Detection Limit of TEM/STEM-EDS
Abstract: Limit of detection (LOD) for As dopant in Si wafer was explored using a transmission electrom microscope (TEM) equipped with two large-sized silicon drift detectors. The LODs of As were estimated to be 30 ppm (1.5 x 1018 atoms/cc) with As(Kα) line and 70 ppm(3.5 x 1018 atoms/cc) with As(Lα) line based on the results of secondary ion mass spectroscopy. These concentrations come into a range of practical dopant concentration in a semiconductor device. In case where there are two lines such as K and L of As in an X-ray spectrum, LOD is lower for a line with lower background than a line with higher background. To avoid increasing background of spectra due to contamination during acquisition of X-ray, it is effective to apply plasma or ion cleaner to specimen before installing into the TEM. We measured LODs at 200 and 100 kV to be 30 ppm and 20 ppm respectively. These values tell that LOD is improved at lower accelerating voltage as well as longer acquisition time, larger beam current, since the X-ray generation probability is higher at lower acceleration voltage, though the background is higher for a line at lower accelerating voltage.
Key words: Transmission electron microscopy, Energy dispersive X ray spectroscopy, Limit of detection