Experimental ultrasonic characterization of machining damage from loose abrasive processes

Abstract

Currently, there is a need for a nondestructive technique which can characterize surface damage in chemical mechanical planarization (CMP). This process is used in the manufacturing of semi-conductors for interconnect structures. Other potential applications for methods of characterizing surface damage include in situ monitoring of planarization, lapping, and vapor deposition. The ability of plate modes with surface displacements (PMSD), or surface acoustic waves (SAW), to penetrate to a fixed depth in the material, makes the technique well suited to characterize surface damage and near-surface properties. Measurements made in this manner are sensitive to the effective elastic constants of the surface layer. The objective of this work is to verify modeling which has been performed by other investigators. The models show that surface damage for one-dimensional parallel surface-breaking cracks may be characterized by measuring the dispersion of surface acoustic waves. Presently, investigations into use of dispersion of PMSD and SAW have been primarily numerical. Little experimental data is available to verify the models and to test the method in applications. The measured dispersion of PMSD and SAW associated with surface damage suggests that the ultrasonic technique may be suitable for detection of surface damage. While a number of practical difficulties remain before a method can be in general use, an “index of state” can be obtained for the characterization of the surface damage. Results indicate that sensitivity exists both to the existence and scale of the surface damage.