Lamb Wave Sensors in Semiconductor Process Control and Nondestructive Evaluation
In situ measurement of processing parameters has become increasingly important as integrated circuit (IC) feature size continues to shrink steadily and close-looped process control is considered critical in many stages of IC manufacturing. Many of today's technological advances in processing an limited by the lack of adequate in situ temperature and film thickness sensors. Lamb wave pin transducers are a recent addition to the various type of ultrasonic transducers in the field of non-destructive evaluation (NDE). With their simplicity and effective Lamb wave generation in plate-like materials, they have been shown as a viable solution to many aspects of IC process sensing. In this dissertation, we demonstrate an in situ sensor that is capable of measuring simultaneously the wafer temperature and the film thickness during processing with Lamb wave transducers. Based on the principle that the velocity of an ultrasonic Lamb wave propagating in a silicon wafer changes by both the wafer temperature and the thin film coating on the wafer surface. We have developed a proper inversion method that enables us to achieve a temperature measurement accuracy of 0.15°C and an aluminum film thickness resolution of 170A. In the second part of this dissertation, we present the Lamb wave transducers as an alternative plate thickness sensor. Current ultrasonic plate thickness sensing techniques are mostly based on bulk wave excitation and interface sensing. The major limitation of these techniques is that intimate contact between the transducer and the plate under test is required. In cases where the plate surface is rough or carries curvature different from that of the transducer surface, a liquid couplant is usually required to obtain efficient sound coupling. We employ a pair of Lamb wave transducers to excite and receive A0 mode Lamb waves in a plate. Any change in plate thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of the A0 mode. This technique employs only dry contact between the transducer buffer pins and the test surface, and therefore, can be applied to any hestile environment such as high t e m p e m and radioactive. We have achieved a better than 1% thickness accuracy and easily detected erosion in a pipe elbow that is removed from service. By analyzing the phase response of the receiver signal in a reasonably wide frequency range, we are also able to measure the transducer to transducer distance. This distance information alleviates many constraints in a tomographic imaging system that utilizes transducer arrays. With the filtered back projection algorithm, we present the result of a tomographic imaging system utilizing scanning Lamb wave transducers. An accurate image of a defect with sub-wavelength thickness resolution has been obtained.
