Boston University (Boston, MA) Center for Nanoscience and Nanobiotechnology Mechanical Engineering Professor Todd W. Murray has devised an apparatus for analyzing micro-and nano scale materials and thin surface layers. The metrology device earned U.S. Patent 7,649,632,
An acoustic wave generating laser beam is amplitude modulated with continuous wave modulation of a frequency in the megahertz to gigahertz range and an optical system directs the modulated radiation to a surface of a thin surface layer. This in turn causes an acoustic wave that is sensed and analyzed to provide an indication of properties of thin surface layer.
A narrow bandwidth laser based system which uses a high frequency modulated continuous wave laser source to generate narrow bandwidth acoustic waves combined with a narrow-bandwidth detection scheme. An acoustic microscopy technique is presented which uses a continuous wave (CW) amplitude modulated laser source for the generation of narrow band acoustic waves for use in analysis of thin materials and micro- and nanoscale plates, membranes, and coatings.
An acoustic wave generating laser beam is amplitude modulated with continuous wave modulation of megahertz-gigahertz frequency range and an optical system directs the modulated radiation to a surface of a test specimen. This in turn causes an acoustic wave that is optically sensed using an interferometer and analyzed in the time domain and/or frequency domain to provide an indication of properties of the test specimen.
This invention allows for the displacement sensitivity to be improved over other laser based ultrasonic inspection techniques through a narrowing of the bandwidth of the detection system. The energy in the generated acoustic signal is centered at the frequency of modulation of the laser generation source. The effective bandwidth of the acoustic signal is inversely proportional to the length of time that the surface is illuminated. The bandwidth of the optical detection system may then be reduced to match that of the acoustic signal, thus allowing for a substantial improvement in the signal to noise ratio of the detection system.
This invention allows for the displacement sensitivity to be improved over other laser based ultrasonic inspection techniques through a narrowing of the bandwidth of the detection system. The energy in the generated acoustic signal is centered at the frequency of modulation of the laser generation source. The effective bandwidth of the acoustic signal is inversely proportional to the length of time that the surface is illuminated. The bandwidth of the optical detection system may then be reduced to match that of the acoustic signal, thus allowing for a substantial improvement in the signal to noise ratio of the detection system.
This narrow band measurement is made, for example, using a lock-in amplifier or vector network analyzer, and the bandwidth can be easily selected based on the signal to noise ratio requirements for a given application. This system is capable of modulating the amplitude of the laser source, and hence generating acoustic waves, over a broad range of frequencies from the low megahertz to tens of gigahertz.
Murray’s invention allows for the generation of high frequency acoustic waves with short wavelengths that are suitable for inspecting small scale systems such as thin films and coatings. It is well suited to measure the mechanical properties of thin films such as the elastic moduli and density, as well as the dimension properties such as thickness.
Murray’s invention allows for the generation of high frequency acoustic waves with short wavelengths that are suitable for inspecting small scale systems such as thin films and coatings. It is well suited to measure the mechanical properties of thin films such as the elastic moduli and density, as well as the dimension properties such as thickness.
It is also suitable for the inspection of other small-scale structures such as micro- or nanoscale beams, membranes, or plates. The invention also can be used to generate and detect acoustic waves in macroscopic systems for nondestructive evaluation and determination of physical and mechanical properties. These applications include the detection of subsurface or surface breaking cracks and the detection of subsurface voids or disbonds.
