Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
1998-10-06
2001-04-03
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S156000, C428S216000, C428S336000, C428S408000, C428S457000
Reexamination Certificate
active
06210780
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a diamond wafer and a surface acoustic wave (SAW) device produced on the diamond wafer. This invention relates more specially to a method of estimating diamond wafers which judges whether a diamond wafer is suitable for producing SAW devices or not. The estimation selects diamond wafers that have a low density of surface defects which enables manufacturers to produce a low loss SAW device. The diamond wafers can also be used as the substrates of microelectronic devices with microscopic wiring or the substrates of micromachines with microscopic structures.
This application claims the priority of Japanese Patent Application No.9-290262 (290262/97) filed Oct. 6, 1997 which is incorporated herein by reference.
2. Description of Related Art
Diamond enjoys the highest sound velocity among all the natural materials. The hardness is also the highest. The thermal conductivity is large. The band gap of diamond is 5.5 eV which is an extremely high value among all known materials. Diamond is excellent in dynamical property, electrical property and electronic property. Diamond is used for dynamic devices and electronic devices that can take advantage of its excellent properties. Improvements have been made by taking advantage of the outstanding properties of diamond in the technical fields of acoustics, optics and semiconductor. Exploitation of diamond will be effective for improving various properties of electronic, acoustic devices or for enlarging margins of operation of the devices.
A surface acoustic wave device (SAW device) is a good candidate for the use of diamond for improving its characteristics. Surface acoustic wave devices can be configured to be, for example, a radio frequency filter, a phase shifter, a convolver, an amplifier, etc. The SAW filter acts as an IF (intermediate frequency) filter of television sets or various filters of communication devices. A surface acoustic wave device is a device having a rigid base, a piezoelectric film stuck to the rigid base and interdigital transducers formed on both ends of the piezoelectric film. Application of an AC voltage on the interdigital transducer causes an AC electric field on the piezoelectric film which deforms in proportion to the electric field. Since the electric field oscillates, the piezoelectric film alternatively expands and contracts in the horizontal direction between two interdigital transducers at the same frequency as the AC voltage.
The piezoelectric film oscillates in the horizontal direction with the frequency of the AC signal. Since the piezoelectric film adheres to the rigid base, the rigid base also oscillates at the same frequency in the same manner. Since the rigid base repeats expansion and contraction at the interdigital transducer (IDT), the oscillation propagates as a longitudinal elastic wave on the surface. The AC voltage applied on the interdigital transducer generates an elastic wave. The wavelength is determined by the period of the interdigital transducer. The elastic wave spreads from one interdigital transducer to the other interdigital transducer. The piezoelectricity is reversible. At the receiving interdigital transducer, the deformation oscillation induces an AC voltage between the components of the electrode. As a whole, the AC signal propagates from one interdigital transducer to the other interdigital transducer by the elastic wave. The wave is called surface acoustic wave (SAW), because it propagates on the surface of the device.
The period of the interdigital transducer uniquely determines the wavelength &lgr; of the surface acoustic wave. The rigidity and density of the rigid base determine the velocity v of the SAW. The more rigid and lighter base brings about the higher SAW velocity. SAW velocity v is different from sound velocity which is equal to a square root of Young's modulus divided by density &rgr;. As with sound velocity, the SAW velocity is higher for the rigid base of higher Young's modulus and lower density. A sound wave is an elastic wave passing through an inner portion of a material. SAW is another elastic wave propagating only on the surface of the material. SAW differs from sound wave. SAW velocity is, in general, higher than sound velocity.
Since the wavelength &lgr; and the velocity v have been predetermined by the interdigital transducer and the physical property of the rigid base, the frequency f is also surely determined as f=v/&lgr;. This is a unique value. Since f is a unique value, it is denoted by f
0
. Namely, the SAW device has a filtering function which selectively allows only the SAW of f
0
to pass the device. SAWs of frequency different from f
0
attenuate. Transmittable SAW has a definite frequency f
0
which is determined by the material of the rigid base and the spatial period of the interdigital transducers. SAW devices have been applied to TV filters having a low allowable frequency of several megahertzs to tens of megahertzs. Hopefully, SAW devices will be applied to far higher frequency filters, for instance, optical communication filters of 2.488 GHz or wireless LAN filters in near future.
Raising frequency f
0
requires either narrowing a spatial period of interdigital transducers or increasing a SAW velocity v. The spatial period of interdigital transducers is limited by the current lithography technology. The only way is the increase of velocity v Diamond, as a rigid base, exhibits the highest SAW velocity among all natural materials. The application of diamond to SAW devices attracts attention. Diamond endows the SAW devices with the highest velocity which affords a moderately wide spatial period to the interdigital transducers.
High velocity is not the only requisites for a material used for SAW devices. Low propagation loss is another important requirement for SAW materials. Loss is a key concept of the present invention. There are different losses in addition to the propagation loss. Losses are now briefly clarified. One is the Joule loss &Dgr;Er by the resistance of the interdigital transducers to which electric power is supplied. Another is an electromechanical conversion loss &Dgr;Ec of energy accompanying the expansion and contraction of the piezoelectric material by the AC electric field. The loss depends on the electromechanical coefficient of the piezoelectric film. The interdigital transducer which converts electric power into mechanical power through the piezoelectric film has no selectivity of direction. The surface acoustic waves propagate in both directions perpendicular to the stripes of the interdigital transducers. Just half of the mechanical power spreads toward the counterpart interdigital transducer. Another half (6 dB) is a loss. This is called a bisection loss &Dgr;Eb. Now a SAW starts from one interdigital transducer and some of the SAW arrives at the other interdigital transducer. The difference between the starting SAW power and arriving SAW power is the propagation loss &Dgr;Ep. The aim of the present invention is a reduction of the propagation loss &Dgr;Ep. At the other interdigital transducer, the piezoelectric film converts the mechanical power of SAW into electric power of AC voltage with a conversion loss &Dgr;Ec. The current flows in the receiving interdigital transducer with a resistance loss &Dgr;Er.
Total loss is a sum, 2 &Dgr;Er+2 &Dgr;Ec+&Dgr;Eb+&Dgr;Ep, of the resistance loss 2 &Dgr;Er, the conversion loss 2 &Dgr;Ec, the bisection loss &Dgr;Eb and the propagation loss &Dgr;Ep. &Dgr;Er is contingent upon interdigital transducers. &Dgr;Ec is ruled by the piezoelectric material. Geometry decides &Dgr;Eb. Only the propagation loss &Dgr;Ep depends upon the insulating material (rigid base). This invention aims at alleviating &Dgr;Ep.
The insulator which has been most widely used as the material of the rigid base is glass. Glass is an inexpensive and low-loss insulator. ZnO/glass SAW filters have been popularly employed as TV intermediate frequency filters. Zinc oxide (ZnO) is a piezoele
Fujii Satoshi
Higaki Kenjiro
Kitabayashi Hiroyuki
Nakahata Hideaki
Seki Yuichiro
Pillsbury & Winthrop LLP
Sumitomo Electric Industries Ltd.
Turner Archene
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