Acoustic mirror materials for acoustic devices

Details Acoustic mirror materials for acoustic devices Acoustic mirror materials for acoustic devices

2000-05-23

2003-08-05

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

Reexamination Certificate

active

06603241

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to bulk acoustic wave devices such as acoustic resonators, and more particularly to acoustic mirror materials used in these devices.
DESCRIPTION OF THE RELATED ART
In recent years, much research has been performed in the development of bulk acoustic wave devices, primarily for use in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems. There is a trend in such systems for operation at increasingly higher carrier frequencies, principally because the spectrum at lower frequencies has become relatively congested, and also because the permissible bandwidth is greater at higher frequencies. Piezoelectric crystals have provided the basis for bulk acoustic wave devices for filtering or frequency control such as oscillators, acoustic resonators and filters, operating at very high radio frequencies (on the order of several gigahertz).
In many high-frequency applications, filters such as band pass and/or band stop filters are based on dielectric-filled electromagnetic cavity resonators with physical dimensions that are large, since they are dictated by the wavelength of the resonating electromagnetic wave. Due to the interaction between electrical charge, stress, and strain in a piezoelectric material, such material acts as a transducer, which converts energy back and forth between electromagnetic and acoustic (i.e., mechanical) energy. Thus, a piezoelectric material incorporated in a structure designed to have a strong mechanical resonance provides an electrically resonant device.
The velocity of an acoustic wave, however, is approximately {fraction (1/10000)} that of the velocity of an electromagnetic wave. This relationship between the wave's velocity and device dimensions thus allows a reduction of roughly this factor in the size of certain devices, including acoustic resonators, employing this material. In other words, an electrical filter based on acoustic waves may be much smaller than one based on electromagnetic waves. To achieve this size reduction, one must use a mechanical material that transduces between electromagnetic and mechanical energies. For example, such may be a piezoelectric, magnetostrictive or electrostrictive material. Although the discussion below focuses on piezoelectric materials for transduction, the transduction is not limited to these materials, but may by attained using one of the other above-noted materials, and/or a combination thereof.
Acoustic resonator devices containing piezoelectric materials, such as thin film resonators (hereinafter “TFR”) are typically used in high-frequency environments ranging from several hundred megahertz (MHz) to several gigahertz (GHz).
FIG. 1
illustrates a side view of a conventional TFR component
100
. In
FIG. 1
, TFR component
100
includes a piezoelectric material
110
interposed between two conductive electrode layers
105
and
115
, with electrode layer
115
formed on a support structure
120
.
The support structure
120
can be a plurality of alternating reflecting layers (acoustic mirrors) provided on a solid semiconductor substrate which may be made of silicon, sapphire, glass or quartz, for example. The support structure
120
can alternatively be removed after device fabrication leaving a free standing membrane type device. The piezoelectric material is commonly selected from the group comprising at least ZnO, CdS and AlN. Electrode layers
105
and
115
are generally formed from a conductive material, such of Al, but may be formed from other conductors as well.
These acoustic resonator devices are often used in filters, more particularly in TFR band pass and/or band stop filter circuits applicable to a myriad of communication technologies. For example, TFR filter circuits may be employed in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems.
In these acoustic devices, any sympathetic vibration or mode has a response curve showing how at a certain frequency the amplitude of mechanical response goes through a peaked maximum for fixed excitation strength. Because of the coupling of mechanical and electrical energy in a piezoelectric film, there is also an electrical current response peak where the film produces a maximum current for a fixed voltage since the mechanical motion produces charge at the surface of the piezo. This peak defines the “zero” resonant frequency of a acoustic resonator (i.e., the term refers to the zero or low impedance to the flow of current.)
The piezoelectric film also produces a polarization current like a capacitor, since it is a dielectric. This current increases linearly with frequency for a fixed voltage, and since the mechanical resonance is narrow (about 1 part in 100 changes in frequency) the polarization current can be essentially thought of as constant while the piezoelectric current goes through its peak.
In view of the above, a second electrical feature, the “pole” frequency or frequency of highest impedance can now be understood. The pole frequency is the frequency at which the piezoelectric and polarization currents most nearly or substantially cancel each other out. Since the piezoelectric current has a peak and reverses its polarity through resonance, its value essentially cancels that of the nearly constant polarization current at one frequency—the pole frequency. Therefore, the separation between pole and zero resonant frequencies is dependent in one respect on the properties of the piezoelectric material, and the mechanical resonance that is explained in more detail below.
As discussed above, the separation in frequency between pole and zero resonant frequencies in an acoustic device such as a TFR is determined by properties of the piezoelectric film, and more particularly by the mechanical and electrical properties and/or characteristics of the piezoelectric. For a piezoelectric film comprised of AlN, the fractional separation is about 3% of the resonance frequency for a plate geometry suspended in air, such as in a membrane type TFR (e.g. the lateral dimension is large as compared to the film's thickness).
FIG. 2
illustrates a conventional Bragg reflector stack constituting acoustic mirror layers of an acoustic device such as a TFR. The reflecting stack
125
of
FIG. 2
corresponds somewhat to substrate
120
of
FIG. 1
, and illustrates the make up of the acoustic mirrors in more detail. Referring to
FIG. 2
, the conventional acoustic mirror arrangement or Bragg reflecting stack
125
comprises a plurality of alternating mirror layers of a high acoustic impedance material, such as AlN layers
130
a-d,
and a low acoustic impedance material such as SiO
2
layers
135
a-d,
that are provided on a substrate
140
such as silicon, for example. Other conventional reflecting stacks used in acoustic devices such as TFRs typically include acoustic mirror layer combinations of Si
3
N
4±x
/SiO
2
as well.
Acoustic impedance is the product of a material's density and speed. This relation is important because, as a sound wave passes between two unlike materials, the portion of the wave reflected by the interface therebetween is larger for a greater difference in impedance between the differing materials. Accordingly, a common theme of the inventors is that, in order to fabricate a good acoustic mirror, and hence acoustic resonant device, it is desirable to place materials having as dissimilar impedance as possible against each other for maximal reflections.
However, there is at least one drawback in fabricating acoustic resonator devices such as TFRs with the reflecting stack
125
illustrated in FIG.
2
. The mechanical resonance, and hence the piezoelectric current response peak of the resonator is altered when a device is fabricated on an acoustic mirror, as compared to the case where the device is fabricated as a membrane. Unfortunately, for a resonator solidly mounted on an acoustic mirror, the pole fr

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