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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electrical resonators employing a mechanical
transducer and more particularly to a method for fine tuning such
resonators following batch fabrication.
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.
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 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 by etching material off the top electrode of
a resonator. With current technology, however, etching is not as
controlled a process as deposition. Etching tends to be less uniform,
smooth or reproducible than deposition. In fact prolonged etching may in
cases change the composition, morphology, grain nature or roughness of
thin films. Accurate etching processes require precise knowledge of the
rate at which material is removed to permit stopping at the exact moment
that sufficient material has been removed to produce the desired resonant
frequency. To a certain extent lack of precise control of the etching rate
may be alleviated by monitoring the device frequency during the etching
process.
When removal of material is done in a dry etching process it is usually
possible to monitor the resonant frequency of the device during the
etching process. However, monitoring of the resonant frequency during
etching is not possible when wet etching processes are used. Wet processes
are desirable as they are much faster than dry processes.
There is thus still a need for a process to accurately fine tune a
mechanical resonator to a desired frequency without concern for possible
over-etching and without need to monitor the frequency during the etching
process.
SUMMARY OF THE INVENTION
The above object is obtained in accordance with this invention by a method
for adjusting the resonant frequency of a mechanical resonator, the method
comprising using alternating selective etching to remove distinct
adjustment layers from an electrode comprising a plurality of stacked
adjustment layers, each of said adjustment layers having distinct etching
properties from any adjacent adjustment layers.
Such a process alleviates the need to know the precise rate of etching in a
particular process because etching will stop when the etching process
removes all the material of one layer and reaches the next layer that is
selected to be impervious to the etching process. In other words the
etching stops each time at the barrier, i.e. the change from one material
to another, as each layer is sequentially removed.
Because the stacked layers have been created by deposition of material on
the top electrode, complete removal of each layer maintains the uniformity
of the remaining layer obtained during the deposition of this layer. The
composition and morphology of the unetched layer film remains ideal.
In more detail, the proposed method is a method of manufacturing a
mechanical resonator having a desired resonant frequency by a process
comprising:
(a) forming a first electrode;
(b) forming a transducer layer over the first electrode;
(c) forming a second electrode with a plurality of discreet layers of known
thickness, each having etching properties different from at least one
other;
(d) sequentially etching a calculated number of the discreet layers thereby
incrementally reducing the resonator overall thickness by a known amount
to adjust the resonator resonant frequency to the desired resonant
frequency.
The distinct layers are composed of materials that have different etching
properties and have thickness calculated to represent a selected
fractional increment of the resonant frequency.
More particularly the present method includes first forming the second
electrode with an initial conductive electrode layer having a thickness
calculated to produce a resonator having a first resonant frequency that
is higher than the desired resonant frequency. Subsequently, calculating a
desired thickness for an adjustment layer such that when the adjustment
layer is placed over the first conductive layer the resonant frequency of
the resonator is reduced by a selected frequency increment. This selected
frequency increment is a small fraction of the desired frequency
correction for the resonator.
Having determined the thickness and number of adjustment layers to produce
over the conductive layer sufficient to bring the top electrode thickness
to a point such that the resonant frequency of the resonator is below the
desired resonant frequency, each of these layers is created using
materials having etching properties different from the etching properties
of any adjacent adjustment layers. Then, the actual resonant frequency of
the resonator is measured and the number of adjustment layers to be
removed to incrementally adjust the actual resonator frequency to the
desired resonant frequency determined.
Once this number is known, the process according to this invention
comprises sequentially selectively etching the calculated number of
adjustment layers to adjust the resonator resonant frequency to a desired
frequency.
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, so that one material can be
removed completely without substantially effecting the other. Thus
selective etching is the process of subjecting two or more materials to an
etching process that effects only one of the materials.
It is a further objective of this invention to provide a method as
hereinabove described, wherein there are at least two resonators
electrically connected and wherein the step of forming said adjustment
layers comprises forming a first plurality of stacked alternating
adjustment layers having first and second etching properties on one of
said at least two resonators, and forming a second plurality of stacked
alternating adjustment layers having third and fourth etching properties,
and alternatively selectively etching said first and said second
pluralities of alternating stacked layers to remove said calculated number
of adjustment layers to adjust the resonator resonant frequency to a
different desired frequency for each of said at least two resonators.
It is also an object of the present invention to provide a mechanical
resonator comprising a first electrode, a transducer and a second
electrode wherein the second electrode comprises a conductive layer and a
plurality of distinct stacked adjustment layers, each of the adjustment
layers having distinct etching properties from any adjacent adjustment
layers. Preferably, the first electrode is a bottom electrode placed over
a supporting substrate, and the second electrode is a top electrode over
the transducer.
The mechanical resonator resonant frequency is a function of the resonator
thickness and the stacked adjustment layers each have a thickness such
that removal of an adjustment layer increases the resonant frequency by a
known increment.
The adjustment layer thickness may be uniform for all layers, or may
decrease for adjustment layers closest to the conductive layer.
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 cross section of a typical resonator.
FIG. 2 shows a cross section of a resonator in which the top electrode is
structured with multiple layers in accordance with an embodiment of the
present invention.
FIG. 3 shows the resonator according to FIG. 2 after it has been adjusted
to a desired resonant frequency according to this invention.
FIG. 4 shows a cross section of a resonator of an alternate embodiment of
this invention in which the top electrode is structure with multiple
layers of different thickness.
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 a mechanical
resonator 10 on a support 12. The resonator structure comprises a
substrate 12 having an upper planar surface 14. Substrate 12 can be any
convenient material that is easily workable, e.g. any of the well known
semiconductor materials. In the present specific example, substrate 12 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 A1N, ending with a SiO2 uppermost layer, form an acoustic
reflective mirror 16. 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 conductive layer forming bottom electrode 18 is deposited and patterned
(if required) on the surface of the acoustic mirror. A mechanical
transducer layer 20, such as a piezoelectric layer, is next coated over
the bottom electrode, and a conductive layer 22 is coated over the
transducer layer and patterned to form the resonator 10.
In the figures used to explain the present invention the different layers
have been shown as co-extensive layers extending only in the area of the
resonator. This is done to avoid cluttering the illustrations. In most
applications, as is well known to the person skilled in this art, the
piezoelectric layer is coated as a continuous conforming layer over the
bottom electrode, the acoustical mirror, if present, and the support.
Similarly the acoustic layers may extend past the bottom electrode on
either side. The transducer is defined by the combination of elements
between the top and bottom electrodes in the area under the top electrode.
With the exception of the top electrode any of these elements may be
layers extending outside the top electrode covered area with little effect
on the resonator characteristics.
The top electrode may be a single conductive layer 22 as shown or a
composite of more than one preferably coextensive layers, at least one of
which is conductive, preferably the layer in contact with the transducer
layer.
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.
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 resonator 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.
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.
FIG. 2 illustrates the first step in adjusting the resonant frequency of a
batch produced resonator according to the present invention. The batch
produced resonator will have a structure similar to the structure shown in
FIG. 1. The combined thickness of the bottom electrode 18, the transducer
layer 20 and the top conductive layer 22 are calculated such that the
resonant frequency of the resonator 10 as batch produced is above a
desired frequency, f.sub.d. The actual frequency is next, if so desired,
measured and a thickness of the top electrode sufficient to bring the
resonant frequency to a second frequency f.sub.s below the desired
frequency calculated. In the alternative, the second frequency may be
simply estimated, without measuring the actual batch produced resonator
frequency, by providing a sufficient number of stacked layer to reduce the
resonant frequency to well below the desired one. Next a number of layers
of material preferably co-extensive with the top electrode are deposited
on the top electrode. This can be preferably achieved by depositing all
layers and then masking once and patterning the entirety in one etching
sequence. The thickness of each of the deposited layers is calculated to
produce a known incremental change in the resonant frequency. Thus as
shown in FIG. 2 five additional layers have been deposited over the top
electrode 22 bringing the resonant frequency of the resonator below the
desired resonant frequency.
As illustrated in FIG. 2 the added layers are distinct layers of materials
having different etching properties. Thus for example layer 18 may be an
aluminum layer, layer 24 a gold layer, then again layer 26 an aluminum
layer, layer 28 a gold layer, and again layer 30 an aluminum layer and
layer 32 a gold layer.
Because the incremental effect of each layer to the resonant frequency of
the resonator is known, one can now measure the frequency of the resonator
11 with the top electrode layers as shown in FIG. 2 and then determine how
many layers must be removed to obtain the desired frequency for this
resonator. Assuming that four layers have been determined that they must
be removed, the resonator may be first subjected to a first etching
process whereby the process only etches the gold electrode. Thus only one
layer will be removed in this step as shown in FIG. 3. Next the resonator
is subjected to a second etching process removing the now exposed aluminum
layer 30 until the layer is completely removed, and the next gold layer
exposed. The process is repeated as many times as needed to remove the
calculated number of layers resulting in a resonator as shown in FIG. 3
wherein the top electrode is shown as having two layers only.
This process is particularly useful in cases where it is not possible to
monitor the shift in frequency of the resonator during etching to obtain
fine tuning of batch produced resonators, as is typically the case where
wet, or chemical vapor etching is used. The ability to accurately use wet
chemical etching with predictable results allows more flexibility in
materials selection for the top electrode of resonators and higher
manufacturing speeds.
In one application of this technique, resonators having resonant
frequencies that differ by a small amount may be produced in a single
batch, and their differing resonant frequencies easily adjusted for each
by removing different numbers of layers to obtain the slight shift in
resonant frequency required in certain combinations of multiple
resonators.
FIG. 4 shows, in admittedly exaggerated form, an alternate embodiment of
this invention in which the layers added to the top conductive layer 22
have different thickness. Different thickness may be resorted to,
depending on the material used and the ability to control the thickness
uniformity of each layer during the deposition. Thus the gold layer 24'
over the aluminum layer 22 may have a first thickness that is less than
the thickness of the next aluminum layer 26' and so on. According to this
invention the thickness of the adjustment layers will be determined by the
desired end result, the materials and the etching processes available and
does not have to be identical for all layers.
According to the present invention a different material is used for
alternating layers of the top electrodes. In the simplest case different
metals are used for each electrode but the desired effect of selective
etching can also be achieved using both conductive and non conductive
layers, as well as using more than a combination of two different
materials.
Aluminum and gold are etched in different etchants, therefore pairing
aluminum and gold for the top electrode layers allows the eventual
selective etching of each electrode to obtain the necessary incremental
frequency adjustment. The same is true for the pair Aluminum and SiO2.
Removal of excess electrode thickness is done by etching the excess
material from the top layer. Selective etching according to this invention
may be accomplished using RIE with combinations of gasses that etch the
different layers selectively. For example, Chlorine based chemistry, will
not etch SiO2 as fast as Aluminum. Fluorine based chemistry on the other
hand will. One can thus use chlorine to etch the aluminum top layers and
fluorine for the SiO2 sequentially until after a number of pre-calculated
cycles sufficient layers have been removed to obtain the desired resonant
frequency.
Reactive ion etching or vapor phase etching are typically used because they
would permit the simultaneous testing of the resonator while it is being
etched. Testing for resonant frequencies may sometimes be impractical as
for instance in cases where multiple resonators are used in an electrical
circuit and access to a particular resonator may be physically difficult.
The present invention alleviates the need for continuous monitoring of the
etching process since the process terminates automatically when all of the
layer has been removed. Naturally monitoring may still be performed when
using the present invention, and still reap the advantages of automatic
termination of the etching process each time a layer is totally removed,
as discussed in the summary of the invention above.
The present invention, therefore, permits the accurate use of other etching
techniques such as wet and vapor chemical etching.
Wet etching by dipping the parts in solution offers the advantage of speed
and can also be used to practice this invention. A subsequent timed
immersion of sufficient length removes a layer and stops. Next the
resonator is dipped in a different bath and the next layer removed. And so
on until the desired number of layers are removed. The baths may be EDTA
Peroxide to etch a titanium layer, and PAE etch for aluminum, in cases
where the layers are alternating layers of aluminum and titanium. If a
gold layer is used, a potassium iodide/iodine bath can be used for the
gold layer.
Vapor phase etch is another possible process and tools exist and can be
used. Similar chemistry to the wet etch example above can be used.
Etching is well known technology not requiring further discussion herein,
as shown by the following two treatises: Vossen and Kern, Thin film
processes; Academic Press, San Diego 1978 and by the same authors, Thin
film processes II, Academic Press, San Diego 1991.
Single resonators are useful for single frequency applications such as
oscillators or other very narrow frequency applications. In some cases
there is need to tune two resonators to two different frequencies to make
broader bandwidth filters. According to the present invention, in such
filters, one resonator is made of alternating Ti and Al stacked layers
calculated as hereinabove described, and the other is Au and SiO2. By use
of combinations of nonselective removal such as Ar RIE or chemical
mechanical polishing (CMP) and the selectivity of EDTA/peroxide for Ti,
PAE for Al, KI/I for Au, and chlorine based RIE for Al, one can perform
the incremental tuning of this invention on each resonator separately
without interfering with the other. Other pairs of materials and etching
process may of course be used the above been given by way of illustration
rather than limitation.
The invention has heretofore been described with reference to specific
materials and etching processes. Such description is only for the purpose
of explaining our invention and the person skilled in the art will
recognize that there are alternate ways to practice this invention. For
example, while the description of the resonator refers to a top and a
bottom electrode, with the stacked layers comprising the top electrode, it
is also possible in resonator structures where the etching process can be
applied to either electrode that the stacked layers may be part of either
or both electrodes. Such modifications are to be construed as being
encompassed within the scope of the present invention as set forth in the
appended claims wherein we claim:
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