Packaging of Silicon Carbide High Temperature, High Power Devices - Processes and Materials

Abstract

Silicon carbide (SiC) has unique electrical, thermal and physical properties compared to the Si and GaAs conventionally used in microelectronics as it can operate in the temperature range from 350ºC to 500ºC. However, there is a lack of reliable packaging techniques and materials for SiC, in particular substrates, die attach, die metallization and die passivation that can survive temperatures as high as 500ºC. Direct bond copper (DBC) Al2O3 substrates in which a thick Cu foil is cladded on Al2O3 has been used for power electronics for many years because of its acceptable heat dissipation capability, high current carrying capability, low coefficient of thermal expansion (CTE) and high mechanical strength. AlN has a very high thermal conductivity and a CTE approximating that of SiC (AlN = 4.5ppm/°C vs. SiC = 4.4ppm/°C [1]). However, with thick Cu metallization, the AlN is prone to fracture during thermal cycling due to its low flex strength and fracture toughness [2]. While Si3N4 does not have the thermal conductivity of AlN (Si3N4= 60W/m•K vs. AlN= 170-250W/m•K), the high fracture toughness of Si3N4 (2.4X AlN) allows thick Cu metallization and provides better thermal cycling performance. Die attach is another key issue for high temperature operation. This study examined a transient liquid phase bonding technique using eutectic Au-Sn braze with a thick Au (20µm) layer electroplated on the DBC Al2O3 substrate. After brazing and annealing at 400ºC, the Sn from the eutectic preform diffused into the substrate Au, lowering the Sn concentration to less than 10% and raising the braze joint melting point to over 400ºC. Au-Ge and Au-Ge-Ag braze alloys were also evaluated. Die metallization plays a role as an adhesion layer and diffusion barrier. It should not degrade during high temperature storage. A sputtered thin film stack of Ti/Ti-W/Au was examined to assess its performance, including its stability during high temperature storage. In high voltage applications, insulation is required to prevent the breakdown of SiC device passivation or arcing around the edge of the die. Phthalonitrile and polyhedral oligomeric silsesquioxanes (POSS) nanoreinforced polyimides were characterized as high temperature high voltage passivation materials. Ceramic capacitors, one of the passive components needed in a power electronics module, were also examined for high temperature applications up to 300ºC.