Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-12-19
2003-12-09
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C029S025350
Reexamination Certificate
active
06659954
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to ultrasound diagnostic systems that use ultrasonic transducers to provide diagnostic information concerning the interior of the body through ultrasound imaging, and more particularly, to micro-machined ultrasonic transducers used in such systems.
BACKGROUND OF THE INVENTION
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. For example, cardiologists, radiologists, and obstetricians use ultrasonic diagnostic imaging systems to examine the heart, various abdominal organs, or a developing fetus, respectively. In general, imaging information is obtained by these systems by placing an ultrasonic probe against the skin of a patient, and actuating an ultrasonic transducer located within the probe to transmit ultrasonic energy through the skin and into the body of the patient. In response to the transmission of ultrasonic energy into the body, ultrasonic echoes emanate from the interior structure of the body. The returning acoustic echoes are converted into electrical signals by the transducer in the probe, which are transferred to the diagnostic system by a cable coupling the diagnostic system to the probe.
Acoustic transducers commonly used in ultrasonic diagnostic probes are comprised of an array of individual piezoelectric elements formed from a piezoelectric material by the application of a number of meticulous manufacturing steps. In one common method, a piezoelectric transducer array is formed by bonding a single block of piezoelectric material to a backing member that provides acoustic attenuation. The single block is then laterally subdivided by cutting or dicing the material to form the rectangular elements of the array. Electrical contact pads are formed on the individual elements using various metallization processes to permit electrical conductors to be coupled to the individual elements of the array. The electrical conductors are then coupled to the contact pads by a variety of electrical joining methods, including soldering, spot-welding, or by adhesively bonding the conductor to the contact pad.
Although the foregoing method is generally adequate to form acoustic transducer arrays having up to a few hundred elements, larger arrays of transducer elements having smaller element sizes are not easily formed using this method. Consequently, various techniques used in the fabrication of silicon microelectronic devices have been adapted to form ultrasonic transducer elements, since these techniques generally permit the repetitive fabrication of small structures in intricate detail.
An example of a device that may be formed using semiconductor fabrication methods is the micro-machined ultrasonic transducer (MUT). The MUT has several significant advantages over conventional piezoelectric ultrasonic transducers. For example, the structure of the MUT generally offers more flexibility in terms of optimization parameters than is typically available in conventional piezoelectric devices. Further, the MUT may be conveniently formed on a semiconductor substrate using various semiconductor fabrication methods, which advantageously permits the formation of relatively large numbers of transducers, which may then be integrated into large transducer arrays. Additionally, interconnections between the MUTs in the array and electronic devices external to the array may also be conveniently formed during the fabrication process. MUTs may be operated capacitively, and are referred to as cMUTs, as shown in U.S. Pat. No. 5,894,452. Alternatively, piezoelectric materials may be used to fabricate the MUT, which are commonly referred to as pMUTs, as shown in U.S. Pat. No. 6,049,158. Accordingly, the MUT has increasingly become an attractive alternative to conventional piezoelectric ultrasonic transducers in ultrasound systems.
FIG. 1
is a partial cross sectional view of a MUT
1
according to the prior art. The MUT
1
may have a platform that is rectangular, circular, or may be of other regular shapes. The MUT
1
generally includes an upper surface
2
that is spaced apart from a lower surface
3
that abuts a silicon substrate
5
. Alternatively, a dielectric layer
4
may be formed on the substrate
5
that underlies the MUT
1
. When a time-varying excitation voltage (not shown) is applied to the MUT
1
, a vibrational deflection in the upper surface
2
is developed that stems from the electro-mechanical properties of the MUT
1
. Accordingly, acoustic waves
6
are created that radiate outwardly from the upper surface
2
in response to the applied time-varying voltage. The electro-mechanical properties of the MUT
1
similarly allow the MUT
1
to be responsive to deflections resulting from acoustic waves
7
that impinge on the upper surface
2
.
One disadvantage in the foregoing prior art device is that a portion of the ultrasonic energy developed by the MUT
1
may be projected backwardly into the underlying substrate
5
, rather that being radiated outwardly in the acoustic wave
6
, which results in a partial loss of radiated energy from the MUT
1
. Moreover, when ultrasonic energy is coupled into the underlying substrate
5
, various undesirable effects are produced, which are briefly described below.
With reference now to
FIG. 2
, a partial cross sectional view of a MUT array
10
according to the prior art is shown. The array
10
includes a plurality of MUT transducers
1
formed on a silicon substrate
5
. Each transducer
1
is coupled to a time-varying voltage source through a plurality of electrical interconnections formed in the substrate
5
. For clarity of illustration, the voltage source and the electrical interconnections are not shown. An acoustic wave
21
may be conducted into the substrate
5
through a back surface
3
. The wave
21
propagates within the substrate
5
and is internally reflected at a lower surface
18
of the substrate
5
to form a reflected wave
23
that is directed towards an upper surface
19
of the substrate
5
. Consequently, a plurality of reflected waves
23
propagate within the substrate
5
between the upper surface
19
and the lower surface
18
. A portion of the energy present in each reflected wave
23
may also leave the substrate
5
through the surface
18
, to form a plurality of leakage waves
25
. An internal reflection
27
from an end
24
of the array
10
may lead to still further reflected waves
27
and leakage waves
26
.
The propagation of acoustic waves
23
and
27
in the substrate
5
, as described above, permits ultrasonic energy to be cross-coupled between the plurality of MUT transducers
1
on the substrate
5
and produce undesirable “cross-talk” signals between the plurality of MUTs
1
, as well as other undesirable interference effects. Still further, the internal reflection of waves in the substrate
5
may adversely affect the acceptance angle, or directivity of the array
10
.
Various prior art devices have included elements that impede the propagation of waves in the substrate. For example, one prior art device employs a plurality of trenches between the MUTs
1
that extend downwardly into the substrate
5
to interrupt wave propagation within the substrate
5
. Another prior art device employs a similar downwardly projecting trench, and fills the trench with an acoustic absorbing material in order to at least partially absorb the energy in the reflected waves
23
. Other prior art devices minimize lateral wave propagation by controlling still other geometrical details of the array. Although these prior art devices generally reduce the undesired lateral wave propagation in the substrate, they generally limit the design flexibility inherent in the MUT by reducing the number of design parameters that may be independently varied. Furthermore, the additional manufacturing steps significantly increase the manufacturing cost of arrays that use MUTs.
A further disadvantage associated with the prior art devices shown in
FIGS. 1 and 2
is that a relatively large parasitic capacitance may be formed b
Dorsey & Whitney LLP
Jaworski Francis J.
Koninklijke Philips Electronics NV
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