U.S. patent application number 10/027103 was filed with the patent office on 2002-04-18 for surface acoustic wave device and production method thereof.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Inoue, Kazuhiro, Watanabe, Masanobu.
Application Number | 20020043889 10/027103 |
Document ID | / |
Family ID | 18885012 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020043889 |
Kind Code |
A1 |
Inoue, Kazuhiro ; et
al. |
April 18, 2002 |
Surface acoustic wave device and production method thereof
Abstract
A method for producing a surface acoustic wave device includes
the steps of forming a resist pattern corresponding to an
interdigital electrodes transducer onto a piezoelectric substrate;
forming a first electrode layer comprising elemental Ti or a
Ti-based alloy onto the piezoelectric substrate and the resist
pattern R; forming a second electrode layer which comprises
elemental Cu or an Al-based alloy containing at least 2% by weight
of Cu (this layer may have a multilayered structure) onto the first
electrode layer; and removing both the resist pattern R and
unnecessary parts, if any, of the first electrode layer 31 and the
second electrode layer to simultaneously pattern the first
electrode layer and the second electrode layer. A surface acoustic
wave device is constructed according to the above method.
Inventors: |
Inoue, Kazuhiro; (Shiga-ken,
JP) ; Watanabe, Masanobu; (Ishikawa-ken, JP) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
|
Family ID: |
18885012 |
Appl. No.: |
10/027103 |
Filed: |
December 26, 2001 |
Current U.S.
Class: |
310/313B ;
29/25.35; 310/364 |
Current CPC
Class: |
Y10T 29/42 20150115;
H03H 3/08 20130101 |
Class at
Publication: |
310/313.00B ;
29/25.35; 310/364 |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2001 |
JP |
2001-019070 |
Claims
What is claimed is:
1. A method for producing a surface acoustic wave device including
a piezoelectric substrate, and an interdigital electrodes
transducer which is arranged on the piezoelectric substrate and
comprises a first electrode layer comprising elemental Ti or a
Ti-based alloy and a second electrode layer comprising elemental Cu
or an Al-based metal material containing at least 2% by weight of
Cu, the method comprising: forming a resist pattern corresponding
to the interdigital electrodes transducer onto the piezoelectric
substrate; forming the first electrode layer onto both the
piezoelectric substrate and the resist pattern by thin-film
deposition; forming the second electrode layer onto the first
electrode layer; and simultaneously removing the resist pattern
together with the portions of the first and second electrode layers
located on the resist pattern.
2. A method for producing a surface acoustic wave device according
to claim 1, wherein a main part of the interdigital electrodes
transducer has a width of 1 .mu.m or less.
3. A method for producing a surface acoustic wave device according
to claim 1, wherein the second electrode layer has a multilayer
structure.
4. A surface acoustic wave device comprising: a piezoelectric
substrate; and an interdigital electrodes transducer formed onto
the piezoelectric substrate by simultaneously patterning a first
electrode layer and a second electrode layer located on the first
layer using a lift-off procedure, wherein the first electrode layer
comprises elemental Ti or a Ti-based alloy, and the second
electrode layer comprises elemental Cu or an Al-based metal
material containing at least 2% by weight of Cu.
5. A surface acoustic wave device according to claim 4, wherein the
interdigital electrodes transducer has a main part having a width
of 1 .mu.m or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to surface acoustic wave
devices used for surface acoustic wave resonators, surface acoustic
wave filters, and the like, and particularly to high-frequency,
high-voltage surface acoustic wave devices, and to a production
method thereof.
[0003] 2. Description of the Related Art
[0004] An interdigital electrode transducer for a surface acoustic
wave (SAW) device generally includes an elemental Al or Al-based
alloy layer which has low electric resistance and a small mass.
However, since such layers are not sufficiently resistant to stress
migration, hillocks or voids may be formed therein when this
happens. Short circuits may be generated when a high voltage is
applied to the device with the result that the device will be
destroyed.
[0005] In order to solve this problem, the following methods have
been proposed:
[0006] (1) depositing an electrode layer composed of a metal, such
as Ti, which ameliorates the orientation of an elemental Al or
Al-based alloy layer on a base of the SAW device and then
depositing the elemental Al or Al-based alloy layer on the
electrode layer; and
[0007] (2) sandwiching an elemental Cu layer between elemental Al
or Al-based alloy layers.
[0008] Dry etching, such as RIE or ion milling, has been used for
patterning the above electrode layer. Ion milling is effective in
etching elemental Cu layers and Al-based alloy layers containing Cu
but is not effective in etching Ti layers since the rate of etching
elemental Ti layers by ion milling is extremely slow. For this
reason, it is necessary to remove the elemental Ti by Reactive Ion
Etching (RIE) after the elemental Cu layers and Al-based alloy
layers containing Cu are removed by ion milling. Because a
plurality of etching techniques are required, several drawbacks
arise: complexity of the process; the need for a large investment
in different etching equipment; and an increase in production cost
of the surface acoustic wave device due to the complexity of the
process.
[0009] Furthermore, when producing an interdigital electrodes
transducer having a width of 1 .mu.m or less, the following
problems arise: deterioration of device characteristics due to the
damage caused in the piezoelectric substrate during processing;
variation in etching; and low yield.
SUMMARY OF THE INVENTION
[0010] In order to solve the above problem, it is an object of the
present invention to provide a method for producing a surface
acoustic wave device by patterning electrode layers effectively and
to provide a surface acoustic wave device, to which high-frequency
or high-voltage is applied, according to the above production
method.
[0011] According to one aspect of the present invention, a method
for producing a surface acoustic wave device is provided. The
surface acoustic wave device includes a piezoelectric substrate,
and an interdigital electrodes transducer arranged on the
piezoelectric substrate and a first electrode layer including
elemental Ti or a Ti-based alloy and a second electrode layer
including elemental Cu or an Al-based metal material containing at
least 2% by weight of Cu. The method comprises:
[0012] forming a resist pattern corresponding to the interdigital
electrodes transducer onto the piezoelectric substrate;
[0013] forming the first electrode layer onto both the
piezoelectric substrate and the resist pattern by thin-film
deposition;
[0014] forming the second electrode layer onto the first electrode
layer, the second electrode layer including elemental Cu or an
Al-based metal material containing at least 2% by weight of Cu (the
second electrode layer can have a multilayered structure); and
[0015] simultaneously removing the resist pattern and the portions
of the first and second electrode layers located on the resist
pattern to pattern the first electrode layer and the second
electrode layer.
[0016] In the production method of the surface acoustic wave device
according to the present invention, the first electrode layer and
the second electrode layer are preferably deposited on both the
resist pattern and the piezoelectric substrate through the openings
of the resist pattern, and then the unnecessary portions of the
first and second electrode layers located on the resist pattern are
simultaneously removed along with the resist pattern. The foregoing
lift-off procedure allows the first and second electrode layers to
be patterned more effectively than the conventional patterning
which uses a plurality of etching techniques.
[0017] In the above production method, the first electrode layer
includes elemental Ti or a Ti-based alloy and the second electrode
layer is arranged on the first electrode layer. As a result,
orientation in the second electrode layer is excellent and the
formation of hillocks or voids is suppressed. Accordingly,
electrode layers having high voltage resistance are produced.
[0018] The second electrode layer may be a multilayer structure,
e.g., one formed by sandwiching a layer including elemental Cu or
an Al-base alloy containing at least 2% by weight of Cu between Al
electrode layers. Other multilayer structures can also be used.
[0019] The present invention is also directed to a production
method an interdigital electrodes transducer whose main part has a
width of 1 .mu.m or less.
[0020] According to conventional production methods, it is
difficult to produce surface acoustic wave devices having
interdigital electrodes transducers of whose the main part has a
width of 1 .mu.m or less because the piezoelectric substrates are
damaged during the production process. In contrast, the production
method of the present invention effectively provides a surface
acoustic wave device having an interdigital electrodes transducer
whose main part has a width of 1 .mu.m or less without causing the
following disadvantages: damage to the piezoelectric substrate,
deterioration of device characteristics, low yield due to variation
in etching preciseness.
[0021] The present invention is also directed to a surface acoustic
wave device comprising:
[0022] a piezoelectric substrate;
[0023] an interdigital electrodes transducer formed onto the
piezoelectric substrate by simultaneously patterning a first
electrode layer and a second electrode layer formed on the first
layer using a lift-off procedure, wherein the first electrode layer
includes elemental Ti or a Ti-based alloy, and the second electrode
layer includes elemental Cu or an Al-based metal material
containing at least 2% by weight of Cu or the second electrode
layer being arranged on the first electrode layer.
[0024] The surface acoustic wave device of the present invention is
preferably produced by a lift-off procedure in which the first
electrode layer and the second electrode layer are patterned at the
same time. As a result, formation of hillocks or voids on the
piezoelectric substrate is suppressed and the interdigital
electrodes transducer has substantially the same dimensions as the
design value and includes a second electrode layer having an
excellent orientation. Accordingly, the surface acoustic wave
device including the interdigital electrodes transducer has the
desired characteristics and high reliability.
[0025] The present invention is also directed to a surface acoustic
wave device having a main part of a width of 1 .mu.m or less.
[0026] The surface acoustic wave device is reliably produced
without causing decrease in yield due to variation in etching
preciseness because the surface acoustic wave device has the above
structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Other features and advantages of the present invention will
become apparent from the following description of the invention
which refers to the accompanying drawings.
[0028] FIG. 1 is a schematic plan view of a surface acoustic wave
device according to an embodiment of the present invention;
[0029] FIGS. 2A to 2E are views illustrating a process for forming
a resist pattern on a piezoelectric substrate in the production
method of the surface acoustic wave device according to the
embodiment of the present invention;
[0030] FIG. 3 is a sectional view of a laminate in which a first
electrode layer is deposited according to the present
invention;
[0031] FIG. 4 is a sectional view of the laminate in which a second
electrode layer is deposited on the first electrode layer according
to the present invention; and
[0032] FIG. 5 is a sectional view of an interdigital electrodes
transducer formed by lifting off unnecessary portions of the first
and second electrode layers in the production method according to
an embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0033] Embodiments according to the present invention will now be
disclosed in detail.
[0034] FIG. 1 is a schematic plan view showing a surface acoustic
wave device according to an embodiment of the present invention.
The surface acoustic wave device 1 has a structure in which an
interdigital electrode transducer (IDT) 3 and reflector electrodes
4 and 5 are arranged on a piezoelectric substrate 2 containing
LiTaO.sub.3 or LiNbO.sub.3. A presently preferred method for
producing the surface acoustic wave device 1 will be described
below. The invention is not limited to this pattern.
FORMING RESIST PATTERN
[0035] First, a resist pattern R shown in FIG. 2E is formed on the
piezoelectric substrate 2 by lithography. The resist pattern R has
a three layer structure consisting of a first resist layer 21
located on the piezoelectric substrate 2, a second resist layer 22
located on the first resist layer 21, and a third resist layer 23
located on the second resist layer 22.
[0036] Referring to FIGS. 2A to 2E, an exemplary method for forming
the resist pattern R will be described below.
[0037] (1) Referring to FIG. 2A, a first resist layer 21,
preferably having a thickness of 0.12 .mu.m, is formed on the
piezoelectric substrate 2 by spin coating. Then, the piezoelectric
substrate 2 and first resist layer 21 are baked at 170.degree. C.
for 20 minutes. The first resist layer 21 may be composed of an
organic material, for example polydimethylglutarimide, which has
high solubility in organic solvents and high heat resistance. When
heat is applied to the layer composed of such a material during a
process for forming metal wiring, distortion of the resist pattern
does not arise and the resist material maintains high solubility in
organic solvents.
[0038] (2) Referring to FIG. 2B, a second resist layer 22,
preferably having a thickness of 0.16 .mu.m, is formed on the first
resist layer 21 by spin coating. Then, the piezoelectric substrate
2, together with the first and second resist layers 21 and 22, is
baked at 155.degree. C. for 1 minute.
[0039] The second resist layer 22 is preferably composed of an
organic material having high light-absorbance at an exposure
wavelength and high heat resistance. Such a material may be
selected according to the exposure wavelength. The material is
preferably XHRi-16 manufactured by BrewerScience Co. because of its
high light-absorbance when an exposure beam has a wavelength of
365-nm.
[0040] (3) Referring to FIG. 2C, a third resist layer 23,
preferably having a thickness of 0.34 .mu.m, is formed on the
second resist layer 22 by spin coating. Then, the piezoelectric
substrate 2, together with the first, second, and third resist
layers 21, 22, and 23, is baked at 90.degree. C. for 1 minute.
[0041] The third resist layer 23 is preferably composed of an
organic material which is resistant to dry etching and to heat. An
example of such a material is Fi-SP2 manufactured by Fuji Film Olin
Co.
[0042] (4) The composite resist (having the three layer structure
formed as above) is exposed and developed, and then the third
resist layer 23 is patterned as shown in FIG. 2D. Because the
second resist layer 22 absorbs a beam of a wavelength used for
exposure, a pattern having high resolution and micro openings is
formed. Then, UV curing is performed to improve the heat resistance
of the resist pattern.
[0043] (5) Dry etching (RIE) using oxygen plasma is then performed
on the composite resist with the patterned third resist layer 23.
As a result, the second resist layer 22 and the first resist layer
21 are partly removed through the openings of the third resist
layer 23 to complete patterning.
[0044] During this process, the third resist layer 23 is etched at
a low etching rate because it is resistant to dry etching. On the
other hand, the second resist layer 22 and the first resist layer
21 are etched at an ordinary rate, so that the resist pattern R
having an opening 24 and a cross section which tapers toward the
opening 24 as shown in FIG. 2E is completed.
[0045] The dry etching (RIE) is preferably performed under the
following conditions: (1) gas used: oxygen, (2) degree of vacuum:
6.7 Pa, (3) RF power: 300 W, and (4) processing time: 3
minutes.
FORMING INTERDIGITAL ELECTRODES TRANSDUCER
[0046] Referring to FIG. 3, a first electrode layer (a Ti electrode
layer) 31 is formed on the piezoelectric substrate 2 by electron
beam deposition, in which Ti is used as a vapor source. The
deposition is preferably performed under the following conditions:
(1) deposition temperature: 100.degree. C., (2) deposition rate: 1
nm/s, and (3) thickness: 5 nm. In this process, the first electrode
layer 31 is deposited on both the piezoelectric substrate 2 and the
resist pattern R.
[0047] Referring to FIG. 4, a second electrode layer 32 is formed
on the first electrode layer 31 by electron beam deposition, in
which an Al-Cu alloy containing 10% by weight of Cu is used as a
vapor source. The deposition is preferably performed under the
following conditions: (1) deposition temperature: 100.degree. C.,
(2) deposition rate: 1 nm/s, and (3) thickness: 200 nm. In this
process, the second electrode layer 32 is deposited on both the
first electrode layer 31 on the piezoelectric substrate 2 and the
first electrode layer 31 on the resist pattern R.
[0048] The resulting laminate is soaked in acetone (a resist
removal solution) to dissolve the resist pattern R and thereby
simultaneously remove (lift off) both the resist pattern R and the
unnecessary portions of the first and second electrode layers
located on the resist pattern R from the piezoelectric substrate 2.
As a result, as shown in FIG. 5, the interdigital electrodes
transducer 3 having a double layer structure in which the second
electrode layer 32 is arranged on the first electrode layer 31 is
formed. In this exemplary embodiment, the fingers of the
interdigital electrodes transducer 3 have a width W of 0.5 .mu.m.
The reflector electrodes 4 and 5 may be also formed in the
above-described way.
[0049] In the above embodiment, the first electrode layer 31 is
deposited on the piezoelectric substrate 2, the second electrode
layer 32 is deposited on the first electrode layer 31, and then the
unnecessary parts of the first and second electrode layers 31 and
32 located on the resist pattern R are simultaneously removed to
form the interdigital electrodes transducer 3. As a result, the
interdigital electrode transducer 3 consisting of the first
electrode layer 31 and the second electrode layer 32 is effectively
formed, in which the second electrode layer (Al-Cu alloy electrode
layer) 32 has a small mass and high electric conductivity, and the
first electrode layer (Ti electrode layer) 31 ameliorates the
orientation of the Al-Cu alloy electrode layer. Accordingly, a
surface acoustic wave device having high reliability is efficiently
produced without increasing production costs.
[0050] The first electrode layer 31 is preferably formed of Ti to
ameliorate the orientation of the Al-Cu alloy second electrode
layer 32. The second electrode layer 32 is preferably composed of
an Al-Cu alloy containing 10% by weight of Cu to improve stress
migration resistance, and the first electrode layer 31 and the
second electrode layer 32 are patterned at the same time by a
lift-off procedure in which the unwanted portion of the first and
second layers located on the resist pattern is removed, along with
the resist pattern, from the substrate. Thus, the first electrode
layer 31 and the second electrode layer 32 are patterned
efficiently and the interdigital electrodes transducer having
excellent performance is produced in contrast to the conventional
method in which patterning is performed by different etching
processes.
[0051] According to the above embodiment, the surface acoustic wave
device having the above structure is produced by single resist
patterning (albeit multiple resist layers) and by continuous
deposition using only one electron beam deposition apparatus which
reduces costs. As a result, the surface acoustic wave device having
high voltage resistance is efficiently produced at a low cost
compared to the conventional method in which procedures and
conditions of etching differ between layers.
[0052] Although, in the above embodiment, the second electrode
layer 32 is composed of an Al-Cu alloy containing 10% by weight of
Cu, the Cu content may be varied in this embodiment in the range of
at least 2% (i.e., 8%-12%) by weight of Cu. Also, in the present
invention, the second electrode layer 32 may have a multilayered
structure including an electrode layer of elemental Cu or an
Al-based metal material containing at least 2% by weight of Cu. In
such a structure, the first electrode layer 31, which is the base
electrode layer, ameliorates the orientation of the second
electrode layer 32. As a result, the generation of hillocks or
voids is suppressed and, consequently, the interdigital electrodes
transducer having excellent withstand voltage is efficiently
produced.
[0053] Although, in the above embodiment, the first electrode layer
31 is composed of elemental Ti, the first electrode layer 31 may
alternatively be composed of an alloy containing Ti as the main
component.
[0054] Although, in the above embodiment, the second electrode
layer 32 is arranged on the first electrode layer 31, a third
electrode layer may be further arranged on the second electrode
layer 32.
[0055] Although, in the above embodiment, the resist pattern has a
three layer structure, the resist pattern is not limited to such a
structure (one or more layers can be used) and may be formed by
other methods such as an image reverse method.
[0056] The present invention is not limited to the above-mentioned
embodiments in other respects, and various applications and various
modifications may be performed within the scope of the present
invention with respect to an electrode pattern, the shape of a
piezoelectric substrate, and so on.
[0057] Although the present invention has been described in
relation to particular embodiments thereof, many other variations
and modifications and other uses will become apparent to those
skilled in the art. It is preferred, therefore, that the present
invention be limited not by the specific disclosure herein, but
only by the appended claims.
* * * * *