PNNL-13501 - Pacific Northwest National Laboratory

Volumetric Imaging of Materials by Advanced, High Resolution Pulse-Echo Ultrasound
Morris S. Good, Leonard J. Bond, Peter M. Martin, Gerry J. Posakony, James R. Skorpik
Study Control Number: PN98081/1327
Ultrasound is a useful, nondestructive analytical tool for imaging structures that cannot be examined visually by other
means. The advanced capabilities of high-resolution, pulse-echo ultrasound are being extended to very high frequencies
using improved electronics and tranducers. It is now possible using high-resolution ultrasound to examine the
characteristics of living cells, the structure of cell nuclei, the integrity of cell membranes, and the response of cells to
environmental stresses.
Project Description
High resolution, volumetric acoustic microscopy imaging
has evolved into 1) an invaluable tool in establishing the
quality of critical parts and 2) an analysis tool for
biological cells. Ultrasound offers a noninvasive means
of imaging sub-surface structures on the order of
0.5 microns and larger (Bereiter-Hahn 1996). Applications
of interest include intercellular imaging in support of
the cellular observatory, imaging of ceramic materials at
the microstructural level to aid in material design and life
prediction, and the inspection of small solder connections
of the next generation of integrated circuits (a,b) . To
achieve the desired resolution, ultrasonic, pulse-echo
systems must operate at frequencies ranging from 200
MHz to 800 MHz, however restrictions exist in the ability
to fabricate transducers in this range. Two technologies
have been used to fabricate pulse-echo transducers.
Lapping of bulk piezoelectric material to fabricate a thin
wafer has an upper frequency limit in the range of 200
MHz due to material damage incurred during lapping.
Deposition of piezoelectric material has traditionally had
a lower frequency limit of 1 GHz due to difficulties in
maintaining a proper microstructure of the piezoelectric.
This effort leveraged the world-class deposition skills at
Pacific Northwest National Laboratory to overcome this
problem and in fiscal year 1999, fabricated a film
thickness several orders of magnitude greater than
typically deposited and evaluated the resulting ultrasonic
transducers as reported earlier. Due to budget restrictions
in FY 2000, work centered on resolution enhancement
utilizing an existing 50-MHz transducer.
(a) Private communication with Pacific Northwest National
Laboratory staff.
(b) Private communication with a major integrated circuit
manufacturer.
Introduction
Future work should address three fundamental issues that
still hinder imaging at a resolution representative of
milestone frequencies of 100 MHz and 200 MHz. Two
are related to improved material deposition while the third
is improved electronics to achieve adequate signal-tonoise
values. Improving the deposition process to
maintain a (002) columnar orientation within a 2°
tolerance and thickness uniformity across the transducer
diameter are the first two issues. Issue 1: The (002)
columnar microstructure of the Pacific Northwest
National Laboratory piezoelectric film has been observed
at an orientation ranging from 0° to 15° relative to the
surface normal. Inclinations greater than 2° degraded
signal quality by generating shear waves. Shear waves
formed an upper noise baseline and leached energy away
from the primary longitudinal wave. Although several
transducers with a 2° inclination have been obtained, most
microstructures at the end of FY 1999 were inclined at
approximately 10° and therefore resulted in a strong shear
wave component. Issue 2: Thickness uniformity of the
deposited film is needed to ensure a good focus and
generation of the desired frequency. A mild thickness
taper (30% thickness change) was observed across the
transducer face. Thickness uniformity within 5% should
be obtained with changes in the presentation of the
substrate to the sputtering electrode. Issue 3: Refinement
of tone-burst electronics should also continue. A low
voltage (10 Volts peak-to-peak) prototype was
demonstrated, however an order of magnitude
improvement is desired to permit adequate penetration in
both materials of interest and fluids such as water that are
used to couple ultrasound between transducer and part.
Sensors and Electronics 415