High Hydrogen Impurity—A hidden Role Aggravating PECVD Amorphous SiN_x Coating Cracking

Silicon nitride (SiN_x) film has been widely utilized in passivation coating and dielectric devices due to its chemical stability and excellent insulating properties. Films developed by the plasma-enhance chemical vapor deposition (PECVD) approach exhibit superiority with good uniformity and high deposition rate. However, under a normal PECVD process, residual stress usually builds up along with increasing SiN_x thickness ^[1]. High film stress will lead to SiN_x film cracking to restrict the practical applications. Though there are studies about SiN_x film stress aiming to improve film toughness, limited information has been dug out to build up the relationships between chemical composition, atomic arrangement, and film stress.<br/>In this work, we uncovered the hidden role of high H impurity in PECVD amorphous SiN_x films at the initial growth due to low deposition temperature in normal deposition conditions. Thin and thick SiN_x films grown on silicon (Si) wafers were selected to compare H impurity and bond configuration at different deposition stages. Both X-ray photoelectron spectroscopy (XPS) and Fourier-transfer infrared spectrum (FT-IR) spectrums indicated higher H impurity at the incipient growth as N-H bonds which were much shorter than the Si-N bonds as backbone of SiN_x matrix. Those shorter N-H bonds resulted in a shrinking tendency to induce extra tensile strain at the SiN_x/Si interface. Therefore, at critical thickness, the original SiN_x film exhibited severe cracking initiated from the SiN_x/Si interface and propagated vertically through whole SiN_x film. To avoid high H impurity concentration near SiN_x/Si interface, Si wafer was soaked in reaction chamber with sufficient time to make sure it was under stable deposition temperature. Correspondingly, SiN_x films were more homogenous with consistent atomic spacing. As a result, these long-soaking grown SiN_x films could tolerate stress with 42% fewer cracks film compared to original SiN_x films. In addition, crack length statistical analysis indicated that only ~14% of crack length was longer than 4 cm in long-soaking SiN_x compared to ~52% of crack length above 4 cm in original SiN_x film. This work provides a long-soaking strategy to regulate interfacial tensile strain attributed to unreacted N-H bonds and alleviate cracks in thick SiN_x PECVD coating.<br/><b>References:</b><br/>[1] X. Xu*, Q. He, T. Fan, Y. Jiang, L. Huang, T. Ao, C. Ma, Applied Surface Science, 2013, 264, 823-831.