Studies of silicon-nitride (Si3N4) using laser ablation mass spectrometry

A reflectron time-of-flight mass spectrometer combined with pulsed laser ablation and post ablation ionisation techniques was used to investigate the ionic and neutral ablation products from powder and thin film samples of Si3N4 at 266 nm. Two prominent peaks at mq = 28 and 70 were always observed for both the ionic and neutral ablation products. However, on the basis of the isotopic abundance of Si, it is found that the mass peak at mq = 70 should be assigned as Si2N+ rather than (Si3N4)2+ as suggested by Takigawa and Hemminger [Appl. Surf. Sci. 79/80 (1994) 146]. The photo-fragmentation pattern of the neutral ablation product at mq = 70 also strongly supports this argument: when Si2N is dissociated by a 355 nm laser, it fragments into SiN and Si. This result is consistent with the experimental results of Iraqi et al. [J. Phys. Chem. 97 (1993) 11371] using neutralisation-reionisation collisional mass spectrometry. In addition, the ion yields of neutral ablation product Si and Si2N as a function of time delay between the post-ionisation and the ablation laser were also investigated. The result shows the experimental velocity distributions are broader than expected for a Maxwell-Boltzmann distribution.