As one of the fastest-growing technologies over the past half century, integrated circuit (IC) packaging is getting smaller and more complex. For example, typical silicon wafers in modern IC packaging have thicknesses ranging from several to tens of micrometers, and their coating layers are in the range of a few nanometers. Because the silicon wafer is the main substrate in IC packaging, it is important to accurately measure the geometry of a silicon wafer, especially its coating thickness, for process monitoring and quality control. In this study, an ultrafast ultrasonic measurement system is developed using a femtosecond laser for silicon wafer coating thickness estimation. The proposed technique provides the following unique features: (1) an ultrafast ultrasonic measurement system using a femtosecond laser is developed specifically for silicon wafer coating thickness estimation; (2) the developed system can estimate the thickness of a coating layer in the range of sub-micrometer; (3) except for the wave speed in the coating material, coating thickness can be estimated without any other prior knowledge of the coating material properties or substrate characteristics such as optical constants; and (4) the thermal effects on the ultrasonic waves propagating within a thin coating layer are explicitly considered and minimized for coating thickness estimation. Using the developed system, validation tests were successfully performed on gold-coated silicon wafers with different coating thicknesses.

For the non-destructive inspection of carbon fiber-reinforced plastic (CFRP), lasers can be used to generate ultrasonic waves. It is important to optimize the wavelength of the laser to ensure the intense excitation of a usable propagating mode. Real CFRP components used in the construction of airplanes and automobiles are often coated with several types of resin to protect against weathering. These resin layers change the excitation of the ultrasonic waves. Thus, the optimum laser wavelength may be changed by the coating resin. In this paper, we investigated the excitation of ultrasonic waves in a resin-coated CFRP plate using different laser wavelengths. We conducted experiments to convert the laser wavelength using periodically poled LiNbO 3 (PPLN) devices. By injecting mid-infrared laser to a coated sample, we observed excited ultrasonic waves using a laser Doppler vibrometer. We found that transparent resins significantly increase the amplitude of the first-arriving longitudinal wave. Furthermore, when the laser was strongly absorbed in the surface layer, the excitation of longitudinal waves was suppressed. These results were clarified by a one-dimensional model of the thermal generation of ultrasonic waves. We concluded that a laser passing through a resin layer is a viable candidate for the effective inspection of coated CFRP by laser ultrasonic waves.

Aiming at characterizing interfacial roughness of thin coatings with unknown sound velocity and thickness, we derive a full time-domain ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness based on the phase screen approximation theory. The constructed URCPS is used to determine the velocity, thickness, and interfacial roughness of specimens through the cross-correlation algorithm. The effect of detection frequency on the roughness measurement is investigated through the finite element method. A series of simulations were implemented on Ni-coating specimens with a thickness of 400 μ m and interfacial roughness of 1.9–39.8 μ m. Simulation results indicated that the measurement errors of interfacial roughness were less than 10% when the roughness satisfies the relationship of Rq = 1.6–10.0% λ . The measured velocity and thicknesses were in good agreement with those imported in simulation models with less than 9.3% error. Ultrasonic experiments were carried out on two Ni-coating specimens through a flat transducer with an optimized frequency of 15 MHz. Compared with the velocities measured by time-of-flight (TOF) method, the relative errors of inversed velocities were all less than 10%. The inversed thicknesses were in good agreement with those observed by optical microscopy with less than 10.9% and 7.6% error. The averaged interfacial roughness determined by the ultrasonic inversion method was 16.9 μ m and 30.7 μ m, respectively. The relative errors were 5.1% and 2.0% between ultrasonic and confocal laser scanning microscope (CLSM) method, respectively.