High reliable strain measurement for power devices using STEM-CBED method

Scanning transmission electron microscopy convergent beam electron diffraction (STEM-CBED) was applied to strain analysis of deep trench electrodes for devices such as power devices. Source-drain current leak, which was one of the crucial failures of this kind of structure, depends on boron concentration in boron doped poly-Si (BP) layers. TEM/STEM and diffraction analysis showed that the BP layers consist of epitaxial phase and poly-Si phase, and the proportion of these phases depends on the boron concentration in the BP layer. Clear strain distribution around the BP layers was obtained with STEM-CBED. This revealed that the origin of the strain is volume shrinkage of the epitaxial phase in the BP layer, and the poly-Si phase acts as buffer against this strain. Relationship between Si phases and boron concentration in the BP layers was examined with STEM and scanning capacitance microscopy. These analyses suggested that boron segregation occurred in samples having a higher boron concentration, and prevented epitaxial growth in the BP layers. As a result, the core of the BP layer remains as poly-Si or amorphous Si and acts as strain buffer. Our analysis concluded that boron concentration in the BP layer is one of the most important factors enabling high production yield for the structure.