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Improved Phase Measurement Technique for the Acoustic Microscope

IP.com Disclosure Number: IPCOM000099607D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 5 page(s) / 243K

Publishing Venue

IBM

Related People

Meeks, SW: AUTHOR

Abstract

The acousto-elastic effect, the change of acoustic velocity with stress, has been used by many investigators to image stress fields in opaque metals on a macroscopic scale. This same effect can be used to image stress fields on a microscopic scale, however, what is required is a precise measurement of the acoustic velocity on a microscopic scale. The solution to this problem is to use an acoustic lens to focus the beam to a small spot and then use phase-measuring techniques to precisely measure the velocity changes induced by stress.

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Improved Phase Measurement Technique for the Acoustic Microscope

       The acousto-elastic effect, the change of acoustic
velocity with stress, has been used by many investigators to image
stress fields in opaque metals on a macroscopic scale.  This same
effect can be used to image stress fields on a microscopic scale,
however, what is required is a precise measurement of the acoustic
velocity on a microscopic scale. The solution to this problem is to
use an acoustic lens to focus the beam to a small spot and then use
phase-measuring techniques to precisely measure the velocity changes
induced by stress.  A conventional acoustic lens which has isotropic
illumination produces a signal which consists of two components:  a
geometrical or specular component which is produced by the region
near the center of the lens and a Rayleigh (surface acoustic) wave
component which is produced by the off-center area of the lens.  The
specular component will interfere with the Rayleigh wave component to
produce the well known V(z) curve.  If sufficiently short acoustic
pulses are produced, then it is possible to temporally separate the
Rayleigh wave and specular pulses.  One may then measure the phase
shift between these two waves to arrive at a precise velocity
measurement.  This scheme leads to poor spatial resolution since a
substantial defocus is required to produce sufficient temporal
separation between the Rayleigh and specular pulses.  An alternate
scheme is used in this work where a tilted orientation of ZnO is used
to produce both a shear and a longitudinal wave in the quartz buffer
rod, as illustrated in Fig. 1.  The dimensions of the acoustic lens
used in this work are also shown in Fig. 1.  The focal length of the
longitudinal wave was 160 microns and 200 microns for the shear wave.
 The combination of shear and longitudinal waves may alternatively be
produced by Y-cut lithium niobate transducers [*].

      The shear wave produces no specular component at normal
incidence upon the acoustic lens.  This is because there is no mode
conversion between a normally incident shear wave and a longitudinal
wave in the water coupling fluid.  Most of the shear wave energy mode
converts at the quartz/water interface to a pressure wave in the
water coupling fluid, propagating at an angle near 24 degrees.  This
means that these lenses will efficiently excite Rayleigh waves on
substrates which have Rayleigh wave speeds of 3 to 4 Km/sec. Since
there is no specular component for the shear wave, very little
defocus is required to generate the Rayleigh wave.  The Rayleigh wave
propagates along the direction defined by the shear wave polarization
since in the orthogonal direction there is no mode conversion between
a shear wave and a longitudinal wave at the quartz interface. The
longitudinal wave is also focussed by the lens but its focal length
is 40 microns shorter than the 200 micron focal length of the wave
generated by the buffer rod s...