The influence of Hydrogen and Nitrogen impurities on the annealing behaviour of undoped silicon glass

Abstract

In today's semiconductor industry materials and deposition techniques have to fulfill the sophisticated product requirements of modern microelectronic devices. To meet the quality standards of the desired products thin film deposition process have to be tuned very accurately to any avoid contaminations and incorporations of chemical impurities. For that reason a wafer curvature instrument was developed enabling the characterization of the thermomechanical behavior of semiconductor materials from room temperature up to 1000°C. This allowed the investigation of the stress evolution of nitrogen and hydrogen doped undoped silicon glass (USG) SiO2, films during an annealing process which are performed at the same temperatures as industrial deposition. Based on the stress-temperature results and complementary characterization techniques of infrared spectroscopy, nanoindentation, ellipsometry, light microscopy and atomic force microscopy a qualitative model was developed to describe the influence of nitrogen and hydrogen in terms of the stress evolution and resulting crack formation. Films A, B, C and D were USG films deposited via PECVD process with a standardized thickness of 1000 nm. Films B, C and D represent an improved SiO2 film (B), a production silane based USG (C), and a production TEOS based USG film (D). All of the films have a qualitative film composition of about SiOxHy. Using higher silane gas flow and additional ammonia gas flow, Film A resulted in SiOxNyHz film composition. During a thermal cycle nitrogen and hydrogen impurities caused massive tensile stress generation to occur at 550°C leading to maximum tensile stresses of about 1GPa at 1000°C. Crack formation occurred between 700°C and 800°C due to a tensile stress between 430 MPa and 780°C. After one thermal cycle the elastic modulus of Film A increased 38% up to 97 GPa, while the film thickness decreased 11%. The investigation led to the qualitative model where remaining trapped nitrogen and hydrogen species are released beginning at 550°C due the build-up of thermal stresses. Further stress increase is caused by hydrogen bond cleavage from Si-H and N-H species starting around 700°C. Intrinsic stresses are generated by following cross linking reactions between silicon and nitrogen species which leads to massive tensile stresses that result in film cracking after exceeding a critical film stress. The qualitative composition of Film A after one thermal cycle became SiOxNy indicating a silicon oxynitride film with almost no remaining hydrogen. The hydrogen species was the origin of large tensile stress generation and nitrogen species was the dopant agent leading to mechanically harder and stiffer films where the cross linking reactions induce compressive stresses due to the different binding nature between nitrogen and silicon atoms compared to oxygen atoms in USG.