High Temperature High Power SiC Devices Packaging Processes and Materials Development
Type of DegreeDissertation
Electrical and Computer Engineering
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Silicon power devices have reached their theoretical limits in terms of higher temperature and higher power operation by virtue of the physical properties of the material. SiC has been identified as a material with the potential to replace Si devices because of its superior material advantages. However, there is a lack of reliable packaging techniques and materials for SiC, in particular die attach, wire bonding and die passivation that can survive temperature as high as 500ºC. Based on the high melting point of Au-In alloy (81/19 wt%), it was evaluated as a potential high temperature die attach material using a transient liquid phase bonding process in this study. Thermal cycle test results over the temperature range from 35ºC to 400ºC and high temperature storage at 450ºC results are presented. Vertical cracks developed in the die attach on Mo tabs during the thermal cycling tests and indium segregated to the defects (voids and cracks) during the high temperature storage and thermal cycling tests. This segregation appeared to negatively impact the reliability of the die attach. The 6µm of nickel or nickel phosphorous commonly used as a barrier layer in conventional ceramic substrate metallization did not prevent Cu diffusion to the surface at a temperature of 450°C. A multi-layer nickel phosphorous structure was found to serve as a good barrier to prevent Cu diffusion for high temperature applications. The bondability and reliability of large diameter (250µm) gold and platinum wire using thermosonic wedge bonding was investigated. High temperature storage results at 350ºC for wire bonds on the substrate metallization and 300ºC for die metallizations are presented. A simplified FEMA 2D model was used to understand the effects of bond force and die metallization structure on the failure modes, SiO2 cracking and SiC cratering. The results matched the experimental results very well. This work demonstrated the effects of wire and pad stack metallurgy on bond reliability. Polyimide PI2611has been evaluated as a passivation coating material. The results were promising at 300°C; however, higher temperature tests have shown rapid decomposition of the polyimide. In this work, electrical characteristics of VJFET and SIT diode modules were measured over the temperature range from 25°C to 400°C to demonstrate the feasibility of paralleling SiC power VJFETs to develop Si IGBT replacements. Paralleled VJFETs formed an equivalent switch of much greater current than the single VJFET, and showed a positive temperature coefficient of on-resistance. The paralleled SIT diodes resulted in a lower cut-on voltage than the single SIT diode, and the leakage current of the paralleled SIT diodes was less than 70 µA when Vds = 100 V at 400°C, validating the impressive blocking performance of the SIT diode at extreme temperatures.