Evaluation of Cu/Sn-Cu Bump Bonding Processes for 3D Integration Using a Fluxing Adhesive

The study of copper-based bump structures for interconnects in 3D integration applications has been ongoing for several years. Typically, an array of Cu bumps is bonded to an array of Sn-capped Cu bumps or another Cu bump array using a thermocompression bonding process. These processes rely on high pressures and temperatures to facilitate bonding between the bump arrays. In order for this bonding to take place, some method of oxide removal is normally required for the Cu and/or Cu/Sn bump surfaces before bonding. A number of different methods have been investigated by a number of groups, including chemical cleaning, plasma cleaning, self-assembled monolayers, and no-flow underfill (NUF) materials. The use of NUFs is particularly intriguing, since these materials can be formulated with fluxing agents which could reduce surface oxides on Cu and Sn and can be deposited immediately prior to the thermocompression bonding process. In addition, the material provides a protective encapsulant to the interconnect array, protecting it from environmental damage and adding mechanical strength to the assembly. We will present the results of a study to evaluate new fluxing NUF materials in thermocompression bonding processes on full area array test devices with 25 micron bump pitch. The test devices are fabricated with either Cu or Cu/Sn bumps to provide two different bonding options (Cu to Cu or Cu/Sn to Cu). We will compare the NUF bonding process and resulting bonded interfaces to assemblies fabricated using our standard bonding processes, which rely on both chemical and plasma pretreatment processes to prepare the bump arrays before bonding. Mechanical and electrical data will be used to compare the two bonding processes, as well as SEM cross-section analysis of the bonded interfaces.