In-situ inspection of stealth laser diced internal crack lines in silicon wafer using cepstrum signal processing with terahertz electromagnetic waves

Abstract

In this study, terahertz (THz) technology was used to inspect internal crack lines created by stealth laser dicing in silicon wafers. Stealth laser diced wafers with different numbers and depths of diced lines were inspected using a terahertz time-domain spectroscopy (THz-TDS) system in reflection mode. The amplitude of the bottom pulse decreased as the number of stealth laser dicing lines increased. However, direct identification of the dicing line signals was challenging due to their low amplitude. To overcome this limitation, a cepstrum-based signal processing method was developed to enhance the weak internal crack-line signals. The densities and depths of stealth laser dicing lines were successfully analyzed using bottom signal amplitude variations and cepstrum peaks, respectively. The depth and number of dicing lines evaluated by THz signals were validated against optical microscopy, with repeatability and accuracy quantified. The methodology developed in this study provides a non-destructive and highly precise inspection technique for semiconductor manufacturing, enabling the detection and analysis of micro-defects that were previously challenging to identify.