S-EVC Method for Sorting Wafers with Defects that Extend to Bar Shaped SSFs

Abstract

In the previous report [1], we proposed the S-EVC (Selective Expansion-Visualization-Contraction) method (Fig. 1) that effectively screens for malignant BPDs (basal plane dislocations) in the drift and buffer layers, which expand to SSFs (Shockley-type stacking faults), leading to forward voltage degradation. The method intentionally utilizes the REDG (recombination enhanced dislocation glide) mechanism by UV (ultraviolet) irradiation in wafer sorting to replace the so-called burn-in (accelerated current stress) process, which is time-consuming during mass production. In the report, triangular SSFs were examined to verify the effectiveness of the method, but they only occupy a much smaller area of the active region on the chip than bar shaped SSFs. In this study, to improve the S-EVC method to be more practical, we focused on the more serious bar shaped SSFs which have a non-negligible impact on electrical characteristics. The bar shaped SSFs are mostly expanded from TED (threading edge dislocation)-converted BPD at or below the substrate epitaxial layer interface. In PL (photoluminescence) observation by a 710 nm LPF (long-pass filter), the TED-converted BPD and the complete TED extended from the bottom of the substrate are observed as the same dark spot, but it was confirmed that both can be distinguished by the presence or absence of their SSF expansion by UV irradiation. In addition, in order to confirm the validity of the S-EVC method even on the virgin epi wafer, UV irradiation was performed on both the aluminum doped PN structured wafer and the virgin epi wafer, and the similar SSF expansion was observed. Meanwhile, the correlation between UV irradiation and forward voltage degradation was quantified using PiN diodes by comparing the glide velocity of 30°Si (g) core partials for bar shaped SSFs by UV irradiation stress with that by current stress.