Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-06-19
2002-09-10
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000, C333S191000
Reexamination Certificate
active
06448695
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a bulk acoustic wave (BAW) device and its manufacture and particularly a filter comprising solidly mounted BAW resonators. The invention also relates to communications equipment (for example a radio frequency receiver and/or transmitter) comprising such filters.
BACKGROUND OF THE INVENTION
Mobile communications products, such as cellular phone handsets, are required to be small and light. It is predicted that in the future even smaller communication devices will be available integrated into, for example, wrist-watches and clothing. All such products require radio-frequency (RF) filters approximately covering the range 0.5 GHz to 10 GHz to protect the received signal from interference, either from the transmitter in the same handset and/or from unwanted externally generated signals. These filters must have low pass-band insertion loss (typically<2 Db) in order to achieve adequate signal-to-noise ratio. To achieve this, the resonators, which are the basic building blocks of filters, must have high quality factor Q. This is defined as the energy stored per cycle in a resonator at the resonant frequency divided by the energy lost per cycle by the resonator at the same frequency. Typically, values for Q in excess of 500 are desirable and achievable.
Resonators normally employ some form of standing wave in a cavity. Both discrete and distributed reflections may be used. Conventional bulk acoustic wave and surface acoustic wave (SAW) resonators are examples of resonators relying on these two options. To keep size to a minimum, discrete reflections are preferred because the length of the cavity is then typically only ½ a wavelength of the mode employed at the resonant frequency. Thus BAW resonators are potentially much smaller than SAW resonators for which a cavity length of the order of 100 wavelengths may be required, and are preferred for this reason.
Resonators are available that rely on acoustic waves or electromagnetic waves. Acoustic wave resonators are preferred to those employing electromagnetic waves for two principal reasons. Firstly, the velocity of acoustic waves propagating in a material is typically 4 to 5 orders of magnitude lower than the velocity of electromagnetic waves, so that a substantial size reduction is possible for any given frequency. Secondly, achievable mechanical quality factors are typically larger than achievable electrical quality factors for the same materials.
Two general types of BAW resonators have been studied for RF applications. In the first of these a thin membrane forms the resonating cavity. This approach is unattractive because the membranes are fragile and subject to buckling caused by stress. In the second, so-called SMRs (solidly-mounted resonators) are used as shown in FIG.
1
. In devices such as these, one or more acoustically mismatched layers
2
are mounted on a substrate
4
and act to reflect the acoustic wave. Upper
6
and lower
8
electrodes are formed on the substrate
4
separated by a piezoelectric layer
10
. Since the reflector layer(s) are deposited on a solid substrate, the structure of a SMR is robust.
In the BAW resonator shown in
FIG. 1
, the required conversion between electrical and mechanical energy is achieved by the layer
10
of piezoelectric material arranged between two metal layers in which electrodes
6
1
,
6
2
and
8
are formed. Although the SMR employs a more distributed reflection than the thin membrane resonator, resonator size is not significantly increased because thickness is predominantly determined by the substrate in both cases. Each upper electrode
6
1
and
6
2
defines an individual resonator with the underlying piezoelectric layer and lower electrode. These two resonators are effectively electrically connected in series, with the common lower electrode
8
at the junction between them. A resonator is a one-port device. In the construction shown in
FIG. 1
, its two terminals are formed by electrodes
6
1
and
6
2
.
RF filters reported to date have been constructed by electrically connecting SMRs in either a ladder or lattice configuration. Ladder configurations of the filters have demonstrated good performance with passband insertion loss at less than 2 dB and very low levels of spurious response. However, there are a number of disadvantages with such arrangements. For example, at frequencies removed from the acoustic resonances, each resonator appears as a capacitor, so the overall filter stop-band response is essentially that of a capacitor network. This leads to a requirement for additional resonators just to reduce the stop-band. Consequently, both the area occupied and the insertion loss in the pass-band are increased without improving selectivity. A large number of resonators is required for even a moderate stop-band level (e.g. a minimum of 9 resonators for approximately 45 dB stop-band). With the drive towards the miniaturization of filters in RF applications this is a serious problem.
In addition, series and shunt resonators in the ladder configuration are required to be centred on different frequencies due to the arrangement of the individual resonators. This means, for example, that an additional mass-loading layer, of very precise thickness, must be deposited on the shunt resonators to reduce their anti-resonance (minimum-admittance) frequency to the same as the resonance (minimum-impedance) frequency of the series resonators.
FIG. 2
shows a schematic representation of an electrical equivalent circuit model for a conventional BAW resonator such as that shown in
FIG. 1. C
0
is the static capacitance of the resonator, C
m
and L
m
are respectively the motional capacitance and inductance, and R
m
is the motional resistance which characterises the mechanical losses of the resonator. The resonant frequency is given by f
0
=1/[2&pgr;(C
m
L
m
)], and the unloaded quality factor is given by Q
u
=(2&pgr;f
0
L
m
)/R
m
.
The manufacturing process for a thin film bulk acoustic wave resonator will be known by those skilled in the art. For example, International Patent Application number WO98/16957 discloses a thin film bulk acoustic wave resonator and a method of manufacturing the same, the contents of which are incorporated herein by reference.
As explained above, in a filter it is usual to arrange more than one resonator in a lattice or ladder configuration connected electrically to each other to obtain optimum filter characteristics. Connecting a number of filters like the one shown in
FIG. 1
causes an inherent lack of design flexibility due to the presence of the static capacitance C
0
in each resonator. As a consequence, approximations to standard ideal filter types such as Butterworth or Chebyshev are not readily implemented. The electrical connection of the resonators in a ladder configuration also produces the need for the series and shunt resonators to be centred on different frequencies.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a bulk acoustic wave device comprising:
one or more acoustic reflector layers formed on a substrate;
a lower electrode formed on said acoustic reflector layer or layers;
a piezoelectric layer formed on said lower electrode; and,
at least three upper electrodes formed over said piezoelectric layer each upper electrode at least partially overlying the lower electrode and defining with an underlying piezoelectric layer portion and the lower electrode a resonator element, in which said upper electrodes are laterally spaced such that a signal applied between one of said upper electrodes and said lower electrode at a resonant frequency of the device is coupled to the other resonator elements by acoustic coupling between the piezoelectric layer portions and in which the upper electrodes are arranged so that there are two outer and at least one inner upper electrodes and in which the or each inner upper electrode is electrically connected to the lower electrode.
The invention provides a device employing SMRs, which are acoustica
Biren Steven R.
Budd Mark O.
Koninklijke Philips Electronics , N.V.
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