Laser Ultrasonic Testing (LUT) is a noncontact and nondestructive method for inspecting internal and surface defects of materials. Ultrasonic waves are generated by pulsed lasers, reflected or scattered by defects, and detected by an optical interferometer through surface displacements. One of the most significant challenges of LUT is improving the sensitivity of the interferometer. In this paper, we report the development of a highly sensitive interferometer and its application in detecting minute defects within the internal structure and on the surface of metals. The interferometer utilizes a fiber-optic Sagnac configuration, which incorporates a loop of polarization-maintaining fiber components. We enhanced the sensitivity not only by amplifying the optical source power to increase the signal-to-shot-noise ratio, but also by eliminating stray light in the optical head to minimize beat noise. The resulting sensitivity, evaluated by noise-equivalent surface displacement, is 1.1×10−6 nm/ √ Hz, which is nearly half of the best sensitivity achieved by commercial industrial interferometers currently used for ultrasonic measurement. We successfully applied the interferometer to two specific cases: the detection of artificial line defects with a diameter of 100 μm engraved on the backside of a 10-mm-thick SS400 steel, and the detection of surface cracks with a width of 0.5 μm and depth of 10 μm. These results demonstrate the potential of our LUT system in detecting minute defects.
In the field of microlithography, conventional computers are widely used for mask optimization. Recent progress of quantum and quantum-inspired computers has encouraged the development of quantum algorithms for numerous applications. So far, no method has been established for solving mask optimization problems with quantum computers. We introduced a simple model that describes the mask optimization problem as a quadratic unconstrained binary optimization (QUBO) problem, which is easily implemented on these computers. For simplicity, we assume there exists a target image profile on the wafer. The target can be the image of an existing mask or a virtual ideal mask which may be designed as a pixelated mask having a continuous transmission distribution. The solution is evaluated as the difference between the simulated image profile on the wafer surface and the target profile.
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