A Stable Cu Nanoparticles Used for Seed Layer Deposition of through Silicon Via

In recent years, the evolution of two-dimensional (2D) chip packaging will soon reach its limitation. As a result, significant attention is being paid to three-dimensional (3D) chip stacking. 3D chip stacking offers many benefits to improve system performance, the most significant of which is the ability to directly connect heterogeneous devices in the vertical direction. Thus, 3D chip stacking can reduce the size of packaging and increase system efficiency as well as saving capacitance, reducing power and increasing speed.

Through silicon via (TSV) is a key technology for 3D chip stacking, which serves as a bridge to connect multiple devices in z-direction. Comparing with the traditional through silicon via (TSV) metallization, it has to thin the wafer thickness by using chemical mechanical polishing (CMP) after the copper electroplating and go on the chip stacking process. Alternatively, wafer thinning can be processed first before making through holes. Following the wafer thinning and through hole drilling, the through silicon hole (TSH) can be fully filled with copper by direct electroplating without using a conducting template. The metallization of TSHs is a critical step in the manufacture process. Up to known, the traditional semiconductor industry uses sputtering process to deposit a barrier layer and a seed layer on a isolation layer (SiO₂) inside the TSH. However, the sputter machine is very expensive. We introduce a wet process to replace the dry seed layer deposition process.

We made use of a flexible substrate to synthesize copper nanoparticles (CuNPs). The mean particle size of the CuNPs is below 10 nm and its size distribution is from 4 to 6 nm(Fig 1).Therefore, they perform a high activity in Cu electroless deposition. However, the CuNPs were unstable when they exposure to air, such that their activity decreased day by day. For this issue, we use a novel stabilizer to make the CuNPs survive for a long time(Fig 2) and still have good activity to catalyze copper eletroless deposition. Because silicon oxide is a non-conducting material, it has to be metalized by Cu electroless deposition and then by Cu electroplating. For the Cu electroless deposition process, we employed the CuNP as a catalyst for copper electroless deposition onto the sidewall of TSH in order to develop a wet TSH process. These processes ensure the complete electroless copper coverage inside the THS, which is coherent and conductive enough to allow TSH to be fully filling with Cu electroplating.