Metal working – Piezoelectric device making
Reexamination Certificate
1999-05-03
2001-06-12
Young, Lee (Department: 2834)
Metal working
Piezoelectric device making
C029S025350, C029S593000, C310S320000, C310S321000, C310S333000, C310S359000, C310S360000, C310S368000
Reexamination Certificate
active
06243933
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric resonator or filter and a method for fabricating the same.
2. Description of Prior Art
A piezoelectric resonator is used mainly as an oscillator or as a clock element in computers, various apparatuses with a microprocessor, and other various digital apparatuses. A piezoelectric resonator comprises a piezoelectric plate cut from a single crystal such as quartz or a piezoelectric ceramic and driving electrodes formed appropriately on the plate. The resonator uses strong resonance generated by applying a driving voltage to the driving electrodes at a frequency around the resonance frequency determined by the sound velocity and the size of the piezoelectric plate. The piezoelectric resonators are used widely because they have superior properties though they have a simple structure.
The resonator traps vibration energy below the driving electrodes, while it is fixed at portions outside the driving electrodes. Then, it can be mounted in a package or on a print circuit board without effecting vibrations. This type of resonators is called as energy trapping type resonator.
Recently, various information apparatuses such as a personal computer perform high speed processing. Then, it is demanded to increase clock frequency for information apparatuses and peripherals thereof such as hard disk drives and CD-ROM drives. For a frequency range from ten to a few tens of MHz used in these apparatuses, resonators use thickness vibration such as thickness shear vibration, thickness twist vibration or thickness-extensional vibration having vibration frequency in reverse proportion to the thickness of the piezoelectric material. As the frequency becomes higher, the piezoelectric material becomes thinner. For example, the thickness is 100 &mgr;m for frequencies exceeding 40 MHz. Then, various problems occur such as decrease in relative precision on forming, decrease in mechanical strength and increase in cost.
It is suggested in Japanese Patent laid open Publication 63-311808/1988 to form layers of lithium niobate with a reversed polarization in order make the thickness of the piezoelectric material for a particular frequency twice the counterpart in a previous resonator in correspondence to the frequency.
FIGS. 1A-1D
show side views for illustrating processes for forming layers
102
,
109
with a reversed polarization. A piezoelectric resonator
101
has driving electrodes
103
and
104
formed on opposing principal planes (top and bottom plane such as Z plane)
102
a
,
102
b
of a piezoelectric plate
102
cut from a lithium niobate single crystal.
In
FIG. 1A
, a wafer
105
is sliced from a lithium niobate single crystal subjected to poling, or the wafer
105
is sliced in a direction oblique by an appropriate angle relative to a polarization direction generated by the poling. A thin film
106
of titanium (Ti) is deposited on a plane of +c axis (or the top plane or +Z′ plane in
FIG. 1A
) if the direction of the spontaneous polarization P
S
is in the direction of an arrow shown in
FIG. 1A
such as an upward direction.
Next, it is heated at a temperature between Curie temperature (about 1250° C.) of lithium niobate and 1100° C. to diffuse titanium in the titanium thin film
106
into the wafer
105
, and a domain
109
with a reversed polarization is formed, as shown enlarged in FIG.
1
B.
If the depth of the domain
109
with the reversed polarization is denoted as “t”, surface charges generated in the wafer
105
under diffusion have a balanced state when the depth “t” is equal to a half of the thickness T
3
of the wafer
105
. Then, the depth “t” of the domain
109
with the reversed polarization extending from the top plane stops to increase further at about a half of the thickness T
3
of the wafer
107
, and the direction of the polarization P
S
′ of the domain
109
becomes reverse to that of the polarization P
S
. Next, as shown in
FIG. 1C
, a plurality of driving electrodes
103
and
104
are formed with patterning on the top and bottom planes of the wafer
107
. Then, the wafer
107
is cut along dash and dot lines shown in
FIG. 1C
so that each element has the opposing electrodes
103
and
104
. Thus, a piezoelectric resonator
101
shown in
FIG. 1D
is completed.
The piezoelectric plate
102
having the polarization P
S
and the reverse polarization P
S
′ has a thickness about twice that of a prior art single domain piezoelectric resonator for the same frequency. For example, if the thickness of the prior art piezoelectric resonator is about 150 &mgr;m for vibration frequency of 26 MHz, that of the plate having the layers with a reversed polarization is about 300 &mgr;m. This is ascribed that half wavelength resonance is excited in the former while one wavelength resonance is excited for the latter.
For a resonator using lithium tantalate, as described for example in Japanese Patent 1-158811/1989, a proton exchange layer is formed for reversed polarization, and a part of the polarization is reversed selectively. The resonator also intends to enhance the upper limit of frequency twice, similarly to the above-mentioned lithium niobate resonator.
FIG. 2A
shows a piezoelectric plate
112
cut from a 0±10° rotation X plate of lithium tantalate single crystal which has a polarization P
S
directed from one principal plane (+X′ plane)
112
a
to another principal plane (−X′ plane)
112
b
. Then, as shown in
FIG. 2B
, a polyimide layer (mask)
113
of thickness of about 5 &mgr;m is applied to the +X′ plane
112
a
by using for example spin coating. Then, as shown in
FIG. 2C
, it is immersed in a liquid for proton exchange processing heated at 250° for about one hour. Then, a proton exchange layer
115
is formed extending from the −X′ layer
112
b
. Then, the piezoelectric plate
112
removed from the liquid
114
and cleaned is heated at a high temperature, for example between 560 and 610° C., below the Curie temperature of 620° C. of the lithium tantalate, for an appropriate time. Then, as shown in
FIG. 2D
, a layer
112
c
with a reversed polarization having spontaneous polarization P
S
′ with a direction reverse to the polarization P
S
is formed from the −X′ plane
112
b
to a half of the depth of the piezoelectric plate
112
. Then, as shown in
FIG. 2E
, driving electrodes
116
and
117
are formed on the opposing principal planes (+X′ and −X′ planes)
112
a
and
112
b
. Thus, a piezoelectric resonator
111
is completed.
It is a problem for a resonator made of lithium niobate or lithium tantalate having a high Q and a large electro-mechanical coupling coefficient that spurious mode is liable to occur due to unnecessary vibration modes. Then, in order to excite a pure vibration mode, a resonator is fabricated by selecting a cut angle which forces thickness-extensional vibrations having principal displacement in the thickness direction and thickness shear vibrations having principal displacement parallel to the plate.
A thickness-extensional mode resonator couples weakly with other vibration modes. Then, by using this property, a resonator having small spuriouses inherently can be provided. When an optimum cut angle is selected for lithium niobate and lithium tantalate, the electromechanical coupling coefficient of thickness shear vibration mode is zero and only the thickness-extensional mode is excited. However, energy of first order wave (fundamental wave) is not trapped between the electrodes at the cut angle, and the resonator uses resonance of third order harmonic wave (third overtone) This is ascribed to the Poisson ratio of the lithium niobate or lithium tantalate being equal to or less than a third, and the first order resonance energy in the thickness-extensional mode cannot be trapped.
In the resonator using third order resonance, vibrations around the fundamental wave or the first order resonance are recognized as unnecessary vibratio
Kawasaki Osamu
Sugimoto Masato
Takeda Katsu
Tomita Yoshihiro
Kim Paul D
Matsushita Electric - Industrial Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
Young Lee
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