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DESCRIPTION
1. Technical Field
This invention relates to bulk wave resonators, and more particularly to
improved resonators and filters employing longitudinal bulk acoustic waves
operating in a fundamental (half wavelength) thickness mode.
2. Background Art
Technological advances in integrated circuit manufacturing processes and
techniques have provided the capability for implementing systems and
subsystems in microelectronic chips. Whereas the size of integrated
circuits continues to diminish, the size of external resonators and
filters, which may be used for frequency control in conjunction with
integrated circuits, has not decreased accordingly. Therefore, the
disparity in size between integrated circuits and related filters and
resonators continues to increase. Additionally, there appears to be an
ever increasing need for higher quality devices operating at carrier
frequencies in the hundreds of megahertz frequency regime. Although higher
frequencies may be achieved by use of frequency multipliers, the use of
frequency multipliers, and the commensurately-required sideband and
subharmonic filters, further aggravates the size disparity between
resonators and filters and integrated circuits which they may be used
with, as described hereinbefore.
To partially satisfy this need, there are a large number of crystal
resonators known in the art. Some of these achieve the desired high
frequencies (hundreds of megahertz) by working at a frequency which is an
harmonic of the device fundamental frequency. However, if one uses the
fourth harmonic, for instance, then one must separate it from the third
and the fifth harmonics by means of band-pass filtering. On the other
hand, use of the fundamental mode is desirable since many of the harmonics
are not generated in fundamental mode devices, depending on the
characteristics of the particular device, and such harmonics as are
generated can be eliminated simply with low pass filtering. Additionally,
the bandwidth of filters comprising resonators operating at harmonics is
severely limited, due in part to the inefficiency of the higher ordered
modes. Examples of higher order mode bulk wave resonators are described in
Page, D. J., Proceedings IEEE, October 1968, pp. 1748, 1749 and Sliker, T.
R., et al, Journal of Applied Physics, April 1967, pp. 2350-2358. The
devices reported there employ a cadmium sulphide layer on top of silicon
or quartz and require that the cadmium sulphide layer be a half wavelength
at the desired frequency, there being additional half wavelengths in the
underlying crystal substrate. These devices tend to have relatively low Q
(resonant quality factor) and are reported to be difficult to produce in
desired size ranges without additional problems such as acoustic
scattering.
A more recent addition to the art is a fundamental mode resonator capable
of operation in excess of 100 megahertz. These devices are formed by ion
milling of lithium niobate or quartz crystals. In general, relatively good
devices have been produced in this way. And, filters employing a pair of
coupled resonators in the same crystal have been successfully fabricated.
However, the utilization of lithium niobate or quartz does not foster the
utilization of such filters with electronic devices that can be fabricated
by normal semiconductor integrated circuit processing techniques on a
single monolithic substrate. For providing two or more resonators on the
same crystal so as to fabricate filters, it is necessary that the devices
be capable of implementation in a closely spaced relationship in order to
have an adequate coupling coefficient between resonators. The success of
such devices may depend on the modes (shear or longitudinal) which are
utilized. Although a shear mode would seem to lend itself to high coupling
coefficients, the coupling coefficient between shear mode resonators in
quartz has been reported to be in the range of 0.0001 to 0.001 (Sykes, R.
A., et al, IEEE Convention Record 1967, pp 78-93).
In addition to compatibility with integrated circuit technology
(semiconductor processing), desirable features of crystal resonator
manufacturing processes include high repeatability of smooth parallel
surfaces, desired thicknesses, and minimum conductivity of the
piezoelectric material. Furthermore, the processes involved should be
inexpensive and simple, and lend themselves to mass production.
DISCLOSURE OF INVENTION
Objects of the invention include provision of fundamental mode resonators
and filters operable in the hundreds of megahertz regime, which are
capable of being fabricated in a form compatible with semiconductor
processing or other considerations, and which are readily fabricated
utilizing mass production techniques on a reliably reproducible basis.
According to the present invention, a fundamental, longitudinal, thickness
mode crystal resonator comprises a substrate having a thin diaphragm
etched therein, a counter electrode disposed over the diaphragm, a thin
film of highly piezoelectric, high resistivity material disposed over the
secondary electrode, and a metal primary electrode disposed on the
piezoelectric material. In accordance further with the invention, there
are two metal electrodes disposed on the piezoelectric material in close
proximity, whereby to provide two adjacent resonators which are mutually
coupled to form a filter. According further to the invention, the
piezoelectric material comprises zinc oxide. According to the invention
still further, an isolation layer, such as silicon dioxide, may be
disposed between the substrate and the counter electrode layer, thereby to
provide crystallographic and/or chemical isolation between the secondary
electrode and the substrate. In still further accord with the invention,
the substrate may be silicon.
Resonators and filters incorporating the present invention may be
fabricated utilizing batch processing techniques well known in the
semiconductor integrated circuit industry. Many devices may be made on a
single wafer, and such devices may be made directly on the same chips as
are circuits to which such devices are to be connected for use. A variety
of techniques for assuring the requisite characteristics of such devices
may be employed, all of which are well known in the art, in the light of
the teachings which follow hereinafter.
The foregoing and other objects, features and advantages of the present
invention will become more apparent in the light of the following detailed
description of exemplary embodiments thereof, as illustrated in the
accompanying drawing.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is a plan view of an exemplary embodiment of the invention; and
FIG. 2 is a sectioned side elevation view taken on the line 2--2 in FIG. 1.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring now to FIGS. 1 and 2, a single device which incorporates the
present invention is shown as it might appear after fabrication on a large
wafer along with a number of other such devices, or circuits with which it
may be related on the wafer. A silicon substrate 10 is etched to provide a
thin membrane 12 surrounded by thicker regions 14 which provide support.
Photolithographic masking and etching, well known in the semiconductor
industry, may be used to provide the thin diaphragm 12. On the other hand,
a chemical etch stop process, employing a solution of ethylene
diamine-water-pyrocatechol, at a temperature near 105.degree. C., may be
utilized. This process is described in Bohg, A., Journal Electrochemical
Society: Electrochemical Technology, February 1971, pp. 401, 402 and in
Guckel, H., et al: Applied Physics Letters, Nov. 1, 1977, pp. 618, 619.
This process involves the selective etching based upon the conductivity of
the silicon, which etches free or lightly doped silicon at a relatively
high rate (near 125 micron/hour), but is virtually incapable of attacking
silicon having a doping density near 7.times.10.sup.19 /cm.sup. 3. In
order to employ that process, it is necessary that the silicon substrate
be prepared with the desired conductivity regions, including the region
above the dot-dash line 16 in FIG. 2, which is heavily doped and the
region below that line which is lightly doped or free silicon. This may be
best achieved by using a wafer prepared by epitaxial growth in boron,
provided that the heavy boron doping does not interfere with subsequent
electronic device fabrication on the same wafer. When subsequent
electronic device fabrication is desirable, the region above the dot-dash
line 16 may be rendered highly conductive by ion bombardment, provided a
very thin diaphragm (on the order of several microns or less) is desired.
If a thicker diaphragm 12 is desired, in excess of 3-5 microns) then
normal diffusion techniques can be utilized to provide the desired
conductivity. When diffusion or ion bombardment is utilized, only the area
of each device in the vicinity of the desired diaphragm need have this
special doping provided, leaving the remainder of the wafer undoped so as
to permit device fabrication in the usual fashion on silicon.
In the embodiment described with respect to FIGS. 1 and 2, there is a
silicon dioxide layer 18, which may be on the order of 0.15 micron
thickness. Then, a counter electrode or ground plane 20 may be deposited
over the silicon dioxide layer 18. This may preferentially comprise a gold
layer, which however may be sandwiched between very thin titanium layers
to provide better adhesion between the gold and silicon dioxide and
between the gold and the subsequent layer 22 of zinc oxide. The counter
electrode layer 20 may extend entirely across any substrate which is not
to have electronic devices formed therein, or may be limited to the
diaphragm area, as shown. The silicon dioxide layer 18 may provide both
chemical and crystallographical isolation between the zinc oxide and the
silicon substrate 10. For instance, the silicon dioxide layer 18 can
prevent an undesirable solid state reaction between the silicon and the
gold which can occur at the deposition temperature (about 325.degree. C.)
of the zinc oxide layer 22. The reaction between the gold and silicon
degrades the surface of the counter electrode quality, which in turn
degrades the surface smoothness and uniformity of crystalline orientation
of the zinc oxide. Further, the silicon dioxide layer 18, being an
amorphous structure, masks the (100) symmetry of the underlying silicon
substrate. This allows reliable growth of the gold counter electrode with
a (111) orientation so that the C-axis zinc oxide film 22 can readily be
RF diode sputtered under conditions yielding the same three-fold symmetry.
In other words, the silicon dioxide isolates the (111) gold counter
electrode from the (100) symmetry of the silicon substrate, so that the
gold counter electrode serves as a seed for achieving a high quality
C-axis zinc oxide crystalline orientation. If used, the titanium layers
deposited both before and after deposition of the gold film may be on the
order of 300 Angstroms thick. The quality of the surfaces upon which the
zinc oxide is grown is significant because the longitudinal acoustic wave
propagates only along the zinc oxide C-axis, and any misorientation
thereof not only reduces the efficiency in exciting the longitudinal wave,
but also allows undesirable shear wave components to be excited, which
increases the spurious frequency response and decreases the quality factor
(Q) of the resonator.
The zinc oxide film 22 may be RF diode sputtered utilizing a relatively
pure (eg., 0.999) zinc oxide target. The wafer or devices being sputtered
are side mounted, preferably at a slight angle (such as 36.degree. off the
vertical) so as to provide uniformity of average thickness to on the order
of 1.5% per centimeter across an overall wafer, which corresponds to a
uniformity of 0.3% per centimeter over a single one millimeter square
diaphragm 12. It has also been found that sputtering with premixed 20%
oxygen/argon ambient and careful pressure control are helpful. For
instance, if the oxygen/argon is not premixed, the variations can alter
the ultimate characteristics of the device. Furthermore, although devices
can readily be sputtered utilizing pressures in the range of
6.times.10.sup.-3 Torr to 12.times.10.sup.-3 Torr, the lower pressures
tend to make the resulting deposited zinc oxide have high insulative but
low piezoelectric qualities and may result in buckling (rippling) of the
diaphragm 12. On the other hand, deposition at the higher end of the
pressure range (12.times.10.sup.-3 Torr) causes the resulting deposited
zinc oxide to be highly conductive and have high piezoelectric qualities,
and there is much less buckling. Obviously, the properties at the higher
pressure are desirable except that the conductivity results in very high
insertion losses. Thus, RF diode sputtering in the oxygen/argon ambient at
pressures on the order of 8 or 9.times.10.sup.-3 Torr have been found most
desirable. The sputtering is preferentially achieved with the substrate
temperature at about 325.degree. C.
In the filter embodiment shown in FIGS. 1 and 2, there are a pair of
closely spaced electrodes 24, 25, with corresponding connectors 26, 27
leading to bonding pads 28, 29. The pads 28, 29 should preferably be over
the regions 14 where there is sufficient substrate thickness to endure
suitable bonding of wires 30, 31 to the pads 28, 29. Additional wires 32,
33 may be bonded to the counter electrode 20 to form ports 34, 35. But the
pads 28, 29 and counter electrode 30 may be connected directly to circuits
formed on the same substrate, when appropriate. Various processes may be
utilized to provide the metallization 24-29, such as chemical etching or
lift-off processes. Because it is difficult to find a chemical etch for a
suitable metal which is selective with respect to the zinc oxide, it is
preferable to use a pattern-developed, photoresist coating, followed by
vacuum deposition of about 1500 Angstroms of aluminum, or possibly a few
hundred Angstroms of chromium, a thousand Angstroms of aluminum, and a few
hundred Angstroms of chromium. The vacuum evaporated Cr-Al-Cr film appears
to provide the best electrode definition, allowing the electrodes 24, 25
to be reliably reproduced with about a 2 micron spacing between them. This
may provide better coupling between the two resonators of the filter,
particularly at higher frequencies (say in excess of 400 MHz). The use of
sputtered electrode metallization is also possible; however, it is not
preferred because of unreliable pattern definition. Utilizing the
processing described hereinbefore, it is possible to provide zinc
oxide/silicon diaphragms as thin as 5 or 15 microns (corresponding to
fundamental frequencies of about 200-500 MHz) without any diaphragm
buckling. Because there is little or no diaphragm buckling, alignment of
successive masks is best achieved by accurately positioning the photo
masks to reference edges of the waver on which devices are being produced.
If found desirable in any given case, the connectors 26, 27 and the bonding
pads 28, 29 may be made thicker (on the order of 5000 Angstroms). However,
for maximal frequency, it is desirable to maintain electrodes 24, 25 which
are as thin as is compatible with quality assurance, since the fundamental
mode will be defined by the distance from the upper surface of the
electrodes 24, 25 to the bottom surface of the diaphragm 12. Minimal area
of the total metallization 24-29 is desirable in order to reduce parasitic
capacitance.
Once the device of FIGS. 1 and 2 has been fabricated, it is possible to
fine tune the fundamental mode frequency response of the device in a
variety of ways. For instance, ion milling of the bottom side of the
silicon diaphragm may be performed through an aperture in the package in
which the device is mounted. Since the ion milling process does not
perturb the electrical connections made to the electrodes on the zinc
oxide side of the resonator, the resulting resonant frequency change can
be monitored while the ion milling proceeds. It is possible to reduce, for
instance, an 8 micron silicon diaphragm 12 to a 3 micron diaphragm by ion
milling to achieve a frequency tuning of over 100 MHz. In fact, the ion
milling may provide an even smoother surface than the chemical etch stop
process. Alternatively, use of a suitably thick coat of photoresist to
protect the zinc oxide surface will permit use of a standard, low
temperature etchant to remove a desired amount of silicon from the
diaphragm 12, without creating any buckling or other adverse effects.
Thus, the frequency can be increased after fabrication and mounting. On
the other hand, if desired, the diaphragm 12 can be thickened by vapor
depositing a thin metal film on the bottom surface in a well known
fashion, while taking suitable precautions known to those in the art,
thereby to decrease the frequency of the final device. Placement of
acoustic absorbing material at the periphery of the electrodes and the
removal of zinc oxide at the periphery of the electrodes (but not between
electrodes of acoustically coupled resonators) may serve to enhance the
resonance Q factor, reduce unwanted sidelobe response, and/or improve
filter efficiency.
Devices of the type described with respect to FIGS. 1 and 2 hereinbefore
have been fabricated with zinc oxide layer thicknesses ranging from about
2 microns to 10 microns and silicon diaphragm 12 thicknesses ranging from
about 5 microns to 15 microns. The ratio of the thickness of the zinc
oxide layer to the silicon diaphragm 12 has been found to be an important
criteria. If the zinc oxide and silicon are of about equal thickness,
there may be spurious frequencies on both sides of the resonant frequency.
If the silicon is thinner than zinc oxide, there are spurious sidelobe
responses at frequencies below resonant frequencies; but if the zinc oxide
is thinner than the silicon, then the spurious sidelobe responses are at
frequencies above the resonant frequency. It has been found that the
absolute thickness ratio of zinc oxide to silicon might best be on the
order of between 0.3 and 1.0. The normalized thickness, expressed in terms
of the acoustic path length, taking into account the different acoustic
velocity of the two mediums (the velocity in silicon being approximately
8430 m/sec and that in zinc oxide being about 6300 m/sec) is perhaps a
more useful tool. It has been found that normalized acoustic thickness
ratios of zinc oxide to silicon on the order of 0.4 to 1.3 are preferred.
For maximum inter-resonator coupling in a filter of the type illustrated
in FIGS. 1 and 2, normalized zinc oxide/silicon acoustic thickness ratio
should be near unity. This can provide effective piezoelectric coupling
coefficient between the electrodes 24, 25 of about 0.28. Reducing the
thickness of the zinc oxide layer reduces the coupling coefficient.
Devices of the type described have been produced with insertion losses in
the 5 db-10 dB regime. However, optimization of electrode shaping, and
resonator area (which is confined by the related electrode 24, 25 and the
lower surface of the diaphragm 12) along with reduction in parasitic
series resistance and bonding pad capacitance, should yield devices with
insertion loss as low as 2 dB.
Although the embodiment shown in FIGS. 1 and 2 include a pair of resonators
defined by the corresponding electrodes 24, 25, it should be understood
that a single resonator may be employed, whereby only a single electrode
24 (or 25) would be provided. Also, by suitable design (such as having the
connectors 26, 27 parallel to the gap between the electrodes 24, 25, more
electrodes could be provided so as to provide a three, four or five pole
device, consisting of acoustically coupled resonators, thereby permitting
design of multipole filters. The equivalent circuit analysis of devices in
accordance with the present invention may be performed utilizing known
techniques, such as those described in Rennick, R. C., IEEE Transactions
On Sonics And Ultrasonics, October 1973, pp. 347-354.
The device described with respect to FIGS. 1 and 2 heretofore employs zinc
oxide on silicon. However, other materials than the zinc oxide may be
employed, if desired, within the spirit of the invention. All that is
necessary is that the material be highly piezoelectric and have a
sufficiently high resistivity. For instance, cadmium sulphide could be
employed if desired, although its characteristics are not as well known
and defined as those of zinc oxide, and there appears to be no advantage
in the resultant processing. Or, aluminum nitride may be employed on
silicon, or on an aluminum oxide substrate. For higher frequencies, the
high acoustic velocity of aluminum nitride in the layer 22 might be
advantageous. And if high temperature is a more paramount consideration
than electronic device compatibility, aluminum oxide may be preferred over
silicon.
As described hereinbefore, one of the advantages of the invention is that
resonators and/or filters may be fabricated on silicon substrates, thereby
permitting fabrication of electronic circiuits with frequency-determining
elements in the same monolithic substrate. However, where desired for
other purposes, the invention may be practiced with other substrate
materials.
If desired, gold may be used alone for the reference electrode 20, titanium
or perhaps platinum may be used alone if desired. And, the silicon dioxide
layer 18 may be eliminated if desired, where the resultant product does
not require the advantages thereof. The only thing significant is that
there is provided a substrate material having semiconductive, high
temperature, or other desired properties, which is readily processed to
form a viable thin diaphragm 12, with a counter electrode disposed
thereon, and a highly piezoelectric, sufficiently resistive material
disposed over the counter electrode, together with one or more metal
electrodes thereon. Of course, the piezoelectric material needs to be
present only in the region of the wave, between the electrodes 24, 25 and
the diaphragm. Similarly, although the invention has been shown and
described with respect to exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and various
other changes, omissions and additions may be made therein and thereto,
without departing from the spirit and the scope of the invention.
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