Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-04-08
2004-05-11
Wells, Kenneth B. (Department: 2816)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S188000, C029S025350
Reexamination Certificate
active
06734763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin-film piezoelectric resonator and a method of making the same. The present invention also relates to a band-pass filter utilizing a thin-film piezoelectric resonator (called “piezo-resonator” below).
2. Description of the Related Art
With the rapid spread of mobile telecommunications equipment such as portable telephones, small and light band-pass filters, as well as resonators needed to make such filters, are in great demand. As known in the art, thin-film piezo-resonators are suitable for producing high-power filters.
A typical thin-film piezo-resonator includes a substrate and a resonator assembly mounted on the substrate. The resonator assembly includes a piezoelectric film and a pair of thin electrodes sandwiching the piezoelectric film from above and below. The substrate is formed with a cavity below the lower electrode of the resonator assembly.
When an AC voltage is applied to the upper and the lower electrode of the piezo-resonator, the sandwiched piezoelectric film vibrates in its thickness direction (which is known as the inverse piezoelectric effect). On the other hand, by the direct piezoelectric effect, a mechanical vibration or elastic wave is converted into a corresponding electrical signal. The elastic wave is a longitudinal wave whose main displacement occurs in the thickness direction of the piezoelectric film. In such a thin-film piezo-resonator, the resonator assembly resonates when its thickness H is equal to n/2 times the wavelength of the elastic wave (where n is an integer). Supposing that the propagation velocity of the elastic wave is V (which depends on the material used), the resonance frequency F is expressed by a formula F=nV/2H. This means that a piezo-resonator having desired frequency characteristics can be obtained by adjusting the thickness H of the resonator assembly. Further, by connecting such resonators in a ladder configuration, it is possible to produce a band-pass filter which allows only those electric waves lying within a certain frequency range to pass.
In the above-described thin-film piezo-resonator, desired resonance characteristics are attained by providing a cavity or hole below the lower electrode. Techniques suitable to making such a cavity are disclosed in “ZnO/SiO
2
-DIAPHRAGM COMPOSITE RESONATOR ON A SILICON WAFER” (K. NAKAMURA, ELECTRONICS LETTERS Jul. 9, 1981 Vol. 17 No. 14 p507-509), JP-A-60 (1985)-189307, JP-A-2000-69594, U.S. Pat. No. 6,060,818 and U.S. Pat. No. 5,587,620 for example.
FIG. 20
shows, in section, a thin-film piezo-resonator disclosed in the above-mentioned “ZnO/SiO
2
— DIAPHRAGM COMPOSITE RESONATOR ON A SILICON WAFER”. The thin-film piezo-resonator (generally indicated by reference numeral
700
) includes a (100)-cut silicon substrate
710
and a resonator assembly
720
supported by the substrate
710
. The resonator assembly
720
is made up of a lower electrode
721
, a piezoelectric film
722
, and an upper electrode
723
. The silicon substrate
710
has an upper surface upon which a SiO
2
film
711
is formed by thermal oxidation. The resonator assembly
720
is placed directly on the SiO
2
film
711
. The silicon substrate
710
is formed with a cavity
710
a
whose upper opening is closed by the SiO
2
film
711
. The cavity
710
a
can be produced by anisotropic etching with respect to the (100) surface of the silicon substrate. The anisotropic etching is performed from below the silicon substrate
710
with the use of KOH solution or EDP solution (ethylenediamine+pyrocatechol+water) for example.
The above anisotropic etching relies upon the fact that the etching rate with respect to the (100) surface of the substrate
710
is significantly greater than the etching rate with respect to the (111) surface. Therefore, the resonator assembly is to be provided only on the (100) surface of the substrate
710
. However, such positional limitation is disadvantageous since it restricts the option of the material suitable for making the components of the resonator assembly
720
, while also depriving the resonator assembly components of freedom of orientation. Another disadvantage is that the lower opening of the cavity
710
a
tends to be unduly large due to the nonupright side wall
710
a
′ of the cavity
710
a
. In the illustrated arrangement, the side wall
710
a
′ corresponds to the (111) surface of the substrate
710
and has an inclination of 54.7 degrees with respect to the (100) surface of the silicon substrate
710
. Due to this, the cavity
710
a
has a large opening in the bottom surface of the silicon substrate
710
. For instance, when the substrate
710
has a thickness of 300 &mgr;m, the lower opening of the cavity
710
a
is larger than the upper opening by more than 420 &mgr;m. Unfavorably, such a large opening of the cavity
710
a
reduces the mechanical strength of the piezo-resonator
700
. In addition, it contributes to an increase in the overall size of the piezo-resonator
700
. With the use of such oversize piezo-resonators, a compact band-pass filter cannot be obtained. Specifically, when the thickness of the substrate
710
is 300 &mgr;m, the lower opening of the cavity
710
a
is larger than the upper opening by more than 420 &mgr;m, as noted above. Thus, the distance between the neighboring upper openings should be more than 420 &mgr;m. Further, as the distance between the adjacent upper openings increases, the length of the wiring pattern for connecting the adjacent resonator assemblies should also increase. This leads to an increase in the resistance of the wiring pattern. A greater resistance of the wiring pattern can be a major factor that prevents the improvement of the filter characteristics in a high-frequency band.
FIG. 21
shows a thin-film piezo-resonator disclosed in JP-A-60-189307. The piezo-resonator
800
includes a substrate
810
, and a resonator assembly
820
which is made up of a lower electrode
821
, a piezoelectric film
822
, and an upper electrode
823
. A cavity
830
is provided between the substrate
810
and the resonator assembly
820
. According to the Japanese document, the piezo-resonator
800
is fabricated in the following manner. First, a sacrifice layer for the cavity
830
is formed in a pattern on the substrate
810
. Next, a SiO
2
film
840
is formed on the sacrifice layer
840
in a manner such that part of the sacrifice layer is exposed. Then, the resonator assembly
820
is provided on the SiO
2
film
840
. Finally, the sacrifice layer is removed by wet etching, so that the cavity
830
appears below the resonator assembly
820
. According to this method, the cavity
830
is kept from becoming too large with respect to the resonator assembly
820
.
In the thin-film piezo-resonator utilizing a longitudinal vibration in the thickness direction, a high-orientation piezoelectric film is required to provide excellent resonance characteristics. According to the technique disclosed in JP-A-60-189303, however, it is difficult to give a sufficiently high orientation to the piezoelectric film
822
. The cavity
830
below the resonator assembly
820
has a length L
15
, which needs to be at least a few micron meters when a twist and oscillation displacement of the resonator assembly
820
are taken into consideration. Unfavorably, the sacrifice layer, formed to have a thickness corresponding to the length L
15
, has a greater surface roughness than that of the silicon substrate
810
. This deteriorates the orientation of the lower electrode
821
and the piezoelectric film
822
both of which are grown on the sacrifice layer via the SiO
2
film
840
. As a result, it is difficult to obtain good resonance characteristics with the thin-film piezo-resonator.
FIG. 22
is a sectional view of a thin-film piezo-resonator disclosed in JP-A-2000-69594. The thin-film piezo-resonator
900
includes a silicon substrate
910
, and a resonator assembly
920
made up of a lower electrode
921
, a piezoelectric film
922
and an upp
Kimachi Rei
Miyashita Tsutomu
Nishihara Tokihiro
Sakashita Takeshi
Yokoyama Tsuyoshi
Armstrong Kratz Quintos Hanson & Brooks, LLP
Fujitsu Limited
Wells Kenneth B.
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