diameter. This creates the need for subnicron alignment toleranc between the waveguides. Such severe toleranc requirements are satisfied, in the case of single mode fiber connectors, through the use of precision ceramic ferrules. Achieving the same performance, when interconnecting fibers and waveguide devices, is somewhat less straightforward. Several approaches are currently under study. One such approach uses precisely etched silicon V-grooves. Another approach uses self-aligned grooves or slots that are formed in the substrate material through ion-milling or laser-assisted etching. Components used in multimode fiber systems use fiber with fifty to one thousand micron core diameters, and are therefore more forgiving to misalignment. Despite this, careful design is required to achieve satisfactory performance. The misalignment penalties associated with a number of interconnect situations are illustrated and discussed in terms of alignment tolerances and fabrication difficulties.

The use of lithium niobate integrated optic devices outside of the research laboratory is predicated on the development of a sound packaging method. We present a discussion of the many issues that face the development of a viable, robust packaging technology. We emphasize the interaction of lithium niobate's physical properties with available packaging materials and technologies. The broad range of properties (i.e. electro-optic, piezo-electric, pyro-electric, photorefractive…) that make lithium niobate an interesting material in many device applications also make it a packaging challenge. The package design, materials and packaging technologies must isolate the device from its environment so that lithium niobate's properties do not adversely affect the device performance.

Recently Lightwave Device Packaging Department at AT&T Bell Laboratories has demonstrated that ceramic materials can provide cost effective and high quality packages to house optical and electronic components for lightwave communication applications. In this paper we examine the material requirements for optical packages. We also study the material properties of metals and ceramics with a potential application in optical packages. In particular, we review hermeticity, thermal conductivity, thermal expansion coefficient, dielectric constant, electrical resistivity, sintering temperature and mechanical strength of these materials. Our study will provide a data base and useful guidelines for designers to make uniformed decisions on material selection for optical package. We also review the mixing rules to predict the resultant property of a composite from the known attributes of its constituents and the use of new composite materials will provide a new degree of flexibility in the optical package design.

The applicability of Nb as a Schottky barrier on GaAs depends to a large extent on the thermal stability of the contacts. In this study, bulk diffusion couple and phase diagram studies in addition to thin film studies were completed to understand the stability of and the reactions at the Nb/GaAs interface. Nb thin films were deposited onto GaAs substrates by dc magnetron sputtering and were annealed in the temperature range 300 to 1000°C. Analysis was done using plan-TEM and XTEM. The Nb/GaAs interface was found to break down into a series of binary compounds above 500°C. Bulk diffusion couples annealed at 600°C were analyzed using an electron microprobe. The stable sequence of phases formed in the couple, i.e., the diffusion path, was determined and was used to rationalize the observed compound formation in the thin film contact system.

Research has been undertaken in several major research laboratories to achieve a commercially successful GaAs via hole etching process. This work presents the achievement of results with the desired characteristics for through-wafer GaAs interconnects or vias, for production of MMICs.

We report vias acceptable for use in the production of microwave GaAs field effect transistors fabricated by a reactive ion etching process. Using conventional resist masks and non toxic gases, aspect ratios of up to 10:1 have been achieved with via sizes as small as 40μm × 40μm. Typical wafer thicknesses were greater than 100μm and the process showed high selectivity to the frontside metallization. The etching conditions had to be varied as a function of overall substrate area and exposed GaAs. To determine optimum process parameters, the RF power and resultant DC bias; gas flow rates and ratios; and system pressure were systematically varied. The effects of changes in area of wafer to be etched and of the composition of the electrode material were also studied. Details are presented of the optimum etching parameters for use in 2″ and 3″ GaAs MMIC production.

Dielectric permittivity and loss tangent were measured on: 1) pressed disks of four different calcium aluminate cements shear mixed with water and poly(vinyl alcohol) and cured at room temperature and 2) laminates of SECAR 71 cement sintered at 1450°C. Post resonance and perturbation methods were used for microwave frequency measurements. Far infrared dielectric response of the cements was determined by FTIR in the diffuse reflectance mode. The real and imaginary parts of the relative dielectric permittivity were obtained via a conventional Kramers-Kronig analysis.