Technology for Controlling Trench Shape in SiC Power MOSFETs

As the worldwide consumption of energy is increases and the global environment steadily deteriorates, society must find more efficient ways to utilize energy in order to achieve sustainable advances. Accordingly, the field of power electronics is of vital importance, and huge advances in power semiconductor devices are anticipated. Power semiconductor devices that use silicon (Si) are said to be approaching the limits of their inherent characteristics, and although technical developments such as the SJ-MOSFET (super junction metal-oxide-semiconductor field-effect transistor) and FS-IGBT (field stop insulated gate bipolar transistor) have resulted in improved characteristics, further improvement will be difficult to realize. Silicon carbide (SiC), a compound that combines silicon and carbon, has physical properties that are superior to those of Si, and is capable of achieving high voltage, low on-resistance, low loss and high-speed operation. The application of SiC devices enables power supplies to be made smaller in size, have lower loss, operate at high temperatures (with a simplified the cooling mechanism), and also enables electric power energy to be utilized more effectively. As devices that use SiC material, some Schottky barrier diodes are being sold commercially, and as switching devices, MOSFETs have been much researched, but are not yet being used in practical applications. Trench-type MOSFET (UMOSFET) devices promise to realize lower on-resistance than the planar types, but require SiC-specific dry etching technology. Being physically hard and chemically stable, SiC material is difficult to etch, and even with an etching machine that uses high density plasma, the etching rate is slow and the etching shape is difficult to control. A trench sidewall or bottom surface formed by dry etching exhibits surface roughness and areas with sharp angles remain at the opening portion. In a UMOSFET, because a channel is formed in the trench sidewall, the smoothness affects electron mobility and the presence of sharp angles in the trench invites decreased withstand voltage capability due to the concentration of electric fields. Thus, smoothing of the trench inner wall and rounding of the corner shapes of the trench opening and bottom are essential. However, it is difficult to obtain an ideal shape and smoothness by only optimizing the dry etching conditions. On the other hand, high-temperature annealing in a hydrogen (H2) atmosphere is known to improve the trench shape and smoothness of Si trench devices.(1)(2) SiC is also stable at high temperatures, and there have been no reports of this type of approach applied to SiC. This paper examines technology for simultaneously performing high-temperature annealing of SiC trenches and performing process optimization in order to control trench shape and improve sidewall smoothness.