U.S. patent number RE47,989 [Application Number 15/655,800] was granted by the patent office on 2020-05-12 for acoustic wave device.
This patent grant is currently assigned to TAIYO YUDEN CO., LTD.. The grantee listed for this patent is TAIYO YUDEN CO., LTD.. Invention is credited to Yoshiki Iwazaki, Tokihiro Nishihara, Yosuke Onda, Tsuyoshi Yokoyama.
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United States Patent |
RE47,989 |
Yokoyama , et al. |
May 12, 2020 |
Acoustic wave device
Abstract
An acoustic wave device includes: a piezoelectric film made of
an aluminum nitride film containing a divalent element and a
tetravalent element, or a divalent element and a pentavalent
element; and an electrode that excites an acoustic wave propagating
through the piezoelectric film.
Inventors: |
Yokoyama; Tsuyoshi (Takasaki,
JP), Iwazaki; Yoshiki (Takasaki, JP),
Nishihara; Tokihiro (Takasaki, JP), Onda; Yosuke
(Takasaki, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
TAIYO YUDEN CO., LTD. |
Chuo-ku, Tokyo |
N/A |
JP |
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Assignee: |
TAIYO YUDEN CO., LTD. (Tokyo,
JP)
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Family
ID: |
49137145 |
Appl.
No.: |
15/655,800 |
Filed: |
July 20, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
13787497 |
Mar 6, 2013 |
9087979 |
Jul 21, 2015 |
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Foreign Application Priority Data
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Mar 15, 2012 [JP] |
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2012-058441 |
Nov 14, 2012 [JP] |
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2012-250535 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
41/0805 (20130101); H03H 9/02015 (20130101); H03H
9/54 (20130101); H03H 9/173 (20130101); H03H
9/173 (20130101); H03H 9/02015 (20130101); H01L
41/0805 (20130101); H03H 9/54 (20130101); H03H
9/174 (20130101); H01L 41/187 (20130101); H03H
9/02228 (20130101); H03H 9/0222 (20130101); H03H
9/02102 (20130101); H01L 41/187 (20130101); H03H
9/175 (20130101); H03H 9/174 (20130101); H03H
9/584 (20130101); H03H 9/02102 (20130101); H03H
9/175 (20130101); H03H 9/0222 (20130101); H03H
9/584 (20130101); H03H 9/02228 (20130101) |
Current International
Class: |
H01L
41/18 (20060101); H01L 41/08 (20060101); H03H
9/54 (20060101); H01L 41/187 (20060101); H03H
9/58 (20060101); H03H 9/02 (20060101); H03H
9/17 (20060101) |
Field of
Search: |
;310/328,330,358,312
;333/133,187,189,195 ;252/62.9R |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2002-344279 |
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Nov 2002 |
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JP |
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2009-010926 |
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Jan 2009 |
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JP |
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Other References
Non-Final Office Action issued by U.S. Patent and Trademark Office,
dated Dec. 24, 2014, for related U.S. Appl. No. 13/787,497. cited
by applicant .
Rajan S. Naik, and 10 others, "Measurements of the Bulk, C-Axis
Electromechanical Coupling Constant as a Function of AIN Film
Quality", IEEE Transactions on Ultrasonics, Ferroelectrics, and
Frequency Control, 2000, vol. 47, p. 292-296. cited by
applicant.
|
Primary Examiner: Ton; My Trang
Attorney, Agent or Firm: Law Office of Katsuhiro Arai
Claims
What is claimed is:
1. An acoustic wave device comprising: a piezoelectric film made of
an aluminum nitride film containing a divalent element and one of a
tetravalent or pentavalent element; and electrodes connected to the
piezoelectric film to excite an acoustic wave propagating through
the piezoelectric film, wherein the divalent element and one of the
tetravalent or pentavalent element are substituted for aluminum
atoms of the aluminum nitride film.
2. The acoustic wave device according to claim 1, wherein the
piezoelectric film is the aluminum nitride film containing the
divalent element and the tetravalent element, and contains at least
one of titanium, zirconium, and hafnium as the tetravalent
element.
3. The acoustic wave device according to claim 2, wherein the
piezoelectric film contains at least one of calcium, magnesium,
strontium, and zinc as the divalent element.
4. The acoustic wave device according to claim 1, wherein the
piezoelectric film is the aluminum nitride film containing the
divalent element and the pentavalent element, and contains at least
one of tantalum, niobium, and vanadium as the pentavalent
element.
5. The acoustic wave device according to claim 4, wherein the
piezoelectric film contains at least one of magnesium and zinc as
the divalent element.
6. The acoustic wave device according to claim 1, wherein the
piezoelectric film has a piezoelectric constant e.sub.33 greater
than 1.55 C/m.sup.2.
7. The acoustic wave device according to claim 1, wherein the
piezoelectric film has a ratio of a lattice constant in a c-axis
direction to a lattice constant in an a-axis direction less than
1.6.
8. The acoustic wave device according to claim 1, wherein the
piezoelectric film is the aluminum nitride film containing the
divalent element and the tetravalent element, and a total of
concentrations of the divalent element and the tetravalent element
is greater than or equal to 3 atomic % and less than or equal to 35
atomic % when a total number of atoms of the divalent element, the
tetravalent element, and aluminum in the aluminum nitride film
defines 100 atomic %.
9. The acoustic wave device according to claim 1, wherein the
piezoelectric film is the aluminum nitride film containing the
divalent element and the tetravalent element, and a ratio of a
concentration of the tetravalent element to a total of
concentrations of the divalent element and the tetravalent element
is greater than or equal to 0.35 and less than or equal to 0.75
when a total number of atoms of the divalent element, the
tetravalent element, and aluminum in the aluminum nitride film
defines 100 atomic %.
10. The acoustic wave device according to claim 1, wherein the
piezoelectric film has a crystal structure with a c-axis
orientation.
11. The acoustic wave device according to claim 1, further
comprising a temperature compensation film having a temperature
coefficient of an elastic constant opposite in sign to a
temperature coefficient of an elastic constant of the piezoelectric
film.
12. The acoustic wave device according to claim 11, wherein the
temperature compensation film contacts the piezoelectric film.
13. The acoustic wave device according to claim 11, wherein the
temperature compensation film contains mainly silicon oxide.
14. The acoustic wave device according to claim 1, wherein the
electrode includes an upper electrode and a lower electrode facing
each other across the piezoelectric film.
15. The acoustic wave device according to claim 14, further
comprising: a temperature compensation film having a temperature
coefficient of an elastic constant opposite in sign to a
temperature coefficient of an elastic constant of the piezoelectric
film; and conductive films that are located on a top surface and a
bottom surface of the temperature compensation film and
electrically connected to each other.
16. The acoustic wave device according to claim 1, further
comprising: a first piezoelectric thin film resonator and a second
piezoelectric thin film resonator, each including the piezoelectric
film and the electrode that includes an upper electrode and a lower
electrode facing each other across the piezoelectric film, wherein
the first piezoelectric thin film resonator and the second
piezoelectric thin film resonator are stacked, and a decoupler film
is located between the upper electrode included in the first
piezoelectric thin film resonator and the lower electrode included
in the second piezoelectric thin film resonator.
17. The acoustic wave device according to claim 1, wherein the
electrode is a comb-shaped electrode located on the piezoelectric
film.
18. The acoustic wave device according to claim 17, wherein the
acoustic wave excited by the electrode when the element is added to
the aluminum nitride constituting the film which is then charged
with electricity is a surface acoustic wave or a Lamb wave.
19. An acoustic wave device comprising: a piezoelectric film made
of an aluminum nitride film containing an element capable of at
least increasing a permittivity or decreasing an acoustic velocity
when the element is added to the aluminum nitride constituting the
film which is then charged with electricity; and electrodes
connected to the piezoelectric film to excite an acoustic wave
propagating through the piezoelectric film, wherein a resonance
frequency of the acoustic wave is less than or equal to 1.5 GHz,
and the element is substituted for aluminum atoms of the aluminum
nitride film.
20. The acoustic wave device according to claim 19, wherein the
piezoelectric film has a permittivity .di-elect cons..sub.33
greater than 8.42.times.10.sup.-11 F/m.
21. The acoustic wave device according to claim 19, wherein the
piezoelectric film has an acoustic velocity less than 11404
m/s.
22. The acoustic wave device according to claim 19, wherein the
piezoelectric film contains a divalent element and a tetravalent
element as the element.
23. The acoustic wave device according to claim 22, wherein the
divalent element is at least one of calcium, magnesium, strontium,
and zinc, and the tetravalent element is at least one of titanium,
zirconium, and hafnium.
24. The acoustic wave device according to claim 19, wherein the
piezoelectric film contains a trivalent element as the element.
25. The acoustic wave device according to claim 24, wherein the
trivalent element is at least one of yttrium and scandium.
26. The acoustic wave device according to claim 19, wherein the
piezoelectric film has a crystal structure with a c-axis
orientation.
27. The acoustic wave device according to claim 19, further
comprising a temperature compensation film having a temperature
coefficient of an elastic constant opposite in sign to a
temperature coefficient of an elastic constant of the piezoelectric
film.
28. The acoustic wave device according to claim 27, wherein the
temperature compensation film contacts the piezoelectric film.
29. The acoustic wave device according to claim 27, wherein the
temperature compensation film contains mainly silicon oxide.
30. The acoustic wave device according to claim 19, wherein the
electrode includes an upper electrode and a lower electrode facing
each other across the piezoelectric film.
31. The acoustic wave device according to claim 30, further
comprising: a temperature compensation film having a temperature
coefficient of an elastic constant opposite in sign to a
temperature coefficient of an elastic constant of the piezoelectric
film; and conductive films that are located on a top surface and a
bottom surface of the temperature compensation film and
electrically connected to each other.
32. The acoustic wave device according to claim 19, further
comprising: a first piezoelectric thin film resonator and a second
piezoelectric thin film resonator, each including the piezoelectric
film and the electrode that includes upper electrode and a lower
electrode facing each other across the piezoelectric film, wherein
the first piezoelectric thin film resonator and the second
piezoelectric thin film resonator are stacked, and a decoupler film
is located between the upper electrode included in the first
piezoelectric thin film resonator and the lower electrode included
in the second piezoelectric thin film resonator.
33. The acoustic wave device according to claim 19, wherein the
electrode is a comb-shaped electrode located on the piezoelectric
film.
34. The acoustic wave device according to claim 33, wherein the
acoustic wave excited by the electrode when the element is added to
the aluminum nitride constituting the film which is then charged
with electricity is a surface acoustic wave or a Lamb wave.
35. An acoustic wave device comprising: a piezoelectric film is an
aluminum nitride film containing a divalent element and a
tetravalent element, and electrodes connected to the piezoelectric
film to excite an acoustic wave propagating through the
piezoelectric film, wherein a total of concentrations of the
divalent element and the tetravalent element is greater than or
equal to 3 atomic % and less than or equal to 35 atomic % when a
total number of atoms of the divalent element, the tetravalent
element, and aluminum in the aluminum nitride film defines 100
atomic %.
36. An acoustic wave device comprising: a piezoelectric film is an
aluminum nitride film containing a divalent element and a
tetravalent element, and electrodes connected to the piezoelectric
film to excite an acoustic wave propagating through the
piezoelectric film, wherein a ratio of a concentration of the
tetravalent element to a total of concentrations of the divalent
element and the tetravalent element is greater than or equal to
0.35 and less than or equal to 0.75 when a total number of atoms of
the divalent element, the tetravalent element, and aluminum in the
aluminum nitride film defines 100 atomic %.
.Iadd.37. An acoustic wave device comprising: a piezoelectric film
made of an aluminum nitride film containing a divalent element and
one of a tetravalent or pentavalent element; and electrodes
connected to the piezoelectric film to excite, by the inverse
piezoelectric effect, or cause, by a strain due to the
piezoelectric effect, an acoustic wave propagating through the
piezoelectric film, wherein the divalent element and one of the
tetravalent or pentavalent element are substituted for aluminum
atoms of the aluminum nitride film. .Iaddend.
.Iadd.38. The acoustic wave device according to claim 37, wherein
the piezoelectric film is the aluminum nitride film containing the
divalent element and the tetravalent element, and contains at least
one of titanium, zirconium, and hafnium as the tetravalent element.
.Iaddend.
.Iadd.39. The acoustic wave device according to claim 38, wherein
the piezoelectric film contains at least one of calcium, magnesium,
strontium, and zinc as the divalent element. .Iaddend.
.Iadd.40. The acoustic wave device according to claim 37, wherein
the piezoelectric film is the aluminum nitride film containing the
divalent element and the pentavalent element, and contains at least
one of tantalum, niobium, and vanadium as the pentavalent element.
.Iaddend.
.Iadd.41. The acoustic wave device according to claim 40, wherein
the piezoelectric film contains at least one of magnesium and zinc
as the divalent element. .Iaddend.
Description
.Iadd.The present reissue application is a reissue application of
U.S. application Ser. No. 13/787,497, filed Mar. 6, 2013, now U.S.
Pat. No. 9,087,979. .Iaddend.
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority
of the prior Japanese Patent Application Nos. 2012-058441 filed on
Mar. 15, 2012 and 2012-250535 filed on Nov. 14, 2012, the entire
contents of which are incorporated herein by reference.
FIELD
A certain aspect of the present invention relates to acoustic wave
devices.
BACKGROUND
Diffusion of wireless communication devices including mobile phones
has encouraged development of filters formed by combining acoustic
wave devices using a surface acoustic wave (SAW) or bulk acoustic
wave (BAW). The filter using a SAW or BAW has a small outer shape
and a high Q compared to a dielectric filter, and thus is suitable
for a high-frequency component in a wireless communication device
such as a mobile phone required to be small in size and have a
steep skirt characteristic. Moreover, there has been suggested an
acoustic wave device using a Lamb wave as a developed device of the
acoustic wave device using a SAW or BAW.
In recent years, filters are required to have high performance. For
example, a bandwidth of a filter characteristic is required to be
widened. The bandwidth of the filter characteristic can be widened
by increasing an electromechanical coupling coefficient of an
acoustic wave device used in the filter. Use of a piezoelectric
film with a high electromechanical coupling coefficient can
increase the electromechanical coupling coefficient of the acoustic
wave device.
When an aluminum nitride film is used as the piezoelectric film,
the electromechanical coupling coefficient of the acoustic wave
device can be improved by controlling a c-axis orientation of the
aluminum nitride film as disclosed in Rajan S. Naik, and 10 others,
"Measurements of the Bulk, C-Axis Electromechanical Coupling
Constant as a Function of AlN Film Quality", IEEE TRANSACTIONS ON
ULTRASONICS, FERROELECTRICS AND FREQUENCY CONTROL, 2000, vol. 47,
p. 292-296 (Non-Patent Document 1), for example. For example, the
electromechanical coupling coefficient of the acoustic wave device
can be improved by using an aluminum nitride film containing an
alkali earth metal and/or a rare-earth metal for the piezoelectric
film as disclosed in Japanese Patent Application Publication No.
2002-344279 (Patent Document 1). Moreover, piezoelectric response
of the acoustic wave device can be improved by using an aluminum
nitride film containing scandium at a content rate in a
predetermined range for the piezoelectric film as disclosed in
Japanese Patent Application Publication No. 2009-10926 (Patent
Document 2).
However, the art disclosed in Non-Patent Document 1 aims to improve
the electromechanical coupling coefficient of the aluminum nitride
film, and thus fails to obtain an electromechanical coupling
coefficient higher than that obtained from a material
characteristic of the aluminum nitride film. In addition, the art
disclosed in Patent Document 1 aims to improve the
electromechanical coupling coefficient by increasing a bond
concentration of a grain boundary between c-axis oriented aluminum
nitride particles, and thus fails to obtain an electromechanical
coupling coefficient higher than that obtained from a material
characteristic of the aluminum nitride film.
The acoustic wave device grows in size and thus increases cost as a
resonance frequency decreases.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided
an acoustic wave device including: a piezoelectric film made of an
aluminum nitride film containing a divalent element and a
tetravalent element, or a divalent element and a pentavalent
element; and an electrode that excites an acoustic wave propagating
through the piezoelectric film.
According to another aspect of the present invention, there is
provided an acoustic wave device including: a piezoelectric film
made of an aluminum nitride film containing an element that
achieves at least one of an increase in a permittivity and a
decrease in an acoustic velocity; and an electrode that excites an
acoustic wave propagating through the piezoelectric film, wherein a
resonance frequency is less than or equal to 1.5 GHz.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a structure of aluminum nitride used for
simulations;
FIG. 2A is a top view of an acoustic wave device in accordance with
a first embodiment, FIG. 2B is a cross-sectional view taken along
line A-A in FIG. 2A, and FIG. 2C is a cross-sectional view taken
along line B-B in FIG. 2A,
FIG. 3A through FIG. 3H are cross-sectional views for explaining a
fabrication method of the acoustic wave device of the first
embodiment;
FIG. 4A illustrates simulation results of a resonance
characteristic of a first FBAR, and FIG. 4B illustrates simulation
results of a resonance characteristic of a second FBAR;
FIG. 5 illustrates simulation results of a band structure of first
doped aluminum nitride;
FIG. 6 illustrates simulation results of a band structure of second
doped aluminum nitride;
FIG. 7 illustrates simulation results of a band structure of third
doped aluminum nitride;
FIG. 8 illustrates a relationship between a piezoelectric constant
e.sub.33 and an electromechanical coupling coefficient k.sup.2;
FIG. 9 illustrates a relationship between a ratio (c/a) of a
lattice constant in a c-axis direction to a lattice constant in an
a-axis direction and an electromechanical coupling coefficient
k.sup.2;
FIG. 10A illustrates a dependence of an electromechanical coupling
coefficient k.sup.2 on substitutional concentrations when magnesium
is used as a divalent element and hafnium is used as a tetravalent
element, and FIG. 10B illustrates a dependence of an
electromechanical coupling coefficient k.sup.2 on substitutional
concentrations when magnesium is used as a divalent element and
titanium is used as a tetravalent element;
FIG. 11 is a cross-sectional view of an acoustic wave device in
accordance with a first variation of the first embodiment;
FIG. 12A illustrates simulation results of a resonance
characteristic of a third FBAR, and FIG. 12B illustrates simulation
results of a resonance characteristic of a fourth FBAR;
FIG. 13 illustrates simulation results of a resonance
characteristic of a fifth FBAR;
FIG. 14 illustrates simulation results of a band structure of
fourth doped aluminum nitride;
FIG. 15 illustrates simulation results of a band structure of fifth
doped aluminum nitride;
FIG. 16 illustrates a relationship between a piezoelectric constant
e.sub.33 and an electromechanical coupling coefficient k.sup.2;
FIG. 17 illustrates a relationship between a ratio (c/a) of a
lattice constant in a c-axis direction to a lattice constant in an
a-axis direction and an electromechanical coupling coefficient
k.sup.2;
FIG. 18 illustrates a dependence of an electromechanical coupling
coefficient k.sup.2 on substitutional concentrations when magnesium
is used as a divalent element and tantalum is used as a pentavalent
element;
FIG. 19 illustrates simulation results of a resonance
characteristic of a sixth FBAR;
FIG. 20A illustrates a relationship between a total of
substitutional concentrations of Mg and Zr and a normalized
piezoelectric constant, and FIG. 20B is a diagram that extracts
data of which a ratio of substitutional concentrations of Mg to Zr
is around 1:1 from FIG. 20A;
FIG. 21A illustrates a relationship between a ratio of a
substitutional concentration of Zr to a total of substitutional
concentrations of Mg and Zr and a normalized piezoelectric
constant; and FIG. 21B is a diagram that extracts data of which a
total of substitutional concentrations of Mg and Zr is greater than
or equal to 3 atomic % and less than or equal to 10 atomic % from
FIG. 21A;
FIG. 22A and FIG. 22B illustrate relationships between a total of
substitutional concentrations of a divalent element and a
tetravalent element and a normalized piezoelectric constant;
FIG. 23 illustrates a relationship between a total of
substitutional concentrations of Mg and Zr and a c/a ratio;
FIG. 24A illustrates a relationship between a resonance frequency
and a normalized film thickness of a resonance portion of an FBAR
of a third comparative example, and FIG. 24B illustrates a
relationship between a resonance frequency and a normalized area of
the resonance portion;
FIG. 25A illustrates a relationship between a substitutional
concentration of Sc and a permittivity .di-elect cons..sub.33, and
FIG. 25B illustrates a relationship between a substitutional
concentration of Sc and an acoustic velocity V;
FIG. 26A illustrates a relationship between a resonance frequency
and a normalized film thickness of a resonance portion of an FBAR
of a fourth embodiment, and FIG. 26B illustrates a relationship
between a resonance frequency and a normalized area of the
resonance portion;
FIG. 27A is a cross-sectional view of an acoustic wave device in
accordance with a first variation of the fourth embodiment; and
FIG. 27B is a cross-sectional view of an acoustic wave device in
accordance with a second variation of the fourth embodiment;
FIG. 28A illustrates relationships between a resonance frequency
and a normalized film thickness of a resonance portion in the first
variation and the second variation of the fourth embodiment; and
FIG. 28B illustrates a relationship between a resonance frequency
and a normalized area of the resonance portion;
FIG. 29A is a cross-sectional view of an FBAR in accordance with a
first variation of the embodiments, FIG. 29B is a cross-sectional
view of an FBAR in accordance with a second variation of the
embodiments, and FIG. 29C is a cross-sectional view of an SMR;
FIG. 30 is a cross-sectional view of a CRF;
FIG. 31A is a top view of a surface acoustic wave device, and FIG.
31B is a cross-sectional view taken along line A-A in FIG. 31A,
FIG. 31C is a cross-sectional view of a Love wave device, and FIG.
31D is a cross-sectional view of a boundary acoustic wave device;
and
FIG. 32 is a cross-sectional view of a Lamb wave device.
DETAILED DESCRIPTION
Hereinafter, a description will be given of embodiments of the
present invention with reference to the attached drawings.
First Embodiment
A description will now be given of a simulation for aluminum
nitride (AlN) conducted by the inventors. The simulation was
conducted with a method called a first principle calculation.
Methods of calculating an electronic state without using fitting
parameters or the like are collectively referred to as the first
principle calculation, which can calculate the electronic state by
using only atomic numbers and coordinates of atoms constituting a
unit lattice or molecule. FIG. 1 illustrates a structure of AlN
used for the simulation. As illustrated in FIG. 1, used for the
simulation is AlN with a wurtzite-type crystal structure that is a
supercell containing sixteen aluminum atoms 10 and sixteen nitrogen
atoms 12 obtained by doubling a unit lattice containing two
aluminum atoms 10 and two nitrogen atoms 12 in a-axis, b-axis, and
c-axis directions. The first principle calculation is performed to
the AlN with the wurtzite-type crystal structure by moving an
atomic coordinate, a cell volume, and a cell shape simultaneously,
and the electronic state of the AlN in a stable structure is
calculated.
Table 1 presents a lattice constant in the a-axis direction, a
lattice constant in the c-axis direction, and a ratio (c/a) of the
lattice constant in the c-axis direction to the lattice constant in
the a-axis direction calculated from the electronic state of the
AlN in the stable structure obtained by the first principle
calculation. Table 1 also presents experimental values obtained by
actually forming an AlN film by sputtering and measuring the AlN
film by X-ray diffraction.
TABLE-US-00001 TABLE 1 Lattice constant in Lattice constant in
a-axis direction [.ANG.] c-axis direction [.ANG.] c/a Calculated
value 3.11 4.98 1.60 Experimental value 3.11 4.98 1.60
As presented in Table 1, both the calculation value and the
experimental value are 3.11 [.ANG.] with respect to the lattice
constant in the a-axis direction, 4.98 [.ANG.] with respect to the
lattice constant in the c-axis direction, and 1.60 with respect to
the c/a ratio. This result demonstrates that the above-described
simulation using the first principle calculation is valid.
A description will now be given of a simulation for doped AlN doped
with an element other than aluminum (Al) and nitrogen (N).
Hereinafter, AlN that is not doped with an element other than Al
and N is referred to as non-doped AlN. The simulation is performed
to doped AlN with a crystal structure formed by substituting a
divalent element in one of the aluminum atoms 10 and substituting a
tetravalent element in another one of the aluminum atoms 10 in
non-doped AlN with the wurtzite-type crystal structure described in
FIG. 1. That is to say, the simulation is performed to the doped
AlN with the wurtzite-type crystal structure containing fourteen
aluminum atoms, one divalent element, one tetravalent element, and
sixteen nitrogen atoms formed by substituting a divalent element
and a tetravalent element in a part of aluminum sites. Here,
referred to as a substitutional concentration is an atomic
concentration of a substitution element when a total of the number
of aluminum atoms and the number of atoms of the substitution
element defines 100 atomic %. Thus, the divalent element and the
tetravalent element contained in the doped AlN for the simulation
have substitutional concentrations of 6.25 atomic %. Calcium (Ca),
magnesium (Mg), strontium (Sr), or zinc (Zn) is used as the
divalent element, and titanium (Ti), zirconium (Zr), or hafnium
(Hf) is used as the tetravalent element.
As is the case with the non-doped AlN, the first principle
calculation can calculate an electronic state of the doped AlN in
the stable structure, and the calculated electronic state allows a
lattice constant in the a-axis direction, a lattice constant in the
c-axis direction, and a c/a ratio to be calculated. The first
principle calculation can also calculate piezoelectric constants,
elastic constants, and permittivities of the non-doped AlN and the
doped AlN from minor change of total energy caused by a small
strain forcibly applied to the crystal lattices of the non-doped
AlN and the doped AlN in the stable structure. A relational
expression (Expression 1) holds true among a piezoelectric constant
e.sub.33, an elastic constant C.sub.33, and a permittivity
.di-elect cons..sub.33 in the c-axis direction and an
electromechanical coupling coefficient k.sup.2 (hereinafter,
referred to as k.sup.2). Therefore, electromechanical coupling
coefficients k.sup.2 of the non-doped AlN and the doped AlN can be
calculated by calculating piezoelectric constants e.sub.33, elastic
constants C.sub.33, and permittivities .di-elect cons..sub.33 of
the non-doped AlN and the doped AlN respectively.
.times..times..times. ##EQU00001##
Table 2 presents calculated piezoelectric constants e.sub.33 and
k.sup.2 calculated from Expression 1 of the non-doped AlN and the
doped AlN. As presented in Table 2, the obtained results
demonstrate that the doped AlN doped with a divalent element and a
tetravalent element (Case 1 through Case 10) have piezoelectric
constants e.sub.33 and electromechanical coupling coefficients
k.sup.2 greater than those of the non-doped AlN (Non-doped AlN in
Table 2). A combination of the divalent element and the tetravalent
element may be Ca--Ti, Ca--Zr, Ca--Hf, Mg--Ti, Mg--Zr, Mg--Hf,
Sr--Hf, Zn--Ti, Zn--Zr, or Zn--Hf as presented in Table 2, and may
be other combinations.
TABLE-US-00002 TABLE 2 Electro- mechanical Piezoelectric coupling
Combi- Divalent Tetravalent constant e.sub.33 coefficient nation
element element [C/m.sup.2] k.sup.2 [%] Case 1 Ca Ti 1.77 9.68 Case
2 Ca Zr 1.85 10.3 Case 3 Ca Hf 2.17 14.2 Case 4 Mg Ti 2.09 12.9
Case 5 Mg Zr 2.13 13.5 Case 6 Mg Hf 2.46 17.6 Case 7 Sr Hf 1.96
11.3 Case 8 Zn Ti 2.08 12.5 Case 9 Zn Zr 2.01 12.4 Case 10 Zn Hf
2.32 11.1 Non-doped -- -- 1.55 7.12 AlN
As presented above, the inventors have newly found that doped AlN
containing a divalent element and a tetravalent element has an
electromechanical coupling coefficient k.sup.2 greater than that of
non-doped AlN. Thus, a description will now be given of a first
embodiment capable of obtaining an acoustic wave device having a
high electromechanical coupling coefficient k.sup.2 based on the
above knowledge.
FIG. 2A is a top view of an acoustic wave device in accordance with
the first embodiment, FIG. 2B is a cross-sectional view take along
line A-A in FIG. 2A, and FIG. 2C is a cross-sectional view taken
along line B-B in FIG. 2A. The first embodiment describes an FBAR
(Film Bulk Acoustic Resonator) that is one of piezoelectric thin
film resonators. As illustrated in FIG. 2A through FIG. 2C, an FBAR
20 includes a substrate 22, a lower electrode 24, a piezoelectric
film 26, and an upper electrode 28.
The substrate 22 may be an insulative substrate such as a silicon
(Si) substrate, a glass substrate, a gallium arsenide (GaAs)
substrate, or a ceramic substrate. The lower electrode 24 is
located on the substrate 22. The lower electrode 24 may be a metal
film including at least one of aluminum (Al), copper (Cu), chrome
(Cr), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt),
ruthenium (Ru), rhodium (Rh), and iridium (Ir) for example. The
lower electrode 24 may have a single layer structure or a
multilayer structure.
The piezoelectric film 26 is located on the substrate 22 and the
lower electrode 24. The piezoelectric film 26 is an aluminum
nitride (AlN) film containing a divalent element and a tetravalent
element, and has a crystal structure with a c-axis orientation that
has a c-axis as a main axis. The divalent element and the
tetravalent element are substituted in aluminum sites of the
aluminum nitride film. The upper electrode 28 is located on the
piezoelectric film 26 so as to have a region facing the lower
electrode 24. A resonance portion 30 is a region where the lower
electrode 24 and the upper electrode 28 face each other across the
piezoelectric film 26. The upper electrode 28 may be a metal film
including at least one of Al, Cu, Cr, Mo, W, Ta, Pt, Ru, Rh, and Ir
described for the lower electrode 24. The upper electrode may have
a single layer structure or a multilayer structure.
A dome-shaped air-space 32 is located between the substrate 22 and
the lower electrode 24 in the resonance portion 30. The dome-shaped
air-space 32 has a height that becomes higher as it becomes closer
to a center of the air-space 32. An introduction path 34 formed by
introduction of etchant for forming the air-space 32 is located
below the lower electrode 24. The piezoelectric film 26 or the like
does not cover a vicinity of a tip of the introduction path 34, and
the tip of the introduction path 34 forms a hole portion 36. The
hole portion 36 is an inlet for introducing etchant to form the
air-space 32. An aperture 38 is formed in the piezoelectric film 26
to provide an electrical connection with the lower electrode
24.
When a high frequency electrical signal is applied between the
lower electrode 24 and the upper electrode 28, an acoustic wave
excited by the inverse piezoelectric effect or an acoustic wave
caused by a strain due to the piezoelectric effect is generated in
the piezoelectric film 26 sandwiched by the lower electrode 24 and
the upper electrode 28. The above-described acoustic wave is fully
reflected at surfaces exposed to air of the lower electrode 24 and
the upper electrode 28, and thus becomes a bulk acoustic wave
having a main displacement in a thickness direction. That is to
say, the lower electrode 24 and the upper electrode 28 function as
electrodes exciting an acoustic wave propagating through the
piezoelectric film 26.
A description will now be given of a fabrication method of the
acoustic wave device of the first embodiment with reference to FIG.
3A through FIG. 3H. FIG. 3A through FIG. 3D are cross-sectional
views corresponding to a cross-section taken along line A-A in FIG.
2A, and FIG. 3E through FIG. 3H are cross-sectional views
corresponding to a cross-section taken along line B-B in FIG.
2A.
As illustrated in FIG. 3A and FIG. 3E, a sacrifice layer 39 is
formed on the substrate 22 by sputtering or evaporation. The
sacrifice layer 39 is made of magnesium oxide (MgO) for example,
and is formed in at least a region in which the air-space 32 is to
be formed. The sacrifice layer 39 may have a film thickness of 20
nm for example. A metal film is then formed on the substrate 22 and
the sacrifice layer 39 by sputtering in an argon (Ar) gas
atmosphere for example. The metal film is selected from at least
one of Al, Cu, Cr, Mo, W, Ta, Pt, Ru, Rh, and Ir as described
previously. Then, the metal film is shaped into a desired shape by
exposure and etching to form the lower electrode 24. At this point,
a part of the lower electrode 24 has a shape that covers the
sacrifice layer 39.
As illustrated in FIG. 3B and FIG. 3F, the piezoelectric film 26
made of an aluminum nitride (AlN) film is formed on the substrate
22 and the lower electrode 24 by sputtering an Al alloy target
formed by incorporating a divalent element and a tetravalent
element into Al in a mixed gas atmosphere of argon and nitrogen.
Instead of sputtering the Al alloy target formed by incorporating a
divalent element and a tetravalent element into Al, an Al target, a
divalent element target, and a tetravalent element target may be
simultaneously sputtered by multiple sputtering. In this case,
atomic concentrations of the divalent element and the tetravalent
element contained in the piezoelectric film 26 can be controlled by
changing electrical power applied to each target.
As illustrated in FIG. 3C and FIG. 3G, a metal film is then formed
on the piezoelectric film 26 by sputtering in an argon gas
atmosphere for example. The metal film is selected from at least
one of Al, Cu, Cr, Mo, W, Ta, Pt, Ru, Rh, and Ir as described
previously. The metal film is then shaped into a desired shape by
exposure and etching to form the upper electrode 28. In addition,
the piezoelectric film 26 is also shaped into a desired shape by
exposure and etching for example. Furthermore, the hole portion 36
is formed by selectively etching the lower electrode 24 and the
sacrifice layer 39.
After the above process, as illustrated in FIG. 3D and FIG. 3H,
etchant is introduced from the hole portion 36 to etch the
sacrifice layer 39. Here, the stress to a multilayered film formed
by the lower electrode 24, the piezoelectric film 26, and the upper
electrode 28 is set to a compression stress by adjusting a
sputtering condition. Thus, when the etching of the sacrifice layer
39 is completed, the multilayered film bulges out, and the
dome-shaped air-space 32 is formed between the substrate 22 and the
lower electrode 24. The introduction path 34 connecting the
air-space 32 to the hole portion 36 is also formed. The above
described fabrication process forms the acoustic wave device
illustrated in FIG. 2.
A description will now be given of a simulation conducted to
investigate an effective electromechanical coupling coefficient
k.sub.eff.sup.2 (hereinafter, referred to as k.sub.eff.sup.2) of
the FBAR of the first embodiment. The simulation uses calculated
values by the first principle calculation for the piezoelectric
constant, the elastic constant, and the permittivity of the
piezoelectric film 26 made of an aluminum nitride film containing a
divalent element and a tetravalent element. A description will now
be given of a simulation performed to a first FBAR and a second
FBAR having the following configuration.
The first FBAR uses a multilayered metal film including Cr with a
film thickness of 100 nm and Ru with a film thickness of 225 nm
stacked in this order from the substrate 22 side for the lower
electrode 24. The piezoelectric film 26 is an aluminum nitride film
that contains Mg as a divalent element and Hf as a tetravalent
element and has a film thickness of 1000 nm. Substitutional
concentrations of Mg and Hf are set to 6.25 atomic %. The upper
electrode 28 is a multilayered metal film including Ru with a film
thickness of 225 nm and Cr with a film thickness of 30 nm stacked
in this order from the substrate 22 side. In addition, a silicon
dioxide (SiO.sub.2) film with a film thickness of 50 nm is located
on the upper electrode 28.
The second FBAR uses an aluminum nitride film having a film
thickness of 1000 nm and containing Mg as a divalent element and Ti
as a tetravalent element for the piezoelectric film 26. Other
configurations are the same as those of the first FBAR.
Substitutional concentrations of Mg and Ti are set to 6.25 atomic
%.
For comparison, the simulation is also performed to a first
comparative example that has the same configuration as those of the
first FBAR and the second FBAR except that a non-doped aluminum
nitride film with a film thickness of 1150 nm is used for the
piezoelectric film.
FIG. 4A illustrates simulation results of a resonance
characteristic of the first FBAR, and FIG. 4B illustrates
simulation results of a resonance characteristic of the second
FBAR. A solid line indicates the resonance characteristic of the
first FBAR in FIG. 4A and the resonance characteristic of the
second FBAR in FIG. 4B, and dashed lines indicate a resonance
characteristic of the first comparative example. As illustrated in
FIG. 4A and FIG. 4B, an interval between a resonance frequency and
an anti-resonance frequency is wide in the first FBAR and the
second FBAR compared to that in the first comparative example.
Effective electromechanical coupling coefficients k.sub.eff.sup.2
of the first FBAR, the second FBAR, and the first comparative
example are 17.5%, 12.9%, and 7.22%, respectively.
In addition, simulated are FBARs using various kinds of elements
for the divalent element and the tetravalent element contained in
the piezoelectric film 26 in the same manner. Table 3 presents
simulation results. Substitutional concentrations of the divalent
element and the tetravalent element are set to 6.25 atomic %, and
configurations other than the kinds of the divalent element and the
tetravalent element are made to be the same as those of the first
FBAR and the second FBAR.
TABLE-US-00003 TABLE 3 Anti- Resonance resonance Combi- Divalent
Tetravalent frequency frequency nation element element [MHz] [MHz]
k.sub.eff.sup.2 [%] Case 1 Ca Ti 1928.9 2011.9 9.77 Case 2 Ca Zr
1895.8 1983.2 10.4 Case 3 Ca Hf 1875.7 1998.3 14.2 Case 4 Mg Ti
1930.3 2043.8 12.9 Case 5 Mg Zr 1911.9 2030.3 13.5 Case 6 Mg Hf
1886.9 2043.9 17.5 Case 7 Sr Hf 1901.5 1998.3 11.4 Case 8 Zn Ti
1940.1 2050.4 12.6 Case 9 Zn Zr 1888.3 1995.0 12.5 Case 10 Zn Hf
1887.5 2027.7 15.9 Aluminum -- -- 1963.0 2024.0 7.22 nitride
As presented in Table 3, the obtained results demonstrate that the
acoustic wave devices using an aluminum nitride film containing a
divalent element and a tetravalent element for the piezoelectric
film (Case 1 through Case 10) have effective electromechanical
coupling coefficients k.sub.eff.sup.2 greater than that of the
acoustic wave device using a non-doped aluminum nitride film for
the piezoelectric film (Table 3: Aluminum nitride). A combination
of the divalent element and the tetravalent element may be Ca--Ti,
Ca--Zr, Ca--Hf, Mg--Ti, Mg--Zr, Mg--Hf, Sr--Hf, Zn--Ti, Zn--Zr, or
Zn--Hf as presented in Table 3, and may be other combinations.
The first embodiment demonstrates that an acoustic wave device
having a high electromechanical coupling coefficient can be
obtained by using an aluminum nitride film containing a divalent
element and a tetravalent element for the piezoelectric film
26.
The piezoelectric film 26 contains one of Ca, Mg, Sr, and Zn as the
divalent element in the simulation results presented in Table 3,
but may contain two or more of these divalent elements. Moreover,
the piezoelectric film 26 contains one of Ti, Zr, and Hf as the
tetravalent element, but may contain two or more of these
tetravalent elements. That is to say, the piezoelectric film 26 may
contain at least one of Ca, Mg, Sr, and Zn as the divalent element
and at least one of Ti, Zr, and Hf as the tetravalent element. In
addition, the piezoelectric film 26 may contain a divalent element
and a tetravalent element other than those cited in Table 3.
A description will now be given of an insulation property of doped
AlN doped with a divalent element and a tetravalent element
(hereinafter, referred to as first doped AlN). The insulation
property was evaluated by calculating an electronic state of the
first doped AlN by the first principle calculation, and drawing a
band diagram. For comparison, evaluated were an insulation property
of doped AlN doped with only a divalent element (hereinafter,
referred to as second doped AlN) and an insulation property of
doped AlN doped with only a tetravalent element (hereinafter,
referred to as third doped AlN) in the same manner. The first doped
AlN, the second doped AlN, and the third doped AlN have the
following crystal structures.
The first doped AlN is doped AlN formed by substituting a divalent
element in one of the aluminum atoms 10 and substituting a
tetravalent element in another one of the aluminum atoms 10 in the
non-doped AlN with the wurtzite-type crystal structure described in
FIG. 1. Thus, a ratio of substitutional concentrations of the
divalent element to the tetravalent element is 1:1. Mg is used as
the divalent element, and Hf is used as the tetravalent
element.
The second doped AlN is doped AlN formed by substituting a divalent
element in one of the aluminum atoms 10 in the non-doped AlN with
the wurtzite-type crystal structure described in FIG. 1. Mg is used
as the divalent element.
The third doped AlN is doped AlN formed by substituting a
tetravalent element in one of the aluminum atoms 10 in the
non-doped AlN with the wurtzite-type crystal structure described in
FIG. 1. Hf is used as the tetravalent element.
FIG. 5 illustrates simulation results of a band structure of the
first doped AlN. FIG. 6 illustrates simulation results of a band
structure of the second doped AlN. FIG. 7 illustrates simulation
results of a band structure of the third doped AlN. In FIG. 5
through FIG. 7, solid lines indicate energy levels, a band of
energy levels at a lower side represents a valence band, and a band
of energy levels at an upper side represents a conduction band. A
forbidden band is between the valence band and the conduction band.
A dashed line indicates Fermi energy (hereinafter, abbreviated as
Ef).
When AlN is doped with only Mg as a divalent element, the Fermi
energy Ef is located below a top of the valence band, and thus lies
in the valence band as illustrated in FIG. 6. This reveals that the
insulation property degrades when AlN is doped with only a divalent
element. When AlN is doped with only Hf as a tetravalent element,
the Fermi energy Ef is located above a bottom of the conduction
band, and thus lies in the conduction band as illustrated in FIG.
7. This reveals that the insulation property also degrades when AlN
is doped with only a tetravalent element.
On the other hand, when AlN is doped with Mg as a divalent element
and Hf as a tetravalent element at a ratio of 1:1, the Fermi energy
Ef lies in the forbidden band between the top of the valence band
and the bottom of the conduction band as illustrated in FIG. 5.
This reveals that the insulation property can be maintained by
doping AlN with a divalent element and a tetravalent element, and
making a ratio of substitutional concentrations of the divalent
element to the tetravalent element 1:1. This is because an electric
property of the doped AlN can remain neutral by making a ratio of
substitutional concentrations of the divalent element to the
tetravalent element 1:1 because both the divalent element and the
tetravalent element are substituted in trivalent aluminum sites.
FIG. 5 illustrates a case where Mg is used as the divalent element
and Hf is used as the tetravalent element, but the insulation
property can be also maintained when other divalent elements and
tetravalent elements are used.
Therefore, an acoustic wave device that can maintain the insulation
property of the piezoelectric film 26 and have a high
electromechanical coupling coefficient can be obtained by using an
aluminum nitride film containing a divalent element and a
tetravalent element at a ratio of 1:1 for the piezoelectric film 26
in the FBAR of the first embodiment. The ratio of substitutional
concentrations of the divalent element and the tetravalent element
is preferably 1:1 to the extent that the electric property of the
piezoelectric film can remain neutral.
Next, a description will be given of a relationship between a
piezoelectric constant e.sub.33 and an electromechanical coupling
coefficient k.sup.2 of doped AlN doped with a divalent element and
a tetravalent element. The piezoelectric constant e.sub.33 of the
doped AlN is calculated by the first principle calculation, and the
electromechanical coupling coefficient k.sup.2 is calculated from
Expression 1. FIG. 8 illustrates a relationship between a
piezoelectric constant e.sub.33 and an electromechanical coupling
coefficient k.sup.2 with respect to the doped AlN of Case 1 through
Case 10 presented in Table 2 and the non-doped AlN. In FIG. 8, the
open circle indicates a result of the non-doped AlN, and black
circles indicate results of the doped AlN. As illustrated in FIG.
8, all doped AlN doped with a divalent element and a tetravalent
element have piezoelectric constants e.sub.33 greater than that of
the non-doped AlN, and the electromechanical coupling coefficient
k.sup.2 increases as the piezoelectric constant e.sub.33 increases.
This reveals that the FBAR of the first embodiment preferably uses
an aluminum nitride film containing a divalent element and a
tetravalent element and having a piezoelectric constant e.sub.33
greater than 1.55, which is the piezoelectric constant e.sub.33 of
aluminum nitride, for the piezoelectric film 26. The above
configuration allows the piezoelectric film 26 to have a high
electromechanical coupling coefficient, and accordingly allows an
acoustic wave device having a high electromechanical coupling
coefficient to be obtained.
As illustrated in FIG. 8, the piezoelectric film 26 preferably has
a piezoelectric constant e.sub.33 greater than 1.6, more preferably
1.8 because the electromechanical coupling coefficient k.sup.2
increases as the piezoelectric constant e.sub.33 increases.
A description will now be given of a relationship between a crystal
structure and an electromechanical coupling coefficient k.sup.2 of
doped AlN doped with a divalent element and a tetravalent element.
The crystal structure of the doped AlN is evaluated with a ratio
(c/a) of a lattice constant in the c-axis direction to a lattice
constant in the a-axis direction calculated by the first principle
calculation. The electromechanical coupling coefficient k.sup.2 is
calculated by assigning calculated values of the piezoelectric
constant and the like of the doped AlN by the first principle
calculation to Expression 1. FIG. 9 illustrates a relationship
between a c/a ratio and an electromechanical coupling coefficient
k.sup.2 with respect to the doped AlN of Case 1 through Case 10
presented in Table 2 and the non-doped AlN. In FIG. 9, the open
circle indicates a result of the non-doped AlN, and black circles
indicate results of the doped AlN. As illustrated in FIG. 9, all
doped AlN doped with a divalent element and a tetravalent element
have c/a ratios less than that of the non-doped AlN, and the
electromechanical coupling coefficient k.sup.2 increases as the c/a
ratio decreases. This reveals that the FBAR of the first embodiment
preferably uses an aluminum nitride film containing a divalent
element and a tetravalent element and having a c/a ratio less than
1.6, which is the c/a ratio of aluminum nitride, for the
piezoelectric film 26. The above configuration allows the
piezoelectric film 26 to have a high electromechanical coupling
coefficient, and accordingly allows an acoustic wave device having
a high electromechanical coupling coefficient to be obtained.
As illustrated in FIG. 9, the piezoelectric film 26 preferably has
a c/a ratio less than 1.595, more preferably 1.59 because the
electromechanical coupling coefficient k.sup.2 increases as the c/a
ratio decreases.
A description will now be given of a dependence of an
electromechanical coupling coefficient k.sup.2 on substitutional
concentrations of doped AlN doped with a divalent element and a
tetravalent element. The dependence of the electromechanical
coupling coefficient k.sup.2 on substitutional concentrations is
evaluated by calculating a size of the supercell of the
wurtzite-type crystal structure described in FIG. 1 and electronic
states of doped AlN with different numbers of aluminum atoms
substituted by a divalent element and a tetravalent element by the
first principle calculation. Substitutional concentrations of the
divalent element and the tetravalent element are made to be equal
to each other to make the electric properties of the doped AlN
neutral.
FIG. 10A illustrates a dependence of an electromechanical coupling
coefficient k.sup.2 on substitutional concentrations when Mg is
used as the divalent element and Hf is used as the tetravalent
element, FIG. 10B illustrates a dependence of an electromechanical
coupling coefficient k.sup.2 on substitutional concentrations when
Mg is used as the divalent element and Ti is used as the
tetravalent element. FIG. 10A and FIG. 10B reveal that the
electromechanical coupling coefficient k.sup.2 of the doped AlN
increases as the substitutional concentrations increase not only
when Mg and Hf are used but also when Mg and Ti are used. This
result reveals that the electromechanical coupling coefficient
k.sup.2 can be controlled to be a desired value by controlling the
substitutional concentrations. For example, doped AlN with an
electromechanical coupling coefficient k.sup.2 of 10% can be
obtained by controlling a total of substitutional concentrations of
Mg and Hf to be approximately 4 atomic %, or by controlling a total
of substitutional concentrations of Mg and Ti to be approximately 7
atomic %. The simulation uses Mg as the divalent element and Ti or
Hf as the tetravalent element, but other divalent elements and
tetravalent elements may be used.
Thus, an acoustic wave device having a desired electromechanical
coupling coefficient can be obtained by controlling substitutional
concentrations of the divalent element and the tetravalent element
contained in the piezoelectric film 26 in the FBAR of the first
embodiment.
A description will now be given of an acoustic wave device in
accordance with a first variation of the first embodiment. FIG. 11
illustrates a cross-sectional view of the acoustic wave device of
the first variation of the first embodiment. As illustrated in FIG.
11, an FBAR 40 of the first variation of the first embodiment
includes a temperature compensation film 42 inserted so as to be
sandwiched by piezoelectric films 26a and 26b. The temperature
compensation film 42 is located between the piezoelectric films 26a
and 26b, and contacts the piezoelectric films 26a and 26b. The
temperature compensation film 42 is formed of a material having a
temperature coefficient of an elastic constant opposite in sign to
those of the piezoelectric films 26a and 26b. For example, when the
temperature coefficients of the piezoelectric films 26a and 26b are
negative, the temperature compensation film 42 with a positive
temperature coefficient is used. Other configurations are the same
as those of the first embodiment, and thus a description thereof is
omitted.
Provision of the above described temperature compensation film 42
allows a temperature characteristic of the FBAR 40 to be improved.
A silicon oxide (SiO.sub.2) film is an example of the temperature
compensation film 42. Instead of the SiO.sub.2 film, a film mainly
containing silicon oxide, e.g. a silicon oxide film doped with an
element such as fluorine (F), may be used. Here, "a film mainly
containing . . . " means a film that contains an element to the
extent that the temperature coefficient of the elastic constant of
the temperature compensation film 42 becomes opposite in sign to
those of the piezoelectric films 26a and 26b.
A description will be given of a simulation conducted to
investigate an effective electromechanical coupling coefficient
k.sub.eff.sup.2 of the FBAR 40 of the first variation of the first
embodiment. As with the first embodiment, the calculated values by
the first principle calculation are used for the piezoelectric
constants, the elastic constants, and the permittivities of the
piezoelectric films 26a and 26b that are aluminum nitride films
containing a divalent element and a tetravalent element. A
description will be given of a simulation performed to a third FBAR
and a fourth FBAR having the following configurations.
The third FBAR uses a multilayered metal film including Cr with a
film thickness of 100 nm and Ru with a film thickness of 225 nm
stacked in this order from the substrate 22 side for the lower
electrode 24. The piezoelectric films 26a and 26b are aluminum
nitride films having a film thickness of 400 nm and containing Mg
as a divalent element and Hf as a tetravalent element.
Substitutional concentrations of Mg and Hf are set to 6.25 atomic
%. A SiO.sub.2 film with a film thickness of 50 nm is used for the
temperature compensation film 42. The upper electrode 28 is a
multilayered metal film including Ru with a film thickness of 225
nm and Cr with a film thickness of 30 nm stacked in this order from
the substrate 22 side. A SiO.sub.2 film with a film thickness of 50
nm is located on the upper electrode 28.
The fourth FBAR uses an aluminum nitride film having a film
thickness of 400 nm and containing Mg as a divalent element and Ti
as a tetravalent element for the piezoelectric films 26a and 26b.
Other configurations are the same as those of the third FBAR.
Substitutional concentrations of Mg and Ti are set to 6.25 atomic
%.
For comparison, the simulation is also performed to a second
comparative example that has the same configuration as those of the
third FBAR and the fourth FBAR except that a non-doped aluminum
nitride film with a film thickness of 475 nm is used for the
piezoelectric film.
FIG. 12A illustrates simulation results of a resonance
characteristic of the third FBAR, and FIG. 12B illustrates
simulation results of a resonance characteristic of the fourth
FBAR. A solid line indicates the resonance characteristic of the
third FBAR in FIG. 12A and the resonance characteristic of the
fourth FBAR in FIG. 12B, and a dashed line indicates a resonance
characteristic of the second comparative example. As illustrated in
FIG. 12A and FIG. 12B, an interval between a resonance frequency
and an anti-resonance frequency is wide in the third FBAR and the
fourth FBAR compared to that in the second comparative example. The
effective electromechanical coupling coefficients k.sub.eff.sup.2
of the third FBAR, the fourth FBAR, and the second comparative
example are 12.0%, 8.78%, and 5.01%, respectively.
Also simulated are FBARs using various kinds of elements for the
divalent element and the tetravalent element contained in the
piezoelectric films 26a and 26b in the same manner. Table 4
presents simulation results. Substitutional concentrations of the
divalent element and the tetravalent element are set to 6.25 atomic
%, and the configurations other than the kinds of the divalent
element and the tetravalent element are the same as those of the
third FBAR and the fourth FBAR.
TABLE-US-00004 TABLE 4 Anti- Resonance resonance Combi- Divalent
Tetravalent frequency frequency nation element element [MHz] [MHz]
k.sub.eff.sup.2 [%] Case 1 Ca Ti 1973.7 2029.8 6.63 Case 2 Ca Zr
1948.0 2007.2 7.05 Case 3 Ca Hf 1938.8 2018.9 9.40 Case 4 Mg Ti
1978.5 2054.4 8.78 Case 5 Mg Zr 1964.4 2043.9 9.22 Case 6 Mg Hf
1949.0 2054.3 12.0 Case 7 Sr Hf 1953.8 2019.1 7.71 Case 8 Zn Ti
1986.1 2059.6 8.49 Case 9 Zn Zr 1944.0 2016.4 8.53 Case 10 Zn Hf
1947.5 2041.8 10.9 Aluminum -- -- 1965.3 2007.0 5.01 nitride
As presented in Table 4, even when the temperature compensation
film 42 is provided, the acoustic wave devices using an aluminum
nitride film containing a divalent element and a tetravalent
element for the piezoelectric film (Case 1 through Case 10) have
effective electromechanical coupling coefficients k.sub.eff.sup.2
greater than that of the acoustic wave device using a non-doped
aluminum nitride film for the piezoelectric film (Table 4: Aluminum
nitride). A combination of the divalent element and the tetravalent
element may be Ca--Ti, Ca--Zr, Ca--Hf, Mg--Ti, Mg--Zr, Mg--Hf,
Sr--Hf, Zn--Ti, Zn--Zr, or Zn--Hf as presented in Table 4, but may
be other combinations.
The first variation of the first embodiment demonstrates that an
acoustic wave device having a high electromechanical coupling
coefficient can be obtained by using an aluminum nitride film
containing a divalent element and a tetravalent element for the
piezoelectric films 26a and 26b even when the temperature
compensation film 42 is included.
Second Embodiment
A second embodiment is an exemplary acoustic wave device that uses
an aluminum nitride film containing a divalent element and a
pentavalent element for the piezoelectric film. A description will
first be given of a simulation performed to doped AlN doped with a
divalent element and a pentavalent element with the first principle
calculation. The simulation is performed to doped AlN with a
crystal structure formed by substituting a divalent element in two
of the aluminum atoms 10 and substituting a pentavalent element in
another one of the aluminum atoms 10 in the non-doped AlN with the
wurtzite-type crystal structure described in FIG. 1. That is to
say, a part of the aluminum sites is substituted by a divalent
element and a pentavalent element, and simulated is the doped AlN
with the wurtzite-type crystal structure containing thirteen
aluminum atoms, two divalent elements, one pentavalent element, and
sixteen nitrogen atoms. Therefore, the substitutional concentration
of the divalent element is 12.5 atomic %, and the substitutional
concentration of the pentavalent element is 6.25 atomic %. Mg or Zn
is used as the divalent element, and tantalum (Ta), niobium (Nb),
or vanadium (V) is used as the pentavalent element.
Table 5 presents calculated values of piezoelectric constants
e.sub.33 and electromechanical coupling coefficients k.sup.2
calculated from Expression 1 of the non-doped AlN and the doped
AlN. As presented in Table 5, the obtained results demonstrate that
the doped AlN doped with a divalent element and a pentavalent
element (Case 1 through Case 6) have piezoelectric constants
e.sub.33 and electromechanical coupling coefficients k.sup.2
greater than those of the non-doped AlN (Table 5: Non-doped AlN). A
combination of the divalent element and the pentavalent element may
be Mg--Ta, Mg--Nb, Mg--V, Zn--Ta, Zn--Nb, or Zn--V as presented in
Table 5, but may be other combinations.
TABLE-US-00005 TABLE 5 Electro- mechanical Piezoelectric coupling
Combi- Divalent Pentavalent constant e.sub.33 coefficient nation
element element [C/m.sup.2] k.sup.2 [%] Case 1 Mg Ta 2.52 19.3 Case
2 Mg Nb 2.22 14.4 Case 3 Mg V 2.33 18.1 Case 4 Zn Ta 2.22 14.3 Case
5 Zn Nb 2.12 13.6 Case 6 Zn V 2.12 10.8 Non-doped -- -- 1.55 7.12
AlN
As described above, the inventors have newly found that doped AlN
containing a divalent element and a pentavalent element also has an
electromechanical coupling coefficient k.sup.2 greater than that of
non-doped AlN. A description will now be given of the second
embodiment capable of obtaining an acoustic wave device having a
high electromechanical coupling coefficient k.sup.2 based on the
above knowledge.
The acoustic wave device of the second embodiment has the same
configuration as that of the first embodiment except that the
piezoelectric film 26 is an aluminum nitride film containing a
divalent element and a pentavalent element, and thus a description
thereof is omitted. The divalent element and the pentavalent
element are substituted in aluminum sites of the aluminum nitride
film. The piezoelectric film 26 has a crystal structure having a
c-axis orientation as with that of the first embodiment.
A fabrication method of the acoustic wave device of the second
embodiment is the same as that of the first embodiment except that
the piezoelectric film 26 is formed with an Al alloy target formed
by incorporating a divalent element and a pentavalent element into
Al, and thus a description thereof is omitted. As described in the
first embodiment, the multiple sputtering technique that sputters
an Al target, a divalent element target, and a pentavalent element
target simultaneously may be used.
A description will now be given of a simulation conducted to
investigate an effective electromechanical coupling coefficient
k.sub.eff.sup.2 of an FBAR of the second embodiment. The simulation
uses calculated values by the first principle calculation for the
piezoelectric constant, the elastic constant, and the permittivity
of the piezoelectric film 26 that is an aluminum nitride film
containing a divalent element and a pentavalent element. A
description will be given of a simulation performed to a fifth FBAR
having the following configuration.
The fifth FBAR uses a multilayered metal film including Cr with a
film thickness of 100 nm and Ru with a film thickness of 225 nm
stacked in this order from the substrate 22 side for the lower
electrode 24. The piezoelectric film 26 is an aluminum nitride film
having a film thickness of 850 nm and containing Mg as a divalent
element and Ta as a pentavalent element. The substitutional
concentration of Mg is set to 12.5 atomic %, and the substitutional
concentration of Ta is set to 6.25 atomic %. The upper electrode 28
is a multilayered metal film including Ru with a film thickness of
225 nm and Cr with a film thickness of 30 nm stacked in this order
from the substrate 22 side. A SiO.sub.2 film with a film thickness
of 50 nm is located on the upper electrode 28.
FIG. 13 illustrates simulation results of a resonance
characteristic of the fifth FBAR. A solid line indicates the
resonance characteristic of the fifth FBAR. For comparison, a
dashed line indicates the resonance characteristic of the first
comparative example described in FIG. 4A and FIG. 4B. As
illustrated in FIG. 13, an interval between a resonance frequency
and an anti-resonance frequency is wide in the fifth FBAR compared
to that in the first comparative example. The effective
electromechanical coupling coefficient k.sub.eff.sup.2 of the first
comparative example is 7.22%, whereas the effective
electromechanical coupling coefficient k.sub.eff.sup.2 of the fifth
FBAR is 17.6%.
Also simulated are FBARs using various kinds of elements for the
divalent element and the pentavalent element contained in the
piezoelectric film 26 in the same manner. Table 6 presents
simulation results. The substitutional concentration of the
divalent element is set to 12.5 atomic %, the substitutional
concentration of the pentavalent element is set to 6.25 atomic %,
and the configuration other than the kinds of the divalent element
and the pentavalent element is the same as that of the fifth
FBAR.
TABLE-US-00006 TABLE 6 Anti- Resonance resonance Combi- Divalent
Pentavalent frequency frequency nation element element [MHz] [MHz]
k.sub.eff.sup.2 [%] Case 1 Mg Ta 1910.3 2086.1 17.6 Case 2 Mg Nb
1977.0 2107.6 14.3 Case 3 Mg V 1835.5 1993.4 16.8 Case 4 Zn Ta
1968.3 2096.9 14.2 Case 5 Zn Nb 1926.9 2047.1 13.6 Case 6 Zn V
2080.1 2179.4 10.7 Aluminum -- -- 1963.0 2024.0 7.22 nitride
As presented in Table 6, the acoustic wave devices using an
aluminum nitride film containing a divalent element and a
pentavalent element for the piezoelectric film (Case 1 through Case
6) have effective electromechanical coupling coefficients
k.sub.eff.sup.2 greater than that of the acoustic wave device using
a non-doped aluminum nitride film for the piezoelectric film (Table
6: Aluminum nitride). A combination of the divalent element and the
pentavalent element may be Mg--Ta, Mg--Nb, Mg--V, Zn--Ta, Zn--Nb,
or Zn--V as presented in Table 6, but may be other
combinations.
The second embodiment demonstrates that an acoustic wave device
having a high electromechanical coupling coefficient can be also
obtained by using an aluminum nitride film containing a divalent
element and a pentavalent element for the piezoelectric film
26.
In Table 6, the piezoelectric film 26 contains Mg or Zn as the
divalent element, but may contain both of them. In addition, the
piezoelectric film 26 contains one of Ta, Nb, and V as the
pentavalent element, but may contain two or more of them. That is
to say, the piezoelectric film 26 may contain at least one of Mg
and Zn as the divalent element, and contain at least one of Ta, Nb,
and V as the pentavalent element. Furthermore, the piezoelectric
film 26 may contain a divalent element and a pentavalent element
other than those cited in Table 6.
A description will now be given of an insulation property of doped
AlN doped with a divalent element and a pentavalent element
(hereinafter, referred to as fourth doped AlN). The insulation
property is evaluated by calculating an electronic state of the
fourth doped AlN by the first principle calculation and drawing a
band diagram. For comparison, an insulation property of doped AlN
doped with only a pentavalent element (hereinafter, referred to as
fifth doped AlN) is also evaluated in the same manner. The fourth
doped AlN and the fifth doped AlN have the following crystal
structures.
The fourth doped AlN is doped AlN formed by substituting divalent
elements in two of the aluminum atoms 10 and substituting a
pentavalent element in another one of the aluminum atoms 10 in the
non-doped AlN with the wurtzite-type crystal structure described in
FIG. 1. Thus, a ratio of the substitutional concentration of the
divalent element to that of the pentavalent element is 2:1. Mg is
used as the divalent element, and Ta is used as the pentavalent
element.
The fifth doped AlN is doped AlN formed by substituting a
pentavalent element in one of the aluminum atoms 10 in the
non-doped AlN with the wurtzite-type crystal structure described in
FIG. 1. Ta is used as the pentavalent element.
FIG. 14 illustrates simulation results of a band structure of the
fourth doped AlN. FIG. 15 illustrates simulation results of a band
structure of the fifth doped AlN. As described for FIG. 6, the
Fermi energy Ef lies in the valence band and the insulation
property degrades when AlN is doped with only Mg as a divalent
element. As illustrated in FIG. 15, when AlN is doped with only Ta
as a pentavalent element, the Fermi energy Ef is located above the
bottom of the conduction band, and thus lies in the conduction
band. This reveals that the insulation property also degrades when
AlN is doped with only a pentavalent element.
On the other hand, when AlN is doped with Mg as a divalent element
and Ta as a pentavalent element at a ratio of 2:1, the Fermi energy
Ef lies in the forbidden band between the top of the valence band
and the bottom of the conduction band as illustrated in FIG. 14.
This reveals that the insulation property can be maintained by
doping AlN with a divalent element and a pentavalent element, and
making a ratio of substitutional concentrations of the divalent
element to the pentavalent element 2:1. This is because an electric
property of the doped AlN can remain neutral by making a ratio of
substitutional concentrations of the divalent element to the
pentavalent element 2:1 because both the divalent element and the
pentavalent element are substituted in trivalent aluminum sites as
described in the first embodiment. FIG. 14 illustrates a case where
Mg is used as the divalent element and Ta is used as the
pentavalent element, but the insulation property can be maintained
even when other divalent elements and pentavalent elements are
used.
Therefore, an acoustic wave device that maintains the insulation
property of the piezoelectric film 26 and has a high
electromechanical coupling coefficient can be obtained by using an
aluminum nitride film containing a divalent element and a
pentavalent element at a ratio of 2:1 for the piezoelectric film 26
in the FBAR of the second embodiment. A ratio of substitutional
concentrations of the divalent element to the pentavalent element
is preferably 2:1 to the extent that the electric property of the
piezoelectric film can remain neutral.
A description will be given of a relationship between a
piezoelectric constant e.sub.33 and a k.sup.2 of doped AlN doped
with a divalent element and a pentavalent element. The
piezoelectric constant e.sub.33 and the k.sup.2 of the doped AlN
are calculated in the same way as that described in FIG. 8 of the
first embodiment. FIG. 16 illustrates a relationship between
piezoelectric constants e.sub.33 and k.sup.2 with respect to the
doped AlN of Case 1 through Case 6 presented in Table 5 and the
non-doped AlN. In FIG. 16, the open circle indicates a result of
the non-doped AlN, and black circles indicate results of the doped
AlN. As illustrated in FIG. 16, all doped AlN doped with a divalent
element and a pentavalent element have piezoelectric constants
e.sub.33 greater than that of the non-doped AlN, and the
electromechanical coupling coefficient k.sup.2 increases as the
piezoelectric constant e.sub.33 increases. This reveals that the
FBAR of the second embodiment preferably uses an aluminum nitride
film containing a divalent element and a pentavalent element and
having a piezoelectric constant e.sub.33 greater than 1.55, which
is the piezoelectric constant e.sub.33 of aluminum nitride, for the
piezoelectric film 26 in. The above-described configuration allows
the piezoelectric film 26 to have a high electromechanical coupling
coefficient, and accordingly allows an acoustic wave device having
a high electromechanical coupling coefficient to be obtained.
As illustrated in FIG. 16, the piezoelectric constant e.sub.33 of
the piezoelectric film 26 is preferably greater than 1.6, more
preferably 1.8 because the electromechanical coupling coefficient
k.sup.2 increases as the piezoelectric constant e.sub.33
increases.
A description will now be given of a relationship between a crystal
structure and an electromechanical coupling coefficient k.sup.2 of
doped AlN doped with a divalent element and a pentavalent element.
The crystal structure of the doped AlN is evaluated with a c/a
ratio as described in FIG. 9 of the first embodiment. The
electromechanical coupling coefficient k.sup.2 is calculated in the
same way as that described in FIG. 9 of the first embodiment. FIG.
17 illustrates a relationship between a c/a ratio and an
electromechanical coupling coefficient k.sup.2 with respect to the
doped AlN of Case 1 through Case 6 presented in Table 5 and the
non-doped AlN. In FIG. 17, the open circle indicate a result of the
non-doped AlN, and black circles indicate results of the doped AlN.
As illustrated in FIG. 17, all doped AlN doped with a divalent
element and a pentavalent element have c/a ratios less than that of
the non-doped AlN, and the electromechanical coupling coefficient
k.sup.2 increases as the c/a ratio decreases. Therefore, the FBAR
of the second embodiment preferably uses an aluminum nitride film
containing a divalent element and a pentavalent element and having
a c/a ratio less than 1.6, which is the c/a ratio of aluminum
nitride, for the piezoelectric film 26. The above-described
configuration allows the piezoelectric film 26 to have a high
electromechanical coupling coefficient, and thus allows an acoustic
wave device having a high electromechanical coupling coefficient to
be obtained.
As illustrated in FIG. 17, the piezoelectric film 26 preferably has
a c/a ratio less than 1.595, more preferably 1.59 because the
electromechanical coupling coefficient k.sup.2 increases as the c/a
ratio decreases.
A description will be given of a dependence of an electromechanical
coupling coefficient k.sup.2 on substitutional concentrations of
doped AlN doped with a divalent element and a pentavalent element.
The dependence of the electromechanical coupling coefficient
k.sup.2 on substitutional concentrations is evaluated in the same
way as that described in FIG. 10A and FIG. 10B of the first
embodiment. The ratio of substitutional concentrations of the
divalent element to the pentavalent element is set to 2:1 so that
the electric property of the doped AlN is neutral.
FIG. 18 illustrates a dependence of an electromechanical coupling
coefficient k.sup.2 on substitutional concentrations when Mg is
used as the divalent element and Ta is used as the pentavalent
element. FIG. 18 reveals that the electromechanical coupling
coefficient k.sup.2 increases as the substitutional concentrations
increase. This reveals that the electromechanical coupling
coefficient k.sup.2 of the doped AlN can be controlled to be a
desired value by controlling the substitutional concentrations as
is the case with the first embodiment. For example, doped AlN with
an electromechanical coupling coefficient k.sup.2 of 10% can be
obtained by controlling the total of substitutional concentrations
of Mg and Ta to be approximately 7 atomic %. The simulation uses Mg
as the divalent element and Ta as the pentavalent element, but
other divalent elements and pentavalent elements may be used.
Therefore, an acoustic wave device with a desired electromechanical
coupling coefficient can be obtained by controlling the
substitutional concentrations of the divalent element and the
pentavalent element contained in the piezoelectric film 26 in the
FBAR of the second embodiment.
A description will now be given of an acoustic wave device in
accordance with a first variation of the second embodiment. The
acoustic wave device of the first variation of the second
embodiment uses an aluminum nitride film containing a divalent
element and a pentavalent element for the piezoelectric films 26a
and 26b. Other configurations are the same as those of the first
variation of the first embodiment, and thus a description thereof
is omitted.
A description will be given of a simulation conducted to
investigate an effective electromechanical coupling coefficient
k.sub.eff.sup.2 of an FBAR of the first variation of the second
embodiment. As is the case with the second embodiment, calculated
values by the first principle calculation are used for the
piezoelectric constants, the elastic constants, and the
permittivities of the piezoelectric films 26a and 26b that are
aluminum nitride films containing a divalent element and a
pentavalent element. A description will now be given of a
simulation performed to a sixth FBAR having the following
configuration.
The sixth FBAR uses a multilayered metal film including Cr with a
film thickness of 100 nm and Ru with a film thickness of 225 nm
stacked in this order from the substrate 22 side for the lower
electrode 24. The piezoelectric films 26a and 26b are aluminum
nitride films having a film thickness of 375 nm and containing Mg
as a divalent element and Ta as a pentavalent element. The
substitutional concentration of Mg is set to 12.5 atomic %, and the
substitutional concentration of Ta is set to 6.25 atomic %. A
SiO.sub.2 film with a film thickness of 50 nm is used for the
temperature compensation film 42. The upper electrode 28 is a
multilayered metal film including Ru with a film thickness of 225
nm and Cr with a film thickness of 30 nm stacked in this order from
the substrate 22 side. A SiO.sub.2 film with a film thickness of 50
nm is located on the upper electrode 28.
FIG. 19 illustrates simulation results of a resonance
characteristic of the sixth FBAR. A solid line indicates the
resonance characteristic of the sixth FBAR. A dashed line indicates
the resonance characteristic of the second comparative example
described in FIG. 12 for comparison. As illustrated in FIG. 19, an
interval between a resonance frequency and an anti-resonance
frequency is wide in the sixth FBAR compared to the second
comparative example. The effective electromechanical coupling
coefficient k.sub.eff.sup.2 of the second comparative example is
5.01%, whereas the effective electromechanical coupling coefficient
k.sub.eff.sup.2 of the sixth FBAR is 13.1%.
Also simulated are FBARs using various kinds of elements for the
divalent element and the pentavalent element contained in the
piezoelectric films 26a and 26b in the same manner. Table 7
presents simulation results. The substitutional concentration of
the divalent element is set to 12.5 atomic %, the substitutional
concentration of the pentavalent element is set to 6.25 atomic %,
and the configuration other than the divalent element and the
pentavalent element is the same as that of the sixth FBAR.
TABLE-US-00007 TABLE 7 Anti- Resonance resonance Combi- Divalent
Pentavalent frequency frequency nation element element [MHz] [MHz]
k.sub.eff.sup.2 [%] Case 1 Mg Ta 1895.3 2008.2 13.1 Case 2 Mg Nb
1941.8 2024.8 9.69 Case 3 Mg V 1832.8 1936.6 12.5 Case 4 Zn Ta
1935.0 2016.7 9.59 Case 5 Zn Nb 1901.3 1978.6 9.26 Case 6 Zn V
2018.4 2078.8 6.96 Aluminum -- -- 1965.3 2007.0 5.01 nitride
As presented in Table 7, the acoustic wave devices using an
aluminum nitride film containing a divalent element and a
pentavalent element for the piezoelectric film (Case 1 through Case
6) have effective electromechanical coupling coefficients
k.sub.eff.sup.2 greater than that of the acoustic wave device using
a non-doped aluminum nitride film for the piezoelectric film (Table
7: Aluminum nitride) even when the temperature compensation film 42
is included. A combination of the divalent element and the
pentavalent element may be Mg--Ta, Mg--Nb, Mg--V, Zn--Ta, Zn--Nb,
or Zn--V as presented in Table 7, but may be other
combinations.
The first variation of the second embodiment demonstrates that an
acoustic wave device having a high electromechanical coupling
coefficient can be obtained by using an aluminum nitride film
containing a divalent element and a pentavalent element for the
piezoelectric films 26a and 26b even when the temperature
compensation film 42 is included.
The first variation of the first embodiment and the first variation
of the second embodiment insert the temperature compensation film
42 between the piezoelectric films 26a and 26b, but the temperature
compensation film 42 may be located in other locations as long as
it contacts the piezoelectric film. For example, the temperature
compensation film 42 may be located between the upper electrode 28
and the piezoelectric film 26b, or between the lower electrode 24
and the piezoelectric film 26a.
Third Embodiment
A third embodiment describes an experiment performed to an aluminum
nitride film formed so as to contain a divalent element and a
tetravalent element. The aluminum nitride film containing a
divalent element and a tetravalent element is formed as follows.
Doped AlN films with different concentrations of Mg and Zr are
formed by sputtering an Al target, a Mg target, and a Zr target
simultaneously in a mixed gas atmosphere of Ar and N.sub.2 with
varying electrical power applied to each target.
A description will be given of measurement results of piezoelectric
constants of the fabricated doped AlN films. A piezoelectric
constant is measured with a piezometer under a condition that a
load is 0.25N and a frequency is 110 Hz. FIG. 20A illustrates a
relationship between a total of substitutional concentrations of Mg
and Zr and a normalized piezoelectric constant, and FIG. 20B is a
diagram that extracts data of which a ratio of substitutional
concentrations of Mg to Zr is around 1:1 from FIG. 20A. In FIG. 20A
and FIG. 20B, the normalized piezoelectric constant (vertical axis)
is a piezoelectric constant normalized by the piezoelectric
constant of the non-doped AlN. Circles indicate measurement results
of the fabricated doped AlN films. Rectangles indicate calculation
results of the first principle calculation as a reference.
As illustrated in FIG. 20A and FIG. 20B, the doped AlN films
containing Mg and Zr have piezoelectric constants greater than that
of the non-doped AlN when they have a total of substitutional
concentrations of Mg and Zr greater than or equal to 3 atomic % and
less than or equal to 35 atomic %. In addition, not only when the
ratio of substitutional concentrations of Mg to Zr is around 1:1,
but also when it is shifted from 1:1, the piezoelectric constant is
high as long as the total of substitutional concentrations of Mg
and Zr is greater than or equal to 3 atomic % and less than or
equal to 35 atomic %.
FIG. 21A illustrates a relationship between a ratio of a
substitutional concentration of Zr to a total of substitutional
concentrations of Mg and Zr and a normalized piezoelectric
constant, and FIG. 21B is a diagram that extracts data of which a
total of substitutional concentrations of Mg and Zr is greater than
or equal to 3 atomic % and less than or equal to 10 atomic % from
FIG. 21A. In FIG. 21A and FIG. 21B, the normalized piezoelectric
constant (vertical axis) is a piezoelectric constant normalized by
the piezoelectric constant of the non-doped AlN. A horizontal axis
represents a ratio of a substitutional concentration of Zr to a
total of substitutional concentrations of Mg and Zr (substitutional
concentration of Zr/(total of substitutional concentrations of Mg
and Zr)).
As illustrated in FIG. 21A and FIG. 21B, the doped AlN films
containing Mg and Zr have piezoelectric constants greater than that
of the non-doped AlN when they have a ratio of a substitutional
concentration of Zr to a total of substitutional concentrations of
Mg and Zr greater than or equal to 0.35 and less than or equal to
0.75. In addition, when the total of substitutional concentrations
of Mg and Zr is greater than or equal to 3 atomic % and less than
or equal to 10 atomic %, the piezoelectric constant is almost
constant as long as the ratio of a substitutional concentration of
Zr to a total of substitutional concentrations of Mg and Zr is
greater than or equal to 0.35 and less than or equal to 0.75.
Here, a description will be given of a dependence of a
piezoelectric constant on substitutional concentrations of doped
AlN doped with Mg or Zn as a divalent element and Hf, Ti, or Zr as
a tetravalent element. The dependence of the piezoelectric constant
on substitutional concentrations is evaluated by calculation by the
first principle calculation. FIG. 22A and FIG. 22B illustrate
relationships between a total of substitutional concentrations of a
divalent element and a tetravalent element and a normalized
piezoelectric constant. In FIG. 22A and FIG. 22B, the normalized
piezoelectric constant (vertical axis) is a piezoelectric constant
normalized by the piezoelectric constant of the non-doped AlN. FIG.
22A illustrates cases where AlN is doped with Mg as a divalent
element and Hf, Ti, or Zr as a tetravalent element, and FIG. 22B
illustrates cases where AlN is doped with Zn as a divalent element
and Hf, Ti, or Zr as a tetravalent element.
As illustrated in FIG. 22A and FIG. 22B, the piezoelectric constant
monotonically increases with increase in substitutional
concentrations whether Mg or Zn is used as the divalent element and
Hf, Ti, or Zr is used as the tetravalent element to dope AlN. This
result dictates that the same tendency will be obtained when other
elements are used although FIG. 20A through FIG. 21B illustrate
measurement results when Mg is used as the divalent element and Zr
is used as the tetravalent element.
Therefore, when an aluminum nitride film containing a divalent
element and a tetravalent element is used for a piezoelectric film
in an acoustic wave device, a total of substitutional
concentrations of the divalent element and the tetravalent element
is preferably greater than or equal to 3 atomic % and less than or
equal to 35 atomic % as illustrated in FIG. 20A and FIG. 20B. The
above-described configuration can make the piezoelectric constant
of the piezoelectric film large, and thus allows the acoustic wave
device to have a high electromechanical coupling coefficient. To
make the piezoelectric constant of the piezoelectric film large,
the total of substitutional concentrations of the divalent element
and the tetravalent element is preferably greater than or equal to
5 atomic % and less than or equal to 35 atomic %, and more
preferably greater than or equal to 10 atomic % and less than or
equal to 35 atomic %.
As illustrated in FIG. 21A and FIG. 21B, the ratio of the
substitutional concentration of the tetravalent element to the
total of substitutional concentrations of the divalent element and
the tetravalent element is preferably greater than or equal to 0.35
and less than or equal to 0.75. The above configuration can make
the piezoelectric constant of the piezoelectric film large, and
allows the acoustic wave device to have a high electromechanical
coupling coefficient. To maintain the insulation property of the
piezoelectric film, the ratio of the substitutional concentration
of the tetravalent element to the total of substitutional
concentrations of the divalent element and the tetravalent element
is preferably greater than or equal to 0.4 and less than or equal
to 0.6, and more preferably greater than or equal to 0.45 and less
than or equal to 0.55, and further preferably equal to 0.5.
A description will now be given of measurement results of a ratio
(c/a) of a lattice constant in the c-axis direction to a lattice
constant in the a-axis direction in the fabricated doped AlN films.
FIG. 23 illustrates a relationship between a total of
substitutional concentrations of Mg and Zr and a c/a ratio. Circles
indicate measurement results of the fabricated doped AlN films. For
comparison, a rectangle indicates a calculation result of c/a of
non-doped AlN by the first principle calculation. FIG. 23
demonstrates that the doped AlN films containing Mg and Zr have c/a
ratios less than that of the non-doped AlN when they have a total
of substitutional concentrations of Mg and Zr greater than or equal
to 3 atomic % and less than or equal to 35 atomic %.
Thus, the total of substitutional concentrations of the divalent
element and the tetravalent element is preferably greater than or
equal to 3 atomic % and less than or equal to 35 atomic % to make
the c/a ratio of the piezoelectric film small and the
electromechanical coupling coefficient of the acoustic wave device
high.
Fourth Embodiment
A fourth embodiment first describes a relationship between a
resonance frequency and a size of a resonance portion of an
acoustic wave device. For example, an acoustic wave device with an
impedance of 50.OMEGA. has a relationship between a resonance
frequency fr and a capacitance C expressed with
fr=1/(2.pi..times.C.times.50). As described above, the capacitance
increases as the resonance frequency becomes lower in the acoustic
wave device. The capacitance is proportional to an area of the
resonance portion, and accordingly the resonance portion becomes
larger as the resonance frequency becomes lower. In addition, a
frequency f and a wavelength .lamda. of an acoustic wave have a
relationship of f=V/.lamda.. V represents the acoustic velocity of
the acoustic wave. The wavelength .lamda. is equal to a period of a
comb-shaped electrode when a surface acoustic wave is used, and is
equal to the double of total film thickness of a multilayered film
of the resonance portion when a bulk acoustic wave is used. The
acoustic velocity V of the acoustic wave depends on a material to
be used, and accordingly, the wavelength becomes longer and the
resonance portion becomes larger as the resonance frequency becomes
lower.
A description will now be given of a simulation conducted to
investigate a relationship between a resonance frequency and a size
of a resonance portion in an acoustic wave device. The simulation
is performed to an FBAR of a third comparative example that uses a
non-doped AlN film for the piezoelectric film 26 in the FBAR having
the structure illustrated in FIG. 2A through FIG. 2C of the first
embodiment. The non-doped AlN is assumed to have a permittivity
.di-elect cons..sub.33 of 8.42.times.10.sup.-.sub.11 F/m, and an
acoustic velocity V of 11404 m/s. These values are calculated by
the first principle calculation. A resonance frequency is 2 GHz
when Ru with a thickness of 240 nm is used for the lower electrode
24 and the upper electrode 28 and a non-doped AlN film with a
thickness of 1300 nm is used for the piezoelectric film 26 to
configure the resonance portion 30. An area of the resonance
portion 30 is 2.455.times.10.sup.-8 m.sup.2 when the FBAR has an
impedance of 50.OMEGA.. Here, to investigate a relationship between
a resonance frequency and a film thickness of the resonance
portion, a resonance frequency is varied by changing a total film
thickness with keeping a ratio of film thicknesses of the lower
electrode 24, the piezoelectric film 26, and the upper electrode 28
the same. In addition, to investigate a relationship between a
resonance frequency and an area of the resonance portion, the area
of the resonance portion 30 is changed so that the FBAR has an
impedance of 50.OMEGA. at each resonance frequency.
FIG. 24A illustrates a relationship between a resonance frequency
and a normalized film thickness of a resonance portion of the FBAR
of the third comparative example, and FIG. 24B illustrates a
relationship between a resonance frequency and a normalized area of
the resonance portion. In FIG. 24A and FIG. 24B, the normalized
film thickness and the normalized area (vertical axis) are a film
thickness and an area normalized by a film thickness and an area
when a resonance frequency is 2 GHz, respectively. As illustrated
in FIG. 24A and FIG. 24B, a film thickness and area of the
resonance portion become larger as the resonance frequency becomes
lower. As described above, the acoustic wave device grows in size
as the resonance frequency becomes lower. Especially, when the
resonance frequency is less than or equal to 1.5 GHz, the acoustic
wave device drastically grows in size, and when the resonance
frequency is less than or equal to 1.0 GHz, the acoustic wave
device further drastically grows in size.
As described above, a capacitance increases as a resonance
frequency becomes lower. The capacitance is proportional to an area
of the resonance portion of the acoustic wave device, and is also
proportional to a permittivity of the piezoelectric film used in
the acoustic wave device. Therefore, use of a piezoelectric film
with a high permittivity in the acoustic wave device can reduce an
area of the resonance portion to obtain a desired capacitance and
prevent the acoustic wave device from growing in size. Moreover,
the above described relational expression of f=V/.lamda. suggests
that use of a piezoelectric film with a low acoustic velocity can
shorten the wavelength .lamda. to obtain a desired frequency f, and
prevent the acoustic wave device from growing in size. Accordingly,
a description will now be given of a simulation conducted to obtain
a piezoelectric film having a high permittivity and a low acoustic
velocity.
The simulation is performed to doped AlN with a crystal structure
formed by substituting a trivalent element in one of the aluminum
atoms 10 in the non-doped AlN with the wurtzite-type crystal
structure illustrated in FIG. 1 of the first embodiment. That is to
say, simulated is the doped AlN having the wurtzite-type crystal
structure containing fifteen aluminum atoms, one trivalent element,
and sixteen nitrogen atoms. The trivalent element has a
substitutional concentration of 6.25 atomic %. Scandium (Sc) or
yttrium (Y) is used as the trivalent element. In addition, as is
the case with the first embodiment, doped AlN doped with a divalent
element and a tetravalent element is also simulated. The divalent
element and the tetravalent element have substitutional
concentrations of 6.25 atomic %. Ca, Mg, Sr, or Zn is used as the
divalent element, and Ti, Zr, or Hf is used as the tetravalent
element.
Table 8 presents calculated values of permittivities .di-elect
cons..sub.33 in the c-axis direction and acoustic velocities V of
the non-doped AlN and the doped AlN. As presented in Table 8, the
doped AlN doped with a trivalent element (Case 1 and Case 2) and
the doped AlN doped with a divalent element and a tetravalent
element (Case 3 through Case 14) have high permittivities .di-elect
cons..sub.33 and low acoustic velocities V compared to the
non-doped AlN (Table 8: Non-doped AlN). The trivalent element, the
divalent element, and the tetravalent element are not limited to
those presented in Table 8, and may be other elements.
TABLE-US-00008 TABLE 8 Acoustic Di- Tri- Tetra- Permittivity
velocity Combi- valent valent valent .sub.33 V nation element
element element [.times.10.sup.-11 F/m] [m/s] Case 1 -- Sc -- 8.96
10815 Case 2 -- Y -- 8.92 10914 Case 3 Ca -- Ti 9.58 10182 Case 4
Ca -- Zr 10.10 10026 Case 5 Ca -- Hf 9.98 10108 Case 6 Mg -- Ti
9.71 10357 Case 7 Mg -- Zr 9.77 10283 Case 8 Mg -- Hf 9.79 10358
Case 9 Sr -- Ti 10.10 10315 Case 10 Sr -- Zr 10.70 10089 Case 11 Sr
-- Hf 10.20 10108 Case 12 Zn -- Ti 9.82 10393 Case 13 Zn -- Zr 9.78
10090 Case 14 Zn -- Hf 9.85 10269 Non-doped -- -- -- 8.42 11404
AlN
As presented previously, the inventors have found that the doped
AlN containing a trivalent element and the doped AlN containing a
divalent element and a tetravalent element have high permittivities
.di-elect cons..sub.33 and low acoustic velocities V compared to
the non-doped AlN. Here, a description will be given of dependences
of a permittivity .di-elect cons..sub.33 and an acoustic velocity V
on a substitutional concentration of doped AlN doped with a
trivalent element. The dependences of the permittivity .di-elect
cons..sub.33 and the acoustic velocity V on the substitutional
concentration are evaluated by calculation by the first principle
calculation using Sc as the trivalent element. FIG. 25A illustrates
a relationship between a substitutional concentration of Sc and a
permittivity .di-elect cons..sub.33, and FIG. 25B illustrates a
relationship between a substitutional concentration of Sc and an
acoustic velocity V. FIG. 25A and FIG. 25B demonstrate that the
permittivity .di-elect cons..sub.33 increases and the acoustic
velocity V decreases with increase in the substitutional
concentration of Sc. The result having the same tendency is
obtained not only when AlN is doped with Sc, but also when it is
doped with the trivalent element, or the divalent element and the
tetravalent element presented in Table 8. As described above, the
concentration of the element with which AlN is doped can change the
permittivity .di-elect cons..sub.33 and the acoustic velocity V of
the doped AlN. Thus, based on the above described knowledge, a
description will be given of an acoustic wave device that is
prevented from growing in size even when the resonance frequency is
less than or equal to 1.5 GHz.
An acoustic wave device of the fourth embodiment has the same
configuration as that illustrated in FIG. 2A through FIG. 2C of the
first embodiment except that the piezoelectric film 26 is an
aluminum nitride film containing a trivalent element, and thus a
description thereof is omitted. The piezoelectric film 26 has a
crystal structure with a c-axis orientation.
A description will be given of a simulation conducted to
investigate a relationship between a resonance frequency and a size
of a resonance portion of an FBAR of the fourth embodiment.
Simulated is an FBAR using Ru for the lower electrode 24 and the
upper electrode 28, and an aluminum nitride film containing Sc with
a substitutional concentration of 30 atomic % for the piezoelectric
film 26. As with the simulation described in FIG. 24A and FIG. 24B,
the resonance frequency is varied to investigate the relationship
between the resonance frequency and the film thickness of the
resonance portion 30 by changing a total film thickness with
keeping a ratio of film thicknesses of the lower electrode 24, the
piezoelectric film 26, and the upper electrode 28 the same. In
addition, the relationship between the resonance frequency and the
area of the resonance portion 30 is investigated by varying the
area of the resonance portion 30 so that the FBAR has an impedance
of 50.OMEGA. at each resonance frequency. The doped AlN doped with
Sc with a substitutional concentration of 30 atomic % is assumed to
have a permittivity .di-elect cons..sub.33 of 1.18.times.10.sup.-10
F/m and an acoustic velocity V of 8646 m/s. These values are
calculated values by the first principle calculation.
FIG. 26A illustrates a relationship between a resonance frequency
and a normalized film thickness of a resonance portion of the FBAR
of the fourth embodiment, and FIG. 26B illustrates a relationship
between a resonance frequency and a normalized area of the
resonance portion. Solid lines indicate simulation results of the
fourth embodiment, and for comparison, dashed lines indicate
simulation results of the third comparative example. In FIG. 26A
and FIG. 26B, the normalized film thickness and the normalized area
(vertical axis) are a film thickness and an area normalized by a
film thickness and an area when a resonance frequency is 2 GHz in
the FBAR of the third comparative example, respectively. As
illustrated in FIG. 26A and FIG. 26B, at the same resonance
frequency, the resonance portion 30 of the fourth embodiment has a
smaller film thickness and a smaller area than that of the third
comparative example. For example, when the resonance frequency is
700 MHz, the normalized film thickness of the resonance portion is
2.84 in the third comparative example, whereas that of the fourth
embodiment is 2.15 and reduced by approximately 24%. The normalized
area of the resonance portion is 8.07 in the third comparative
example, whereas that of the fourth embodiment is 4.40 and reduced
by about 45%.
The fourth embodiment uses an aluminum nitride film containing a
trivalent element that increases a permittivity .di-elect
cons..sub.33 and decreases an acoustic velocity V for the
piezoelectric film 26. This configuration allows the resonance
portion to have a small film thickness and a small area as
illustrated in FIG. 26A and FIG. 26B, and can prevent the acoustic
wave device from growing in size even when the acoustic wave device
has a resonance frequency less than or equal to 1.5 GHz.
As presented in Table 8, the permittivity .di-elect cons..sub.33
increases and the acoustic velocity V decreases when AlN is doped
with a divalent element and a tetravalent element. Therefore, an
aluminum nitride film containing a divalent element and a
tetravalent element may be used for the piezoelectric film 26. When
an aluminum nitride film containing the trivalent element presented
in Table 8 is used for the piezoelectric film 26, at least one of
Sc and Y may be contained. When an aluminum nitride film containing
the divalent element and the tetravalent element presented in Table
8 is used, at least one of Ca, Mg, Sr, and Zn may be contained as
the divalent element, and at least one of Ti, Zr, and Hf may be
contained as the tetravalent element.
The acoustic wave device can be prevented from growing in size by
achieving at least one of an increase in the permittivity .di-elect
cons..sub.33 and a decrease in the acoustic velocity V in the
piezoelectric film 26. Therefore, the piezoelectric film 26 is not
limited to an aluminum nitride film containing a trivalent element,
or a divalent element and a tetravalent element, and may be an
aluminum nitride film containing an element that can achieve at
least one of an increase in a permittivity .di-elect cons..sub.33
and a decrease in an acoustic velocity V. Moreover, when a
trivalent element, or a divalent element and a tetravalent element
are contained, elements other than the elements presented in Table
8 may be contained.
When the resonance frequency is less than or equal to 1.5 GHz, the
acoustic wave device drastically grows in size, and when the
resonance frequency is less than or equal to 1.0 GHz, the acoustic
wave device further drastically grows in size. This fact leads a
conclusion that an aluminum nitride film containing an element that
contributes to at least one of an increase in the permittivity
.di-elect cons..sub.33 and a decrease in the acoustic velocity V is
preferably used for the piezoelectric film 26 of the acoustic wave
device with a resonance frequency less than or equal to 1.0
GHz.
To prevent the acoustic wave device from growing in size, the
permittivity .di-elect cons..sub.33 of the piezoelectric film 26 is
preferably greater than 8.42.times.10.sup.-11 F/m, which is the
permittivity of the non-doped AlN. The acoustic velocity V is
preferably less than 11404 m/s, which is the acoustic velocity of
the non-doped AlN.
A description will now be given of acoustic wave devices in
accordance with a first variation and a second variation of the
fourth embodiment. FIG. 27A is a cross-sectional view of the
acoustic wave device in accordance with the first variation of the
fourth embodiment, and FIG. 27B is a cross-sectional view of the
acoustic wave device in accordance with the second variation of the
fourth embodiment. As illustrated in FIG. 27A, an FBAR of the first
variation of the fourth embodiment includes the temperature
compensation film 42 located between the piezoelectric film 26 and
the upper electrode 28 and contacting the piezoelectric film 26 and
the upper electrode 28. An aluminum nitride film containing a
trivalent element is used for the piezoelectric film 26. Other
configurations are the same as those of the first variation of the
first embodiment, and thus a description thereof is omitted.
As illustrated in FIG. 27B, an FBAR of the second variation of the
fourth embodiment includes the upper electrode 28 including a lower
layer 28a and an upper layer 28b. The temperature compensation film
42 is located between the lower layer 28a and the upper layer 28b.
As described above, the upper electrodes 28 are formed on the top
surface and the bottom surface of the temperature compensation film
42, and mutually electrically short-circuited. Accordingly, a
capacitance of the temperature compensation film 42 fails to
electrically contribute, and the effective electromechanical
coupling coefficient can be made high. An aluminum nitride film
containing a trivalent element is used for the piezoelectric film
26 as is the case with the first variation of the fourth
embodiment. Other configurations are the same as those of the first
variation of the first embodiment, and thus a description thereof
is omitted.
A description will be given of a simulation conducted to
investigate a relationship between a resonance frequency and a size
of a resonance portion in the FBARs of the first variation and the
second variation of the fourth embodiment. Simulated is an FBAR
using Ru for the lower electrode 24 and the upper electrode 28, an
aluminum nitride film containing Sc with a substitutional
concentration of 30 atomic % for the piezoelectric film 26, and a
SiO.sub.2 film for the temperature compensation film 42.
In the first variation of the fourth embodiment, the resonance
frequency is 2 GHz when the lower electrode 24 has a thickness of
160 nm, the piezoelectric film 26 has a thickness of 870 nm, the
temperature compensation film 42 has a thickness of 100 nm, and the
upper electrode 28 has a thickness of 160 nm. In addition, the area
of the resonance portion 30 is 1.595.times.10.sup.-8 m.sup.2 when
the FBAR has an impedance of 50.OMEGA..
In the second variation of the fourth embodiment, the resonance
frequency is approximately 40 MHz lower than that in the first
variation of the fourth embodiment when the lower electrode 24 has
a thickness of 160 nm, the piezoelectric film 26 has a thickness of
870 nm, the temperature compensation film 42 has a thickness of 100
nm, the lower layer 28a of the upper electrode 28 has a thickness
of 20 nm, the upper layer 28b has a thickness of 160 nm.
As with the simulation described in FIG. 24A and FIG. 24B, the
resonance frequency is varied to investigate a relationship between
the resonance frequency and the film thickness of the resonance
portion 30 by changing a total film thickness with keeping a ratio
of film thicknesses of layers constituting the resonance portion 30
the same. In addition, the area of the resonance portion 30 is
varied so that the FBAR has an impedance of 50.OMEGA. at each
resonance frequency in order to investigate a relationship between
the resonance frequency and the area of the resonance portion
30.
FIG. 28A illustrates relationships between a resonance frequency
and a normalized film thickness of a resonance portion in the first
variation and the second variation of the fourth embodiment, and
FIG. 28B illustrates relationships between a resonance frequency
and a normalized area of the resonance portion. In FIG. 28A and
FIG. 28B, the normalized film thickness and the normalized area
(vertical axis) are a film thickness and an area normalized by a
film thickness and an area when a resonance frequency is 2 GHz in
the FBAR of the first variation of the fourth embodiment,
respectively. Solid lines indicate simulation results of the first
variation of the fourth embodiment, and dashed lines indicate
simulation results of the second variation of the fourth
embodiment. FIG. 28A and FIG. 28B demonstrate that the film
thickness of the resonance portion 30 is almost the same in the
first variation and the second variation of the fourth embodiment
at the same resonance frequency, but the area of the resonance
portion 30 is small in the second variation of the fourth
embodiment compared to that in the first variation. For example,
when the resonance frequency is 700 MHz, the normalized area of the
resonance portion is 8.20 in the first variation of the fourth
embodiment, whereas that of the second variation of the fourth
embodiment is 5.90 and reduced by approximately 28%.
The first variation of the fourth embodiment demonstrates that an
effect of temperature compensation can be obtained and the acoustic
wave device can be prevented from growing in size by using an
aluminum nitride film containing an element that achieves at least
one of an increase in the permittivity .di-elect cons..sub.33 and a
decrease in the acoustic velocity V for the piezoelectric film 26,
and including the temperature compensation film 42. The second
variation of the fourth embodiment demonstrates that both a
temperature compensation and an increase in the effective
electromechanical coupling coefficient can be achieved and the
acoustic wave device can be prevented from growing in size by
including conductive films that are formed on the top surface and
the bottom surface of the temperature compensation film 42 and
mutually short-circuited.
In the first variation of the fourth embodiment, the temperature
compensation film 42 may be inserted into the piezoelectric film
26, or may be located between the lower electrode 24 and the
piezoelectric film 26. In addition, the second variation of the
fourth embodiment uses the upper electrodes 28 as conductive films
that are formed on the top surface and the bottom surface of the
temperature compensation film 42 and mutually short-circuited, but
may use the lower electrode 24. When the temperature compensation
film 42 is inserted into the piezoelectric film 26, new conductive
films that are mutually electrically short-circuited may be formed
on the top surface and the bottom surface of the temperature
compensation film 42.
As illustrated in FIG. 2B, the first embodiment through the fourth
embodiment describes the air-space 32 formed by a dome-shaped bulge
between the substrate 22 and the lower electrode 24, but the
air-space 32 may have a structure illustrated in FIG. 29A through
FIG. 29B. FIG. 29A illustrates a cross-section of an FBAR of a
first variation of the embodiments, and FIG. 29B illustrates a
cross-section of an FBAR of a second variation of the embodiments.
As illustrated in FIG. 29A, an air-space 32a is formed by removing
a part of the substrate 22 below the lower electrode 24 in the
resonance portion 30 in the FBAR of the first variation of the
embodiments. As illustrated in FIG. 29B, an air-space 32b is formed
so that it pierces through the substrate 22 below the lower
electrode 24 in the resonance portion 30 in the FBAR of the second
variation of the embodiments.
In addition, the acoustic wave device is not limited to a
piezoelectric thin film resonator of FBAR type, and may be a
piezoelectric thin film resonator of SMR (Solidly Mounted
Resonator) type. FIG. 29C illustrates a cross-section of an SMR. As
illustrated in FIG. 29C, the SMR includes an acoustic reflection
film 50 formed by alternately stacking a film 52 having a high
acoustic impedance and a film 54 having a low acoustic impedance
with a film thickness of .lamda./4 (.lamda. is the wavelength of
the acoustic wave) under the lower electrode 24.
Furthermore, the acoustic wave device may be a piezoelectric thin
film resonator of CRF (Coupled Resonator Filter) type. FIG. 30
illustrates a cross-section of a CRF. As illustrated in FIG. 30,
the CRF includes a first piezoelectric thin film resonator 92 and a
second piezoelectric thin film resonator 94 stacked on the
substrate 22. The first piezoelectric thin film resonator 92
includes the lower electrode 24, the piezoelectric film 26, and the
upper electrode 28. The second piezoelectric thin film resonator 94
includes the lower electrode 24, the piezoelectric film 26, and the
upper electrode 28. A decoupler film 90 with a single layer is
located between the upper electrode 28 of the first piezoelectric
thin film resonator 92 and the lower electrode 24 of the second
piezoelectric thin film resonator 94. The decoupler film 90 may be
a film containing silicon oxide, such as a silicon oxide film or
silicon oxide film containing an additive element.
The acoustic wave device may be a surface acoustic wave device or
Lamb wave device. FIG. 31A is a top view of a surface acoustic wave
device, and FIG. 31B is a cross-sectional view taken along line A-A
in FIG. 31A. FIG. 31C is a cross-sectional view of a Love wave
device, and FIG. 31D is a cross-sectional view of a boundary
acoustic wave device. As illustrated in FIG. 31A and FIG. 31B, a
piezoelectric film 62 is formed on a support substrate 60 made of
an insulative substrate such as a Si substrate, a glass substrate,
a ceramic substrate, or a sapphire substrate. The piezoelectric
film 62 is an aluminum nitride film containing a divalent element
and a tetravalent element, an aluminum nitride film containing a
divalent element and a pentavalent element, or an aluminum nitride
film containing a trivalent element or other elements, and is made
of the same material as that of the piezoelectric film 26 described
in the first embodiment and the second embodiment, or the fourth
embodiment. A metal film 64 such as Al or Cu is formed on the
piezoelectric film 62. The metal film 64 forms reflectors R0, an
IDT (Interdigital Transducer) IDT0, an input terminal T.sub.in, and
an output terminal T.sub.out. The IDT0 includes two comb-shaped
electrodes 66. One of the comb-shaped electrodes 66 is coupled to
the input terminal T.sub.in, and the other one is coupled to the
output terminal T.sub.out. The input terminal T.sub.in and the
output terminal T.sub.out form external connection terminals. The
reflectors R0 are located at both sides of the IDT0 in the
propagation direction of the acoustic wave. The comb-shaped
electrodes 66 and the reflectors R0 include electrode fingers
arranged at intervals corresponding to the wavelength .lamda. of
the acoustic wave. The acoustic wave excited by the IDT0 propagates
through the surface of the piezoelectric film 62, and is reflected
by the reflectors R0. This allows the surface acoustic wave device
to resonate at a frequency corresponding to the wavelength .lamda.
of the acoustic wave. That is to say, the comb-shaped electrodes 66
located on the piezoelectric film 62 function as electrodes that
excite the acoustic wave propagating through the piezoelectric film
62.
Plan views of the Love wave device and the boundary acoustic wave
device are the same as FIG. 31A, and thus a description thereof is
omitted. The Love wave device includes a dielectric film 68 formed
so as to cover the metal film 64 and contact the top surface of the
piezoelectric film 62 as illustrated in FIG. 31C. The dielectric
film 68 can function as a temperature compensation film when the
dielectric film 68 is formed of a material having a temperature
coefficient of an elastic constant opposite in sign to that of the
piezoelectric film 62. The dielectric film 68 may be a film mainly
containing silicon oxide such as SiO.sub.2. The boundary acoustic
wave device further includes a dielectric film 70 formed on the
dielectric film 68 as illustrated in FIG. 31D. The dielectric film
70 may be an aluminum oxide film for example. To confine the
acoustic wave in the dielectric film 68, the dielectric film 70 has
an acoustic velocity faster than that in the dielectric film
68.
FIG. 31A through FIG. 31D illustrate the piezoelectric film 62
located on the support substrate 60, but the piezoelectric film 62
may be made to have a large thickness so that it has a supporting
function as a substrate instead of the support substrate 60.
FIG. 32 is a cross-sectional view of a Lamb wave device. The Lamb
wave device includes a second support substrate 82 located on a
first support substrate 80. The second support substrate 82 is
bonded to the top surface of the first support substrate 80 by
surface activated bonding or resin bonding for example. The first
support substrate 80 and the second support substrate 82 may be an
insulative substrate such as a Si substrate, a glass substrate, a
ceramic substrate, or a sapphire substrate. A piezoelectric film 84
is located on the second support substrate 82. The piezoelectric
film 84 is an aluminum nitride film containing a divalent element
and a tetravalent element, an aluminum nitride film containing a
divalent element and a pentavalent element, or an aluminum nitride
film containing a trivalent element or other elements, and is made
of the same material as that of the piezoelectric film 26 described
in the first embodiment and the second embodiment, or the fourth
embodiment. Hole portions piercing through the second support
substrate 82 in a thickness direction are formed, and the hole
portions function as air-spaces 86 between the first support
substrate 80 and the piezoelectric film 84. An electrode 88 is
located on the piezoelectric film 84 and in a region located above
the air-spaces 86. The electrode 88 is an IDT, and reflectors (not
illustrated) are located at both sides of the IDT. The acoustic
wave excited by the electrode 88 is repeatedly reflected between
the top and bottom surface of the piezoelectric film 84, and
propagated through the piezoelectric film 84 in a lateral
direction.
The Lamb wave device may also include a dielectric film formed so
as to cover the electrode 88 and contact the top surface of the
piezoelectric film 84 as illustrated in FIG. 31C. The dielectric
film can function as a temperature compensation film when it is
formed of a material having a temperature coefficient of an elastic
constant opposite in sign to that of the piezoelectric film 84.
Although the embodiments of the present invention have been
described in detail, it is to be understood that the various
change, substitutions, and alterations could be made hereto without
departing from the spirit and scope of the invention.
* * * * *