TAP-NDE is employed to perform an experimental study on silicon wafers of different thicknesses to determine the maximum possible resolution of TAP-NDE towards temperature sensitivity, and to demonstrate the ability to differentiate between wafers of different deposition layer thickness at temperatures up to 600°C. Temperature resolution is demonstrated for ±10°C resolution and for ±5°C resolution; while thickness differentiation is carried out with wafers carrying 4,000Å and 8,000Å of aluminum deposition layer. The experimental group velocities of a set of selected frequency components extracted using the Gabor wavelet time-frequency analysis compare favorably to their corresponding theoretical group velocities. It is shown that TAP-NDE is a feasible tool for identifying and characterizing thickness and temperature changes simultaneously during thermal annealing that can replace the current need for separate characterization of these two important parameters in semiconductor fabrication.

The feasibility of Thermo-Acousto-Photonic Nondestructive Evaluation (TAP-NDE) as an alternative to current pyrometric and ellipsometric techniques to simultaneously measure temperature and thickness during single crystalline silicon wafer processing is investigated. TAP-NDE is a non-contact, non-invasive, laser-based ultrasound technique employed to profile the thermal and spatial characteristics of the wafer using Lamb waveguide modes. A theoretical formulation viable for identifying the frequencies sensitive to temperature and thickness changes is presented. The group velocity of the frequency components and their corresponding changes with respect to temperature for different thicknesses are modeled by considering a titanium bonding layer between aluminum/copper and silicon. It is shown both theoretically and experimentally that there exists a correlation for differentiating thickness variation as a function of varying group velocities for different frequencies at different temperatures, thus establishing the groundwork for the optically generated Lamb wave thermometric methodology for silicon wafer.

Traditional transducer-based techniques for Non-Destructive Evaluation (NDE) are limited by fixed frequency-bandwidth for generation and sensing, and thus provide unsatisfactory resolution for certain types of material defects. Thermo-Acousto-Photonic NDE (TAP-NDE) is a proven alternative that is non-invasive and non-contact, and suited for real-time applications. This paper focuses on employing TAP-NDE to examine the presence of microcracks and fissures in multi-layered composites. Tests were performed on layered composite panels of specific epoxyresin composition and constant thickness to identify localized delaminations formed by subjecting the specimen to cryogenic cycling. Interrogation of the undamaged specimen using laser-generated broadband surface waves revealed a standard reference knowledge base, as seen in the instantaneous frequency-time domain. Tests were repeated after each specimen was subjected to a set number of cycles of liquid nitrogen cycling, which caused damages at the micron scale in the bulk material. Analyses showed changes in the time of wave arrival and absence of prominent high frequency components. Wave velocity and dispersion characteristics of the cycled specimen were altered. Thus, the specimen, on cryogenic cycling, was found to undergo a decrease in stiffness, which is speculatively the result of micro-voids, fissures or delaminations between layers. Hence, when combining with the basic notion of instantaneous frequency, TAP-NDE acts as an effective broadband generation and sensing technique, demonstrating feasibility and greater versatility for inspecting layered composites as against contemporary narrowband techniques.