Acoustic transducer and method of making the same

Details Acoustic transducer and method of making the same Acoustic transducer and method of making the same

1999-05-20

2001-08-07

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S365000, C310S369000

Reexamination Certificate

active

06271620

ABSTRACT:

BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to the field of acoustic transducers. More specifically, the present invention relates to a novel electrostatic ultrasonic transducer capable of operating in high frequency ranges, and novel methods of fabricating such a transducer.
II. Description of the Related Art
An acoustic transducer is an electronic device used to emit and receive sound waves. An ultrasonic transducer is a type of acoustic transducer that operates at a frequency range beyond that of human perception, about 20 KHz Acoustic transducers are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of acoustic transducers are piezoelectric transducers, which operate in low and narrow band frequencies. Piezoelectric transducers are not efficient in the conversion between electric and acoustic energy in air. Furthermore, the operating frequencies of piezoelectric transducers in air are quite low.
Air coupled ultrasonic transducers with higher operating frequencies, which rely on certain microfabrication techniques, are described by Haller et al. in U.S. Pat. No. 5,619,476 entitled “Electrostatic Ultrasonic Transducer,” issued Apr. 9, 1997, and Ladabaum et al. in U.S. Pat. No. 5,870,351 entitled “Broadband Microfabricated Ultrasonic Transducer and Method of Fabrication,” issued Feb. 9, 1999. Published material known in the art also demonstrates that immersion transducers can be made with similar techniques. Air-coupled transducers are usually resonant, while liquid-coupled transducers are typically not. As shown in
FIGS. 1A and 1B
taken from the '476 patent, the transducer disclosed therein is made of a substrate
11
and a gold contact layer
14
that forms one one plate of a capacitor, and a membrane including a nitride layer
13
and a gold contact layer
14
B that form the other plate of the capacitor (while the gold contact layer
14
is the electrode, with the nitride layer
13
being an insulator, the reference to electrode
13
/
14
will be used so as to distinguish the other electrode
11
/
14
that has a gold contact layer
14
adjacent the conductive substrate
11
as illustrated in the above-mentioned patents). Holes
16
etched in the nitride layer
13
and the gold layer
14
are used to etch away portions of the sacrificial oxide layer
12
, while remaining posts of the sacrificial layer
12
support the membrane. By noting the change in capacitance between the two electrodes
13
/
14
and
11
/
14
, the ultrasonic resonance of the membrane can be detected.
Such microfabricated ultrasonic transducers use resilient membranes that have very little inertia. The momentum carried by approximately half of a wavelength of air molecules is able to set the membrane in motion and visa versa. Electrostatic actuation and detection enable the realization and control of such resonant membranes. When distances are small, electrostatic attractions can exert very large forces on the actuators of interest.
Microfabricated ultrasonic transducers of this design have practical problems that prohibit their use at high frequencies, typically above about 10 MHz, and that reduce their efficiency at any frequency range. It has been realized by the present inventor that there are various reasons that prohibit the use of microfabricated ultrasonic transducers. One reason is that the electrodes
13
/
14
and
11
/
14
are each formed as a conductive sheet. As illustrated in
FIG. 1A
, while the gold contact layer
14
covers the voids where the sacrificial layer
12
has been etched away, the gold contact layer
14
also entirely covers the posts which support the membrane. Similarly, the substrate
11
and the gold contact layer
14
associated therewith is another conductive sheet. Accordingly, at areas other than where sacrificial etch access holes
15
exist, there is no area where the electrodes
13
/
14
and
11
/
14
do not overlap. This overlap causes a parasitic capacitance, which is exacerbated due to the fact that the dielectric constant of the semiconductor insulators between the areas of the sacrificial layer
12
posts can be approximately one order of magnitude larger than that of the air/vacuum gap at the center of the membrane. As frequencies become higher, the parasitic capacitance becomes significant and sometimes even a dominant factor in transducer performance. Thus, even if the overlap at the areas of the sacrificial layer
12
posts accounts for only {fraction (1/10)} of the active area of the transducer, such overlap may account for half the capacitance.
Furthermore, the spacing between the top electrode
13
/
14
and the bottom electrode
11
/
14
is a further reason that the parasitic capacitance increases. In particular, the membrane has a thickness that, due to physical constraints, needs to be at least about 2,500 Angstroms thick. Thus, when the gold contact layer
14
is placed over the nitride layer
13
, there is additional parasitic capacitance due to the thickness of the nitride layer.
As a result of the above-mentioned parasitic capacitances, transducers such as those described in Haller et al or Ladabaum et al are not able to operate at higher frequencies, and operate less efficiently than ultimately possible at lower frequencies. Accordingly, there is the need for an improved acoustic transducer.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ultrasonic transducer capable of operating at higher frequencies.
It is a further object of the present invention to provide an ultrasonic transducer capable of operating more efficiently than previously known ultrasonic transducers.
It is a further object of the present invention to provide an ultrasonic transducer that has reduced parasitic capacitance between the electrodes used to alternatively detect and excite the sound wave.
It is a further object of the present invention to form a transducer of a plurality of transducer cells that have an interconnect structure that reduces parasitic capacitance.
It is a further object of the present invention to provide a method for fabricating an ultrasonic transducer that has the above-mentioned characteristics.
The present invention achieves the above objects, among others, with an ultrasonic transducer comprised of a plurality of transducer cells, and conductive interconnects between the cells. Each transducer cell contains a bottom electrode formed on a layer of insulator material, a lower insulating film portion formed over the bottom electrode, a middle insulating film portion that includes an air/vacuum void region, and an upper insulating film portion that includes a top electrode formed within a portion of the upper insulating film portion. A first layer of interconnects electrically connect the bottom electrodes of each transducer cell and a second layer of interconnects electrically connect the top electrodes of each transducer cell. The top and bottom layers of interconnects are patterned to avoid overlap between them, thus reducing the parasitic capacitance.
Further, as noted, the top electrode is preferably formed within the upper insulating film portion, closer to the air/vacuum void than to the top surface of the insulating film, to increase the electric field for a given voltage.
Still furthermore, the electrodes within each transducer cell are preferably formed to have dimensions that are smaller than the overall surface area of the insulating film that they excite.
A method of fabricating the ultrasonic transducer according to the present invention is initiated by depositing and forming a pattern of the bottom electrode and interconnects. Thereafter, the lower insulating film portion of insulator material is deposited. A sacrificial layer is then deposited over the lower insulating film portion and etched to a desired pattern. The middle insulating film portion of insulator material is deposited over the sacrificial layer pattern, followed by the depositing and forming of the top layer of electrode and interconnects. Thereaf

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