Method of producing thin-film bulk acoustic wave devices

Details Method of producing thin-film bulk acoustic wave devices Method of producing thin-film bulk acoustic wave devices

2001-04-12

2003-04-15

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S365000

Reexamination Certificate

active

06548943

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to bulk acoustic wave resonators and filters and, more particularly, to the fabrication of resonators operated in the piston mode.
BACKGROUND OF THE INVENTION
It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical/acoustic wave (produced by the RF signal) is about twice the thickness of the piezoelectric layer. Although the resonant frequency of a BAW device also depends on other factors, the thickness of the piezoelectric layer is the predominant factor in determining the resonant frequency. As the thickness of the piezoelectric layer is reduced, the resonance frequency is increased. BAW devices have traditionally been fabricated on sheets of quartz crystals. In general, it is difficult to achieve a device of high resonance frequency using this fabrication method. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5-10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. There are primarily two types of FBARs, namely, BAW resonators and stacked crystal filters (SCFs). An SCF usually has two or more piezoelectric layers and three or more electrodes, with some electrodes being grounded. The difference between these two types of devices lies mainly in their structures. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel FBAR makes up one section of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based device may have one or more protective layers commonly referred to as the passivation layers. A typical FBAR-based device is shown in
FIGS. 1
a
and
1
b.
As shown in
FIGS. 1
a
and
1
b,
the FBAR device comprises a substrate
110
, a bottom electrode
120
, a piezoelectric layer
130
, and a top electrode
140
. The FBAR device may additionally include a membrane layer
112
and a sacrificial layer
114
, among others. The substrate can be made from silicon (Si), silicon dioxide (SiO2), Galium Arsenide (GaAs), glass or ceramic materials. The bottom electrode and top electrode can be made from gold (Au), molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium (Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), or aluminum (Al). The piezoelectric layer
130
can be made from zinc oxide (ZnO), zinc sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO
3
) or other members of the so-called lead lanthanum zirconate titanate family. The passivation layer is typically made from a dielectric material, such as SiO2, Si3N4, or polyimide, to serve as an electrical insulator and to protect the piezoelectric layer. It should be noted that the sacrificial layer
114
in a bridge-type BAW device is, in general, etched away in the final fabrication stages to create an air interface beneath the device. In a mirror-type BAW device, there is an acoustic mirror structure beneath the bottom electrode
120
. The mirror structure consists of several layer pairs of high and low acoustic impedance materials, usually quarter-wave thick. The bridge-type and the mirror-type BAW devices are known in the art.
The desired electrical response in an FBAR-based device is achieved by a shear or longitudinal acoustic wave propagating in the vertical thickness through the device. Besides these wave modes, there exist other modes, known as the Lamb waves, that may deteriorate the electrical response. In quartz crystals, the strength of these spurious modes is controlled by adjusting the thickness and the width of the top electrode. In an FBAR-based device, the dimension in thickness direction is so small that it renders thickness adjustment difficult and impractical. A possible solution to the problems associated with the spurious modes is to thicken the edge of the top electrode. As disclosed in Kaitila et al. (WO 01/06647 A1, hereafter referred to as Kaitila), a frame-like structure
150
is formed on top of the top electrode
140
to thicken the edge thereof. As shown in
FIGS. 1
a
and
1
b,
the frame-like structure
150
is a rectangular frame for defining a first zone and a second zone for acoustic wave excitation. The first zone is the area under the rectangular frame
150
, and the second zone
148
is the area surrounded by the rectangular frame
150
. With such a structure, the cut-off frequency of the piezoelectrically excited wave modes in the first zone and that of the second zone are different. When the width of the frame-like structure and the acoustic properties of the layer structure are properly arranged, the displacement relating to the strongest of the piezoelectrically excited resonance modes is substantially uniform in the second zone. Thus, the spurious resonances in the electric response of the bulk acoustic wave device are suppressed, and the FBAR is said to operate in a piston mode.
It should be noted that, as disclosed in Kaitila, the frame-like structure may be circular, square, polygonal, regular or irregular. Also, the frame-like structure can have different configurations, as shown in
FIGS. 2 and 3
, to achieve the piston mode. As shown in
FIGS. 2 and 3
, part of the piezoelectric layer
130
is covered by a passivation layer
160
, and part of the passivation layer is sandwiched between the piezoelectric layer
130
and the frame-like structure
150
extended upward from the edge of the top electrode
140
. In
FIGS. 2 and 3
, the frame-like structure
150
is basically where the top electrode
140
overlaps with the passivation layer
130
. It should be noted that,
FIG. 1
a
is a cross section view of a BAW device, as viewed in the lateral direction and the top, while FIG.
2
and
FIG. 3
are cross section views of a BAW device, as viewed in the horizontal direction.
Traditionally, the frame-like structure is fabricated by forming an electrically conducting layer on top of the passivation layer and an exposed part of the piezoelectric layer, and removing part of the electrically conducting layer, as shown in
FIGS. 4
a
-
4
e.
In
FIGS. 4
a
-
4
e,
only the top few layers are shown. As shown, the device has a patterned passivation layer
160
, which covers most of the piezoelectric layer
130
but leaves a section
132
of the top surface exposed. A top metal layer
128
is formed on top of the passivation layer
160
and the exposed portion
132
of the piezoelectric layer
130
. The portion of the top metal layer
128
that is in direct contact to the piezoelectric layer
130
is denoted by reference numeral
148
, as shown in
FIG. 4
b.
As shown in
FIG. 4
c,
an etching mask
200
, such as a photoresist mask, is provided on top of the device. As shown in
FIG. 4
c,
the mask
200
is skewed to the left in reference to the center portion
148
. The exposed parts of the top metal layer
128
can be removed with an etching process to form an upper electrode
140
, as shown in
FIG. 4
c.
FIG. 4
d
shows the device after the etching mask
200
has been stripped. As shown, the upper electrode
140
has a frame-like structure similar to the structure
150
as shown in FIG.
3
. However, the left section
150
′ of the top electrode
140
is much broader than the right section
150
″, and this is not the intended result. For example, the intended result is that the left section
150
′ and the right section
150
″ are substantially the same. As such, the operation of the device in piston mode may be compromised. The unintended result is due to the misalignment of the mask
200
.
FIG. 4
c
is used to illustrate the disadva

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