Novel Electroformed Ni-Au Alloy for High Temperature Semiconductor Test Structures

Abstract

The ongoing reduction in semiconductor and package pitch leads to challenges for engineering suitable miniaturized test and measurement contact vehicles. Traditional probe card approaches for miniaturization may reach fundamental limits for reduction in scale and pitch. Direct electroforming of conductive electrical contacts at pitch using LIGA technology can allow for direct registration to the test points, the simultaneous formation of all contacts and an opportunity to extend the roadmap for miniaturization. We report here on a new and novel nickel-gold alloy (one to three percent gold by weight) that can be produced by electroforming. This new material was computationally designed to optimize the thermodynamics of the two alloy components such that the alloy has a temperature stable nanocrystalline grain structure which produces highly desirable mechanical and physical properties. Specifically, the alloy is highly electrically and thermally conductive with a resistivity of 10 micro-ohm-cm, while also having a yield strength in excess of 1.5 GPa and an elastic modulus of 130 GPa. This combination of high strength and moderate elastic modulus creates and optimum for mechanical elastic stored energy in a formed contact beam. That is to say, contacts engineered from this material are flexible and strong enough to remain in the elastic regime and have sufficient normal force to maintain low and stable contact resistance, which boosts their long term service life. Further, the alloy shows a fixed cantilever beam bending fatigue limit of more than 1 GPa, further enhancing its viability for long term service. For applications requiring even more strength a 2 GPa yield strength version has also been produced, however this version leads to minor trade-offs in terms of ductility and electrical resistivity and cost. Since these contacts will be used at elevated temperature, the alloy has been designed to be nearly as electrically conductive as plated unalloyed nickel and thermodynamically stable under long term high temperature storage conditions of 400 C. Characterization of the microstructure shows that it is a supersaturated solid solution of Au in a Ni host matrix with evidence of preferential Au segregation to the Ni grain boundaries. Xray diffraction analysis confirms a nanocrystalline grain structure whose size varies as a function of the alloying content. By segregating the Au to the grain boundaries, we extend the electron mean free path and boost electrical conductivity. We also reduce the grain size of the metal leading to improvements in mechanical properties such as yield and fatigue. The electroplating solution is environmentally friendly with no significantly hazardous materials and is REACH compliant. We have shown chemical compatibility with a limited test set of standard photo-resists and masks used for the LIGA process. The plating process is reasonably fast and has shown the ability to plate layers as thick as 400 um without cracks, defects or voids. For some applications it may be desirable to add more plated layers to engineer the contact surface. Gold and other precious metals can be plated on top of the Ni-Au layer with good adhesion and lead to improvements in long term low level contact resistance.