Direct metal-to-metal thermocompression bonding studies

Abstract

Three-dimensional integration, which involves stacking devices vertically, promises several attractive benefits such as high functionality, performance and low system power consumption due to shorter signal delays with the elimination of long wires. One of the common bonding techniques used is metal fhermocompression bonding. The greatest stumbling block for using this technique is the need for high bonding temperature and/or pressure to create strong and reliable bonds. This thesis begins with a study on how temperature and pressure affect bonding characteristics and the mechanical integrity of direct gold joints. One important finding of these experiments is the identification of a critical temperature and pressure below which no bonds can be formed. The criteria for bonding to occur include the removal of surface barrier film between contacting sites through sufficient interfacial mechanical and chemical actions. Freshly exposed surfaces are thus activated thermally, leading to an increase in bond strength. Higher bonding pressure increases bond strength but to a maximum. With further geometrical measurements and fracture analysis, the bonding mechanism is elucidated. This thesis also presents, for the first time, a method to lower the required bonding temperature in gold thermocompression bonded joints with the help of self-assembled monolayers [SAMs]. A comparative study on alkanethiols of various chain lengths in influencing its bondability at several temperatures is evaluated.