Acoustic microscopy from 10 to 100 MHz for industrial applications

Abstract The development of a system is described here that, for the first time, utilizes acoustic microscopy techniques to evaluate materials and processes on a scale practical for support of automated manufacture. The properties of acoustic microscopy attractive for this application are the ability to inspect the elastic structure of the surface and the subsurface of materials. In the past, several barriers have prevented its use except for near-surface inspection of a very limited area (a few square millimetres). These barriers include critical alignment requirements, very shallow penetration, and limitations in resolution and the size of the workpiece. Presented here is a unique configuration that differs from high-frequency or conventional acoustic microscopy methods in these ways: (a) images are formed directly by displaying amplitude of broadband acoustic pulses (centre frequencies: 10-100 MHz; bandwidths: 80% -120%), rather than by displaying amplitude variations resulting from interference between narrowband pulses (carrier frequencies: 0.5-4.0 GHz; bandwidths: 0.5% -1.0% ); (b) the short pulses, having only a 2-4 wavelength duration, are readily time resolved by gating the surface wave and thus eliminating interference from the direct reflection. This technique avoids the complex analytical problem of separating the surface waves from the direct reflection; (c) a single-crystal silicon acoustic lens is used instead of the sapphire acoustic lens conventionally used. Incorporating these methods, a large-scale, 10-100 MHz scanning microscopy system has been developed with the following advances over previously reported ultrasonic non-destructive testing systems and acoustic microscopes: (a) reliable detection and display of surface-breaking cracks is possible at all orientations for non-destructive evaluation purposes; (b) imaging accuracy is independent of small variations in the water gap (the distance between lens and workpiece); in contrast, such variations are a major consideration in determining the imaging quality of conventional acoustic microscopes; (c) magnifications of 2-20 times instead of 1000 times allow interrogation of much larger areas and volumes of material; (d)the compound resolution of large against small features is theoretically explained for the first time by a combined ray tracing-diffraction model; (e) a practical method for the inspection of volumes and even interior surfaces results from the use of longer wavelength signals, where previously the use of shorter wavelength signals limited penetration to the first few micrometres of the subsurface; (f) shear-wave images of subsurface features, which give the most dependable information on the integrity of bonded interfaces, are available, as well as longitudinalwave images. Practical applications are presented, such as inspection for near-surface inclusions in aircraft engine materials and inspection of heat-sink bonds in semiconductor power devices. The broadband-wave method presented here allows scanning acoustic-microscopy methods, previously available only on a scale useful for the materials scientist, to be used on a scale practical for industrial materials evaluation, with a capability for an interrogated volume 12 orders of magnitude greater than that possible with the higher frequency methods.