Phase Velocity Scanning Method Research Articles

Finite Element Method Analysis of Evaluation of Surface Micro Cracks Using Laser Ultrasound Generated by Phase Velocity Scanning Method

Nondestructive evaluation of defects that reduce material strength is critical to maintain the reliability of micro electro mechanical system (MEMS) components. We developed a new laser ultrasonic method, the phase velocity scanning (PVS) method, for this purpose. In a feasibility study, 200-µm-long micro standard defects (slits) on a Si wafer were fabricated with various depths (6.8 to 28.7 µm) and widths (2 to 5 µm), and scattered waves from a 6.8-µm-deep slit were successfully detected using 60 MHz surface acoustic waves. To analyze the directivity of acoustic waves generated by the PVS method and scattered waves from the slits, we developed 2D and 3D finite element method (FEM) programs. Using these programs, we confirmed that scattered waves are generated from a slit of 200 µm length, 10 µm depth, and 5 µm width. A comparison with the simulation of a conventional laser ultrasonic method showed that the PVS method has better directivity to suppress disturbing echoes generated in small objects.

Generation and Detection of 400-MHz-Band Surface Acoustic Waves Using the Phase Velocity Scanning Method

We succeeded in generating and detecting high-frequency surface acoustic waves (SAWs) up to 400 MHz by improving an apparatus for the scanning interference fringes (SIF) approach of the phase velocity scanning method using an optical knife-edge technique with a single lens. Using the new SIF apparatus, we confirmed that we are able to measure the SAW velocity anisotropy of a single-crystal Si(001) wafer within 0.1% relative error. We also measured the velocity anisotropy of Sezawa waves on a Si(001) wafer with a 1430-nm-thick Cu film and estimated Young's modulus and thickness of the Cu film by an inverse analysis. The estimated Young's modulus was close to that of bulk Cu (129.8 GPa). On a single-crystal Si(001) wafer with thinner Cu films (270–700 nm), we measured the SAW velocity along the [110] direction of the Si substrate by the time-of-flight method. The measured velocities decreased with increasing film thickness. The measured velocity for the thicker sample approximated the group velocity calculated from the estimated value for 1430-nm-thick Cu film by the inverse analysis.

Increasing the frequency of surface acoustic waves generated by phase velocity scanning of laser interference fringes

To nondestructively detect surface defects that reduce the strength of ceramics and structural materials of micromachines, we previously developed the phase velocity scanning method. By using scanning interference fringes generated by two laser beams whose frequencies are shifted, the method can generate a 110 MHz surface acoustic wave (SAW). However, the frequency of the SAW depends on the frequency of the Bragg cell used, thus making the upper-limit frequency about 110 MHz. In this study, we propose multiple use of Bragg cells in series, and successfully generated a 200 MHz SAW.

Surface Acoustic Wave Velocity and Attenuation Dispersion Measurement by Phase Velocity Scanning of Laser Interference Fringes

We developed a noncontact, nondestructive surface acoustic waves (SAW) velocity and attenuation measurement method in the frequency range of 30–110 MHz. This method is based on the phase velocity scanning method previously proposed by the authors that uses laser interference fringes scanned at the phase velocity of SAW to generate single-mode SAW with high intensity and directivity. In order to verify the validity and accuracy of the method, we measured SAW velocity and attenuation in Al and AISI 304 steel. The error in SAW velocity measurement was less than 1% in Al. Also, the attenuation was obtained with high precision in the AISI 304 steel. Then, we applied this method to porous silicon (PS) films on Si wafers as an example of a layered medium. The SAW velocity in PS films decreased with increasing porosity. By fitting the calculated dispersion curve to the measured one, the elastic stiffness of PS films was determined with a relative accuracy of 0.2%, which is thought to be the highest using laser ultrasonic methods. The attenuation of PS films was found to be 6–80 dB/cm in the frequency range of 30–70 MHz. The frequency dependence of attenuation was obtained with high precisely.

Analysis of excitation and coherent amplitude enhancement of surface acoustic waves by the phase velocity scanning method

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocity V is equal to the phase velocity νR of the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocity V, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity νf of the fringes is determined by the intersection angle and the frequency difference ωa of the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency ω depends upon a characteristic time t*, defined as a propagation time of the SAW across the laser beam spot. The SAW frequency ω is identical to the frequency difference ωa when the laser pulse width T is longer than the characteristic time t*. But, the SAW frequency ω is determined as a product kfνR of the wave number of the SIF and the SAW velocity when the laser pulse width is shorter than the characteristic time. Precise frequency measurement provided by the amplitude enhancement effect and the narrow frequency bandwidth in the SIF approach make the PVS method particularly promising for the noncontact SAW velocity measurement.