Communications – electrical: acoustic wave systems and devices – Signal transducers
Reexamination Certificate
1999-10-01
2001-09-25
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Reexamination Certificate
active
06295247
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to ultrasonic transducers. More particularly, it relates to micromachined capacitive ultrasonic transducers for generating and detecting Rayleigh, Lamb, and bulk waves in solids.
BACKGROUND ART
Ultrasonic transducers are used to generate and detect acoustic waves in solid and fluid media. They are found in a wide variety of applications, including chemical sensing, signal processing, nondestructive evaluation, and medical imaging. Based on the desired application, they can be tailored to produce and detect particular frequencies in a suitable medium. Chemical sensors, for example, excite ultrasonic waves in a solid substrate. The waves are generated in one region of the substrate, and pass through an area subject to chemical adsorption. The resultant density change in the adsorbing region alters the wave's propagation speed in the material, and this change is then detected at the other end of the sensor.
Conventional ultrasonic transducers are used to excite and detect Rayleigh (surface), Lamb (plate), and bulk waves in solid media. Transducer design and operating conditions are chosen to select a particular mode required by the application. For generation of waves in solids, three types of transducers are commonly found: piezoelectric, electromagnetic, and thermal. Each type has some drawbacks, either in efficiency, bandwidth, or ease of manufacture. Piezoelectric transducers, the most common type, require the deposition of piezoelectric material on a substrate before a voltage can be applied to the material to generate the waves. Creating the circuitry and depositing the piezoelectric material require independent manufacturing processes, making the overall process costly and inefficient. Electromagnetic transducers combine magnetic field application with fringing eddy currents to generate a sufficient force to excite ultrasonic waves. Thermal excitation transducers use lasers operating in the ablation regime to generate the waves. These transducers are clearly more complex and expensive to operate than piezoelectric transducers.
For applications requiring generation and detection of waves in air or other fluids, capacitive ultrasonic transducers are much more efficient than the above types. Examples of capacitive transducers are described in U.S. Pat. No. 5,287,331, issued to Schindel et al.; U.S. Pat. No. 5,619,476, issued to Haller et al.; and U.S. Pat. No. 5,894,452, issued to Ladabaum et al. In general, capacitive transducers contain a substrate and an array of membranes, usually circular, supported above the substrate by a dielectric material. Both the membranes and substrate have a conductive region, and can be thought of as plates of a capacitor. When an electrical signal is applied across the plates, an electric field between the substrate and membrane causes the membrane to vibrate, generating acoustic waves in the air at the frequency of the applied signal. Similarly, when an airborne acoustic wave is received by the membrane, it generates a corresponding electrical signal. If the substrate is silicon, the entire transducer can be produced using well-known silicon processing techniques, allowing for micron-scale accuracy. Capacitive transducers are particularly attractive because they can be produced inexpensively and are easily integrated with the required electronics.
Capacitive transducers are not very efficient in transferring energy from the membrane to the surrounding fluid. In fact, in some cases up to 90% of the energy is instead transferred to the support structure. However, because they are designed for excitation and detection of fluid waves, existing transducers cannot be used to excite acoustic modes of the solid support structure efficiently, and the energy coupled into the solid is wasted.
There is a need, therefore, for a capacitive ultrasonic transducer that can selectively excite and detect Rayleigh, Lamb, or bulk modes in solids.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide a capacitive ultrasonic transducer for exciting and detecting particular Rayleigh, Lamb, and bulk acoustic modes in a solid.
It is a further object of the invention to provide an ultrasonic transducer that can be produced by standard integrated circuit manufacturing techniques. These well-developed methods provide excellent control, precision, scalability, and repeatability.
It is an additional object of the invention to provide an ultrasonic transducer that can be easily integrated with the required electronics, even on the same wafer, without requiring additional materials.
It is another object of the present invention to provide a capacitive ultrasonic transducer than can replace existing piezoelectric transducers in many applications including sensing, filtering, and imaging.
SUMMARY
These objects and advantages are attained by a capacitive ultrasonic transducer for selectively generating and detecting Rayleigh (surface), Lamb (plate), and bulk wave modes in a solid. The transducer includes a support structure, formed from a substrate, preferably a silicon wafer, with a conductive back electrode, and a support material on the substrate; a membrane fixed to and supported by support regions of the support material; and a conductive film in communication with the membrane. A voltage applied between the conductive back electrode and conductive film generates an attractive force between the membrane and substrate, causing the membrane to vibrate with the applied frequency. Membrane vibration couples stresses into the support structure, generating ultrasonic waves in the solid.
Conversely, existing waves in the solid cause membrane vibration and generation of an electrical signal.
The support material contains support regions, which are defined by a height h
s
and a thickness t
S
and have at least two extended parallel edges. The membrane is fixed above the parallel edges and supported above the substrate to define at least one elongated gap between the membrane, support regions, and substrate. Preferably, there are more than one gaps defined by more than two parallel edges, and more than one membranes are supported above the gaps. The gaps are separated by a predetermined distance d. Preferably, unsupported free regions of the membranes above the gaps are substantially rectangular. They may also be curved, to focus the ultrasonic waves. The membranes have a thickness t
m
and are capable of vibrating above the gaps at a predetermined frequency; h
s
, t
s
, t
m
, and d are chosen to couple energy of the vibrating membranes to a particular acoustic mode of the support structure. The acoustic modes propagate in a direction perpendicular to the parallel edges.
To select an acoustic mode of the transducer, a particular value of the distance d between adjacent gaps and unsupported free regions of the membranes is chosen. d may be equal to a wavelength, half-wavelength, or one-third of a wavelength of the chosen acoustic mode. Alternately, the gaps may be divided into subsets, with each subset having gaps that are separated by a negligible distance. The subsets themselves are separated by a distance equal to a wavelength or half-wavelength of the chosen acoustic mode.
The present invention also provides a method for using a capacitive ultrasonic transducer. The method includes two steps: vibrating the membranes above the gaps to generate acoustic waves with particular propagation characteristics in the conductive substrate and support material, and selecting the propagation characteristics of the acoustic waves. Preferably, the support regions have substantially parallel edges, and the waves propagate in a direction perpendicular to the parallel edges. The membranes may be vibrated by applying an electrical signal between the back electrode and the conductive film. The electrical signal has an AC component and a DC component, and excites a Rayleigh, Lamb, or bulk acoustic wave mode. Different regions of the membrane may be excited with different phases, or with different v
Degertekin F. Levent
Khuri-Yakub B. T.
Lumen Intellectual Property Services Inc.
Pihulic Daniel T.
The Board of Trustees of the Leland Stanford Junior University
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