Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-05-08
2002-05-07
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S187000
Reexamination Certificate
active
06384697
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to acoustic resonators and more particularly to approaches for supporting an acoustic resonator on a substrate.
BACKGROUND ART
Acoustic resonators that are formed of thin films may be used in a number of applications that require a precisely controlled frequency. A Thin Film Bulk Acoustic Resonator (FBAR) or a Stacked Thin Film Bulk Acoustic Resonator (SBAR) may be used as a filter in a cellular telephone or other device in which size, cost and frequency stability are important factors.
An FBAR includes a thin film of piezoelectric material between two conductive electrodes, while an SBAR includes additional layers of piezoelectric material, with each such layer separating two electrodes. The active layers of an FBAR or SBAR are suspended in air by supporting the layers around the perimeter. The air/resonator interfaces at both sides of the stack of layers trap the energy that is generated during operation.
When a time-varying electrical field is created by applying a signal across two electrodes that are separated by a piezoelectric layer, the piezoelectric material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electrical field and are reflected at the air/resonator interfaces. For a properly fabricated FBAR or SBAR, the sound waves will have a particular mechanical resonance.
An FBAR or SBAR can be used as a filter, since it will function as an electronic resonator when allowed to operate at its mechanical resonant frequency. At this mechanical resonant frequency, the half wavelength of the sound waves propagating through the resonator is approximately equal to the total thickness of the resonator for a given phase velocity of sound in the piezoelectric material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be compact. A resonator for applications in which a frequency in the gigahertz range is desired may be constructed with physical dimensions less than 100 microns in diameter and a few microns in thickness.
An FBAR is conventionally fabricated on the surface of the substrate by depositing the bottom electrode, the piezoelectric film, and then the top electrode. Therefore, a top air/resonator interface exists and only the bottom interface requires some design selections. There are several known approaches for obtaining the desired characteristics at the bottom interface.
The first approach involves etching away the substrate material from the back side of the substrate. If the substrate is silicon, the silicon is etched away from the back side using a KOH etch, which is a strong base etchant. This approach is described in detail in U.S. Pat. No. 5,587,620 to Ruby et al. With reference to
FIG. 1
, a layer
10
of Si
3
N
4
may be deposited on a top surface of a silicon substrate
12
. The back side of the substrate
12
is then etched using the KOH. Preferably, approximately 80% of the silicon substrate is removed during the etching step, leaving a remainder
14
that provides structural stability. The metallization of the bottom electrode
16
is then formed on the Si
3
N
4
layer
10
. Aluminum nitride (AlN) may then be deposited as the piezoelectric layer
18
. A second electrode
20
is subsequently formed on the surface of the piezoelectric layer
18
. As shown in
FIG. 2
, if an SBAR is to be fabricated, rather than an FBAR, a second piezoelectric layer
22
and a third electrode
24
are also formed.
After completing the fabrication of the third electrode
24
, the remainder
14
of the silicon within the previously etched cavity is removed by a slow etching process that is more easily controlled than the KOH etch. For example, a tetra-methyl-ammonium hydroxide (TMAH) etching solution may be used, since it is less likely to attack the AlN of the piezoelectric layers
18
and
22
. The result is that an SBAR
26
of
FIG. 2
is formed.
One concern with this first approach is that it results in a relatively low fabrication yield. The cavities that are formed through the wafer
12
render the wafer very delicate and highly susceptible to breakage. Furthermore, the wet etching using KOH forms side walls having 54.7° slopes. This limits the ultimate density of fabricating the acoustic resonators on a given sized wafer. For example, devices with lateral dimensions of approximately 150 &mgr;m×150 &mgr;m that are built on a standard 530 &mgr;m thick silicon wafer require a back side etch hole that is roughly 450 &mgr;m×450 &mgr;m. Consequently, only approximately 11% of the wafer can be productively utilized.
A second approach to forming the air/resonator interfaces is to create an air bridge type FBAR/SBAR device. Typically, a sacrificial layer is deposited and the acoustic resonator layer stack is then fabricated on top of the sacrificial layer. At or near the end of the process, the sacrificial layer is removed. Since all of the processing is completed on the front side, this approach does not suffer from having two-sided alignment and large area back side cavities. However, this approach has other inherent difficulties. First, the method is difficult to practice on large devices. Typically, the sacrificial layer is thermally grown silicon dioxide that is subsequently removed using hydrofluoric (HF) gas, which has an etch rate in the order of 1000 to 3000 Å/minute. To etch beneath device areas that are in the order of 150 &mgr;m×150 &mgr;m or larger, an etch time greater than 500 minutes is required. In addition to being excessively long, the exposure of the metal electrodes to the etchant for periods in excess of 30 minutes leads to the delamination of the metal electrodes from the piezoelectric material.
A third approach is sometimes referred to as the “solidly mounted resonator” (SMR), since there are no air gaps at the bottom of the device. A large acoustic impedance at the bottom of the device is created by using an acoustic Bragg reflector. The Bragg reflector is made of layers of alternating high and low acoustic impedance materials. Each thickness is fixed to be at the quarter wavelength of the resonant frequency. With sufficient layers, the effective impedance of the piezoelectric/electrode interface is much higher than the device acoustic impedance, thereby trapping the sound waves effectively within the piezoelectric layer. While this approach avoids some of the problems discussed with regard to creating a free-standing membrane, it includes difficulties. The choice of materials used in the Bragg reflector is limited, since metal layers would form parasitic capacitors that would degrade the electrical performance of the device. Moreover, the degree of difference in the acoustic impedance of layers made by the available dielectric materials is not large, so that more layers are needed. This complicates the fabrication process as the stress on each layer must be well controlled. After many layers, the device is not conducive to integration with other active elements, since making vias through a large number of holes is difficult. Furthermore, devices of this type are reported to have significantly lower effective coupling coefficients than devices having air bridges.
Acoustic resonators may be used alone or in combination. For example, a bandpass filter is formed by electrically connecting several resonators to provide a desired filter response. Several filter topologies are possible. One favored topology is the half-ladder topology, where a group of resonators are connected in series (series resonators) and in between the series resonators are shunt resonators that are connected to ground. The series resonators are fabricated such that their resonant frequency is approximately 2% to 3% higher than the shunt resonators. Since the thickness of the piezoelectric layer can be the same for the series and shunt resonators, the piezoelectric deposition is often “shared” between resonators. In fact, it is tempting for a designe
Agilent Technologie,s Inc.
Pascal Robert
Takaoka Dean
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