Contrast Mechanisms in the Thermoacoustic Microscope

The thermoacoustic microscope [1–9] uses a modulated particle beam (electrons, photons or ions) as a heat source on (or near) the surface of the sample, and a piezoelectric transducer attached elsewhere on the sample as a detector of acoustic waves generated by the beam. The modulation frequency is typically 103 to 107 Hz, so the acoustic signal is in the sonic to ultrasonic range. These acoustic signals are used to produce images of surface and near surface features of the solid by scanning the source over the face of the sample. In order to make full use of this type of microscope as a quantitative NDE tool, one must be able to interpret the images in terms of the physical properties of the features being imaged. The interpretation of an image resulting from variations in, say, the elastic constants as if it were caused by variations in, say, thermal expansion coefficient, could lead to totally incorrect conclusions about the nature of a defect. This paper summarizes a theoretical analysis which can form a basis for assessing the relative importance of different contrast mechanisms

Process Control in IC Manufacturing with Thermal Waves

In today’s semiconductor market, manufacturers face a daunting challenge. Product concepts evolve rapidly in response to rapidly changing markets while design rules, i.e., device geometries, become increasingly smaller and wafers become larger. Devices must run faster, reliability must improve and the resultant increasing complexity in IC design and fabrication technology intensifies the need for tighter controls of process variables. To compete effectively in this market, manufacturers must improve both product development and product manufacturing processes

Beam profile reflectometry: a new technique for thin film measurements

In the manufacture of semiconductor devices, it is of critical importance to know the thickness and material properties of various dielectric and semiconducting thin films. Although there are many techniques for measuring these films, the most commonly used are reflection spectrophotometry [1,2] and ellipsometry [3]. In the former method, the normal- incidence reflectivity is measured as a function of wavelength. The shape of the reflectivity spectrum is then analyzed using the Fresnel equations to determine the thickness of the film. In some cases, the refractive index can also be determined provided that the dispersion of the optical constants are well known. The latter method consists of reflecting a beam of known polarization off the sample surface at an oblique angle. The film thickness, and in some cases the refractive index, can be determined from the change in polarization experienced upon reflection

Progress on Ultrasonic Flaw Sizing in Turbine Engine Rotor Components: Bore and Web Geometries

The application of generic flaw sizing techniques to specific components generally involves difficulties associated with geometrical complexity and simplifications arising from a knowledge of the expected flaw distribution. This paper is concerned with the case of ultrasonic flaw sizing in turbine engine rotor components. The sizing of flat penny shaped cracks in the web geometry will be discussed and new crack sizing algorithms based on the Born and Kirchhoff approximations will be introduced. Additionally we propose a simple method for finding the size of a flat, penny shaped crack given only the magnitude of the scattering amplitude. The bore geometry is discussed with primary emphasis on the cylindrical focussing of the incident beam. Important questions which are addressed include the effects of diffraction and the position of the flaw with respect to the focal line. The appropriate deconvolution procedures to account for these effects will be introduced. Generic features of the theory will be compared with experiment. Finally, the effects of focused transducers on the Born inversion algorithm are discussed

Buried Thermoplastic Layer Diagnostics by the Use of Photo-Thermo-Acoustic Radiometry

Nondestructive evaluation of buried layers finds important applications in industry. In this paper we describe a fast, non-contact photothermal technique for thermoacoustic characterization of a thermoplastic layer sandwiched between two metal foils used in heat sealed food containers. For this particular application a non-contact measurement was required of the thickness of the polymer layer between two layers of aluminum in a heat sealed container rim at a speed of four locations of the container in 60 sec

Comparison of Matrix Methods for Elastic Wave Scattering Problems

In the last ten years several numerical methods have been developed for the solution of elastic wave scattering problems that have found application in quantitative flaw definition. Before the development of these methods, due to the complexity of Navier’s equation which governs wave motion in an elastic continuum, numerical results were available only for circular cylinders and spheres. The elastic wave equation is separable only in polar and spherical coordinates. For other geometries, three types of numerical methods have been developed. They were all originally developed for acoustic and electromagnetic problems governed by the scalar and vector wave equations respectively

Response of Laser-Induced Thermal Lens Effect at Solid Surface

Recently Kuo et al. [1,2] and Satio et al.[3] presented the surface-thermal lens (STL) technique, this novel photothermal deformation technique has attracted great attention because it is a highly sensitive, noncontact and nondestructive measurement[4–6]. In this technique, a modulated pump beam is focused on the sample surface to produce the surface deformation and a cw probe beam is incident at the deformation region. Differing from the conventional photothermal deformation techniques, the spot size of the probe beam at the sample surface is much larger than the pump beam one. Then the probe beam reflected from the surface produces a diffraction pattern at the detection plane. More recently, STL technique has been successfully applied to study the temperature dependence of the thermal conductivity of semiconductor materials[5], weak absorption of optical thin films[6] and characterization of the solid materials[7,8]. However, the mechanism of STL phenomena has not been completely understood. Most theoretical models took no account of the influence of the air-thermal lens (ATL), although some experiment showed that the air significantly affected the detected diffraction pattern[2]. In addition, it is necessary to characterize frequency responses of signals because the response is used to determine the thermal property of the solid materials[5]

Picosecond Transient Thermoreflectance: Time-Resolved Studies of Thin Film Thermal Transport

The advent of new and sophisticated material growth processes (molecular beam epitaxy, chemical vapor deposition and ion sputter deposition) has produced new exotic materials such as amorphous alloys and compositionally modulated structures [1]. The atomic level structure of these materials can be proved by techniques such as x-ray diffraction. The electrical and thermal transport properties are also used to characterize these materials, which are usually deposited as thin films onto supporting substrates. Although the substrate may be electrically isolated from the film, complete thermal isolation is more difficult to achieve and thermal transport measurements are complicated.</p

China\u27s hegemonic intentions and trajectory: Will it opt for benevolent, coercive, or Dutch‐style hegemony?

China\u27s unprecedented economic growth led some scholars to conclude that it will replace the United States as the future global hegemon. However, China\u27s intentions in exercising future global leadership are yet unknown and difficult to extrapolate from its often contradictory behaviour. A preliminary overview of China\u27s island building in the South China Sea reveals its potentially coercive intentions. This inference is consistent with the analysis of those who prognosticate China\u27s violent rise. Conversely and simultaneously, China\u27s participation in peacekeeping operations and its global investments evince its benevolent hegemonic intentions, which are congruent with the argument of those who predict China\u27s peaceful hegemonic ascent. Confronted with these divergent tendencies in China\u27s recent international relations, and assuming its continued rise, it is, thus, essential to examine China\u27s strategic intentions and how these may ultimately project its violent or peaceful hegemonic rise. This article argues that the “Third Way” or “Dutch‐style” hegemony is highly instructive in this context and, thus, should be examined and added to the existing debate on China\u27s rise as either a benevolent or coercive hegemon. We argue that Dutch‐style hegemony may be the most viable way for China to proceed in its global hegemonic ascendancy

Deep Thermoacoustic Imaging Using Scanning Electron Acoustic Microscopy

There has developed over the past few years some controversy [1] over the meaning and use of terms such as “thermal wave imaging” vs. “thermoacoustic imaging” or “Thermal Wave Microscopy” as opposed to “Scanning Electron Acostic Microscopy” (SEAM). The issue in question is the extent of the role played by acoustics directly on the imaging. In the present work, SEAM studies of a prepared test standard are presented which show that macroscopic elastic properties can play an important role in imaging. The SEAM technique uses Coordinate Modulation (CM) of the electron beam [2] rather than the more commonly used intensity modulation via beam blanking. Defects as deep as 26 thermal diffusion lengths in stainless steel have been clearly imaged with this technique. This work strongly supports the Jackson -Amer thermal bending model [3] for low frequency (up to approximately 100kHz in small specimens) acoustic detection in the solid as compared to the theory of Opsal and Rosencwaig [4] which does not incorporate total specimen response to the thermal source. This problem has more recently been theoretically investigated by Favro [5,6], who has developed a more general theory encompasing both the Opsal-Rosencwaig short wavelength limit and the Jackson-Amer long wavelength limit. Favro’s theory also accounts for features in the image shown