Applied Surface Science, Vol.284, 348-353, 2013
Fundamental properties of a-SiNx: H thin films deposited by ICP-PECVD for MEMS applications
In this study, the impact of deposition conditions on the properties of amorphous hydrogenated silicon nitride (a-SiNx:H) films using an inductively coupled plasma enhanced chemical vapor deposition technique (ICP-CVD) is evaluated. Due to the large number of experiments - even when taking only the most important synthesization parameters into account such as the total pressure in the deposition chamber, the substrate temperature, the ICP power and the flow rate ratio of N-2/SiH4 - a design of experiments-based approach is chosen. As expected, the deposition rate strongly depends on the ICP power and the N2/SiH4 flow rate ratio, respectively. Films in the field of investigation deposited with a high flow rate of N-2 labeled as Type I show relatively low mechanical stress values between -50 and +200 MPa, but exhibit a strong drift behavior toward compressive stress. Layers deposited at low nitrogen flow rates (Type II), however, yield large compressive stress and are stable as a function of time. The wet etch rate in hydrofluoric acid shows a gap of over two orders of magnitude when comparing the two a-SiNx: H types, indicating strong differences in the chemical composition. Fourier-transform infrared measurements demonstrate that in Type I films the hydrogen is mainly bonded to nitrogen, in contrast to Type II films, where Si-H bonds dominate. Surface related X-ray photoelectron spectroscopy measurements show that Type II layers have higher relative silicon content, while depth profiles yield that the oxygen content Type I films is above 10 at.%. This high oxygen content is proposed to be the result of diffusion of H2O into the layer, causing oxidation, and, as a consequence, the drifting behavior of the intrinsic film stress. (C) 2013 Elsevier B. V. All rights reserved.
Keywords:ICP-CVD;Silicon nitride;Deposition rate;Residual stress;Wet etching;Infrared spectroscopy;X-ray photoelectron spectroscopy