Pressure-assisted reaction bonding between W and Si80Ge20 alloy with Ni as the interlayer

Abstract

The conditions and reaction mechanism of W/Ni/Si80Ge20 hot-press bonding have been studied. It was found that a Ni/Si80Ge20 bond can be formed using low pressure, 19.6 MPa, in the temperature range between 780 and 900 °C in a short time. The kinetics follows a parabolic pattern, suggesting it is a diffusion-controlled process. The activation energy is 2.7 eV and the parabolic rate constant is given by KP = 4.0 × 1014 exp(−3.2×104/T) (μm2/min). The bonding interface has a multilayered structure. A phenomenological mechanism of the bonding formation has been proposed based on scanning electron microscopy observations and energy dispersive spectroscopy. The cracking problem due to thermal stress is discussed based on Oxx’s equation. It was found that bonds free from cracks in the Si80Ge20 alloy are formed when the Ni consumption (as measured by the thickness of the nickel layer) is sufficiently small (<40–45 μm) and the post-hot-press cooling is slow. On the other hand, tungsten and nickel react during hot pressing at higher temperature, forming WNi4. As an interlayer, nickel can join the tungsten sheet and the Si80Ge20 together. It has been also demonstrated that a thin nickel layer formed by vapor deposition on a tungsten sheet may be used as the interlayer in place of nickel sheet.