Evaluation of standard defects using surface acoustic waves generated by phase velocity scanning of laser interference fringes

In order to maintain the reliability of structural materials such as ceramics and micromachines, it is important to nondestructively evaluate micrometer-order surface defects that reduce the strength of these materials. Standard defects whose size and shape are precisely defined are needed but until now, only Vickers and Knoop indentation cracks were available. However, since indentation cracks are created stochastically, it is difficult to define their precise size and shape. Alternatively, we tried to make standard defects on a Si (001) wafer using the focused ion beam machining process. Four grooves of 200 μm length and 6.8, 7.8, 12.4 and 28.7 μm depth were formed by a 90-nm-diameter Ga ion beam on Si surface. We used iodine gas for making fine edges. We then tried to evaluate these defects by using the phase velocity scanning (PVS) method which generates surface acoustic waves (SAW) with excellent directivity. We used 60-110 MHz SAW generated by the PVS method and the distance from defects to probe of SAW was about 2 mm. We succeeded in detecting the scattered wave from the 6.8-μm-deep slit (d/λ approximately 0.08) and confirmed the usefulness of these standard defects. Next, we compared the theoretical reflection coefficient of an infinite-length crack with our experimental result and confirmed that they agree qualitatively.