Title: Micromachined piezoelectric microspeaker and fabricating method thereof
Abstract: A micromachined piezoelectric microspeaker and its fabricating method are disclosed. The micromachined piezoelectric microspeaker comprises a diaphragm and a plurality of contact pads. The diaphragm comprises an active area which is flat, and a non-active area which is wrinkled and surrounds the active area. The plurality of contact pads for electrodes are located outside of the diaphragm and over a wafer. And, the method comprises the steps of forming a compressive film on a wafer, forming a bottom electrode on a predetermined part of the compressive film of the front side of the wafer, forming a piezoelectric film on the bottom electrode and on the compressive film of the front side of the wafer, forming a bottom insulator film on the piezoelectric film, forming a top electrode on a predetermined part of the bottom insulator where the top electrode is located over some part of the bottom electrode, forming a top insulator film on the top electrode and on the bottom insulator film, forming contact pads for the bottom electrode and top electrode at an outside part of each electrode, and removing a predetermined part of the wafer which is located between wafer parts located under the each contact pads.
Patent Number: 7,003,125 Issued on 02/21/2006 to Yi, et al.
| Inventors:
|
Yi; Seung-Hwan (410-304 Jugong Apt., Neutimaeul, Jeongja2-dong, Bundang-gu, Gyeonggi-do, KR);
Kim; Eun-Sok (26944 Grayslake Rd., Rancho Palos Verdes, CA 90275)
|
| Appl. No.:
|
243958 |
| Filed:
|
September 12, 2002 |
| Current U.S. Class: |
381/190; 381/173 |
| Current Intern'l Class: |
H04R 25/00 (20060101) |
| Field of Search: |
367/155,157,180,181
310/311,322
381/173-175,190-191
257/254,415-416,418-419
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Ni; Suhan
Attorney, Agent or Firm: Jones Day
Parent Case Text
This application claims the benefit of U.S. Provisional Application No. 60/322,331,
filed on Sep. 12, 2001.
Claims
What is claimed is:
1. A micromachined piezoelectric microspeaker comprising: a diaphragm which comprises:
a piezoelectrically active area which is flat; and
a piezoelectrically non-active area which is wrinkled and surrounds the active
area; and
a plurality of contact pads for electrodes which are located outside of the diaphragm
and over a wafer.
2. The micromachined piezoelectric microspeaker according to claim 1, wherein
the active area comprises a plurality of electrode films and at least one piezoelectric
film, and the non-active area comprises at least one compressive film.
3. The micromachined piezoelectric microspeaker according to claim 1, wherein
the active area comprises:
a compressive film;
a bottom electrode on the compressive film;
a piezoelectric film on the bottom electrode;
a bottom insulator film on the piezoelectric film;
a top electrode on the bottom insulator; and
a top insulator on the top electrode, and
the non-active area comprises:
a compressive film;
a piezoelectric film on the compressive film; and
an insulator film on the piezoelectric film.
4. The micromachined piezoelectric microspeaker according to claim 3, wherein
the compressive film in the active area and non-active area is a compressive silicon
nitride film.
5. The micromachined piezoelectric microspeaker according to claim 3, wherein
the bottom electrode and top electrode are Al films.
6. The micromachined piezoelectric microspeaker according to claim 3, wherein
the piezoelectric film in the active area and non-active area is a piezoelectric
ZnO film.
7. The micromachined piezoelectric microspeaker according to claim 3, wherein
all the insulator films are Parylen-D films.
Description
FIELD OF THE INVENTION
This invention relates to the micromachined acoustic transducers and their fabrication
technology. More particularly this invention relates to piezoelectric microspeaker
with compressive nitride diaphragm.
BACKGROUND OF THE INVENTION
The prior art provides various examples of piezoelectric transducers. Examples
of such piezoelectric transducers are disclosed in U.S. Pat. Nos. 6,140,740; 6,064,746;
5,956,292; 5,751,827; 5,633,552; 4,654,554, and 4,979,219. In many cases, the known
piezoelectric vibrating plate comprises a single thin metal sheet on one or both
sides of which is or are laminated a piezoelectric sheet or sheets consisting of
a round thin piece of 20 to 30 mm in diameter. A conventional piezoelectric speaker
has a construction in which a vibrating film or sheet is stretched on a frame while
being applied tension and a plurality of piezoelectric ceramics are directly stuck
on the film. However, ceramic is so fragile that it is very difficult to make thin
sheet and also it is not economical in terms of mass production with on-chip circuitry
for signal conditioning.
Recently, there has been increasing interest in micromachined acoustic
transducers based on the following advantages: size miniaturization with extremely
small weight, potentially low cost due to the batch processing, possibility of
integrating transducers and circuits on a single chip, lack of transducer "ringing"
due to small diaphragm mass. Especially, these advantages make the micromachined
acoustic transducers, such as microspeaker and microphone attractive in the applications
for personal communication systems, multimedia systems, hearing aid and so on.
Micromachined acoustic transducers are provided with a thin diaphragm
by deposition system and several diaphragm materials that must be compatible with
high temperature semiconductor process, such as low stress silicon nitride and
silicon have been applied as diaphragm. However, micromachined acoustic transducers
made by these conventional diaphragm materials suffer from a relatively low output
pressure and sensitivity, which are mainly because of the high stiffness and low
deflection of these diaphragm materials in case of transducers application. So,
in some cases, a conventional piezoelectric speaker used fiber reinforced epoxy,
polyester, or ABS resin diaphragm in order to increase the deflection of diaphragm
reported in U.S. Pat. No. 5,751,827.
In order to implement the micromachined microspeaker transducers with competitive
performance with conventional microspeaker, it is necessary to find the new diaphragm
materials that have large deflection with small driving voltage and compatibility
with semiconductor process at the same time. Also, proper material and technique
should be investigated to cause large deflection of diaphragm.
For the foregoing reasons, there is a need for a micromachined piezoelectric
microspeaker which has a new diaphragm materials that have large deflection with
small driving voltage and compatibility with semiconductor process at the same time.
SUMMARY OF THE INVENTION
The present invention is directed to a micromachined piezoelectric microspeaker
and its fabricating method that satisfies this need. The micromachined piezoelectric
microspeaker comprises a diaphragm and a plurality of contact pads. The diaphragm
(102) comprises an active area (104), which is flat, and a non-active
area (106), which is wrinkled and surrounds the active area (104).
The plurality of contact pads (108) for electrodes are located outside of
the diaphragm (102) and over a wafer (110).
And, the method comprises the steps of forming a compressive film (202,204)
on a wafer (110), forming a bottom electrode (206) on a predetermined
part of the compressive film (202) of the front side of the wafer (110),
forming a piezoelectric film (208) on the bottom electrode (206)
and on the compressive film (202) of the front side of the wafer, forming
a bottom insulator film (210) on the piezoelectric film (208), forming
a top electrode (212) on a predetermined part of the bottom insulator (210)
where the top electrode (212) is located over some part of the bottom electrode
(206), forming a top insulator film (214) on the top electrode (212)
and on the bottom insulator film (210), forming contact pads (108)
for the bottom electrode (206) and top electrode (208) at an outside
part of each electrode (206,208), and removing a predetermined part
of the wafer (110) which is located between wafer parts located under the
each contact pads (108).
As a novel idea, micromachined piezoelectric microspeaker has successfully been
fabricated on a 1.0 μm thick compressive nitride diaphragm (5,000 μm2
for flat square diaphragm, grand cross type, circle shape type with 3 mm diameter,
which are shown in FIG. 1A) with electrodes and a piezoelectric ZnO film. The piezoelectric
microspeakers are tested with various applying voltage and frequency ranges. The
experimental results showed that it has a comparable sound output as a commercial,
rather bulky, piezo-ceramic speaker. The sound output of the microspeaker (fabricated
with a relatively simple and robust process) is even higher than a cantilever-based
piezoelectric microspeaker patented on May 27, 1997 (U.S. Pat. No. 5,633,552).
The key to this breakthrough is the usage of a diaphragm that has a very high
compressive residual stress, high enough to cause the diaphragm to be wrinkled.
And we maintain flatness in the speaker active area through a mild tensile stress
in the electrode layers, though the non-active area is wrinkled. This way, we can
produce a large diaphragm deflection (without being hindered by the diaphragm stretching
effect) with good control over a flat, active area where the electromechanical
transduction is happening.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention
will become better understood with regard to the following description, appended
claims, and accompanying drawings where:
FIG. 1A shows piezoelectric microspeaker built on a wrinkled diaphragm (photo
of fabricated speakers);
FIG. 1B shows a cross-sectional view of a schematic of piezoelectric microspeaker
built on a wrinkled diaphragm;
FIG. 2 shows fabrication process flows for the piezoelectric microspeaker;
FIG. 3 shows photo taken from the front side of a completed 3" silicon wafer
that contains various acoustic transducers;
FIG. 4 shows a schematic diagram of the experimental set-up for the measurement
of microspeaker frequency response;
FIG. 5 shows a speaker output pressure versus input voltage measured at 1 kHz
(without an acoustic coupler); and
FIG. 6 shows a speaker output pressure versus frequency between 0.4 and 12 kHz
(without an acoustic coupler).
DETAILED DESCRIPTION OF THE INVENTION
Microelectromechanical Systems (MEMS) technology has been
used to fabricate tiny microphones and microspeaker [1,2,3] on silicon wafer. This
method of fabricating acoustic transducers on silicon wafer has the following advantages
over the more traditional methods: potentially low cost due to the batch processing,
possibility of integrating sensor and amplifier on a single chip, and size miniaturization.
Compared to more popular condenser-type MEMS transducers, piezoelectric
MEMS transducers are simpler to fabricate, free from the polarization-voltage requirement,
and responsive over a wider dynamic range [4,5,6]. However, piezoelectric MEMS
transducer suffers from a relatively low sensitivity, mainly due to high stiffness
of the diaphragm materials used for the transducer. The thin film materials for
diaphragm strictly restricted to use such as silicon nitride, silicon, and polysilicon
though these materials have high stiffness and residual stress. It is because of
the considerations of compatibility with high temperature semiconductor process.
High temperature semiconductor process hinders the usage of more flexible materials
such as polymer films and metal foils as diaphragm materials though many conventional
bulky acoustic transducers use polymer diaphragm to improve the performance.
As a novel idea for building micromachined acoustic transducers, we used a diaphragm
that has a very high compressive residual stress, high enough to cause the diaphragm
to be wrinkled as shown in FIG. 1A. By using a high compressive silicon nitride
diaphragm, however, we maintain flatness in the speaker active area, through a
mild tensile stress in the electrode layers, though the non-active area is wrinkled
as described in FIG. 1B. This way, we can produce a large diaphragm deflection
(without being hindered by the diaphragm stretching effect) with good control over
a flat, active area where the electromechanical transduction is happening.
FABRICATION AND TESTING RESULTS
Four masks are used in the fabrication process for the piezoelectric microspeaker
shown in FIG. 2. First, 1 μm thick compressive silicon nitride film is deposited
by Low Pressure Chemical Vapor Deposition (LPCVD) system on bare silicon wafers.
An Al film is next deposited on the front side of the wafers for contact pads and
electrodes. The film is approximately 0.5 μm thick, patterned by lithography
to form bottom contact pads and electrodes, wet etched by using a potassium ferrocyanide
(K3Fe(CN)6)/potassium hydroxide (KOH) solution. After depositing about 0.5 μm
thick piezoelectric ZnO film by RF (Radio Frequency) magnetron sputtering system
at 400 watts 275° C. substrate temperature, approximately 0.2 μm thin
Parylene-D film is deposited with Parylene-deposition system only onto the front
side of wafers at 8 mtorr for one and half hours (the weight of Parylene-D dimmer
vaporizer is around 0.8 gram). In order to secure good contact, Parylene-D covered
contact pads are patterned by lithography and dry etched by RIE (Reactive Ion Etching)
system at 60 watts oxygen plasma ambient for 5 min. Then, 0.5 μm thick Al
film is deposited to form top electrodes and contact pads, wet etched by using
same etchant mentioned above. Since the Parylene-D has a low stiffness (one hundred
times lower than silicon nitride film), the diaphragm was mechanically strengthened
without critical changing of stiffness by depositing 1.0 μm thick Parylene-D
(the weight of Parylene-D dimmer vaporizer is around approximately 4.0 gram.) onto
front side only, which increases the yield by preventing breakage of diaphragms
during cutting wafers into small chips. After Parylene-D patterning by lithography,
which is dry etched by RIE system for 10 min at 100 watts oxygen plasma. Then,
the ZnO film that is covered above bottom Al contact pads is wet etched by diluted
phosphoric acid (H3PO4) solution (H3PO4:H2O=1:100). The back side silicon nitride
is patterned by lithography, and dried etched by RIE system with CF4 plasma ambient
at 100 watts for 30 min. And then, silicon substrate is removed by KOH solution
under IR lamp [7] in order to release the diaphragm. After the silicon substrate
is cleaned by flowing DI (De-Ionized) water and dried by nitrogen blowing, the
wafer is cut into small chips in order to test its performance.
FIG. 3 shows the photo of a fabricated 3" silicon wafer that contains the microspeakers
(built on wrinkled diaphragms except the active regions sandwiched by Al electrodes).
We have designed and fabricated various kinds of piezoelectric microspeakers (on
a 5×5 mm2 diaphragm) with electrode shapes of circles (2 to 3 mm in diameter),
grand cross (1.67 mm wide and with its four edges clamped to silicon), and rectangle
(with its wide edge clamped to silicon). The labeling for the tested microspeakers
is indicated in FIG. 3.
FIG. 4 describes an experimental set-up for the fabricated microspeaker according
to present invention. The fabricated microspeaker is put into an acoustic chamber
shown in FIG. 4 and is actuated by applying sinusoidal wave (6 VPEAK-TO-PEAK) with
function generator. The output frequency response has been measured without an
acoustic coupler by reference microphone (B&K 4135 microphone) connected to the
spectrum analyzer. The data has been normalized by the characteristic value of
reference microphone.
FIG. 5 shows the microspeaker output pressure as a function of input voltages.
As can be seen in FIG. 5 that shows the speaker sound output as a function of an
input voltage at 1 kHz, the linearities of most of the fabricated microspeakers
are very good over a wide range. The microspeaker labeled as UH MEMS4 (a grand
cross type, which is shown in FIG. 1) produces about 26.1 mPa, while a circular
type (#82
—4
—4, which is shown in FIG. 3) produces
10.4 mPa at 6.0 Vpeak-to-peak. The frequency responses of the microspeakers have
also been measured between 400 Hz and 12 kHz, and are shown in FIG. 6 along with
that of a commercial piezoelectric speaker (SMAT
—21). In the frequency
range between 0.4 and 1.5 kHz, the microspeaker (UH MEMS4) produces comparable
sound pressure as the commercial one. We, indeed, qualitatively observed several
times higher sound output than what is quantitatively reported in FIG. 6.
Although the present invention has been described in considerable detail
with reference to certain preferred versions thereof, other versions are possible.
Therefore, The sprit and scope of the appended claims should not be limited to
the description of the preferred versions contained herein.
*
