Cantilevered air-gap type thin film piezoelectric resonator

A method and apparatus directed to a thin film piezoelectric resonator deposited on a support layer of dielectric material and being configured in the form of a cantilever structure having a base portion formed on an integrated circuit substrate and having an overhanging beam portion joined at one end to the base portion, with the opposite end being unsupported with an air-gap intermediate the beam portion and the substrate. In the fabrication of the resonator, a thin film of dielectric material, e.g. zinc oxide (ZnO) is first deposited on the substrate, but is subsequently removed from beneath the beam portion resulting in the air-gap which is open on three sides and thus results in a greater ease of fabrication. A completely monolithic chemical detector is implemented utilizing a matched pair of these cantilevered air-gap thin film resonators, with one of the resonators further including a coating on one of its electrodes and which is responsive to a predetermined chemical to cause a frequency shift in the presence of the chemical to be detected. In one embodiment of the chemical detector, the frequency outputs of the two resonators are compared with the difference frequency providing an indication of the chemical being detected.

FIELD OF THE INVENTION 
This invention relates generally to analog signal processing and 
piezoelectric thin film resonators and more particularly to a cantilevered 
air-gap type thin film piezoelectric resonator and a chemical detector 
implemented with such resonators. 
BACKGROUND OF THE INVENTION 
Thin film resonators having a diaphragm consisting of a piezoelectric zinc 
oxide film on a silicon or silicon dioxide film are generally known and 
have received considerable attention due to the fact that such devices 
exhibit a low temperature coefficient at the resonant frequency which is 
determined by the thickness of the piezoelectric film. Resonant 
frequencies in the VHF/UHF frequency range have been developed and have 
received considerable attention inasmuch as these devices are compatible 
with integrated circuit structures in their fabrication processes and 
device size. Further, a self-supported thin film resonator with a very 
thin air-gap between the semiconductor surface and the diaphragm bottom 
surface has been developed and is disclosed, for example, in a publication 
entitled "Air-Gap Type Piezoelectric Composite Thin Film Resonator", H. 
Satoh et al, Proceedings of the 39th Annual Symposium On Frequency 
Control, 1985, pp. 361-366. While such resonators are presumed to operate 
as intended, certain inherent limitations nevertheless exist, particularly 
as it relates to their fabrication. 
It is an object of the present invention, therefore, to provide an 
improvement in piezoelectric resonators. 
It is a further object of the invention to provide an improvement in thin 
film piezoelectric resonators. 
It is still another object of the invention to provide an improvement in 
air-gap type piezoelectric thin film resonators and their relative ease of 
fabrication so as to provide a higher device and circuit yield. 
It is still yet another object of the invention to apply an improvement in 
air-gap type thin film resonators having a cantilever construction to 
implement a monolithic chemical sensor. 
SUMMARY OF THE INVENTION 
Briefly, the foregoing and other objects are achieved by a method and 
apparatus for providing a thin film piezoelectric resonator deposited on a 
support layer of dielectric material in the form of a cantilever structure 
having a base portion formed on a substrate for integrated circuits and an 
overhanging beam portion joined at one end to the base portion, with the 
opposite end being unsupported with an air-gap intermediate the beam 
portion and the substrate. In the fabrication of the resonator, a thin 
film of dielectric material is first deposited on the substrate, but is 
subsequently removed from beneath the beam portion to provide the air-gap 
which is open on three sides and thus results in a greater ease of 
fabrication. A completely monolithic chemical detector is implemented 
utilizing a matched pair of these cantilevered air-gap thin film 
resonators, with one of the resonators further including a coating on one 
of its electrodes and which is responsive to a predetermined chemical to 
cause a frequency shift in the presence of the chemical to be detected. In 
one embodiment of a chemical detector, the frequency outputs of the two 
resonators are compared with the difference frequency providing an 
indication of the chemical being detected.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings and more particularly to FIG. 1, shown 
thereat is an embodiment of a cantilevered air-gap type thin film 
piezoelectric resonator. As shown, reference numeral 10 denotes a 
substrate used for integrated circuits and is shown comprising gallium 
arsenide (GaAs) or silicon (Si). On the upper surface 12 of the substrate, 
there is formed a cantilever support structure 14 comprised of insulator 
material such as silicon dioxide (SiO.sub.2). The cantilevered support 
structure 14 is comprised of a base portion 16 from which there extends a 
beam portion 18 under which there is an air-gap 20 which is open on three 
sides. The beam portion 18 is shown having a generally uniform thickness 
extending parallel over the top surface 12 of the substrate 10 and defines 
a generally rectangular surface area on the top surface 22. The base 
portion 16, on the other hand, is generally triangular in shape, having a 
sloping or an inclined upper surface 24. Over the top of the base portion 
including the top surface portion 22 and 24, there is formed a first or 
bottom metallic electrode 26. On top of the metallic electrode 26 over the 
top surface portion 22 of the beam portion 18 there is formed a thin film 
28 of polycrystalline piezoelectric material such as aluminum nitride 
(AlN) or zinc oxide (ZnO). On top of the piezoelectric thin film layer 28 
there is formed a second or top metallic electrode element 30. 
Both the piezoelectric film 28 and the top electrode 30 are illustrated as 
being of a generally square configuration with the top electrode 30 being 
relatively smaller in size than the piezoelectric film 28. However, the 
relative size and shape of these two elements is selectively chosen by the 
designer for the particular application. 
Upon the application of an excitation voltage across the electrodes 26 and 
30, a compressional wave is generated in the piezoelectric material 28, 
with the thickness of the piezoelectric film 28 determining the frequency 
of operation in the fundamental mode. The thickness of the thin 
piezoelectric film 28 represents a half wavelength at the fundamental 
frequency. Operation of the device can occur at higher frequencies, 
however, by operating at high overtones. 
The resonator structure shown in FIG. 1 is fabricated in the manner shown 
in FIGS. 2A-2F. In the first step as shown in FIG. 2A, a thin film of 
dielectric material, such as non-piezoelectric ZnO, is deposited on the 
substrate 10 as shown by reference numeral 11. The thickness of the ZnO 
film 11 defines the height of the air-gap 20 shown in FIGS. 1 and 3. Next 
a layer of SiO.sub.2 is deposited, as shown in FIG. 2B, which as noted 
above serves as a structural support for the thin film resonator. Next the 
layer of metallic electrode 26 forming the bottom electrode is deposited 
on the top surfaces 22 and 24 of the support structure 14. This is 
followed by a deposition of the piezoelectric film 28 as shown in FIG. 2D 
and which is then followed by the deposition of the second or upper 
electrode layer 30, as shown in FIG. 2E. Finally, the original supporting 
ZnO layer 11 is etched away using standard wet chemistry techniques upon 
completion of the device to form the air-gap 20 as shown in FIG. 2F. 
The major advantage of the air-gap type thin film resonator resulting from 
fabrication steps shown in FIGS. 2A-2F, is that the cantilever 
construction permits faster, more reliable fabrication of the air-gap 
structure which is vital to the operation of this type of device. The 
geometry permits three sides of the support structure 14 to be accessible 
to etching and permits for an accelerated etching process, since the 
etching recedes from three sides instead of two as in the known prior art 
devices, thereby decreasing the possibility of damage to any other active 
components formed on the same integrated circuit substrate 10. Also the 
formation of a cleaner, well defined air-gap 20 provides for the ability 
to etch smaller gaps which add to the structural integrity and reliability 
of the resonator device. 
Although a single resonator device is shown in FIG. 1, clusters of matched 
devices can be produced around and over a single air-gap support layer 11 
of ZnO which when removed, results in air-gaps 20 being provided Such a 
configuration is shown in FIG. 3 where two resonator devices 32.sub.1 and 
32.sub.2 are depicted. Clusters of these resonant structures can be used 
to form, for example, multi-resonator filters or multiple frequency 
sources operating singly or in combination. 
This now leads to consideration of a practical application of the 
cantilevered resonator device shown in FIG. 1 and comprises the 
implementation of a chemical detector which, for example, utilizes the 
pair of matched resonators 32.sub.1 and 32.sub.2 as shown in FIG. 3. One 
of the resonators 32.sub.1, however, includes an additional element in the 
form of a top most layer 34 which comprises a chemical coating selectively 
chosen that it either adsorbs, absorbs or chemisorbs a chemical to be 
detected such that it will cause a frequency shift in the cantilevered 
resonant frequency via mass loading of the active surface area of the 
piezoelectric film 28 overlaying the beam portion 18 of the SiO.sub.2 
support structure 14. 
The chemical detector device operates on the same basic principle as other 
bulk acoustic wave devices in that the thickness of the piezoelectric film 
28 determines the frequency of operation in the fundamental mode such that 
the film thickness represents a half wave at the fundamental frequency as 
referred to earlier. For a bulk wave device, the following relationship 
can be shown for relating a change in detector operating frequency for a 
change in the mass of the film coating the detector: 
##EQU1## 
where M.sub.s is the mass of the film coating the detector surface, f is 
the operating frequency of the device, A is the surface area of the 
piezoelectric material, g is the density of the piezoelectric film, and v 
is the velocity of propagation of a transverse wave in the crystal plane. 
Therefore, any change in the mass loading caused by the detector coating 
34 interaction with the desired contaminant will cause a corresponding 
change in the operating frequency. It can be shown that such a detector as 
described has a sensitivity of parts per billion. From equation (1) the 
increased sensitivity of the cantilevered chemical detector is apparent. 
The reduced active surface area A of the device will increase the 
frequency shift for any given mass of sorbed material, thereby increasing 
its sensitivity. Also, the ability to operate at higher frequencies also 
increases the sensitivity. 
Referring now to FIG. 4, there is shown a schematic diagram of a monolithic 
chemical detector comprised of the matched pair 32.sub.1 and 32.sub.2 of 
cantilevered air-gap type thin film resonators which form the sensor 
circuit and where one of the resonators 32.sub.2 serves as a reference 
frequency source while the other resonator 32.sub.1 acts as the chemical 
detector The reference frequency source 32.sub.2 is hermetically sealed in 
packaging 36 with the remainder of the active components including but not 
limited to, for example, a pair of RF amplifiers 36 and 38 which have 
their respective outputs coupled to a differential output amplifier 40. 
The detector resonator 32.sub.1 includes a film 34 which remains exposed 
to the chemical(s) to be detected. 
As shown, the output from the two resonator structures 32.sub.1 and 
32.sub.2 feed into respective RF amplifiers 36 and 38 with a portion of 
their respective outputs being fed back as feedback to the electrodes 26 
while the other part of the signal serves as the data signal. The two data 
signals from the resonators 32.sub.1 and 32.sub.2 are fed to the 
differential amplifier 40 which provides a difference frequency output 
which is representative of the difference in mass loading between the two 
resonators 32.sub.1 and 32.sub.2 and which provides an output signal at 
circuit terminal 42 which acts as the detector output for the chemical 
sensor. 
Thus what has been shown and described is an improved piezoelectric thin 
film resonator which provides the basis for monolithic integration of very 
small filters and frequency sources which are required for highly compact 
RF/microwave integrated circuits. 
Having thus shown and described what is at present considered to be the 
preferred embodiments of the invention, it should be noted that the same 
has been made by way of illustration and not limitation. Accordingly, all 
modifications, alterations and changes coming within the spirit and scope 
of the invention are herein meant to be included.