Thermal Attachment
Alloy composition refers to a specific mixture of gold and silicon that melts at a temperature far below the melting point of either constituent. The gold silicon eutectic governs the high-reliability die attachment of micro-sensors to ceramic substrates. It stops applying once the junction is formed and remains solid below the eutectic temperature of approximately 363 degrees Celsius.
Mechanical Strength
Joint formation provides both electrical conductivity and rigid mechanical support for the sensor. The resulting gold silicon eutectic bond is brittle but offers high thermal conductivity, which is necessary for dissipating heat. It remains the preferred attachment method for aerospace sensors that endure high vibrations.
Interfacial Integrity
Oxide layers on the silicon die can prevent the proper formation of the alloy. If oxides are present, the gold silicon eutectic cannot form a uniform bond, resulting in voids that concentrate stress and lead to micro-cracks. Sputtering a thin layer of gold onto the back of the wafer helps to facilitate the bonding process.
Process Control
Temperature profiles must be strictly managed during the die attach phase. If the heating cycle is too short, the gold silicon eutectic does not fully diffuse, resulting in a weak interface. Conversely, excessive heating can cause the silicon to migrate to the surface, causing contamination.
