Off-eutectic Au-Ge die-attach - Microstructure, mechanical strength, and electrical resistivity

Abstract

In this paper, off-eutectic Au–Ge joints were formed between Si substrates to investigate their high-temperature compatibility. High-quality joints made with small bond pressure, 53 kPa, were fabricated. The joints comprised three different types of morphologies: 1) a layered structure of Au/Au–Ge/Au; 2) a layered structure of Au/Au–Ge/Au where some sections of the central Au–Ge band have been replaced by an Au section that extended across the entire section; and 3) a roughly homogenous Au layer comprising the primary $\alpha $ phase. The average Ge concentration was 10 ± 2 at%. Joints formed with a higher bond line pressure, 7.6 MPa, were of a reduced quality with voids and cracks at the original bond line. Annealing at 400 °C for 1000 h transformed the microstructure into an Au–Ge–Si compound with Au precipitates. The shear strength of the fabricated joints was found to be at least 50 MPa, and the fracture mode was an adhesive fracture between the adhesion layer and the die or substrate. Heated dies detached from the substrates at 460 °C, i.e., more than 100 °C above the eutectic melting point of the binary Au–Ge system. Electrical resistivity measurements confirmed a melting process at the eutectic melting point by an abrupt increase in resistivity.