Electrical generator or motor structure – Non-dynamoelectric – Charge accumulating
Reexamination Certificate
1999-12-06
2002-03-19
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Charge accumulating
C310S306000, C310S311000, C310S334000, C310S001000
Reexamination Certificate
active
06359367
ABSTRACT:
BACKGROUND
The present invention relates generally to transducer arrays, and more particularly, to spiral transducer arrays manufactured using micromachining fabrication technologies.
Capacitive micromachined ultrasonic transducers (CMUTs) in particular have been fabricated in this manner. Spiral sparse arrays have been described in various publications, Spiral sparse arrays are discussed in U.S. Pat. No. 5,808,962 entitled “Ultrasparse, Ultrawideband Arrays” and a technical paper by Sumanaweera et al. entitled “A Spiral 2D Phased Array for 3D Imaging” published in the Proceedings of the IEEE International Ultrasonic Symposium, 1999.
Capacitive micromachined ultrasonic transducers (CMUTs) have also been described in various publications. Such transducers are described in U.S. Pat. No. 5,619,476 entitled “Electrostatic Ultrasonic Transducer”, U.S. Pat. No. 4,262,339 entitled “Ferroelectric Digital Device”, and U.S. Pat. No. 4,432,007 entitled “Ultrasonic Transducer Fabricated as an Integral Part of a Monolithic Integrated Circuit”.
Finally, the following papers report the use of micromachining technologies in the fabrication of conventional ultrasound transducer designs: (1) R. A Noble et al., “Novel silicon nitride micromachined wide-bandwidth ultrasonic transducers”, presented at the 1998 IEEE International Ultrasonics Symposium in Sendai, Japan, (2) X. C. Jin, “Micromachined capacitive transducer arrays for medical ultrasound imaging”, presented at the 1998 IEEE International Ultrasonics Symposium in Sendai, Japan, (3) I. Ladabaum, “Miniature drumheads: microfabricated ultrasonic transducers”, Ultrasonics 36 (1998) 25-29, and (4) H. T. Soh, “Silicon micromachined ultrasonic immersion transducers”, Appl. Phys. Lett. 69 (24), Dec. 9, 1996.
However, heretofore, the use of micromachining has not been applied to the fabrication of spiral arrays and sparse spiral arrays in particular. Spiral arrays, previously recognized as offering unique beam-forming advantages such as sidelobe elimination, have not been rendered manufacturable using conventional transducer construction methods. Inventors of the present invention recognize the unique abilities of micromachining are now able to solve this problem in advantageous manners disclosed herein.
In the past, conventional two-dimensional arrays (areal arrangements of piezoelements) have been fabricated using piezoelectric ceramic materials such as PZT. Although the typical ceramic PZT materials used in medical ultrasound transducer arrays have a high dielectric constant, the electrical impedance of a small two-dimensional array element is very high. This prevents effective transmission of the transmission pulse signals through the transducer cable without using buffer amplifiers at the probe end of the cable.
In addition, the electrical connection to the small areal piezoelectric ceramic elements is generally done using multilayer flexible circuits, which comprise a layered structure of polymer and metal support materials, typically Kapton™ and copper. Kapton, having a low acoustic impedance, and copper having a high acoustic impedance, form a highly undesirable acoustic loading to the high acoustic impedance piezomaterial. This in effect increases the internal undesired reflections within the transducer and compromises the necessary temporal compactness of the transducer's acoustic output in order to get good axial resolution.
It would be desirable to have a transducer structure wherein electrical connections do not significantly compromise the acoustic signal quality. It would also be desirable to have a transducer structure manufactured using micromachining fabrication techniques and materials that overcome the limitations of conventional arrays. It would also be desirable to have improved ultrasound imaging systems employing such transducer structures.
SUMMARY OF THE INVENTION
The present invention provides for spiral, or substantially spiral, transducer arrays manufactured using micromachining techniques and materials, with the arrays preferably being capacitive micromachined ultrasonic transducer arrays. Capacitive micromachined ultrasonic transducers (CMUTS) have been demonstrated to have sensitivities that are equivalent to piezoelectric ceramic elements.
Before proceeding the terms “micromachining” and “multilayer interconnects” used herein shall be defined.
Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining which we utilize-1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface-and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.
Multilayer interconnects are defined as including interconnects made in the manner of IC or integrated circuit interconnects or interconnects found on hybrid circuit substrates. More particularly, the transducer embodiments disclosed herein incorporate at least two of the following known items or two instances of one of the known items:
(1) Thin-film interconnect layer such as those deposited by PVD, CVD, LPCVD, electroplating, electroless plating, screen-printing, pattern-forming dispensing techniques or damascene-type CMP or chemical-mechanical-polishing techniques,
(2) Diffused interconnect layer or ion-implanted interconnects,
(3) Silicide metal-based interconnect layer,
(4) Vias or contact through-hole layer as formed by wet-etching, dry plasma etching, laser drilling, chemical photodevelopment of a photosensitive polymer dielectric or screen-printing,
(5) Interlayer insulating dielectric layer such as thin-film PECVD glasses, SOGs and spin-on polyimide, and
(6) Overcoat layer such as hermetic oxynitride or nitride protective and insulating layers used in combination with one or more of (1-5).
It is to be emphasized that the interconnects and vias may be either (or both) surface features (limited to the surface films as in a typical IC) or bulk features such as through-the-wafer vias and interconnects found in micromachined silicon pressure sensors sold by the millions.
By eliminating conventional fabrication processes and using micromachined processes and materials, the inventors of the present invention even more importantly realized that the difficult interconnection routing problem inherent to spiral arrays can be solved in addition to getting rid of the above impedance mismatch problems. Micromachining technologies are utilized for the fabrication of acoustic elements and related IC multilayer interconnection technologies to also solve the interconnection routing problem among those elements and their supporting electronics. Specific arrangements of such multilayer interconnects are described in support of micromachined spiral arrays and sparse spiral arrays
Ayter Sevig
Sliwa, Jr. John W.
Sumanaweera Thilaka S.
Acuson Corporation
Brinks Hofer Gilson & Lione
Budd Mark O.
Summerfield Craig A.
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