Novel Die-to-Wafer Interconnect Process for 3D-IC Utilizing a Thermo-Decomposable Adhesive and Cu-Cu Thermo-Compression Bonding

Die-to-wafer interconnect offers key advantages for 3D-IC including heterogeneous die population using devices from different process lines, and higher yield by incorporating only known-good die. Attaching the dies to a wafer, however, typically involves serial processing whereby each die is aligned and bonded one at a time. This can be prohibitively time consuming particularly when using a slow, high-temperature process such as Cu-Cu thermo-compression for each die in succession. Also, the high temperatures applied locally can adversely affect neighboring sites by oxidizing the Cu bonding surface if not protected in an oxygen-free environment. We have developed a novel die-to-wafer interconnect process that circumvents these problems using a die-tacking and global-bonding approach. Using a high-accuracy die placement tool, individual dies are aligned and tacked onto a wafer that is coated with a thermo-decomposable adhesive layer. The low-temperature tacking process avoids oxidation of the Cu bonding surfaces, minimizes thermal cycling, and increases throughput significantly. Once the wafer is fully populated, it is then processed in a closed-chamber wafer-bonding tool, which provides an oxygen-free environment. All of the dies are bonded in parallel using a high-temperature Cu-Cu bonding process to globally apply the required heat and force. Once the adhesive is heated past its critical decomposition temperature, it cleanly vaporizes away and allows the Cu-Cu bonding to proceed. We have successfully demonstrated this process on a 300mm platform using custom-designed test dies and wafers containing through-silicon via (TSV) chains and Kelvin test structures providing 4-point resistance measurements. Experimental results including TSV chain yield, electrical resistance, alignment accuracy, and cross-sectional analysis will be presented. A discussion will also be given on the potential cost savings and future technical challenges of this approach.