Theory And System Study Of Laser Generated Surface Acoustic Wave Method For Film's Young's Modulus Measurement

For the existing LSAW (Laser-generated surface acoustic wave), a technique used to measure thin film's Young's modulus, this dissertation improved it from two aspects including both theoretical calculation algorithm and experimental detection technique, in order to improve its measurement resolution, measurement efficiency and measurement automation, making it more suitable for thin film's, especially for Low-k dielectric film's Young's modulus detection. This dissertation completed the following works:1. Based on finite-difference time-domain (FDTD) method, aimed at LSAW's theoretical dispersion curve's calculation, its special symmetric characteristic was studied and its calculation's space domain specialty was explored, based on which a new Symmetric Boundary-Vary Space Step FDTD (SB-VSS FDTD) method was developed by this dissertation. An Isotropic Film/Substrate calculation model was proposed based on SB-VSS FDTD. Especially for the Low-k dielectric film incorporated with periodic nano pores, an Anisotropic Film/Substrate calculation model was developed. Film's porosity, Young's modulus, density, thickness, Poisson's rate's influence on SAW's dispersion curve was also studied.2. A Differential-Confocal Light-Deflection-Sensitive (DC-LDS) SAW detection system was designed and constructed. Its detection principle was studied and its key techniques were explained. The repeatability of this DC-LDS system was also tested. On this basis, the reliability of the system's measurement result was studied by comparing with that obtained by commercial nano-indentation instrument.3. An improved piezoelectric SAW detection system was designed and constructed, which works as a comparison testing system of DC-LDS. A Four-Quadrant piezoelectric plane SAW (FQ-PZT-pSAW) detection technique was proposed based on the theoretical study of the front characteristics of SAW excited by line source. This FQ-PZT-pSAW technique eliminated not only the direction deflection error of traditional PZT SAW detection system but also the fractionated measurement error of experimental dispersion curves, which ensured the reliability and sophisticate of the comparison testing system.4. Using hard film and porous Low-k dielectric film as the testing samples, a comparison test between the DC-LDS system and FQ-PZT-pSAW system was completed. The bandwidth of the two systems was compared. Based on the DC-LDS system's ability to detect high frequency SAW, a scheme that fitting dispersion curves in high frequency band was proposed to improve LSAW's measurement resolution. The experiment results showed that DC-LDS technique can improve LSAW's measurement resolution dramatically.5. The characteristic of SAW's dispersion curves of the same kind samples was studied. And it is proposed to evaluate curves matching goodness according to the extreme criterion of its autocorrelation sequence. On such basis, an autocorrelation-dichotomy trace-approaching (ADTA) algorithm was developed to calculate Young's modulus's measurement results automatically, which improved its computing efficiency logarithmically.