The Design, Fabrication, and Characterization of Waffle-substrate-based n-channel IGBTs in 4H-SiC

Abstract

Power semiconductor devices play an important role in many areas, including householdappliances, electric vehicles, high speed trains, electric power stations, and renewable energyconversion. In the modern era, silicon based devices have dominated the semiconductormarket, including power electronics, because of their low cost and high performance. Theapplications of devices rated 600 V - 6.5 kV are still dominated by silicon devices, but theyare nearly reaching fundamental material limits. New wide band gap materials such as siliconcarbide (SiC) offer significant performance improvements due to superior material propertiesfor such applications in and beyond this voltage range. 4H-SiC is a strong candidateamong other wide band gap materials because of its high critical electric field, high thermalconductivity, compatibility with silicon processing techniques, and the availability of highquality conductive substrates.Vertical DMOSFETs and insulated gate bipolar transistors (IGBT) are key devices forhigh voltage applications. High blocking voltages require thick drift regions with very lightdoping, leading to specific on-resistance (RON,SP ) that increases with the square of blockingvoltage (VBR). In theory, superjunction drift regions could provide a solution because of alinear dependence of RON,SP on VBR when charge balance between the pillars is achievedthrough extremely tight process control. In this thesis, we have concluded that superjunctiondevices inevitably have at least some level of charge imbalance which leads to a quadraticrelationship between VBR and RON,SP . We then proposed an optimization methodology toachieve improved performance in the presence of this inevitable imbalance.On the other hand, an IGBT combines the benefits of a conductivity modulated driftregion for significantly reduced specific on-resistance with the voltage controlled input of aMOSFET. Silicon carbide n-channel IGBTs would have lower conduction losses than equivalentDMOSFETs beyond 6.5 kV, but traditionally have not been feasible below 15 kV. Thisis due to the fact that the n+ substrate must be removed to access the p+ collector of theIGBT, and devices below 15 kV have drift layers too thin to be mechanically self-supporting.In this thesis, we have demonstrated the world’s first functional 10 kV class n-IGBT witha waffle substrate through simulation, process development, fabrication and characterization.The waffle substrate would provide the required mechanical support for this class of devices.The fabricated IGBT has exhibited a differential RON,SP of 160 mohm.cm2, less than half ofwhat would be expected without conductivity modulation. An extensive fabrication processdevelopment for integrating a waffle substrate into an active IGBT structure is describedin this thesis. This process enables an entirely new class of moderate voltage SiC IGBTs,opening up new applications for SiC power devices.