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.

Applications of Ultrasonic Microscopy to the Evaluation of Ceramic Materials

“…We may expect that a void considerably smaller than the full sound beam width might be detectable, for example, provided that its echo signal is above noise, whereas an inclusion would be less detectable. Table I summarizes beam properties for several typical CAUM/C-SAM transducers; note that to a first approximation, the beam diameter is the same [8] in water and solids, but the focal depth is shortened in solid in proportion to the ratio of the sound velocities in these two materials.…”

“…The upper limit has now been passed, using liquid helium as a couplant to reduce attenuation, allowing frequencies up to 8 GHz, and it is now recognized that the lower limit was arbitrary and unrealistic. Gilmore (7,8] appears to have been the first to realize that it was in the frequency region below 100 MHz that most practical industrial applications lay, and that "ultrasonic microscopy" might better be defined as the formation of images using sound beams with widths comparable to or smaller than the features being imaged, independent of frequency. In reality, there is a continuum between lower frequency ultrasonic C-scan images and the ultra-high frequency SAM images.…”

Rayleigh Wave Velocity Mapping Using Scanning Acoustic Microscope

“…An alternative methodology is to utilize a measurement of difference in propagation time between the direct reflected signal and the Rayleigh wave signal [2,3,4,5,6,7]. This procedure can not be implemented in SAM working with tone bust signals because the width of the burst will encompass both the direct and Rayleigh wave signal, more over only in this situation V(z) curve can be generated.…”

Acoustic Microscopy: Materials Art and Materials Science

“…Originally introduced by Quate [11, this technology has been established by Weglin [21,Kino [31,Wickramasinghe [4], Bertoni [51, and Quate [6] as a powerful tool for materials characterization and development. The work described here [7] goes beyond that cited: it utilizes time-resolved acoustic signals of much greater bandwidth, and does not rely on V(z) behavior to form images. Instead only the digitized amplitudes of the spatially and temporally resolved acoustic signals are processed and displayed to form the images.…”

“…The beams were focused by highvelocity lenses of fused quartz or [111] cut silicon at apertures from F/0.8 to F/7.0, which resulted in single-point resolution spot sizes from 20 to 300 microns. Work reported elsewhere [7] suggests that the resolution for surface wave imaging is determined by the frequency, the surface wave velocity, and the diameter of the entry circle ( Figure 1) due to the intersection of the cone of convergence caused by the Rayleigh incident angle and the entry surface of the material. The maximum dynamic range of the images is controlled by the 8-bit, single-pulse (up to 80 MHz) gated peak detector; however, most of the image files were attenuator adjusted to contain maximum amplitude signals between 7 and 8 bits.…”