BACKGROUND OF THE INVENTION
The present invention relates to a thin-film bulk acoustic resonator
(FBAR) and fabrication method therefore. More particularly, the present invention
relates to an air-gap type thin-film acoustic resonator including a substrate having
an air-gap formed in an upper portion of the substrate and a fabrication method
therefor, wherein the air gap is formed by etching the substrate through a via hole
extending into the substrate from a lower surface thereof.
Recently, mobile communication devices such as mobile phones have
become widely used, and there is a need for smaller-sized and light-weight filters
for use in such devices. FBARs are known as being suitable for such small-sized
and light-weight filters, and are advantageous in that they can be mass produced
at minimum cost and in small size. Further, such FBARs are advantageous in that
they can be fabricated to have a high quality factor Q which is a primary characteristic
of such filters, for use in micro-frequency bands, and, particularly fabricated
to cover even bands for personal communication systems (PCSs) and digital cordless
In general, the FBAR device is fabricated by forming a lower electrode,
a piezoelectric layer, and an upper electrode which are deposited in order over
a substrate. The operation principle of the FBAR device is as follows: Resonance
is generated by applying electric energy to the electrodes to induce within the
piezoelectric layer an electromagnetic field which varies with time to generate
bulk acoustic waves in the vibration direction of the resonance part.
FIG. 1A is a cross-sectioned view of a Bragg-reflector FBAR which
is a type of FBAR. In FIG. 1A, the Bragg-reflector FBAR has a substrate 10, reflection
layers 11, a lower electrode 12, a piezoelectric layer 13, and an upper electrode
14. In the Bragg-reflector FBAR, acoustic waves generated from the piezoelectric
layer 13 are not propagated in the substrate direction, but are reflected from the
reflection layers 11, so that effective resonance can be generated. In its fabrication
process, first, substances having a large acoustic impedance difference therebetween
are deposited over the substrate 10 to form the reflection layers 11, and then the
lower electrode 12, piezoelectric layer 13, and upper electrode 14 are deposited
in order, so that a resonance part is formed over the reflection layers 11. The
Bragg-reflector FBAR is sturdy in structure and does not exhibit stress upon being
bent, but has a drawback in that it is difficult to precisely form more than four
reflection layers for total reflection and much time and expense is required for
Thus, the air-gap type FBAR has been investigated which uses an air
gap instead of a reflection layer, to isolate the substrate from the resonance part.
FIG. 1B and FIG. 1C are cross-sectional views which illustrate the structure of
a conventional air-gap type FBAR.
The FBAR having a structure shown in FIG. 1B has an air gap 21 under
the resonance part formed with a lower electrode 23, a piezoelectric layer 24, and
an upper electrode 25 deposited in order, isolating the resonance part from the
substrate 20. In the process of fabricating the FBAR, first, a sacrificial layer
(not shown) is deposited and patterned over the substrate 20, so predetermined portions
of the layer remain on the substrate 20. Next, an insulation layer 22 is deposited
on a sacrificial layer and substrate 20, and a lower electrode 23, piezoelectric
layer 24, and upper electrode 25 are deposited in order to form a resonance part.
The insulation layer 22 serves as a membrane layer supporting the resonance part.
Finally, the sacrificial layer is removed to form an air gap 21. That is, a via
hole is formed from the outer surface of the substrate to the inner sacrificial
layer, and the sacrificial layer is removed by injecting etching solution through
the via hole, so that an air gap 21 is formed in place of the sacrificial layer.
Further, U.S. Patent No. 6,355,498 discloses applying an anti-etching material on
the substrate 20 which enables the air gap to be adjusted in size and position when
fabricating an air-gap type FBAR having a structure as shown in FIG. 1B.
However, such FBAR fabrication process is complicated because a sacrificial
layer is needed. Further, the filter design is restrained since the via hole has
to be formed on the membrane layer around the resonator. Further, chemical damage
can occur to the resonance part since etching is performed through a via hole formed
just near the resonance part.
On the other hand, FIG. 1C is a cross-sectional view of the air-gap
type FBAR disclosed in U.S. 6,060,818. In FIG. 1C, a photoresist layer is used to
form a cavity 35 when etching is applied to a predetermined portion of the substrate
30. Next, an insulation layer 31 is deposited over the entire upper surface of the
substrate 30 on which the cavity 35 is formed. Next, after filling a sacrificial
substance in the cavity 35, the lower electrode 32, piezoelectric layer 33, and
upper electrode 34 are deposited in order over the sacrificial layer and insulation
layer 31 to form a resonance part. Next, a via hole is formed through the insulation
layer 31 near the resonance part, and the sacrificial substance is etched away through
the via hole to form the air gap 35. However, use of a sacrificial substance makes
the process complicated, and the resonance part is subject to chemical damage as
well due to the etching. Further, part of the insulation layer can remain below
the resonance part, which can degrade resonance characteristics.
SUMMARY OF THE INVENTION
According to the invention, there is provided an air-gap type thin-film
bulk acoustic resonator, comprising: a substrate having a cavity formed at a predetermined
portion of an upper surface thereof; a resonance part comprising a first electrode,
a piezoelectric substance, and a second electrode deposited in order, the resonance
part being spaced at a predetermined distance from a lower surface of the cavity;
and at least one via hole penetrating a lower surface of the substrate and connecting
to the cavity.
An area of the substrate occupied by the cavity is preferably larger
than that of the resonance part.
Further, the air-gap type thin-film bulk acoustic resonator further
preferably comprises a packaging substrate bonded on the lower surface of the substrate
to close the via hole and prevent foreign material from entering into the cavity.
Further, the air-gap type thin-film bulk acoustic resonator further
preferably comprises an insulation layer deposited over the upper surface of the
substrate exclusive of an area on which the cavity is formed. The insulation layer
separates the first and second electrodes constituting the resonance part from the
Furthermore, in a preferred embodiment, the resonance part is supported
by the first electrode and the piezoelectric layer, each of which are in contact
with the upper surface of the insulation layer.
The invention also provides a method for fabricating an air-gap type
thin film bulk acoustic resonator which comprises the steps of (a) depositing an
insulation layer over an upper surface of a substrate; (b) depositing a first electrode,
a piezoelectric layer, and a second electrode in order over the insulation layer
to form a resonance part; (c) forming at least one via hole penetrating a lower
surface of the substrate; and (d) etching the substrate and insulation layer formed
below the resonance part through the via hole to form a cavity.
Preferably, the method of the invention further comprises a step of
bonding a packaging substrate on the lower surface of the substrate to close the
Further, step (d) preferably comprises etching the substrate so that
an area of the substrate occupied by the cavity is larger than that of the resonance
Further preferably, at least one of the first and second electrodes
constituting an air-gap type thin-film bulk acoustic resonator comprises at least
one of aluminum (Al), tungsten (W), gold (Au), platinum (Pt), nickel (Ni), titanium
(Ti), chromium (Cr), palladium (Pd), and molybdenum (Mo).
The invention thus provides an air-gap type FBAR and fabrication method
therefor in which a via hole is formed through the lower surface of the substrate
to form an air-gap, so that the FBAR can be fabricated using a simplified process
yet provide excellent resonance characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
The above aspects and features of the present invention will be more
apparent by describing certain embodiments of the present invention with reference
to the accompanying drawings, in which:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- FIG. 1A is a cross-sectional view of a conventional Bragg reflector FBAR;
- FIG. 1B and FIG. 1C are cross-sectional views of an air-gap type FBAR fabricated
in a conventional process;
- FIG. 2 is a cross-sectional view for an air-gap type FBAR according to an embodiment
of the present invention; and
- FIG. 3A to 3D are cross-sectional views illustrating a process for fabricating
an air-gap type FBAR of FIG. 2.
Hereinafter, the present invention will be described in detail with
reference to the accompanying drawings. However, the present invention should not
be construed as being limited thereto.
FIG. 2 is a cross-sectional view of an air-gap type thin-film bulk
acoustic resonator (FBAR) according to an embodiment of the present invention. In
FIG. 2, the air-gap type FBAR has a substrate 110 having a cavity 170, an insulation
layer 120, a first electrode 130, a piezoelectric layer 140, and a second electrode
A resonance part 200 is formed over the cavity 170, having the first
electrode 130, piezoelectric layer 140, and second electrode 150 deposited in order.
As discussed above, the resonance part 200 filters a wireless signal utilizing the
piezoelectric effect of the piezoelectric layer 140. That is, a radio frequency
(RF) signal applied through the second electrode 150 can be outputted in the direction
of the first electrode 130 through the resonance part 200. Since the resonance part
200 has a constant resonance frequency depending on the vibration occurring in the
piezoelectric layer 140, only a signal resonant with a resonance frequency of the
resonance part 200 is outputted from the input RF signal.
Meanwhile, the first electrode 130, piezoelectric layer 140, and second
electrode 150 constituting the resonance part 200 contact the insulation layer 120
deposited over the substrate surface around the cavity 170. Accordingly, the resonance
part 200 is supported by the insulation layer 120. As shown in FIG. 2, the first
electrode 130 and piezoelectric layer 140 can be formed so as to be in contact with
the insulation layer 120. Further, the piezoelectric layer 140 present on a certain
part of the first electrode 130 can be removed to form a pad electrically connected
to an external electrode.
As discussed above, the piezoelectric layer 140 exhibits a piezoelectric
effect which converts electric energy into mechanical energy of acoustic form. Aluminum
nitride (AlN), zinc oxide (ZnO), and the like piezoelectric substances can be used
to form the piezoelectric layer 140.
Meanwhile, an insulation layer 120 made of a certain insulating substance
is formed between the first and second electrodes 130 and 150 and the substrate
110 in order to isolate the first and second electrodes 130 and 150 and the substrate
110. Silicon dioxide (SiO2), alumina (Al2O3), and
the like insulating substances can be used to form the insulation layer 120. Further,
a RF magnetron sputtering method or evaporation method can be used to deposit insulation
layer 120 over the substrate 110.
A conductive substance known to those skilled in the art such as a
metal can be used to form the first and second electrodes 130 and 150, including,
for example, aluminum (Al), tungsten (W), gold (Au), platinum (Pt), nickel (Ni),
titanium (Ti), chromium (Cr), magnesium (Mg), palladium (Pd) and molybdenum (Mo).
Meanwhile, a silicon substrate, for example, can be used for the substrate
110. The cavity 170 is formed over a certain part of the substrate 110. In this
embodiment, in order to form the cavity 170, a via hole 160 is formed through the
lower surface of the substrate 110 where the resonance part 200 is not formed. Etching
solution or gas is injected through the via hole 160 to etch away a portion of the
substrate 110 below the resonance part 200, so that the cavity 170 is formed. Since
the via hole 160 is formed at the lower surface of the substrate 110, the etching
gas is absorbed in the piezoelectric layer 140 during the etching process, to solve
the problem of possible chemical damage to the piezoelectric layer 140.
Meanwhile, the etching process is performed for a period of time long
enough to completely etch away the insulation layer 120 present at the lower surface
of the resonance part 200. Consequently, a part of the substrate 110 is also etched
so that it is spaced at a distance d1 or d2 from resonance part 200, which can enhance
the resonance characteristics.
Further, in order to prevent dust or the like from entering the via
hole 160 formed at the lower surface of the substrate 110, a packaging substrate
180 can be bonded to the lower surface of the substrate 110. A silicon substrate
can be used for the packaging substrate 180. It is possible to use a direct bonding
method of heating and bonding the packaging substrate 180, an anodic bonding method
of applying voltages to and bonding the same, a method of using an adhesive such
as epoxy or the like to bond the same, an eutectic bonding method of using metal
to bond the same, or the like, but it is preferable to use the adhesive or eutectic
bonding method allowing for a low-temperature process. This is because the direct
bonding method and the anodic bonding method require a relatively high-temperature
FIG. 3A to FIG. 3D are cross-sectional views illustrating step by
step a process of fabricating the air-gap type FBAR shown in FIG. 2. In FIG. 3A,
the insulation layer 120 is deposited over the entire upper surface of the substrate
110. The substance and method of depositing the insulation layer 120 are described
Next, as shown in FIG. 3B, the first electrode 130, piezoelectric
layer 140, and second electrode 150 are deposited in order over the insulation layer
120. To do so, the first electrode 130 is deposited over the entire upper surface
of the insulation layer 120, and patterned, in order to expose a portion of the
insulation layer 120. The patterning will typically employ a photoresist masking
layer and a development and etching process of forming a predetermined pattern by
etching away only exposed portions not protected by the photoresist.
Next, the piezoelectric layer 140 is deposited over the entire surface
of the exposed insulation layer 120 and first electrode 130, and patterned so that
a certain portion of the first electrode 130 is exposed. Next, a patterning process
is used to deposit the second electrode 150 over a predetermined portion of the
piezoelectric layer 140. FIG. 3B shows second electrode 150 deposited over the entire
upper surface of the piezoelectric layer 140, but it is possible to deposit the
second electrode 150 only over the piezoelectric layer 140 under which the first
electrode 130 is placed. The resonance part 200 is thereby formed when the first
electrode 130, piezoelectric layer 140, and second electrode 150 are deposited in
FIG. 3C is a cross-sectional view showing a process of forming the
air gap 170 by use of the via hole 160. In FIG. 3C, at least one via hole 160 is
formed at the lower surface and in the thickness direction of the substrate 110.
A reactive ion etching (RIE) method can be used to form the via hole. The RIE method
is an etching method by which a volatile substance is generated by etching with
a chemically reactive, activated plasma. Particularly, an inductively coupled plasma
reactive ion etching method (hereinafter, referred to as 'ICP-RIE' method) can be
used in which an inductively coupled plasma (ICP) serves as an activation source.
The ICP-RIE method is a kind of dry etching method and exhibits no etching anisotropy,
providing an advantage of greatly increasing the degree of design freedom in forming
a structure as compared to use of a wet etching method (which typically will result
in etching anisotropy).
Next, via hole 160 thus formed is used to etch the substrate 110 and
insulation layer 120 formed below the resonance part 200, and such etching forms
an air gap 170. A wet etching method or dry etching method can be used. The wet
etching method introduces a chemical solution such as acetic acid solution, hydrofluoric
acid, phosphoric acid solution, or the like into the via hole, and the dry etching
method refers to an etching method of using gas, plasma, ion beams, or the like.
Herein, the etching is performed for a long enough period of time so that substrate
110 is spaced at distances d1 and d2 from the lower surface of the resonance part
FIG. 3D is a view showing a process of bonding the packaging substrate
180. The bonding of the packaging substrate 180 can prevent foreign material from
entering the air gap 170 through the via hole 160.
Thus, the air-gap type FBAR shown in FIG. 2 can be finally fabricated.
The fabricated air-gap type FBAR filters only RF signals of a predetermined frequency
band. Accordingly, a band-pass filter can be implemented to have a certain center
frequency and a frequency bandwidth when plural air-gap type FBARs are properly
combined in series and in parallel. Further, a duplexer can even be implemented
by combining such band-pass filters with a phase shifter constructed with inductors
As stated above, according to the present invention, the air-gap type
FBAR can be fabricated by etching the substrate by means of a via hole formed in
the lower surface of the substrate. Thus, the air-gap type FBAR can be fabricated
to have the desired function using a simple process and without the need for a sacrificial
layer. Further, the present invention can minimize damage to the piezoelectric layer
and other structures when forming an air gap, and enhance resonance characteristics
by completely removing an insulation substance formed under the resonance part.
The foregoing embodiments and advantages are merely exemplary and
are not to be construed as limiting the present invention. The present teachings
can be readily applied to other types of apparatuses. Also, the description of the
embodiments of the present invention is intended to be illustrative, and not to
limit the scope of the claims, and many alternatives, modifications, and variations
will be apparent to those skilled in the art.