## 4 Application as Interposer RDL Dielectric  
To verify the feasibility of the composite to be used as interposer RDL dielectric, interposer test samples were fabricated. Moreover, to evaluate the effects of the composite on the improvement of interposer properties, thermal-mechanical simulations and thermal distribution tests were performed. The fabrication process of the interposer RDL is shown in Figure 8. The routing layers were fabricated by Cu electroplating and the dielectrics were formed by spin-coating. It is worth noting that the PI matrix was no longer patentrable due to the insoluble reinforcement materials. Thus, holes (openings in the dielectric to connect the adjacent routing layers) could not be formed by patterning. An electroplating-coating-polishing process was developed for the connection of the adjacent routing layers. First, connecting pillars were fabricated on the lower routing layer. Then the composite was \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Thermal** & **Coefficient of** & **Tensile** & **Young’s** & **Elongation** & **Relative** & **Volume Resistivity** \\  & **Conductivity** & **Thermal Expansion** & **Strength** & **Modulus** & **at Break** & **Permitivity** & **at 1 MHz** \\  & **(W/m-K)** & **(ppm/\({}^{\circ}\)C)** & **(MPa)** & **(GPa)** & **(\%)** & **at 1 MHz** & **(\(\Omega\)-cm)** \\ \hline PI & 0.19 & 55.6 & 81 & 2.5 & 7.2 & 3.5 & \(4.3\times 10^{7}\) \\ DN/S-CW/PI & 1.63 & 16.7 & 145 & 8.4 & 3.3 & 4.6 & \(2.1\times 10^{7}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The properties of the composite (10 wt% DNs, 7 wt% SCWs) compared with the PI matrix. Figure 7: Coefficient of thermal expansion (CTE) of the composite versus different DN concentrations. spin-coated to cover the routing layer and the connecting pillars. At last, a polishing process was performed to reveal the connecting pillars for the connection with the upper routing layer. Interposer samples with both PI dielectric and the composite dielectric (10 wt% DNs and 7 wt% SCWs) were fabricated, as shown in Figure 9. Cu routing layers, substrates and TSVs of the two samples were exactly the same. TSVs were arranged on the periphery of the interposer and two Cu routing layers were fabricated on the front side. It can be seen that, the DN/SCW/PI composite dielectric is opaque, unlike the transparent PI dielectric. The specifications for the components of the interposer are listed in Table 2.  
Finite element analyses (FEA) were performed to investigate the temperature distribution, von Mises stress distribution and the warpage of the two different interposers. Comparisons were made to evaluate the effects of the composite dielectric on the properties enhancement of the interposer. In the simulation model, a heating chip with a power density of 20 W/cm\({}^{2}\) was assembled on the interposer. The heat was supposed to dissipate from the backside of the interposer which was set to be at the environment temperature of 17 \({}^{\circ}\)C. The thermal convection and radiation were neglected in the simulation, since the die surface in a real packaged chip is always covered with a thick layer of protecting polymer which hinders the thermal convection and radiation. As shown in Figure 10, the DN/SCW/PI composite RDL dielectric significantly enhanced the thermal-mechanical properties of the interposer compared with the PI dielectric. The maximum temperature of the interposer could be reduced from 47 \({}^{\circ}\)C to 26.8 \({}^{\circ}\)C and the temperature difference (Tmax - Tmin) decreased from 20 \({}^{\circ}\)C to 9.8 \({}^{\circ}\)C, mainly due to the significant increase of the thermal conductivity of the dielectric material. The maximum von Mises stress decreased from 72.8 MPa to 16.9 MPa and the warpage decreased from 1.13 \(\upmu\)m to 0.15 \(\upmu\)m. These two improvements could be attributed to three factors, the reduction of the thermal gradient, the reduction of the CTE and the increase of the Young's modulus of the dielectric [15].