K. Liang

Material Characterization by the Inversion of V(z)

Absfmet-It is demonstrated that the reflectance function R(@) of a liquid-solid interface can be obtained by inverting the complex V(z) data collected with an acoustic microscope. The inversion algorithm is based on a nonparaxial formulation of the V(z) integral, which establishes the Fourier transform relation between R(@) and V(z). Examples are given to show that with this measurement technique, the acoustic phase velocities of the propagating modes in the solid medium can easily be determined and material losses can be estimated. The same technique is also used for characterizing imaging performance of focused systems. Applications In thin-6lm measurement are also discussed.

Precise Phase Measurements with the Acoustic Microscope

Abstmct-The measurement and the use of phase in acoustic microscopy are discussed. It is demonstrated that in many applications phase can be used to provide sensitivity and information unparalleled by amplitude-only measurement methods. A technique capable of highaccuracy measurement of the phase of short RF acoustic pulses is described. The power of this phase measurement technique is illustrated in a number of applications. Surface material property measurements such as the Rayleigh-wave velocity and the inversion of the complex V(z) to obtain the reflectance function of a liquid-solid interface are considered. Surface topography mapping based on phase measurement is examined. A Fourier transform approach for precision determination of linewidths comparable to the resolution spot size is also presented.

Fourier transform approach to materials characterization with the acoustic microscope

A general method is presented for characterization of elastic properties of materials with the reflection acoustic microscope. The material acoustic reflectance function and the acoustic microscope focus curve [V(z)] are shown to be a Fourier transform pair. By measurement of both phase and amplitude, the complex V(z) curve can be recorded. These data can then be inverted to obtain an estimate of the reflectance function. This method allows calibration of individual acoustic lenses and is experimentally demonstrated with a 50‐MHz acoustic microscope and a synchronous phase detection system.

Reflection acoustic microscope for precision differential phase imaging

An acoustic microscope comprising a transducer for transmitting acoustic signals towards the surface to be studied, and means for receiving at least one reflected signal from the surface; in many embodiments of the invention, signals are received from two separate points. The signals received are passed to a synchronous phase detection system for analysis. The signals may be received at the same phase detector input and separated according to their expected time of receipt relative to their time of transmission, or they may be received at separated points on the transducer related to their separated points of transmission. The separated return signals are compared on the basis of phase (and in certain embodiments, magnitude) differential either to each other or to an internally generated reference signal to analyze the surface characteristics of the material.

Phase Measurements in Acoustic Microscopy

We have carried out experiments using a 50 MHz acoustic microscope to demonstrate the utility of phase information in acoustic microscopy. Phase perturbation was used to characterize surface r esidual stress on a glass disk sample. topography of different samples. inversion technique was used to obtain the reflectance function of a liquid/solid from the corresponding V (z) d ata measured in both amplitude and phase. Phase images were made to map the surface An exact

Precision phase measurement with short tone burst signals in acoustic microscopy

A new phase measurement system based on a sychronous detection scheme is described. It is capable of yielding high precision phase data with very short tone bursts. A theoretical analysis of the system is given and system performance characteristics are also discussed. This phase measurement technique is especially suited for acoustic microscopy applications. Examples in material characterization and surface topography mapping are presented.

A Three-Dimensional Synthetic Focus System

Synthetic focus imaging techniques suitable for reconstructing 3-D acoustic images of flaws inside silicon nitride are described. A 50 MHz imaging system consisting of a precision scanner, a microcomputer controller, and a minicomputer image processor has been developed for this purpose. A square synthetic aperture is used to image flaws in flat disc samples and a cylindrical synthetic aperture is used in the cylindrical rod case. We have developed the theory to predict the imaging performance of the two aperture geometries. The respective Point Spread Functions are simulated and agree well with theoretical results. Special attention is given to reconstructing images of specular reflectors. Computer simulations based on theoretical flaw models have been carried out.

High temperature phased array ultrasonic system with integrated front end electronics

This paper presents a novel 2-D phased array ultrasonic imaging system consisting of a cylindrical transducer array with 800 elements and its associated electronics, in a miniaturized package, with the capability of electronic beam scanning and beam focusing. The system can operate in harsh environments with temperature up to 175°C and pressure around 1,400 bars.

A 50 MHz Synthetic Focus System

Last year at this conference we discussed a number of difficulties which can occur in synthetic aperture and other short pulse imaging systems. In this paper, we have carried out studies in both a linear and cylindrical format to understand some of these problems more clearly and to arrive at solutions for them. We have studied the effects of a finite number of digital phase samples on the sidelobe level, the effect of apodization on grating and sidelobe levels, the effect of a finite number of transducer elements, problems of imaging specular reflectors, and the use of selective back projection methods to improve images of specular reflectors. We show here that the use of this latter technique may be of great importance because it provides an alternative approach of imaging, which does not make use of the normal phase cancellation effects employed in conventional imaging systems.

3A-2 Cylindrical Ultrasonic Array for Borehole Applications

A cylindrical ultrasonic array has been developed for operating environments that can reach extremes of 175 C and 20,000 psi. The array is a key component of the PharUSIT (Phased Array Ultrasonic Transducer for Inspection of Tubing), a research demonstrator developed for borehole applications. The full array consists of 800 elements (10 rings of 80 elements each) and can provide a whole range of beam-forming versatilities and capabilities in 3D, such as variable focusing, beam steering, electronic scanning, etc, all accomplished without mechanical movements. Special piezocomposites have been developed for the transduction layer, and new polymeric composites have been formulated for the backing material. The center frequency was chosen to be about 500 kHz to accommodate attenuation of the propagation media. A novel technique utilizing custom flexible circuit provides electrical connections between the array and the front-end electronics. Special fabrication processes have been developed to construct the array in a cylindrical geometry. A customized testing protocol has been implemented to demonstrate the survivability of the array technology and to evaluate the performance characteristics of individual elements under high-temperature/high-pressure conditions. Data from electroacoustic measurements such as electrical impedance, bandwidth, sensitivity, angular directivity, and inter-element cross-talks will be shown