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In-situ Gold-Silicon Eutectic Mixture Formation

A common way to bond wafer in semiconductors is to use a metal such as gold that forms a eutectic mixture with the silicon wafer. [1] The gold-silicon system can melt at a temperature as low as 363 °C at 18.6% silicon, much lower than the melting point of gold (1064 °C) or silicon (1414 °C). [2] Previous research has shown that the fraction of silicon is inversely proportional to the bond strength of the material, though higher percentages are common due to processing conditions where silicon is in excess. [3] At high Si concentrations there are large inclusion of silicon that form leading to a lower bonding rate and brittle structures above 57.5% silicon. [3] This study aims to understand the formation mechanism of the silicon and gold eutectic mixture through using in-situ heating in the environmental scanning electron microscope (ESEM). In-situ heating experiments on the melting behavior of small sections of gold wire on single crystalline silicon were performed in the Quattro ESEM using the high vacuum heating stage. Heating of the samples near the transitions were performed at 1.5 °C/min to observe the phase transitions and determine the melting point. Two different experiments were performed to understand the process of formation and the subsequent product formed. The first experiment determined the melting point of the gold and the resulting melting point of the eutectic mixture formed. The second experiment involved a long gold wire that was only in contact with the silicon wafer at the edges, with the middle elevated. The sample from the second experiment was analyzed using the HeliScan microCT to characterize the diffusion of gold into silicon and vice versa. Data analysis and visualization were performed in the Avizo software suite. Additional analysis was performed by cutting cross-sections in the sample using the Helios NanoLab 600 dualbeam and characterizing the exposed surfaces using the EDAX Octane Elect Super Energy Dispersive X-ray