Spin-coated polymer dielectrics are widely used in 3D IC integration for the advantages of low cost, low relative permittivity (leading to low parasitic capacitance and low resistance-capacitance delay) and the convenience in planarization (especially for multilevel metallization). However, there are several disadvantages for the use of the polymer as RDL dielectric. One serious drawback is its poor thermal conductivity that works as a thermal barrier to heat flow. As the integration density increases, more localized heat is produced and reliability problems become critical. The low thermal conductivity of the polymer dielectric aggravates these problems related to thermal concentration and thermal stress [11; 12]. It is worth noting that, although the dielectric layer occupies only a small fraction of the Si interposer in thickness, it provides the majority of the thermal resistance through the heat conduction path (R\({}_{\text{interposer}}\) = R\({}_{\text{Rdl}}\) + R\({}_{\text{sub/TSV}}\), as illustrated in Figure 1b). According to the study presented by [PERSON] et al. [13], the 3 \(\upmu\)m thick polymer dielectric film of the chip has a thermal resistance (2.31 \({}^{\circ}\)C/W) four times larger than that of the Si substrate of 500 \(\upmu\)m thick (0.46 \({}^{\circ}\)C/W). This indicates that more than 80% of the thermal resistance is provided by the polymer dielectric in this polymer dielectric/Si substrate structure. Thus, improving the thermal conductivity of the polymer dielectric is crucial to the heat dispassion of Si interposer. Another disadvantage of the polymer dielectric is its large coefficient of thermal expansion (CTE) (e.g., PI: \(\sim\)30-60 ppm/\({}^{\circ}\)C) compared with the interposer substrate (Si: 2.6 ppm/\({}^{\circ}\)C) and routing layer (Cu: 17 ppm/\({}^{\circ}\)C) [9]. The CTE mismatch among RDL dielectric, substrate and routing layer intensifies the thermal stress in the 3D integration [14; 15]. Therefore, spin-coated polymer-based dielectrics with high thermal conductivity and low CTE has very good application prospects in the 3D IC integration. At present, there are many studies focusing on improving the thermal conductivity of polymer by adding in high thermal conductivity particles (also lead to a drop in the CTE due to the low CTE of the particles) [16; 17; 18; 19; 20; 21; 22; 23; 24]. [PERSON] et al. proposed a BCB-carbon nanotube (CNT) composite as a bonding interface material [16]. An 80% improvement in thermal conductivity is achieved with the CNT concentration at 1.5 wt%. [PERSON] et al. developed a novel polymer composite as thermal interface material with silicone matrix and fillers of diamond and hexagonal boron nitride platelets [17]. The composite has a maximum thermal conductivity of 6.1 W/m-K with the filler concentration at 90 wt%. [PERSON] et al. presented a PI/AlN composite with an enhanced thermal conductivity of 0.8 W/m-K compared with 0.22 W/m-K of PI [18]. From these studies, it can be concluded that the thermal conductivity of the polymer can be significantly improved by adding in high thermal conductivity particles. However, the composites presented in these studies are mainly developed for bonding interface, thermal interface, surface coating or electrical packaging, most of which are not suitable to be used as RDL dielectric. For example, in [PERSON]'s research, the insulation property of the composite is impaired by the CNT fillers, which will induce a large leakage current when the composite is used as RDL dielectric. For the composite developed by [PERSON], the size of the diamond particles (more than 9 \(\upmu\)m) is too large for the thin dielectric layer and the viscosity of the composite is too high for spin-coating process (due to the  
Figure 1: (**a**) Si interposer integration and (**b**) its heat conduction path. high filler concentration).