Acoustic methods for thickness measurements and monitoring multi-layered structures

Intensive studies have been performed on both theory and application of wave propagation in multi-layer solid media. Two types of wave propagation are employed and shown effective for non-destructive evaluation of multi-layered structures.; Incident waves normal to the surface of the multi-layer structure were employed for thickness measurements at specified local spots. A novel frequency domain thickness measurement method for multilayered thin polymer structures is introduced. Specific challenges due to the strong frequency dependent attenuation in polymers are discussed. Unlike the popular techniques which employ the impulse response of the layer structure regardless the wave propagation phenomenon inside, the proposed technique is based on a transfer matrix formulation and pulse/echo boundary conditions, thus is physics based. An experimental procedure is developed for determining the natural frequencies of a multilayer structure consisting of highly attenuative polymer layers. Extraction of layer thicknesses from a set of natural frequencies is also demonstrated. One unique feature of the current technique is that it requires low frequency acoustic equipment.; Guided wave traveling along the direction of the plate was employed to monitor the thickness of a layered structure in an overall manner. In this dissertation, the guided wave is employed in monitoring the thickness of the sputtering target erosion profile. Sputtering target is widely used high purity engineering material. Manual inspection of the target life causes substantial system downtime and energy loss, and over use of the target endangers the wafer and the sputtering tool. The proposed ultrasonic non-destructive evaluation system provides an important in situ method to monitor the life of the target. In the design of the ultrasonic evaluation system, several researches were conducted. Dispersion curve analysis was performed to give important insights on the transducer arrangement and signal processing algorithm. Finite element analysis was performed to simulate the mechanical design and confirm the findings from solid mechanics. Onsite in situ measurement on the sputtering tool was performed to implement the designed system.