Direct bonding of metals to ceramics: Interface investigations

There is a growing interest in metal/ceramic bonding for a wide range of applications from electronic packaging to biomedical implants. In this research work, results are reported for direct bonding of copper to ceramic (e.g., $\rm Al\sb2O\sb3$ and ZrO) in a furnace under inert atmosphere (e.g., N$\sb2$ and Ar$\sb2).$ Other, metals such as Cu, Ni, SS-316 were directly bonded to ceramics (e.g., $\alpha$-$\rm Al\sb2O\sb3,$ sapphire) using laser heating (e.g., 247 nm and 10.6 $\mu$m wavelengths) in the presence of N$\sb2$ atmosphere. Cu flakes have bean bonded to industrial alumina ceramic and sapphire in the presence of methyl, ethyl and isopropyl alcohols using a CO$\sb2$ laser. All these experiments were performed by heating the metal or metal-organic media member for a sufficient time in order to create a metal-metal oxide eutectic melt at the interface with the ceramic substrate. Thermal wave imaging (TWI) was used to investigate the bonding at the metal/ceramic interface. It was found that the method of direct bonding of metals to ceramics using lasers performed better than the furnace. The properties of the copper bonded layer on alumina ceramic was investigated using scanning electron microscopy (SEM). Also, the elemental distribution at the metal/ceramic interface was analyzed, using energy dispersive x-ray spectroscopy (EDS). With the help of x-ray diffraction (XRD), the phase present at the copper/industrial alumina ceramic interface was determined to be $\rm CuAl\sb2O\sb4.$ This was different from the $\rm CuAlO\sb2$ phase found at the copper/sapphire interface for the furnace bonding case. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) was also used to investigate the aspects of metal/ceramic interfaces. It was found that the samples processed by furnace heating and by laser beam heating have a diffused transition interface. The electron diffraction patterns revealed the phase present at the interface (Cu/$\rm \alpha$-$\rm Al\sb2O\sb3)$ to be a cubic one, with the $\rm CuAl\sb2O\sb4$ crystallographic structure. The TEM images show that the samples heated using excimer laser have an amorphous top copper layer. The HRTEM images of the samples heated with the excimer laser beam under N$\sb2$ atmosphere revealed that the inter-atomic spacing at the $\rm Cu/\alpha\ Al\sb2O\sb3$ interface region to be 0.365 nm in average. In order to investigate inter-diffusion between the copper layer and the sapphire and also to measure the percentage of diffused of copper in sapphire, Rutherford backscattering spectroscopy (RBS) was performed. Based on heat transfer theory and on diffusion theory, a model was derived to explain and predict the behavior of furnace and laser beam bonding of metals to ceramics. The predictions of the model were in agreement with the experimental percentage values of diffused copper in sapphire obtained in the RBS investigations. Experimental investigations and the theoretical model concluded that the direct bonding phenomenon is a thermal effect. The direct bonding phenomenon is similar to transient liquid phase bonding (TLP). The major advantage of direct bonding is that no filler element is required between the metallic member and the ceramic substrate. Finally, the method of direct bonding of metals to ceramics using laser beam is capable of directly drawing a metallic pattern on a ceramic substrate