Through Silicon Vias (TSVs) promise to provide shorter interconnect length and higher electrical connection density with reduced signal delay and power consumption for both multichip interconnection and packaging. To achieve void free electrolytic TSV gapfill, continuous seed is critical. While the TSV dimension scales down and the aspect ratio increases, continuous Cu seed coverage is harder to achieve by conventional PVD process. Giving the conformal nature of electrochemical process, direct electrolytic Cu deposition on barrier is an alternative to enable continuous seed coverage.
The challenges for direct wet seed are: (1) the barrier layer, typically TiN and TaNx, is too resistive to achieve uniform thickness distribution by conventional copper plating processes. (2) the nucleation of copper on barrier oxides is difficult and the copper growth is dendritic.
Designed for direct seeding on barriers, a novel chemistry has been developed. This alkaline (pH 9~10) bath has been demonstrated with TSV wafers with different barriers including Ta, TaNx, Ti, TiN. The process has shown to provide high nucleation density (10Exp11~10Exp12 nuclei/cm2) and continuous seed coverage can be achieved between 10 and 25nm thickness on most barriers. The deposition takes place at very negative plating potential while the current efficiency is maintained at >90%. The strong polarization enables in-situ barrier oxide reduction for some barriers, and pretreatment becomes not necessary. Elimination of oxides along with high nucleation density significantly enhanced the bonding strength between copper and barriers. The wet seed on all barriers including Ta, TaNx, Ti, TiN has passed ASTM D3359 cross-cut tape test. Electrolytic TSV gapfill on the wet seed is demonstrated for patterned TiN barrier TSV wafer. Void-free fills are achieved for the features with dimensions between 20 and 200 microns.
In summary, the novel chemistry for direct copper wet seed deposition on barriers has been demonstrated to yield high nucleation density and uniform seed distribution with adequate adhesion, and the wet seed enables subsequent void free TSV gapfill.
Downlink Connection Density Maximization for NB-IoT Networks using NOMA with Perfect and Partial CSI
