Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-02-04
2002-08-20
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S191000, C310S312000
Reexamination Certificate
active
06437667
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to bulk acoustic resonator devices, more particularly to tuning thin film resonator filters.
DESCRIPTION OF THE RELATED ART
Bulk acoustic wave devices such as thin film resonators (hereinafter “TFR”) are typically used in high-frequency frequency control and filtering applications ranging from several hundred megahertz (MHz) to several gigahertz (GHz). A TFR typically is comprised of a piezoelectric material interposed between two conductive electrodes, one of which can be formed on a support structure. The support structure can be a membrane formed by removal of material beneath it, or a plurality of alternating acoustic reflecting layers formed on a semiconductor substrate such as silicon or quartz, for example. The piezoelectric material is typically AIN, but may also be formed of ZnO or CdS amongst other piezoelectric material. The electrodes are formed from a conductive material, preferably of Al, but may be formed from other conductors as well. These films are deposited and lithographically patterned into their useful form in much the same way modern integrated circuits are made.
TFRs are often used in electronic signal filters, more particularly in TFR 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.
The desire to render these increasingly complicated communication systems portable, even hand-held, places significant demands on filtering technology, particularly in the context of the increasingly crowded radio frequency spectrum. TFR filters must meet strict physical requirements which include: (a) being extremely robust, (b) being readily mass-produced and (c) being small while maintaining the required strict rejection and transmission characteristics. Restated, there is a simultaneous need for low passband insertion loss and for a large stopband attenuation in order to effectively clean up, for example, signals at the front-end of an RF radio. Some cellular phone applications for these TFR filters require passband widths up to 4% of the center frequency (for example, for a 2 GHz center frequency, this would be a bandwidth of about 80 MHz). This is not easily accomplished using common piezoelectrics such as AIN, and careful design and manufacture steps must be taken to keep filter bandwidths as wide as possible.
The piezoelectric material in TFR resonators converts electrical to mechanical energy and vice versa such that at its mechanical resonance frequency, the electrical behavior of the device abruptly changes. Electrical signals of particular frequencies easily pass thorough the resonators, while others will not be transmitted. These particular frequencies can be dictated by choosing resonator size and design. Resonators of certain sizes and design frequencies can be networked in appropriate combinations, such that they will impose desired filtering functions on signals passing through the network. A standard approach to designing filters out of resonators is to arrange them in a ladder configuration alternately in a series-shunt relationship. A series element in this sense carries signal from an input toward an output, whereas a shunt element provides an alternative path for the signal to ground. The transmission or blocking characteristics of both series and shunt elements affect the final signal reaching output from input, somewhat analogous to how branching of water pipes can affect the flow through the main water line.
Currently, the conventional way of designing TFR ladder filters is to design simple building blocks of TFR components having moderate selectivity, and then to concatenate these building blocks together (connected or linked up in a series or chain) to obtain a stronger filtering characteristic. In a simplified view, concatenation helps to achieve a larger stopband attenuation for the filter because each individual linked up section in the chain successively filters the signal more as it passes through the chain.
To make wide bandwidth filters from piezoelectric resonators, it is known that resonators of at least two differing frequencies are required. The difference in the frequencies will be similar to the required filter bandwidth. Numerous strategies are employed depending whether bandpass, bandstop, or any number of other filter shapes is required. Designs can be complicated and require more than a simple pair of frequencies. We shall illustrate an advantageous way to produce, in a batch fabricated manner similar to making integrated circuits, resonators on a single substrate of differing frequencies for use in any number of filtering applications. We shall describe the technique in the light of making a bandpass filter, but it will be realized the technique is applicable to making any number of filters requiring a multiplicity of differing frequency resonators.
FIG. 1
illustrates schematically illustrates this simple building block, commonly known as a T-Cell. Referring specifically to
FIG. 1
, a schematic of a T-Cell building block
100
includes three TFR components
110
,
120
and
130
. TFR components
110
and
120
comprise the “series arm” portion of the T-Cell block, being connected in series between an input port
115
and an output port
125
of T-Cell
100
. TFR component
130
comprises the “shunt leg” portion of T-Cell
100
, being connected in shunt between node
135
and ground. A TFR T-Cell itself may define a filter; although a TFR ladder filter typically has a plurality of these T-cells concatenated together.
FIGS. 2A-2C
graphically illustrate how a bandpass filter response for bulk acoustic wave devices such as resonator filters are conventionally achieved. Each of the shunt and series TFR components
110
,
120
and
130
in the schematic T-Cell of
FIG. 1
has a set of characteristic frequencies: a “pole” frequency and a “zero” frequency. The terms refer to the magnitude of the impedance to current flow through the device; impedance is low at the zero and high at the pole. The series and shunt arms in a filter typically have zero and pole frequencies slightly shifted from each other. As will be explained further below, the current method of achieving an acceptable bandpass filter response has been to shift the frequencies of the shunt TFR component down in frequency.
Providing resonator components having desirable impedance characteristics is a necessary requirement for building a TFR-based filter.
FIG. 2A
illustrates a typical transmission response for a series TFR component of a TFR filter. Referring to
FIG. 2A
, a single, series-wired TFR component will have the voltage transmission response S
21
(as shown in
FIG. 2A
, signal magnitude (y-axis in dB) as a function of frequency (z-axis GHz) shown at its output.
FIG. 2A
illustrates the following characteristics: the signal maximum (nearest the vertical zero, greatest transmission) occurs at about 1.90 GHz. This point is known as the resonator zero because of the nearly zero impedance to current flow. The point of least transmission is at about 1.94 GHz; this is the resonator's pole, where it has the highest impedance to the flow of electrical current.
FIG. 2A
illustrates the behavior of a device whose transmission of an electrical signal varies as a function of the frequency which is the basic definition of a filter. However, this single TFR component by itself does not have the characteristics desired in typical filters, like high rejection away from the pass band, or a flat pass band in which transmission is uniform.
FIG. 2B
illustrates a typical response for a shunt TFR component of a TFR filter. The difference between
FIGS. 2A and 2B
is that in
FIG. 2A
, the signal moving from input to output must flow through the TFR, whereas in
FIG. 2B
, any signal flowing through the shunt TFR will not reach the output since it shunts to ground. Referring to
FIG. 2B
, a circuit executed
Barber Bradley Paul
Fetter Linus Albert
Rittenhouse George E.
Zierdt Michael George
Agere Systems Guardian Corp.
Summons Barbara
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