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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical filters employing a mechanical
transducer and more particularly to a method for fine tuning such filters
following batch fabrication of the filters.
2. Description of Related Art
The need to reduce the cost and size of electronic equipment has led to a
continuing need for ever smaller filter elements. Consumer electronics
such as cellular telephones and miniature radios place severe limitations
on both the size and cost of the components contained therein. Many such
devices utilize filters that must be tuned to precise frequencies. Hence,
there has been a continuing effort to provide inexpensive, compact filter
units.
One class of filter element that meets these needs is constructed from
mechanical resonators such as acoustic resonators. These devices use
acoustic waves, bulk longitudinal waves for example, in thin film
material, typically but not exclusively piezoelectric (PZ) material. In
one simple configuration, a layer of PZ material is sandwiched between two
metal electrodes. The resonator may be suspended in air, supported along
its rim, or may be placed on an acoustic mirror comprised of a plurality
of alternating layers of high and low acoustic impedance (the product of
speed and density), usually silicon dioxide and aluminum nitride. When an
electric field is applied between the two electrodes via an impressed
voltage, the PZ material converts some of the electrical energy into
mechanical energy in the form of sound waves. For certain crystal
orientations, such as having the c axis parallel to the thickness of an
Aluminum Nitride film, the sound waves propagate in the same direction as
the electric field and reflect off of the electrode/air or
electrode/mirror interface.
At a certain frequency which is a function of the resonator thickness the
forward and returning waves add constructively to produce a mechanical
resonance and because of the coupling between mechanical strain and charge
produced at the surface of a piezoelectric material, the device behaves as
an electronic resonator; hence, such devices combined in known
architectures can act as a filter. The fundamental mechanical resonant
frequency is that for which the half wavelength of the sound waves
propagating in the device is equal to the total thickness of the
piezoelectric plus electrode layers. Since the velocity of sound is many
orders of magnitude smaller than the velocity of light, the resulting
resonator can be more compact than dielectric cavity resonators.
Resonators for 50 Ohm matched applications in the GHz range may be
constructed with physical dimensions approximately 100 micrometers in
diameter and few micrometers in thickness.
Combinations of such resonators may be used to produce complex filters for
band pass applications as disclosed inter alia in U.S. Pat. No. 5,910,756
issued to Ella. This patent describes the use of multiple acoustic
resonators in constructing ladder and T type band pass filters.
The resonant frequency of the resonator is a function of the acoustic path
of the resonator. The acoustic path is determined by the distances between
the outer surfaces of the electrodes. When batch producing resonators on a
substrate, the thickness of the transducing material and the electrodes is
fixed at fabrication; hence, the resultant resonance frequency is also
fixed. Since there are variations in thickness from device to device
resulting from manufacturing tolerances, some method for fine tuning the
resonance frequency of each device is needed.
To compensate for this inability to reliably and inexpensively mass produce
resonators and therefore filters with the proper resonance
characteristics, it is known to intentionally produce resonators having a
lesser thickness than the thickness indicated to achieve a desirable
resonant frequency, and then deposit excess material on at least one of
the electrodes to change the overall thickness of the device and thereby
fine tune the device. As this deposition of material may be done while the
device is subjected to an input signal and simultaneously tested for
resonance this method has produced acceptable results.
This method is not, however without problems as the presence of a mask
needed to control the deposition over the desired electrodes creates
problems of its own. If the mask, for instance is in contact with the
electrode, the mask mass is added to the device mass and alters the
resonance characteristics of the device. On the other hand if the mask is
not in contact with the device the control of the deposition area suffers.
Such masking techniques have been successful with quartz type resonators
that are much larger, but have not been as successful with resonators of
the order of less than one millimeter.
It has also been proposed to remove material from the device in order to
adjust its resonant frequency.
Whether deposition or removal of electrode material is used in fine tuning
a resonator, in producing a filter that uses more than one resonator as is
the typical case, more than one resonator frequency must be adjusted and
that involves a multiplicity of steps wherein each resonator is masked and
fine tuned in separate process steps.
There is thus still a need for a process to fine tune more than one
resonator to different desired frequencies without need to move and
re-mask the resonators.
SUMMARY OF THE INVENTION
The above object is obtained in accordance with this invention by a method
for adjusting different resonant frequencies of a plurality of mechanical
resonators formed on a common substrate, wherein the resonant frequencies
of said resonators are a function of each resonator thickness, the method
comprising forming said resonators with an etchable layer comprising a
material having different etching properties for each of said resonators
having different resonant frequencies and selectively etching said
etchable layers to adjust the resonant frequencies of said resonators.
The terms different etching properties and selective etching as used herein
mean that the materials used may be etched using an etching process for
one that does not effect the other, or that effects the other at a
different rate so that one material can be etched for the purpose of this
invention while both are exposed to the same etching process without
effecting the other, or effecting the other to a degree that does not
interfere with the purpose of this invention. Thus selective etching is
the process of subjecting two or more materials to an etching process that
effects only one of the materials, or that effects one of the materials
differently, i.e. at a different rate, than the others.
In somewhat more detail, the method comprises adjusting the resonant
frequencies of at least two mechanical resonators comprising a filter, to
a first and a second desired frequency. Each of the resonators comprises a
mechanical energy transducer between a top and a bottom electrode, and its
resonant frequency is a function of the overall resonator thickness. The
frequency adjustment comprises the following process:
Forming the top electrode over the transducer material for each of the
resonators, having a thickness selected such that the combined bottom
electrode thickness, transducer thickness and top electrode thickness is
in excess of the required total thickness for each resonator to resonate
at a desired frequency.
The top electrode of the first resonator comprises a first material and the
top electrode for the second resonator comprises a second material. These
materials are selected to have different etching properties, that is each
is etched by a different etchant that does not effect the other.
The process next comprises selectively etching the top electrodes of the
first and second resonators while applying a signal and monitoring each of
the resonator frequencies, and stopping the etching process for each of
resonators when the monitored resonant frequency is the desired resonant
frequency for each resonator. Thus there is no need to either move or mask
the resonators during the frequency adjusting process where more than one
resonator on a common support must be adjusted, each to a different
frequency.
Alternatively, the method further includes forming the top electrode of at
least one resonator by first forming a first conductive layer over the
film of transducer material and subsequently adding a topmost adjustment
layer over the first conductive layer. The topmost adjustment layer is
selected to have different etching properties than the top electrode of
the other resonators. For instance, the adjustment topmost layer of one
top electrode can be SiO.sub.2 placed over an aluminum conductive layer,
and the top electrode of another resonator simply an Aluminum conductive
layer. In the alternative all electrodes may be metals such as gold for
one, aluminum for another, titanium for a third and so on.
In a typical single T-cell filter structure there are three resonators two
of which have substantially the same frequency, the third being different.
In this case the same material, i.e. aluminum, may be used for the two
same resonant frequency resonators and a second material, such as gold for
the third. In the alternative, aluminum may be used as a conductive
electrode layer in all three resonators and a silicon dioxide
(SiO.sub.2)layer may be added to the aluminum electrode of one of the
resonators to alter its thickness. Fine tuning may then be accomplished
according to the present invention for two different frequencies, by
reactive ion etching of the aluminum electrodes with chlorine ions and by
reactive ion etching of the SiO.sub.2 topmost layer of the third resonator
top electrode with fluorine ions.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood from the following description
thereof in connection with the accompanying drawings described as follows.
FIG. 1 shows a typical cross section of a resonator comprising an acoustic
mirror and a piezoelectric resonator.
FIG. 2 shows a top view of a three resonator structure forming a single
T-cell band pass filter.
FIG. 3 shows a typical arrangement for selective etching using a reactive
ion etching process.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Throughout the following detailed description, similar reference characters
refer to similar elements in all figures of the drawings. Depending on the
thin film materials used, additional layers of insulation, protective
films, encapsulation, etc. may be required and all such layers and films
have been omitted herein for simplification and better understanding of
the invention. The specific structure and fabrication method illustrated
is for exemplary purposes only and other methods of fabricating a
resonator and or filter in accordance with the present invention can be
devised including but not limited to substrate etching, adjustment layers,
reflecting impedance matching layers, etc. U.S. Pat. No. 5,373,268, issued
Dec. 13, 1994, with the title "Thin Film Resonator Having Stacked Acoustic
Reflecting Impedance Matching Layers and Method", discloses a method of
fabricating thin film resonators on a substrate.
Referring now to FIG. 1, there is shown a typical structure of two
mechanical resonators 20 and 20' on a common support of the type used in
forming an electrical filter. The resonator structure comprises a
substrate 10 having an upper planar surface 12. Substrate 10 can be any
convenient material that is easily workable, e.g. any of the well known
semiconductor materials. In the present specific example, substrate 10 is
a silicon wafer normally used for fabricating semiconductor products.
Other materials useful as resonator supports include, inter alia, glass,
quartz, sapphire or high resistivity silicon In the example illustrated in
FIG. 1, a plurality of alternating layers of SiO2 and AlN, ending with a
SiO2 uppermost layer, form an acoustic reflective mirror 14. Each of the
mirror layers has a typical thickness that is a 1/4 wavelength of the
filter's central frequency. For PCS cellular phone applications this
frequency is 1.9 gigahertz.
The use of an acoustic mirror of course, is not the only way to make a
resonator. What is needed, and what the acoustic mirror provides, is good
acoustic reflection at the boundaries of the transducer layer. Other
techniques to achieve this are known in the art, including using a solid
to air interface. Air against most solids produces the required acoustic
reflection. For example, one can also make an acoustic resonator by thin
film deposition of the resonator material on a substrate of Si and
subsequent removal of the layers beneath the resonator by: a) back etching
away the Si or b) deposition of a sacrificial layer beneath the resonator
which is removed by subsequent preferential etching. The present invention
is directed to resonator tuning by selective etching techniques, and
applies to all resonators regardless of their structure.
A bottom electrode, which may be patterned to define distinct electrodes
for each resonator structure, (not shown) or may be a common bottom
electrode 22 (shown in FIG. 1) is deposited and patterned (if required) on
the surface of the acoustic mirror. A mechanical transducer layer 18, such
as a piezoelectric layer, is next coated over the bottom electrode. In
most applications, the piezoelectric layer is coated as a continuous
conforming layer over the bottom electrodes, the acoustical mirror, if
present, and the support.
Top electrodes 24 and 24' complete the basic resonator structure. The term
electrode as used in this description denotes both single conductive layer
electrodes and/or electrodes comprising more than one layer at least one
of which is conductive.
The manner of fabrication of the above described layers and resonator
structure is well known in the resonator fabrication art. The different
layers can for example be fabricated utilizing any of the well known
techniques, such as, vacuum deposition of a convenient material,
electroless deposition, etc., followed by masking and etching to created
desired patterns. In FIG. 1, the transducer layer 18 is shown as a
continuous layer, however this layer may, depending on the particular
application, be masked and etched so that it exists only between the top
and bottom electrode defined areas.
Still as shown in FIG. 1, a plurality of individual piezoelectric
resonators are fabricated on a single wafer and, since each resonator is
relatively small (on the order of a few hundred microns on each side) and
the plurality of resonators are formed close together, each resonator will
be very similar to each adjacent resonator. A required number of
piezoelectric resonators are fabricated on a single substrate or wafer and
electrically connected to form a desired piezoelectric filter
configuration. The electrical connections are typically patterned on the
wafer at the same time that electrodes 22, 24 and 24' are patterned on the
wafer.
Because piezoelectric materials are the most commonly used transducer
materials, we describe this invention using a piezoelectric material for
the transducer. Such use is not, however, intended to limit the invention
to piezoelectric transducers. Other transducers such a magnetostrictive or
electrostrictive may equally well be used in filter designs and the
teachings of this invention apply equally well to structures that
incorporate different transducer materials. What is significant is that
the transducer material used results in a resonator having a resonant
frequency that is dependent on the overall thickness of the resonator,
which thickness includes both the transducer thickness and the electrode
thickness.
FIG. 2 is a top view of a basic T-cell type filter structure utilizing
three resonators, 30, 32, and 34. A three resonator T-cell filter
structure is a simple case used for illustrating the present invention.
Other, more complex designs are also well known in the art. Frequently,
many T-cells are concatenated to form more complex filters. There are also
lattice filters, and "L" filters. The present invention is applicable in
all cases, and is not limited to T-cells.
In cases where there are more than two resonators present in a filter
structure, there may be more that two resonant frequencies to which
resonators must be tuned. The use of selective etching according to the
present invention is therefore not limited to two resonant frequencies.
The two frequencies discussed herein are used only to illustrate this
invention.
Each resonator in FIG. 2 has the structure of the resonators shown in FIG.
1. A T-cell filter structure can thus be achieved by providing through
wire 36 an input connection to the resonator 30, and through wire 38 an
output. The shunt resonator 32 is connected to common or ground through
wire 39. Bottom electrode 22 serves as the common point between all
resonators. In this example, resonators 30 and 34 are designed to have the
same resonant frequency while resonator 32 has a different resonant
frequency. To obtain this difference in frequency the total
electrode/transducer combined thickness is the same for resonators 30 and
34 but different (higher for bandpass filters) for resonator 32.
In order to facilitate the batch manufacture of the resonators, it is
advantageous to initially form the bottom electrode and the piezoelectric
layer with the same thickness for all three resonators and vary their
resonant frequencies by varying the thickness of the top electrodes 40, 42
and 44. In this example electrodes 40 and 44 have the same thickness while
electrode 42 has a different thickness.
FIG. 3, illustrates the selective etching process according to the present
invention as applied to produce the three resonator T-cell filter
structure of FIG. 2. For illustration purpose the filter structure shown
is a simplified cross section of such structure. The acoustic mirror is
not illustrated because its presence is not essential in explaining this
invention. The person skilled in the art will recognize that the
resonators may be substantially more complex than illustrated, however the
structure as represented is sufficient to explain the invention, any
omitted features such as details of the resonator supports, connections,
protective layers etc. being well known in the art as previously
mentioned.
According to the present invention a different material is used for the top
electrodes, or for the uppermost portion of the top electrodes, for each
resonator having a desired different resonant frequency. In the simplest
case different metals are used for each electrode but the desired effect
of selective etching can also be achieved by making all electrodes from
the same material then adding a second layer, conducting or
non-conducting, to the top electrode of the resonator desired to function
at a lower frequency. FIG. 3 illustrates this composite electrode case.
This approach is advantageous as it permits a manufacturing process where a
uniform thickness single conductive layer is deposited over the transducer
layer. The conductive layer is next patterned to produce conductive layers
40, 41 and 44. Layers 40 and 44 serve as top electrodes for two of the
resonators. Next an added layer 46 is deposited and patterned leaving
additional material only over the corresponding top conductive layer of
the resonator meant to have a lower frequency using a material that has
different etching properties from the material used for the conductive
electrode layers 40, and 44. Such material may be a conductive layer of a
different metal or a non conductive material.
In this example, the top electrodes 40 and 44 for resonators 30 and 34
respectively, are formed by an aluminum layer, while the top electrode 42'
of resonator 32' is a combination of an aluminum layer 41 and a superposed
SiO.sub.2 layer 46 which is, preferably, co-extensive with the aluminum
layer. This SiO.sub.2 layer serves as an adjustment layer which may be
etched to adjust the resonant frequency of this resonator. If the topmost
electrode 46 of a resonator is non-conducting such as the case of SiO2, a
small exposed region of layer 41 must be provided to which connection 60
must be made for good electrical contact (not shown in FIG. 3). For better
performance this electrical connection can be made just outside the edge
of the resonator by leaving a small region of electrode 41 outside of the
boundary of bottom electrode 22 on top of which no electrode 46 remains
after device preparation.
In this manner the top conductive layer may deposited in a single step over
all resonators then patterned and additional material deposited only over
the patterned conductive layer forming the top electrodes of selected
resonators.
The support with the resonators is next placed in a vacuum chamber 48 for
reactive ion etching. Typically such chamber comprises as a minimum, an
article support 50 connected to a high voltage source (not shown), an
exhaust port 54 and a gas intake port 52. Test and measurement connections
60, 61 and 62 to the resonators of a sample are also provided whereby a
test signal may be selectively applied to each resonator to monitor the
resonator resonant frequency during the etching process. A vector network
analyzer (VNA) 64 can be used to measure the filter's response. Known
de-embedding techniques can be used to extract responses of the individual
resonators. See for example Pozar, Microwave Engineering, Addison-Wesley,
Reading, Mass., 1990.
Connecting to the proper resonator in the simple example illustrated in
FIG. 3 is of course easy. The filter input, output and ground connections
are conveniently available for testing connections. In complex structures
with filters having a plurality of interconnected resonators such easy
access may not always be available. In such cases simple small diagnostic
resonators may be constructed adjacent to and simultaneously with the
actual filter resonators and these diagnostic resonators can be used to
monitor the etching process. Since these diagnostic resonators are
constructed same as the actual filter resonators and are etched
simultaneously with the filter resonators, both will end up with the same
frequency during the etching process. Thus they provide a convenient way
to monitor the tuning of the actual filter resonators without access to
the filter resonators.
In a filter that comprises a plurality of resonators, such as the three
resonator T-cell filter shown in FIGS. 2 and 3, the top electrode of the
shunt resonator that is shown comprising an adjustment SiO2 layer may be
etched first using fluorine ions while the resonator is excited and its
resonant frequency monitored. When the proper thickness of the top
electrode has been reduced by sufficient etching of the adjustment SiO2
layer to obtain the desired resonant frequency, the etching process is
stopped and the fluorine evacuated from the vacuum chamber. Chlorine is
next introduced in the chamber and etching of the aluminum electrodes
begins without need to mask or move the sample in the vacuum chamber. A
test signal is now applied to these resonators and when the resonant
frequency is reached for the resonators with the aluminum top electrode,
the etching process is terminated. Thus both resonators have been adjusted
to proper different frequencies with high accuracy and without need to
mask or move the sample during the process.
Alternatively, the three resonators may be fabricated with the top
electrodes 40 and 44 out of aluminum and the top electrode 42 out of gold.
Aluminum and gold are etched in different etchants, therefore pairing
aluminum and gold for the top electrodes allows the eventual selective
etching of each electrode to obtain the necessary frequency adjustment for
each resonator.
Removal of excess electrode thickness is done by etching the excess
material from the top electrodes, preferably by dry etching as described
above. However, selective etching may also be accomplished using RIE with
gasses that may etch both top electrode materials in combination with
gasses that etch top electrode materials selectively. For example, argon
gas under high voltage etches everything simply by mechanical process.
Chlorine, on the other hand will not etch Gold as fast as Aluminum if at
all. One can thus use the nonselective argon gas to etch all resonators
until one subset of resonators, those having gold as the top electrode,
are at their proper frequency leaving the others, with aluminum top
electrodes still below their desired frequencies. Then use chlorine gas to
etch the aluminum top electrodes of the remaining subset of the resonators
to tuning them to their desired resonant frequency.
Reactive ion etching is preferred because it permits the simultaneous
testing of the resonator while it is being etched. However other etching
techniques can be applied within the scope of this invention.
Wet etching by dipping the parts in solution offers the advantage of speed
and can also be used to practice this invention. In such case, a
measurement of the resonator frequency is made prior to dipping the
resonator in an etching bath. A subsequent timed immersion removes desired
amounts of material. For example, in a structure with three resonators
where three different resonant frequencies are desired, the top electrodes
(or the topmost layer of the top electrodes) of the three resonators may
be respectively titanium, gold and aluminum. The baths then may be EDTA
Peroxide to etch the titani | | |
