Reliability and Challenge of SiC Substrate Thinning

In recent years, substrate thinning has become a clear development direction in the evolution of SiC MOSFET technology, and multiple industry-leading companies have successively launched corresponding thin substrate device products. PAM-XIAMEN can provide SiC substrate & epitaxy thinning processing services, for specific processing capabilities, please consult [email protected]. Thinned SiC substrates can effectively reduce conduction resistance and thermal resistance while maintaining the breakdown voltage of the device at a relatively constant level. The mechanism of decreasing on resistance is relatively direct: a decrease in substrate thickness means a reduction in its own resistance component, which in turn drives the overall resistance to decrease. Thermal resistance describes the obstacles encountered during the process of heat dissipation from the junction region to the outside of the device. Thinning substrate shortens the heat conduction path, thereby improving heat dissipation efficiency and reducing thermal resistance.

1. Thinned SiC Wafer in Device

In a typical SiC device structure, the wafer is mainly composed of heavily doped substrate and lightly doped epitaxial layer, with a conventional total thickness of about 350-380um. The thickness of the epitaxial layer is related to the withstand voltage level. For example, 1200V devices usually require 10-12um of epitaxial layer, while the rest is the substrate part.

The breakdown voltage is mainly determined by the thickness and doping level of the lightly doped drift region. The substrate itself contributes very little to the withstand voltage capability, but introduces additional thermal resistance and a certain resistance. Taking JBS diodes as an example, when the substrate is thinned to 150um, the average forward voltage drop decreases by about 0.08-0.1V. Therefore, while maintaining sufficient mechanical stability, reducing the substrate thickness can help improve the thermoelectric performance.

At present, some companies have achieved mass production of SiC wafers with a thickness of 110um, and the most advanced technology can reach 70um. The mainstream thickness range in the industry is 150-175um.

 2. Reliability Challenges of Device based on Thinned SiC Wafer

Thinned wafers have also introduced some engineering challenges. On the one hand, the weakening of substrate mechanical strength may make it difficult for it to withstand stress caused by solid pressure or temperature changes during the packaging stage, thereby affecting reliability. Especially in harsh working conditions such as automotive electronics, thin substrate structures are more prone to damage in the face of vibration or impact. On the other hand, the mismatch in thermal expansion coefficient between the substrate and other materials further increases the interface thermal stress after thinning, which also increases the probability of device damage.

3. Warping Mechanism Caused by Thinning

It is worth noting that thinning treatment may not only weaken the mechanical strength of the wafer, but also lead to warping deformation. Further research has shown that the wafer center bow (C-bow) generated by 4H-SiC thinning is closely related to its surface morphology. Researchers thinned a 350um thick 4H-SiC substrate to a thickness of 180um, and observed and statistically analyzed dislocations in the 4H-SiC substrate by KOH wet etching technology. They found a clear correspondence between C-bow and dislocation density.

In addition, high-resolution full wafer images were obtained through a fully automated optical microscope, with detailed records of dislocation counting and classification. Bottom dislocations also have a certain correlation with C-bow. This study suggests that dislocations, as an important pathway for releasing internal stress in thin films, typically SiC wafer have lower C-bow values as their density increases. This mechanism provides an important microscopic explanation for understanding the warping behavior of crystal circles during the thinning process.

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