Non-destructive Evaluation Of Mechanical Strength And Interfacial Adhesion For UlSI Interconnect Films By LSAWs

The poor mechanical strength and interfacial adhesion of low-k film limit its application in modern ultra-large scaled integrated circuit (ULSI) fabrication. Hence, fast, accurate and non-destructive technique should be developed to characterize the mechanical properties of ULSI interconnect films. Laser-generated surface acoustic waves (LSAWs) technique utilizes the dispersion phenomenon of surface waves propagating in layered structure to determine the Young’s modulus, Poisson’s ratio and interfacial adhesion of low-k films without destruction. This technique is a promising approach for the online detect of ULSI interconnect dielectrics.In theory, wave motion equation of surface waves propagating in solid media was induced, and a fast algorithm to calculate the frequency-velocity dispersion curve was proposed. Single- /Multi-layered low-k films/ Si substrate model was established to obtain the relationship between the dispersion property of surface waves and the mechanical parameters of films. Due to the different performance of dispersion curves with variation of Poisson’s ratios when surface waves propagated along different directions, a two-direction measurement was proposed to determine the Young’s modulus and Poisson’s ratio of ultra-thin films. The interfacial adhesion was described by introducing the spring boundary model which located between the film and the substrate.In experiment, the whole optical system was optimized, and the effect of laser source for the surface waves was analyzed. Electrical translation equipment was employed in the experimental system to improve the level of automation. The external interference was reduced by applying copper net as the electromagnetic shielding, and surface waves signals with signal-to-noise ratio up to 50: 1 was achieved. After the fast Fourier transformation (FFT) to the processed signals, smooth dispersion curves of surface waves were obtained.In the aspect of experimental and theoretical dispersion curves matching, a new method was improved based on the original least square matching method. This approach can revise the error in the measurement, which makes the results more accurate. In this paper, three low-k film sample sets were characterized. First, porous SiO2 sample with Cs doping was measured, and the result indicated that the Young’s modulus of low-k films increased with the increase of Cs doping concentration. Second, the Young’s modulus and Poisson’s ratio of ultra-thin porous Black DiamondTM films were determined by two-direction measurement. Finally, the adhesion property of low-k films was characterized, and compared with the traditional scratching method, which verify the accuracy of LSAWs technique. The measured results indicate that LSAWs technique can be successfully applied in the determination of mechanical properties of ultra-thin low-k films.