Study On Residual Stress And Roughness Of ULSI Interconnection Film By Surface Acoustic Wave

Laser-induced surface acoustic wave(LSAW)technology is widely used to measure Young's modulus,thickness,adhesion and other properties of thin films because of its rapid,nondestructive and quantitative advantages.Surface acoustic waves(SAWs)are dispersive on the layered structure.SAW method can characterize the mechanical properties of thin films by matching the experimental dispersion curve with the theoretical dispersion curve cluster.In the previous studies,the theoretical dispersion curve of surface wave is calculated by Green's function and matrix method,and residual stress and surface and interface roughness of the film are ignored.Therefore,in this paper,the residual stress and roughness in the SAW technology are deeply studied,which expands the application range of surface acoustic wave technology and improves the reliability of SAW technology.Based on the theory of acoustic elasticity,the residual stress model of surface wave propagation in layered structure is established,and the theoretical dispersion curve of SAW under different residual stress is obtained.The residual stress of a group of silica thin films is measured by LSAW measurement system,and compared with the measured results of curvature method,the correctness of the established residual stress model and the feasibility of surface acoustic wave method to characterize the residual stress of thin films are verified.The influence of residual compressive stress and residual tensile stress on the mechanical properties of silica films by SAW is studied.It is found that the influence of residual tensile stress on the characterization of Young's modulus is less than 5%,which can be ignored;when the film thickness is 1000 nm and the residual compressive stress is-400 MPa,when the SAW propagates along the Si[110] and Si[100] crystal direction,the influence of residual stress is 3.9% and 5.36%,respectively.Through a lot of numerical analysis,the application range of the ideal theoretical calculation model and the modified formula of Young's modulus measurement by the ideal theoretical model are obtained,so as to reduce the influence of residual stress on the SAW measurement.In addition,it is found that when the SAWs propagating along the Si[110] direction,the influence of residual stress on the measurement is smaller than that of the SAWs propagating along the Si[100] direction.By means of Monte Carlo method,random surfaces with different roughness degrees are simulated by changing two key parameters,root mean square height and correlation length.Through the finite element method,the rough model of surface wave propagation is established.Through the modal analysis,the influence of different rough surfaces and interfaces on dispersion curve and Young's modulus measurement is studied.The results show that the influence of surface and interface roughness on the surface wave method for characterizing the Young's modulus of silica film and low-k film are less than 0.5 GPa and 0.05 GPa,respectively,which can be ignored.