Measuring and testing – Vibration – By mechanical waves
Patent
1981-01-23
1983-04-05
Birmiel, Howard A.
Measuring and testing
Vibration
By mechanical waves
73644, G01N 2900
Patent
active
043786995
DESCRIPTION:
BRIEF SUMMARY
It is known that an image of a sample can be provided by causing a focussed beam of high frequency acoustic radiation to scan across the sample surface, and converting the transmitted or reflected acoustic radiation into an optical signal. The sample is coupled to a suitable transducer by a liquid, such as water, liquid helium or liquid argon. In such a system, there are very large signal losses, mainly in the coupling liquid which has a high absorption coefficient. For typical liquids the absorption coefficient is proportional to the square of the frequency up to and beyond 10 GHz, and the acoustic wavelength varies inversely as the frequency. It can be shown that the resolution of a scanning acoustic microscope is about 2/3 of the acoustic wavelength in the coupling medium.
Suppose the transit time and loss through the coupling fluid are fixed at T sec and N nepers respectively. Further suppose that the fluid path length is L and the attenuation coefficient is .alpha. at a frequency f.
If V is acoustic velocity and .lambda. is acoustic wavelength, then V=f.lambda.. Substituting in equation [1]. ##EQU1## coupling medium, the theoretically attainable resolution is proportional to the product of (the acoustic velocity in liquid).sup.3/2 and the square root of a constant of the liquid.
It is also known that in a gas at atmospheric pressure, while the acoustic velocity is typically 5 to 10 times lower than in liquids, thus increasing resolution, values of X are typically 100 to 1000 times higher than in water which offsets the gain in resolution as indicated by equation [2]; use of a gas as a coupling medium in a scanning acoustic mircoscope therefore did not seem practical because predicted resolution was lower than for a liquid-coupled system.
However, we now believe that use of a gas may, under certain conditions, be advantageous.
According to the invention, a scanning acoustic microscope comprises transducer means to provide a convergent beam of acoustic radiation; means to cause relative movement in the focal plane of the focus of said beam and a sample under investigation; transducer means to receive acoustic radiation modulated by the sample near the beam focus; and means to supply a pressurised gas to a volume surrounding the transducer means and the sample.
Preferably the gas is at a pressure of at least 10 atmospheres, and usually a pressure of at least 100 atmospheres will be required. The gas may be a monatomic gas such as argon or xenon.
When the microscope is used in a transmission mode, a transmitting transducer will provide the convergent beam and a separate receiving transducer will sense the modulated radiation. When the microscope is used in a reflection mode, a single transmitting/receiving transducer can be used.
A microscope according to the invention can be used in any of the conventional modes. Phase and amplitude information can be provided, pulsed or continuous wave operation is possible, and dark field imaging may be used.
The present invention is based on the discovery, theoretically predicted by the inventor, that for a monoactomic gas at a pressure of about 100 atmospheres, the constant X is of the same order as its value for liquids. This discovery, in conjunction with the classical equation showing that velocity of sound in a gas is independent of pressure, finds practical application for the first time in a novel and inventive scanning acoustic microscope in which a pressurised gas is used as a coupling medium. It is believed that this is the first time that the effect of gas pressure on absorption of sound in a gas has been considered either theoretically or practically in acoustic microscopy.
In classical gas theory considering behaviour of an ideal gas, two absorption mechanisms are applicable to explain absorption of sound by the gas, i.e. viscous damping, and heat conduction caused by direct transfer of atomic momentum. These two effects are about the same order of magnitude for argon and xenon.
Considering acoustic absorption .alpha. due to viscous damping and thermal co
REFERENCES:
patent: 3832888 (1974-09-01), Langlois
patent: 4028933 (1977-06-01), Lenvars et al.
"The Acoustic Microscope", Quate, Scientific American, Oct. 1979, vol. 241, No. 4, pp. 62-70.
Birmiel Howard A.
National Research Development Corporation
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