A combination of pulsed-source interferometry and acoustic diffraction has been proposed for use in imaging subsurface microscopic defects and other features in such diverse objects as integrated-circuit chips, specimens of materials, and mechanical parts. A specimen to be inspected by this technique would be mounted with its bottom side in contact with an acoustic transducer driven by a continuous-wave acoustic signal at a suitable frequency, which could be as low as a megahertz or as high as a few hundred gigahertz (see figure). The top side of the specimen would be coupled to an object that would have a flat (when not vibrating) top surface and that would serve as the acoustical analog of an optical medium (in effect, an acoustical "optic").
Microfeatures within the specimen would diffract the acoustic wave. The diffracted wave would propagate through the acoustical "optic," forming a vibration pattern on the top surface. The vibration pattern would be measured twice by use of a pulsed-source optical interferometer; the first measurement would be taken in phase, the second 90° out of phase with the acoustic signal at its source. The amplitude and phase of the vibration pattern, and thus of the acoustic field, would be computed from the two measurements. Then by use of a diffraction formula, the acoustic pattern would be computationally propagated back into the specimen to obtain an acoustic image of the internal microfeatures.
The pulsed-source interferometer has already been demonstrated, in a different application, to afford an amplitude resolution as small as 1 nm. With refinements in design and operation, it should be possible to resolve amplitudes an order of magnitude smaller. If, in addition, the acoustic frequency were at least 30 GHz, then it should be possible to image features as small as 30 nm. The ability to image at such high resolution would be a significant contribution to the art of nondestructive microscopy. Of course, lower acoustic frequencies could be used to image larger features in applications in which the highest resolution is not needed.
This work was done by Kirill Shcheglov, Roman Gutierrez, and Tony K. Tang of Caltech for NASA's Jet Propulsion Laboratory.
NPO-20478
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Pulsed-Source Injterferometry in Acoustic Imaging
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Overview
The document is a technical support package prepared under the sponsorship of the National Aeronautics and Space Administration (NASA), specifically focusing on the topic of pulsed-source interferometry in acoustic imaging. It is associated with NASA Contract No. NAS 7-918 and is part of the NASA Tech Brief, Volume 27, No. 3, dated March 1, 2003. The report is identified as JPL New Technology Report NPO-20478 and was developed by a team of inventors including Roman C. Gutierrez, Kirill V. Shcheglov, and Tony K. Tang, all affiliated with the Jet Propulsion Laboratory (JPL) at the California Institute of Technology.
Pulsed-source interferometry is a technique that utilizes the principles of interference to analyze acoustic signals, which can be particularly useful in various applications such as imaging and sensing. The document likely discusses the methodologies, experimental setups, and potential applications of this technology in fields such as aerospace, environmental monitoring, and materials testing.
The report emphasizes that the work was conducted at JPL under a contract with NASA, highlighting the collaborative nature of the research and the importance of government-sponsored innovation in advancing scientific knowledge and technology. It also includes a notice that the United States Government, nor any individuals acting on its behalf, assumes liability for the use of the information contained in the document, nor does it guarantee that such use will be free from privately owned rights.
Additionally, the document contains disclaimers regarding the mention of specific commercial products or services, clarifying that such references do not imply endorsement by the U.S. Government or JPL. This is a common practice in technical reports to maintain objectivity and neutrality.
Overall, the document serves as a technical resource for researchers and professionals interested in the advancements of pulsed-source interferometry and its applications in acoustic imaging, showcasing the innovative work being done at JPL and its significance in the broader context of aerospace and technology development.
