Grounding of TFR ladder filters

Details Grounding of TFR ladder filters Grounding of TFR ladder filters

2000-02-04

2001-11-27

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S191000

Reexamination Certificate

active

06323744

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to grounding of thin film resonator (TFR) ladder filters.
DESCRIPTION OF THE RELATED ART
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 the general cross-section 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
which may be formed on a membrane or sets of reflecting layers deposited on a solidly mounted semiconductor substrate
120
which may be made of silicon or quartz, for example. The piezoelectric material is typically a dielectric, preferably one selected from the group comprising at least ZnO, CdS and AlN. Electrode layers
115
and
105
are formed from a conductive material, preferably of Al, but may be formed from other conductors as well.
These TFR components are often used in electrical signal filters, more particularly in TFR filters applicable to a myriad of communication technologies. For example, TFR filters 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 and even hand-held place significant demands on filtering technology, particularly in the context of increasingly crowded radio frequency resources. TFR filters must meet strict performance requirements which include: (a) being extremely robust, (b) being readily mass-produced and (c) being able to sharply increase performance to size ratio achievable in a frequency range extending into the gigahertz region. However, in addition to meeting these requirements, there is a need for low passband insertion loss simultaneously coupled with demand for a relatively large stopband attenuation. Moreover, some of the typical applications noted above for these TFR filters require passband widths up to 4% of the center frequency, which is not easily achieved using common piezoelectrics such as AlN.
A standard approach to designing TFR filters out of resonators is to arrange them in a ladder configuration alternately in a series-shunt relationship (i.e., a “shunt” resonator connected in shunt at a terminal of a “series” resonator). Currently, the conventional way of designing TFR ladder filters is to design simple building blocks of TFR components which are then concatenated together (connected or linked up in a series or chain).
FIG. 2
illustrates a simple building block in circuit form, commonly known as a T-Cell. Referring specifically to
FIG. 2
, a T-Cell
125
includes three TFR components
130
A,
130
B and
135
. TFRs
130
A and
130
B each are “series arm” portions of the T-Cell block. They are connected in series between an input port
132
and node
136
, and node
136
to an output port
134
of T-Cell building block
125
. Further, TFR components
130
A or
130
B may be part of a series arm for an adjacently connected T-Cell, as will be shown later. Resonator element
135
comprises the “shunt leg” portion of T-Cell building block
125
, being connected in shunt between terminal
136
and ground. A plurality of these T-Cells
125
chained together form a TFR ladder filter.
FIGS. 3A and 3B
illustrate ideal and conventional grounding patterns for TFR ladder filters. Ideally, TFR ladder filters would like to see perfect isolated grounds paths from each of is shunt legs to the final external ground of a package or carrier that the die rests on, so that there are no avenues of feedback or coupling between the shunt resonators. The die is an integral base on which the individual serially and shunt-coupled TFR components of the ladder filter are fabricated on (i.e., the semiconductor substrate of
FIG. 1
, for example). The die typically rests upon or is situated within a carrier or package. Such an ideal grounding arrangement is illustrated by the TFR ladder filter circuit
150
shown between input port
149
and output port
151
of FIG.
3
A. As shown in
FIG. 3A
, shunt TFR elements
152
and
153
are directly grounded to the final external ground
155
of a carrier or package. Since all of the ground nodes of the ladder filter are top electrodes and are usually grouped next to each other, it is common practice to tie all the grounds together into one large ground pad, or “bus”, and then wirebond this pad to the final package ground with one or more wirebonds. Such a grounding arrangement is illustrated in FIG.
3
B. In
FIG. 3B
, the die grounds of shunt elements
162
and
163
of TFR ladder filter
160
are “tied” together to form a single metal strip
164
(i.e., a common die ground from the top metal electrodes) which is connected to the final external ground
166
on the carrier by wirebond
165
. Although this provides somewhat adequate grounding, there is significant degradation of ladder filter performance in the stopband near the passband edges, due to the aforementioned coupling or feedback between the shunt resonators caused by this common die bus.
These stopband performance “glitches” near the passband of a TFR ladder filter can be somewhat minimized by adding multiple wirebonds.
FIG. 4A
illustrates a simplified view of a TFR ladder filter circuit using multiple wirebonding, and
FIG. 4B
depicts a three-dimensional physical representation of the multiple wirebond arrangement of FIG.
4
A. In
FIG. 4A
, the TFR ladder filter
200
consists of two T-cells
205
and
215
concatenated together, T-cell
205
having serially-coupled TFR elements
207
and
209
and shunt TFR element
210
, T-cell
215
having serially coupled TFR elements
217
and
219
and shunt TFR element
220
. Similar to
FIG. 3B
, the die grounds of the shunt TFR elements
210
and
220
are tied together to form a single metal strip
230
; however, instead of a single wirebond, two wirebonds
225
and
235
connect the common die ground to the final external ground of the carrier or package (not shown).
FIG. 4B
is a three-dimensional physical representation of the TFR ladder filter
200
of FIG.
4
A. Specifically, in die
250
there is shown the arrangement of top and bottom metal electrodes corresponding to the TFR elements in T-cells
205
and
215
of
FIG. 4A
, as well as the wirebond connections to the final external ground. Specifically, top electrodes
255
and
265
correspond to series TFR elements
207
and
219
, top electrode
260
represents a common top metal electrode for series TFR elements
209
and
217
, and top metal electrode
270
is a common die ground electrode for shunt TFR elements
210
and
220
(analogous to the singular metal strip
230
connecting the shunt TFR elements of the ladder filter
200
). Connectors
281
and
282
connect the TFR ladder filter to other components adjacent thereto within a particular system (not shown). Bottom electrodes
280
and
290
are common to respective TFR elements in T-cells
205
and
215
respectively. As can be seen from
FIG. 4B
, two wirebonds
295
A and
295
B (corresponding to wirebonds
225
and
235
in
FIG. 4A
) are for connecting the common die ground electrode of the adjacent TFR shunt elements to the final external ground on a carrier or package that the die rests on (not shown).
The use of multiple wirebonds somewhat improves the stopband glitches near the passband edges of the TFR ladder filter, as compared to using the single wirebond shown in FIG.
3
B. This is because by increasing the number of wirebonds to a final external ground, the overall inductance and resistance is lowered, which in turn helps to isolate the common die bus from the final package ground. This somewhat limits the deteriorating effects due to the feedback/coupling phenomena. However, the improvement is still unacceptable when compared to the response achievable by employing an ideal grounding arrangement as illustrated in FIG

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